ISOPRENE SYNTHASE - FROM BIOCHEMICAL-MECHANISM TO EMISSION ALGORITHM

Interest in modeling isoprene emission from vegetation has led to the development and widespread use of an emission algorithm (referred to h ere as the G93 algorithm) that estimates instantaneous leaf isoprene e mission as a function of basal isoprene emission rate with corrections for the effects of light and temperature. Here we address the questio n: Does the G93 algorithm reflect the biochemical and physiological pr ocesses responsible for leaf isoprene emission? In particular, the act ivity of the enzyme isoprene synthase, thought to be responsible for l eaf isoprene emission, is examined. Recent findings show that isoprene synthase activity in willow leaves is located in chloroplasts, where both soluble and thylakoid-bound forms of the enzyme occur. Short-term changes in the activity of the isoprene synthases in response to temp erature are consistent with the temperature correction term of the G93 algorithm. The light dependence term of the algorithm, which predicts light saturation of leaf isoprene emission, is consistent with the pl astidic location of the isoprene synthases and light-driven processes responsible for activation of enzymes. However, the complete mechanism for the light activation of isoprene synthase(s) has not been determi ned. Long-term changes in the amount of active isoprene synthase(s) co rrelate with the variations in basal isoprene emission rate seen as le aves mature and adapt to differences in leaf growth environment. Despi te these correlations, recent reports that the G93 algorithm can be us ed to model light-dependent formic acid, 2-methyl-3-buten-2-ol and mon oterpene emissions raise questions about the algorithm's specificity a nd underlying assumptions.