Changes in carbohydrate and enzyme levels during the sink to source transition of leaves of Cucumis sativus L., a stachyose translocator

The sink to source transition of expanding leaves of cucumber was studied by sampling leaves sequentially from the growing stem apex toward the base of the plant. Leaf fresh weight and photosynthetic rate increased in the progression to lower nodes. Sucrose and raffinose concentrations were higher in sink leaves than in source leaves. Galactinol and stachyose concentrations increased during leaf expansion. Both alkaline and acidic α-galactosidase activity declined during leaf expansion, whereas galactinol synthase activity increased abruptly in leaves beginning in the leaf at the fourth node below the apex. This increase in activity corresponded temporally to marked changes in the oligosaccharide composition of the leaves in favor of galactosyl-saccharides, particularly stachyose. Across all leaf positions, galactinol synthase correlated positively with galactosyl-oligosaccharide concentration (r = 0.90) and correlated negatively with sucrose concentration (r = −0.82). The ratio of stachyose to raffinose correlated positively with the ratio of galactinol synthase to α-galactosidase (r = 0.95). The results point to the importance of changes in enzyme levels per se as determinants of changes in soluble carbohydrate levels associated with acquisition of export capability during leaf expansion.