DIURNAL CYCLE OF EMISSION OF INDUCED VOLATILE TERPENOIDS HERBIVORE-INJURED COTTON PLANTS

Cotton plants attacked by herbivorous insect pests emit relatively lar ge amounts of characteristic volatile terpenoids that have been implic ated in the attraction of natural enemies of the herbivores. However, the composition of the blend of volatile terpenes released by the plan ts varies remarkably throughout the photoperiod. Some components are e mitted in at least 10-fold greater quantities during the photophase th an during the scotophase, whereas others are released continuously, wi thout conforming to a pattern, during the entire time that the plants are under herbivore attack. The diurnal pattern of emission of volatil e terpenoids was determined by collecting and analyzing the volatile c ompounds emitted by cotton plants subjected to feeding damage by beet armyworm larvae in situ. The damage was allowed to proceed for 3 days, and volatile emission was monitored continuously. During early stages of damage high levels of lipoxygenase-derived volatile compounds [e.g ., (Z)-3-hexenal, (Z)-3-hexenyl acetate] and several terpene hydrocarb ons [e.g., alpha-pinene, caryophyllene] were emitted. As damage procee ded, high levels of other terpenes, all acyclic [e.g., (E)-beta-ocimen e, (E)-beta-farnesene], were emitted in a pronounced diurnal fashion; maximal emissions occurred in the afternoon. These acyclic terpenes fo llowed this diurnal pattern of emission, even after removal of the cat erpillars, although emission was in somewhat smaller amounts. In contr ast, the emission of cyclic terpenes almost ceased after the caterpill ars were removed.