Effect of Photoinhibitory Treatments on the Activity of Light-Activated Enzymes of C3 and C4 Photosynthetic Carbon Metabolism

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Abstract
Exposure of maize leaves to a 3-h photoinhibitory treatment (photon flux rate of 2000 .mu.einsteins m-2 s-1, CO2-free air) resulted in lower activities of the light-activated enzymes NADP malate dehydrogenase, pyruvate, Pi dikinase and ribulose-5-phosphate kinase. The activities could be recovered partially either by incubating enzyme extracts with dithiothreitol or by illuminating the treated leaf in air. Several enzymes which are not light-activated were not affected by the treatment. Ribulose-5-phosphate kinase activity was also reduced when bean [Phaseolus vulgaris cv. Hawkesbury Wonder] plants grown in low light were subjected to a similar photoinhibitory treatment. Although the reactivation of these enzymes may be correlated with the short-term increase of CO2 uptake capacity observed when photoinhibited leaves are returned to illumination in air, inactivation of these enzyes does not contribute significantly to the long-term in vivo expression of photoinhibition observed after 2-4 h. An example of partial inactivation of light-activated enzymes under illumination equivalent to full sunlight is provided.

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