Phloem Unloading within the Seed coat ofPisum sativumObserved using Surgically Modified Seeds

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Abstract
Phloem unloading in pea seed coats was observed by removing the embryos from developing seeds and washing the attached coats with a weakly buffered solution. The quantity of labelled photosynthate appearing in the washing solution varied immediately when the solute concentration was changed, and is shown to be an osmotic response. This response is predicted by the Münch theory of phloem transport with concentration dependent unloading. Respiratory inhibitors and the sulphydryl modifying reagent PCMBS had a slow effect upon the washout of tracer, which arrived within the seed coat prior to inhibitor application, but completely stopped any washout of tracer arriving after its application. This time-course suggests that the inhibitors were not directly inhibiting unloading, but preventing further tracer from entering the region of unloading within the seed coat. Phloem unloading within the seed coats of Pisum appears to be passive and not dependent upon a PCMBS-sensitive carrier.