Functional anatomy controls ion distribution in banana leaves: significance of Na+ seclusion at the leaf margins

Typical salt stress symptoms appear in banana ( Musa sp., cv. 'Grand Nain' AAA) only along the leaf margins. Mineral analysis of the dry matter of plants treated with increasing concentrations of KCl or NaCl revealed significant accumulation of Na⁺, but not of K⁺ or Cl^-, in the affected leaf margins. The differential distribution of the three ions suggests that water and ion movement out of the xylem is mostly symplastic and, in contrast to K⁺ and Cl^-, there exists considerable resistance to the flow of Na⁺ from the xylem to the adjacent mesophyll and epidermis. The parallel veins of the lamina are enclosed by several layers of bundle sheath parenchyma; in contrast, the large vascular bundle that encircles the entire lamina, and into which the parallel veins merge, lacks a complete bundle sheath. Xylem sap containing a high concentration of Na⁺ is 'pulled' by water tension from the marginal vein back into the adjacent mesophyll without having to cross a layer of parenchyma tissue. When the marginal vein was dissected from the lamina, the pattern of Na⁺ distribution in the margins changed markedly. The distinct anatomy of the marginal vein plays a major role in the accumulation of Na⁺ in the margins, with the latter serving as a 'dumping site' for toxic molecules.