Sodium Substitution of Potassium in Photosynthesis Processes in 'Natrophilic' Plant Species.

Potassium (K) is macronutrient understood to play a role in the physiological performance of plants and considered an enhancer of abiotic stress resistance. In some plant species, sodium (Na) can partially substitute K. Although photosynthetic enhancement by K has been well established, the mechanisms by which K or Na affects photosynthesis are not fully understood. The objective of the present study was to evaluate the role of K and possible replacement by Na in photosynthetic mechanisms and stomatal regulation. Olive trees supplied with either K, Na or no fertilizer were grown in soilless culture and were exposed to gradual water stress. Gas exchange and chlorophyll fluorescence were measured under conditions of manipulated levels of CO2, O2 and radiation. In the presence of Na, trees were tolerant of extremely low K concentrations. Depletion of K and Na resulted in about 50% reduction in CO2 assimilation rate as compared to sufficiently fertilized control plants. Sodium was able to recover 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. Moreover, stomatal control machinery was not significantly impaired by K deficiency. The enhanced performance of K, and to a lesser extent, Na-supplied trees was found to be related mainly to modification of non-stomatal limitation. This indicates that K deficiency promotes inhibition of enzymatic-photochemical processes. Potassium deficiency was also found to diminish photoprotection mechanisms due to reduced photosynthetic and photorespiratory capacity. Sodium replacement considerably diminished the negative effect of K deficiency on photoprotection mechanisms. The positive effect of Na on assimilation and photoinhibition indicates that Na may play a metabolic role in addition to its acknowledged osmotic role.