Estimation of Photorespiration Based on the Initial Rate of Postillumination CO2 Release

Abstract

An open system associated with an IR gas analyzer was employed to study transients in CO2 exchange generated on darkening preilluminated leaf discs of tobacco (Nicotiana tabacum cv. John Williams Broadleaf and cv. 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 CO2 exchange rate. A computer was used to evaluate parameters of the leaf CO2 release rate to provide an estimate of the initial rate of postillumination CO2 evolution and to produce maximal agreement between predicted and observed analyzer responses. In 21% O2, the decline in rate of CO2 evolution on darkening followed 1st order kinetics. Initial rates of CO2 evolution following darkening were relatively independent of the prior ambient CO2 concentrations. Rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO2 concentration at 21% O2. Under normal atmospheric conditions, photorespiration was 45-50% of the net CO2 fixation rate at 32.degree. C and high irradiance. The rapid initial CO2 evolution observed on darkening at 21% O2 was absent in 3% O2. Rates of photorespiration under normal atmospheric concentrations of CO2 and O2 as measured by the postillumination burst were highly dependent on temperature (observed activation energy = 30.1 kc/mol). The results are discussed with respect to previously published estimates of photorespiration in C3 leaf tissue.