RELATIONSHIPS AMONG ISOPRENE EMISSION RATE, PHOTOSYNTHESIS, AND ISOPRENE SYNTHASE ACTIVITY AS INFLUENCED BY TEMPERATURE

Isoprene emissions from the leaves of velvet bean (Mucuna pruriens L. var utilis) plants exhibited temperature response patterns that were d ependent on the plant's growth temperature. Plants grown in a warm reg imen (34/28-degrees-C, day/night) exhibited a temperature optimum for emissions of 45-degrees-C, whereas those grown in a cooler regimen (26 /20-degrees-C, day/night) exhibited an optimum of 40-degrees-C. Severa l previous studies have provided evidence of a linkage between isopren e emissions and photosynthesis, and more recent studies have demonstra ted that isoprene emissions are linked to the activity of isoprene syn thase in plant leaves. To further explore this linkage within the cont ext of the temperature dependence of isoprene emissions, we determined the relative temperature dependencies of photosynthetic electron tran sport, CO2 assimilation, and isoprene synthase activity. When measured over a broad range of temperatures, the temperature dependence of iso prene emission rate was not closely correlated with either the electro n transport rate or the CO2 assimilation rate. The temperature optima for electron transport rate and CO2 assimilation rate were 5 to 10-deg rees-C lower than that for the isoprene emission rate. The dependence of isoprene emissions on photon flux density was also affected by meas urement temperature in a pattern independent of those exhibited for el ectron transport rate and CO2 assimilation rate. Thus, despite no chan ge in the electron transport rate or CO2 assimilation rate at 26 and 3 4-degrees-C, the isoprene emission rate changed markedly. The quantum yield of isoprene emissions was stimulated by a temperature increase f rom 26 to 34-degrees-C, whereas the quantum yield for CO2 assimilation was inhibited. In greenhouse-grown aspen leaves (Populus tremuloides Michaux.), the high temperature threshold for inhibition of isoprene e missions was closely correlated with the high temperature-induced decr ease in the in vitro activity of isoprene synthase. When taken togethe r, the results indicate that although there may be a linkage between i soprene emission rate and photosynthesis, the temperature dependence o f isoprene emission is not determined solely by the rates of CO2 assim ilation or electron transport. Rather, we propose that regulation is a ccomplished primarily through the enzyme isoprene synthase.