Effect of Phosphlnothricin (Glufosinate) on Photosynthesis and Chlorophyll Fluorescence Emission by Barley Leaves Illuminated Under Photorespiratory and Non-Photorespiratory Conditions

Abstract

The effect of phosphinothricin (PPT), an inhibitor of glutamine synthetase, on several aspects of photosynthesis has been studied in primary leaves of barley (Hordeum vulgare L.). When photorespiration was suppressed, either by increasing CO₂ concentration to 0.7%, or by decreasing O₂ concentration to 1%, feeding the illuminated leaves with 0·5 or 1·0 mM PPT did not affect photosynthesis to a noticeable extent. Conversely, when PPT-fed leaves were illuminated in air, CO₂ uptake decreased continuously. Modification of the components of chlorophyll fluorescence quenching indicated increased reduction of Q_A, the primary acceptor of photosystem II, and increased chloroplast energization. Feeding of PPT to leaves illuminated in air increased the quantum requirement of photosynthesis and decreased photosynthetic rate of oxygen evolution in saturating [CO₂] and high light intensity. It is concluded that the effect of PPT on the photochemical processes is indirect, through the inhibition of CO₂ assimilation probably caused by the depletion of intermediates of the reductive pentose phosphate cycle.