Changes in Enzymes of Sucrose Metabolism and the Activation of Certain Chloroplast Enzymes During Wheat Leaf Senescence

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Abstract
Relatively little change was observed in the partitioning of photosynthetically fixed carbon between sucrose and starch during senescence of vegetative leaves of wheat (Triticum aestivum L. cv. Roy). The maximum activities of sucrose phosphate synthase and cytoplasmic fructose-1, 6-bisphosphatase in leaf extracts approximated photosynthetic sucrose formation in leaf segments and the three parameters declined together with age. Changes in activities of sucrose synthase and uridine-5'-diphosphatase, and in leaf phosphate content did not parallel changes in sucrose formation with age. Chloroplast fructose-1, 6-bisphosphatase activity, activated in vivo with light and in vitro with dithiothreitol, declined continually throughout senescence of the second vegetative leaf. The ability of light to activate chloroplast fructose-1, 6-bisphosphatase did not, however, appear to change with age. Ribulose-l, 5-bisphosphate carboxylase activity also declined continually with age, but in vivo-activated ribulose-l, 5-bisphosphate carboxylase was a minimum of three fold greater than needed to account for photosynthetic activity. The percentage of ribulose-l, 5-bisphosphate carboxylase activated in vivo was essentially constant during senescence. In general, activities of soluble chloroplast enzymes declined continually from the point of full leaf expansion throughout senescence, whereas the activities of cytoplasmic enzymes that were studied remained relatively constant until the latter stages of senescence.