In water-scarce regions including Israel’s Negev Highlands, olives are commonly irrigated with non-conventional water sources containing high concentrations of salts. While olive trees are understood to be relatively tolerant to abiotic stress-causing factors, including salinity, there is evidence of damage following extreme or long-term exposure to salinity and of negative repercussions of deficit irrigation with water high in salts. We studied whole tree olive response to root zone salinity coming as a function of irrigation water salinity and irrigation (leaching) level in order to evaluate leaching requirements and reveal salinity stress coping mechanisms. Nine treatments representing five source salinity levels (electrical conductivity, EC of 0.5, 2.5, 5.0, 7.5 and 10.5 dS/m at a constant, high leaching fraction) and five leaching fractions (100, 120, 140, 160, and 180% return of actual daily tree-scale evapotranspiration with irrigation water EC of 5 dS/m) were applied to large free-standing weighing-drainage lysimeters at the Gilat Research Center. Olive trees (cv Barnea) were grown for three years under the treatments, including two seasons with fruit production.
Measurements and monitoring of individual trees included evapotranspiration (ET) calculated daily from water balance, deep drainage water quality and quantity, vegetative growth, fruit yield and oil yield and oil quality parameters. Physiological properties and responses including stem water potential, stomatal conductance, carbon exchange rate, and canopy temperature were measured periodically. Root profile dynamics were monitored using a minirhizotron camera in two access tubes located in each lysimeter. Diagnostic leaves and representative roots were sampled annually for mineral analyses and structural traits. After three years, the trees were removed, separated into trunk, branches, stems, leaves and roots for quantification of biomass and mineral mass balance.
Soil salinity, measured as EC of saturated soil paste extract (ECe) and maintained at stable levels over time, decreased tree water consumption and tree size by 40-60% as it increased from 1.2 to around 3.5-4.0 dS m-1. Further increases in ECe to as high as 7.5 dS m-1 brought these parameters to 20-30% of the treatment with low salinity. Fruit yield also decreased with increasing salinity, albeit with less drastic effects. Increased ECe decreased root growth, most significantly between the lowest and next lowest salinities (caused by irrigation with EC=0.5 compared to EC = 2.5 dS/m). Root structural traits were altered as increased salinity led to reduced specific root length and increased root diameter, periderm thickness, and specific hydraulic conductivity. Sodium was found to be compartmentalized within the periderm. The trees demonstrated a remarkable capacity for excluding potentially toxic ions. The highest concentrations of sodium and chloride ions were found in the youngest roots and not found to accumulate in mature tissues. This indicates the dominance of osmotic stress and responses and that any toxic effects of salinity were limited to young roots.
