PHOTORESPIRATION IS ESSENTIAL FOR THE PROTECTION OF THE PHOTOSYNTHETIC APPARATUS OF C3 PLANTS AGAINST PHOTOINACTIVATION UNDER SUNLIGHT

CO2 assimilation, transpiration and modulated chlorophyll fluorescence of leaves of Chenopodium bonus-henricus (L.) were measured in the lab oratory and, at a high altitude location, in the field. Direct calibra tion of chlorophyll fluorescence parameters against carbon assimilatio n in the presence of 1 or 0.5% oxygen (plus CO2) proved necessary to c alculate electron transport under photorespiratory conditions in indiv idual experiments. Even when stomata were open in the field, total ele ctron transport was two to three times higher in sunlight than indicat ed by net carbon gain. It decreased when stomata were blocked by subme rging leaves under water or by forcing them to close in air by cutting the petiole. Even under these conditions, electron transport behind c losed stomata approached 10 nmol electrons m(-2) leaf area s(-1) at te mperatures between 25 and 30 degrees C. No photoinactivation of photos ystem II was indicated by fluorescence analysis after a day's exposure to full sunlight. Only when leaves were submerged in ice was apprecia ble photoinactivation noticeable after 4 h exposure to sunlight. Even then almost full recovery occurred overnight. Electron transport behin d blocked stomata was much decreased when leaves were darkened for 70 min (in order to deactivate light-regulated enzymes of the Calvin cycl e) before exposure to full sunlight. Brief exposure of leaves to HCN ( to inhibit photoassimilation and photorespiration) also decreased elec tron transport drastically compared to electron transport in unpoisone d leaves with blocked stomata. Non-photochemical fluorescence quenchin g and reduction of Q(A), the primary electron acceptor of photosystem II was increased by HCN-poisoning. Very similar observations were made when glyceraldehyde was used instead of HCN to inhibit photosynthesis and photorespiration. In HCN-poisoned leaves, residual electron trans port increased linearly with temperature and showed early light satura tion revealing characteristics of the Mehler reaction. During short ex posure of these leaves to photon flux densities equivalent to 25% of s unlight, no or only little photoinactivation of photosystem II was obs erved. However, prolonged exposure to sunlight caused inactivation eve n though non-photochemical quenching of chlorophyll fluorescence was e xtensive. Simultaneously, oxidation of cellular ascorbate and glutathi one increased. Inactivation of photosystem II was reversible in dim li ght and in the dark only after short times of exposure to sunlight. Gl yceraldehyde was very similar to HCN in increasing the sensitivity of photosystem II in leaves to sunlight. We conclude from the observation s that the electron transport permitted by the interplay of photoassim ilatory and photorespiratory electron transport is essential to preven t the photoinactivation of photosynthetic electron transport. The Mehl er and Asada reactions, which give rise to strong nonphotochemical flu orescence quenching, are insufficient to protect the chloroplast elect ron transport chain against photoinactivation.