Photorespiratory Ammonia Does Not Inhibit Photosynthesis in Glutamate Synthase Mutants of Arabidopsis

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Abstract
Exposure of ferredoxin-dependent glutamate synthase (EC 1.4.7.1) mutants of Arabidopsis thaliana to photorespiratory conditions resulted in the accumulation of NH4+ and the inhibition of photosynthesis. However, upon transfer from 2% O2, 350 microliters per liter CO2, to 21% O2, 350 microliters per liter CO2, net photosynthesis declined at a slower rate in methionine sulfoximine treated leaf discs relative to controls. The recovery of photosynthesis was also more rapid in MSO-treated leaf discs although ammonia levels were more than threefold higher. Photosynthesis in leaf discs treated with azaserine was inhibited more than controls when transferred to 21% O2 and recovered less than controls when returned to 2% O2 although NH4+ levels were not significantly different. The results obtained are consistent with the view that the rapid inhibition of photosynthesis in the glutamate synthase mutants in photorespiratory conditions is not due to the accumulation of NH4+ but rather to the depletion of amino donors for glyoxylate and the consequent effects of glyoxylate on the lack of return of carbon to the chloroplast.