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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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4.9
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.10
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t1.11
N-F
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r
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t1.13
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ssh
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Err
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19
75
5t1.15
t1.16
TN
Cs
(gT
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/gle
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gh
t)t1.17
Mo
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.83
7.5
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.001
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Blo
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2.9
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.001
t1.21
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4.8
0.0
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96
.80
.0t1.22
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29
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.00
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10
.02
0.8
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0.8
0.5
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.92
1.3
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0.1
3t1.24
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r
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1.0
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0.0
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1.5
0.0
01
4.0
0.0
2t1.25
Wat
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0.0
60
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.00
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6.3
0.0
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Gra
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t1.27
Err
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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 = 22.06, P /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 150 /GrayImageDepth -1 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputCondition () /PDFXRegistryName (http://www.color.org?) /PDFXTrapped /False
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