ANATOMICAL AND ULTRASTRUCTURAL-CHANGES ASSOCIATED WITH SINK-TO-SOURCETRANSITION IN DEVELOPING MAIZE LEAVES

We studied the anatomical and ultrastructural changes accompanying sin k-to-source transition in developing maize (Zea mays L. cv. W273) leav es, in which sink, transition, and source regions had been identified by whole-leaf autoradiography. In the leaves examined, a complete stru ctural gradient existed from nonimporting to importing regions of the blade. Although all components, except metaxylem elements, of the larg e (transport) bundles reach maturity before their counterparts in inte rmediate and small (loading) bundles, the final events in structural m aturation are uniform for all bundle types across the blade. Among the very last structures to mature are the plasmodesmata at the interface s between mesophyll cells and between mesophyll cells and bundle sheat h cells. Maturation of the plasmodesmata coincides with maturation of the thick-walled sieve tubes, the last components of the vascular bund les to mature. Significantly, the vasculature reaches structural matur ity in advance of cessation of import, and maturation of bundles invol ved with phloem loading is not closely correlated with initiation of e xport from the blade. Deposition of suberin lamellae in the walls of t he bundle sheath cells coincides with the deposition of secondary wall s in the metaxylem vessels. It is suggested that a primary role of the suberin lamellae may be to prevent leakage of sucrose from the bundle s.