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

Abstract

Mutant barley plants containing only 8%, 16% or 38% of the wild type level of glutamine synthetase activity have been isolated. The level of glutamine synthetase activity in the roots of the mutant containing only 8% leaf activity was not affected by this mutation. The plants accumulated high levels of ammonia in leaves exposed to air and although they were able to carry out photosynthetic CO₂fixation normally at low levels of atmospheric O₂, they were unable to maintain wild type rates of CO₂fixation in air. The extent of this inhibition and the extent to which ammonia accumulated in the leaves was dependent on the photon fluence rate intercepted by the plant. When leaves from the mutant plant were fed glutamine under non-photorespiratory conditions for 40 min before they were transferred to air, the plants exhibited wild type rates of CO₂ fixation in air but the ammonia content of the leaves increased to an even higher level. At least in the short term, therefore, ammonia accumulation was not responsible for the dramatic decline in the fixation rate of these mutants in air. The most probable explanation is that as the supply of potential amino donors diminished on transfer to air, there was a restriction on the return of glycerate to the Calvin cycle within the chloroplast.