Effect of L-Methionine Sulphoximine on the Products of Photosynthesis in Wheat (Triticum aestivum) Leaves

Abstract

Methionine sulphoximine, an inhibitor of glutamine synthetase, caused ammonia accumulation in detached wheat leaves. The rate was increased by increased oxygen in the atmosphere and by simultaneously supplying glycine or giving extra nitrate; it was decreased by isonicotinyl hydrazide. Ammonia production was light-dependent and continued at a constant rate in air for at least 2 h. Photosynthesis was progressively inhibited after the first hour; this inhibition was not because of increased stomatal resistance. Leaves supplied with 30 mol m⁻³ ammonium chloride, without methionine sulphoximine, accumulated more ammonia than leaves treated with the inhibitor but showed less inhibition of photosynthesis. The inhibitor decreased synthesis of [¹⁴C] amino acids from ¹⁴CO₂ in the light but increased the synthesis of [¹⁴C] malate and, relatively, the incorporation of ¹⁴C into sugar phosphates. In the absence of inhibitor, nitrate increased and ammonium ion decreased synthesis of malate. Methionine sulphoximine, by causing a shortage of amino acids, probably inhibited photosynthesis in part by decreasing the recycling of carbon from the photorespiratory cycle back to the Calvin cycle.