Ontogenetic Variation in Plant-to-Plant Signaling Effects on Direct and Indirect Defenses in Wild Cotton (Gossypium hirsutum).

Various plant-related factors have been shown to influence volatile organic compound (VOC)-mediated plant-to-plant interactions, but effects of plant ontogeny have been rarely considered. Here, we tested if these interactions differ between two ontogenetic stages in wild cotton, Gossypium hirsutum. We used 2.5- and 4.5-month-old plants (seedlings and juveniles, respectively), arranged in triplets where one plant acted as an emitter and two as receivers (all plants within a cage were of the same developmental stage). Half of the emitters were subjected to herbivory by Alabama argillacea caterpillars, while the emitters of the other triplets were left unharmed. After 48 h of VOC exposure, we collected the VOCs from a subset of the emitter plants (for 2 h), and one receiver from each cage was removed to measure extrafloral nectar (EFN) production and to collect leaves for total phenolic analyses. The remaining receivers were subjected to herbivory to assess signaling effects on defense induction. Emitter seedlings released larger quantities of VOCs in response to caterpillar damage than juveniles, and ontogenetic stages also differed in the composition of the emitted VOC blend. Accordingly, we found trends for ontogenetic variation in signaling effects: seedling receivers exposed to damaged emitters tended to produce more EFN both prior to and following damage compared to those exposed to undamaged emitters, whereas juvenile receivers showed no EFN response to emitter damage. Similarly, seedling receivers exposed to damaged emitters exhibited higher phenolic concentrations, whereas no such signaling effect was observed in juveniles. These findings indicate that VOC-mediated signaling of defenses is strong in young cotton plants but weakens during development in pre-reproductive juvenile plants, and that this is reflected in both direct and indirect defense responses.