Ribulose Bisphosphate Carboxylase and Proteolytic Activity in Wheat Leaves from Anthesis through Senescence

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Abstract
Changes in ribulose bisphosphate carboxylase (RuBPCase) and proteolytic activity were followed in the flag leaf and 2nd leaf of field-grown winter wheat (cv. Arthur). These changes were followed in relation to changes in leaf chlorophyll, protein, and photosynthesis, and seed development. Levels of RuBPCase were determined by rocket immunoelectrophoresis as described previously (Wittenbach). RuBPCase constituted 40-45% of the total soluble protein in the flag leaf and an even higher percentage of the soluble protein in the 2nd leaf. This ratio remained unchanged until senescence when RuBPCase protein was apparently lost at a faster rate than total soluble protein. No change in the specific activity of RuBPCase on either a mg protein or RuBPCase basis was observed until senescence. A close correlation existed among the various indices of senescence in the field, namely, the decline in chlorophyll, protein, photosynthesis and RuBPCase activity. In addition, proteinase activity increased with the onset of senescence. These enzymes readily degraded RuBPCase, exhibiting a pH optimum of 4.8-5.0 and a temperature optimum of 50 C. Proteinase activity was modified by sulfydryl reagents suggesting the presence of sulfydryl groups at or near the active sites.