Effects of salinity on water transport of excised maize (Zea mays L.) roots

The root pressure probe was used to determine the effects of salinity on the hydraulic properties of primary roots of maize (Zea mays L. cv Halamish). Maize seedlings were grown in nutrient solutions modified by additions of NaCl and/or extra CaCl2 so that the seedlings received one of four treatments: Control, plus 100 millimolar NaCl, plus 10 millimolar CaCl2, plus 100 millimolar NaCl plus 10 millimolar CaCl2. The hydraulic conductivities (Lpr) of primary root segments were determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder. Exosmotic hydrostatic Lpr for the different treatments were 2.8, 1.7, 2.8, and 3.4.10(-7) meters per second per megapascals and the endosmotic hydrostatic Lpr were 2.4, 1.5, 2.7, and 2.3.10(-7) meters per second per megapascals, respectively. Exosmotic Lpr of the osmotic experiments were 0.55, 0.38, 0.68, and 0.60.10(-7) meters per second per megapascals and the endosmotic Lpr were 0.53, 0.21, 0.56, and 0.54.10(-7) meters per second per megapascals, respectively. The osmotic Lpr was significantly smaller (4-5 times) than hydrostatic Lpr. However, both hydrostatic and osmotic Lpr experiments showed that salinization of the growth media at regular (0.5 millimolar) calcium levels decreased the Lpr significantly (30-60%). Addition of extra calcium (10 millimolar) to the salinized media caused ameliorative effects on Lpr. The low Lpr values may partially explain the reduction in root growth rates caused by salinity. High calcium levels in the salinized media increased the relative availability of water needed for growth. The mean reflection coefficients of the roots using NaCl were between 0.64 and 0.73 and were not significantly different for the different treatments. The mean values of the root permeability coefficients to NaCl of the different treatments were between 2.2 and 3.5.10(-9) meters per second and were significantly different only in one of four treatments. Cutting the roots successively from the tip and measuring the changes in the hydraulic resistance of the root as well as staining of root cross-sections obtained at various distances from the root tip revealed that salinized roots had mature xylem elements closer to the tip (5-10 millimeters) compared with the controls (30 millimeters). Our results demonstrate that salinity has adverse effects on water transport and that extra calcium can, in part, compensate for these effects