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) h ad narrower protoxylem and metaxylem cells than controls. Leaf width and cr oss-sectional area were also reduced, so that the salt treatment had no eff ect on the area of protoxylem per area of leaf cross section. Dye uptake st udies suggested that in controls most of the veins, but in salt-affected le aves 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 surfac e area. Some decreases in flow rates per unit leaf mass or area were also p roduced by salinity, particularly in late developmental stages. Not surpris ingly, 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 relat ed to the size of the protoxylem elements: the rates of water deposition in to 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.