Auxin control of photoassimilate transport to and within developing grains of wheat

Rates of C-14-labelled photoassimilate and dry matter import into developing grains of wheat (Triticum turgidum cv. Franshawi) were enhanced by indolyl-3-acetic acid (IAA) applied in an agar gel infused into the flag internodal lacunae of intact plants or as an aqueous solution to detached, aseptically-cultured ears. Enhancement of assimilate import occurred in the absence of any changes in the net rates of flag leaf and ear photosynthesis and the relative rates of C-14-labelled photoassimilate export from the flag leaves, Sucrose concentrations and osmolalities of saps collected from the rachis phloem and bulk maternal tissues of the grain crease region also were unaffected. However, both the sucrose concentration and osmolality of the endosperm cavity sap was elevated by IAA. The sucrose concentration of the bulk filial sap increased without any detectable change in sap osmolality. Auxin stimulated accumulation rates of [C-14]sucrose from the endosperm cavity of detached intact grain and by filial tissue slices enriched in the cells of the modified aleurone/subaleurone layers. Auxin-enhanced [C-14]sucrose accumulation was abolished by para-chloromercuribenzenesulfonic acid and erythrosin B. We conclude that IAA stimulates photoassimilate transport to and within developing wheat grains by a primary action on the membrane exchange of solutes to and from the endosperm cavity.