Research Aspects for Crop Salt Tolerance under Irrigation with Special Reference to Root Environment

Due to the lack of good quality water for human consumption and industry, in many countries irrigated agriculture is exposed to increasing pressure to expand the use of brackish and even high-saline waters for crop production. The application of lower quality waters requires special management practices, which are not yet fully understood with respect to optimization of plant growth on saline soils. Generally, the evaluation of crop salt tolerance and the calculation of irrigation water requirement for brackish water is based on crop yields related to the average salinity of the rooted soil layer (FAO and USDA). However, this concept often fails when brackish water is often applied frequently (e.g. drip irrigation). The approach presented in this paper considers not only the effect of water applications on the vertical salt movement (salt leaching), but also reflects the effect of lateral water and salt movement of saline soil solutions induced by roots, which differ in their root morphology. This lateral flow of water is caused by the transpiration of plants, which results in a flow of distant saline soil solution directed to the water absorbing root surface. As most salts are excluded from root uptake, they usually accumulate in the rhizospheric soil (soil in direct contact with the root surface including the cylinder formed by root hairs). A concentration gradient between distant and rhizospheric soil solution develops, the rhizospheric soil solution may become several times more saline than the average or distant soil solution (bulk soil). The effects of dynamic processes between bulk and rhizospheric soil under irrigation and with respect to root morphology are not yet understood. It is postulated that roots forming a large rhizocylinder (deeply rooting, high rooting density, long root hairs) are contributing to a higher salt tolerance as roots forming a smaller rhizocylinder. It can be expected that a more profound understanding of these processes may essentially contribute to optimize brackish water irrigation and to define goals for breeding more salt tolerant crops.