A LONG-LASTING PHOTORESPIRATION IN CO2-FREE AIR, MEASURED AS THE POSTIRRADIATION CO2 BURST, INDICATES MOBILIZATION OF STORAGE PHOTOSYNTHATES

In CO2-free air, the CO2 postirradiation burst (PIE) in wheat leaves w as measured with an IRGA in an open gas exchange system to ascertain i ts potential role in alleviating photoinhibition of photorespiratory c arbon oxidation (PCO) under a CO2 deficiency. A pre-photosynthesized l eaf having been transferred into CO2-free air exhibited a typical CO2 PIE following darkening which could last, with a rate substantially hi gher than that of dark respiration, over a long time period (at least more than 2 h) of continuously alternate irradiation (2 min)-dark (2 m in)-light transitions. The rate and the time of PIE maintenance, altho ugh unaffected by the exogenous dark respiration inhibitor iodoacetic acid, were stimulated largely by increasing irradiance and O-2 level, and suppressed by DCMU and N-ethyl-maleimide (NEM). They also showed a large photosynthates-loading dependence. In a darkened leaf, the irra diation-induced PIE in the CO2-free air was clearly revealed and it wa s characterized by an initial net uptake of respiratory CO2. The light -induced PIE was accelerated by increasing irradiance, and delayed by prolonging the period of darkening the leaves. Hence, the origin of ca rbon needed for a long-term CO2 evolution in the CO2-free air might no t only be derived directly from the pool of intermediates in the Calvi n cycle, but it might also arise indirectly from a remotely fixed rese rve of photosynthates in the leaf via a PCO-mediated, yet to be furthe r clarified, mobilization process. Such mobilization of photosynthates probably exerted an important role in coordination of photochemical r eactions and carbon assimilation during photosynthesis in C-3 plants u nder the photoinhibitory conditions.