Leaf development and phloem transport in Cucurbita pepo: Transition from import to export.

The capacity of a growing leaf blade of Cucurbita pepo L. to import (14)C-labelled photoassimilate is lost in a basipetal direction. Import into the lamina tip stops when the blade is 10% expanded. Development of the leaf progresses linearly with time and the lamina base stops importing when the blade is 45% expanded. Export capacity also develops basipetally and follows immediately the loss of import capacity, at least in the lamina base. The small amount of material initially exported from the leaf tip is redistributed to the still-importing leaf base, delaying export from the lamina until the blade is 35% expanded. Loss of import capacity by the petiole is both basipetal and dorsoventral. The proximal, adaxial portion of the petiole is the last region to cease importing (14)C. Leaves of Beta vulgaris L. and Nicotiana tabacum L. also lose import capacity in a basipetal direction.