Changes in tolerance and resistance of a plant to insect herbivores under variable water availability

Water availability is an important factor that influences plant-insect interactions. While the influence of water limitation on plant resistance traits has received much attention, how water availability affects plant tolerance to herbivory is rarely tested. Here we show that lower water availability reduced tolerance capacity of tomato plants as measured by above ground regrowth and flower development after herbivory. In contrast to a reduced ability to tolerate herbivory, lower water availability increased the constitutive and induced levels of two defensive proteins, trypsin protease inhibitor and polyphenol oxidase, indicative of an increased investment on resistance under water limitation. The increase in defense proteins was paralleled with lower performance of a specialist caterpillar, Manduca sexta, and lower consumption of plant tissues. Although the performance of generalist, Helicoverpa zea, was unaffected by water availability, we observed a high mortality of H. zea that suggests strong resistance of tomato against H. zea. The findings revealed an unexpected case where water limitation decreases tolerance but increases resistance of a plant, suggesting a potential tradeoff between these strategies. This plasticity may benefit herbaceous plants by balancing growth and defense under variable water availability.