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

An open system associated with an infrared gas analyzer was employed to study transients in CO2 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 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 aVnd observed analyzer responses. In 21% 02, the decline in rate of CO2 evolution upon darkening followed first order kinetics. Initial rates of CO2 evolution following darkening were relatively independent of the prior ambient CO2 concentrations. However, rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO2 concentration at 21% 02. Under normal atmospheric conditions, photorespiration was 45 to 50% of the net CO2 fixation rate at 320C and high irradiance. The rapid initial CO2 evolution observed upon darkening at 21% 02 was absent in 3% 02. Rates of photorespiration under normal atmospheric concentrations of CO2 and 02 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 C3 leaf tissue. in a closed system, immediately after darkening (the PIB'). This rate of CO2 evolution declined rapidly with time. The principle of this assay is attractive because it minimizes the problem of refixation of photorespiratory CO2 during photosynthesis, can be conducted over a wide range of CO2 and 02 concentrations, and is technically and conceptually straightforward. A serious limitation of closed systems for measuring the CO2 released was that photorespiration was based on an estimate of the tangent to the initial increase in CO2 concentration versus time in the recirculated gas (3, 4). Even under ideal conditions, this approach would somewhat underestimate the rate. Also, both open and closed systems operate under nonsteady state conditions (14, 16, 19). The IRGA response model presented in the previous paper (16) enables estimation of transient rates of CO2 exchange under nonsteady state conditions characteristic of the early stage of the PIB. Most studies of the effects of environmental conditions on photorespiration indicate that increasing 02 concentrations or temperatures enhance this process (4, 7, 8, 21). Increasing C02 concentrations appear to diminish the importance of photorespiration when compared to net photosynthesis (1, 2, 4, 12, 13). In this paper, the IRGA response model presented previously (16) has been employed to reexamine these fundamental relationships and further verify the model. Observed responses of photosynthesis and photorespiration to C02, 02, and temperature in tobacco leaf discs provided data that are consistent with many earlier reports. Hence, the IRGA response model produces reliable results. Also, the experiments suggest that photorespiration accounts for about 50% of net photosynthesis under normal atmospheric conditions and high temperature and irradiance. In spite of considerable effort aimed at elucidating the physiology, identity of metabolic intermediates, and regulation of photorespiration over the past 20 years, much uncertainty remains concerning the magnitude of this important process in C3 leaves. Existing assays are indirect and those which are conducted in light certainly underestimate photorespiration due to refixation of the released CO2 before it can escape the leaf and be detected (24). Although of comparative value, some methods such as CO2 evolution into C02-free air are conducted under nonphysiological conditions. Other assays, such as the CO2 compensation point as an indicator of photorespiration, are ambiguous or insensitive (8). Inhibition of net CO2 uptake by 21% 02 versus 3% 02 has been used, and the results are largely due to increased photorespiration but also include other inhibitory effects of high 02 concentrations (7). Decker (3, 4) reported that preilluminated leaves of C3 plants displayed vigorous CO2 evolution, detectable by IR gas analysis MATERIALS AND METHODS These are described in the companion paper (16). Unless stated otherwise, all correlation coefficients reported here are significant at P < 0.01.