Front Cover
 Title Page
 Table of Contents
 List of Tables
 List of Figures
 Literature review
 Materials and methods
 Yield potential and drought...
 Summary and conclusions

Group Title: Effects of water stress imposed at mid-pod filling upon yield and dry matter partitioning in dry beans (Phaseolus vulgaris L.)
Title: Effects of water stress imposed at mid-pod filling upon yield and dry matter partitioning in dry beans ( Phaseolus vulgaris L. )
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00086758/00001
 Material Information
Title: Effects of water stress imposed at mid-pod filling upon yield and dry matter partitioning in dry beans ( Phaseolus vulgaris L. )
Physical Description: Book
Language: English
Creator: Samper, Catalina
Publisher: Catalina Samper
Place of Publication: East Lansing, Mich.
Publication Date: 1984
General Note: Thesis for the degree of M. S., Michigan State University
 Record Information
Bibliographic ID: UF00086758
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 15231594

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Title Page
        Title Page
        Page i
        Page ii
        Page iii
    Table of Contents
        Page iv
        Page v
    List of Tables
        Page vi
        Page vii
        Page viii
    List of Figures
        Page ix
        Page x
        Page 1
        Page 2
    Literature review
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
    Materials and methods
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 44a
        Page 45
        Page 45a
        Page 46
        Page 47
        Page 47a
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
        Page 92
    Yield potential and drought susceptibility
        Page 93
        Page 94
        Page 95
        Page 96
        Page 97
        Page 98
        Page 99
        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
        Page 110
        Page 111
    Summary and conclusions
        Page 112
        Page 113
        Page 114
        Page 114a
        Page 115
        Page 116
        Page 117
        Page 118
        Page 119
        Page 120
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        Page 123
        Page 124
Full Text

(Phaseolus vulgaris).

Thesis for the Degree of M. S.






(Phaseolus vulgaris)




Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of


Department of Crop and Soil Science


To Dad, Mom and Ivan

for their love, generosity and
encouragement of my growth


I cannot find the words to express my gratitude to my

major professor, Dr. M. Wayne Adams. During the course of my

studies he has been not only a major professor, but a friend

and a colleague. He was always at my side, sharing moments

of joy and sadness, giving me encouragement and

unconditional support. The challenging discussions that we

had, his encouragement of my independent thinking and his

deep honesty, will always be with me as an example to

follow. I had the privilege to work with a great scientist

and a true teacher but best of all, with a great human


I want to express my gratitude to:

Dr. Andrew Hanson, a member of my committee, for his

encouragement of my development as an independent thinker,

his thorough and constructive criticism of my work, his

generosity and kind support. His conceptualization of

science and his fine example as a continues learner and

actively involved researcher are qualities that I greatly


Dr. Al Smucker, committee member, for reviewing this

manuscript and for his help on the progress of my research


Dr. Peter Graham, Dr. Rogelio Lepiz, Dr. Ronald

Ferrera, Mr. Jorge Acosta and Mr. Abelardo NuHez, for

their help and interest in this project.

Special thanks to Greg, Nasrat, Sue, Joe, Rhea, Francisco,

Earl and to all the graduate students, the professors and

technicians of the "bean program", for their intellectual

stimulation, the hard work and the good times that we shared


Last but not least, to my brothers Juan, Gordo and Bite,

my sister Cuca, and my friends Kim, Touran and Regina for

their love and unconditional support.


LIST OF TABLES . . . . .




Yield Constraints . . .

Allocation of Assimilates .

Biological Nitrogen Fixation

Photosynthate Partitioning .

Drought Tolerance Mechanisms



I. Water Effects . . .

A. Biological- Yield .

B. Economic Yield . .

C. Harvest Index . .

D. Seed Size . .

E. Seed Number . . .

F. Length of- Vegetative and
Reproductive Stages .

G. Leaf Dropping . .


S. . vi


* 1

. 3

* 0 3

* 7

. 14

* 12

* 30

. 30

S . . 18

.* 30

S 37

* 37

0 37

. 40

.* 42

* 47

H. Plant Dry Weight at Physiological
Maturity .

I. Plant Dry Weight Changes:
Remobilization . . . .

J. Starch Analysis . . .

. 50

. 52

. 56


K. 20 Upper and Lower Pods:
Seed number and Size. . . . 60

II. Nitrogen Effects . . . . . 64

A. Non-significant Effects . . . 66

B. Plant Dry Weight . . .66

C. Plant Dry Weight Changes:
Remobilization . . . . 69


1. Crop Growth Rate . . . . .75

2. Partitioning 79

3. The Filling Period . . . . 90


1. Drought Susceptibility Index . . . 93

2. Relationship between Control and
Stress Yields 99

3. Geometric Mean of Stress and Control

Yields as a Selection Criterion for
Drought Tolerance . . . . .



APPENDIX I : Durango Experiment: Experimental Design
and Yield Data . . . .

APPENDIX II : Starch Analysis . . . .

. 103

. 112

. 115

. 122














Biological Yield (kg/ha) under two water
treatments. Iguala, 1982-3.. .

Economic Yield (kg/ha) under two water
treatments. Iguala, 1982-3. . . .

Harvest Index under two water treatments.
Iguala, 1982-3. .

Weight of 100 seeds (grs) under two water
treatments. Iguala, 1982-3. . . .

Seed number (seeds/mt2) under two water
treatments. Iguala, 1982-3. . . .

Days between flowering and physiological
maturity under two water treatments. Iguala,
1982-3. *

Leaf dropping under two water treatments.
Iguala, 1982-3. . .. .

Stem and Pod % of total dry weight at
physiological maturity under two water
treatments. Iguala, 1982-3. . . .

Mean values of starch (mgrs/gr dry wt) at
three different physiological stages under
two water treatments. Iguala, 1982-3. .

Seed number of 20 upper and lower pods
under two water treatments. Iguala, 1982-3.

Seed weight (mgrs/seed) of 20 upper and
lower pods under two water treatments.
Iguala, 1982-3. . .* *

Shoot:Root ratio under two Nitrogen
treatments at two physiological stages.
Iguala, 1982-3. .

. 33


. 39

. 41

* 43

. 49

. 51

. 61

LIST OF TABLES (Continued)














Average Crop Growth Rates (kg/ha/day)
form planting to flowering. Iguala, 1982-3.

Average Crop Growth Rates (kg/ha/day)
from flowering to maturity under two water
treatments. Iguala, 1982-3. . . .

Average Fruit Growth Rate from flowering
to physiological maturity (kg/ha/day)
under two water treatments. Iguala, 1982-3.

Partitioning Factor under two water
treatments. Iguala, 1982-3. . .

Comparison between Grain yield, Fruit
Growth Rate, Seed number and Effective
seed filling period under two water
treatments. Iguala, 1982-3. . . .

Individual cultivar drought susceptibility
indices. Iguala 1982-3 and Durango 1983.

Group drought susceptibility indices -S-
Iguala 1982-3 and Durango 1983. . .

Group ranking by drought susceptibility
index (S). Iguala 1982-3 and Durango 1983

Ranking by drought susceptibility index (S)
of the eight cultivars planted in Iguala
and Durango . . . . . .

. 78

. 80

. 81

. 91

. 95

. 96

. 97


Yield differential, Arithmetic mean and
Geometric mean for the Iguala experiment

Yield differential, Arithmetic mean and
Geometric mean for the Durango experiment .

. 104

. 106

Cultivar ranking for the Iguala experiment
using four different selection criteria . 107

Cultivar ranking for the Durango experiment
using four different selection criteria

. 108

LIST OF TABLES (Continued)

TABLE 26. Mean yields of the selected top 20%
cultivars, using two different selection
criteria. Iguala experiment. . . . 110

TABLE 27. Mean yields of the selected top 20%
cultivars, using two different selection
criteria. Durango experiment. . . 111














Maximum and minimum daily temperatures.
Iguala, 1982-3. . .

. 24

Flowering dates and maximum daily
temperatures. Iguala, 1982-3. . . 45

Biological yield under irrigation and
flowering dates. Iguala, 1982-3. . . 46

Economic yield under irrigation and
flowering dates. Iguala, 1982-3. . . 48

Stem, Root, Pod and Leaf dry weights
(grs/mt2) over three physiological
stages. Iguala, 1982-3. . . . 53

Changes in Stem-Starch contents (grs/mt2)
over three physiological stages under
two water treatments. Iguala, 1982-3.

Changes in Pod-Starch contents (grs/mt2)
over three physiological stages under
two water treatments. Iguala, 1982-3. . 59

Stem % of total plant dry weight at
flowering time. Iguala, 1982-3.

. 67

Stem, Root, Pod and Leaf weights at
flowering and at 15 dap. under two
different levels of added Nitrogen.
Iguala, 1982-3. . . . . 70

Proportion of fruit growth that can
be accounted for by post-anthesis
photosynthesis under irrigated
conditions. Iguala, 1982-3. . . 84

Proportion of fruit growth that can
be accounted for by post-anthesis
photosynthesis under stress conditions.
Iguala, 1982-3. . . . 85

. 58





Relationship between change in stem
and leaf weight from anthesis to
maturity and grain yield under
irrigated conditions. Iguala, 1982-3. . 88

Relationship between change in stem
and leaf weight from anthesis to
maturity and grain yield under
stress conditions. Iguala, 1982-3. . 89

Relationship between control and
stress yield. Iguala, 1982-3. . . 100

FIGURE 15. Relationship between control and
stress yield. Durango, 1983. . . 101


Varietal differences in the amount of starch present at

flowering time,grain filling and physiological maturity in the

dry bean (Phaseolus vulgaris) have been previously reported

(4,30). The capacity of certain genotypes to store and remobilize

starch to the grain may be an advantage when the plants are

subjected to stress and their photosynthetic activity is reduced.

The general objective of the project of which this thesis is

part of, was to study the relationship between photosynthate

partitioning, remobilization, and the seed filling processes in

several genotypes of P. vulgaris grown under different stress

conditions. Initial objectives were: 1) to determine the effect

of drought stress imposed during the latter part of the seed

filling period on the yield performance of 22 different bean

cultivars and to relate their performance under stress to their

ability to accumulate and remobilize non-structural

carbohydrates; 2) to compare the effect of nitrogen fertilizer

versus biologically fixed nitrogen (BNF) under stress conditions,

and subsequently to determine the relationship between total

amount of non-structural carbohydrates and their remobilization

with the plant's ability to buffer the adverse environmental

conditions; 3)to identify bean genotypes having tolerance to

drought and high BNF potentials and to relate their performance

under stress conditions with the patterns of accumulation and

remobilization of starch and soluble sugars; 4)to identify

specific traits or physiological characteristics that could be

associated with better cultivar performance under drought

conditions, 5)to use the information and genetic materials

obtained during the course of this research as sources of new

improved germplasm in the development of varieties with

resistance to drought and with the capacity to fix nitrogen under

water stressed conditions.

For these purposes an experiment was conducted in

Iguala,Mexico, from December of 1982 to April of 1983. This

experiment was intended to study and exploit the genetic

potential for differential storage and remobilization of non-

structural carbohydrates and the genetic capacity for higher

levels of BNF, and to relate this to the yield performance of

cultivars under conditions of drought and low soil nitrogen.

A second experiment was conducted in the summer of 1983 in

Durango, Mexico to provide further data on varietal performance

under stress.

Yield constraints

Over forty five per cent of the world production of dry

edible beans (Phaseolus vulgaris) is consumed in Latin America.

Nevertheless,low yields of this crop are limiting the traditional

role beans play as a staple food in the diets of poor and middle

income consumers of this region. Although bean yields of over

4000 kg./ha. have been reported from experimental plots at the

Centro Internacional de Agricultura Tropical (CIAT) in

Colombia,the average bean yield in Latin America remains near 600

kg./ha. (51). In the dryland production region of Mexico the long-

term yields are reported to average less than 300 kg./ha. (1). A

significant closing of the gap between current yields and

potential yields must be achieved if this crop is to fulfill its

role in meeting the nutritional needs of the population.

In large areas of Latin America and Africa, where beans

constitute major source of dietary protein, production is

limited mainly because beans are a crop of the small farmer and

the conditions under which the crop is usually grown are typified

by low soil fertility, and minimal technical inputs, such as

irrigation, fungicides and insecticides .

Approximately 20% of all potentially arable land in the

world is in arid and semiarid zones, and about 16% of the world's

population lives on these lands (40). Research and development in

arid and semi-arid agriculture has, therefore, global

significance. Arid and semi-arid lands have been defined in a

number of different ways. Their main characteristic is a low

and variable seasonal rainfall, a condition which is often

directly exacerbated by other variable elements of the climate,

such as temperature, sunshine, wind and humidity conditions.

Beans, among a few other crops, are dryland staples in many

developing countries, providing a major source of affordable

protein and carbohydrate. Bean breeding over the years has

focused on improving agronomic adaptation along with disease

resistance, with less direct emphasis upon yield itself. It is

acknowledged by bean plant breeders that there have been no

decisive breakthroughs in yield, excepting increases originating

from disease resistance or favorable maturity adjustments (2).

Increasing yield is imperative, but this objective must be

integrated with the genetic improvement of adaptation and

resistance to stresses brought about by diseases,insects and

physical causes. The improvement of both agronomic characters and

yield could maximize the responses of the bean plant to available

resources characteristic of the site and local production system.

This is especially important for the Latin American small

farmers,because the conditions under which the crop is usually

grown are typified by the lack of irrigation systems,little or no

use of fungicides and insecticides, and small amounts of

fertilizers. In Mexico, where 1.7 million hectares are planted

annually with beans, 24 out of the 30 states that produce beans

raise them under rainfed conditions. During 1970 to 1975

approximately 1.2 million hectares were planted annually with

beans,and the average yield was around 545 kg./ha (35). This was

enough for the internal demand, but from there on only during

1978 and 1980 was production considered to be at its normal

level. As a consequence, during 1980 Mexico had to import more

than 250,000 tons of beans to satisfy the internal demand.

This production shortfall originated basically because of

adverse climatic effects such as drought and early frost. In

Mexico, beans are planted twice a year, during the Spring-Summer

and the Autumn-Winter cycles. During the Spring-Summer cycle when

the majority of the total production is obtained, about 1.4

million ha. are planted and 530,000 tons are harvested. In this

cycle typical low yields of 387 kg/ha are caused by adverse

environmental factors such as drought -scarce or irregular

rainfall- and early frost in the northern part of the country.

During the Autumn-Winter cycle about 260,000 ha. are planted and

246,000 tons are harvested; this corresponds to 31% of the total

national production and 16% of the total planted area. It is

interesting to note that with only 16% of the total planted area

almost one third of the total national production is obtained,

with average yields being 933 kg./ha. In states such as Nayarit,

Sinaloa and Baja California where irrigation is widely practiced,

average yields are over 1100 kg./ha., while for the country as a

whole 88% of the area that produces beans is rainfed only and the

average yields are about 350 kg./ha. Of this, 88% or

approximately 1 million ha. are in the states of Aguascalientes,

San Luis Potosf, Zacatecas and Chihuahua. These areas are

frequently affected by either scarce or badly distributed rain

throughout the growing season. In the state of Durango, nearly

30% of the planted beans are lost annually due to insufficient

water,and in bad years such as 1979 the losses can reach up to

60% of the total planted area (1).

In Colombia, in the states of Huila, Narifo and Antioquia

where the average farm sizes are 29.5 9.2 and 4.4hectares,

respectively, the percentage of farms that use irrigation is 2,

0, and 0 and the average yields are 680, 467 and 533 kg./ha. (44).

On the other hand, in the state of Valle del Cauca, where the

average farm size is 48.0 ha., 45% of the farms use irrigation

and the average yield is 906 kg./ha.

Among factors other than drought tolerance that could

contribute to improved crop yields the availability of fixed

nitrogen to crops is probably one of the greatest importance. In

1974 40 x 106 tons of fertilizer nitrogen with an approximate

value of 8 billion dollars were used as opposed to the 3.5 x
106 tons that were used annually twenty five years ago (28). The

scarcity of nitrogen fertilizers and their increased selling

price has produced a tremendous interest in the search for

alternative technologies. Inoculation of legumes with Rhizobium

at the farm level appears to offer promise as a possible

substitute for nitrogen fertilizers. Recent reports from farm

trials performed in Colombia by CIAT show that in the absence of

any nitrogen amendments, inoculation of a local variety with a

mixture of Rhizobium strains gave yields that were not

significantly different from a farmers' technology treatment, in

which 20 kg./ha. of chemical nitrogen in the form of urea and 2

tons/ha. of chicken manure were applied. Substitution for

nitrogen fertilizers by a Rhizobium inoculant would reduce total

costs of production by 34%, while the net return per peso

invested would rise from 5.5 to 7.7 pesos.

A bean breeder who wants to develop a variety for the small

farmers of Latin America should be aware that increased yields

must be obtained with very limited cash inputs.

Allocation of Assimilates

In recent years breeders have been considering the

development of plant ideotypes (14). In dry beans an ideotype for

production under monoculture has been proposed by Adams (2), who

suggested that productivity increases in dry beans could be

obtained if a more efficient allocation of assimilates into the

economic sink is developed by breeding.

Two of the principal physiological processes that can be

considered for improvement of crop yields are photosynthate

production and photosynthate partitioning to the economically

important organs. However, the importance of transpiration as a

central factor in explaining the influence of water limitation on

productivity, as pointed out by Fischer and Turner (20) cannot be

overlooked. It depends on the inevitable association between

water loss and C02 assimilation. Dry matter production over a

given period of time is a function of the total transpiration for

the given period and the water use efficiency. The importance of

respiration rates in determining the net accumulation of dry

matter is commonly overlooked. As Gifford et. al. (22) pointed

out, in leaves the fixed carbon is partitioned between its

retention in the plant and its photorespiratory release. Over

long periods, a full understanding of productivity requires

consideration of how each increment of dry weight is allocated to

both vegetative and reproductive sinks.

The potential for increasing crop productivity by optimizing

canopy structure has been documented by experimental research,

modeling, and computer simulation (50). Assimilate partitioning

is a dynamic process and varies with the stage of plant

development. In the vegetative stage of the dry bean plant, the

distribution of assimilates is dominated by the proximity between

the source and the sink. After flowering, when the developing

pods become major sinks, there is a more complex pattern,

although the relationship between leaves and pods in their own

axils still predominates (3).

Use of 14C as a tracer, and changes in dry weight of

specific organs have been important techniques in helping to

understand assimilate distribution. However, many important

aspects of this process, like the mechanisms of regulation,

accumulation and remobilization of storage assimilates under

different conditions, still remain to be studied in order to

provide guidelines for the increase of yields by manipulation of

photosynthate partitioning.

Large varietal differences in the ability to translocate

14c- assimilates and a clear trend for varieties with the higher

translocation rates to have higher photosynthetic rates, were

reported by Adams and Reicosky (3). From data obtained on

carbohydrate translocation patterns in beans, they suggested that

the two facets upon which breeding studies might be focused were

rate of translocation and direction of partition to sinks of the

assimilate. They also suggested that these characters may be

under genetic control and might be used in a plant breeding


In recent years, attention has been given to carbohydrate

production and partitioning in plants as factors related to crop

yield; carbohydrate mobilization may be especially important

under stress conditions (6). Varietal differences in carbohydrate

accumulation or partitioning may be related to maintenance of a

high rate of seed filling during periods of temporary

environmental stress when photosynthesis is adversely affected.

Evans (18) considers that whereas photosynthesis during the

storage phase can be an important determinant of yield,

photosynthesis prior to that contributes to the determination of

storage capacity and generates reserves that may be mobilized

during the storage phase.

Gifford et al. (22) reviewing the partition of

photoassimilates and crop productivity, examined the

photosynthetic basis for increasing yield of major field crops in

terms of improving the partitioning of photoassimilates to organs

of economic interest. Although little is known about the

regulation of carbohydrate partitioning between starch storage

(for later utilization) and sucrose synthesis (for immediate

export), they afirm that sink demand plays a very important role.

The partitioning of photosynthetically fixed carbon is important

for plant growth not only because the formation of sucrose

partially determines the carbon export from photosynthesizing

leaves, but also because leaf starch is mobilized to sucrose when

current photosynthesis is low relative to sink demand for

assimilates. In their discussion they suggest that photosynhtesis

and the mechanism of phloem loading determines the amount of

photosynthetic assimilate available fro translocation, while the

mechanism and kinetics of unloading into competing sinks

determines the partition of loaded materials.

Carbohydrates reach a maximum concentration in the plant's

vegetative parts around flowering time, after which they start to

decrease. Yoshida (58) found that stored carbohydrate could be

translocated into the rice grain, thus contributing to grain

filling, or it could be consumed as a substrate for respiration.

The carbohydrate loss from the vegetative parts during grain

filling provides only a maximum estimate of the contribution of

the stored carbohydrates to the grain.

Evidence that stored carbohydrate can be translocated into

the grain has been obtained for rice and wheat by labeling the

stored carbohydrate with 14c. Cock and Yoshida (11) showed that

under normal field conditions 60% of the stored carbohydrate was

translocated into the grain. When photosynthesis during the

ripening period is restricted by shading or defoliation, the

stored carbohydrate appears able to support the grain growth of

rice and corn at almost the normal rate for some time. Perhaps

the stored carbohydrate can serve as a buffer to support normal

grain growth despite weather fluctuations (58). Whether the yield

capacity or assimilate supply limits the grain yield is not

clear. However, defoliation and shading experiments in rice at or

after heading clearly demonstrate that impaired photosynthesis

during the ripening period can severely limit the grain yield

(48). Assimilate supply may limit grain yield under stress

conditions: if photosynthetic activity is limited by shading, or

if translocation of assimilates into the grain decreases, a

certain portion of the grains may remain unfilled (58).

Biological Nitrogen Fixation

Beans are a crop of small farmers in much of the third

world, and are often produced on marginal soils deficient in

nitrogen. In a world of rising fertilizer prices, the need for

cultivars with improved ability to fix nitrogen is especially

important. The identification of genetic variability for

biological nitrogen fixation (BNF) in beans has made selection

for enhanced BNF possible (5).

Graham (25) suggested that at least three factors could

contribute to the variability in N-fixation observed in

P.vulgaris : a) supply of carbohydrates to the nodules, b)

relative rates of nitrogen uptake from soil, and c) time to

flowering. Hardy and Havelka (29) indicated that the amount of

photosynthate available to the nodules may be the most

significant factor limiting N-fixation. They examined factors

that affect photosynthate availability to the nodule such as

light intensity, size of photosynthetic source, competitive

sinks, CO2 enrichment and photosynthate translocation. With

respect to the effect of variation of each of these parameters on

N-fixation in soybeans, N-fixation correlates directly with the

amount of photosynthate available to the nodule. Nodules in

general maintain low reserves of readily utilisable carbohydrates

relative to their requirements for fixation, so they probably

rely for their growth and functioning on photosynthetic products

currently translocated from the leaves, or on carbohydrate

reserves mobilized from other regions of the plant (41).

Experimental evidence is consistent with this view, since a very

close relationship has been observed between photosynthesis,

amount of photosynthate and N-fixation. Reducing light or

defoliation decreases fixation, while supplemental light

increases it (7,26,27,29,41,46,47,49) ; pod removal increases N-

fixation (7,27,28) presumably by leaving more photosynthate

available for the nodules. Lawn and Brun (33) indicated that the

decline in soybean nodule activity was associated with the

development of the pods as a competing assimilate sink. The fact

that the decline in nodule activity coincided with the time when

pod growth rate first exceeded total top growth rate is an

indication of mobilization of previously assimilated material

into the pods.

Factors, both genotypic and environmental, which tend to

lessen competitive effects by enhancing the photosynthetic

source-sink ratio,may be expected to minimize a decline in N-

fixation and should be considered in the future development of

higher yielding varieties with high BNF potential.

In order to understand how varietal differences in N-

fixation might relate to carbohydrate supply and availability in

the bean nodule, Graham and Halliday (23) planted fourteen

commercial varieties, inoculated and sampled at initiation of

fixation and at the beginning of decline in fixation rates.

Marked varietal differences were found, and a highly active N-

fixing variety (P590) showed a higher soluble carbohydrate

percentage in all organs and also partitioned more of its total

carbohydrate to the nodule as compared to an inactive N-fixing

line (P635). In this study, climbing varieties which had been

previously reported to be good N-fixers (24) were found to hold

more of their carbohydrate in the soluble form.

The ontogenetic development of four dry bean cultivars with

reference to the relationships that may exist between symbiotic

nitrogen fixation and the energy supply (in the form of

carbohydrates ) to the nodules was studied by Martinez (38). His

data are consistent with the hypothesis that carbohydrate supply

to the nodules limits fixation. He showed that an increase of

total photosynthate available to the symbiotic system, achieved

through C02 enrichment, resulted in higher rates of nitrogen

fixation. The nitrogen in the bean plant is stored temporarily in

the leaves, and it is suggested that mobilization of this

nitrogen to the seeds results primarily from leaf aging. Martinez

(38) showed a similar phenomenon of mobilization of carbohydrates
temporarily stored in the stems and leaves.

Wilson et al. (56) performed experiments to study the

nonstructural carbohydrates, the nitrogen content of plant

tissues and the nitrogenase activity throughout the development

of male sterile and male fertile soybean plants. Male sterile

plants set approximately 85% fewer pods than the male fertile

plants, and reduced pod set was found to increase carbohydrate

accumulation in the leaf and root systems. Although roots of male

sterile plants contained greater quantities of carbohydrate, a

decline in nitrogenase activity occurred after flowering. The low

percentage of soluble carbohydrates in roots of either type (male

sterile and male-fertile) during the pod filling stage might be

one of the many possible explanations for the similar trends

observed in male sterile and male fertile nodule activity.

In efforts to increase N-fixation it is not necessary to
restrict selection only to genetic factors that affect

nodulation, increase nitrogenase activity or generate larger

amounts of accumulated nitrogen. Certain genotypes may be

superior to others in their allocation of assimilatory resources

to the various plant parts (27,29,41) The functional economy of

whole plants and the interactions of their organs during growth

should be considered in order to determine the plant factors that

are responsible for the variation in nodulation and nitrogen

fixation (57).

Effective photosynthate partitioning

The selection of cultivars with more effective partitioning

of nitrogen and carbon assimilates to the reproductive organs

than older cultivars was thought to be the key factor for the

improvement of yield in other crops, namely rice (58), peanuts

(16) and cotton (55).

Genotypic variation in carbohydrate and nitrogen

remobilization during periods of environmental stress when

photosynthesis is adversely affected, may enable maintenance of a

high rate of seed filling and may buffer and stabilize yields.

Photosynthate partitioning has been shown to be under genetic

control in cereals (15), soybeans (31) and sugar beets (45). In

beans, Adams et. al. (4) showed genetic variation for

accumulation of starch during reproductive development. Izquierdo

(30) also showed that differences in sugars and starch (

total nonstructural carbohydrates ) and nitrogen were associated

with cultivars and physiological stages over the entire

reproductive growth period. Izquierdo (30) showed genetic

variation of seed filling parameters (rate and duration) in this

crop and the relationship of these parameters to patterns of

assimilate partitioning among genotypes. He concluded that yield

differences among cultivars are more associated with the length

of the seed filling period than with the rate of seed growth.

Constable and Hearn (12) performed a series of experiments

with sorghum and two soybean varieties (Ruse and Bragg) under two

different water treatments. Sorghum and Ruse soybean showed a

significant (17-25%) loss in stem dry weight during grain filling

under both treatments. In Bragg soybeans, only the stressed

plants had a loss in stem dry weight during grain filling. One

can infer that in sorghum and in Ruse, the significant loss in

stem dry weight during grain filling could have been a

consequence of relocation of dry matter from the stem to the

developing grain. This agrees with Yoshida's conclusion (58) that

the weight loss from vegetative parts during grain filling sets

an upper limit to the possible contribution of stored

carbohydrates to the grain. An apparently large difference

between soybean cultivars in the effect of water treatment on the

contribution of stem storage to yield was reported by Constable

and Hearn (12); in cultivar Ruse an estimated 25% of grain dry

weight could have come from the stem, while in Bragg only the

stressed plants appeared to use stem reserves. This suggests that

Bragg was sink limited and had little requirement for storage

carbohydrates, except during stress. Rawson et al. (42)

substantiate Constable and Hearn's conclusion that when water

deficits restrict current photosynthesis during grain

development, the plant may buffer yield by drawing heavily on

reserves. Also Egli and Leggett (17) have suggested that

soybean seed growth rates are not closely related to rates of

photosynthate production because storage carbohydrate acts as a

buffer between seed growth and photosynthesis.

Evidence supporting the idea that plant growth rate and seed

yield are not directly affected by total photosynthesis was

reported recently by Ford et al. (21). They used soybean lines

divergently selected for rates of 14C02 uptake per unit leaf area

and tested the effect of this divergent selection for leaf total

photosynthesis on crop growth rate and seed yield. Their data

showed that selection for improved photosynthesis per unit area

did not necessarily enhance seed yield.

The effects of drought on nodulation and nitrogen fixation

in field grown cowpeas were studied by Zablotowicz et al. (59).

The nodulation process was inhibited by drought and maximum

nodulation was observed at mid-pod fill in the drought regime

while plants from the well watered regime showed maximum

nodulation at early flowering. As the plants matured beyond mid-

pod fill,there was no significant difference in nodule mass

between water treatments. Droughted plants failed to form nodules

of high nitrogenase activity during the early stages of

development, and at the reproductive stages the N-fixation

capacity of the crop decreased, probably because there was

insufficient carbohydrate to support high activity at this stage.

Field canopies of two semi-dwarf wheat genotypes were

subjected to water stress that caused visible wilting during the

grain filling stage, and the distribution of photosynthesis

within canopies and the patterns of translocation of labeled

assimilates following 14c02 uptake were determined (32). In

stressed plants 24 hours after labeling, 46% of the 140 was

found in the grain compared to 35% in the control plants. Of the

total 14C recovered from the shoots at maturity, 83% was found in

the grain of stressed plants and 69% in control plants. The lower

percentage of 140 in grain of control plants at maturity was due

to its accumulation in stem segments, primarily in the form of

structural carbohydrates. Fischer and Turner (20) stated that

water stress during seed filling has its major effect upon

current assimilation through reductions in assimilatory activity

and assimilatory surface. They concluded that water stress not

only increases the proportion of current assimilate translocated

to the seed, but also may increase the contribution from

assimilate stored prior to seed filling.

In the broad bean Vicia faba N-fixation has been found

to be severely suppressed once flagging of the lower leaves has

commenced (46). If flagging of the lower leaves takes place,

photosynthesis is likely to be arrested and since these leaves

are likely to be the main providers of C to the nodules, it is

possible that the first reduction of N-fixation during drought

will be caused by reduction in assimilate supply (41) .

Drought tolerance mechanisms

Plant responses to water stress can be classified broadly

into escape, avoidance or tolerance mechanisms (36,53). Escape

can be achieved through more rapid development and through

developmental plasticity, whereby the coincidence of critical

developmental stages with periods of drought is avoided (34).

Water stress avoidance involves mechanisms either to reduce water

loss or increase water uptake. Water stress tolerance implies the

ability to survive large water deficits and may involve

mechanisms such as osmotic adjustment.

Lawn (34) evaluated the response of four different grain
legumes, soybean (Glycine max), black gram (Vigna mungo), green

gram (Vigna radiata) and cowpea (Vigna unguiculata) to water

stress under field conditions. These four legume species

responded to the stress in several ways, but the degree of

expression varied substantially between species. Each cultivar

exhibited some tendency to escape through faster development in

response to stress; the effect was small in soybean and large in

the Vigna cultivars, particular in the flowering to maturity

period. Each cultivar also exhibited to some degree two

mechanisms which served to avoid dehydration by reducing plant

water loss. The most important of these was stomatal control of

water loss in response to declining leaf water potentials, for

which there appeared to be substantial differences in response

between cultivars. Finally under stress each cultivar showed

some paraheliotropic leaf movement ; in these studies there was

some suggestion that paraheliotropy helps to lower leaf

temperatures under stress and presumably further restrict water


Developmental plasticity can be seen as a mechanism that

facilitates the matching of crop growth and development to the

constraints of the environment, especially in terms of minimizing

the occurrence of the critical reproductive phase during drought

periods. Faster development may allow the successful completion

of the plant's life cycle before the existing water supply is

exhausted. Turk et al. (52) growing cowpeas under water

stress, observed that drought resulted in earliness when present

at moderate levels, but severe drought delayed reproductive

activity. This provides the plant with two possible adaptive

responses. Under moderate drought the plant produces early pods

which may mature before the soil water is depleted. If there is

severe drought at early flowering, the plant remains in a

vegetative stage but has the ability to continue reproductive

activity if water is supplied. Determinate types flower, whether

water levels are optimum or not, while indeterminate types remain

in a vegetative stage under adverse conditions. Once rains start,

the latter enter into the reproductive phase while the former can

start a whole new cycle. It can be speculated that more

determinate cultivars may have less capacity for recovery after

mid-season drought.

Leaf movements which orient the leaf parallel to the sun's

rays, leaf flagging, and rolling are common features of response

in dry situations especially once leaf water potential begins to

fall (20). It is unknown whether these leaf movements are

beneficial to the plant. Shackel and Hall (43) considered that

leaf movements in cowpeas could substantially reduce heat load

and water deficits in cowpeas by minimizing transpiration. On the

other hand Lawn (34) states that paraheliotropic leaf movements

act to reduce total light interception by the canopy, implying a

reduction in photosynthesis. Recently, Ludlow and Bjorkman (37)

reported that the paraheliotropic movement of water stressed

Macroptilium atropurpureum cv. Siratro protected the primary

photosynthetic reactions from damage by excess light

(photoinhibition), heat, and the interactive effects of excess

light and high leaf temperatures. They concluded that even though

heat damage is more severe when it occurs, photoinhibition may be

a more common phenomenon during drought, unless paraheliotropic

leaf movements reduce the amount of solar radiation incident on

water stressed Siratro leaves.

The importance of root morphology for maintaining a supply

of water to the plant should not be overlooked. Under drought

conditions, an extensive root system is a characteristic that

enables the plant to exploit a higher proportion of the available

soil water without incurring severe plant water deficits (8).

Deeper root penetration of soybean was particularly evident in

the drought periods (Lawn, 34). He suggested that perhaps this

root system is related to the tendency of soybeans to keep

stomata open longer into the drought periods, thus maintaining a

supply of photosynthate for continued root growth.

No definite conclusions can be reached about the "best"

strategy to overcome drought stress. However one can conclude

that there is no absolute character to "drought resistance".

Rather, there are several alternative and perhaps inter-related

mechanisms, and their relative success depends on the seasonal

pattern of water availability, on soil type and depth, and on

other factors. The most appropriate strategy for a particular

environment presumably will be the one that simultaneously

maximizes production and minimizes risk in that environment.

Identifying the appropriate strategy requires assessment of the

probability of particular seasonal patterns of water availability

for that particular environment.

One effective approach to breeding for higher yield under

stress would be to identify physiological and morphological

components causing varietal differences in economic yield in the

presence and absence of stress, and to gain an understanding of

their genetic control. Evidence indicates that genetic

variability exists for all such components (54). If physiological

genetic data are used in selecting parents, it should be possible

to select directly for yield, using standard selection and

breeding procedures. Knowledge of the physiological genetics of

yield will improve the plant breeder's understanding of

desirable plant types and habit, and appropriate selection and

breeding methods can then be used. As world food demand


increases, production of drought tolerant beans may become

increasingly important to make optimal use of water-limited



An experiment was conducted at the Campo Agricola

Experimental de Iguala-CAEIGUA-, Iguala, Mexico. The station is

located in the state of Guerrero, at the meridian 990 45'

longitude West and the parallel 180 30' latitude North. The

altitude at the station is 739 meters above sea level. The

average minimum temperature is 7C and the average maximum is 42

C, with an annual average of 24.5 The average annual

precipitation is 1155 millimeters, and the rainy season starts

during the last part of June. Eighty percent of the total annual

precipitation occurs between the months of May and October. The

experiment was planted in the second week of December, and the

final harvest was taken in the last week of March. Precipitation

and temperatures were recorded during the course of the

experiment, and are shown in Figure 1.

The experimental plots were on a silty clay soil, with a

high alkaline pH that varied between 8.25 and 8.75. The organic

matter content as well as total nitrogen were low the

percentage of organic matter being 1.05 and the total nitrogen

0.112 ppm. The levels of potassium, calcium and magnesium were

high, but the phosphorus content was relatively low (10.22 ppm).

Before planting, 40 kg. of phosphorus per hectare were applied to

all plots and 40 kgs. of nitrogen per ha. (in the form of Urea)

were applied to half of the plots. At planting time .all plots

were inoculated with a commercial granular Rhizobium inoculant,

NITRAGIN L x 441 for dry beans, obtained from the Nitragin

35 12a


Temp. C .= .


0 20 40 60 80 110

Days After Planting

Figure 1. Maximum and Minimum Dally Temperatures. Iguala, 1982-3.

a=Mllltmetere of rain

Company in Wisconsin ; 1.5 gms. of inoculant per meter of row

were applied. Twenty one dry bean genotypes were selected on the

basis of their performance under drought as well as on their

nitrogen fixation capabilities. They included:

a) three good nitrogen fixing lines from the University of

Wisconsin: 23-61, 21-58, and 21-54.

b) five CIAT lines reported to have some tolerance to drought:

BAT 332, BAT 85, BAT 47, A-162, and BAT 798.

c) seven Mexican lines with some tolerance to drought: Pinto

Nacional 1, Durango 222, Ojo de Cabra, Bayo Madero, C-5, 1213-2,

and LEF-2-RB.

d) two Michigan State varieties with good architecture and high

yielding potential: Neptune and 61065.

e) two Michigan State lines that showed leaf flagging under

severely dry conditions and were high yielders: 81017 and 800122.

f) two Michigan State lines that showed leaf flagging under

severe dry conditions and were poor yielders: 790131 and 800205.

A Tepary bean, Phaseolus acutifolius was also planted.

Each plot consisted of 6 rows 4 meters long; the distance

between rows was 75 cms. and the distance between plants within a

row was 10 cms. Two empty rows were always left between adjacent

plots in order to facilitate water management. The experimental

units were arranged in a split plot design with three

replications. The combination of nitrogen source and water level

was the whole plot factor and cultivars were the split plot


All plots were flood-irrigated every two weeks starting

before the planting day, until flowering time. Individual plots

of each cultivar were treated as separate units for water

management. After flowering, only the so-called "plus" water

plots continued to receive water.

A commercial micronutrient foliar spray was applied 42 and

50 days after planting. Insects were controlled by spraying once
a week with available commercial insecticides. Two center rows of

each plot were used for periodic sample collection, two were used

for final harvest, and the two outer rows were discarded.

Flowering notes were recorded and when 50% flowering was reached,

the first sample was taken. Ten of the twenty two planted

cultivars were selected for detailed sampling. This selection was

based on previous information regarding differences in N-fixation

potential and drought tolerance. The 10 cultivars chosen for more

detailed study included 8 drought tolerant lines ( 4 from Mexico,

2 from CIAT and 2 from MSU ), one good N-fixing line from

Wisconsin and one drought susceptible line from MSU.

Each sample consisted of five plants that had uniform
competition, they were dug up trying to get as much of the roots

as possible. Each sample was separated into stems, roots and

leaves; this material was placed in an oven at 800 C for one hour

and then was left out in the sun for completion of drying. After

dry weights were recorded, the tissue from each sample was ground

in a Wiley mill and saved for starch and soluble sugars

determinations. At the same time a 2-plant sample was taken

(plants were kept entire); after drying they were ground and

saved for total Kjeldahl nitrogen determinations. The second

sample was taken 15 days after flowering, the time when the

stress was expected to become effective. The third and last

sample was taken at physiological maturity. The sampling

procedures for the second and third samples were the same as for

the first sample, except that in the last two samples pods were

also separated.

Additional observations and notes such as occurrence of leaf

flagging, leaf dropping and leaf yellowness were recorded. A

leaf dropping scale from 1 to 5 was adapted, where 1 was no

defoliation and 5 was complete defoliation. Scores for each plot

were taken 85 days after flowering (before physiological

maturity was reached). At harvest time both economic and

biological yield were recorded and the Harvest Index was


A random sample of 10 plants was taken at harvest time to observe

if there were any differences for seed weight and seed number

between the plant's upper and lower pods. For this purpose the

two lowest pods as well as the two highest pods of each sampled
plant were taken and their seed number and weight recorded.

Starch contents were determined with a colorimetric method with

perchloric acid. This technique involved 3 basic steps which are

described as follows.

1. Reagents.

a. Colorimetric Solution: 80 grs. of NaCl disolved in 250 ml.

of distilled water and 750 ml. of ethanol.

b. Diluted perchloric acid: 300 ml. of perchloric acid (70%)

and 224 ml. of distilled water.

c. Potassium Iodine: 20 grs. of KI dissolved in 20 ml. of

distilled water, and 2 grs. of Iodine diluted to 1 It. with

distilled water.

2. Calibration Curve.

One gram of starch is dissolved in 10 ml. of perchloric acid

solution and diluted to 100 ml. with distilled water. Aliquotes

of 2,3,4,5,6,7,8,9and 10 ml.are taken and 0.5 ml. of perchloric

acid solution are added and diluted to 100 ml. with distilled

water. The solutions will have 20,30,40,50,60,70,80,90 and 100

mgms./ml. of starch. To 5 ml. of each solution 4.5 ml.

of distilled water and 0.5 ml. of KI solution are added. The

final concentrations being attained are then

100,150,200,250,300,350,400,450 and 500 ,/gms. of starch in 10 ml.

of solution. After 20 minutes, absorbance is read at 600 nm.

with the spectrophotometer. The calibration curve is plotted

calculating the mgs. of starch that correspond to one unit of

absorbance (F).

3. Determination of starch in the sample.

Fifty mgs. of dry ground sample (duplicate samples) are taken;

after centrifugation, 12.5 mls. of the colorimetric solution are

added. The sample is placed in a water bath at 720C for 10
minutes, and centrifuged for 10 minutes at 2000 rpm. The

supernatant is discarded and to the residue 5 mls. of perchloric

acid solution are added. After letting the sample stand for 10

minutes, 5 mls. of distilled water are added and then it is

centrifuged at 15000 rpm for 20 minutes. An aliquot of the

supernatant is taken and the color is developed as in the

callibration curve. Then, absorbance is read at 600 nm. The

amount of starch is expressed as grs./100 grs. of sample, and is

calculated using the following formula:

% of starch =

A x F x V x D
a x m x 10

where A = absorbance

F = absorbance factor taken from the curve

V = volume

D = dilution

a = aliquot

m = sample weight

10 = conversion factor


I. Water Effects

The objective of this experiment was to determine the effect

of drought stress imposed during the latter part of the seed

filling period on the yield performance of 22 different

cultivars and to relate their performance under stress with the

ability to translocate non-structural carbohydrates. To

accomplish this objective, the different genotypes were irrigated

every two weeks from planting until anthesis. Working under the

assumption that with high temperatures and high solar radiation

the potential evaporation was high, we expected the irrigation

water to be depleted at about two weeks after it was added. Based

on these assumptions, withholding the water at anthesis

presumably would cause an effective stress in the middle of the

seed filling period, defined as 2 weeks after flowering. The

control plots were continuously irrigated every two weeks

throughout the entire growing season. Different cultivars were

treated independently, meaning that each plot was considered as a

separate unit for irrigation purposes.

The first evident symptom of water deficit was premature

defoliation; it started to occur two weeks after the plants were

expected to be under stress. A two-week lapse between the time

that we had intended to have the stress and the first visible

signs of stress might indicate that we did not actually impose

the stress at the physiological stage that we had originally

intended. However, we can not assure that the plants were not

under stress before this time because measurements that would

have indicated that, such as stomatal closure, osmotic adjustment

and photosynthesis reduction, were not taken. Another indication

of the presence of the water stress in the crop consisted in the

reduction of the length of the seed filling period ( days from

flowering to physiological maturity ), in the water stressed

plots as compared to the irrigated plots. Perhaps a faster

development allows the completion of the reproductive stage

before soil water is completely exhausted.

It is evident that we did have a water stress, but what we

can not assure is the degree of the stress or its precise timing.

The degree of correlation between control and stress yields has

been considered to be an indication of the severity of the stress

( 9,10). A mild drought stress reduces yield, but the grain yield

of the stressed plots is highly correlated with the yield

potential in the absence of the stress. Severe stress provokes

very different responses among genotypes with similar yield

potential, and the correlation between grain yield under stress

and yield potential is weaker. Since in this case the correlation

of control yield vs. stress yield was found to be positive and

highly significant ( calculated r= 0.895 ), we can infer that

the stress was moderate rather than severe.

Since there were not significant effects of N-treatment, the

water effects described in here are based on both the plus and

minus N treatments.

A. Biological Yield

A significant cultivar effect as well as a significant water

effect were indicated by the Analysis of Variance. Twelve of the

twenty two cultivars had a significant reduction of Biological

Yield under water stress, while only two cultivars, MSU 800122

and Mexico LEF-2-RB, showed a significant increase for this trait

under stress (Table 1). The other eight cultivars didn't show any

significant differences between treatments, but except for

cultivars MSU 61065 and CIAT BAT 332 Biological Yield was

reduced under water stress. In the case of 800122 and LEF-2-RB we

have no evidence that will allow a reasonable explanation.

The size of the biological yield reduction in some cultivars was

unexpectedly high, considering that the stress was not effective

until late in the season. In fact, in cultivar Bayo Madero this

reduction was more than 50%.

Differences in magnitude for Biological Yield were observed;

the high values of 81017, Ojo de Cabra and Bayo Madero contrast

with the relatively low values of Durango 222 and Pinto

Nacional. It is interesting to note that entry 22, the Tepary

bean, P. acutifolius known to be a drought tolerant line,

showed no difference in Biological Yield between the plus and

minus water treatments.

Table 1.

Biological yield (kg/ha) under two water
treatments. Iguala, 1982-3.

Entry No. Identification Irrigated Stress

Wisec 23-61
Wisec 21-58
Wisc 21-54
Bayo Madero
BAT 332
BAT 85
BAT 47
BAT 798
Pinto Nacional 1
Durango 222
Ojo de Cabra


3443 **
3761 **
3536 **
4254 **
5280 **
4308 **
5298 **
2951 **
2515 *
3371 **
3761 *
3019 **
2316 **
3827 **

* = LSD at .10 (672)
** = LSD at .05 (802)

B. Economic Yield

The Analysis of Variance revealed a significant cultivar

effect and also a significant water effect at the 5% level. Only

six of the twenty two entries showed a significant yield

decrease under the water stress conditions, as compared to the
non stressed plots (Table 2). Of these six cultivars, four

were Mexican lines ( 1213-2, C-5, Bayo Madero and Pinto Nacional

1) and two were MSU lines ( 800122 and 800205 ). Among the other

sixteen entries, ten showed a non-significant decrease in

Economic Yield under stress conditions and six had a non-

significant yield increase.

Water stress, when imposed during the seed filling period,

seemed to have a greater effect on the economic yield of the

cultivars that retained more assimilates in the stems at

maturity. A significant negative correlation of 0.443 between

economic yield and stem % of dry weight at Physiological Maturity

under water stress supports this assertion. Inherently low

yielding genotypes such as 790131 and Durango 222 displayed a

decrease in stem weight only under stress at P.M.; under non-

stress stem weight was not reduced. The correlation between stem

% of total dry weight and economic yield for both cultivars was

non-significant under the plus water treatment, while under water

stress for both 790131 and Durango 222 there was a significant

negative correlation, with values of 0.850 and 0.975

respectively. Even though these are low yielding genotypes, their

economic yield was not significantly reduced under stress. This

Table 2.

Economic yield (kg/ha) under two water
treatments. Iguala, 1982-3.

Entry No. Identification Irrigated Stress

1 Wise 23-61 1536 1500
2 Wise 21-58 1887 1743
3 Wise 21-54 1330 1440
4 Neptune 1554 1673
5 61065 1815 1938
6 800122 1571 1178 **
7 81017 2086 2001
8 800205 1822 1487 **
9 790131 1225 1064
10 LEF-2-RB 1964 .1775
11 1213-2 1514 1200 **
12 C-5 1502 1181 **
13 Bayo Madero 1183 742 **
14 BAT 332 1779 1919
15 BAT 85 1966 1751
16 BAT 47 1843 1780
17 A-162 1833 1744
18 BAT 798 1081 1044
19 Pinto Nacional 1 1262 889 **
20 Durango 222 1102 985
21 Ojo de Cabra 628 693
22 Tepary 1521 1484

* = LSD at 0.10 (217)
** = LSD at 0.05 (259)

is consistent with the hypothesis that remobilization is

enhanced under stress conditions.

Bayo Madero and 800122 incurred significant reductions in

economic yield under stress as well as a significant increase in

stem dry weight suggesting either a poor remobilization and a

low capacity to buffer adverse environmental effects, or a weak

sink that does not have the ability to utilize the stored

assimilates. 800122, a late maturity cultivar, did not flower

until late in the season; for this reason as shown in Figure 2,

during the reproductive stage it was subjected to high

temperatures. I believe that the high temperatures during this

stage of development kept this particular variety from

remobilizing and senescing normally, and as a consequence yield

was significantly affected.

The three Wisconsin cultivars 23-61, 21-58 and 21-54, as

well as Neptune A-162 BAT 85 and Ojo de Cabra had a

significant reduction in biological yield under stress, however,

their economic yield was not significantly reduced This is

supportive of the hypothesis that in those genotypes that have

the ability to remobilize assimilates from stems ,storage

photosynthates act as a buffer between seed growth and

photosynthesis. However, we can not determine if the contribution

to seed yield is coming mainly from assimilates that were

produced before the plants were subjected to the water stress, or

if the seeds were filled with photosynthates produced after the

onset of stress.

C. Harvest Index

The AOV reveals a significant cultivar and water effect on

Harvest Index (H.I.) at the 1% level. Seven entries showed a

significant increase in H.I. under water stress as compared to

the non-stressed plots (Table 3). They include the Wisconsin

lines 23-61 and 21-54, the MSU lines Neptune and 81017 the

Mexican lines Ojo de Cabra and Bayo Madero and the CIAT line BAT

798. Significant reductions of biological yield under stress

account for differences in Harvest Index for these cultivars. In

the case of Bayo Madero, reductions in both economic and

biological yield occurred.

Two lines, MSU 800122 and Mexico LEF-2-RB, had a significant

reduction in HI under the minus water treatment. Nine out of the

thirteen remaining lines had a non-significant HI increase under

stress, while the other four had a non- significant decrease. It

is interesting to note that the Tepary bean (entry 22) had almost

the same value for HI under both stress and non stress


D. Seed Size

The AOV for this trait, measured as weight of 100 seeds,

reveals a significant effect for cultivars and water treatment.

All entries incurred a reduction in single seed weight of about

one centigram due to stress (Table 4). However, only three out of

the twenty two entries had a statistically significant reduction

Table 3. Harvest Index under two
treatments. Iguala, 1982-3.


Entry No. Identification Irrigated Stress

1 Wisec 23-61 0.33 0.43 **
2 Wisec 21-58 0.41 0.46
3 Wisec 21-54 0.31 0.41 **
4 Neptune 0.32 0.40 **
5 61065 0.47 0.48 **
6 800122 0.39 0.22 **
7 81017 0.36 0.46 **
8 800205 0.42 0.42
9 790131 0.30 0.32
10 LEF-2-RB 0.44 0.37 **
11 1213-2 0.40 0.41
12 C-5 0.43 0.47
13 Bayo Madero 0.15 0.22
14 BAT 332 0.45 0.46
15 BAT 85 0.49 0.51
16 BAT 47 0.39 0.44
17 A-162 0.41 0.46
18 BAT 798 0.27 0.34 **
19 Pinto Nacional 1 0.39 0.38
20 Durango 222 0.30 0.30
21 Ojo de Cabra 0.15 0.24 **
22 Tepary 0.39 0.38

* = LSD at 0.10 (0.60)
** = LSD at 0.05 (0.70)

Table 4.

Weight of 100 seeds (grs) under
treatments. Iguala, 1982-3.

two water

Entry No. Identification Irrigated Stress

Wisec 23-61
Wisec 21-58
Wisc 21-54
Bayo Madero
BAT 332
BAT 85
BAT 47
BAT 798
Pinto Nacional 1
Durango 222
Ojo de Cabra

11 .96

41.48 **
33.10 **
45.07 **

* = LSD at 0.10 (1.55)
** = LSD at 0.05 1.85)

in the weight of 100 seeds under the water stress treatment.

These three entries were all Mexican lines (Bayo Madero, Pinto

Nacional and Durango 222).

Variations in economic yield due to water stress in the

cultivars Bayo Madero and Pinto Nacional were due to reduction

of seed size as well as seed number. Smaller seeds under a low

water regime are the consequence of incomplete filling,

indicating lower photosynthesis and/or an inadequate reallocation

of carbohydrates during the seed filling process.

E. Seed Number

The AOV for the number of seeds per square meter indicated a

significant cultivar effect as well as a significant cultivar -

water interaction, but the water effect itself was not

statistically significant. Since the water stress did not become

effective until the late part of the growing season, when the

number of seeds had already been determined, no water treatment

effect is to be expected for this trait. Only two of the twenty

two entries had a significant reduction in the number of seeds

under the water stress treatment, while three had a significantly

larger number of seeds under stress (Table 5) The three

cultivars that had a significant increment in the number of seeds

(Neptune, 61065 and BAT 332) had a reduction of seed weight under

stress. This might be an indication of component compensation, in

which the reduction of seed weight is caused by the increment in

seed number. Although the reductions in seed number for Bayo

Table 5. Seed number (seeds/mt2) under two water
treatments. Iguala, 1982-3.

Entry No. Identification Irrigated Stress

1 Wisec 23-61 842 862
2 Wise 21-58 902 868
3 Wisec 21-54 663 752
4 Neptune 946 1051 *
5 61065 994 1108 *
6 800122 957 787 **
7 81017 1013 1049
8 800205 1093 944 *
9 790131 534 474
10 LEF-2-RB 732 735
11 1213-2 421 347
12 C-5 434 353
13 Bayo Madero 269 178
14 BAT 332 1052 1181 **
15 BAT 85 949 894
16 BAT 47 688 682
17 A-162 940 907
18 BAT 798 531 545
19 Pinto Nacional 1 354 269
20 Durango 222 221 216
21 Ojo de Cabra 159 176
22 Tepary 1272 1293

= LSD at 0.10 (101)
** = LSD at 0.05 (121)


Madero and Pinto Nacional were not statistically significant,

they can be considered large enough to explain the economic yield

loss observed under water stress.

F. Length of Vegetative and Reproductive Stages

The AOV for the length of the seed filling period, measured

as the number of days between 50% Flowering and Physiological

Maturity, showed a significant cultivar effect as well as a

significant water effect. All cultivars, without exceptions,

incurred a reduction in the length of the seed filling period

under the water stress treatment (Table 6). However, these

reductions turned out to be significant only for 15 of the 22


Variability in seed filling duration between cultivars and

between treatments within cultivars is shown in the data. Since

seeds compete for available assimilates, if the sink capacity at

the initiation of the seed filling stage is higher than the

source supply, extending the duration of seed filling under non-

stress conditions theoretically should provide more available

photosynthate to the seed which in turn will produce heavier

seeds and higher yields.

Earliness has been associated with improved adaptation in

crops subjected to drought probably as a mean of escaping

the stress through faster development. If this reduction in the

length of the reproductive stage under adverse conditions induces

an earlier partition of carbohydrates to the seeds, early

genotypes should be able to buffer adverse environmental effects

Table 6. Days between flowering and physiological maturity
under two water treatments. Iguala, 1982-3.

Entry No. Identification Irrigated Stress

1 Wisec 23-61 37 33 **
2 Wisc 21-58 38 34 **
3 Wisec 21-54 37 34 **
4 Neptune 37 34 **
5 61065 35 34
6 800122 34 34
7 81017 41 38 **
8 800205 37 36
9 790131 30 28 *
10 LEF-2-RB 39 36 **
11 1213-2 42 40 *
12 C-5 45 45
13 Bayo Madero 52 48 **
14 BAT 332 30 29
15 BAT 85 33 33
16 BAT 47 41 40
17 A-162 41 38 **
18 BAT 798 45 33 **
19 Pinto Nacional 1 41 39 *
20 Durango 222 50 47 **
21 Ojo de Cabra 52 49 **
22 Tepary 44 41 **

= LSD at 0.10 (2)
** = LSD at 0.05 (

in a more efficient manner. This is supported by the data shown

here where the cultivars Ojo de Cabra, Bayo Madero, Durango 222

and C-5 were among the lowest yielding lines and had the longest

seed filling period under stress. All the top yielding cultivars

81017 61065, LEF-2-RB, BAT 85 and BAT 332 had a relatively

short reproductive phase under water stress.

High temperatures during the reproductive stage as occurred

in Iguala (Figure 2), probably reduced the length of the period

from Flowering to P.Maturity by inducing an earlier partition

of carbohydrates that, in turn, hastened leaf senescence.

Developmental plasticity helps plants cope with an adverse

environment, especially in terms of delaying or accelerating the

onset of the reproductive phase so as to escape the more severe

periods of adversity. Faster development allows the completion of

the reproductive stage before the soil water is exhausted.

The length of the vegetative stage,expressed as the number of

days between planting and 50% flowering, is expected to be

closely associated with the accumulation of total dry matter. One

would also expect a positive correlation between Biological Yield

and days to flower in the irrigated cultivars, since the longer

they grow the greater the possibility for accumulation of dry

matter Figure 3 illustrates this point, and shows that only 2

of the 7 cultivars that flowered in less than 50 days after

planting had a high Biological Yield, while all the late

flowering cultivars had a high Biological Yield.

A greater accumulation of dry matter before the onset of

stress might act as a reserve pool of assimilates that can be

drawn upon when photosynthesis is reduced. If pre-anthesis

-M S..L
-'M h

u u

* i


Days After Planting

-- Indlcates 50% Flowering Date


Temp. C








Ii 11

.~a, &
'4~t~) *

11 ii I

Temp. C 21. 70 Gooo.-
35 "

1000- o

Is 1 4

,25 2

2000 .

0 20 40 60 80 110

Days After Planting

--* Indicates 50% Flowering Date

assimilates are being utilized to fill the seeds, the cultivars

that have a longer vegetative growth would be expected to have a

greater source of assimilates and maybe a greater potential to

buffer adverse environmental effects. Figure 4 shows that the top

yielding cultivars from this experiment are all within the group

that flowered 60 days or more after planting.

G. Leaf Dropping

Significant effects for the water treatment as well as

significant differences between cultivars were detected by the

AOV. All twenty two entries, without exception, had a significant

increase in the leaf dropping score under stress (Table 7), which

simply reveals that plants under stress had early defoliation.

However, it is important to note that different degrees of

defoliation were expressed. The leaf dropping scores under the

irrigated plots ranged between 2.3 and 3.8 (little to moderate

defoliation), while the scores for the water stress plots ranged

from 2.8 to 4.8 (moderate to almost complete defoliation). Under

water stress, varieties such as Ojo de Cabra, LEF-2-RB and

Neptune had corresponding scores of 2.8, 3.1 and 3.3 while BAT

85 1213-2 and C-5 had respective values of 4.8, 4.7 and 4.6.

Defoliation occurred in all cultivars under water stress and its

correlation with economic yield turned out to be a non

significant -0.161.

o s

1S eS

T a p U 0 5 0 U

10 0 90 or11
\ 1 Pa000--t

14 *5 e O

5 IAO 1000lat i

0 20 40 60 80 110

Days After Planting

-** Indicates 50% Flowering Date

Table 7.

Leaf dropping under two water
treatments. Iguala, 1982-3.

Entry No. Identification Irrigated Stress

1 Wisec 23-61 2.7 3.6 **
2 Wisec 21-58 2.8 3.5 **
3 Wisec 21-54 2.5 3.8 **
4 Neptune 2.5 3.3 **
5 61065 3.0 3.6 *
6 800122 2.0 3.5 **
7 81017 2.5 3.8 **
8 800205 3.0 3.6 **
9 790131 3.6 4.5 **
10 LEF-2-RB 2.6 3.1 *
11 1213-2 3.3 4.7 **
12 C-5 3.8 4.6 **
13 Bayo Madero 2.5 3.8 **
14 BAT 332 2.6 3.5 **
15 BAT 85 3.5 4.8 **
16 BAT 47 2.3 3.3 **
17 A-162 2.3 4.0 **
18 BAT 798 3.1 3.6 *
19 Pinto Nacional 1 3.6 4.8 **
20 Durango 222 3.8 4.5 **
21 Ojo de Cabra 2.3 2.8 *
22 Tepary 2.3 4.1 **

Based on a scale from 1 to 5, where 1= No defoliation and
5= Complete defoliation.

= LSD at 0.10 (0.5)
** = LSD at 0.05 (0.6

H. Plant Dry Weight at Physiological Maturity

Significant differences between cultivars were detected by

the AOV for total plant dry weight. When individual components of

total plant weight expressed as percent of the total dry weight

were examined, not only significant differences between cultivars

were evident, but also the water effects were significant for

stem and leaf % of total dry weight. One must be cautious in

interpreting these data. It is important to remember the

inexplicable increase of total dry matter in the cultivars 800122

and LEF-2-RB under stress (Table 1). One must be aware that the %

values may reveal some trends not shown by the total dry matter


In Table 8, one can see that of the 10 genotypes sampled

only three had significant differences for stem % of total dry

weight between the stress and the non-stress treatments. LEF-2-RB

had a significant reduction of stem weight under water stress ,
while 800122 and Bayo Madero had a significant stem weight

increase under stress. The other seven cultivars sampled showed a

tendency towards decreasing stem weight under water stress, with

the exception of 790131 and BAT 85.

At physiological maturity, the stems of Bayo Madero and

800122 under normal irrigation constituted 22.3 and 27.4 % of

the total plant dry weight, indicating a high accumulation of dry

matter in the stems. Their corresponding values under stress

were 33.3 and 30.0 % ; these figures might indicate that these

two genotypes do not have the capacity to remobilize the stored

Table 8. Stem and Pod % of total dry weight at Physiological
Maturity under two water treatments. Iguala, 1982-3.

Stem %a Pod %b
Identification Irrigated Stress Irrigated Stress

Wisc 23-61 17.27 15.95 70.83 71.63
61065 17.35 17.94 71.90 71.00
800122 27.40 33.39 ** 56.93 51.61 **
790131 17.37 17.46 66.20 70.85 **
LEF-2-RB 18.12 15.31 68.55 74.09 **
1213-2 14.40 13.65 70.09 77.05 **
Bayo Madero 22.33 30.04 ** 65.48 54.60 **
BAT 332 18.10 16.53 69.39 72.61
BAT 85 15.19 16.12 70.96 71.67
Durango 222 16.12 15.84 70.75 72.67

a = LSD at 0.10 (1.86)
** = LSD at 0.05 (2.23)
a = LSD at 0.10 (3.78)
** = LSD at 0.05 (4.52)

assimilates from stems to the seeds. This inability to remobilize

then could be the cause of the significant reduction of economic

yield under stress for both Bayo Madero and 800122. However, this

could also mean that the reduction of seed number under stress,

although not statistically significant, resulted in an

insufficient sink demand to require remobilization.

The high yielding cultivar LEF-2-RB had a different behavior;

under no stress the stem weight corresponded to 18.1 % of the

total dry weight, while under stress it was only 15.3 %. The

changes in dry weight induced by the stress though not large by

these figures, may be enough to sustain the seed filling process

temporarily and as a consequence economic yield under stress was

not significantly reduced.

Pod % of total plant dry weight showed a significant

reduction under minus water in the low yielding cultivars 800122

and Bayo Madero, indicating that reduction in economic yield in

these two cultivars probably was due not only to the decrease in

single seed weight but also to the reduction of seed number.

I. Plant Dry Weight Changes: Remobilization

Figure 5 shows the changes in stem, root, leaf and pod dry

weights over the three different sampling times (Flowering, 15

days after Flowering and P.Maturity) for six different cultivars.

From Flowering to 15 d.a.f. stem weight increased in all

cultivars, however, the size of this increment varied among

cultivars. One can see fairly large increments in cultivars such

I = Flowering
0= Mid-pod-fill
I = Physiological Maturity
* = Stress



10o f




61065 790131 1213-2 B. Madero BAT 85 Dgo222

Fig 5. Stem. Root. Pod and Leaf dry weights
three physiological stages. *lguala, 1982-3.

(grs/mt2) over


as 61065 and BAT 85, while 790131,1213-2 and Bayo Madero showed

only a relatively small increase. The changes from 15 d.a.f. to

P.Maturity differed in the sampled cultivars. BAT 85, 61065 and

790131 had a reduction in stem dry weight, while Durango 222,

1213-2 and Bayo Madero had an increase.

When comparing the stem dry weight reduction in the water

stress versus the non-stress plots, one can observe a general

tendency towards a greater reduction in dry weights under the

stress treatment. Durango 222 and 1213-2 incurred a reduction of

dry weight under the stress treatment, while no reduction

occurred under the plus water conditions. Bayo Madero showed an

increase in stem dry weight at P.Maturity with respect to 15

d.a.f., however, the increment was slightly smaller under stress.

BAT 85, as noted before, incurred a reduction in stem dry weight

from 15 d.a.f. to P.Maturity, the reduction being greater under

stress. 790131 had a small reduction from 15 d.a.f. to P.Maturity

but no differences were seen between the stress and the non-

stress treatments.

The high yielding lines BAT 85 and 61065 had the largest

stem weights at 15 d.a.f. Their respective losses from 15 d.a.f.

to P.Maturity might indicate that remobilization of stored

assimilates has taken place.

The low yielding line Bayo Madero, although it had high

values for stem weight at 15 d.a.f., apparently did not

remobilize its stored carbohydrates to the seeds. In the case of

Durango 222 remobilization occurred only under stress. The lack

of remobilization under non-stress conditions was probably due to

the lack of need to utilize the stored carbohydrates because

assimilate demand by the seeds was being satisfied by currently

produced photosynthates. Only when photosynthesis is adversely

affected would the seed filling process depend upon the stored

assimilates and their reallocation .

A significant correlation of 0.394 between root weight at

flowering time and economic yield points out the importance of

the root system in relation to yield. However, one must be

careful when interpreting these results, because a low

correlation even though statistically significant, still leaves a

great deal of yield variance unaccounted for that has to be

explained by other factors. The top yielding cultivars BAT 85 and

61065 had the highest values for root dry weight at flowering,

while the bottom yielding line Durango 222 had the smallest

value. No significant correlations were found between economic

yield and root weight at either 15 d.a.f. or P.Maturity. Large

values of leaf weight at flowering and at 15 d.a.f. represent a

large photosynthetic area and therefore a substantial

carbohydrate manufacturing site. The data show that the highest

values for leaf weight at these physiological stages were

produced by the top yielding lines BAT 85 and 61065. Changes in

leaf weight from 15 d.a.f. to P.Maturity show that under stress

the reduction of leaf weight is greater as compared to the non-

stress values. However, as pointed out before when describing the

leaf dropping results, economic yield and defoliation did not

show a significant correlation.

In general one can observe that the pod dry weight data presented

in Fig. 5, as expected, are in broad agreement with the economic

yield data given in Table 2. Small discrepancies such as the

higher pod weight in BAT 85 which was outyielded by cultivar

61065 (Table 2) might indicate a greater dry weight of the pod

walls in BAT 85 which are not included in the economic yield

data. Nevertheless, these discrepancies are small and not

statistically significant.

J. Starch Analysis

Significant cultivar differences for the amount of starch

present at flowering, 15 d.a.f. and physiological maturity in the

stems, roots and pods were detected by the AOV. Table 9 shows the

starch percentage ( mgrs of starch per gr of dry weight ) for the

different plant components at three physiological stages. The

estimated values are very low as compared to starch

determinations previously reported for dry beans (30,39). These

low values might be the result of the lack of sensitivity of

the method used, starch being determined by the colorimetric

method already described, or from high respiration rates caused

by high temperatures that prevailed during the growing season.

Figures 6 and 7 illustrate the changes in starch content in the

stems and pods over the 3 sampling times. One can see that the

starch content is always lower under the stress treatment as

compared to the irrigated plots. The AOV revealed significant

cultivar and treatment differences for the amount of starch

present in the stems at physiological maturity. Figure 6

illustrates the seasonal variation in starch content in the

stems. Bayo Madero was the only cultivar that had an increment in

Table 9. Mean values of starch ( mgrs / gr dry wt. ) at three
different physiological stages under two water
treatments. Iguala 1982-3.

Identification Stage* Stems
Irrigated Stress

Wisconsin 21-34






Bayo Madero

BAT 332

BAT 85

Durango 222

















* F= flowering
MPF= mid-pod filling
PM = physiological maturity


- -- Stress




1 2 3



1 2 3


1 2 3


1 2 3


1 2 3


1 2 3 1 2 3



1 2 3

BAT 332

1 2 3 1 2 3

BAT 85

DGO 222


2 Mid-Pod-Fill

3-Physiological Maturity

Figure 6. Changes in Stem Starch Contents ( grs/mt2 ) Over Three Physiological

Stages Under Two Water Treatments. Iguala, 1982 3.

m Irrigated
V --- Stress



1 2 3



1 2 3


1 2 3



1 2 3


1 2 3


1 2 3 1 2 3 1 2 3



BAT 332

1 2 3

BAT 85

1 2 3

DGO 222


3-Physiological Maturity

Figure 7. Changes In Pod Starch Contents ( grs/mt2 ) Over Three Physiological

Stages Under Two Water Treatments. Iguala, 1982 3.




the amount of starch in the stem from m.p.f. to PM, and this

increment was smaller under the water stress treatment. A greater

utilization of assimilates stored in the stems under water

stress may constitute a strategy by which the plants cope with

adverse environmental effects. A negative and significant

correlation between the starch present in the stems at P.M. and

seed yield supports this hypothesis. The calculated correlation

was -0.5 under water stress, while under irrigation the

corresponding value was -0.3. This might indicate that the seed

filling process, when photosynthesis is adversely reduced,

utilizes the stored carbohydrates, and that the greater the

capacity to remobilize them, the higher the seed yield would be.

Under irrigation, when photosynthesis is not drastically reduced,

there is a continues supply of assimilates so the plant does not

have to draw upon the reserves so heavily.

K. Twenty Upper and Lower Pods: Seed Number and Size

Significant differences were found among cultivars for

seed number in both the upper and lower pods, but no significant

water effect was indicated by the AOV. When the number of seeds

of the upper pods versus the lower pods was compared, no

significant differences were observed (Table 10).However, for

seed weight the AOV revealed a significant cultivar effect as

well as a significant water by cultivar interaction. Seed weight

in the upper and lower pods was reduced under the minus water

treatment (Tables 4 and 11) seeds from the upper pods being

Table 10. Seed number of 20 upper and lower pods under two
water treatments. Iguala, 1982-3.

20 lower podsa 20 upper podsb
Identification Irrigated Stress Irrigated Stress

Wise 23-61 102 113 111 119
61065 104 114 109 111
800122 109 107 108 112
790131 71 68 73 65
LEF-2-RB 87 106** 95 108
1213-2 61 56 70 61
Bayo Madero 57 61 62 65
BAT 332 96 108 84 99
BAT 85 106 107 113 106
Durango 222 50 45 52 52

a = LSD at 0.10 (17)
** = LSD at 0.05 (20)
b = LSD at 0.10 (16)
** = LSD at 0.05 (19)

Table 11. Seed weight (mgrs/seed) of 20 upper and lower pods
under two water treatments. Iguala, 1982-3.

20 lower podsa 20 upper podsb
Identification Irrigated Stress Irrigated Stress

Wisc 23-61 210.7 199.9 193.5 188.5
61065 190.5 186.7 174.2 165.6
800122 163.9 158.5 168.5 153.1
790131 230.1 231.1 218.1 217.5
LEF-2-RB 251.2 254.0 242.2 242.9
1213-2 360.6 341.2 347.0 354.1
Bayo Madero 446.0 391 .1** 409.7 373.3**
BAT 332 171.7 163.3 169.2 161.1
BAT 85 219.2 209.6 211.7 181.7**
Durango 222 519.4 422.8** 481.2 469.8

a = LSD at 0.10 (27.7)
** = LSD at 0.05 (33.1)
b = LSD at 0.10 (19.9)
** = LSD at 0.05 (23.8)


smaller than those from the lower pods. Since the lower pods are

the first ones to be formed during the plant's developmental

processes they were at a more advanced seed filling stage when

the stress became effective. This probably implies that either

they did not have to rely upon the stored carbohydrates to fill

their seeds because they were being filled with currently made

photosynthates, or due to closer proximity they were the first

ones to use the stored carbohydrates; 'they had essentially

completed filling before the stress became severe.

II.Nitrogen Effects

A secondary objective of this experiment was to determine

the nitrogen fixation potential of the 22 cultivars, and to

relate their potential with the effect of drought stress imposed

during the latter part of the seed filling period and with the

ability to translocate non-structural carbohydrates. To

accomplish this objective, the twenty two cultivars were planted

under two contrasting nitrogen levels and under two water

regimes. As described in the Materials and Methods section, the

experimental plots were on a Silty Clay soil in which the organic

matter and total N contents were low (%of organic matter = 1.05,

total N = 0.112 ppm). Before planting the so called plus N plots

were fertilized with 40 kgs of N per hectare, applied in the form

of urea, while the minus N plots did not receive any N

fertilizer. At the time of planting all plots, except 2 border

rows that ran the length the field, were inoculated with a

commercial granular Rhizobium inoculant.

We expected to see differences due to the N treatment, but

we could not observe any visual symptoms of N deficiency in the

non- fertilized plots; also, the observed nodulation throughout

the season was considered fairly poor for plants grown under the

two different N treatments. The only clear N deficiency

symptoms, such as severe yellowness and reduced growth, were

observed in the two border rows that had neither fertilizer nor

inoculant. We have no definite explanations for the lack of

difference between the plus and minus N treatments. Whether the

soil analysis was faulty and the actual content of N in the soil

was higher than shown by the analysis is unknown. We can't answer

this question now because after harvesting this experiment the

soil was plowed and uniformly fertilized for the next crop to be

planted. Another possible explanation, though it may be remote,

might be found in the levels of N03 present in the irrigation

water. If such levels were high enough to provide the plants with

sufficient N for their vegetative growth, it is possible that no

N treatment differences did in fact exist. The fact that only two

border rows, which were at the end of the field and therefore did

not receive as much water as the other plants did, supports this


The quality of the applied inoculant turned out to be poor,

having only 5.8 x 104 Rhizobia per gram. A good quality inoculum

should have at least 108 Rhizobia per gram. Nevertheless, this

does not imply that there were not enough bacteria in the soil

sufficient to have established a symbiotic relationship with the

host plants. Countings of the native Rhizobia population existing

in the soil before inoculation ranged from 1.8 x 103 to 1.7 x

107 colonies per gram of soil.

Since the water stress was not effective until the late pod

filling stage and the data herein described refers to

earlier physiological stages, the results are based on only one

water treatment.

A.Non significant effects

The individual AOVs for Biological Yield, Economic Yield,

Harvest Index, Weight of 100 seeds, Length of seed filling

period, Leaf dropping, Seed number and weight from the 20 upper

and lower pods and % of Nitrogen did not show a significant

Nitrogen effect. However, some Nitrogen x Cultivar interactions

as well as Nitrogen x Water and Nitrogen x Water x Cultivars

interactions were significant. These interactions will be

referred to in the next section.

The effect of added Nitrogen on N-fixation depends on the

specific cultivar; different cultivars show different responses

to N fertilizer. In the section, Interaction Effects, the

differential response of the genotypes used in this study will be

discussed and I will attempt to reach some conclusions from this


B. Plant dry weight

The AOV for total plant dry weight at the three sampling

times (Flowering, 15 d.a.f., and Physiological Maturity) did not

detect any significant differences due to N effect. However, with

respect to the individual components of plant dry weight over the

three different samples, a significant Nitrogen effect was given

for stem % of total plant dry weight at flowering time.

Figure 8 illustrates the differential responses of the 10

sampled cultivars over the two different levels of Nitrogen. In

genotypes such as BAT 332, 61065, and 800122, the stem

constituted over 42 % of their total plant dry weight, while in

- i ........l ~ J .1 ~ I &~ a a a .*~ I I a I a a ~. -' -. -. -.

Wisc 23-61 61065

800122 790131 LEF-2-RB 1213-2

Bayo BAT 332 BAT 85 Durango
Madero 222

* I= 40 kg/ha added N. CV.
E2 = no added N.

Figure 8. Stem % of total plant dry weight at flowering time. *Iguala 1982,3.

50 -


% 030


Durango 222 the corresponding value was less than 30 %. Even

though the correlation between stem % of dry weight at flowering

and Economic yield was expressed as a significant r value of

0.408, the data show that in high yielding lines such as BAT 85

the stem % of total dry weight was approximately the same as in

the low yielding lines Bayo Madero and 790131.

Different cultivars showed different responses to N

fertilizer. A line previously selected for high BNF potential,

Wisconsin 23-61, showed an increase in stem weight when 1i was not

added; BAT 332, also previously reported by CIAT to be a good N-

fixer, showed an opposite response. No conclusions can be drawn

from these results except that if stem % of total dry weight is

positively correlated with yield, good N-fixers should have high

stem weights at flowering time. The carbohydrates that are stored

in the stems can be remobilized and utilized in the later stages

of plant development when photosynthetic activity is reduced,

particular in the lower (shaded) portion of the canopy where

carbohydrates might be needed for supporting N-fixation. In the

case of N-fixation, a great amount of photosynthate is required

by the nodules in order to maintain their growth and to

facilitate the organic binding of the fixed Nitrogen. Since the

nodules store very few reserves, they depend on the supply of

assimilates that is available to them. Genotypes like BAT 332,

61065 and 800122 should have a greater BNF potential than 1213-2

and Durango 222.

The roots constitute a potential site for carbohydrate

storage and a possible supplier of assimilates to the nodules. A

positive and significant correlation between the economic yield

and the root weight at flowering time (when N-fixation is

supposed to be at its maximum activity) was found, the calculated

r-value being a significant 0.394.

C. Plant dry weight changes: remobilization

Figure 9 shows the changes in stem, root, leaf and pod dry

weights that occurred from flowering to 15 d.a.f., the time that

we consider to be the middle of the pod filling stage. This

figure illustrates the differences in dry weights of six

genotypes under added N as well as under non added N.

Stem weight increased in all cultivars, from flowering to 15

d.a.f. but the s ze of the increment varied for the different

cultivars. The incremental changes were essentially the same for

N-fertilized and non-fertilized treatments. Root weights remained

approximately the same from flowering to 15 d.a.f.; only 61065

and Durango 222 showed a noticeable increase in root dry weight,

but no overall differences were seen between the two N

treatments. With respect to pod weight at 15 d.a.f., a general

tendency towards a greater pod weight under no added N was

observed. These data suggest that in this experiment N fixation

or soil N supply was sufficient to maintain a large number of

flowers which developed into pods. Leaf weight increased over

time, but again no differences due to N source were seen.

Since no differences were observed between N treatments for

the individual components of plant dry weight, a Shoot:Root ratio

(S:R) was calculated in order to try to understand the behavior

a L

100 1
40 *

-40 -
SO -

g1 1 2 1 2 I

ri 1

r-m -I


- -- ruf-u


j 4

*0 40 kthe alddedN. I 1 PNleAld
* a-MddedLN. I 1- AIMp.




61065 700131 1213-2 B. Madero BAT85 DOo222
Fig. 9 Stern, Root, Pod and Leaf weights at Flowering and at 15 dap.
under two different levels of added NItrogen. *Igiial 1982-3.


1 2

of the different genotypes. This ratio, as a measure of the

pattern of differential growth, can provide an index for the

performance of each plant organ under different growth

conditions. An increase in the S:R ratio might be the result of a

greater utilization of carbohydrates by the shoot at the expense

of the root, possibly bringing about a shortage in carbohydrate

supply to the nodules that will translate into poor or reduced N-

fixation. The effects of added N on S:R ratio as shown in Table

12 varied with plant genotype and stage of development. When

reading this table one must be aware that very high values of S:R

ratio probably reflect very incomplete harvest of the root


At flowering time, the cultivars 61065, 1213-2 and Bayo Madero

had a higher S:R ratio under non-added N, while Durango 222 and

790131 remained the same. Only BAT 85 had a higher S:R ratio

under added N at flowering time.

At 15 d.a.f. the cultivars 61065, 1213-2, Bayo Madero and

Durango 222 showed no differences in S:R ratio due to added N.

However, 790131 which showed no differences at flowering time,

did show an increase of S:R ratio at 15 d.a.f. under non-added N.

Also, BAT 85 had an increase of S:R ratio at 15 d.a.f. under the

non fertilized treatment. Higher S:R ratios at flowering under

non-fertilized conditions, are caused by an increase in shoot dry

weight. Shoot growth was enhanced when no N was added and it was

diminished under added N. One possibility is that the applied

dosage of N fertilizer (40 kgs/ha) was enough to inhibit N

fixation but at the same time it was not enough to maintain a

Table 12. Shoot:Root ratio under two Nitrogen treatments
at two physiological stages. Iguala, 1982-3.

Identification Stage*1 Treatment*2 S:R ratio

61065 P + 6.7
MPF + 17.0
790131 F + 9.9
MPF + 25.9
1213-2 F + 14.9
MPF + 18.8
Bayo Madero P + 17.3
MPF + 16.9
BAT 85 F + 12.7
MPF + 32.3
Durango 222 P + 22.5
MPF + 16.6

*1 F= flowering
MPP= mid-pod filling
*2 + = added N
= non-added N

continued vigorous growth. On the other hand, the plants that

were grown under non-added N conditions were able to maintain

high levels of N-fixation which resulted in shoot growth. A

second possible explanation is that temperatures were too high

for maintaining high levels of the enzyme Nitrate Reductase,

whose presence and activity is necessary for the utilization of

N-fertilizer. In the case of BAT 85, the applied dosage of N

fertilizer was either enough to satisfy the plant's requirements

and promote optimum growth or it acted as a "starter" and

stimulated nodulation and plant growth at flowering time.

However, in a later stage of development, BAT 85 had a higher S:R

ratio under non added N as compared to the N-fertilized plots.

This illustrates that the effect of N fertilizer varies not only

among cultivars but also between stages of development within the

same cultivar.

The data presented herein suggest that N availability in

both N-fertilized and inoculated plants, was sufficient to

support vegetative growth. Economic yield was not affected by N

treatments for any of the sampled cultivars, nevertheless small

differences were seen between different combinations of genotype

and N treatment.


One of the main purposes of this experiment was to try to

identify physiological changes during the course of the growing

season that can be responsible for maintaining normal yields

under stress conditions. It seems reasonable to think that a

better understanding of the basis of differences among cultivars

and the relationship between these differences and their yield

potential should provide basic information that would be very

valuable in choosing a drought tolerance breeding strategy. As

suggested by Duncan et al. (16), three plausible explanations for

differences in yield between cultivars can be given. The first

one is a difference in photosynthetic efficiency of leaf

canopies, which would result in differences in the amount of

carbon fixed over the growing season. This efficiency could

result from better canopy geometry resulting in better light

interception, or from greater leaf area duration. A second reason

for yield differential could be the proportion of daily produced

assimilates that is partitioned to the economic sink. It is

likely that a higher yielding cultivar either partitions more of

the daily assimilate production to the seeds or is capable of

utilizing stored assimilates to fill the seeds. As a result, a

greater number of seeds (increased sink size) and heavier seeds

can be attained. The third reason could be the duration of the

vegetative and reproductive periods, the latter commonly known as

the seed filling period. Seed yield is the result of the rate and

duration of the filling period times the size of the economic

sink. With this experiment we sought to answer the following

questions: 1. Is there storage capacity in the stems of all

cultivars ? 2. When stem weight declines, does this correspond to

a non-structural carbohydrate loss? 35. What proportion of the

seed dry weight increase can be accounted for by changes in dry

weights, particular by stem dry weight loss? 4. Are there

cultivar and treatment differences in the contribution of storage

assimilates to seed yield? Having these questions in mind, the

following discussion is organized around the three possible

reasons for yield differences that were mentioned above.

1. Crop Growth Rate

Ground cover by the leaf canopy and rate of accumulation of

dry weight generally increase exponentially until light

interception is complete (16). In dry beans a fully closed

canopy is achieved at around flowering time, and this was the

case for the 22 cultivars planted in this experiment. Given that

after reaching a closed canopy full light interception is

attained, a further increase in LAI should not have any effect on

light interception. The data presented here show that total leaf

area (expressed as total leaf dry weight) continued to increase

after flowering. However, for the reasons stated above, no

further gains in light interception were expected after flowering


Total dry matter accumulation or net photosynthesis,

expressed as kgs of dry matter per hectare, is simply the

difference between the total amount of carbon fixed by

photosynthesis and the respective carbon losses due to growth and

maintenance respiration. Net photosynthesis as well as average

crop growth rates for 10 different cultivars during the

vegetative stage are given in Table 13. It is evident that the

late flowering cultivars had both a greater accumulation of dry

matter and a higher growth rate during their vegetative phase of

development. However, the CIAT line BAT 85 stands out for its

high photosynthetic efficiency ( expressed as kgs. of dry matter

accumulated per hectare per day ) given that it was not included

among the late flowering cultivars, and that the cultivars that

flowered at the same time as BAT 85 (63 to 64 days after
planting) had lower crop growth rates. The efficiency of BAT 85

can not be explained further with our current data; we can not

determine whether high photosynthetic capability, low respiratory

losses or both are responsible for the high dry matter

accumulation that occurred during the vegetative phase. The

photosynthates accumulated during the reproductive stage of

development and the crop growth rates for that period for the 10

sampled cultivars are given in Table 14. Crop growth rates

increased in all cases in the reproductive stage as compared to

the vegetative stage. Cultivar responses to the stress differed:

BAT 332, LEF-2-RB, 800122 and 61065 had higher growth rates

under the minus water treatment as compared to the irrigated

plots. The other 6 cultivars had lower crop growth rates under

stress. It becomes evident from these data that not only do

cultivar differences for crop growth rate exist but also that

different cultivars react differently under water stress.

Table 13. Average Crop growth rates( kg/ha/day) from
planting to flowering. Iguala, 1982-3.


Days to

Total dry matter
at Flow.

Average Crop
growth rate from
planting to Flow.

Wisec 23-61 64 1198 18.7
61065 64 1286 20.1
800122 70 1969 28.1
790131 63 1309 20.7
LEF-2-RB 64 1369 21.3
1213-2 48 838 17.4
Bayo Madero 50 1022 20.4
BAT 332 70 1910 27.3
BAT 85 63 1816 28.8
Durango 222 43 688 16.0

Table 14. Average Crop growth rates ( kg/ha/day ) from
flowering to maturity under two water treatments.
Iguala, 1982-3.

Identification Total dry matter Average Crop growth
at PM (kg/ha) rates from Fl. to PM
Irrigated Stress Irrigated Stress

Wisc 23-61 4464 3536 88.2 68.7
61065 3908 4012 74.9 80.1
800122 4305 5280 68.7 97.3
790131 4021 3457 90.4 76.7
LEF-2-RB 4360 5298 76.6 98.2
1213-2 3853 2952 71.7 52.8
Bayo Madero 7592 3371 126.3 48.9
BAT 332 3862 4044 65.0 73.5
BAT 85 4049 3482 67.6 52.0
Durango 222 3642 3288 59.0 55.3

Net photosynthesis is the result of a biological input-

output system that has several constraints. Identifying and

quantifying the relevant constraints would help the plant breeder

achieve a maximization of photosynthetic production.

Physiological and morphological components which determine the

crops efficiency of light conversion in a particular environment,

such as rapid establishment of a closed leaf canopy, efficient

canopy photosynthesis and effective distribution of assimilates

to the relevant economic sinks for as long a period as possible,

and the genetic variation associated with them, should be the

focus of detailed study.

2. Partitioning

The most important determinant of economic yield, as Donald

and Hamblin (15) stated, is not total crop photosynthesis, but

the way in which assimilates are distributed within the plant,

either for continued vegetative growth or for accumulation in

storage organs, seeds or fruits. However, it is not clear how

this allocation is regulated. It can be regulated by the supply

of assimilates (source strength), by the ability of the sink to

make use of the assimilates (sink strength) or by the rate of

translocation. How far sink strength can influence photosynthetic

rate is still an unanwered question. The term partitioning, as

used here, indicates the allocation of assimilates between

reproductive and vegetative plant parts. It is a dynamic day-to-

day process, that differs with cultivars and with physiological

Table15. Average Fruit growth rate from flowering to
physiological maturity (kgs/ha/day) under two water
treatments. Iguala, 1982-3.

Identification Irrigated Stress

Wisc 23-61 80.6 109.8
61065 94.0 101.5
800122 84.2 86.4
790131 77.0 88.6
LEF-2-RB 77.5 116.9
1213-2 56.9 62.5
Bayo Madero 56.7 37.3
BAT 332 91.0 123.2
BAT 85 123.0 109.4
Durango 222 57.6 46.8

Table 16. Partitioning Factor under two water1
treatments. Iguala, 1982-3. Calculated %

Identification Irrigated Stress

Wisec 23-61 91.4 159.7
61065 125.5 126.6
800122 114.9 88.7
790131 85.2 115.5
LEF-2-RB 101.0 119.0
1213-2 79.3 118.3
Bayo Madero 44.9 76.2
BAT 332 139.8 167.4
BAT 85 181.7 210.1
Durango 222 97.5 76.4

*1 %=(fruit growth rate/crop growth rate)x 100

stages such as early or late pod filling. The partitioninig of

assimilates between new vegetative tissue and storage can be very

important for plant performance under environmental stresses such

as temperature or water stress.

Table 15 illustrates the fruit growth rates of the 10

sampled cultivars under irrigated and stress conditions.

Differences not only among cultivars but also among treatments

were obtained. This indicates that the daily partitioning of

assimilates to the fruits was determined by the genotype and the

water treatment. When comparing the data shown in Table 14 (Crop

gowth rates CGR-) with the corresponding values in Table 15,

one can see that Fruit growth rates -FGR- exceeded CGR in 7

cultivars under water stress, while under irrigation FGR exceeded

CGR in 5 cultivars.

One of the basic questions to be answered is whether or not

the water stress treatment induces a greater partitioning of

stored assimilates to the fruit. The division of FGR by CGR

during a given period of time gives the average fraction of net

photosynthate partitioned to fruit growth. If all fruit growth

can be explained by current photosynthesis, net accumulation of

dry matter should be greater than or equal to fruit growth; if

not, one can assume that fruit growth was sustained in part with

photosynthates that were fixed in an earlier developmental stage.

A calculated partitioning factor shown in Table 16, indicates

that in 7 out of the 10 sampled cultivars under stress, the

calculated partitioning ratio exceeded 100%, and under irrigation

' in 5 entries the ratio exceeded 100%. Whether this can be

extrapolated to the extent that we can be sure that water stress

induces a greater partition of assimilates to the fruit is not

clear, but it is clear that treatment and cultivar differences in

partitioning exist.

A greater partition of assimilates can be the result of a

greater fruit load, or what was called before, namely "sink

strength". We have no conclusive evidence to say that this in

fact is the case, but the data show a very consistent trend in

which the cultivars with a high partitioning factor such as BAT

332 and BAT 85 had between 149 and 231 grs of seed/mt2, while in

cultivars with a low partitioning factor such as Durango 222 and

Bayo Madero sink size varied from 80 to 111 gra of seed/mt2.

Considering the change in plant dry matter between anthesis

and maturity as an indicator of net plant photosynthesis during

this period, and comparing this increment with the corresponding

increment in fruit weight, seems to be a reasonable way of

illustrating the proportion of pre-anthesis and post-anthesis

assimilates that were utilized for fruit growth under the 2 water

treatments. In Figures 10 and 11 plus water and minus water

treatments respectively, the X-axis is the net post-anthesis

photosynthesis expressed in kgs/ha and the Y-axis is the fruit

growth from anthesis to maturity also expressed in kgs/ha. The

1:1 line shows the position where a cultivar would lie if all the

assimilate produced after anthesis had gone into the grain. A

cultivar that lies further from and below the line is fixing more

carbon and it is using it for fruit growth at a slower rate than

it is produced. On the other hand in a cultivar that is

positioned above the 1:1 line fruit growth exceeds total growth









1 = Wisc 23-61
2= 61065
3 = 800122
4 = 790131
7 = B. Madero
8 = BAT 332
9= BAT 85
10= Dgo222



3000 4000 5000 6000
Net post-anthesis photosynthesis (kgs/ha)

Fig. 10 Proportion of fruit growth that can be accounted for by post-
anthesis photosynthesis under irrigated conditions. Iguala,

1:1 line.




*8 93



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