Sodium-potassium synergism in Theobroma cacao: stimulation of photosynthesis, water-use efficiency and mineral nutrition

In ecological setting, sodium (Na+) can be beneficial or toxic, depending on plant species and the Na+ level in the soil. While its effects are more frequently studied at high saline levels, Na+ has also been shown to be of potential benefit to some species at lower levels of supply, especially in C4 species. Here, clonal plants of the major tropical C3 crop Theobroma cacao (cacao) were grown in soil where potassium (K+) was partially replaced (at six levels, up to 50% replacement) by Na+, at two concentrations (2.5 and 4.0 mmolc dm-3). At both concentrations, net photosynthesis per unit leaf area (A) increased more than twofold with increasing substitution of K+ by Na+. Concomitantly, instantaneous (A/E) and intrinsic (A/gs) water-use efficiency (WUE) more than doubled. Stomatal conductance (gs) and transpiration rate (E) exhibited a decline at 2.5 mmol dm-3, but remained unchanged at 4 mmol dm-3. Leaf nitrogen content was not impacted by Na+ supplementation, whereas sulfur (S), calcium (Ca2+), magnesium (Mg2+) and zinc (Zn2+) contents were maximized at 2.5 mmol dm-3 and intermediate (3040%) replacement levels. Leaf K+ did not decline significantly. In contrast, leaf Na+ content increased steadily. The resultant elevated Na+/K+ ratios in tissue correlated with increased, not decreased, plant performance. The results show that Na+ can partially replace K+ in the nutrition of clonal cacao, with significant beneficial effects on photosynthesis, WUE and mineral nutrition in this major perennial C3 crop.