CHANGES IN SOYBEAN FRUIT CA2+ (SR2+) AND K+ (RB+) TRANSPORT ABILITY DURING DEVELOPMENT

Mineral uptake by soybean (Glycine max [L.] Merrill) seeds during deve lopment can significantly affect seed quality and value. Little is kno wn about seed mineral transport mechanisms and control processes, alth ough it is clear that each mineral displays a characteristic accumulat ion pattern. ion-specific accumulation patterns could result from chan ges in source availability, in transport kinetics through the seed pod and seed coat, or in the mineral uptake capability of the embryo. Ca2 + and K+ have negligible and high phloem mobilities, respectively. Ca2 + accumulation lags behind dry matter (C and N) and K+ accumulation in soybean embryos. To eliminate source availability influences, the Ca2 + and K+ uptake ability of isolated embryos and of seeds in pod cultur e was examined during seed development. Sr2+ and Rb+ were used as tran sport analogs of Ca2+ and K+, respectively. Sr2+ and Rb+ uptake rates by isolated embryos increased with seed fresh weight, indicating that the embryo was not limiting Ca2+ accumulation. However, the pod-cultur ed embryo Sr2+ and Rb+ uptake rate trends differed: Rb+ uptake increas ed with seed fresh weight, whereas Sr2+ uptake rates remained constant or decreased slightly. Ovule Sr2+ influx data suggest that the pod an d seed coat impose a transport barrier that could account for the rela tive decline in embryo Ca2+ content during development.