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.

A pressure dehydration technique for recovering sap from the apoplast of the pericarp tissue of developing tomato fruit has recently been developed. Samples of this sap from two cultivars have now been analysed for sugars, amino acids, organic acids, ammonia and inorganic ions. The measured solutes accounted for 92 and 97% of the osmolality of the apoplast sap from the two cultivars. The osmotic potential of the apoplast samples was similar in the two cultivars, and the apoplast samples were distinctly different in osmolality and in composition from samples of the bulk sap obtained after thawing frozen tissue. Hexoses and inorganic compounds, principally potassium and chloride, accounted for 75% of the osmotic potential of the apoplast samples. There is little prior information on the composition of the apoplast in fruit. The impact of this new knowledge is discussed in relation to the uptake of solutes into fruit cells, the partitioning of solutes between apoplast and symplast, and the ionic environment of the cell wall and wall-bound enzymes.

14C-labelled photosynthates, on reaching developing ovules of Phaseolus vulgaris plants, were transported rapidly and evenly throughout the highly vascularized seed coats. Apoplastic transfer of 14C-labelled photosynthates from the seed coats to the developing embryos occurred over the entire inner surface of the seed coats. Further lateral transfer of the photosynthate through the cotyledons exhibited characteristics consistent with a diffusional process. The 14C-labelled photosynthates transported from the seed coat to cotyledon symplasts were principally composed of sucrose. The free space of the cotyledons was fully permeated by photosynthates at concentrations in the range of 100-200 mM sucrose equivalents. This pool of photosynthates was continuously maintained by transfer from the seed coats. Furthermore, initial rates of depletion of the free-space pool of photosynthates, following removal of the seed coats, accounted for cotyledon growth rates. These observations are consistent with photosynthate movement being largely restricted to the apoplast of the cotyledons.

The exchange of solutes between a plant and invading microorganism involves transport across both plant and microorganism membranes separated by a common apoplast. An empirical analysis of the interrelationship between these two membrane transport steps is undertaken with emphasis on transfer of reduced carbon from host to microorganism. The analysis leads to the conclusion that solute efflux from the plant partner has the potential to exert significant control over net transport of solutes from the plant to microorganism. The nature of solute efflux from the plant partner was evaluated for examples of pathological, mycorrhizal and Rhizobium associations. Estimates of solute efflux across the perceived interfacial membrane of the plant were greater than those of non-infected tissues. The principal factors contributing to the enhanced solute efflux in the presence of a microorganism were considered to be elevation of solute concentrations in the plant cytoplasm and modification of membrane transport. Possible modes of action by the invading microorganism on these factors are examined with particular attention being paid to the modification of membrane transport.