REPAIR MECHANISMS FOLLOWING PHOTOINHIBITION IN CITRUS

The repair mechanisms seemed to depend, in terms of their reaction and acceleration, on leaf age. Mature leaves, which had developed in the previous sunny autumn, were rich in chlorophyll and starch and responded to photoinhibition within a week and recovered quicker from photoinhibition, but to a lesser extent, than young leaves. Young leaves, which had developed during the winter in the greenhouse, were short in chlorophyll and starch but rich in glucose. They were also photo-inhibited within a week. However, they showed delayed recovery from photoinhibition, but to a greater extent than the mature leaves with a recovery of their photosynthesis of up to 170%. The first mechanism results in rapid de-novo synthesis of the granal D1 protein of photosystem II (PS II) after his initial degradation during photoinhibition. In the second mechanism, violaxanthin in the xanthophyll cycle scavenges excess energy, thereby protection the chlorophyll from oxidation and degradation. The research project has shown an increased (over-compensated) photosynthesis of the young Citrus lemon leaves after photoinhibition. This was probably due to glucose-enhanced expression of the psb A2 gene which resulted in an efficient D1 protein repair and accelerated D1 protein turnover rate after photoinhibition.

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