ULTRASTRUCTURE OF THE CELL-WALLS OF THE TRANSPORT PATHWAY FOR GIBBERELLIC-ACID IN BARLEY ALEURONE LAYER

Transmission and scanning electron microscopic studies of the scutellu m-aleurone junction of barley suggest that, in the absence of plasmode smata connections, the transport of gibberellic acid from the scutellu m to the aleurone layer is effected by apoplastic transport across 3-4 mu m thick cell walls. Transport of gibberellic acid along the aleuro ne could be apoplastic as well as symplastic because large numbers of plasmodesmata are present in the 4 mu m thick cell walls, through whic h the hormone could pass from cell to cell. Structurally, the scutellu m is separated from the starchy endosperm by a 30 mu m thick layer of compressed cell wall material across which transfer of either gibberel lic acid to the aleurone or small quantities of non-specific beta-gluc anase enzymes (E1) from the scutellum to the starchy endosperm would b e restricted. No such barrier is present between the aleurone layer an d the starchy endosperm, where cell wall thickness is about 4 mu m, an d the release of endosperm-degrading enzymes into the starchy endosper m is not restricted.