Tomato Defenses Under Stress: The Impact of Salinity on Direct Defenses Against Insect Herbivores

Plant responses to multifactorial stress combinations are seldom studied, but combinations of abiotic and biotic stresses may cause drastic yield reductions in crops. While an abiotic stress, the salinization of agricultural soil, affects crop production, added pressures of insect herbivores could lead to further losses. In this paper, we investigate the effects of salinity on an insect herbivore, the corn earworm caterpillar ( Helicoverpa zea ), feeding on tomato ( Solanum lycopersicum cv. Better Boy) plants. We show that salt-stressed tomato plants are poor hosts for H. zea , impacting caterpillar growth rates, caterpillar feeding preference, and moth oviposition. We further show that these observations are best explained by reductions in both relative leaf water content and leaf total protein content, along with ionic toxicity and imbalance. We observe that salt stress does not influence anti-insect herbivory defense protein (PPO and TPI) levels. Finally, we observe that salt treatment leads to differences in specific volatiles, with lower emissions of 2-carene, α-phellandrene, β-phellandrene, α-humulene, and β-caryophyllene in salt-treated plants. We demonstrate that salt exposure changes tomato plant quality and chemical composition, which in turn negatively affects insect herbivores feeding on these plants.