Leaf development and phloem transport in Cucurbita pepo: Maturation of the minor veins.

Young leaves of Cucurbita pepo L. were examined by whole-leaf autoradiography and serial paradermal sections were examined by light microscopy to determine whether commencement of sugar export depends upon the minor vein phloem achieving structural maturity. Maturation of these veins develops progressively from the largest toward the smallest elements with the minor veins in the distal region of the leaf maturing before those in the proximal region. Commencement of sugar export is coincident with maturation of the abaxial phloem of the minor veins delimiting the areoles. The abaxial phloem elements of the larger minor veins, which are probably capable of vein loading too but border only relatively few areoles, mature before export starts. The adaxial phloem surrounding the areoles and the xylem elements, mature in advance of the abaxial phloem and well before the beginning of sugar export. It is therefore considered unlikely that structural development alone directly governs the initiation of export. The results suggest that some other rate controlling step is involved.