<%BANNER%>

Effect of time if harvest and storage conditions on the development of lenticel disorders of tablestock potato (Solanum ...

Permanent Link: http://ufdc.ufl.edu/UFE0042644/00001

Material Information

Title: Effect of time if harvest and storage conditions on the development of lenticel disorders of tablestock potato (Solanum tuberosum)
Physical Description: 1 online resource (92 p.)
Language: english
Creator: Makani, Mildred
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: disorder, enlarged, halo, lenticel, physiological, potato, proliferation, solanum, suberization
Horticultural Science -- Dissertations, Academic -- UF
Genre: Horticultural Science thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Enlarged lenticel is a physiological disorder affecting potato (Solanum tuberosum L.) periderm and is known to be caused by excessive moisture conditions. In storage, the lenticel cells expand, forcing the aperture to increase in size. The area immediately surrounding the lenticel aperture may also be raised or darken, forming a halo around the lenticel aperture. The objective of this research was to evaluate the effects of harvest time, storage temperature and relative humidity (RH) on the development of enlarged lenticel disorder and lenticel suberization in two fresh market potato cultivars, Fabula and Red LaSoda . In addition, the study aimed at characterizing lenticel suberization in relation to harvest time and lenticel diameter. Potato tubers were harvested at four different times, from pre-vine kill, to 3 weeks after vine kill (Harvests 1 to 4). The freshly harvested tubers were stored for 12 d at 10?C or 20?C, and low (65%) or high (95%) RH (10L, 10H, 20L, 20H), simulating commercial conditions. Incidence and severity of the enlarged lenticel and halo disorders were rated by the average diameter of each. This study confirmed that the incidence and severity of the disorder varies according to cultivar. Wet soil conditions increased the incidence of proliferated lenticels before harvest. In storage, non-vine killed tubers (Harvest 1) for both varieties were more susceptible to the enlarged lenticel disorder, and they had a higher degree of quantitative and qualitative losses. An interaction of higher temperature and high relative humidity (20H) triggered development of the disorder in Fabula tubers. Red LaSoda tubers developed enlarged lenticels under all four storage conditions, although the disorder was more apparent at high humidity conditions. The enlarged lenticel disorder developed during the first 3 d in storage. There was no accumulation of free moisture on any of the tuber surfaces during storage. Incidence of halos increased when Fabula tubers were stored in low humidity (10Land 20L), appearing as darkened areas due accumulation of phenol compounds. The highest severity of the raised halo disorder was observed at 20H in Red LaSoda due to cell enlargement. The study also indicated that suberization of the lenticel filling cells may be a response to stress signals, such as vine killing and lenticel enlargement, since more suberization occurred after vine kill with an increase in lenticel diameter. Based on the results from this study, incidence of the lenticel disorders can be minimized if these varieties are harvested two weeks after vine kill when there is a higher probability of high rainfall events occurring during the growing season. To avoid severe incidence of enlarged lenticels, and minimize weight and moisture loss, Fabula tubers should be stored at 10H, while Red LaSoda at 10L.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Mildred Makani.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Sargent, Steven A.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042644:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042644/00001

Material Information

Title: Effect of time if harvest and storage conditions on the development of lenticel disorders of tablestock potato (Solanum tuberosum)
Physical Description: 1 online resource (92 p.)
Language: english
Creator: Makani, Mildred
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: disorder, enlarged, halo, lenticel, physiological, potato, proliferation, solanum, suberization
Horticultural Science -- Dissertations, Academic -- UF
Genre: Horticultural Science thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Enlarged lenticel is a physiological disorder affecting potato (Solanum tuberosum L.) periderm and is known to be caused by excessive moisture conditions. In storage, the lenticel cells expand, forcing the aperture to increase in size. The area immediately surrounding the lenticel aperture may also be raised or darken, forming a halo around the lenticel aperture. The objective of this research was to evaluate the effects of harvest time, storage temperature and relative humidity (RH) on the development of enlarged lenticel disorder and lenticel suberization in two fresh market potato cultivars, Fabula and Red LaSoda . In addition, the study aimed at characterizing lenticel suberization in relation to harvest time and lenticel diameter. Potato tubers were harvested at four different times, from pre-vine kill, to 3 weeks after vine kill (Harvests 1 to 4). The freshly harvested tubers were stored for 12 d at 10?C or 20?C, and low (65%) or high (95%) RH (10L, 10H, 20L, 20H), simulating commercial conditions. Incidence and severity of the enlarged lenticel and halo disorders were rated by the average diameter of each. This study confirmed that the incidence and severity of the disorder varies according to cultivar. Wet soil conditions increased the incidence of proliferated lenticels before harvest. In storage, non-vine killed tubers (Harvest 1) for both varieties were more susceptible to the enlarged lenticel disorder, and they had a higher degree of quantitative and qualitative losses. An interaction of higher temperature and high relative humidity (20H) triggered development of the disorder in Fabula tubers. Red LaSoda tubers developed enlarged lenticels under all four storage conditions, although the disorder was more apparent at high humidity conditions. The enlarged lenticel disorder developed during the first 3 d in storage. There was no accumulation of free moisture on any of the tuber surfaces during storage. Incidence of halos increased when Fabula tubers were stored in low humidity (10Land 20L), appearing as darkened areas due accumulation of phenol compounds. The highest severity of the raised halo disorder was observed at 20H in Red LaSoda due to cell enlargement. The study also indicated that suberization of the lenticel filling cells may be a response to stress signals, such as vine killing and lenticel enlargement, since more suberization occurred after vine kill with an increase in lenticel diameter. Based on the results from this study, incidence of the lenticel disorders can be minimized if these varieties are harvested two weeks after vine kill when there is a higher probability of high rainfall events occurring during the growing season. To avoid severe incidence of enlarged lenticels, and minimize weight and moisture loss, Fabula tubers should be stored at 10H, while Red LaSoda at 10L.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Mildred Makani.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Sargent, Steven A.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042644:00001


This item has the following downloads:


Full Text

PAGE 1

1 EFFECT OF TIME OF HARVEST AND STORAGE CONDITIONS ON THE DEVELOPMENT OF LE NTICEL DISORDERS OF TABLESTOCK POTATO ( Solanum tuberosum L.) By MILDRED N. MAKANI A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2010

PAGE 2

2 2010 Mildred N. Makani

PAGE 3

3 To my mom with love

PAGE 4

4 ACKNOWLEDGMENTS I would like to thank my advisor, Dr. Steven Sargent, for his continuous guidance, support, and encouragement throughout my research project. I would also like to thank my committee members, Dr. Donald Huber and Dr. Jerry Bartz for their guidance and suggestions. My sincere gratitude goes to the Hastings team Dr. C had Hutchinson, Douglas h to accommodate my research Thank you for all the support and ensuring access to all necessary resources. I also owe my sincer e thanks to Adrian Berry, Kim Co rdasco Dr. Marcio Pereira, Gabriella Maia, and fellow graduate students for their assistance with laboratory methods and statistical analysis. I would also like to express my sincere gratitude to Dr. Cecilia Nunes, for givi ng me full access to her lab for my storage experiments. Finally, I would like to thank my family and friends fo r their continuous support and unconditional love.

PAGE 5

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 LIST OF ABBREVIATIONS ................................ ................................ ............................. 9 ABSTRACT ................................ ................................ ................................ ................... 11 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 13 2 LITERATURE REVIEW ................................ ................................ .......................... 17 Botanical Characteristics of Potato ................................ ................................ ......... 17 U.S. Potato Industry ................................ ................................ ................................ 18 Production and Consumption of Florida Potatoes ................................ ................... 20 Harvesting and Post Harvest Handling of Fresh Potatoes ................................ ...... 21 Extent of Physiological Disorders Associated with Lenticels ................................ ... 25 Research Objectives ................................ ................................ ............................... 29 3 POTATO AS AFFECT ED BY HARVEST TIME AND STORAGE CONDITIONS .... 30 Introduction ................................ ................................ ................................ ............. 30 Materials and Methods ................................ ................................ ............................ 31 Plant Material ................................ ................................ ................................ ... 31 Harvest Times ................................ ................................ ................................ .. 32 Preparation and Treatments ................................ ................................ ............. 32 Lenticel and Halo Rating ................................ ................................ .................. 33 Weight Loss ................................ ................................ ................................ ...... 33 Moisture Content ................................ ................................ .............................. 33 Specific Gra vity ................................ ................................ ................................ 33 Statistical Analysis ................................ ................................ ............................ 34 Results ................................ ................................ ................................ .................... 34 Lenticel and Halo Rating ................................ ................................ .................. 34 Weight Loss ................................ ................................ ................................ ...... 36 Moisture Content ................................ ................................ .............................. 36 Specific Gravity ................................ ................................ ................................ 37 Discussion ................................ ................................ ................................ .............. 37 Conclusions ................................ ................................ ................................ ............ 41

PAGE 6

6 4 CONDITIONS ................................ ................................ ................................ ......... 56 Introduction ................................ ................................ ................................ ............. 56 Materials and Methods ................................ ................................ ............................ 57 Plant Material ................................ ................................ ................................ ... 57 Statistical Analysis ................................ ................................ ............................ 57 Results ................................ ................................ ................................ .................... 57 Lenticel and Halo Rating ................................ ................................ .................. 57 Weight Loss ................................ ................................ ................................ ...... 58 Moisture Content ................................ ................................ .............................. 59 Specific Gravity ................................ ................................ ................................ 59 Discussion ................................ ................................ ................................ .............. 6 0 Conclusions ................................ ................................ ................................ ............ 62 5 CHARACTERIZATION OF POTATO SUBERIZATION IN RELATION TO VINE KILLING AND ENLARGED LENTICELS ................................ ................................ 72 Introduction ................................ ................................ ................................ ............. 72 Materials and Methods ................................ ................................ ............................ 73 Plant Material ................................ ................................ ................................ ... 73 Sectioning and Fixation ................................ ................................ .................... 73 Infiltration and Dehydration ................................ ................................ ............... 73 Clearing and Staining Procedure ................................ ................................ ...... 74 Photography ................................ ................................ ................................ ..... 74 Results ................................ ................................ ................................ .................... 74 Discussion ................................ ................................ ................................ .............. 76 Conclusion ................................ ................................ ................................ .............. 77 6 CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH ..................... 83 LIST OF REFERENCES ................................ ................................ ............................... 87 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 92

PAGE 7

7 LIST OF TABLES Table page 3 1 Average environmental conditions in Hastings, Florida from February 2010 to June 2010 from FAWN weather database ................................ .......................... 51 3 2 Rating scale for e nlarged lenticel disorder according to lenticel diameter. ......... 51 3 3 Rating Scale For Halo Disorder According to Diameter. ................................ ..... 51 3 5 and stored for 12 d in four different storage treatments. ................................ ..... 53 3 6 in storage. ................................ ................................ ................................ ........... 53 3 7 harvest times. ................................ ................................ ................................ ..... 54 3 8 different times and stored for 12 d in four different storage treatments. ............. 55 4 1 Analysis of variance showing effects of harvest time, storage condition and storage period on development of lenticel disorders, weight loss and ................................ .................. 69 4 2 times and stored for 12 d in four different storage treatments ........................... 70 4 3 in storage for 12 days ................................ ................................ ......................... 70 4 4 The average initial (freshly dug) and final (after 12 d storage) peel and pulp m ................ 71 5 1 tubers before (initial rating 2, 3) and after 12 d storage at 10C and 20C, 95% RH storage for marke(ratings 2, 3) and unmarke(ratings 4, 5, 6). ............. 79 5 2 Average thickness of suberized lenticel cell layers before (initial rating 2,3) and after 12 days at 10C and 20C, 95% RH storage for marke(rating 2, 3) ................................ ... 79

PAGE 8

8 LIST OF FIGURES Figure page 2 1 Lenticel disorders. A) Proliferated lenticels. B)Enlarged lenticels with halos. ..... 28 3 1 Potato preparation and storage. Potatoes packaged into mesh bags and placed in storage chambers. ................................ ................................ .............. 43 3 2 Severity of the lenticel disorder. A) Lenticel rating 2, B) Lenticel rating 6. .......... 44 3 3 Halos appearing as a dark, swollen area surrounding the lenticel aperture. ...... 45 3 4 Lenticel rating and stored under four different storage conditions for 12 d. ............................... 46 3 5 and stored in four different st orage conditions for 12 days. ................................ 48 3 6 kill (Harvest 1). ................................ ................................ ................................ .... 50 4 1 ................................ .................. 64 4 2 Differences in potato skin set at harvest time, despite careful A) One week after vine kill (Harvest 2), and B) Three weeks after vine kill (Harvest 4). .......... 64 4 3 and stored under four different storage conditions for 12 d. ............................... 65 4 4 times and stored in four different storage conditions for 12 d. ............................ 67 5 1 95% RH), stained dark red and viewed under a light microscope (magnification 10X), A) Small lenticel, rating 2 or 3 (suberized zone thickness appro x. 37 m), B) Enlarged lenticel, rating 4, 5, or 6 (suberized zone thickness approx. 90 m). ................................ ................................ .................. 80 5 2 storage, stained dark red and viewed under a light microscope (magnification 10X), ................................ ................................ ................................ ................... 81 5 3 icels, after 12 day storage at 20C, 95% RH. ................................ ............................. 82

PAGE 9

9 LIST OF ABBREVIATION S 10L 10 C, 65% RH 10H 10 C, 95% RH 20L 20 C, 65% RH 20 H 20 C, 95% RH pts. points cwt centum weight C Degrees Celsius RH Relative Humidity d Day g/cm 3 grams per cubic centimeter ml milliliters lbs pounds Ha hectares lbs.ha 1 pounds per hectare kg kilograms mm millimeters cm centimeters cm 2 square centimeters m micrometers in inches MT Metric Tons VPD Vapor Pressure Deficit U.S. United States of America FAWN Florida Automated Weather Network

PAGE 10

10 FAO Food and Agricultural Organization FAA Formalin Acetic Alcoho l

PAGE 11

11 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science EFFECT OF TIME OF HARVEST AND STORAGE CONDITION ON THE DEVELOPMENT OF LENTICEL DISORDERS OF TABLESTOCK POTATO ( Solanum tuberosum L.) By Mildred N. Makani December 2010 Chair: Steven A. Sargent Major: Horticultural Science Enlarged lenticel is a physiological disorder aff ecting potato ( Solanum tuberosum L.) periderm and is known to be caused by excessive moisture conditions. In storage, the lenticel cells expand, forcing the aperture to increase in size. The area immediately surrounding the lenticel aperture may also be ra the lenticel aperture. The objective of this research was to evaluate the effects of harvest time, storage temperature and relative humidity ( RH) on the development of enlarged lenticel disorder and lenticel suberiza tion in two fresh market potato cultivars, In addition, the study aimed at characterizing lenticel suberization in relation to harvest time and lenticel diameter. Potato tubers were harvested at four different times, from pre vin e kill, to 3 weeks after vine kill (Harvests 1 to 4). The f reshly harvested tubers were stored for 12 d at 10 65 %) or hi gh (95%) RH (10L, 10H, 20L, 20H), simulating commercial conditions. Incidence and severity of the enlarged lenticel and halo disorders were rated by the average diameter of each.

PAGE 12

12 This study confirmed that the incidence and severity of the disorder varies according to cultivar. Wet soil conditions increased the incidence of proliferated lenticels before harvest. In stor age, non vine killed tubers (Harvest 1) for both varieties were more susceptible to the enlarged lenticel disorder, and they had a higher degree of quantitative and qualitative losses. An interaction of higher temperature and high relative humidity (20H) t the disorder was more apparent at high humidity conditions. The enlarged lenticel disorder developed durin g the first 3 d in storage. There was no accumulation of free moisture on any of the tuber surfaces during storage. (10Land 20L), appearing as darkened areas due accumulation of phenol compounds. due to cell enlargement. The study also indicated that suberization of the lenticel filling cells may be a response to stress signals, such as vine killi ng and lenticel enlargement, since more suberization occur r ed after vine kill with an increase in lenticel diameter. Based on the results from this study, incidence of the lenticel disorders can be minimized if these varieties are harvested two weeks after vine kill when there is a higher probability of high rainfall events occurring during the growing season. To avoid t 10L.

PAGE 13

13 CHAPTER 1 INTRODUCTION The U S potato industry has two main markets: the fresh and processing market. Since the late 1980s, the consumption of potato as food has shifted from fresh potatoes, to primarily processed products, such as frozen fries and chips (Pavilista, 2005). Irrespective of the decline in demand, fresh, or table stock, potatoes still account for a big portion of the national produce, with 28% being utilized as fresh potatoes (National Potato Council, 2010). F resh potato varieties are classified mainly according to their t uber shape, flesh and skin color, and smoothness of the skin. Hsieh et al (2009) classified the varieties into four major categories: red, white, minor colored, and russet type potatoes. The National Potato Council further classified fresh potatoes into se ven main types: russet, round white, long white, red, yellow, blue or purp le, and fingerlings (United States Potato Board 2010). In Florida, potato yield, resistances to pests and diseases, and horticultural quality at time of harvest are some of the key factors that determine variety se lection (Hutchinson et al., 1999 ). Growers tend to favor short season varieties that widely grown red skinned variety in the state. Othe r popular red or purple skinned skinned varieties, and consumers (Hut chinson et al. 2006). The planting season in Florida extends from October till February, depending on the production region (Olson et al., 1995). Due to their relatively rapid loss of quality, the tubers are stored for very short periods before being distributed to their respective U.S

PAGE 14

14 an d Canad ian markets (Emekandoko et al., 2006; Klassen et al., 2006). The marketability of fresh potatoes is highly determined by the quality of the tuber. Hiller (1985) defined quality as the nutritive value, texture, skin and flesh color, including interna l and external appearance of the tuber. A study conducted by Jemison et al. (2008) determined that skin quality was one of the key characteristics affecting consumer preference when purchasing fresh potatoes. Therefore, tubers that are free from any type o f blemishes tend to fetch a higher market price. However, like any other crop, the potato tuber is prone to pathogenic and non pathogenic diseases during growth, or while in storage. Sorting and grading of freshly dug tubers before packaging and storage he lps to eliminate low quality tubers. However, some diseases or physiological disorders tend to develop or worsen during storage or in transit to the markets. Enlarged lenticel is one of the physiological disorders that have been reported to develop wheneve r the field or storage conditions are not ideal. Lenticels permit gaseous exchange necessary for respiration through the relatively impermeable periderm (Evert and Eichhorn, 2006). In their normal state, they appear as tiny slits across the tuber surface, with tubers having an average of about one lenticel per 0.3 cm to 2 cm (Burton, 1989). Unfavorable field or storage conditions can induce the lenticels to proliferate, enlarging the aperture, and in some cases turning darker (Eddins et al., 1946). The di sorder leaves skin blemishes which lowers the cosmetic appearance of the tubers, affecting their marketability. Physiological disorders of lenticels have not received much attention in literature. To date, much of the research on lenticels has focused on t heir role as portals of entry for disease causing pathogens.

PAGE 15

15 Adams (1975) analyzed their development and structure, and mentioned that dry soil conditions promote suberization of the lenticel. As the tuber expands during growth, the suberized lenticel stre tches out into an oval shaped aperture. Other researchers have mentioned that suberization of lenticels increases as the tuber ages (Scott et al., 1996), sometimes forming a closing layer at the base of the lenticel (Lulai, 2007). Suberin has an active rol e in preventing dehydration of the internal tissues and sealing off any potential portals of entry such as lenticels and wounds (Lulai and Corsini, 1998). Tyner (1997) examined how the suberized closing layer can be ruptured when the cells beneath it proli ferate under unfavorable conditions. Under wet field conditions, they appear as domed centers of loosely packed cells, while in storage the aperture te nds to enlarge, with a raised or flat plateau in the center (Bezuindenhout, 2005). All authors have attributed this enlarged lenticel disorder to high humidity or oxygen depletion in the environment surrounding the tuber. Most potato varieties are repor ted to be more susceptible (Hutchinson and Gergela, 2006). Another factor that potentially affects proliferation is tuber age, with lenticels proliferating less readily in older tubers, even if the relative humidity reaches saturation (Adams, 1975). Flori da potatoes are normally harvested two to three weeks after their plant tops (vines) are killed by herbicide application, allowing tuber skin maturation before harvest (Mossler and Hutchinson, 2008). However, the actual time of harvest should be determined by the time it takes for the periderm to set on the tuber. This depends on the variety, weather conditions, and type of desiccant used (Haderlie et al., 1985). No

PAGE 16

16 examination of how this phenomenon affects the lenticel proliferation has been reported in t he literature. The goal of this research was to examine the effect of time of harvest on the appearance of the disorder. In addition, it was of interest to determine how different storage conditions affect the incidence and severity of the disorder. The d egree of lenticel and periderm suberization is determined by tuber age and subsequent storage conditions. Therefore, it was also necessary to analyze if suberization was correlated to the shape or size of the lenticels of tubers. Taken together, this knowledge can allow growers to get a better understanding of the pre and postharvest conditions that affect the development of the disorder. In addition, further research on ways to prevent or minimize the disorder can be carried out using this informati on.

PAGE 17

17 CHAPTER 2 LITERATURE REVIEW Botanical Characteristics of Potato A native of the Andean highlands of Peru, the white or Irish potato ( Solan um tuberosum L.) is an annual belonging to the Solanaceae family, together with the tomato ( Lycorpersicon esculentum Mill.), the eggplant ( Solanum melongena L.) and the pepper ( Capsicum frutescens L.). Potatoes are believed to have been first cultivated in Peru around 2000 B.C., later spreading out throughout the world temperate zones. The cool s eason plant was introduced in North America in 1691 (FAO, 2008). There are eight species of the cultivated potato belonging to the genus Solanum, with S. tuberosum being the most frequently cultivated (Burton, 1989). The aerial parts of the potato plant ra nge from 30 to 80 cm in length, with some cultivars reaching two meters. White, yellow, purple, blue or variegated inflorescences are borne on the stems; with many cultivars producing fruit (Linsinka and Leszczynki, 1989). The seed found in the fruit can b e used for propagation, although the most common method of propagation is vegetative, using tubers. The tuber, which is the edible portion, is an enlarged underground stem that forms at the end of stolons. It maintains the characteristics of the above grou nd stem, such as nodes, internodes, scale leaves, and lenticel pores. There are two ends to a tuber the bud end and stem end, the latter of which is attached to the stolon. The tuber consists of four primary zones of tissue (Dean, 1994).The first zone, t he periderm or skin, is the outermost layer and cultivars range in color from brown russet, white, red, pink or yellow. The skin texture ranges from smooth to netted. The cortex, which lies between the periderm and the vascular tissue, is the second tissue zone. It contains the highest

PAGE 18

18 starch concentration. Beneath the cortex lies the perimedullary zone, which comprises the largest amount of storage tissue. The pith is the central part of the tuber, and has the lowest starch concentration. The tuber flesh, comprising of the cortex, perimedullary zone and pith can be white, cream, yellow, purple or striated in color (Burton, 1989). Tuber shapes are round, oval, oblate, or a combination. Nutritionally, the tuber is a low fat, high energy vegetable, consisting of an average of about 18% carbohydrates and only 0.1% fat. It also has about 2% high quality protein, and 80% water (McCay et al., 1987). Raw potatoes have an average energy content of about 80 kcal per 100 g fresh weight basis (Li, 1985). Small amounts o f antioxidant carotenoids are also present in the flesh of all tubers, ranging from 50 to100 g per 100 g fresh weight, in white fleshed tubers. Yellow to orange fleshed tubers have a considerably higher amount, averaging 2000 g per 100 g fresh weight (Br own, 2005). Significant amounts of iron, thiamine, nicotinic acid and riboflavin are also found in the vegetable crop. The potato is a major food source in many parts of the world largely because of its ability to produce large yields per unit land area (D ean, 1994). U.S. Potato Industry Potatoes are grown in over 130 countries across the globe (CIP, 1984), with China being the leading producer, producing 64,837,389 Metric Tons (MT) in 2009 (FAO, 2009). According to FAO, world production of the potato over the last ten years has seen a 4.5% increment, ranking it in fourth place in terms of volume of production, after wheat, rice and corn. In 2007, over 350 million MT of potato were consumed worldwide, making it one of the most important food crops (FAO, 200 9). The U.S. is the fourth largest producer, with the crop being grown commercially in 36 states.

PAGE 19

19 In 2009, the U.S. produced 43.1 billion pounds of tubers on over 400, 000 ha of harvested land (USDA, 2009). Location of potato production in the U S is base d on consumer need, new markets and technological developments (Dean, 1994). Most of the crop is grown in the fall, with the northern states being the leading producers. In 2009, Idaho, Washington, and Wisconsin produced a total of over 23.5 billion pounds about fifty percent of the national crop. There has been an increase of about 15% in the value of the potato produced over the last five years, with the country generating over $3.5 billion from potatoes grown in 2009. This is largely due to an increase in potato price which went up from $7.04 in 2005, to $8.00 per 100 pounds in 2009 (Potato Statistical Yearbook, 2006, 2009). This makes potato the top vegetable crop grown in the USA. The crop has many cultivars with different maturity periods, ranging fr om early, medium to late season (Kay, 1973). The early cultivars mature in less than four months, medium take between 4 and 5 months, while the late cultivars can take up to 7 months, depending on the prevailing weather conditions. Most commercial growers in the northern states grow a russet type potato, which takes about four months to mature, and is harvested when the tuber skin is set and the vines are dying down. In the southern states, such as Florida, early cultivars are favored as they mature before the inclement weather conditions of high rainfall amounts and high temperatures set in (Park, et al., 2005). According to the Economic Research Service (2009), annual per capita consumption in 2009 averaged at 130 pounds, in the form of fresh or processed tuber. The largest proportion of potatoes produced in the U.S. are processed into frozen fries

PAGE 20

20 (34%), followed by fresh potatoes which use 27% of tubers produced. Potatoes are also processed into chips, starch, dehydrated and canned products. A smaller per centage is used for seed, animal feed and other agro industrial products (USDA/NASS, 2007). Production and Consumption of Florida P otatoes With a $135 million market value, the Florida potato industry is relatively small when compared to the leading producers such as Idaho and Washington, valued at an average of $753 million and $628 million, respectively. However, Florida contributes a third of the high value crop produced in the winter or spring season (Mossler and Hutchinson, 2008). With the 2009 s hundred pounds, and the national average at $8.00, Florida potato currently ranks as the 11th highest value potato producing state (USDA, 2009). The majority of the crop is grown in the Tri County Agricultural Area of Putnam, St. Johns, and Flagler counties in northeast Florida. In 2009, 13.2 hectares was harvested in Florida, with total sales of $135,201,000.00; an increase of over $4 million dollars from the previous year (USDA/NASS, 2009). Potatoes produced in Florida fall mainly into two main marketing niches, fresh and processed market. Florida is one of the top shipping states in the nation, with most of the growers establishing pre season contracts mainly for the processing industry (USDA, 2010). Fresh po tatoes are also high value crops, with grower production marketing costs at an average of $4,140 per acre (Van Sickle et al., 2009). Attributes of the variety which help determine final use of the tubers include skin type, set, color, tuber shape and speci fic gravity. Other desirable characteristics include yield, adaptability and disease resistance (Pack et al., 2005). Varieties with a high specific gravity, such as Atlantic and Harley Blackwell are produced commercially for use in the

PAGE 21

21 chipping industry. T he leading table stock varieties grown in Florida include Red LaSoda, LaRouge and LaChipper, which are all early to medium season varieties (Hutchinson et al., 1999). Potato plants are grown commercially when the days are getting warm and the nights cool. The crop grown in Florida has an average season of between 90 to 110 areas, and conti nuing through mid March in the northern counties. Split applications of Nitrogen and Potassium fertilizers a re required at planting and 3 to 4 weeks after planting (Hutchinson et al., 2008 ). Supplemental irrigation is also required during the warm spring m onths. summer in other southern and western states, have a thinner periderm which is easily damaged during handling (Suslow and Voss, 1996). Due to a greater probability of moisture loss and infection during storage, most of the tubers are kept for very short periods before being distributed to their respective markets. Harvesting and P os t Harvest Handling of Fresh P otatoes The life cycle of the potato plant can be divided into five main growth stages (Rowe, 1993), with early potatoes reaching maturity at an average of 100 days, while late varieties mature in about 150 days (Klenkopf, 1983). The first two growth stages involve sprout emergence and vegetative growth. Tuber i nitiation occurs in the third growth stage, followed by tuber bulking due to cell division and expansion. The final growth stage is the maturation stage. Maturity indices of potatoes include a peak in

PAGE 22

22 tuber dry weight, desirable tuber size, senescence of t he plant tops, and setting of the tuber skin (Suslow and Voss, 1996). a desirable market size, still have a thin, poorly developed skin that rubs off easily during harves t and packaging (Sabba and Lulai, 2002). This can potentially lower the market value of fresh market potatoes as appearance is one of the key factors influencing consumer preference. In addition, exposure of the internal tissue increases the occurrence of water loss and disease pathogen entry. The periderm is a protective tissue comprising of three parts: the phellogen (cork cambium), the phellem (cork), and the phelloderm. The phellogen, which comprises of actively dividing meristematic cambium cells ori ginates from the epidermis, and produces phellem cells outwardly and phelloderm cells towards the cortex (Fahn, 1990). In the early stages of tuber development, the periderm is living and the cells are unsuberized (Burton 1989 ). As phellem cells develop, they become suberized and die, have a very active phellogen, making the skin more prone to scuffing. Suberin, a complex biopolyester, is deposited on the cell walls whe n plants are exposed to environmental stress stimuli (Schreiber et al., 2005). It is comprised of two major domains, a phenolic domain and an aliphatic domain, in addition to soluble waxes found within the matrix (Lulai and Corsini, 1998). Suberization pro vides a barrier to desiccation of internal tissues and impedes microbial invasion. As the tuber matures, new skin layers are continuously added by cell division.

PAGE 23

23 It is, therefore, necessary for the tubers to develop a mature skin prior to harvest, to incr ease resistance to bruising, and decrease water loss and decay in transit or storage. In order to promote skin set, and reduce skinning or scuffing of the tubers, the potato plant vines are killed about three weeks before harvest, depending on the cultivar Vine kill also induces tuber maturity, encourages the tubers to loosen from the stolons, and helps ease harvest by reducing the vine quantity (Mossler and Hutchinson, 2008). Vines can be killed mechanically by vine pulling or cutting, chemically using herbicides, or a combination of both methods (Haderlie, et al., 2009). In Florida, the most common method used commercially is chemical vine kill, with the herbicide Pi quet being the standard used by the potato growers (Vavrina, 1999). Split applications of the herbicide are applied at the first indication of tuber maturity, when the plant tops begin to senesce. Diquat (Syngenta, NC)) has a fast desiccation rate. It requires at l east seven days between its application and subsequent tuber harvest (Hutchinson, et al., 1999). However, due to a number of other factors such as variety and plant growth stage, the actual number of days from application of the desiccant to the digging of potatoes ranges from 14 to 21 days. Other herbicides commonly used include endothal, pelagonic acid and carentrazone. By inducing plant stress, vine killing promotes suberin deposition onto the periderm, making the tuber more resistant to various quality issues that are caused by inadequate skin set (Lulai, 2007). Mechanical harvesting is the common method used commercially by potato growers. Sample tubers are tested for adequate skin set before harvest by rubbing the skin if the skin is not easily remov ed, the tubers area considered ready for harvest

PAGE 24

24 (Mossler and Hutchinson, 2008). After the washing, drying and removal of culls on the packing line, potatoes are packaged into bulk and consumer packages, depending on buyer specifications or the primary use of the tubers. Tubers for processing are normally packed into 2000 pound bins or tote bags, while those destined for the fresh market go in 5, 10, or 50 pound packages, generally netted or paper bags. Market quality of the harvested tubers is determined l argely by the absence of external defects, in addition to the shape, color, size, or quality of the tubers. Depending on the external appearance of the potatoes, tubers are graded as Extra No. 1, No. 1, Commercial, or No. 2 (Voss, 2005). Florida potatoes, being early to mid season varieties with minimally suberized periderms at harvest time, are very perishable compared to the late varieties. They are associated wi th the varieties. In contrast, the periderm of late varieties are well suberized at maturity, with vine kill and curing in storage further promoting skin set. Most of the late crop produced is stored for long periods to ensure adequate supply throughout th e year. In storage, it is critical to maintain quality of the tubers by minimizing water loss and respiration, keeping sugars at a minimum and maintaining external appearance (Plissey, 1976). Regulation of temperature and humidity is very important as the tuber is made up of about 80% water, and therefore quality can easily be compromised through water loss (Rastovski et al., 1981). According to Phillips and Armstrong (1967) storage of early potatoes at 10C and 85 90% RH keeps the tubers for a few days. Lowering the tem perature to 4 5C can prolong storage by 3 to 8 weeks (Snowdon, 1991). To maximize the time they can be

PAGE 25

25 stored, early varieties can also be cured for 8 days at 15C and 95% relative humidity. Curing promotes wound healing and periderm sub erization, which helps reduce qualitative and quantitative tuber losses during storage. The tubers can then be stored fo r up to 5 months at 4C and 95 to 98% relative humidity (Suslow and Voss, 1996; Thompson, 2003). However, long term storage of the ear ly varieties grown in Florida is not a common practice. Because of high early season prices in Florida, tubers are stored briefly, if at all, before being packed and transported to their respective markets. Many diseases and disorders affect tubers before harvest and in storage. Bacteria, fungi, nematodes, and viruses are the main vectors of pathogenic diseases of plants or tubers. Certain diseases, such as nutrient imbalances and physio ogical disorders of the plant or tuber, can appear in the absence of in fectious pathogens. The non parasitic diseases are a result of factors such as extreme temperatures and genetic disorders. These diseases can further predispose the potato plant or tuber to pathogenic diseases (Loria, 2001). An example is when lenticels en large, becoming portals of entry of pathogens, such as Erwinia carotovora which causes soft rot in tubers. Extent of Physiological Disorders Associated with L enticels When the tubers are young, with an average diameter of 1 centimeter or less, the tuber i s covered by an epidermis, with breathing pores called stomata scattered all over the surface (Lulai, 2001). As the tuber size increases due to further cell expansion, the epidermal skin is replaced by the periderm. The rectangular shaped phellem cells of the periderm lack intercellular spaces, forming a barrier which limits the amount of moisture movement or gaseous exchange occurring through the periderm.

PAGE 26

26 In order to maintain adequate oxygen supply to the interior tissues, parts of the phellogen undergo p ericlinal cell divisions, producing loose masses of small, thin walled and rounded cells called complimentary or filling tissue (Mauseth, 1988). The complimentary tissue develops under a stoma or a group of stomata. As the cells continue to divide, the gua rd cells of the stoma become raised and are eventually shed off. This region of loosely arranged cells with an opening to the external environment is called a lenticel. Since it is comprised of cells with intercellular spaces, the lenticel takes over the r ole of gas exchange and oxygen diffusion into the tuber for aerobic respiration (Peterson et al., 1985). Lenticels are capable of promoting oxygen diffusion through the periderm (Burton, 1989). Although essential for movement of water vapor and gas exchang e across the periderm, lenticels are also associated with physiological disorders or pathogenic diseases of tubers. Lenticels are usually inconspicuous, appearing as minute slits scattered all over the tuber surface. The number and size of lenticels found on a fully grown tuber range from an average of 1 to 3 per 1.0 cm (Burton, 1989). However, the number and size of lenticels per unit area of tuber can be influenced by soil type and the prevailing weather conditions during growth (Rastovski et al., 1981) According to Adams (1975), dry soil conditions promote suberin deposition on the walls of the lenticel filling cells, with the lenticels becoming flat and saucer shaped as the tuber expands during growth. A physical barrier of suberized cells may form at the base of the lenticel, retarding water loss from the interior tissue. Waxes with fungicidal properties found on the suberized closing barrier of cells are believed to aid in keeping pathogens out (Tyner, et al., 1997).

PAGE 27

27 Oxygen depletion in the environ ment surrounding the tuber can causes the parenchymatous lenticel cells to proliferate, rupture through the protective suberized layer, and erupt through the lenticel aperture. In this condition, the lenticel openings become ports of entry, predisposing th e tuber to diseases, including pectolytic bacteria that cause soft rots. Other pathogenic diseases caused by bacterial entry through the lenticel openings include powdery scab, stem rot, charcoal rot and gangrene (Talburt et. al 1987). The proliferated or hypertrophied cells appear on the tuber surface as white spots (Fig ure 2 1A), rising to about 1.5 mm under high field moisture conditions (USDA Handbook 479, 1978). This disorder can also occur during tuber storage if unfavorable conditions cause free moi sture to form on the tuber surface. When triggered by excessive moisture, this condition is commonly known as water spots or water scab (Lulai, 2001). Enlarged lenticels can also turn darker during storage (Eddins et al., 1946).The area immediately surroun ding the lenticel aperture may also be raised, ure 2 1B). Vapor pressure deficit, which is determined by the interaction between temperature, humidity and air ventilation, is one of the key factors affecti ng the development of the disorder in storage. Enlarged lenticels resemble scab lesions, with the distinction being that the lenticel disorder has smaller and lighter lesions. Enlarged lenticels can increase to over 6.0 mm in diameter, lowering the cosmeti c appearance of the tuber (Burton, 1989).

PAGE 28

28 A B Figure 2 1. Lenticel disorders. A) Proliferated lenticels. B)Enlarged lenticels with halos. To date, much of the research on lenticels has focused on their role as ports of pathogen entry. However, physiological disorders of the lenticels are also important, particularly for fresh market potatoes, by increasing packing costs due to more g rading and leading to rejected loads when the tubers become out of grade. Hiller (1985) refers to tuber quality as the nutritive value, texture, color of flesh and skin, external and internal appearance of the tuber. More potato recipes now being prepared with the tuber skin included, which is a good source of fiber and phenolic compounds. A survey conducted in Maine, USA, showed that tuber skin quality is one of the key characteristics that determine consumer preference (Jemison et al., 2008). All potato c ultivars have been reported as susceptible to the enlarged lenticel disorder. Therefore, there is great need to produce blemish free tubers which attract the cons umers and help the growers obtain the best market price. A number of precautionary measures ha ve been suggested to reduce the probability of development of the disorder before and after harvest. These include

PAGE 29

29 maintaining good field drainage, and avoiding overwatering and soil compaction (Lulai, 2001). Tuber surfaces should be adequately dried befor e being packaged, and storage conditions should ensure no build up of free surface moisture on the tubers. If the disorder occurs in the field, affected tubers can be easily removed during grading. However, when it develops after packaging, there will be n eed for the tubers to be graded once again. It is therefore necessary to examine how different preharvest and postharvest factors affect the development of enlarged lenticels and associated halos. Postharvest storage temperature, relative humidity and thei r interactions may potentially affect the incidence and severity of this disorder. This information would be of significant benefit to growers and storage operators, as they would gain a better understanding of the conditions that trigger or worsen the dis order. The knowledge can then be used to develop improved post harvest handling practices and storage facilities which minimize the incidence of the disorder during storage or in transit to the respective markets. Research Objectives The null hypothes es of this research were: 1) Time of harvest and degree of lenticel suberization have no effect on the development of enlarged lenticels in the field. 2) Temperature and humidity conditions in storage have no effect on development of the disorder during s torage of two potato cultivars. The objectives of the research were as follows: 1) To evaluate the effect of harvest time and storage conditions on the incidence and severity of enlarged lenticels and halos in two potato cultivars. 2) To characterize lenticel suberization in relation to harvest time and lenticel diameter.

PAGE 30

30 CHAPTER 3 DEVELOPMENT OF ENLAR GED LENTICELS AND HA TO AS AFFECTED BY HARVE ST TIME AND STORAGE CONDITIONS Introduction Lenticel disorders, like other tuber skin disorders, affect the price of fresh market potatoes by lowering the skin quality. Symptoms of the disorder range from white tufts of proliferated lenticels in water logged soils (Fahn, 1990) to darkened, enlarged lenticels in storage (Eddins et al., 1946; Lulai, 2001). In some cases, the periderm immediately surrounding the lenticel aperture can swell or grow darker. Adams (1975) attributed development of lenticel disorders to one or a combination of the following factors: tuber age, high moisture levels, and cultivar. Tuber maturity is assessed subjectively primarily on tuber size and degree of skin set (Suslow and Voss, 1996). Once the tubers reach maturity, plant vines are killed off with chemical desiccants to speed up the skin set process and improve the quality of tubers for easier postharvest handling. The standard harvest time in Florida ranges from 14 to 21 days after vine kill (Mossler and Hutchinson, 2008). Previous studies suggest that freshly dug tubers harvested later in the season took a longer time to proliferate when placed in unfavorable environments, compared to tubers harvested at an earlier date in the season (Adams, 1975). Therefore it is expected that time of harvest, which is related to tuber maturity, would have an effect on the development of the disorder during postharvest handling. Since the disorder may also develop or worsen in storage or during transit to the market, an understanding of the postharvest parameters affecting the disorder is essential. Temperature and humidity interactions are some of the key factors determing the water vapor pressure deficit (VPD) in storage. VPD is a measure of the difference

PAGE 31

31 between the actual water vapor present in the atmosphere, and the potential amou nt that could exist at the same temperature, without condensation. Surface moisture condensation, which increases the development of enlarged lenticels on tubers, occurs at a low VPD (Li, 1985). Variety trials done on white skinned fresh market potatoes sh owed that some grown in soils with high moisture levels (Hutchinson and Gergela, 2006). The objective of this study was to determine the effect of harvest time and subsequen t storage condition and duration on the incidence and severity of the disorder Materials and Methods Plant Material T was grown in the spring season of 2010 at the UF/IFAS Plant Science and Education Unit, Yelvington Farm in Hastings, Florida. The site is characterized by Ellzey fine sands ( taxonomic classification sandy, siliceous, hyperthermic Areni Endoaqualfs), which are poorly drained, moderately permeable soils (Ou et al., 1995; www.nts.usda.gov ). The planting date was February 18, 2010, with the experimental layout following a complete randomized design. Plant rows were 1 m (40 in) wide, with the production beds being separated by water furrows, used for seepage irrigation and drainage. Granular nitrogen fertilizer was de dressed two weeks after emergence of plants. A pre emergent application of the herbicide Boundary 6.5 EC (Sy genta, NC) was also applied. Upon maturity, the plant tops were killed off using a split applicatio n of the chemical desiccant,

PAGE 32

32 Di quat (Sygenta, NC). The herbicide was applied at a rate of 700ml/acre when the tubers reached horticultural maturity, with a second spray being carried out 5 days later. Harvest Times F our harvests were carried out from 18 May till 15 June, 2010. An initial harvest (Har vest 1) was done at 88 d after planting, when the tubers had reached maturity and a desirable marketable size. Shortly after, the plants were vine killed, and a second harvest (Harvest 2) was done one week after the second desiccant application (100 d afte r planting) Harvest 3 and 4 were carried out at two (107 d after planting) and three weeks after vine kill (114 d after planting) respectively. Weather data were recorded through the Florida Automated Weather Network (FAWN) (Table 3 1) At each harvest, 300 tubers were hand dug in the morning and placed in well ventilated plastic crates. The tubers were transported shortly after digging to the Postharvest Horticulture Laboratory at the University of Florida, Gainesville. Preparation and Treatments Upon a rrival, tubers were carefully hand washed, placed on paper towels and allowed to dry for not more than 30 minutes, using fans. Average sized, blemish free tubers were then selected and stored overnight at 20C. The following morning, tubers were randomly s elected and placed in mesh bags (Figure 3 1). The bagged tubers (n=20) were assigned to four storage chambers (Figure 3 1B) which were maintained at 10 or 20C, each with a low relative humidity (RH) of 65% (expressed as 10L for 10C, 65% RH; 20L for 20C, 65% RH) or high of 95% (10H for 10C, 95% RH; 20H for 20C, 95% RH). Data loggers were used to record the temperature and relative humidity conditions throughout the 12 days (d) in storage.

PAGE 33

33 Tubers were rated for development of the disorder, and other qua lity assessments done by analyzing the weight loss and moisture content over the 12 d in storage. Lenticel and Halo R ating For each tuber, an area with the average lenticel diameter not exceeding 1.0 mm was marked out and tracked throughout the 12 d in sto rage. Each tuber was assigned a rating every 3 d using the average diameter of the lenticels or halos. Rating was done using a subjective scale for lenticels (Table 3 2) and halos (Table 3 3 ), with the ratings ranging from 1 to 6. Tubers with indistinct le nticels were rated 1, while those that had clearly enlarged lenticels with average diameters >2.0 mm were rated 6 (Fig ure 3 2). Tubers that were defined as severely affected were rated 4, 5, or 6. Any halo ratings of 2 7 classified the tubers as unmarketab le. Weight Loss Tuber weight loss was calculated by weighing the fresh weight of 20 tubers from each of the four storage treatments. This was done every 3 d and weight loss was expressed as a percentage of the initial fresh weight Moisture Content The m oisture content of the peel was assessed using approximately 10 g of peeled potato skin and a portion of the pulp cut with a sharp knife (n=4). The tissue was placed in aluminum weighing boats and dried for 48 h in an oven set at 70C. Moisture content was expressed as a percentage of the initial fresh weight. Specific Gravity Tuber specific gravity was measured at each harvest time by weighing the sample in air and under water. A single, 2 kg sample of randomly selected tubers was weighed on a scale, tran sferred to another tared weighing basket and weighed under

PAGE 34

34 water. The weight in air and in water was used to calculate the specific gravity (Specific Gravity = Weight in air / (Weight in air Weight in water)). Statistical Analysis The experiment was con ducted using a 4 by 2 by 2 factorial design. Data was transformed to arcsine values and analyzed using PROC GLM with SAS Software for PC (Institute Inc., Cary, N.C.). Significant differences (p<0.05) between treatment ltiple Range Test Results Lenticel and Halo Rating During the 2010 potato growing season in Hastings, significant amounts of rainfall were received. A total of 11.58 and 8.66 cm of rainfall was received in the months of May and June, respectively, coinci ding with the harvest periods for this study (Table 3 1). Just prior to the first harvest, 6.55 cm of rainfall was recorded (FAWN, 2010), leading to water logged soil conditions. Although Harvest 1 tubers were dug from water logged soils, drainage improved with subsequent harvests. The four way interaction of the time of harvest and subsequent storage condition (Temp x RH x Storage Time) had a highly significant e ffect on the development of enlarged lenticels and halos on the tubers (Table 3. 4 ). Due to the high rainfall just prior to Harvest 1 tubers had marked lenticel proliferation at the time of digging. The wet conditions were persistent throughout the season, causing significant rots and a high degree of proliferation. Harvests 1 and 4 had the highest mean lenticel rating of 2.7 and 2.6, respectively, at the time of harvest (Table 3 5) No halos were observed at digging on any of the tubers from the four harvest times.

PAGE 35

35 In storage, enlarged lenticels and halos were observed in at least one storage condition at each of the four harvests (Fig ure 3 4 ). The effect of the storage treatments on the incidence of lenticel disorders was examined in more detail at each harvest time (Table 3 5 ). The interaction of temperature and RH had a significant effect on the tubers from Harvest 1, where lenticels enlarged significantly in the 20L and 20H treatments after 3 d in storage. The average lenticel rating in the 20H increased from 2.6 to 3.6, a full point (pt.) in severity. The 20L tubers increased in severity by 0.7 pts. There were no significant changes in the lenticels after 3 d in 20L, while the rating in 20H dropped significantly. A similar trend was observed in tubers from Harvest 3, where the average lenticel rating increased after 3 d in both the 20L and 20H treatments, by 0.5 pts. and 0.4 pts., respectively. Harvest 2 tubers had a significant change in severity only in those kept at 20H, with the average rating reaching 2.8, an increase by 0.6 pts. from the initial rating of 2.2. Incidence of the disorder on tubers from Harvest 4 appeared to show some dependence on RH; enlarged lenticels were observed in the 10H and 20H conditions. Lenticel rating in 10H increased by 0.8 pts. by 3d, while that in 20H went up by 0.6 pts. Generally, lenticels of tubers kept at lower RH became depressed as storage duration increased, likely due to desiccation, while those in the high RH remained slightly raised, giving the tuber skin a bumpy appearance. The lenticel aperture also darkened as storage time increased, irrespectiv e of the storage environment. No free moisture was observed on the surface of tubers in any of the storage conditions. There were significant differences in the mean halo size found in each storage treatment at each harvest (Fig ure 3 5 ). No halos appeared in the 10H treatment for any

PAGE 36

36 harvest. However, they appeared in all the other treatments, becoming noticeable after 6 d, and darkening as storage time progressed. Significant increments in the halo rating occurred during storage for Harvest 1 tubers in 10L and 20L. Harvest 2 tubers increased in halo size as storage progressed in all treatments except 10H, which had none. A significant difference in halo appearance was also observed at 20L for both Ha rvest 3 and Harvest 4 tubers. For each harvest, the highe st mean halo rating occurred in the 20L storage treatment by the end of storage. Halos appeared mainly as dark areas of the periderm immediately surrounding the lenticel aperture (Figure 3 3) although a few raised ones were observed in the high humidity c onditions. Weight Loss Tubers lost more weight in all treatments as storage time increased. For all harvests, there were significant differences between storage treatments, with the highest mean weight loss occurring in the 2 0L and 10L treatments (Table 3 6 ). Tubers from Harvest 1 lost the most weight, 7.1% and 8.7% in the 10L and 20L, re spectively by 3 d in storage. By comparison, Harvest 4 tubers lost 2.6% and 2.2% at 3 d in the same storage conditions. By the end of the 12 d storage, the most weight loss was observed at 20L for Harvest 1 tubers, (14.9% of their fresh weight), while Harvest 4 tubers only lost 5% of their fresh weight under the same conditions. The tubers in the low humidity storage conditions became less turgid to the touch and the skin sh riveled, while those in the high humidity remained fairly firm. Moisture Content There were significant differences in the moisture content of freshly dug tubers between the four harvests. The initial peel moisture content of Harvest 3 was significantly hi gher (92.1%) than that of the previous two harvests. On the other hand,

PAGE 37

37 Harvest 2 had significantly lower initial pulp moisture content (83.1%) compared to the other harve sts (Table 3 7 ). All tubers lost moisture in both the peel and pulp during storage. T here were significant differences in the peel moisture content between storage conditions for each harvest period, with most of the moisture loss occurring in the low humi dity conditions (Table 3 8 ). In the 10L, Harvest 1 had the lowest peel moisture conte nt of 84.2% at the end of storage, while Harvest 3 had approximately 6% more moisture. The peel moisture content of Harvest 3 and 4 tubers also declined by 1.6% and 4.6%, respectively, by the end of storage in 20L. There were no significant differences in the pulp moisture content of tubers in the different storage conditions at each harvest time. The only exception was Harvest 1 tubers, which lost significantly more moisture in the low humidity compared to the high humidity. A high amount of skinning or e xcoriation was observed in tubers from Harvest 1 which had not be en vine killed (Fig ure 3 6 ). Specific Gravity Although not replicated, the initial specific gravity of 1.052 g/ cm 3 decreased with time of harvest (Table 3 7 ). Potatoes from Harvest 4 had the lowest of all harvests (1.044 g/cm3). Discussion Unfavorable field or storage conditi ons, such as high moisture levels, have been shown to induce lenticel proliferation, enlarging and causing the aperture to darken at times, as storage time progressed ( Ed dins et al., 1946) Anatomical studies have indicated that lenticel proliferation decreases with increased tuber maturity (Adams, 1975). Sabba et al. (2007) defined tuber maturity as a combination of chemical,

PAGE 38

38 physiological and physical maturity. Therefo re, in this experiment Harvest 4 tubers were expected to be more mature than Harvest 1 (non vine killed). The highest lenticel rating of freshly dug tubers was observed for Harvest 1 (rating 2.7) implying the immature tubers were more susceptible. However Harvest 4 (3 weeks after vine kill) tubers also had a high rating of 2.6 Adams (1975) reported that lenticels from older tubers can still proliferate if soil moist ure levels remain high, which is likely to explain the proliferated lenticels in Harvest 4 The rainfall received during the harvesting period caused soil water logging in the first few days, with the soils remaining considerably wet throughout the until the final harvest (Table 3 1). Lenticel apertures of the tubers lying in the moist soils we re proliferated, with the apertures enlarging, and some of the tubers succumbing to rot. In stor age, enlarged lenticels developed in tubers from all four harvest times for those in the 20C, 95% storage treatment (20H). These results confirm those of Lulai (2001) that enlarged lenticels are more likely to develop if stored under high humidity conditions. The higher temperature and high relative humidity in the 20H storage treatment created a low VPD, which facilitated enlargement of the lenticel cells in th e moist environment. In this storage treatment, Harvest 1 tubers had the greatest increase in severity of the disorder, the rating increasing by 1.05 while increases for the other harvests ranged b etween 0.5 to 0.6 (Table 3 5 ). Incidence of the lenticel disorder in the 20L was likely due to desiccation of lenticel cells. This occurrence, however, was not consistent with all harvests. These differences in the development of the disorder during storage found among the four harvest times confirm the findings of Adams (1975), who demonstrated that lenticel proliferation or

PAGE 39

39 enlargement during storage is caused, not by a single factor, but by an interaction of soil moisture levels tuber maturity and cultivar. Lenticel cells of some cul tivars tend to proliferate more readily when stored under high moisture levels, with the less mature tubers being more susceptible to proliferation and subsequent enlargement of the lenticel aperture. The result presented for the halo disorder suggests tha t occurrence of raised halos was possibly due to the infiltration of water into the lenticel aperture before harvest. Studies by Bartz et al. (1985) indicated that water can infiltrate into the lenticel, phelloderm and cortex because such tissue has large intercellular spaces. The cells then expand, causing the periderm area immediately surrounding the aperture to swell. Accumulation of phenol compounds as storage progressed is the likely explanation of the darkening of the halos. A similar case was report ed in the lenticels of avocado fruit (Everett et al, 2008). Although a fruit, avocado lenticels have similar morphologies to those of potato lenticels both are comprised of loose complimentary cells. In the case of avocado lenticels, the lenticel cell e xpansion caused the cell membranes to rupture and release phenolic compounds. In the present study there were no differences among harvests in the appearance of halos because of the high soil moisture levels from wh ich the tubers had been dug The higher w eight loss in the 10L and 20L storage treatments for all harvests was due to the high desiccation rate promoted by the environments. Tuber weight loss in storage is affected by the respiration rate, temperature, relative humidity, the quality of tuber skin suberization, sprouting and storage ventilation (Rastovski et al., 1981). This is

PAGE 40

40 supported by data obtained in this study, which indicated that the weight loss occurred due to an interaction of these factors. In addition to the drying effect of the stor age air, the skin properties of the tuber also have a huge influence on the water lost through transpiration in storage. This was clearly shown by the high amounts of weight loss that occurred in the Harvest 1 tubers, which had a less mature periderm. Thes e tubers, because of their thin, loosely bound skin, were more susceptible to scuffing (Fig ure 3. 6 ). Scuffing exposes the cortical tubers naturally have a high er respiration rate compared to more mature tubers due to large r concentrations of starch which act as a respiratory substrate (Lisinska and Leszczynski, 1989). Cargill (1976) suggested that when potato weight loss reaches 5 % of the original fresh weight, the tuber wi ll shrink and become soft. This is clearly supported by data from this study where the minimum amount of weight loss in the low humidity was 5.5% (Table 3 6 ), while those in the high humidity lost a maximum of 3.5%. This explains the softening of tubers th at was observed in the low humidity. The results also show that m oisture lost during tuber storage is highly dependent on the permeability of water through the periderm and degree of suberization (Lulai, 2006). This helps account for the low peel and pulp moisture contents observed in the first two harvests, which had less suberized ski ns and therefore lost more water The average tuber consists of approximately 75% water, in bound form and as free water (Lisinska and Leszczynski, 1989). Loss of this moistu re can occur through the periderm (98%) or the lenticels (2%). It was also observed that tubers stored in the high humidity

PAGE 41

41 treatments (10H and 20H) maintained higher moisture content, as these conditions prevented excessive water loss. The pulp had lower moisture content than the peel possibly due to a greater proportion of dry matter content found in the internal tissue. According to Anzaldua Morales, et al. (1992), dry matter content was higher in the inner tissues of the tuber, with the cortical tissue having 3 to 6 % more dry matter than the outer pith tissue Dry matter content is also related to the tuber sp ecific gravity which affects the cooking (baking, boiling, steaming) quality of fresh potatoes. A high specific gravity indicates high dry matt er content, producing a tuber with a mealy texture (Lulai et al, 1979; Smith et al., 1940). Specific gravity is also an important indicator of tuber maturity (Shetty, 2009); with more mature tubers having a higher specific gravity or percent solids. Howeve r, in this study, Harvest 1 tubers had the highest specific gravity, and density decreas ed at each subsequent harvest. An explanation to this could be the effect of vine killing on the tuber quality. Stark et al. (2009) noted that the longer the tubers rem ain in the soil after vine kill the lower the specific gravity at harvest due to loss of starch through respiration. This confirms the observations in this study. Conclusions The development of lenticel disorders in storage was not attributed to a single environmental or anatomical factor, but rather an interaction of preharvest and postharvest factors. Non vine killed tubers were more susceptible to the enlarged lenticel dis order, in addition to a higher degree of quantitative and qualitative losses being associated with these tubers. However, the incidence of the enlarged lenticels varied in the harvests following vine kill. This indicated that time of harvest after vine kil l had a minimal effect

PAGE 42

42 on the development of the disorder in storage. Storage condition had a great effect on development of the disorder, with an interaction of high temperatures and high relative humidity levels triggering the disorder at all harvest tim es. With this information, it can be concluded that, during a potato growing season marked by high rainfall amounts, as is common in Florida, a greater incidence of enlarged lenticels can be expected at any harvest time if stored at 20C, 95% RH. Since th ese early season potatoes have high respiration rates, changes in temperature and RH should be monitored in the various forms of packaging, especially in bulk packaging where there is minimum ventilation. Lenticels did not enlarge when stored at 10C, 65% RH for all harvests. However, this condition led to other quality losses such as weight loss and shriveling, in addition to halos appearing at the low humidity. Based on the results from this study, storage at 10C, 95% RH would lessen the incidence and se verity of both disorders and other qualitative losses. It was also noted that the first 3 d in storage were the most critical, with any changes in the disorder being observed within that period. Previous studies have attributed development of the enlarged lenticel disorder to presence of free moisture on the tuber surface. This study show ed that an interaction of high humidity and temperature conditions (20C, 95% RH), without any moisture condensation, is still capable of inducing lenticel enlargement.

PAGE 43

43 Figure 3 1 Potato preparation and storage. Potatoes packaged into mesh bags and placed in storage chambers.

PAGE 44

44 A B Figure 3 2. Severity of the lenticel disorder. A) Lenticel rating 2, B) Lenticel rating 6.

PAGE 45

45 Figure 3 3. Halos appearing as a dark, swollen area surrounding the lenticel aperture.

PAGE 46

46 A B Figure 3 4. intervals and stored under four different storage conditions for 12 d. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 3 6 9 12 LENTICEL RATING DAYS IN STORAGE Harvest 1 10L 10H 20L 20H 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 3 6 9 12 LENTICEL RATING DAYS IN STORAGE Harvest 2 10L 10H 20L 20H

PAGE 47

47 C D Fig ure 3 4. Continued 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 3 6 9 12 LENTICEL RATING DAYS IN STORAGE Harvest 3 10L 10H 20L 20H 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 3 6 9 12 LENTICEL RATING DAYS IN STORAGE Harvest 4 10L 10H 20L 20H

PAGE 48

48 A B Figure 3 5. and stored in four different storage conditions for 12 days. 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0 3 6 9 12 HALO RATING DAYS IN STORAGE Harvest 1 10L 10H 20L 20H 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0 3 6 9 12 HALO RATING DAYS IN STORAGE Harvest 2 10L 10H 20L 20H

PAGE 49

49 C D Figure 3 5. Continued 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0 3 6 9 12 HALO RATING DAYS IN STORAGE Harvest 3 10L 10H 20L 20H 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0 3 6 9 12 HALO RATING DAYS IN STORAGE Harvest 4 10L 10H 20L 20H

PAGE 50

50 Figure 3 6. vine kill (Harvest 1).

PAGE 51

51 Table 3 1. Average environmental conditions in Hastings, Florida from February 2010 to June 2010 from FAWN weather database Period Air Temp. 2m ( F) Soil Temp 10cm. Rel. Hum. 2m Total Rainfall at 2 m (F) (%) (cm) Feb 52.0 55.5 77 9.4 Mar 57.6 58.9 75 10.2 Apr 67.2 68.4 79 1.8 May 75.5 76.9 83 11.6 Jun 80.4 81.9 82 8.7 Florida Automated Weather Network ( www.fawn.ifas.ufl.edu ) Tabl e 3 2. Rating scale for enlarged lenticel disorder according to lenticel diameter. Rating Lenticel diameter (over 50% of lenticels with diameter) 1 No visible lenticels 2 0.5 mm 3 0.51 1.0 mm 4 1.1 1.5 mm 5 1.51 2.0 mm 6 2.0 mm Tabl e 3 3. Rating Scale For Halo Disorder According to Diameter. Rating Halo diameter (over 50% of halos with diameter) 1 No visible halos 2 2.0 mm 3 2.1 3.0 mm 4 3.1 4.0 mm 5 4.1 5.0 mm 6 5.0 mm

PAGE 52

52 Table 3 4 Analysis of Variance showing e ffects of harvest time, storage conditions and storage time on development of lenticel disorders, weight loss and moisture Main effects Lenticel Halo Weight loss Peel MC Pulp MC Harvest NS Temp. NS NS NS NS RH NS * Storage time NS NS Interactions: Harvest x Temp. * Harvest x RH * Harvest x Storage time * Temp. x RH * Temp. x Storage time NS NS RH x Storage time * Harvest x Temp x RH x Storage time * NS,* Not significant and significant at P<0.05 respectively.

PAGE 53

53 Table 3 5 and stored for 12 d in four different storage treatments. Storage Average Lenticel Rating Harvest Temp RH (C) (%) Storage Time (days) 0 d 3 d 6 d 9 d 12 d 1 10 65 95 20 65 95 2.6a z A y 2.9abA 3.1aA 3.1aA 3.1aA 2.9aA 3.5aA 3.3aA 3.4aA 3.4aA 2.8aB 3.5abA 3.5aA 3.6aA 3.6aA 2.6aC 3.6aA 3.1aA 3.1aB 3.1aB 2 10 65 95 20 65 95 2.4aA 2.6abA 2.4aA 2.4aA 2.4aA 2.3aA 2.6abA 2.5aA 2.3aA 2.3aA 2.2aA 2.3bA 2.3aA 2.3aA 2.3aA 2.2aB 2.8aA 2.5aB 2.5aB 2.5bA 3 10 65 95 20 65 95 2.1aA 2.4aA 2.4aA 2.4bA 2.4bA 2.2aA 2.6aA 2.6aA 2.6abA 2.6abA 2.2aB 2.7aA 2.7aA 2.9aA 2.9Aa 2.3aB 2.3aB 2.7aA 2.7abA 2.7abA 4 10 65 95 20 65 95 2.5aA 3.1aA 3.1aA 3.1aA 3.1aA 2.7aB 3.5aA 3.5aA 3.4aA 3.4 aA 2.7aA 3.2aA 3.2aA 3.2aA 3.2aA 2.4aB 3.0aA 3.0aA 3.1aA 3.1 aA z,y Means within a column followed by the same small letter at each given storage time, or by the same capital letter within a row at the same level of storage condition do not differ significantly Table 3 6. Weight loss (expressed as % of original in storage. Harvest Time Storage Treatment 10L 10H 20L 20H 1 14.6a z A y 3.3aB 12.6aA 3.5aB 2 6.8bA 2.2bB 6.7bA 2.2bB 3 7.4bA 2.3bB 7.4bA 1.6bB 4 5.5cA 1.8cB 6.0cA 1.7cB z,y Means within a column followed by the same small letter at each given storage condition, or by the same capital letter within a row at each harvest time do not differ significantly according

PAGE 54

54 Table 3 7 The initial moisture content and specific gravity of harvest times. Harvest time Peel MC Pulp MC Specific Gravity (%) (%) (g/cm 3 ) 1 91.56 b z 84.81 a 1.052 2 91.32 b 83.09 b 1.047 3 92.07 a 84.84 a 1.045 4 91. 58 a b 85.41 a 1.044 z Differences in letter per column indicate significant difference between harvests, according to

PAGE 55

55 Table 3 8 different times and s tored for 12 d in four different storage treatments. Moisture Content (%) Harvest Temp RH (C) (%) Peel Pulp Initial Final Initial Final 1 10 65 95 20 65 95 91.6a z 84.2b 84.8a 80.0b 91.6a 90.3a 84.8a 82.5a 91.6a 88.7a 84.8a 79.8b 91.6a 90.0a 84.8a 82.2a Significance (p<0.05) NS NS 2 10 65 95 20 65 95 91.3a 88.8c 83.1a 81.3b 91.3a 91.4ab 83.1a 83.8a 91.3a 90.9b 83.1a 83.6b 91.3a 92.0a 83.1a 84.7a Sig nificance (p<0.05) NS NS NS 3 10 65 95 20 65 95 92.1a 90.8b 84.8a 85.6a 92.1a 92.5a 84.8a 84.9a 92.1a 90.5b 84.8a 85.0a 92.1a 92.4a 84.8a 86.0a Significance (p<0.05) NS NS NS 4 10 65 95 20 65 95 91.6a 85.7b 85.4a 82.7a 91.6a 85.8a 85.4a 81.3a 91.6a 87.0b 85.4a 82.4a 91.6a 86.4a 85.4a 81.0a Significance (p<0.05) NS NS NS z Different letters in the column indicate significant differences within harvests, according to

PAGE 56

56 CHAPTER 4 DEVELOPMENT OF ENLAR GED LENTICELS AND HA POTATO AS AFFECTED B Y HARVEST TIME AND S TORAGE CONDITIONS Introduction Florida grows a wide range of potato varieties, ranging from white or yellow skinned to red skinned varieties. While most USA commercial growers grow russet type potatoes, Florida favors mostly early to mid season varieties (Pack et al., 2003). The inclement weather conditions such as high rainfall and temperatures, which increase as the potato growing season progresses, makes earliness in reaching maturity one of the important qualities considered for Florida production. The tuber s are mainly used for the chipping industry or the fresh market, depending on the variety and its attributes, such as tuber shape, specific gravity and skin set (Hutchinson et al., 1995) One of the leading fresh market varieties in Florida is the red skin ned Red later being released by the USDA and Louisiana Agricultural Experiment Station in 1953 (The Potato Association of America, 2010). The variety matures in 85 to 95 days, with two to three weeks after vine kill required for the tuber skin to set (Hutchinson et al., 2006). The tubers are round or oblong shaped, with a deep red color which fades with maturity The flesh of the tuber is white, and the specific gravity relativ ely low compared to other red skinned varieties. This makes the variety ideal for boiling. bacterial wilt. The cultivar has also been seen to develop enlarged lenticels in wat erlogged soil conditions, decreasing external appearance and making the tubers more prone to soft rot and scab causing pathogens.

PAGE 57

57 The objective of this study was to determine the effect of harvest time, storage temperature/relative humidity and duration of severity of enlarged lenticel and halo disorders and related quality parameters. Materials and Methods Plant Material Science and Education Unit, Yelvington Farm in Hastings, Florida. The planting date was February 18, 2010, with the experimental layout following a Complete Randomized Design. Fertilization, herbicide application and vine kill were carried out as outlined in Chapter 3 The same harvest times, preparation of tubers, and storage treatments were identical to outlined in Chapter 3. Similar procedures as mentioned in Chapter 3 were also used to analyze development of the lenticel disorder and other tuber quality parameters (weight loss, moisture content, and specific gravity). Statistical Analysis Experimental design and data analysis were done as described in Chapter 3. Results Lenticel and Halo R ating A difference in the average lenticel appearance on freshly dug tubers was observed between the four harvest times. Tu bers from Harvest 1 and 4 had the highest mean ratings of 2.6 and 2.3, respectively while Harvest 2 and 3 tubers were rated 2 (diameter below 2.0 mm) at the time of harvest (Table 4 2). The interaction between the time of harvest and subsequent storage conditions had a significant effect on the development of the disorder (Table 4 1).

PAGE 58

58 Differences in the lenticel appearance and size were observed in each of the four storage treatments, for e ach harvest time. The lenticel rating increased after 12 d in all storage treatments (Fig ure 4 2), and the lenticel shape changed from roun d to oval (Figure 4 1). The greatest increase in severity for Harvest 1 and Harvest 4 occurred after 12 d storage at 20H, where the rating of the lenticel disorder increased by 0.7 and 0.8 pts., respectively (Table 4 3). The highest increase in severity for Harvest 2 was observed in 10H. The same storage condition, in addition to 20L, also caused an increase in severity for Harvest 3 tubers. Halos appear as raised areas surrounding the lenticel. No halos were observed at digging on any of the tubers from the four harvest times. However, there were significant differences in the appearance of halos in storage for each harv est time (Fig ure 4 4). The average halo rating for Harvest 4 was significantly different at 20L with a mean halo rating of 1.3 after 6 d in storage. The highest average halo ratings for the other three harvests occurred mainly at 20H where after 6 d Harve st 1 increased in rating to 1.4 and Harvest s 2 and 3 by an average of 0.2 ps Weight L oss There were significant differences in weight loss due to harvest time and the storage condition (Table 4 3). For all of the harvests, the greatest weight loss occurr ed under low RH conditions at either 20L or 10L, increasing over storage time. Harvest 1 tubers lost the most weight, 9.6% at 10L and 7.3 % at 20L while tubers from the other harvests lost from 2. 1 % to 2.7% under the same conditions. Despite the weight lo ss, tubers in these low RH conditions remained reasonably firm to the touch with no marked signs of shrivel after 12 d storage.

PAGE 59

59 Moisture Content Differences between the peel and pulp moisture content (MC) were observed at each of the four harvest times. Th e peel MC remained significantly higher than the pulp MC throughout storage (Table 4 4). There were essentially no d ifferences in initial tuber peel or pulp moisture contents based on harvest time The peel moisture content of freshly dug tubers (initial MC) averaged 87.9%, and pulp moisture content averaged 82.0% (p=0.0129) .At Harvest 3, pulp MC was higher than the other harvests (82.6%). No significant differences were observed in peel or pulp moisture content between treatments at any of the four harves t times, suggesting storage condition had minimal effect on the moisture content. On the other hand, by the end of the 12 d storage (final MC) Harvest 1 tubers generally had the lowest peel moisture content (p=0.045) at each storage condition, with a mean of 85.5 % compared to the other harvest times (Table 4 4). There was also a n indication that harvest time ha d the same effect on pulp moisture content, with Harvest 1 tubers having the lowest amount after 12 d (not quite significant at 5% level, though p = 0.057). Although the extent of tuber skinning was greatest in the Harvest 1 tubers (non vine killed), there were slight differences between harvests after vine killing (Fig ure 4 2). This suggested that the majority of the skin set of periderm suberization had occurred by the time of Harvest 2 (one week after vine kill). Specific G ravity The average tuber density decreased as the days from planting increased. The specific gravity for Harvest 1 was 1.064 g /cm3, while the final harvest was recorded at 1.055 g/cm3.

PAGE 60

60 Discussion The results presented here confirm those of Adams (1975) and Tyner (2001) and show that lenticel pr oliferation and structure are strongly influenced by cultivar A similar pattern to in the appearance of the enlarged lenticel disorder on freshly dug tubers at different harvest times. These results indicate that the proliferation that occurred in tubers from Harvest 4 was due to the prolonged ex posure to wet soils with the soils being moist throughout most of the harvest period (Table 3 1) Lenticels enlarged in al l storage conditions at all four harvest times (Table 4 2), implying er. However, the greatest increase in lenticel enlargement was observed at high humidity in both the 10C and 20C storage conditions for all harvests. These data strongly suggest that high relative humidity conditions may have the greatest effect on lenti cels, causing the existing filling cells to expand (Diriwachter and Parbery, 1990) with the resultant effect of an increase in lenticel aperture size. The highest severity of the raised halo disorder was observed at 20H for all harvests, except the final one. This observation is consistent with that of Everett et al. (2008) who indicated that in high RH conditions, the lenticel cells continue to enlarge, affecting the immediate area surrounding the periderm. On the contrary, the tubers in the low humidi ty conditions lost the most weight by the end of storage. The low humidity conditions in the 10L and 2 0L created a high vapor pressure deficit, increasing weight loss. Harvest 1 (non vine killed) tubers lost the most weight at low humidity, due to l ess sub erization of tuber periderm (refer to data in Chapter 5).

PAGE 61

61 Unlike the late season varieties which are cured before long term storage, Florida fresh market potatoes are only stored briefly before being consumed. Curing heals wounds and increases suberizatio n of the periderm (Thompson, 2003). This helps reduce weight loss and desiccation while in storage. This inadequate curing explains why all tubers at the four different harvest times lost weight in storage (Table 4 3). The general lack of variability in moisture content among harvest times seems to indicate that the degree of periderm suberization had minimal effect on tuber moisture content Moisture content of tubers is influenced by storage conditions, extent of cell suberization, wound periderm format ion, and variations in the growing conditions (Blenkinsop et al., 2002). There was no significant effect of harvest time on the peel moisture content of freshly dug tubers. The pulp moisture content was also not significantly different among Harvests 1, 2 or 4 (Table 4 4 ). However, harvest time ha d a greater effect on both the peel and pulp moisture content s during storage than did storage condition (Table 4 1). Tubers from Harvest 1 had the lowest peel and pulp moisture content, apparently due to the immat ure skin which was not an effective vapor barrier, compared to the later harvested tubers which most likely had more suberin deposits in the periderm significantly lower than the peel, possibly due to a greater amount of dry matter content associated with the inner tissues of tubers (Anzaldua Morales, et al., 1992). Harvest 1 tubers also had the highest specific gravity, with the density decreasing at each subsequent harvest. Vine killing induces pl ant stress which possibly contributes to a decrease in starch accumulation in tubers. Similar correlations were observed when

PAGE 62

62 plants were subjected to heat stress (Sabba et al., 2007). Geisel (2003) also noted that specific gravity decrea sed with increased time interval between vine kill and harvest increases. Conclusions The data presented show ed that development of enlarged lenticels is dependent on an interaction of various factors which include harvest time, storage condition and du rat ion. This study also confirmed that the incidence and severity of the disorder varies according to cultivar. developing in all four storage conditions. This is greatly attri buted to the manner in ed their round shape as they enlarge d ed out diagonally into oval shapes. However, the effects of the storage condition were more apparent when tubers were stored at high humidity, confirming previous reports of enlarged lenticels under similar conditions. An interaction of soil conditions and time of harvest had an impact on the development of proliferated lenticels before har vest. As was observed in Harvest 4 tubers had be en exposed to wet soils through most of the harvest period, thereby developing the lenticel disorder. Harvesting tubers two weeks after vine kill likely minimized the incidence of the disorder, given the high rainfall amounts in the 2010 growing season. In storage, although especially in the low humidity as observed in most other varieties, they maintai n ed

PAGE 63

63 acceptable stored at 10 C 65% RH to avoid development of severe ly enlarged lenticels.

PAGE 64

64 Figure 4 1. A B Figure 4 2. Differences in potato skin set at harvest time, despite careful A) One week after vine kill (Harvest 2), and B) Three weeks after vine kill (Harvest 4).

PAGE 65

65 A B Figure 4 3. different times and stored under four different storage conditions for 12 d. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 3 6 9 12 LENTICEL RATING DAYS IN STORAGE Harvest 1 10L 10H 20L 20H 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 3 6 9 12 LENTICEL RATING DAYS IN STORAGE Harvest 2 10L 10H 20L 20H

PAGE 66

66 C D Figure 4 3. Continued 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 3 6 9 12 LENTICEL RATING STORAGE TIME Harvest 3 10L 10H 20L 20H 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 3 6 9 12 LENTICEL RATING STORAGE TIME Harvest 4 10L 10H 20L 20H

PAGE 67

67 A B Figure 4 4. times and stored in four different storage condi tions for 12 d. b a b a 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 0 3 6 9 12 HALO RATING DAYS IN STORAGE Harvest 1 10L 10H 20L 20H 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 0 3 6 9 12 HALO RATING DAYS IN STORAGE HARVEST 2 10L 10H 20L 20H

PAGE 68

68 C D Figure 4 4. Continued 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 0 3 6 9 12 HALO RATING DAYS IN STORAGE HARVEST 3 10L 10H 20L 20H 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 0 3 6 9 12 HALO RATING DAYS IN STORAGE HARVEST 4 10L 10H 20L 20H

PAGE 69

69 Table 4 1. Analysis of variance showing effects of harvest time, storage condition and storage period on development of lenticel disorders, weight loss and moisture Main effects Lenticel Halo Weight loss Peel MC Pulp MC Harvest z * Temp. NS NS NS RH NS NS Storage time * NS Interactions: Harvest x Temp. * Harvest x RH * Harvest x Storage time * Temp. x RH * NS Temp. x Storage time * RH x Storage time * Harvest x Temp x RH x Storage time * z NS,* Not significant and significant at P<0.05 respectively

PAGE 70

70 Table 4 2. times and stor ed for 12 d in four different storage treatments Storage Average Lenticel Rating Harvest Temp RH (C) (%) Storage Time (days) 0 d 3 d 6 d 9 d 12 d 1 10 65 95 20 65 95 2.5a z B y 2.7aA 2.9aA 2.9aA 2.9aA 2.4aB 2.8aA 2.9aA 2.8aA 2.8aA 3.2aB 3.0aA 3.2aA 3.2aAB 3.2aAB 2.5aB 3.0aA 3.1aA 3.2aA 3.2aA 2 10 65 95 20 65 95 2.0aB 2.7bA 2.6bA 2.6bA 2.4bA 2.0aB 3.0aA 3.0aA 3.0aA 3.0aA 2.0aB 2.6bA 2.4bA 2.4bA 2.4bA 2.0aB 2.8aA 2.7bA 2.6bA 2.5bA 3 10 65 95 20 65 95 2.0aB 2.6aA 2.5aA 2.5aA 2.5aA 2.0a B 2.4aA 2.8aA 2.8aA 2.8aA 2.0aB 2.8aA 2.8aA 2.8aA 2.8 a a 2.0aB 2.5aA 2.5aA 2.5aA 2.5aA 4 10 65 95 20 65 95 2.4aB 2.9aA 2.9aA 3.0aA 2.9aA 2.3aB 2.8aA 2.8aA 2.9aA 2.9 a a 2.4aB 3.0aA 3.0aA 2.6aAB 2.6aAB 2.1aB 2.6aA 2.6aA 2.9aA 2.9aA z, y Means within a column per harvest time, followed by the same small letter at each given storage time, or by the same capital letter within a row at the same level of storage condition do st (p<0.05) (n=20). Table 4 3. kept in storage for 12 days Harvest Time Storage Treatment 10L 10H 20L 20H 1 9.56 a z A y 2.31 aB 7.29 aA 4.25 aB 2 2.74 bA 0.91 bB 3.02 bA 1.09 bB 3 2.38 bcA 1.25 bcB 2.60 bA 0.78 bcB 4 2.07 cA 0.78 cB 2.58 cA 0.79 cB z,y Means within a column followed by the same small letter at each given storage condition, or by the same capital letter within a row at each harvest time do not differ significantly according

PAGE 71

71 Table 4 4. The average initial (freshly dug) and final (after 12 d storage) peel and pulp Harvest time Initial MC (%) Final MC (%) Peel Pulp Peel Pulp 1 87. 8 a A z 81.7 bB 8 5.5 b A 81.0 a B 2 87. 5 a A 81.9 bB 8 7.2 aA 82.3 a B 3 88. 8 a A 82.6 a B 8 6.1 a A 81.8 a B 4 87. 5 a A 81.9 bB 86.2 aA 81.8 a B z Means within a column followed by the same small letter, or by the same capital letter for each pair of rows at either the initial or final MC, do not differ significantly (p < 0.05) (n=4)

PAGE 72

72 CHAPTER 5 CHARACTERIZATION OF POTATO SUBERIZATION IN RELATION TO VINE KILLING AND ENLARGED LENTICE LS Introduction To avoid extensive losses in storage due to disease, dehydration and defects, it is of great importance that tubers have a mature periderm. A mature periderm is characterized by suberized cells which act as barriers, preventing invasion by micro organisms and desiccation of the plant tissue (Walter and Schadel, 1983). Potato tuber maturation is initiated when plant growth ceases and vines start to die off. The process is hastened by chemically or manually killing off the vines, two to three weeks prior to harvest. Vine killing induces plant stress, which in turn encourages suberin deposition on the phellem cell walls of the periderm. The complimentary or filling tissue of the lenticel is also suberized much in the same manner as the periderm. A suberized cl osing layer may form at the base of the lenticel, acting as a barrier to lenticel penetration by water (Lulai, 2007). In high moisture conditions, this barrier may be disrupted by renewed production of proliferated cells by the lenticel meristematic tissue / phellogen. Previous studies have suggested that older, well suberized cells tend to be less permeable to water, compared to younger lenticels (Adams, 1975; Tyner, 2001). Much of the work done on suberization of lenticels has been related to the physica l and chemical properties of the suberized closing layer in preventing pathogen entry. It would be of interest to determine if amount of lenticel suberization is related to the size of the lenticel aperture. The objective of this research was to characteri ze lenticel suberization to the diameter of the aperture in vine killed and non vine killed tubers.

PAGE 73

73 Materials and Methods Plant Material Solanum tuberosum lenticel suberization were grown in the field in the 2010 season and vine killed using standard cultural practices (Chapter 3 and 4). Tubers were harvested before vine kill (Harvest 1), one week after vine kill (Harvest 2), two weeks after (Harvest 3), and three weeks after (Harvest 4). Initial tissue samples (n=3) from marketable sized lenticels (lenticel rating 2 and 3) were collected prior to sto rage for 12 d at 10C, or 20C 95% RH. More samples (n=3) representative of each lenticel rating ( rating 2 to 6) were collected at the end of storag e where the enlarged lenticel disorder had been observed Sectioning and Fixation Lenticel cross section samples for observation were sectioned using a sliding hand microtome (model DK 10, Edmund Scientific, Barrington, N.J.). The 10 m sections were cut and immediately transferred into Formalin Acetic Alcohol (FAA) (50% ethanol, 5% glacial acetic acid, 10% formalin, and 35% deionized water). Tissues were allowed to remain in the fixation formula (FAA) for 24 h at room temperature. Infi ltration and Dehydration The fixed tissue samples were dehydrated for 9 h using a graded tert butyl/ethanol series, according to standard procedures (Ruzin, 1999). Liquid paraffin was then allowed to infiltrate into the tissue before the sections were mou nted on subbed microscope slide s and dried in a 58C oven for 1 d and followed by a second day at room te mperature to ensure full adhesion to the microscope slide.

PAGE 74

74 Clearing and Staining Procedure Tissue was washed twice in xylene to remove paraffin, and th en dehydrated to 70% ethanol in a graded series. A 1% w/v Safranin O solution (Sigma Aldrich, Saint Louis, MO ) was dissolved at room temperature. Tissue sections were stained in the solution for 2 h, before the slides were rinsed in deionized water with ge ntle agitation. The sections were counterstained in 0.15% w/v Fast Green FCF solution (Sigma Aldrich, Saint Louis, MO ), according to standard procedures by Johansen (1940). The procedure allows analysis of suberized cells by staining them into an intense r ed color. Photography The stained sections were examined using standard light microscopy with an Olympus system microscope (model BX51, Olympus America, Inc., Melville, NY). L ight micrographs were taken with a digital camera system (model DP70, Olympus Am erica, Inc., Melville, NY) The light micrographs were taken at the 10x magnification, with the depth of suberized lenticel cell layers and immediate surrounding periderm measured with a caliber fixed in the microscope lens eye. Result s Cross sections of the lenticel filling cells and the phellem cells of the immediate surrounding periderm had an intense red color when viewed under the light microscope after staining with Safranin O solution (Figure 5 1) This indicated presence of suberized cells, while t he non suberized parenchyma cells were not as intensely stained. Generally, at each harvest period the depth of the suberized cell zone of the lenticels was equal to or greater than that of the imme diate surrounding periderm In addition, there seemed to be an effect of cultivar, harvest time, and storage on the thickness of the suberized cell zone.

PAGE 75

75 The thickness of the suberized filling cell layers increased as time after vine kill 2 m before vine kill (Harvest 1), increasing to 14.4 m by 3 weeks after vine kill (Harvest 1) (Table 5 between Harvest 1 and 4 (Table 5 2). The high humidity storage co nditions (10H and 20H) from which the tuber sample tissue had been obtained apparently promoted further suberin deposition onto the lenticel filling cells. The thickness of the suberized layer increased during 12 d in storage irrespective of the harvest ti me o r cultivar (Table 5 1 and 5 2). Lenticels with a wider diameter (higher rating) at the end of 12 d storage had a 1.0 mm (ratings 2, 3) had an average suberized layer thickness of 10.1 m by the end of 12 d storage, while those with a diameter greater than 1.0 mm (ratings 4,5,6) were approximately 11.5 m (Table 5 1). The difference in the thickness increased for vine killed tubers (Harvest 2 to 4). For smaller diamete r lenticels ( < 1.0 mm) the average suberized zone thickness ranged from 17.9 m to 41.4 m, while that for the enlarged lenticels ( > 1.0 mm) ranged from 59.9 m to 82.1 m (Fig ure 5 1). A similar in the larger diameter lenticels was thicker (Figure 5 2A, 5 2B) This suggests that lenticel enlargement during storage is associated with enhanced cell suberization. Histological analysis also showed similarity in the basic organization and suberizatio n of periderm cells in both cultivars at the four harvest times. S uberization of the potato periderm occur red only in the first few outer phellem cell layers (Figure 5 3)

PAGE 76

76 Discussion Diriwachter (1990) defined lenticel maturity as complete suberization of the lenticel filling cells, sealing off the lenticel cavity. Closed lenticels cannot take up water as the suberized cells are lined with wax crystals, which are associated with the suberin complex (Bezuindenhout, 2005). When the lenticel is filled with wat er, gaseous exchange, especially oxygen, is hindered. These high moisture levels stimulate cell division and proliferation of the lenticel fillings cells, causing an enlargement of the lenticel aperture. Results show that lenticel suberization increased as time after vine kill increased. Initial Harvest 4 (three weeks after vine kill) lenticels had suberized cell layer s which were on average 10 m thicker than that observed in initial Harvest 1 (non vine killed) samples. These results indicate that vine killing has an effect of increasing deposition of suberin on both the phellem cells of the periderm, and the complimentary cells of the lenticel. This supports the findings by Lulai (2001), that any form of plant stress enhances suberin deposition on the filling cells. Subsequent storage under the high humidity conditions (10C and 20C, 95% RH) for 12 d, promoted an increase in the thickness of the suberized cell layers. There were no differences in the lenticel s uberization observed at the two storage temperatures, indicating humidity levels had a greater effect on suberization. Environmental conditions have been reported to affect suberization of cells, with mo re suberization occurring at high humidity, whil e cel l desiccation occurs at low humidity (Morris et al., 1989). The present results also indicate that there is a difference in the extent of suberization in lenticels of different diameters after 12 d storage. Generally, the larger lenticels (ratings 4, 5, 6) had a thicker suberi zed layer, while the smaller lenticel s had

PAGE 77

77 fewer suberized continuous cell laye rs. This was observed at all harvest times. Adams (1975), reported that when the lenticel filling cells enlarge or proliferate under wet soil or storage con ditions, the existing suberized cell layer is ruptured, opening up the lenticel. However, disrupted suberized cell layers expected to be associated with lenticel enlargement were not observed in this study. The presence of continuous suberized cell layers by the end of storage was likely due to stress induced suberization that occurred during lenticel enlargement. When the defense system induces stress induced suberiza tion. A similar response occurs when the periderm tissue is wounded, with the rate of the suberization being influenced by the type and severity of the wound (Lulai, 2001). Wound healing in tubers involves the formation of closing layers of suberized cells followed by suberization of newly developed cells under the closing layer. This likely explains why the thickness of the suberized layer increased with an increase in the diameter of the lenticel. Conclusion This study confirms that suberization can b e a response to stress signals. V ine killing induced plant stress, thereby enhanc ing lenticel maturity by increasing cell suberization while the tubers were still in the ground. S uberization continued with increased time after vine kill, as indicated by an increase in thickness of the suberized cell layers This suggest s that le nticel cell proliferation is less likely to occur in wet fields after vine kill. B tubers from Harvests 2 and 3 (one and two weeks after vine kill, respe ctively) had a lower incidence of proliferated lenticels at harvest compared to tubers harvested just prior to vine kill (Harvest 1) (Chapter s 3 and

PAGE 78

78 4). Therefore, harvesting 7 to 14 d after vine kill would be of great benefit to reduce the incidence of tu bers affected by the lenticel disorder. In addition, the results indicated that tubers have a mechanism to maintain the protective suberized lenticel cells. Enlargement of the lenticel aperture likely led to increas ed suberin deposition, resulting in lenticels with a wider diameter having a thicker suberized cell layer.

PAGE 79

79 Table 5 1. tubers before (initial rating 2, 3) and after 12 d storage at 10C and 20C, 95% RH storage for marketable (ratings 2, 3) and unmarketable (ratings 4, 5, 6). Harvest Time After 12 d Storage Initial Marketable Unmarketable 1 10 0.0 10.3 0.2 24.7 2.1 4 20 0.0 19.3 0.9 76.1 0.3 Table 5 2. Average thickness of suberized lenticel cell layers before (initial rating 2,3) and after 12 days at 10C and 20C, 95% RH storage for marketable (rating Harvest Time Average thickness (um) Before storage After 12 d Storage Initial Marketable Unmarketable 1 5 .2 0.1 10 .1 0.1 11 .5 0.4 2 10 0.0 17.9 0.2 59.9 0.2 3 10 0.0 21.9 1.0 69.2 0.8 4 14.4 1.0 41.4 1.2 82.1 0.8

PAGE 80

80 A B Figure 5 1. 95% RH), stained dark red and viewed under a light microscope (magnification 10X), A) Small lenticel, rating 2 or 3 (suberized zone thickness approx. 37 m), B) Enlarged lenticel, r ating 4, 5, or 6 (suberized zone thickness approx. 90 m).

PAGE 81

81 A B Figure 5 2. storage, stained dark red and viewed under a light microscope (magnification 10X), A) Small lenticel, rating 2 or 3 (suberized zone thickness approx. 30 m), B) Enlarged lenticel, rating 4, 5, or 6 (suberized zone thickness approx. 65 m).

PAGE 82

82 A B Figure 5 3. uber lenticels, after 12 day storage at 20C, 95% RH. A) Suberization of top 3 4 phellem/skin cell layers (magnification 20X), B) Light micrograph showing both suberized periderm cells and suberized lenticel filling cells shows suberized lenticel filling cells.

PAGE 83

83 CHAPTER 6 CONCLUSIONS AND SUGG ESTIONS FOR FUTURE R ESEARCH Development of l enticel disorders can affect the marketability of fresh potatoes. Knowledge about the factors leading to the development of th ese disorder s is the first step in possibly controlling the ir incidence and severity during tuber stor age. In this study an interaction of harvest time storage condition and duration affect ed the appearance of the enlarged High amounts of rainfall received just prior to Harvest 1 (before vine kill) led to water logged soil conditions and triggered m arked proliferation on the freshly dug tubers in both varieties. Although the tubers were graded as marketable, the lenticel apertures had clearly enlarged, with a mean lenticel rating of 2. 65 for both varieties Subsequent Harvests 2 and 3 had a lower average lenticel rating compared to Harvest 1, despite th e soils remaining wet throughout the growing season. These results indicate d that tubers with an immature skin (Harvest 1) wer e more susceptible to the enlarged l enticel disorder than vine killed tubers (Harvests 2 and 3) However, prolonged exposure to we t soils triggered cell division and enlargement, causing a higher lenticel rating in Harvest 4 tubers when compared to Harvests 2 and 3. ubers from all four harvest s developed the enlarged lenticel disorder when stored in the 20C, 9 5% RH (20H ) storage treatment Harvest 1 lenticel s were rated the highest for this disorder under these high humidity conditions indicating that non vine killed tubers were more susceptible than those from later harvests. Red La S o da had significant differ ences in the appearance of enlarged lenticels in all storage treatments, which were attributed to the manner in which the lenticels enlarge. Lenticels

PAGE 84

84 from Red La stretch ed diagonally into maintained their rou nd shape during enlargement. The highest increase in severity developed at 10 C and 20 C 95% RH (10H and 20H). These data strongly suggest that high relati ve humidity conditions during storage had the greatest effect on lenticels of both varieties by caus ing the existing filling cells to expand. The critical time for the development of the disorder was after 3 d of storage, as the highest mean lenticel rating for most of the affected tubers was observed by that time. Halos developed during storage in both varieties, appear ing either as dark areas ( due to accumulation of phenols ) in the periderm immediately surrounding the lenticel aperture in the low humidity conditions, or as raised areas in the high humidity conditions tubers from all harvest s the highest incidence of halo disorder occurred by the end of storage at 20C, 6 5% RH ( 20L ) For tubers the highest incidence was observed at 20H for the first three ; whereas for Harvest 4 the highest mean halo rating was ob served at 20L. Cell enlargement was possibly the cause of the raised halos observed in the high humidity conditions. The highest mean weight loss occurred at 10L and 20L storage conditions in both varieties, at each harvest time ; Harvest 1 tubers lost the most weight after 12 d in storage, compared to the subsequent harvests The higher weight loss in the 10L and 20L treatments for all harvests was due to the high desiccation rate promoted by the se environments. The immature tubers from Harvest 1 lost the m ost weight due to the combination of partially suberized periderm, and high er respiration rates due to large concentrations of starch which act as a respiratory substrate.

PAGE 85

85 Generally, t he periderm had higher moisture content than the inner cortical tissue p ossibly due to a greater proportion of dry matter content found in the latter There were also d ifferences in the moisture content of these tissues at each of the four harvest times, for both varieties, where tubers from the earlier harvests lost the most moisture content by the end of stor Harvest s 1 and 2 had the lowest peel and pulp moisture content s by the end of the 12 d storage had the lowest peel moisture content at Harvest 1. This is accounted for by the differences in degree of suberization of the periderm and lenticels, in which the prolonged stay in the ground after vine kill favored suberin deposition and the resulting thicker skin set. A thicker layer of sube rized filling cells was observed with increased time a fter initial, average thickness of 5.2 m before vine kill, increasing to 14.4 m by 3 weeks after vine kill A similar trend was T his coincided with a lower incidence of proliferated lenticels in freshly dug tubers from Harvests 2 and 3, when compared to Harvest 1, suggesting that lenticel cell proliferation is less likely to occur in wet fields after vine kill. However, prolonged ex posure to wet soils led to marked proliferation, as observed in Harvest 4 tubers. Additionally, more suberization was observed at the end of 12 d storage in enlarged lenticels compared to those that had maintained their initial aperture diameter suggesti ng that lenticel enlargement during storage enhanced suberization. Possibly, the lenticel s undergo stress induced suberization w hen the plant is vine killed or when there is development of the enlarged lenticel disorder in storage

PAGE 86

86 Based on the results fro m this study, harvesting two weeks after vine kill should reduce the incidence of tubers affected by the lenticel disorder. at 10C, 95% RH should have lower incidence and severity of both lenticel disorders and other qualitative losses Ho 10 C 65% RH for best external appearance and firmness. More research is needed to determine the effect of field irrigation systems and soil moisture levels on the development of proliferated lenticels prior to harvest. In addition, the effects of changes in storage tempera ture and RH on the development of these disorders should be determined for the various forms of packaging used for commercial shipping. Additional research also needs to be conducted to determi ne the relationship between the extent of lenticel suberization and susceptibility of the tuber to lenticel disorders. These results would provide growers and storage operators with valuable information on the ideal harvest maturity and storage condition r equired to minimize the disorder.

PAGE 87

87 LIST OF REFERENCES Adams MJ. 1975. Potato Tuber Lentice ls Development and Structure. Annals of Applied Biology 79 (3): 265 &. Anzalda M orales, A., B ourne, M. and Shomer I. 1992 Cultivar, Specific Gravity and Location in Tuber Affect Puncture Force of Raw Potatoe s. Journal of Food Science, 57: 135 3 1356. doi: 10.1111/j.1365 2621.1992.tb06855.x Bezuidenhout, J.L.J., Robbertse, P.J. and Kaiser, C. 2005. Anatomical investigation of lenticel development and subsequen t discolouration of 'Tommy Atkins' and 'Keitt' mango ( Mangifera indica L.) fruit. J. of Hort. Sci. and Biotechnology 80:18 22. Brown, C. 2005. Antioxidants in potato American Journal of Potato Research 82 (2): 163 172. Burton W G. 1989. The Potato. Essex, England, Longman Scientific & Technical. Cargill B F. 1976 The Potato And The Storage. The Potato Storage. Design, Construction, Handling and Environmental Control. B. F. Cargill, Michigan State University : 39 45. CIP. 1984 Potatoes For The Developing World. Lima, Peru, International Potato Center (CIP). Dean BB. 1994. Managing the Potato Production System. New York, Food Products Press. Dir iwchter, G. and D. G. Parbery 1990. Infection of pot ato by Spongospora subterranea. Mycological Research 95 (6): 762 764. Eddins A H GD Ruehle and GR Townsend 1946. Potato Diseases In Florida. Gainesville, FL., University of Florida Agricultural Experiment Station. Emeka ndoko A, K. Horti and T. Sray 2006. Chances of keeping quality of f resh, raw, early potatoes, with special regard to st orage in controlled atmosphere Acta Alimentaria 35 (Number 4/ December 2006): 493 500. Evert RF. 2006. Esau's Plant Anatomy, Meristems, Cells, and Tissues of the Plant Body: their Structure, Funct ion, and Development. 3rd edn. 99 (4): 785 786 Annals of Botany Fahn A.1990. Plant anatomy .4th edn.Oxford : Pergamnn Press FAO. 2008 I nternational Year Of The Potato Food And Agriculture Organization Of The United Nations.

PAGE 88

88 FAO. 2009. International Year Of The Potato. New Light On A Hidden Treasure., Food And Agriculture Organization Of The United Nations. Ginzberg, I., G. Barel, R. Ophir, E. Tzin, Z. Tanami, T. Muddarangappa, W. d e Jong and E. Fogelman 2009. Transcriptomic profiling of heat stre ss response in potato periderm. Journal of Experimental Botany 60 (15): 4411 4421. Haderlie, L., J. Halderson, P. Lein o, P. Petersen and R. Callihan 1989. Chemica l desiccation of potato vines. American Journal of Potato Research 66 (2): 53 62. Hiller LK, DC Koller and R E Thornton. 1985. Physiological Disorders of Potato Tubers. Potato Physiology. P. H. Li, Academic Press, Inc. : 389 391. Hsieh, M. F., Mitch ell, P. D. and Stiegert, K. W. 2009. Potato demand in an increasingly organic marketplace. Agribusiness, 25 : 369 394. doi: 10.1002/agr.20209 Hutchinson CM, EH Simonne, GJ Hochmuth, WM Stall, SM Olson, SE Webb, TG Taylor and S A Smith. 1995. Potato Production In Florida. HS733, University of Florida. Institute of Food an Agricultural Sciences. Hutchinson CM, DM Ger gela, T Olczyk, G McAvoy, JM White 2003 Red Skinned Potato ( Solanum tuberosum .L.)Variety evaluation in a sub tropical climate University of Florida. Institute of Food an Agricultural Sciences. J emison J M, P Sexton and ME Camire 2008. Factors influencing consumer preference of fresh potato varieties i n maine (vol 85, pg 140, 2008). American Journal of Potato Research 85 (5): 388 389. J ohansen DA. 1940 Plant microtechnique. New York: McGraw Hill. [Jodrell Lab.] Review article General article, Textbook, Staining Microscopy Technique Kay, DE 1973. Root Crops, TheTropical Products Institute, Foreign and Commonwealth Office (Overseas Development Administration). Kleinkopf, G.E. 1995. Dynamics of the stored potato A management approach. Early season storage. Amer. Potato J. 72:449 462. L isinska, G. and W. Leszczynski 1989 Potato science and technology New York, USA, E lsevier Science Publishers Ltd. Loria R. 2001. Development and Anatomy of the Potato Plant. Compendium of Potato Diseases. Second Edition. W R. Stevenson, R. Loria, G. D. Franc and D. P. Weingartner, The American Phytopathological Society.

PAGE 89

89 Loria R. 2001. Potato Disease Management Strategies. Compendium of Potato Diseases. Second Edition. W. R. Stevenson, R. Loria, G. D. Franc and D. P. Weinga rtner, The American Phytopathological Society : 7 8. Lulai E C. 2001. Tuber Periderm and Disease Resistance. Compedium of Potato Disease. Second Edition. W. R. Stevenson, R. Loria, G. D. Franc and D. P. Weingartner, The American Phytopathological Society : 3 6. Lulai, E C. and D. L. Corsini 1998. Differential deposition of suberin phenolic and aliphatic domains and their roles in resistance to infection during potato tuber (Solan um tuberosum L.) wound healing. Physiological and Molecular Plant Pathology 53 (4) : 209 222. Mauseth JD. 1988 Plant anatomy. Benjamin/Cummings Publ. Co.: Menlo Park, Calif. 560 pp Morris, S. C., M. R. Forbes Smith and F. M. Scriven. 1989. Determination of optimum conditions for suberization, wound periderm formation, cellular desiccation and pathogen resistance in wo unded Solanum tuberosum tubers. Physiological and Molecular Plant Pathology 35 (2): 177 190. Mossler MA and C Hutchinson 1999. Florida Crop/Pest Management Profile: Potatoes. CIR 1237, University of Florida. Instit ute of Food and Agricultural Sciences. National Potato Council. 2010 National Potato Council 2010 Potato Statistical Yearbook Englewood, CO: National Potato Council. Park, D. H., Y. M. Yu, J. S. Kim, J. M Cho, J. H. Hur and C. K. Lim 2003. Characterizati on of Streptomycetes Causi ng Potato Common Scab in Korea. Plant Disease 87 (11): 1290 1296. Peterson RL, WG Barker and MJ Howarth 1985. Development and Structure of Tubers. Potato Physiology. Third Edition. P. H. Li, Academic Press, Inc. : 124 148. Plissey ES 1976. Philosophy of Potato Storage. The Potato Storage. Design, Construction, Handling and Environmental Control. B. F. Cargill, Michigan State University : 7 10. Powelson, M. L. and R. C. Rowe 1993. Biology and Management of Early D ying of Potatoes. An nual Review of Phytopathology 31 (1): 111 126. Rastovski A, A van ES, N Buitelaar, PH de Haan, KJ Hartmans, CP Meijers, JHW van der Schild, PH Sijbring, H Sparenberg, BH van Zwol and DE van der Zaag 1981. Storage of Potatoes. Post harvest behaviour, store design, storage practice, handling. Wageningen, Centre for Agricultural publishing and Documentation.

PAGE 90

90 Rhodes D 2009. HORT410 Vegetable Crops from http://www.hort.purdue.edu/rhod cv/hort410/potat/po00001.htm Ruzin SE. 1999 Plant microtechnique and microscopy. Oxford, UK.: Oxford University Press 322 pp.. Jodrell 578.6 RUZ. Review by N.J. Chaffey (2000) in: New Phytol. 148 Sabba, R., A. Bussan, B. Michaelis, R. H ughes, M. Drilias and M. Glynn 2007. Effect of planting and vine kill timing on sugars, specific gravity and skin set in pr ocessing potato cultivars. American Journal of Potato Research 84 (3): 205 215. Sabba, R. P. and E. C. Lulai 2002. Histological Analys is of the Maturation of Native and Wound Periderm in Potat o (Solanum tuberosum L.) Tuber. Annals of Botany 90 (1): 1 10. Schippers P A. 1976. Biological Characteristics of The Potato In Relationship To Storage., Michigan State University. Schreiber, L., R. Franke and K. Hartmann 2005 Wax and suberin development of native and woun d periderm of potato ( Solanum tuberosum L.) and its relati on to peridermal transpiration. Planta 220 (4): 520 530. Scott, R., J. Chard, M. Ho cart, J. Lennard and D. Graham 1996. Penetration of potato tuber lenticels by bacteria in relation to biologic al control of blackleg disease. Potato Research 39 (3): 333 344. Shetty, K.K. 1998. Potato storage management for disease control. Univ. of Idaho website at www.kimberley.uidaho.edu/potatoes Smith, O. and L. Na sh 1940. Potato quality. I. relation of fertilizers and rotation systems to specif ic gravity and cooking quality. American Journal of Potato Research 17 (7): 163 169. Snowdo wn, A.L., 1991 Color Atlas of Post Harvest Diseases and Disorders of Fruits and Vegetables, Vol. 2: Vegetables. CRC Press, Boca Raton, FL, 416 pp. Stevenson, W.R., R. Loria, G.D. Franc and D.P. Weingartner. 2001. Compendium of potato diseases, 2 nd e dition APS Press, 144 pp. Suslow, T. and R. Voss. 2000. Potato (immature early crop). Recommendations in maintaining postharvest quality. Postharv. Technol. Res. Info. Cntr., Univ. of Calif., Davis CA. Thompson, A. K. 2003 Fruit and vegetables: harvesting, handling and storage Iowa, Blackwell Publishing Ltd.

PAGE 91

91 Tyner DN, MJ Hocart, JH Lennard and DC Graham 1997. Periderm and lenticel characterization in relation to potato cultivar, soil moisture and tuber maturity. Potato Research 40 (2): 181 190. USDA/NASS fro m http://www.nass.usda.gov/QuickStats/index2.jsp United States Potato Board from http://www.potatoesusa.com/articles.php VanSickle JJ, S Smith, R Weldon 2009 Potato Production In Florida. FE794 University of Florida. Institute of Food an Agricultural Sciences

PAGE 92

92 BIOGRAPHICAL SKETCH Mildred N. Makani was born in Masvingo, Zimbabwe in 1980. She did her primary school education at Victoria High School, in the same town. She then proceeded to do Advanced Levels at Victoria High School. In 1999, she enrolled at the University of Zimbabwe, where she graduated with a BSc honors degree in Plant Science. Mildred then joined a private company which specialized in providing virus-free plant material of drought tolerant crops such as sweet potato and cassava. She specialized mostly in the post harvest area, with her work involving research and extension work on the handling, storage and value-addition of the root crops after harvest. In 2009, Mildred decided to pursue her graduate studies, joining Dr. Steven degree in orticultural ciences. Upon completion of physiology and technology of fruits and vegetables.