Inhibition of Photosynthesis in Barley with Decreased Levels of Chloroplastic Glutamine Synthetase Activity

Mutant barley plants containing only 8%, 16% or 38% of the wildtype level of glutamine synthetase activity have been isolated.The level of glutamine synthetase activity in the roots of themutant containing only 8% leaf activity was not affected bythis mutation. The plants accumulated high levels of ammoniain leaves exposed to air and although they were able to carryout photosynthetic CO₂fixation normally at low levels of atmosphericO₂, they were unable to maintain wild type rates of CO₂fixationin air. The extent of this inhibition and the extent to whichammonia accumulated in the leaves was dependent on the photonfluence rate intercepted by the plant. When leaves from themutant plant were fed glutamine under non-photorespiratory conditionsfor 40 min before they were transferred to air, the plants exhibitedwild type rates of CO₂ fixation in air but the ammonia contentof the leaves increased to an even higher level. At least inthe short term, therefore, ammonia accumulation was not responsiblefor the dramatic decline in the fixation rate of these mutantsin air. The most probable explanation is that as the supplyof potential amino donors diminished on transfer to air, therewas a restriction on the return of glycerate to the Calvin cyclewithin the chloroplast. Key words: Ammonia toxicity, photorespiration, photosynthesis, GS-deficient barley