Relationship between Inhibition of CO₂Fixation and Glutamine Synthetase Inactivation inLemna gibbaL. Treated with L-Methionine-D,L-Sulphoximine (MSO)

The rate of net CO₂ fixation in Lemna gibba L. was decreased to 50% by 100–150 min incubation in the presence of 0⋅5 mol m⁻³ L-methionine-D,L-Sulphoximine (MSO), an irreversible inhibitor of glutasnine synthetase (GS). The pattern of inhibition was similar in both 21% O₂ and 2% O₂. The inhibition was accompanied by increased intracellular $NH4+$ levels. Incubation with 10 mol m⁻³$NH4+$ under the same conditions, but without MSO, resulted in even higher $NH4+$ levels but the rate of CO₂ fixation was unaffected. Additions of glutamine, glutamate, glycine or serine delayed the MSO-induced inhibition of CO₂ fixation. The same amino acids delayed the inactivation of GS by MSO. Thus inhibition of CO₂ fixation by MSO in Lemna is neither caused by elevated $NH4+$ levels nor closely related to photorespiration. Possibly, MSO causes shortage of amino-N for maintenance of the functional integrity of the photosynthetic apparatus.