Photosynthesis in Relation to Defoliation Stress in Soybean
I. D. Teare, A.J. Mueller, J. E. Funderburk, and L. G. Higley
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
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.
MATERIALS AND METHODS
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
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.
RESULTS AND DISCUSSION
RAINFALL AND DROUGHT
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).
PHOTOSYNTHESIS vs. DEFOLIATION TREATMENT
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
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
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