-
THE PLANT STRESS HYPOTHESIS AND VARIABLE
RESPONSES BY BLUE GRAMA GRASS
(Bouteloua gracilis) TO WATER,
MINERAL NITROGEN, AND
INSECT HERBIVORY
ANTHONY JOERN1,* and SIMON MOLE2,3
1Division of Biology, Kansas State University, Manhattan, KS
66506, USA2School of Biological Sciences, University of
Nebraska-Lincoln, Lincoln,
NE 68588-0118, USA
(Received January 3, 2005; revised May 9, 2005; accepted May 17,
2005)
AbstractActing simultaneously or sequentially, plants encounter
multiple
stresses from combined abiotic and biotic factors that result in
decreased
growth and internal reallocation of resources. The plant stress
hypothesis
predicts that environmental stresses on plants decrease plant
resistance to
insect herbivory by altering biochemical sourceYsink
relationships and foliarchemistry, leading to more palatable food.
Such changes in the nutritional
landscape for insects may facilitate insect population outbreaks
during periods
of moderate stress on host plants. We examined the plant stress
hypothesis
with field experiments in continental grassland (USA) using the
C4 grass
Bouteloua gracilis. Water, nitrogen fertilizer, and herbivory
from the grass-
feeding grasshopper Ageneotettix deorum were manipulated.
Combined
stresses from water and mineral-N in the soil decreased plant
growth and
altered foliar percent total N (TN) and percent total
nonstructural carbohy-
drate (TNC) concentrations in an additive fashion. Grasshopper
herbivory
affected final biomass only in dry years; plants compensated for
tissue loss
when rainfall was abundant. Foliar TN and TNC concentrations
were dynamic
with respect to variable climatic conditions and treatment
combinations,
showing significant interactions. Grasshopper herbivory had its
greatest
impact on TN or TNC in dry years, interacting with other forms
of stress.
Herbivory as a single factor had strong effects on TNC in years
with normal
precipitation, but not in a dry year. Performance (developmental
rate and
0098-0331/05/0900-2069/0 # 2005 Springer Science + Business
Media, Inc.
2069
Journal of Chemical Ecology, Vol. 31, No. 9, September 2005
(#2005)
DOI: 10.1007/s10886-005-6078-3
* To whom correspondence should be addressed. E-mail:
[email protected] Current address: Boulder, CO, USA.
-
survival) by the grasshoppers Phoetaliotes nebrascensis and A.
deorum were
not greatly affected by plant stress in a manner consistent with
the plant stress
hypothesis.
Key WordsVChewing insects, environmental stress hypothesis,
functional-convergence-to-plant-stress hypothesis, grasshopper,
insect herbivory, total
foliar nitrogen, total nonstructural carbohydrates.
INTRODUCTION
Dynamic biochemical, physiological, and morphological responses
by plants to
environmental conditions are integrated at organ and whole-plant
levels through
a variety of sourceYsink relationships (Mooney and Chiariello,
1984; Bazzazand Grace, 1997). The plant stress hypothesis states
that environmental stresses
on plants decrease plant resistance to insect herbivory by
altering whole-plant
sourceYsink resource allocation schedules and foliar chemistry,
thus changingfood palatability (Rhoades, 1983; Mattson and Haack,
1987; Louda and
Collinge, 1992; White, 1993; Redak and Capinera, 1994; Koricheva
et al.,
1998; Huberty and Denno, 2004). Plant resource acquisition
(light, water,
carbon, elemental nutrients), internal resource allocation among
tissues
(sourceYsink relationships, translocation products), and
partitioning of resourcesto different plant functions (growth,
maintenance, reproduction, repair, defense,
senescence) ultimately prescribe the nature and distribution of
nutritional
constituents within plants to herbivores (Mooney and Gilman,
1982; Bazzaz
et al., 1987; Chapin et al., 1987; Mooney et al., 1991; Aerts
and Chapin,
2000)Voften considered growth optimization processes (Mooney and
Winner,1991). Variation in water and soil nutrient availability
coupled to herbivory may
cause unpredictable levels of stress that alters plant
metabolism in response to
the action of one or all factors with consequences for plant
growth (Trlica and
Cook, 1971; Bokhari, 1978; Mooney et al., 1991; Louda and
Collinge, 1992).
The plant stress hypothesis was proposed as an environmentally
deter-
mined explanation for outbreaks of insect herbivores operating
through plant
condition (Rhoades, 1983; Waring and Cobb, 1992; Watt, 1992;
Koricheva
et al., 1998), in which improved nutritional quality of host
plants experiencing
intermediate levels of stress resulted in increased demographic
performance by
herbivores. Rhoades (1983) extended the hypothesis to also
include reduced
production of chemical defenses under stress conditions in
addition to elevated
nutritional quality. Experimental tests of the plant stress
hypothesis for forest
insects provide little general support of the hypothesis
(Rhoades, 1983; Waring
and Cobb, 1992; Watt, 1992; Koricheva et al., 1998). Although
some insect
feeding guilds (e.g., boring and sucking feeders) responded as
predicted in
experimental tests in woody plants, other groups including
chewing insects did
2070 JOERN AND MOLE
-
not generally respond to plant stress as predicted (Waring and
Cobb, 1992;
Watt, 1992; Koricheva et al., 1998; Huberty and Denno, 2004).
However, about
67% of the examples are consistent with predictions (Waring and
Cobb, 1992)
in observational studies of trees along environmental stress
gradients, although
alternate explanations exist (Watt, 1992). Although this system
may be
prototypical for the action of the plant stress hypothesis, few
tests with grasses
exist (Waring and Cobb, 1992; Redak and Capinera, 1994).
We seek to clarify the nature of interactions among multiple
stresses as
they impact growth and variable leaf chemistry in blue grama
grass, Bouteloua
gracilis (H.B.K.) Lag. ex Griffiths, according to predictions of
the plant stress
hypothesis. B. gracilis is a dominant C4 grass species in
western North
American (USA) grasslands. Two primary predictions of the plant
stress
hypothesis were examined in the short grass B. gracilis
experiencing naturally
occurring and variable abiotic conditions: (1) reduced water or
soil nitrogen
levels coupled to insect herbivory will negatively affect plant
growth and
increase the palatability of tissues to insect herbivores, (2)
chewing insect
herbivores will perform better on stressed host plants with
higher concentrations
of primary nutrients (protein and carbohydrate). In addition, we
examined the
relative contribution to responses of stresses when combined
under field
conditions. We examined direct effects and interactions among
three common
forms of stress to B. gracilis: water availability, plant
nutrient availability, and
grasshopper herbivory within natural levels in the field.
Experiments repeated
over 3 years included a wide range of weather conditions against
which to
gauge plant responses. We expected that the imposition of
moderate water or
nutrient stress should modify plant physiology in such a way
that resistance to
herbivores decreases, with a concomitant increase in
availability of primary
nutrients in leaves to herbivores. As food plant palatability
increases following
moderate stress to B. gracilis, performance by the grass-feeding
grasshoppers
Ageneotettix deorum (Scudder) and Phoetaliotes nebrascensis
Thomas should
be enhanced as levels of primary nutrients in leaf tissues,
especially protein and
carbohydrates, increase. B. gracilis does not produce
allelochemicals that are
expected to influence responses to primary nutrients by
herbivores in this
experiment (Mole and Joern, 1994), allowing us to restrict our
attention to the
nutritional component of the problem.
METHODS AND MATERIALS
Study System. We conducted field experiments at Arapaho Prairie
(Arthur
County, NE, USA), a protected research site in Nebraska
sandhills grassland.
The site is characterized by upland sandhills grassland composed
of large
stabilized sand dunes with steep upper ridges that gradually
slope into broad flat
2071PLANT STRESS HYPOTHESIS
-
valleys. Most plants at Arapaho Prairie experience at least some
water and
nutrient stress in most years (Barnes, 1985; Mole et al.,
1994).
Vegetation at Arapaho Prairie is an open-canopy mixed-prairie,
modified by
sandy substrate (Barnes, 1985). Grasses contribute 80% to total
plant biomass,
with long-term NAPP ranging between 75 and 250 g mj2
(unpublished data). C3and C4 grass species typical of eastern
tallgrass prairie and western shortgrass
steppe grasslands intermingle at the site. Dominant plants in
this sand dune land-
scape form loose but recognizable vegetation associations along
the existing
topographic gradient (Barnes, 1985). The grass canopy is
intermingled with ex-
tensive bare ground, largely because of extensive disturbance
from pocket gophers.
Long-term annual mean precipitation (1951Y1980) recorded 15 km
fromArapaho Prairie at Arthur County, NE, averaged 47.1 cm (SD =
8.98 cm) from
FIG. 1. Precipitation patterns at Arapaho Prairie. (a) Annual
rainfall with mean and 95%
confidence intervals, 1987Y2000. (b) Seasonal pattern of
precipitation illustrated bycumulative amount by date for the 3
years of the study.
2072 JOERN AND MOLE
-
US Weather Bureau records; the recent 14-year record from
Arapaho Prairie
(1987Y2000) averaged 37.3 cm (SD = 11.4 cm). The amount and
timing ofprecipitation at Arapaho Prairie varies greatly among
years (Figure 1). Below-
average precipitation was observed in two of the three years of
this study
(Figure 1a), with rainfall in 1990 equaling the average amount
for the site.
Perhaps more importantly, the seasonal timing of rainfall over
the growing
season differs in important ways among years (Figure 1b). Both
1989 and 1991
received approximately the same amount of precipitation, but
rain fell early in
the season in 1991 compared with late-season rainfall in 1989.
In 1990, rainfall
occurred throughout the growing season, compared with 1989 and
1991, each of
which experienced large periods without significant amounts of
rain.
Arapaho Prairie soils contain 80Y85% sand with low nutrient
concen-trations (Barnes et al., 1984). Total nitrogen in soil in
the top 10 cm ranges from
0.02 to 0.07% of total soil weight according to landscape
position. Valleys
exhibit the highest soil total N levels, but all landscape
positions are generally
low (Alward and Joern, 1993). Nitrate concentrations range from
0.04 to 15
ppm, and ammonium concentrations varied from 0.17 to 3.3 ppm.
Light is
seldom a major limitation to plant growth because of the open
canopy and large
proportion of sunny days at this site.
B. gracilis is an often dominant C4 short-grass species
throughout the
shortgrass steppe of the Rocky Mountain foothills to the
mixed-grass prairies of
the central Great Plains of North America. In Nebraska sandhills
grasslands, it
is commonly found in fine-textured soils typical of dry valleys.
At Arapaho
Prairie, B. gracilis comprises up to 20Y30% of the relative
cover of valleys andmidslope dunes but is nearly absent from dune
ridges (Barnes et al., 1984). B.
gracilis productivity is correlated with soil moisture, and
biomass peaks in early
August although yearly variability exists. B. gracilis is an
important dietary
component of graminivorous grasshopper species at this site,
including A.
deorum and P. nebrascensis (Joern, 1985).
Experimental Design and Statistical Analyses. Overall, two
related experi-
ments were run concurrently, one addressing effects of water, N
fertilizer, and
grasshopper herbivory on plant response, and the other
investigating grasshop-
per performance in response to water and N-fertilizer treatments
on plants.
Rectangular cages (basal area 0.5 m2, 80 cm high) were
constructed of 0.64-cm
mesh and buried 10 cm after severing possible root connections
to neighboring
ramets. Cages were placed over natural stands of B. gracilis
Bturf^ in earlyJune, corresponding to the initiation of growth.
Cages housing treatment com-
binations of both experiments were intermingled randomly within
each block,
but experiments were analyzed separately.
Plant Responses. We manipulated levels of water, nitrogen
fertilizer, and
grasshopper herbivory within natural levels to understand
variation in plant re-
sponses to stress. Biomass accumulation and foliar chemical
responses (% total
2073PLANT STRESS HYPOTHESIS
-
nitrogen, TN; and % total nonstructural carbohydrates, TNC) by
B. gracilis to
multiple stresses was studied using a 3 2 2 full-factorial
treatmentcombination (N fertilizer, water availability, and
grasshopper herbivory,
respectively) experiment in a randomized complete block design,
nested within
each of 3 years. Six sites (blocks) were arbitrarily selected in
a range of natural
habitats for B. gracilis along a gradient stretching from slope
vegetation to
valley vegetation. Sites were selected based on the criterion
that a sufficient
density of B. gracilis was available to set up a full set of
treatment combi-
nations. Treatment combinations were randomly assigned to
predetermined
patches of B. gracilis within each block.
Grasshopper Performance. Grasshopper performance was evaluated
in a
field experiment executed in parallel with the plant stress
experiment by using a
similar experimental design and identical water and mineral-N
fertilizer
additions using cages as described above. Cages were
intermingled randomly
with those of the plant stress experiment. The experimental
design was a 3 2full-factorial treatment combination experiment (N
fertilizer and water
availability, respectively) arrayed in a randomized complete
block design,
nested within each of 3 years. Six blocks were used. A
repeated-measures
analysis of variance (ANOVA) was used to examine grasshopper
survival.
Responses of two grasshopper species to plant stress were
evaluated in different
years (1989, P. nebrascensis; 1990, A. deorum), but specific
responses between
species cannot be compared directly because of overall
differences in naturally
occurring stress between years. Ten fourth instar nymphs were
added to each
cage in late June or early July to match natural phenological
development of
each species in the field. The number of survivors and the
developmental stage
of individuals were determined every 2Y3 d from censuses of
individualsremaining in each cage.
Statistical Analyses. Statistical analyses were performed using
ANOVA,
with treatments evaluated as fixed effects in the ANOVA. To
normalize data,
dependent variables expressed as percent of the total sample
weight were
transformed by applying arcsine(square root) to original data
before statistical
analyses. We present and discuss values in the nontransformed
state. Treatment
variables were treated categorically in analyses.
Manipulations of Plant Stress from Water, Mineral Nitrogen, and
Grasshopper
Herbivory
(1) Water. Two water levels were used: W+, in which water was
added weekly
for the 10-wk duration of the experiment, and W0, where no
additional water
beyond ambient rainfall was added. We considered W0 to be more
stressful
than W+ as water stress is common in grasses (Heinisch, 1981;
Barnes, 1985).
2074 JOERN AND MOLE
-
In the first 2 wk of the experiment, all plots received water in
addition to N
fertilizer if scheduled for that cage. After this, W+ cages
received 2 l mj2
wkj1 of supplemental water over the course of the experiment. No
attempt
was made to standardize the absolute level of plant water stress
among years.
(2) N-Fertilizer. Soil-nitrogen levels were manipulated using
ammonium
nitrate (NH4NO3). Levels included 0, 3, and 6 g N mj2 of N
fertilizer
(N0, N3, and N6 treatments, respectively). N fertilizer was
applied in two
half-strength additions over several days in early June in each
year.
(3) Grasshopper Herbivory. Moderate densities of the B.
gracilis-feeding
grasshopper, A. deorum, were added to cages to assess foliar
responses to
insect herbivory. In the GH+ treatment, we added four adult
grasshoppers to
each cage in late June. This density corresponded to eight
individuals per
square meter, about double the long-term average of all
grasshoppers at
Arapaho Prairie (A. Joern, unpublished data), but about half the
economic
threshold. Moreover, the densities used in the experiments are
routinely
observed in some vegetation patches in most years. No
grasshoppers were
added to cages in the GH0 treatment. Initiation of the
grasshopper treatment
corresponded to the phenological presence of the adult A. deorum
in the
field. Grasshoppers were replaced weekly to maintain relatively
constant
levels of herbivory.
Final Biomass Estimates and Chemical Analyses of Leaf Material.
Leaf
samples of B. gracilis were collected at the end of the
experiment (mid-August)
and prepared for chemical analysis. Initially, a subsample of
green leaf material
[ca. 2Y3 g dry weight (d.w.)] was collected, immediately
flash-frozen in liquidnitrogen in the field, and then prepared for
chemical analyses. Samples were
lyophilized for 48 hr and stored under desiccant in a freezer.
Dried leaf material
was ground with a Wiley Mill (40-mesh sieve) before chemical
analysis. After
collecting leaf material for chemical analyses, remaining plant
biomass in a
cage was clipped, dried (80-C for 24 hr) and weighed.Total
Nitrogen. Total nitrogen was analyzed by using modified micro-
Kjehldahl techniques (AOAC, 1984) with a standard digest on
100-mg samples
of ground leaf material (2 ml H2SO4, a CuSeO4 Kjeltab catalyst
tablet). Total N
was determined by measuring ammonia generated after adding 100
ml of 5 M
NaOH to the digest using a selective ion electrode (Orion). The
ammonium
probe was calibrated daily with an ammonium sulfate
standard.
Total Nonstructural Carbohydrates. Total nonstructural
carbohydrates
were extracted following the method of (Smith, 1981) except for
the use of
amylglucosidase (Sigma A-7255) as the enzyme preparation in the
digest. These
were analyzed by the titrimetric method of Smith (1981) with
glucose as a stan-
dard without the hydrolysis of sucrose. Sucrose averaged about
0.4Y0.5% d.w.
2075PLANT STRESS HYPOTHESIS
-
of plant material compared with 17Y22% d.w. plant material for
TNC asmeasured and did not vary with TNC concentration (S. Mole,
unpublished data).
RESULTS
Total Plant Biomass. On average, total biomass in B. gracilis
plots at the
end of the season (Figure 2) was about 50Y100% greater in an
average rainfall
FIG. 2. End of season B. gracilis biomass (mean, SE) according
to stress treatment
conditions [water (W0, W+), N-fertilization (0, 3 and 6 g N
mj2), and grasshopper
herbivory (GH0, GH+)] for each year of the study.
2076 JOERN AND MOLE
-
year (1990) as in dry years (1989, 1991), which were similar. B.
gracilis
biomass was significantly different among experimental
treatments depending
on the number of stresses applied, indicating that the plants in
this study expe-
rienced varying degrees of overall stress. Both water (1989:
F1,56 = 17.4,
P < 0.001; 1990: F1,56 = 6.6, P = 0.013; 1991: F1,56 = 11.3,
P < 0.001) and N
fertilizer additions (1989: F2,56 = 4.2, P < 0.021; 1990:
F2,56 = 11.2, P < 0.001;
1991: F2,56 = 7.1, P < 0.001) resulted in increased biomass
in all years as
additive, direct effects; no statistical interactions were
detected for water and N
fertilizer in any year (Figure 2).
Feeding by grasshoppers reduced the final B. gracilis biomass in
the dry
years of 1989 and 1991 (67% in 1989, F1,56 = 34.8, P < 0.001;
32% in 1991,
F1,56 = 6.5, P = 0.012), but no effect from grasshopper feeding
was detected in
1990, a year of normal rainfall. This indicates that complete
compensation for
foliage loss was observed in this year with normal rainfall. No
statistical
interactions among grasshopper herbivory, water availability,
and N fertilizer
treatments were observed in their combined effect on final B.
gracilis biomass,
but were additive instead. Although biomass estimates do not
include the
amounts consumed by grasshoppers, these should be similar
between years as
the grasshopper encounter rate was controlled.
Foliar Total Nitrogen. Foliar TN differed significantly among
treatments,
year, and block (Figures 3a and 4a, Table 1). TN concentrations
were highest
for all treatments in 1989, the driest year, a year with almost
no precipitation
occurring early in the growing period (Figure 1b). TN at the end
of the
experiments in August 1989 averaged 1.73% total dry weight in
all treatment
combinations compared with 1.01% (1990) and 1.14% (1991) TN in
subsequent
years, representing a notable decrease in 1990Y1991 compared
with 1989.Foliar TN levels varied in response to both N fertilizer
and water treatments
in some fashion in all years (Figures 3a and 4a, Table 1), with
water addition
explaining the most variation in responses (Figure 5). Depending
on the year, N
fertilizer addition increased foliar TN levels from 5 to 21% dry
mass compared
with no fertilizer addition treatments. An average 13% increase
in foliar TN over
the 3-year period was observed. Differences in foliar TN between
3N vs 6N
treatments were of smaller magnitude (3Y10%), and only
significantly differentin 1991.
Although the main effects of treatments were pronounced in all
cases
(Figure 3), treatment interactions that were important and
insightful to
underlying processes were sometimes detected. W0 treatments
resulted in a
10Y20% higher level of total foliar-N compared with W+
treatments. Theweakest response to water (9.5%) was observed in the
driest year (1989),
possibly because extreme drought stress in that year was not
proportionally
offset by the water addition treatment compared to other years.
A significant N
fertilizer by water interaction existed in 1989 and 1990 but
with different
2077PLANT STRESS HYPOTHESIS
-
responses between the 2 years (Figure 5). In very dry 1989,
higher foliar TN
levels were seen in W0 only for the N0 treatment. No differences
were seen
between W0 and W+ for the N3 and N6 fertilization treatment
levels. In a year
of average rainfall (1990), there was no difference in foliar TN
between water
treatments at N0, but significant and about equal increases in
total N for N3 and
N6 treatments in interaction with water availability.
Grasshopper herbivory affected foliar TN levels significantly as
a main
effect only in 1991. However, grasshopper feeding interacted
with other treat-
ments to influence total foliar TN in all years (Figures 4a and
5). In 1989, there
was an increase in TN up to the maximum level observed at N3, a
TN level that
was reached with N6 with no grasshoppers. In 1990, grasshopper
herbivory
interacting with water availability led to higher TN level that
was reached in the
FIG. 3. Responses (mean, SE) in (a) % total N and (b) % TNC to
main treatments (water
addition, grasshopper herbivory, and N fertilizer) for each
year.
2078 JOERN AND MOLE
-
W0 treatment compared with the W+ treatment for which there was
no
significant difference between grasshopper treatments.
Foliar Total Nonstructural Carbohydrates. Significant responses
in foliar
TNC concentrations were also observed (Table 1, Figures 3b, 5b,
and 6) in
response to combined stresses. Among-year differences averaged
5Y10%, with1989 exhibiting the highest foliar TNC levels.
Differences in responses among
all treatment combinations showed little variation in 1989 and
1991 compared
FIG. 4. Percentage of total variance in foliar nutrient
responses explained by experi-
mental treatments in each year of study. (a) % Total foliar
nitrogen (TN) and (b) % total
nonstructural carbohydrates (TNC). Letters refer to main effects
(N, nitrogen fertili-
zation; W, water; G, grasshopper herbivory) and statistical
interactions (N*W, N*G,
W*G) as indicated in the experimental design of Table 1. B is
the block (site) effect.
Percentage of total variance in response was calculated as the
variance associated with
the treatment combination compared with the total variance of
the experiment.
2079PLANT STRESS HYPOTHESIS
-
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2.6
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92
1.5
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01
4.0
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Wat
er
Gra
ssh
opp
er1
0.0
60
.81
0.1
0.8
11
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.00
11
6.3
0.0
01
t1.26
N-F
erti
lize
r
Wat
er
Gra
ssh
op
per
20
.94
0.3
92
2.2
0.1
21
8.0
0.0
01
7.5
0.0
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t1.27
Err
or
19
75
5t1.28
2080 JOERN AND MOLE
-
with 1990. Total variance in TNC levels among treatments was
1.5Y5 timesgreater in the average rainfall year (1990) than in the
other years. Over all 3 years,
combined nitrogen fertilizer and grasshopper treatments for all
levels were sig-
nificant as main effects, with no significant statistical
interactions. When com-
pared against the N fertilizer treatments, W0/GH0 had the lowest
TNC levels and
W+/GH+ had the highest levels on average, with each decreasing
along the
N-fertilization axis. Levels of TN and TNC in leaves were
uncorrelated in
all years for all treatments combined (1989: r2 = 0.014; 1990:
r2 = 0.001;
1991: r2 = 0.038; P > 0.05 for all years). However, when
years were analyzed
separately, interesting differences were observed.
In general, TNC declined 4Y6.5% with increased N fertilizer in
all years,although no significant differences were observed between
the 3 g and 6 g
FIG. 5. Responses of significant interactions among treatments
for % total foliar N (TN)
for each year of study.
2081PLANT STRESS HYPOTHESIS
-
N fertilizer treatments. When water treatments were significant
(1989 and
1991), TNC was greater in W+ compared with the W0 treatments,
with
differences on the order of about 3Y4%. Generally, grasshopper
herbivory was afactor when interacting with either N fertilizer or
water treatments (Table 1). In
1990, GH+ resulted in a large 23% increase in % foliar TNC, and
important
interactions with N fertilizer and water were detected.
The nature of interactions among sources of plant stress
differed among
years. Numerous interactions were observed in both 1990 and 1991
(Figure 6),
average and below average rainfall years, respectively. In 1990,
all two-way
interactions and a three-way interaction were significant. % TNC
in the N6fertilizer treatment increased in the W0 treatment, but
the trend otherwise was
for TNC to drop with increased N fertilizer. Grasshopper
treatments interacted
with both N fertilizer and water in both 1990 and 1991, but the
TNC responses
were different. In the very dry 1989, no interactions were
detected, and all
contributions to the variance in TNC content were additive.
Inclusion of
grasshoppers resulted in increased TNC in high-resource
environments (N or
FIG. 6. Responses of significant interactions among treatments
for % total nonstructural
carbohydrate (TNC) for each year of study.
2082 JOERN AND MOLE
-
water) compared with the GH0 treatments. In 1991, the opposite
response was
observed where TNC levels under high-resource conditions were
lower if
grasshoppers were present.
Grasshopper Performance. P. nebrascensis. This species was
studied in a
very dry year with late season rainfall. No significant effect
of treatment
combinations was observed for developmental rate although there
is a suggestion
that W0/6N develops faster. Repeated-measures ANOVA of the
number of
FIG. 7. Mean survival of two grasshoppers in response to plant
stress treatments.
Experiments were performed in different years as described in
the text. Data are trans-
formed as natural log of number alive at each census period.
2083PLANT STRESS HYPOTHESIS
-
individuals remaining in cages of P. nebrascensis (Figure 7a)
was significant
(Wilks l = 0.10, P < 0.001). However, although observed
trends in survivalmay be suggestive, no significant effect of water
and N fertilizer treatments
were detected. The significant difference in the
repeated-measures ANOVA re-
flected the decrease in the number of survivors over time, not
treatments.
Ageneotettix deorum. This species was studied in a normal
rainfall year.
No significant effect of water and N fertilizer treatments on
developmental rate
was detected. A. deorum survival (Figure 7b) varied in response
to experimental
treatments (repeated-measures ANOVA, Wilks l = 0.137, F6,21 =
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