Monoterpene and herbivore-induced emissions from cabbage plants grown at elevated atmospheric CO2 concentration

The warming of the lower atmosphere due to elevating CO2 concentration may increase volatile organic compound (VOC) emissions from plants. Also, direct effects of elevated CO2 on plant secondary metabolism are expected to lead to increased VOC emissions due to allocation of excess carbon on secondary metabolites, of which many are volatile. We investigated how growing at doubled ambient CO2 concentration affects emissions from cabbage plants (Brassica oleracea subsp. capitata) damaged by either the leaf-chewing larvae of crucifer specialist diamondback moth (Plutella xylostella L.) or generalist Egyptian cotton leafworm (Spodoptera littoralis (Boisduval)). The emission from cabbage cv. Lennox grown in both CO2 concentrations, consisted mainly of monoterpenes (sabinene, limonene, alpha-thujene, 1,8-cineole, beta-pinene, myrcene, alpha-pinene and gamma-terpinene). (Z)-3-Hexenyl acetate, sesquiterpene (E, E)-alpha-farnesene and homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) were emitted mainly from herbivore-damaged plants. Plants grown at 720 mumol mol(-1) of CO2 had significantly lower total monoterpene emissions per shoot dry weight than plants grown at 360 mumol mol(-1) of CO2, while damage by both herbivores significantly increased the total monoterpene emissions compared to intact plants. (Z)-3-Hexenyl acetate, (E, E)-alpha-farnesene and DMNT emissions per shoot dry weight were not affected by the growth at elevated CO2. The emission of DMNT was significantly enhanced from plants damaged by the specialist P. xylostella compared to the plants damaged by the generalist S. littoralis. The relative proportions of total monoterpenes and total berbivore-induced compounds of total VOCs did not change due to the growth at elevated CO2, while insect damage increased significantly the proportion of induced compounds. The results suggest that VOC emissions that are induced by the leaf-chewing herbivores will not be influenced by elevated CO2 concentration. (C) 2003 Elsevier Ltd. All rights reserved.