Estimation of Photorespiration Based on the Initial Rate of Postillumination CO(2) Release: II. Effects of O(2), CO(2), and Temperature.
An open system associated with an infrared gas analyzer was employed to study transients in CO(2) exchange generated upon darkening preilluminated leaf discs of tobacco (Nicotiana tabacum vars John Williams Broadleaf and Havana Seed). An empirical formula presented previously enabled prediction of the analyzer response under nonsteady state conditions as a function of time and of the leaf CO(2) exchange rate. A computer was used to evaluate parameters of the leaf CO(2) release rate to provide an estimate of the initial rate of postillumination CO(2) evolution and to produce maximal agreement between predicted and observed analyzer responses. In 21% O(2), the decline in rate of CO(2) evolution upon darkening followed first order kinetics. Initial rates of CO(2) evolution following darkening were relatively independent of the prior ambient CO(2) concentrations. However, rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO(2) concentration at 21% O(2). Under normal atmospheric conditions, photorespiration was 45 to 50% of the net CO(2) fixation rate at 32 degrees C and high irradiance. The rapid initial CO(2) evolution observed upon darkening at 21% O(2) was absent in 3% O(2). Rates of photorespiration under normal atmospheric concentrations of CO(2) and O(2) as measured by the postillumination burst were highly dependent upon temperature (observed activation energy = 30.1 kilocalories per mole). The results are discussed with respect to previously published estimates of photorespiration in C(3) leaf tissue.
Peterson
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RB
Department of Biochemistry and Genetics, The Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504.