THE CELLULAR PATHWAY OF PHOTOSYNTHATE TRANSFER IN THE DEVELOPING WHEAT-GRAIN .1. DELINEATION OF A POTENTIAL TRANSFER PATHWAY USING FLUORESCENT DYES

A potential cellular pathway for photosynthate transfer between the cr ease phloem and the starchy endosperm of the developing wheat grain ha s been delineated using fluorescent dyes. Membrane permeable and imper meable dyes have been introduced into the grain through the crease phl oem, the endosperm cavity or the dorsal surface of the starchy endospe rm. The movement of the symplastic tracer 5-(6)-6-carboxyfluorescein ( CF) derived from 5-(6)-6-carboxyfluorescein diacetate (CFDA), from eit her direction between the crease phloem and the endosperm cavity, indi cated that the symplastic pathway was operative from the crease phloem to the nucellar projection. Furthermore, the inward movement of apopl astic tracer trisodium, 3-hydroxy-5,8,10-pyrentrisulphonate (PTS) from the endosperm cavity and that of CF following plasmolysis showed that there was a high resistance to solute transfer within the apoplast of the pigment strand. All dyes entered the modified aleurone and adjace nt subaleurone bordering the endosperm cavity. Subsequent movement of the symplastic tracers CF and sulphorhodamine G (SRG) into and through the endosperm was rapid. However, the movement of apoplastic tracers PTS and Calcofluor White (CFW) was relatively slow and with tissue pla smolysis, CF was confined to the cytoplasm of the modified aleurone an d subaleurone cells. Together, these results demonstrate that there is a high resistance to solute movement within the apoplast of the cells bordering the endosperm cavity. We propose that photosynthate transfe r is via the symplast to the nucellar projection where membrane exchan ge to the endosperm cavity occurs. Uptake from the cavity is by the mo dified aleurone and small endosperm cells prior to transfer through th e symplast to and through the starchy endosperm.