During the last 50 years world irrigation has doubled while water withdrawal has tripled, leading to a global water crisis. As a result, growers around the world are forced to utilize effluent water for irrigation. In Israel the main source for such water is "SHAFDAN", i.e., tertiary-treated waste water, which is characterized by high chloride content that might increase in the future. These days about 33% of Israeli citrus orchards are irrigated with this water, yet its potential effect on citrus orchards is not clear. Citrus is one of the major fruit crops of the world and is relatively salt-sensitive. Recently it was reported that continuous application of KNO 3 might reduce chloride (Cl) uptake, and thus overcome the toxic effects of salinity. We examined the effect of KNO 3 fertilization and rootstock on grapefruit's response to salinized SHAFDAN reclaimed water over four years. Trees were grafted on five different rootstocks and fertigated with two KNO 3 regimes (118 vs. 148 mg 1 -1 NO 3), and exposed to five levels of salinized reclaimed water (from 221 to 765 mg 1 -1 Cl). During the experiment, soil electrical conductivity (EC) ranged from 2 dS m -1 (above the threshold for yield reduction) to 5 dS m -1. Leaf Cl (with the exception of trees grafted on Troyer) usually kept below toxicity level, i.e., >0.4%. Cumulative yield reduction due to salinity was 37.6, 21.3, 18.2, 17.2, and 12.6 kg per tree per dS m -1 of soil paste extract, for trees grafted on 812, SO, Volk, Gau, and Troy rootstocks, respectively, reflecting their sensitivity to the osmotic component of salt stress. Enhanced application of KNO 3 did not overcome the osmotic effect of salinity. These results indicate that while using reclaimed water, the osmotic component of salinity can be of higher significance than its toxic component, and more attention should be given in the future to the rootstock/scion combination's sensitivity to the osmotic component of salinity.
