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