Effects of Irrigation Induced Salinity on Olive Tree Root System

Irrigated olives are increasingly exposed to conditions of high salinity. To advance understanding of salt stress response of olive roots, nine levels of root zone salinity originating from either irrigation water salinity or leaching level were applied to bearing olive (Olea europaea L.) cv. Barnea trees grown in weighing-drainage lysimeters. Minirhizotrons were used to measure count, diameter, length and age (color) of roots while analysis of sampled roots and leaves quantified ion uptake and accumulation. After 3 years of treatments, trees were removed and root morphological and anatomical traits were determined by scanning and free hand sectioning. Increased exposure to salinity caused reduction in number and length of roots and increased root turnover. Concentration of Na⁺ and Cl^- was 5-10 times greater in root compared to leaf tissue. The K⁺/Na⁺ ratio decreased tenfold as root zone salinity increased in both roots and leaves. Average root diameter and specific root length increased linearly with increasing root-zone salinity. The total area of the vascular cylinder of young roots was reduced and the cortex lengthened as salinity increased. The increase in cortex length was not related to increased size of a single parenchyma cell but rather a slight increase in number of cells. The total area of xylem vessel was reduced with salinity resulting from reduction in both vessel number and average single vessel size. In conclusion, restricted ion transport from roots protected aerial tissue from ion toxicity, but at a high cost as root growth decreased and mortality rate increased. Overall, soil salinity was shown to influence root density, longevity, morphology and anatomy. Alterations in root xylem structure are likely to impair root hydraulic conductivity and hence, limit water uptake. We suggest that the ionic component of salt stress is a prevailing force restricting root growth, life span and development in olives.