Influenced by Temperature1

Isoprene emissions from the leaves of velvet bean (Mucuna pruriens L. var utilis) plants exhibited temperature response pattems that were dependent on the plant's growth temperature. Plants grown in a warm regimen (34/280C, day/night) exhibited a temperature optimum for emissions of 450C, whereas those grown in a cooler regimen (26/200C, day/night) exhibited an optimum of 40°C. Several previous studies have provided evidence of a linkage between isoprene emissions and photosynthesis, and more recent studies have demonstrated that isoprene emissions are linked to the activity of isoprene synthase in plant leaves. To further explore this linkage within the context of the temperature dependence of isoprene emissions, we determined the relative temperature dependencies of photosynthetic electron transport, CO2 assimilation, and isoprene synthase activity. When measured over a broad range of temperatures, the temperature dependence of isoprene emission rate was not closely correlated with either the electron transport rate or the CO2 assimilation rate. The temperature optima for electron transport rate and CO2 assimilation rate were 5 to 10°C lower than that for the isoprene emission rate. The dependence of isoprene emissions on photon flux density was also affected by measurement temperature in a pattem independent of those exhibited for electron transport rate and CO2 assimilation rate. Thus, despite no change in the electron transport rate or CO2 assimilation rate at 26 and 340C, the isoprene emission rate changed markedly. The quantum yield of isoprene emissions was stimulated by a temperature increase from 26 to 340C, 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 emissions was closely correlated with the high temperature-induced decrease in the in vitro activity of isoprene synthase. When taken together, the results indicate that although there may be a linkage between isoprene emission rate and photosynthesis, the temperature dependence of isoprene emission is not determined solely by the rates of CO2 assimilation or electron transport. Rather, we propose that regulation is accomplished primarily through the enzyme isoprene synthase. ' This material is based upon work supported by the National Science Foundation under grant ATM 9007849, the Environmental Protection Agency under grant R-815995-01-0, and the United States Department of Agriculture, Competitive Research Grants Office under award 90-37130-5422. Isoprene (2-methyl1,3-butadiene) is emitted from the leaves of numerous plant species in quantities that have an important impact on the oxidation potential of the troposphere (3, 8, 11, 20). Past studies have identified a linkage between isoprene emission rate and photosynthesis rate (9, 10, 13, 14). Although several details of the mechanism(s) underlying this linkage are unknown, there is good evidence that isoprene emission rate is determined, at least to some extent, by the availability of reduced carbon to support the synthesis of isoprenoid skeletons and the availability of chloroplastic ATP to support the energetic needs ofthe mevalonic acid pathway (16). There has been additional speculation that the rate of photosynthetic electron transport has a role in determining the isoprene emission rate (1 1), a view supported by the NADPH requirement of the mevalonic acid pathway. Evidence against a link between isoprene emission rate and photosynthetic electron transport rate was provided in a recent analysis of the responses of CO2 assimilation, isoprene emissions, and Chl fluorescence to incident photon flux density and intercellular CO2 partial pressure (9). However, to date there are such scant data describing the potential relationship of isoprene emission rate to electron transport rate that any broad conclusions must remain tentative. An additional factor to consider in the regulation of isoprene emissions and its linkage to photosynthesis is the role of the enzyme isoprene synthase, which was recently reported in plant tissues and appears to underlie the conversion of DMAPP2 to isoprene (17). With some knowledge of the linkages between photosynthesis and isoprene emission, it has become easier to explain the responses of isoprene emission rate to changes in environmental variables such as photon flux density and C02/O2 concentrations (1 1). One environmental variable that is known to have a profound influence on isoprene emission rate, but has not yet been studied in terms of the mechanistic linkages, is leaf temperature. Isoprene emission rate exhibits a Qlo of 2 or higher at temperatures between 30 and 40°C, and decreases at temperatures above 40 to 45°C (6, 10, 19). The increase in isoprene emission rate at temperatures be2Abbreviations: DMAPP, dimethylallyl diphosphate; F., initial fluorescence; qN, nonphotochemical fluorescence quenching; r*, CO2 compensation point in absence of mitochondrial respiration.