Temperature and Oxygen Effects on 14C-Photosynthate Unloading and Accumulation in Developing Soybean Seeds

Abstract

The environmental sensitivity of the processes associated with the import of photosynthate by developing soybean seeds was investigated within intact fruit and with excised, immature embryos. Intact pods of field-grown (G. max [L.] Merr.) ''Amsoy 71'' soybeans were subjected to localized regimes of 0, 21 or 100% O2 and 15.degree., 25.degree. or 35.degree. C during pulse-chase translocation experiments and, 2.5 h later, the uptake and distribution of 14C-photosynthase among dissected fruit tissues determined. Excised embryos were incubated in [14C]sucrose solutions under various experimental conditions to separate the effects of these treatments on accumulation by the embryos from those which may operate on phloem unloading in the maternal seedcoat. Import of 14C-photosynthate by intact soybean fruit was both temperature- and O2-dependent. This dependency occurred only within the seeds; import by the pod walls was essentially insensitive to fruit temperature or O2 treatments. The embryos of anaerobic fruit were completely unlabeled, regardless of fruit temperature. Under anaerobic in vitro incubation conditions, uptake of [14C]sucrose in excised embryos was only 30% less than that in aerobic in vitro conditions. Evidently, within intact fruit, anoxia prevented sucrose efflux from the seed coat phloem and any subsequent uptake by the embryo. The demonstrated energy dependence of phloem unloading may reflect requirements for membrane integrity or energy metabolism in the companion cell-sieve element complex, consistent with a facilitated unloading process. Environmental regulation of import may occur at both the embryo level and at the phloem terminals within the seed coat.