Ammonia Accumulation and Inhibition of Photosynthesis in Methionine Sulfoximine Treated Spinach

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Abstract
Ammonia accumulation and photosynthetic rate inhibition took place when spinach leaf tissue was supplied with methionine sulfoximine (MSO), an inhibitor of glutamine synthetase. This effect was observed in the absence of significant inorganic nitrogen reduction or an exogenous source of ammonia. Both the time lag prior to the initial photosynthetic rate decrease and the rate of that decrease depend on the O2 and MSO concentrations supplied to the leaf tissue. However, the total rate of ammonia accumulation was similar at both 20% and 2.2% O2. The decline in photosynthetic rate was not caused by stomatal closure but may be a result of ammonia toxicity. The data point out the importance of glutamine synthetase in preventing the poisoning of leaf metabolism by ammonia generated internally through processes not involved in net nitrogen assimilation. The rapidity of the action of MSO in suppressing photosynthesis was unexpected and should not be overlooked in interpreting data from other experiments involving that inhibitor. MSO shows promise as a tool for investigating C-N flow, particularly during photorespiration.