PATTERNS OF INDUCED AND CONSTITUTIVE MONOTERPENE PRODUCTION IN CONIFER NEEDLES IN RELATION TO INSECT HERBIVORY

Studies were conducted to determine whether herbivore-induced synthesi s of monoterpenes occurs in the needles of ponderosa pine (Pinus ponde rosa Lawson), lodgepole pine (P. contorta Douglas var. latfolia Engelm ann), white fir (Abies concolor Lindl. and Gordon) and Engelmann spruc e [Picea engelmanii (Parry) Engelm.]. In the needles of all species ex cept Engelmann spruce, simulated herbivory significantly induced the a ctivity of monoterpene cyclases 4-8 days after wounding. In ponderosa pine, real herbivory by last-instar tiger moth larvae (Halisdota ingen s Hy. Edwards: Lepidoptera) induced a significantly larger response (4 .5-fold increase in monoterpene cyclase activity) than did simulated h erbivory (2.5-fold increase). To our knowledge, this is the first repo rt of herbivore-induced increases in monoterpene synthesis in needle t issue. Despite this increase in monoterpene synthesis, we observed no significant increase in total monoterpene pool size in wounded needles compared to controls. Large increases in the rate of monoterpene vola tilization were observed in response to wounding. We conclude that the volatile losses caused by tissue damage compensate for herbivore-indu ced monoterpene synthesis, resulting in no change in pool size. Tiger moth larvae consume ponderosa pine needles in a pattern that begins at the tip and proceeds downward to midway along the needle, at which po int they move to an undamaged needle. Constitutive monoterpene concent rations and monoterpene cyclase activities were highest in the lower h alf of ponderosa pine needles. The monoterpene profile also differed b etween the upper and lower needle halves, the lower half possessing an additional one to four monoterpene forms. We propose that the increas ing gradient in monoterpene concentrations and number of monoterpenes along the needle from tip to base deters feeding beyond the midway poi nt and provides time for the induction of increased cyclase activity a nd production of new monoterpenes. The induction of new monoterpene sy nthesis may have a role in replacing monoterpenes lost through damage- induced volatilization and preventing extreme compromise of the consti tutive defense system.