Water Relations and Leaf Gas Exchange of Table-Beet in Response to Replacement of Nutrient K with Na

Developing crop-based approaches for sodium recycling may be required for integrating urine and gray water into food production systems for advanced life support. Sodium (Na) is generally considered non-essential for plants, whereas potassium (K) is a macro-nutrient involved in a range of metabolic functions, including maintenance of cell turgor and water relations. This study compared if K can be largely replaced with Na in the nutrient solutions without affecting productivity. Table-beet (Beta vulgaris) variety Ruby Queen was grown at K/Na (mM) ratios of 5.0/0, and 0.1/4.90 (represents 0% and 98% substitution of K with Na in nutrient solutions) in a re-circulating hydroponic system. Plants were grown at elevated CO₂ (i.e. at 1200 μmol mol⁻¹) in growth chamber for 42 days. Leaf photosynthetic rates were measured at 32 days after planting. Leaf relative water content (RWC) and osmotic potential were measured at harvest. Replacing nutrient K with Na had no significant effect on leaf chlorophyll levels or photosynthetic rate. Leaf relative water content was significantly higher at K/Na 0.1/4.90 than 5.0/0 treatment. Osmotic potential of the leaf sap was not significantly affected by Na substitution for K. Sodium concentrations reached to about 300 mM in leaf sap. Results indicated that table-beet has a high degree of tolerance to substitution of tissue K with Na. We conclude that for table beet, K could be managed as a micro-nutrient in the presence of adequate sodium which could take over some of the osmotic functions of K.