Group Title: Research report (North Florida Research and Education Center (Quincy, Fla.))
Title: Photosynthesis in relation to defoliation stress in soybean
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 Material Information
Title: Photosynthesis in relation to defoliation stress in soybean
Series Title: Research report (North Florida Research and Education Center (Quincy, Fla.))
Physical Description: 11 leaves : ; 28 cm.
Language: English
Creator: Teare, I. D ( Iwan Dale ), 1931-
North Florida Research and Education Center (Quincy, Fla.)
Publisher: North Florida Research and Education Center
Place of Publication: Quincy Fla
Publication Date: 1993
Subject: Photosynthesis   ( lcsh )
Defoliation   ( lcsh )
Soybean   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical reference.
Statement of Responsibility: I.D. Teare ... et al..
General Note: Cover title.
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Bibliographic ID: UF00066109
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 71173609

Full Text


Photosynthesis in Relation to Defoliation Stress in Soybean

I. D. Teare, A.J. Mueller, J. E. Funderburk, and L. G. Higley

04f** i/v

North Florida Res. and Educ. Ctr., Univ. of Fla.
Nebraska, Lincoln, NE 68583; Univ of Arkansas;
Exp. Stn. Res. Rep. NF-93-7.

,Quincy,, Fla. 32351; Univ. of
Fayetteville, AR 72701. Agric.


Defoliation by insect pests is a major stress of soybean in the Southern

Region. Quantifying physiological responses of soybean to defoliation is

essential for understanding how insect defoliation reduces soybean yields.

Understanding the relationship between defoliation and plant responses, such as

yield is essential for better pest management.

Researchers have long recognized the importance of defoliating pests to

soybean, and over the past 40 years more than 50 research articles have addressed

soybean defoliation (Ostlie 1984). However, despite this volume of research,

long standing questions regarding defoliation on soybean persists. Compensatory

responses to defoliation are not well documented or understood (particularly

photosynthetic responses),

The objectives of this study are: 1) To characterize effects of simulated

insect defoliation on soybean physiology, 2) To characterize mechanisms of

soybean compensation to defoliation, particularly delayed leaf senescence and

altered leaf photosynthesis.


Soybean were grown on a Norfolk loamy sand (fine, loamy, siliceous thermic

typic Kandidult) with a pH of 5.5 and 1.5 percent organic matter in 1990 and 1991

at the North Fla. Res. and Educ. Ctr. at Quincy, FL.

The experimental area was bottom plowed 3 May 1990 and fertilized at a rate

of 15# N/acre, 45# P/acre and 90# K/acre on 20 May 1990. Weed control consisted

of a preplant treatment of Treflan 4E at 0.75 Ib Ai/acre on 17 May 1990,

postemergence treatment of Classic at 0.5 oz Ai/acre plus Tackle at 0.5 Ib

Ai/acre on 25 June 1990, and a cultivation with a rolling cultivator on 2 July

1990. The experimental area for 30 May 1991 was bottom plowed and fertilized

with 5-10-15 at 500#/acre. Weed control consisted of Treflan at 2 pts/acre and

Sencor at 3/8 AI/A in a preplant treatment on 31 May 1991.

A 2-row cone planter was used both years to plant Braxton soybean in eight

row plots 7 meters long and with 30 cm between rows at a rate of 26 seeds per

row-m on 11 June 2 1990. Row orientation was north-south. Final soybean plant

density was 18 plants/row-m in 1990 and 25 plants/row-m in 1991 (soybean were
over-planted and thinned to provide uniform plant spacing). Soybean cultivar

used was Braxton in southern states.

Insecticides used were Dimilin at 0.5 oz Ai/acre plus Dipel at 1 Ib/acre on

9 Sept 90; Dimilin at 0.5 oz Ai/acre plus Assini at 0.025 oz Ai/acre on 16 Oct

90. The insecticides used in 1991 were Asana at .025 oz AI/A on 15 July, Asana

at .025 AI/A and Dipel 1 1/4# /A on 30 July, 8 oz Asana/A and 1# Orthene/A on 23

Aug, 8 oz Asana and 1# Orthene/A on 6 Sept, Pyrellian at 1 pt/A 19 Sept, and

finally 8 oz/A Asana and 1 pt/A Pyrellian 3 Oct.

The experimental design for soil treatments and for yield was a randomized

complete block containing four replications. Yield estimates each year were

determined for all 11 defoliation treatments. The effects of defoliation

treatments on soybean yield, light interception and photosynthesis were evaluated

by using ANOVA, subsequent treatment comparisons, and regression.

Treatments consist of 4 defoliation patterns (simulated insect defoliation

at stage R2, R4, R2R4, three levels within each pattern (defoliation to produce

a leaf area index of 3.5, 2.5, and 1.5 at R2 and R4), and an undefoliated check.

Defoliation levels were chosen based on the measured leaf area index (LAI) at

late Rl/early R2 and the projected LAI at R4 based on the R4 measurement.

Defoliation levels were chosen to provide final LAIs after defoliation (R4) that

include values above, at, and below the critical LAI (the LAI value at which 95%

include values above, at, and below the critical LAI (the LAI value at which 95%

of incident light is intercepted by a plant canopy, estimated to'be ca. 3.5 for

soybean). Consequently, specific levels of defoliation may vary between

locations, but all locations will provide defoliation treatments that span the

critical LAI. Soybean was defoliated by leaflet, and leaf areas of all leaflets

removed were measured.

Defoliation was be limited to the upper two thirds of the canopy, but

extended into the lower canopy where necessary for high defoliation levels. The

center 4 row-mm of the two middle rows were defoliated. Undefoliated plots

received comparable handling (walking in plots and "fondling" plants) as

defoliated plots (to allow for compaction during defoliation and affects of

touching plants). Areas adjacent to the defoliated region (border rows and ends

of plots) were sham defoliated (stripping leaflets without quantifying area

removed) to approximately the same level as the defoliated area. Border areas

were sham defoliated during the last 8 days of the simulation (when most of the

defoliation is occurring), specifically on days 8 and 12. Sham defoliation

occurred on 17 Aug and 7 Sept in 1990, on 9 Aug and 30 Aug 1991. Treatments were

color coded with wood lath and flags to minimize potential errors in leaf


Target LAI's (3.5, 2.5, and 1.5) were chosen to provide discrete reductions

in light interception based a critical LAI of ca. 3.5. The undefoliated check

provided a treatment above the critical LAI. Because the defoliation levels

depend on these target LAIs at late R4, an estimate of the R4 LAI was needed.

The LAI at R4 was likely to be 1-2 units greater than the LAI at late Rl/early

R2 (when-defoliation is initiated for certain treatments); therefore, the LAI at

R4 was approximated by the measured LAI at late Rl/early R2 + 2. Treatments were

imposed by removing a given amount of leaf area (leaflets) based on the

difference in projected LAIs and the target LAIs. For the R2+4 sequential

defoliation treatments, the total leaf area removed was calculated and half

removed at R2 and half removed at R4 (so that total leaf area removed is the same

as in the R2 and R4 treatments). Although, the amount of leaf area removed

differed between states, the resulting treatments (LAI levels and corresponding

levels of light interception) were comparable. The percent defoliation per

treatment to achieve the target LAI was calculated from the formula:

[projected LAI target LAI)/projected LAI]*100
Defoliation was imposed to approximate insect injury. Ideally, daily

injury rates and duration should be based on temperature driven consumption and

development models. However, because temperature driven models would result in

substantial differences in injury rates among locations, standard durations and

injury rates were employed at all sites. Many soybean defoliators have larval

development times for the latter developmental stages (when >90% of consumption

occurs) of approximately two weeks, at temperatures commonly occurring in mid to

late summer. Consequently, defoliation was imposed over 12 days (Monday of week

1 to Friday of week 2). Defoliation occurred at soybean stages R2 and R4

(depending on treatment) which corresponds with the injury phenology for many

soybean defoliators.

Daily defoliation rates depend on stage specific consumption rates. In

brief, to simulate insect feeding we need to estimate what proportion of the

total defoliation required should occur on each day. The rationale behind the

values chosen was as follows. For this study, two aspects of development and

consumption are pertinent. First, proportion of total larval consumption in a

stage, and second, duration of developmental time in a stage. To determine the

proportion of the total defoliation that should occur in each larval stage, an

estimate of proportion of total consumption by stage was needed. Published data

on this question indicate that the proportion of total consumption by instars

are: GCW 1-2=2%, 3-4=8%, 5-6=90% (Hammond et al., 1979a); SBL 1-2=1%, 3-4=9%, 5-

6=90% (Boldt et al., 1975); and VBC 1-2=3%, 3-4=5%, 5-6=92% (Boldt et al., 1975).

Because so little defoliation occurs in the first two larval stages (<3%), for

this study we will consider defoliation only during the latter stages.

Specifically, we estimated the proportion of defoliation by stage as 3-4=10%, and

5-6=90%, The second question was duration of development time in a stage.

Literature data on green cloverworm (GCW), corn earworm (CEW), soybean looper

(SBL), and velvetbean caterpillar (VBC) were used to determine appropriate values

for this study. The proportion of time spent in various instars are: GCW 1-

2=29%, 3-4=26%, 5-6=45% (Hammond et al., 1979b); and CEW 1-2=23%, 3-4=25%, 5-

6=52% (Boldt et al., 1975). Based on these values, an appropriate estimate of

time spent in each stage is 1-2=25%, 3-4=25%, and 5-6=50%. We estimated

development through stages 3-6 as requiring 12 days. Therefore, the ratio of

development times (25%:50% or 1:2) gave the number of days spent. in each stage;

specifically, stages 3-4=4 days and stages 5-6=8 days. Consequently, to provide

an appropriate simulation of a lepidopteran defoliator of soybean (combining

consumption and development data), we imposed injury over 12 days with 2.5% of

the total defoliation occurring on each of the first 4 days and 11.25% occurring

on each of the last 8 days.

All calculations of total leaf area to be removed (% of total leaf area to

be removed each day per plot; conversion of leaf area to be removed to leaflets

to be removed; and defoliation summaries were provided by a computer program,

DEFOL (written by L. G. Higley for this project). To adjust for possible

discrepancies between projected and actual leaf area removed, all leaf area

removed/plot/day were quantified and entered into the program to allow for daily

adjustments. The program outputted leaf areas and numbers of leaflets to be

removed from each plot on each day. Leaf area removal was based on target

defoliation levels, appropriate injury rate, and previously removed leaf area.

Leaflets to be removed were calculated from leaf area to be removed and a user-

supplied estimate of average leaflet size on the first day. Subsequently, the

program calculates the average leaflet size based on number of leaflets removed

and measured leaf areas. Because the defoliation levels were based on projected

LAIs at R4, it is important to have an idea of actual LAIs during defoliation so

that adjustments can be made if the projections are greatly in error. Measures

of plant leaf area were taken immediately before the defoliation period, which

provide a measure of the actual LAIs. (To convert a mean plant leaf area into

an LAI for 76 cm rows and 25 plants/row-m, multiply the plant leaf area [in cm2)

by 0.00329). Records of total leaf area actually removed (by plot) were

maintained to calculate actual defoliation at end of the defoliation period.

In Arkansas and Florida photosynthesis measurements were taken weekly, from

R2, to examine the soybean compensation to defoliation through altered leaf '

photosynthesis. Leaflets at ca. nodes 6, 9, and 12 were marked, and

photosynthetic rates monitored before, during, and after defoliation. Leaflets

on at least two plants per plot were measured. Measurements were made in full

sunlight at comparable times for each measurement. Measurments were taken at each

R-stage, R2 thru R6 10 Aug, 16 Aug, 24 Aug, 5 Sept in 1990 and 2 Aug, 7 Aug, 20

Aug, 5 Sept, 12 sept, in 1991 respectively. In 1991 the photosynthesis

measurements were greatly hindered by excessive rainfall and cloudy weather.

Statistical procedures used will include analysis of variance and

regression techniques. All data were subjected to analysis of variance. When

the F test was significant, multiple range tests were applied.



Weather must be discussed in relation to defoliation results for 1990 at

Quincy, FL. The summer of 1990 was one of the driest on record. The average

rainfall for the soybean growing of the previous 10 year period was 26 inches

compared to 12 inches for 1990. Soybean were definitely stressed during this

period, particularly from the R4 to R6 stage (Fig. 1).


Defoliation in 1990 at R2 seemed to increase photosynthesis (Fig. 7, top).

Photosynthesis was greatest for 1.5>2.5>3.5
photosynthesis of defoliation treatments below the controls (Fig. 3, bottom) on

Julian day 236. For that day, photosynthesis was greatest for the

control>2.5>3.5=1.5. Following drought at Julian day 247, photosynthesis for R4

defoliation was greatest for 1.5>3.5>2.5>control. Photosynthesis for R2

defoliation on Julian day 247 was similar, 1.5>2.5=3.5>control.

Photosynthesis in 1991 decreased with time when defoliated at the R2 stage

because of the high leaf area over time that covered the preselected

photosynthesis leaves. The R4 defoliation had some reduction in photosynthesis

after defoliation.


The 1991 rainfall for the soybean growing season was 28 inches. Rainfall

distribution was heavy in the early part of the season, but was dry during the

R5 and R6 reproductive stage (Fig. 1).

The quantitative effect of drought on soybean in relation to defoliation

period is shown as a function of 1990 stomatal resistance (RS) in Fig. 2).

Stomatal resistances were statistically significant for defoliation treatments

(F=2.57; df=7,21; P<0.05). Stomatal resistances (RS) were significantly

different in 1991. Defoliation at the R2 stage resulted in RS differences at the

R5 sampling date and defoliation at the R4 stage resulted in RS differences at

the R4 and R5 stage.


Our thanks to E. Brown, Agric. Tech. IV; North Fla. Res. and Educ. Ctr.,
Univ. of Fla., Quincy, FL; for data collection, computer processing, and data


Boldt, P. E., K. D. Biever, and C. M. Ignoffo. 1975. Lepidopteran pests of
soybean: consumption of soybean foliage and pods and development time. J.
Econ. Entomol. 68:480-482.

Hammond, R. B., L. P. Pedigo, and F. L. Poston. 1979a. Green cloverworm leaf
consumption on greenhouse and field soybean leaves and development of a
leaf-consumption model. J. Econ. Entomol. 72:714-717.

Hammond, R. B., F. L. Poston, and L. P. Pedigo. 1979b. Growth of the green
cloverworm and a thermal-unit system for development. Environ. Entomol.

Hutchins, S. H., L. G. Higley, and L. P. Pedigo. 1988. Injury-equivalency as
a basis for developing multiple-species economic injury levels. J. Econ.
Entomol. 81:1-8.

Ingram, K. T., D. C. Herzog, K. J. Boote, J. W. Jones, and C. S. Barfield.
1981. Effects of defoliating pests on soybean canopy CO2 exchange and
reproductive growth. Crop Sci. 21:961-968.

Ostlie, K. R. 1984. Soybean transpiration, vegetative morphology, and yield
requirements following simulated and actual insect defoliation. Ph.D.
dissertation. Iowa State University, Ames, IA.

Pedigo, L. P., L. G. Higley, and P. M. Davis. 1989. Concepts and advances in
economic thresholds for soybean entomology. p. 1487-1493 in A. J. Pascale
(ed.) Proc. World Soybean Res. Conf. IV. Vol. III.

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