Stress herbivore

Nitrogen isotope ratios ( N/ " N) inerease from plants to herbivores to eami-vores and ean be used to estimate the degree of camivory in human diets. Some field studies observe a greater differenee in 5 N between trophie levels in dry, hot habitats than in wet, cool ones. Two hypotheses have been proposed to explain this variation in difference in 8 N between trophic levels. (1) Elevated excretion of -depleted urea in heat/water-stressed animals (2) recycling of nitrogen on protein-deficient diets. Both predict increased diet-tissue 8 N difference under stress. 

Protein stress and recycling of nitrogen could also have the opposite effect, however. If less N-depleted N is excreted as inea, then there should be less overall enrichment in the nitrogen available for tissue synthesis. Moreover, if urea itself is recycled for protein synthesis under protein stress, which often occurs in herbivores, then the diet-tissue difference should be smaller than in unstressed individuals because urea has a substantially lower 8 N value than the diet. 

These data do not provide strong support for the heat water stress/urea recycling model (Ambrose 1991). This model may be incorrect or inapplicable to rats. Were the experimental conditions inappropriate, with temperatures too low and/or protein levels too low or too high In the heat stress experiments that inspired this research animals were kept at a temperature of 40°C for 12 hours each day rather than 36°C in this study. In our heat and water stress experiments the protein content of the diets were set at 20% and 70%. These are relatively high levels compared to those in herbivore diets. It would be necessary to repeat the experiments with ruminant herbivores or lower protein diets to conclusively determine if rats are an inappropriate model. 

Leimu R, Koricheva J (2006) A meta-analysis of tradeoffs between plant tolerance and resistance to herbivores combining the evidence from ecological and agricultural studies. Oikos 112 1-9 Lesser MP (2006) Oxidative stress in marine environments biochemistry and physiological ecology. Annu Rev Physiol 68 253-278... 

The nutritional value of the diet influences the ability of herbivores to detoxify plant secondary metabolites (Schwass and Finley, 1985). For instance, voles in winter have to consume the bark of birch Betula sp.). This causes stress and leads to poor growth and high mortality among young animals. Birch bark contains phenolics and terpenoids. Both are metabolized by cytochrome P450 monooxidases in phase I and conjugated with glucuronic acid in phase II. 

We were also able to show that lupin leaves can increase their alkaloid content by a factor of 2-4 within a few hours after mechanical wounding (which could imitate a herbivoral attack) (29). This means that the defense system is able to respond to environmental stress. 

From the above examples it is thus apparent that a plant challenged by herbivores, by plant pathogens, or otherwise stressed by specific chemicals does not remain biochemically indifferent. The plant responds with more or less drastic metabolic changes, some of which have a profound effect on additional herbivores or other plant pathogens. The chemical... 

White, T.C.R., The abundance of invertebrate herbivores in relation to the availability of nitrogen in stressed food plants, Oecologia, 63, 90, 1984. 

Trindell, R.N., Stress induced intraspecific variation in macrophyte susceptibility to herbivores. Ph.D. thesis, University of North Carolina at Chapel Hill, Chapel Hill, NC, 1991. 

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