Drought stress in tomatoes: Changes in plant chemistry and potential nonlinear consequences for insect herbivores

Insect herbivores respond variably to drought-stressed plants; in some cases herbivore performance (e.g. fecundity, survival) is enhanced, in others it is negatively affected. We used tomato plants to examine the consequences of drought stress intensity on plant chemistry potentially important to insect herbivores in order to gain insights into resolving these conflicting results. In a series of experiments conducted out-of-doors, we measured the concentration of five chemicals previously shown to act as growth reducers against herbivores (2 oxidative enzymes, 2 phenolics, proteinase inhibitor) in control plants and drought-stressed plants over a period of about 14 d. Three of the five defenses increased in response to increasing drought (polyphenol oxidase, rutin, chlorogenic acid), one showed no response (peroxidase) and one was inconsistent in its response (proteinase inhibitor). Simultaneously, we measured three chemicals thought to be important nutrients for insect herbivores (water content, total nitrogen, and soluble carbohydrates). Water content declined with increasing drought stress intensity, total nitrogen increased at severe stress in one experiment and did not respond in a second, and soluble carbohydrates showed a convex response to drought stress. In one experiment we also fed leaf tissue, incorporated into non-nutritive agar, from control and drought-stressed plants to neonate Spodoptera exigua larvae. Growth of these larvae over a 7-d period was negatively affected by drought stress while survival was not significantly affected. This study represents one of the first to assess both chemical defenses and nutrients in plants subjected to drought as it develops from slight stress to a very severe condition. Depending on the direction and shape of response curves for nutrients and defenses, many possible effects on herbivores are predicted, some of them complex and nonlinear. By studying the particular kinetics of defenses and nutrients in plants we may be in a better position to understand why insect herbivores show such variable responses to drought-stressed plants.