Apoplastic and symplastic pathways in the leaf of the grey mangrove Avicennia marina (Forsk.) Vierh

summary Pathways for water and salts were traced from the vein endings to their sites of exit in leaves of Avicennia marina (Forsk.) Vierh. Investigations were conducted to determine whether the water and solutes separate at any point in the transpiration stream, and to identify their subsequent pathways. Leaves were placed with their cut petioles in solution containing one of two fluorochromes chosen to trace the transpiration stream. The apoplastic tracer was B-hydroxypyrene-1, 3, 6-trisulphonic acid (PTS), and the symplastic tracer was riboflavin buffered to pH 6.8. Leaf pieces were freeze-substituted with special care to keep the specimens dry to prevent redistribution of the water-soluble fluorochromes. Stages of the technique were sensitive to atmospheric moisture, so sections were cut in a room dried to 30% relative humidity, and they were stored in a sealed box containing desiccant. Riboflavin was never seen in the apoplast, and was inside the abaxtal (lower) glands within 30 min indicating a low resistance pathway through the symplast to the gland. The veins terminate in heavily pitted xylem vessels, and at the ends of these pits, the junction between the vein ending and the apoplast of the surrounding mesophyll, were high concentrations of PTS indicating regions of separation of the fluorochrome from water – places where the water crosses from the apoplast to the symplast. PTS diffused slowly in cell walls (measured diffusivity: 4.7 × 10−10 cm2 s−1 reaching the abaxial epidermis after 8 h, which was sufficient time for four complete changes of leaf water. It was concluded that the transpiration stream, probably containing the salt destined for secretion, moved symplastically from the xylem vein endings.