Pathway and regulation of phosphate translocation to the pods of soybean explants

Abstract

We investigated the degree to which developing fruit compete directly with leaves for mineral nutrients, e.g. phosphate coming up from the roots. When soybean (Glycine max (L.) Merrill cv. Anoka) explants cut at mid‐late podfill were given a 15‐min pulse of 32Pi via the cut stem and then transferred to distilled water, 75% of the 32P accumulated in the leaves and 21% in stem and petiole during the first hour. The amount of 32P entering the seeds was low (1%) initially, but thereafter increased to 30% in 48 h. An accumulation of 32P in the seed coats preceded its entry into the embryos. Disruption (with hot steam) of the phloem between the leaf and the pods after pulse labelling indicated that more than 80% of the 32Pi pulse moved to the leaf before redistribution to the pods. Increasing “sink” size by adjusting the pod load from 1 to 2–3 did not increase the 32P accumulated by the pods proportionally. Conversely, excision of the seeds after pulse labelling did not prevent translocation of 32P out of the leaves. These results suggest that the rate of transport of phosphate to the pods at mid‐late podfill is controlled primarily by factors in the leaves. The results are consistent with the observation that the relative size of the sink (pod load) does not regulate leaf senescence.