Photoinhibition of Intact Attached Leaves of C 3 Plants Illuminated in the Absence of Both Carbon Dioxide and of Photorespiration

When leaflets of bean and leaves of other species of C(3) plants are illuminated in the absence of CO(2) and at low O(2) partial pressure, the capacity for CO(2) assimilation at saturating light and its efficiency at low light intensities are inhibited. This photoinhibition is dependent on leaflet age and period of illumination. In young leaflets and following short exposure to these photoinhibitory conditions, some recovery of CO(2) assimilation capacity is observed immediately after treatment. Following substantial (70 to 80%) photoinhibition of CO(2) assimilation, recovery in fully expanded leaflets is observed only after 48 hours in normal air. The photoinhibition is largely prevented by providing CO(2) at partial pressures equivalent to the CO(2) compensation point, or by >210 millibars O(2) which permits internal CO(2) production by photorespiration. If leaflets are illuminated in 60 microbars CO(2) and 210 millibars O(2) (the CO(2) compensation point in air), no photoinhibition is observed. Electron transport processes and fluorescence emission associated with photosystem II are inhibited in chloroplast thylakoids isolated from leaflets after illumination in zero CO(2) and 10 millibars O(2). These studies support the hypothesis that CO(2) recycling through photorespiration is one means of effectively dissipating excess photochemical energy when CO(2) supply to illuminated leaves is limited.