Salt Tolerance of Trifolium alexandrinum L. III. Effects of Salt on Ultrastructure of Phloem and Xylem Transfer Cells in Petioles and Leaves

Abstract

The ultrastructure of vascular tissue of petioles, petiolules and major and minor leaf veins from T. alexandrinum plants grown with and without 50 mM NaCl in solution culture was examined at two stages of the salt treatment. Gradual destruction of the cell content in phloem transfer cells of major and minor veins in leaves of salt-treated plants was observed. This destruction was only found in these cells and preceded the development of leaf burn symptoms. First signs of disorganization are myelin-figures, final stages are coagulation of the cellular content and accumulation of large starch grains in the chloroplasts of the surrounding mesophyll tissue. The appearance of these symptoms coincides with the end of a period of increased Na⁺ inflow into the leaves. The obser- vations are discussed in relation to a hypothesis of intraveinal recycling of Na⁺ and Cl^- in leaves as an integral part of the salt-tolerance mechanisms of T. alexandrinum.