Grasshopper crop and midgut extract effects on plants: an example of reward feedback.

Acid extracts and a resultant fraction from solid-phase extraction (SPE) of Romalea guttata crop and midgut tissues induce sorghum (Sorghum bicolor var. Rio) coleoptile growth in 24-h incubations an average of 49% above untreated controls. When combined with plant auxin, indole-3-acetic acid (IAA), the SPE fraction shows a synergistic reaction, yielding increases in coleoptile growth that average 295% above untreated controls and 8% above IAA standards. The interaction lowered the point of maximum sensitivity of IAA 3 orders of magnitude, resulting in a new IAA physiological set point at 10(-7) g/ml. This synergism suggests that contents in animal regurgitants making their way into plant tissue during feeding may produce a positive feedback in plant growth and development following herbivory. Such a process, also known as reward feedback, may exert major controls on ecosystem-level relationships in nature.

Measuring Heterotroph-Induced Source-Sink Relationships in Panicum Coloratum with 11C Technology.

We report a synthesis from three series of experiments on source-sink relationships in Panicum coloratum L., a C"4 tropical grass obtained from the Serengeti grasslands of Africa. Studies on ^1^1C real-time analyses of P. coloratum to determine aboveground effects of grasshopper grazing and belowground effects of mycorrhizal inoculation and nematode feeding provided the database. A series of multi- and univariate statistical investigations of all available experimental data described responses of leaves, stems, and roots to these biological stresses. From a principal components analysis we have shown differences in distribution of C source-sink locations along three principal component axes, which accounted for 84% of the experimental variance. The first and second components (62% of variance) described C allocation to leaf, stem, and root sinks. The third component (22% of variance) showed a metabolic dichotomy between leaf starch sinks and labile carbon pools throughout the plant. We use the three principal components from a ^1^1C three-compartment model describing leaf, stem, and root C source and sink variables to present patterns, or fingerprints, of responses to the experiments. Time of day, treatment class, number of days since transplanting, and ecotype controlled a large amount of the overall variation in plant C fixation and reallocation. A comparison of ^1^2C leaf carbon exchange rates (CER) measured with an infrared gas analyzer and ^1^1C rates showed a high positive correlation. Slopes for grasshopper grazing, mycorrhizal inoculation experiments, and nematode feeding showed almost identical results; however, differences in the intercept developed as a function of ecotype. We noted a significantly lower intercept in morning studies, but no differences in the slope for morning compared to afternoon studies. CER and all ^1^1C variables for grasshopper and nematode experiments showed a lower coefficient of ^1^1C variables and higher for CER. We conclude that ^1^1C experiments provide the base for developing laboratory, field, modeling studies to incorporate aggregations of real-time C transfers within plants responding to biological stresses, including those of heterotrophs.

Maximization of Aboveground Grassland Production: The Role of Defoliation Frequency, Intensity, and History.

Production of tallgrass prairie vegetation was measured on experimental plots in which defoliation intensity and frequency were manipulated by mowing and using movable exclosures on areas chronically grazed by cattle. Defoliation history largely controlled whether or not defoliated plants overcompensated (exhibited enhanced production compared to undefoliated controls) for tissue removal. Plants on chronically grazed sites only compensated for foliage removed by grazers. Production on plots mowed prior to the year of measurement was similar to that on chronically grazed sites, while previously unmowed plots exhibited substantial aboveground overcompensation. Aboveground production was maximized by the most frequent mowing treatment and by intermediate mowing heights. Nitrogen and phosphorus concentrations and amounts in aboveground tissues were increased by mowing and grazing. Current mowing regime was more important than mowing history in determining nitrogen concentrations except very early in the growing season. Effects of grazing and mowing on belowground biomass were inconsistent, but frequent mowing appeared to limit accumulation of belowground N reserves and biomass. In North American grasslands, overcompensation is a nonequilibrium plant response to grazing. Photosynthate that would be stored as reserves and used for root growth and flower and seed production instead is used to replace lost leaf area, thereby resulting in higher foliage productivity. However, under chronic grazing or mowing, vegetation is prevented from maintaining high nutrient and water uptake capacity (large root biomass) and accumulating reserves that allow overcompensation responses.

Herbivory and Its Consequences.

We argue that herbivores often induce nonlinear or biphasic growth and development in plants. Collectively these individual responses translate into a system-level optimization curve wherein at low levels of herbivory overall community responses show increases in production potential, whereas extreme herbivory causes extreme reduction in productivity. The transition between these two states defines a point of optimal herbivory in respect to C and N processes.We present four case examples from the literature demonstrating such nonlinear responses, suggesting a widespread existence for this herbivore-plant phenomenon. The nonlinear responses appear to demonstrate temporal and spatial scale dependencies.

Multi-Level Interactions Arising from Herbivory: A Simulation Analysis of Deciduous Forests Utilizing Foret.

We examined potential effects of herbivory on temperate forest ecosystems using FORET, a forest succession simulation model with the capacity for treating various hierarchical levels for long time periods. Two species of trees were chosen for evaluating herbivory effects: white oak (Quercus alba), a relatively slow-growing shade-tolerant species, and tulip poplar (Liriodendron tulipifera), a fast-growing shade-intolerant species. Both are dominants in the Southeastern U.S. forest system selected as a basis for this study. The study focused on four hierarchical levels, covering individual, phenotypic, interspecific, and community interactions. Simulations of herbivory were arrived at by stochastically varying annual incidence that herbivores were present on each simulation plot over a 500-yr period, and by modifying the proportion of energy allocated to either tree growth, or to its defense from herbivore attack. Two hypothetical tree phenotypes were considered, one that allocated specified amounts of energy to herbivore defense mechanisms at all times, and the other that made the allocation only when herbivores were present according to the stochastic determination made for a specific study period. Thus, one phenotype was fixed in its life history strategy; the other was given a facultative strategy where it switched its growth and defense tactics as a function of variation in herbivore presence. The results of the simulations suggest how deciduous forests may respond to long-term variations in the intensity of herbivore stress on two dominant tree species, and show the importance of hierarchical relationships within the community. White oak tended to show a greater sensitivity to interspecific interactions; tulip poplar showed a higher sensitivity to intraspecific interactions. Changes in growth rates associated with the switching strategies (an ultimate factor) were more important in answering variations in productivity than was impact imparted by annual changes in incidence of herbivore presence (a proximate factor) for these two species, although there were important differences in several statistical interactions. While our results suggest that herbivore stress can explain a larger degree of the variation in long-term community dynamics, ecological interactions between herbivore and climate effects must be more closely linked in such long-term studies.

Mowing and Fertilization Effect on Productivity and Spectral Reflectance in Bromus Inermis Plots.

Experiments were conducted to examine the potential role of grazing on ecosystem-level parameters as part of the NASA-sponsored First Isabela (International Satellite Land Surface Climatology Programmed) Field Experiment (FIFE) conducted at Konza Prairie Research Natural Area in 1987. Here we report results of one experiment conducted in a field consisting primarily of Bromus inermis, a cool season C3 grass. The experiment involved four simulated grazing components (unmowed control, 20-, 10-, and 5-cm mowing heights) and fertilization (untreated control and ammonium nitrate application). The plots were mowed to ground level and raked in April, following which they were mowed seven times during the growing season from May to October. Biomass production, N production, and spectral reflectance data were collected with a hand-held radiometer throughout the growing season, with standing crop estimates taken at two periods (7 August [day 219] and 27 October [day 300]) to correlate with the remote sensing information base. Standing crop values of mowed plots were as much as 67-70% lower than controls, but they produced significantly larger amounts of both biomass and total N. Maximum seasonlong production values in the mowed plots were °43% above controls, with major differences developing as a result of fertilization. Fertilized plots produced 67% more foliage than unfertilized plots. Our data show over-compensatory growth as a result of the simulated grazing treatments. Indexes (NDVI [normalized difference vegetation index] and greenness) derived from the reflectance data were poorly correlated with biomass. The correlation of NDVI with N content of the canopy foliage was somewhat stronger, particularly if stratified by mowing class. NDVI was a better predictor of vegetation status than the greenness indexes, but in plots stimulating heavily grazed areas where leafy vegetation was sparse and soil became more visible from above the canopy its utility decreased significantly.

Carbon partitioning patterns of mycorrhizal versus non-mycorrhizal plants: real-time dynamic measurements using 11 CO2.

Gas exchange and carbon allocation patterns were studied in two populations of Panicum coloratum, an Africa C-4 grass. The plants were grown in split-root pots, containing partially sterilized soil, with one side either inoculated (I) or not inoculated (NI) with a vesicular arbuscular (VA) mycorrhizal Fungus, Gigaspora margarita. Net carbon exchange rates (CER) and stomatal conductances were measured with conventional gas exchange apparatus, and carbon assimilation, translocation, and allocation were measured using photosynthetically-fixed 11 CO2 . Mycorrhizal infection on one half of the split-root system caused a 20%, increase in CER. The effect on CER was less in tillers on the opposite side of the plants from the infected half of the roots. The rate at which photosynthates were stored in the leaves was 45% higher. Sink activity (concentration of labelled photosynthates in stem phloem tissue) more than doubled in 1 versus NI plants. CER and stomatal conductances, along with most of the carbon allocation patterns, were nearly identical between the NI (control) high grazing and low grazing ecotypes. However, VA mycorrhizal fungi caused a greater storage of photosynthates in the low grazing ecotype.

Plant-herbivore interactions in a North American mixed-grass prairie : I. Effects of black-tailed prairie dogs on intraseasonal aboveground plant biomass and nutrient dynamics and plant species diversity.

Research was conducted to determine the effects of a native, sedentary rodent of North American grasslands, the black-tailed prairie dog (Cynomys ludovicianus), on seasonal aboveground plant biomass and nutrient dynamics and plant species diversity. The study was done on a northern mixed-grass prairie site at wind Cave National Park, South Dakota.Peak live plant biomass was greatest (190 g/m2) on the uncolonized part of the study area and least (95 g/m2) on a part of the prairie dog town colonized for 3 to 8 y. Peak live plant biomass (170 g/m2) of the oldest portion of the prairie dog town (colonized >26 y) was not significantly different from that of uncolonized prairie. However, where-as graminoids composed >85% of the total biomass of the latter area, forbs and dwarf shrubs (Artemisia frigida) were >95% of the total of the former. Both standing-dead plant biomass and litter declined markedly as time since colonization increased. Total plant species diversity (H) was greatest in the young prairie dog town (colonized for 3 to 8 y).Nitrogen concentration of plant shoots varied significantly as a function of time since colonization. Shoot-nitrogen was lowest in plants from the uncolonized site and greatest in plants collected from the longest-colonized areas of the prairie dog town. Shoot-nitrogen declined significantly over the growing season and tended to be higher in C3 graminoids than in C4 graminoids. In vitro digestible dry matter showed similar trends; the differences between C3 and C4 digestibilities were greatest during the last half of the growing season.We suggest that prairie dog-induced changes in plant biomass, plant species diversity, plant nutrient content, and forage digestibility may lead to further alterations of nutrient cycling and trophic dynamics in this mixed-grass prairie ecosystem.

Plant-herbivore interactions in a North American mixed-grass prairie : II. Responses of bison to modification of vegetation by prairie dogs.

Studies were conducted during the 1979 growing season to examine how North American bison (Bison bison) use prairie dog (Cynomys ludovicianus) colonies in Wind Cave National Park, South Dakota. Objectives included (1) determining whether bison selected for prairie dog towns parkwide; (2) characterizing in greater detail bison use patterns of a 36-ha colony in Pringle Valley as a function of time since prairie dog colonization; and (3) relating these bison use patterns to measured changes in structure and nutritional value of vegetation on and off the dog town.During midsummer, prairie dog towns were one of the most frequently used habitats by bison parkwide. Day-long observations at Pringle Valley revealed that bison exerted strong selection (nearly 90% of all habitat use and feeding time) for the dog town, which occupied only 39% of the valley. While there, they partitioned their use of the colony by grazing in moderately affected areas (occupied <8 years by prairie dogs) and by resting in the oldest area (>26 years occupation).Prairie dogs facilitate bison habitat selection for a shortgrass successional stage in this mixed-grass community by causing a broad array of compositional, structural, and nutritional changes in the vegetation.

Examination of North American bison saliva for potential plant growth regulators.

A series of laboratory bioassays were utilized to test for the presence of potential plant growth factors in saliva from a large native ungulate, the North American bison (Bison bison L.). Whole saliva enhancedAvena coleoptile growth at high pH, whether alone or in combination with indoleacetic acid (IAA). However, this enhancement was a result of salts in the saliva (primarily NaHCO3) rather than of other compounds acting hormonally, enhancing IAA activity, or inhibiting IAA oxidase activity as possibly occurs with some insect salivas. Additionally, the absence of detectable cytokinins in the saliva was indicated by its failure to enhance cucumber cotyledon expansion. This suggests that biochemical control of plant growth by salivary compounds following grazing is probably not an important component of this ruminant's interactions with its food plants, as has been suggested for some herbivores.

Relative growth rates and the grazing optimization hypothesis.

A mathematical analysis of the changes in plant relative growth rates necessary to increase aboveground production following grazing was conducted. The equation derived gives an isoline where production of a grazed and ungrazed plant will be the same. The equation has four variables (mean shoot relative growth rate, change in relative growth rate after grazing, grazing intensity, and recovery time) and may be analyzed graphically in a number of ways.Under certain conditions, small increases in shoot relative growth rate following grazing will lead to increased aboveground production. Under other conditions, very large increases in relative growth rate after grazing can occur without production being increased over that of ungrazed plants. Plants growing at nearly their maximum potential relative growth rate have little opportunity to respond positively to grazing and potentially can sustain less grazing than plants with growth rates far below maximum. Plants with high relative growth rates at the time of grazing require large increases in growth rate while slow growing plants require only small increases. High grazing intensities are least likely to increase production and high grazing frequencies require greater responses than infrequent grazing events.

Mammalian epidermal growth factor promotes plant growth.

Application of mouse submaxillary gland epidermal growth factor to young sorghum seedlings at low concentrations ( approximately 0.4-4 muM) increased shoot growth significantly over 3- and 6-day periods. The effects were dose dependent.

Plant-herbivore interactions: Examination of potential effects of bison saliva on regrowth of Bouteloua gracilis (H.B.K.) lag.

Laboratory experiments were performed to determine whether regrowth of blue grama was affected by potential growth-promoting substances in saliva of North American bison. We observed no statistically significant effects of foliar application of whole bison saliva on net photosynthesis (PN), root respiration (RR), allocation patterns of photosynthetically fixed 14C, or regrowth rates over a 10-day period following clipping to various heights. In a 10-week experiment, there were no significant effects of saliva on leaf, crown or root growth or tiller production in plants clipped to heights of 6, 4 or 2 cm above crowns. Similarly, nitrogen-stressed plants failed to show significant changes in growth rates or tillering in response to saliva over a 3-week period. Clipped blue grama plants did exhibit significant compensatory growth responses, including higher PN rates from 3-10 days following clipping and allocation of a higher proportion of current photosynthate to synthesis of new leaf tissue with increasing severity of defoliation. Nevertheless, unclipped plants invariably outproduced clipped plants following defoliation.

Net photosynthesis, root respiration, and regrowth of Bouteloua gracilis following simulated grazing.

Net photosynthesis (PN), root respiration (RR), and regrowth of Bouteloua gracilis (H.B.K.) Lag. were examined in the laboratory over a 10-day period following clipping to a 4-cm height to simulate grazing by large herbivores. Net photosynthesis rates of tissue remaining immediately following defoliation were only about 40% as great as preclipping rates. Three days after clipping, PN rates of defoliated plants had increased to values about 21% greater (per unit leaf area) than those of unclipped controls and remained at that level through Day 10. No statistically significant changes in RR occurred following defoliation. Biomass of unclipped plants nearly doubled during the 10-day study period, while that of defoliated plants increased 67%. Over half the new growth of defoliated plants was allocated to new leaf blades and only 18% to new roots, while only 33% of the new growth of control plants was allocated to new leaf blades but 29% went to new roots. As a consequence of increased PN rates and increased carbon allocation to synthesis of additional photosynthetic tissue following defoliation, net CO2 uptake per plant increased from 9% to 80% of that of the controls from Day 0 through Day 10.