Effects of salinity on xylem structure and water use in growing leaves of sorghum

Within the growth zone, salt-affected leaves of sorghum (Sorghum bicolor) had narrower protoxylem and metaxylem cells than controls. Leaf width and cross-sectional area were also reduced, so that the salt treatment had no effect on the area of protoxylem per area of leaf cross section. Dye uptake studies suggested that in controls most of the veins, but in salt-affected leaves only half of the veins, are functional in water transport. Volumetric water flow was greatly diminished in the salt-affected plants. The reduced flow rate was largely explained by the salt-induced decrease in leaf surface area. Some decreases in flow rates per unit leaf mass or area were also produced by salinity, particularly in late developmental stages. Not surprisingly, leaf conductance measured with a diffusion porometer did not appear to be correlated with the diameter of the protoxylem or metaxylem elements. By contrast, published values of water deposition rates are strongly related to the size of the protoxylem elements: the rates of water deposition into the growing leaf tissue are proportional to the square of the protoxylem radius. Thus environmentally produced change of the hydraulic architecture of monocot leaves may cause change in local growth rates.