Photosynthate Unloading from Seed Coats ofPhaseolus vulgarisL.—Nature and Cellular Location of Turgor-Sensitive Unloading

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Abstract
Patrick, J. W., Jacobs, E., Offler, C. E. and Cram, W. J. 1986. Photosynthate unloading from seed coats of Phaseolus vulgaris L.—Nature and cellular location of turgor-sensitive unloading—J. exp. Bot. 37: 1006–1019. Unloading rates of 14C-Photosynthates from excised seed-coat halves of Phaseolus vulgaris L. plants were sharply increased at cell turgor potentials in excess of 5 × 105 Pa. Turgor-sensitive unloading occurred in the absence of any change in the passive permeability of, and active sucrose influx across, the plasmalemma and tonoplast membranes. The proton ionophore CCCP, and low temperature significantly slowed turgor-sensitive unloading while PCMBS, a non-permeating sulphydryl-modifying compound, was without effect. Turgor-sensitive unloading significantly depressed the 14C-Photosynthate content of the ground and branch parenchyma, but had no effect on the 14C-Photosynthate levels in the vascular tissues. Cycling of cell turgor potentials above and below 5 × 105 Pa elicited reproducible responses in the unloading rate of 14C-Photosynthates. Increasing turgor above 5 × 105 Pa resulted in a burst of 14C-Photosynthate unloading. Reversal to turgors less than 5 × 105 Pa caused a rapid depression in unloading rate. It is proposed that turgor-sensitive unloading is facilitated by a specific turgor-sensitive porter located on the plasmalemma of the ground and/or branch parenchyma cells of bean seed coats.