The relationship between the activity and the activation state of RuBP carboxylase and carbon exchange rate as affected by sink and developmental changes.

The purpose of this study was to explore if sink manipulations which affect leaf carbon exchange rate (CER) are mediated by ribulose 1,5-bisphosphate (RuBP) carboxylase activity. Tomato leaf (Lycopersicon esculentum Mill. cv. Vendor) RuBP carboxylase was assayed using a rapid extraction method. Over a diurnal period, leaf CER fluctuated independent of carboxylase activity. Differences in leaf CER induced by fruit pruning in one leaf-one cluster plants were not accompanied by changes in carboxylase activity.During leaf expansion, carboxylase activity and percent enzyme in the active form paralleled the increase and then decrease in leaf carbon exchange rate. Differences in leaf CER induced by root warming at ambient air temperature, were accompanied by parallel changes in carboxylase activity.These results suggest that modifications in leaf CER are not mediated exclusively through changes in carboxylase activity, but rather that modifications in carboxylase activity coincide with overall changes in leaf physiology and morphology in response to sink demand.