Changes in Photosynthate Unloading from Perfused Seed Coats of Phaseolus vulgaris L. Induced by Osmoticum and Ethylenediaminetetraacetate (EDTA)

Photosynthate unloading in Phaseolus vulgaris L. seed coatswas studied by treating perfused seed coats with differing concentrationsof an osmoticum and ethylenediaminetetraacetate (EDTA). Largechanges in osmoticum concentration typically produced rapidchanges in efflux of unlabelled sugar and steady-state-labelled¹⁴C-photosynthate. Osmoticum-induced changes in photosynthateefflux were caused by phloem import stimulation at low cellturgor and net efflux stimulation by high cell turgor. Eventhough rapid changes in sugar and tracer efflux were often inducedby osmoticum treatments, the specific activity of sugar releasedfrom seed coats was not greatly affected by these treatmentsand was similar to the specific activity of sugar remainingin the seed coat after perfusion. Thus, tracer was transportedfrom the phloem throughout the seed coat sugar pool before itwas released to the apoplast. This result is most consistentwith symplastic phloem unloading throughout perfused seed coats,because apoplastic transport between cells within the seed coatwas blocked by perfusion. Photosynthate efflux was stimulatedby simultaneous treatment of seed coats with EDTA and differentconcentrations of an osmoticum; loss of photosynthate from seedcoats did not appear to be tissue-specific. Key words: Phaseolus vulgaris, seed coat, photosynthate unloading, turgor, EDTA