Effects of the glutamine synthetase inhibitor methionine sulfoximine on CO2 fixation inLemna gibba

Summary Net CO₂ fixation inLemna gibba L. was inhibited by 0.5 mM L-methionine D,L-sulfoximine (MSX) both under photorespiratory conditions (21% O₂) and in 2% O₂. The inhibition was noticeably delayed by addition of 5 mM glutamine. Glutamine also delayed MSX-induced inactivation of glutamine synthetase. An increase in intracellular NH ₄ ⁺ concentration was noted in the presence of MSX only, and in the presence of 10 mM NH ₄ ⁺ only. However, presence of 10 mM NH ₄ ⁺ did not cause any inhibition of CO₂ fixation.