Anatomical and ultrastructural changes associated with sink-to-source transition in developing maize leaves

We studied the anatomical and ultrastructural changes accompanying sink-to-source transition in developing maize (Zea mays L. cv. W273) leaves, in which sink, transition, and source regions had been identified by whole-leaf autoradiography. In the leaves examined, a complete structural gradient existed from nonimporting to importing regions of the blade. Although all components, except metaxylem elements, of the large (transport) bundles reach maturity before their counterparts in intermediate and small (loading) bundles, the final events in structural maturation are uniform for all bundle types across the blade. Among the very last structures to mature are the plasmodesmata at the interfaces between mesophyll cells and between mesophyll cells and bundle sheath cells. Maturation of the plasmodesmata coincides with maturation of the thick-walled sieve tubes, the last components of the vascular bundles to mature. Significantly, the vasculature reaches structural maturity in advance of cessation of import, and maturation of bundles involved with phloem loading is not closely correlated with initiation of export from the blade. Deposition of suberin lamellae in the walls of the bundle sheath cells coincides with the deposition of secondary walls in the metaxylem vessels. It is suggested that a primary role of the suberin lamellae may be to prevent leakage of sucrose from the bundles.