Changes in Soybean Fruit Ca2+ (Sr2+) and K+ (Rb+) Transport Ability during Development

Mineral uptake by soybean (Glycine max [L.] Merrill) seeds during development can significantly affect seed quality and value. Little is known about seed mineral transport mechanisms and control processes, although it is clear that each mineral displays a characteristic accumulation pattern. Ion-specific accumulation patterns could result from changes 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 culture was examined during seed development. Sr2+ and Rb+ were used as transport 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-cultured embryo Sr2+ and Rb+ uptake rate trends differed: Rb+ uptake increased with seed fresh weight, whereas Sr2+ uptake rates remained constant or decreased slightly. Ovule Sr2+ influx data suggest that the pod and seed coat impose a transport barrier that could account for the relative decline in embryo Ca2+ content during development.