Cellular pathway of photosynthate transport in coats of developing seed ofVicia fabaL. andPhaseolus vulgarisL. I. Extent of transport through the coat symplast

Abstract

The extent of post-phloem solute transport through the coat symplasts of developing seeds of Vicia faba L. and Phaseolus vulgaris L. was evaluated. For Vicia seed coats, the membrane-impermeant fluorochrome, CF, moved radially from the chalazal vein to reach the chlorenchyma and thin-walled parenchyma transfer cell layers. Thereafter, the fluorochrome moved laterally in these two cell layers around the entire circumference of the seed coat. Transfer of CF from the chalazal vein was inhibited by plasmolysis of attached «empty» seed coats. In contrast, the spread of phloemimported CF was restricted to the ground parenchyma of Phaseolus seed coats. Fluorochrome loaded into the outermost ground parenchyma cell layer was rendered immobile following plasmolysis of excised seed-coat halves. Phloem-imported [ 14 C]sucrose and the slowly membrane permeable sugar, L-[ 14 C]glucose, were partitioned identically between the vascular and non-vascular regions of intact Vicia seed coats. For 14 C-photosynthates, these partitioning patterns in attached «empty» Vicia seed coats were unaffected by PCMBS, but inhibited by plasmolysis. Tissue autoradiographs of intact Phaseolus seed coats demonstrated that a pulse of 14 C-photosynthate moved from the veins to the ground tissues. In excised Vicia seed coats, preloaded with 14 C-photosynthates, the cellular distribution of residual 14 C-label was unaffected by PCMBS. In contrast, PCMBS caused the 14 C-photosynthate levels to be elevated in the veins and ground parenchyma relative to the branch parenchyma of Phaseolus seed coat halves. Based on the above findings, it is concluded that the phloem of Vicia seed coats is interconnected to two major symplastic domains; one comprises the chlorenchyma, the other the thin-walled parenchyma plus thin-walled parenchyma transfer cells. For Phaseolus seed coats, the phloem forms a major symplastic domain with the ground parenchyma