Comparative Hydroecological Studies on Some Hydrophytes, Wet and Dry Halophytes and Xerophytes

Abstract

Summary In Wadi EI-Natrun region, the wide variation in the topography and depth of water table provides a good chance for the occurrence of plants belonging to some ecological groups namely hydrophytes, halophytes (dry and wet) and xerophytes, within a limited area under almost similar climatic conditions. The main factors controlling root extensions in the studied habitats are the depth of water table and the soil moisture content. In the swampy habitat, the ground surface is continuously inundated with water and the roots are weekly developed with very limited extensions. In the salt marsh habitat the water table is shallow and accordingly the soil is wet with high salt content. In this habitat the roots are shallow with limited vertical penetration. In dry salty sites with deeper water table and relatively dry soil, the roots exhibit great vertical and horizontal extensions. In the desert habitat the extremely deep water table and the dry soil favour the great vertical and horizontal extensions of roots as well as the high ratio of root to shoot. The daily march of the stomatal opening runs parallel to that of the transpiration rate in nearly all the cases. The widest stomatal aperture and the highest transpiration rate were exhibited by hydrophytes. The width of stomatal aperture in dry halophytes was slightly greater than that of xerophytic species. In case of wet halophytes, the width of stomatal aperture and rate of transpiration were intermediate between those of hydrophytes and xerophytic species. The lowest osmotic pressure of plant sap (from 5.92 to 6.81 atm.) was shown by hydrophytic species and the highest (from 24.44 to 40.61 atm.) by dry halophytes. In wet halophytes the osmotic pressure of shoot sap was relatively high but generally less than that of dry halophytes and higher than that of xerophytes. The high osmotic pressure of dry and wet halophytes is mainly due to accumulation of salts since roots of these plants grow in saline soils containing a relatively high percentage of salts. In case of hydrophytes and xerophytic species, the odmotically active constituents of the cell sap are mainly nonelectrolytes.