Developing seeds of cereals and grain legumes have proven to be useful experimental models to examine post-sieve element assimilate transport in sink tissues. Morphologically, these seeds offer well-defined sinks in which the processes of sucrose import plus efflux and influx plus metabolism may be examined independently. In all cases, sucrose is delivered through the phloem to the maternal seed tissues. Unloading from the sieve element-companion cell complexes is symplastic. Subsequently, sucrose moves through a symplastic route to cells responsible for sucrose efflux to the seed apoplast. The efflux cells are located at, or near, the maternal/filial interface. Sucrose is retrieved from the seed apoplast by the outermost cell layers of the filial tissues. Subsequent transfer of sucrose to the sites of storage in the filial tissues is confined principally to a symplastic route. Sucrose efflux from the maternal tissues appears to be passive in cereals and energy dependent in grain legumes, possibly through a sucrose/proton antiport system. Sucrose influx across the plasma membranes of the filial cells is energy dependent and, for grain legumes, is energy coupled through a sucrose/proton symporter. Studies on the control of post-sieve element transport of sucrose have focused largely on the membrane transport steps. The role of phytohormones as modulators of sucrose transport is uncertain in grain legumes, efflux from the maternal cells could be regulated by rates of sucrose utilisation in the filial tissues through a turgor homeostat mechanism located in the efflux cells.

In the developing ovule of P. vulgaris, the rapid lateral dissemination of photosynthates throughout the seed coat occurs within a reticulate vasculature of uniform sized veins embedded in a parenchymatous layer. Radial movement of photosynthate from the sieve elements to the cotyledonary apoplast is through vascular, ground and branch parenchyma, epidermis and residual endosperm. For this cellular pathway, there appears to be no barrier within the apoplast to photosynthate transfer and symplastic continuity exists from the sieve elements to the branch parenchyma-epidermal boundary. However, based on estimates of possible sucrose fluxes through the apoplastic and symplastic routes, it seems likely that photosynthates move symplastically from the sieve elements, through the vascular parenchyma to either the adjoining ground parenchyma or inner band of branch parenchyma where membrane transfer to the apoplast occurs. The resemblance of the ultrastructure of the branch parenchyma to that of known secretory cells favours this tissue as the main site of photosynthate exchange to the apoplast. The significance of these findings is discussed in relation to photosynthate transfer within the developing ovule.