Abstract

The seed coat vascular system of the developing seed of Vicia faba consists of a chalazal and two lateral veins. The veins are embedded in parenchymatous tissue which lies beneath the hypodermis and is divided into chlorenchyma, ground parenchyma and thin-walled parenchyma. The thin-walled parenchyma cells and, in old seed coats, the vascular parenchyma of the veins undergo additional secondary wall development to form transfer cells. Thus, transfer cells line the entire inner surface of the seed coat. Initial distribution of 14C-photosynthates and sodium fluorescein within the seed coat was in the vascular system. Subsequent transfer towards the embryo was either radially through vascular parenchyma and thin-walled parenchyma to thin-walled parenchyma/transfer cells, or by lateral spread within the ground and thin-walled parenchyma/transfer cells of the non-vascular region of the seed coat prior to radial transfer. One-third of the 14C-photosynthate delivered to the enclosed embryo was estimated to be transferred via the non-vascular region of the seed coat. The cotyledons consist of a single-layered epidermis enclosing storage parenchyma in which a differentiating reticulate vascular system is embedded. Epidermal cells juxtaposed to the seed coat develop wall ingrowths characteristic of transfer cells. Initial distribution of 14C-photosynthate within the cotyledons reflected the unequal delivery to the seed apoplast from the vascular and non-vascular regions of the seed coat. Subsequent even distribution of photosynthate within the cotyledons possibly occurred by transfer within their vascular system.

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