Effects of Salinity on Water Transport of Excised Maize (Zea mays L.) Roots

Abstract

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 CaCl(2) so that the seedlings received one of four treatments: Control, plus 100 millimolar NaCl, plus 10 millimolar CaCl(2), plus 100 millimolar NaCl plus 10 millimolar CaCl(2). The hydraulic conductivities (Lp(r)) of primary root segments were determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder. Exosmotic hydrostatic Lp(r) 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 Lp(r) were 2.4, 1.5, 2.7, and 2.3.10(-7) meters per second per megapascals, respectively. Exosmotic Lp(r) of the osmotic experiments were 0.55, 0.38, 0.68, and 0.60.10(-7) meters per second per megapascals and the endosmotic Lp(r) were 0.53, 0.21, 0.56, and 0.54.10(-7) meters per second per megapascals, respectively. The osmotic Lp(r) was significantly smaller (4-5 times) than hydrostatic Lp(r). However, both hydrostatic and osmotic Lp(r) experiments showed that salinization of the growth media at regular (0.5 millimolar) calcium levels decreased the Lp(r) significantly (30-60%). Addition of extra calcium (10 millimolar) to the salinized media caused ameliorative effects on Lp(r). The low Lp(r) 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.