Salt stress aggravates boron toxicity symptoms in banana leaves by impairing guttation

ABSTRACT Boron (B) is known to accumulate in the leaf margins of different plant species, arguably a passive consequence of enhanced transpiration at the ends of the vascular system. However, transpiration rate is not the only factor affecting ion distribution. We examine an alternative hypothesis, suggesting the participation of the leaf bundle sheath in controlling radial water and solute transport from the xylem to the mesophyll in analogy to the root endodermis. In banana, excess B that remains confined to the vascular system is effectively disposed of via dissolution in the guttation fluid; therefore, impairing guttation should aggravate B damage to the leaf margins. Banana plants were subjected to increasing B concentrations. Guttation rates were manipulated by imposing a moderate osmotic stress. Guttation fluid was collected and analysed continuously. The distribution of ions across the lamina was determined. Impairing guttation indeed led to increased B damage to the leaf margins. The kinetics of ion concentration in guttation samples revealed major differences between ion species, corresponding to their distribution in the lamina dry matter. We provide evidence that the distribution pattern of B and other ions across banana leaves depends on active filtration of the transpiration stream and on guttation. In banana leaves, the transport of water and solutes from the xylem of the veins to the mesophyll and epidermis is mediated by the vascular bundle sheath tissue, in analogy to the endodermis of the root. As a result molecules such as B and Mn, which may reach the lamina via the xylem sap at higher concentrations are prevented from accumulating in the mesophyll and remain confined to the vascular system. Some of the excess B may accumulate in the margins of the lamina, around the vascular bundle that lucks a complete bundle sheath. But most of the excess B will be released from the lamina daily by guttation. Guttation literally “cleans unwanted” molecules from the xylem vessels.