Induction by caterpillars of stored and emitted volatiles among populations of wild cotton (Gossypium hirsutum)

Upland cotton (Gossypium hirsutum) plants constitutively store volatile terpenes in their leaves, which are steadily emitted at low levels. Herbivory leads to a greater release of these stored volatiles. Additionally, damaged plants increase terpene accumulation and emit other terpenes and additional volatiles. This has been well characterised for cultivated cotton, but little is known about volatile emissions in wild cotton. We investigated how herbivory by the beet armyworm (Spodoptera exigua) affects both leaf-stored and emitted volatiles in wild cotton plants from populations found along the Yucatan coast and assessed different scales of intraspecific variation in induced volatiles. Furthermore, we examined how leaf-terpene chemotypes differ in volatile emissions from intact and damaged plants. Herbivory triggered changes in stored and emitted volatiles in a manner similar to what is known for cultivated cotton. However, we report for the first time the emission of volatile aldoximes by cotton plants, which were only detected in the headspace upon herbivory, and displayed interpopulation variation. Intraspecific variation was also observed in the induced emissions of nitriles and some terpenes. In addition, chemotypes differed in their induction of the (E)-beta-ocimene in leaves. Distinctions between terpene chemotypes were reflected in the emissions from both intact and damaged plants, suggesting differences in how dominant terpenes of each chemotype might affect airborne interactions. However, these variable terpenes showed weak inducibility, raising questions about their functions. This comprehensive insight into herbivore-induced volatiles in wild cotton may help in the development of future pest-management strategies for cotton crops.