Soil salinity effects on inter-plant signalling in wild cotton are contingent on herbivore load and differentially shape direct and indirect defences.

Abstract

<h4>Main conclusion</h4>Inter-plant signalling effects on cotton defences are shaped by the joint effects of emitter herbivore load and soil salinity stress, with latter strengthening such effects on direct defences but weakening those on indirect defences. Volatile-mediated plant-to-plant signalling in response to herbivory is well documented, but its contingency on abiotic factors and variation in herbivore load is poorly understood. To address this gap, we investigated how soil salinity levels and herbivore load may influence signalling effects on plant defences and resistance in wild cotton (Gossypium hirsutum), using the specialist herbivore Alabama argillacea (cotton leafworm). We conducted a greenhouse experiment where plants were grouped in triplets: one acting as the emitter and the other two as receivers. Each triplet was placed inside a mesh cage. Both emitters and receivers were exposed to either ambient (control) or augmented soil salinity. Emitters at each salinity level underwent three treatments: no damage, damage by one or three caterpillars. After 48 h of damage, we collected VOCs from the emitters, which were removed from the cages at 72 h. Afterwards, we damaged the receiver plants to test for the priming effects on the induction of extrafloral nectar (EFN) and terpenoid aldehyde content, as well as on resistance to herbivory. Herbivory increased VOC emissions significantly under high herbivory load, but salinization reduced this effect. Receivers exposed to VOC from plants with high herbivore load induced a greater EFN concentration compared to those exposed to control emitters, an effect that disappeared under augmented salinity. Soil salinity did not affect the signalling effect on terpenoid aldehyde content but did influence herbivore resistance. These findings suggest that signalling depends on herbivore load and soil salinity, with the latter enhancing or affecting the induction depending on the plant defense traits.