LIGHT AND TEMPERATURE-DEPENDENCE OF THE EMISSION OF CYCLIC AND ACYCLIC MONOTERPENES FROM HOLM OAK (QUERCUS-ILEX L.) LEAVES

In a laboratory study, we investigated the monoterpene emissions from Quercus ilex, an evergreen sclerophyllous Mediterranean oak species wh ose emissions are light dependent, We examined the light and temperatu re responses of individual monoterpenes emitted from leaves under vari ous conditions, the effect of heat stress on emissions, and the emissi on-onset during leaf development, Emission rate increased 10-fold duri ng leaf growth, with slight changes in the composition. At 30 degrees C and saturating light, the monoterpene emission rate from mature leav es averaged 4.1 nmol m(-2) s(-1), of which alpha-pinene, sabinene and beta-pinene accounted for 85%, The light dependence of emission was si milar for all monoterpenes: it resembled the light saturation curve of CO2 assimilation, although monoterpene emission continued in the dark . Temperature dependence differed among emitted compounds: most of the m exhibited an exponential increase up to 35 degrees C, a maximum at 4 2 degrees C, and a slight decline at higher temperatures. However, the two acyclic isomers cis-beta-ocimene and trans-beta-ocimene were hard ly detected below 35 degrees C, but their emission rates increased abo ve this temperature as the emission rates of other compounds fell, so that total emission of monoterpenes exponentially increased from 5 to 45 degrees C, The ratio between ocimene isomers and other compounds in creased with both absolute temperature and time of heat exposure. The light dependence of emission was insensitive to the temperature at whi ch it was measured, and vice versa the temperature dependence was inse nsitive to the light regime. The results demonstrated that none of the models currently applied to simulate isoprene or monoterpene emission s correctly predicts the short-term effects of light and temperature o n Q. ilex emissions, The percentage of fixed carbon lost immediately a s monoterpenes ranged between 0.1 and 6.0% depending on temperature, b ut rose up to 20% when leaves were continuously exposed to temperature s between 40 and 45 degrees C.