Erel, R., Institute of Soil, Water and Environmental Sciences, Gilat Research Center, Agricultural Research Organization, Mobile Post Negev, Israel, Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot, Israel
Ben-Gal, A., Institute of Soil, Water and Environmental Sciences, Gilat Research Center, Agricultural Research Organization, Mobile Post Negev, Israel
Dag, A., Institute of Plant Sciences, Gilat Research Center, Agricultural Research Organization, Mobile Post Negev, Israel
Schwartz, A., Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot, Israel
Yermiyahu, U., Institute of Soil, Water and Environmental Sciences, Gilat Research Center, Agricultural Research Organization, Mobile Post Negev, Israel

Potassium (K) is a macro-nutrient understood to play a role in the physiological performance of plants under drought. In some plant species, sodium (Na) can partially substitute K. Although a beneficial role of Na is well established, information regarding its nutritional role in trees is scant and its function under conditions of drought is not fully understood. The objective of the present study was to evaluate the role of K and its possible replacement by Na in olive's (Olea europaea L.) response to drought. Young and bearing olive trees were grown in soilless culture and exposed to gradual drought. In the presence of Na, trees were tolerant of extremely low K concentrations. Depletion of K and Na resulted in ∼50% reduction in CO2 assimilation rate when compared with sufficiently fertilized control plants. Sodium was able to replace K and recover the assimilation rate to nearly optimum level. The inhibitory effect of K deficiency on photosynthesis was more pronounced under high stomatal conductance. Potassium was not found to facilitate drought tolerance mechanisms in olives. Moreover, stomatal control machinery was not significantly impaired by K deficiency, regardless of water availability. Under drought, leaf water potential was affected by K and Na. High environmental K and Na increased leaf starch content and affected the soluble carbohydrate profile in a similar manner. These results identify olive as a species capable of partly replacing K by Na. The nutritional effect of K and Na was shown to be independent of plant water status. The beneficial effect of Na on photosynthesis and carbohydrates under insufficient K indicates a positive role of Na in metabolism and photosynthetic reactions. © The Author 2014. Published by Oxford University Press. All rights reserved.