The ideal percentage of K substitution by Na in Eucalyptus seedlings: Evidences from leaf carbon isotopic composition, leaf gas exchanges and plant growth

Abstract Potassium (K) is the most required macronutrient by Eucalyptus , while sodium (Na) can partially substitute some physiological functions of K and have a positive response on plant growth in K-depleted tropical soils. However, the right percentage of K substitution by Na is not yet known for Eucalyptus seedlings, since a few experiments have only compared treatments receiving K or Na. This study evaluated five levels of Na supply (0, 0.45, 0.90, 1.35 and 1.80 mM) as substitution for K in Eucalyptus seedlings grown in nutrient solution. Plants growth, biomass, K-nutritional status, leaf gas exchange, leaf carbon isotopic composition (δ 13 C ‰), leaf water potential (Ψ w ), leaf area (LA), stomatal density (SD) and water use efficiency (WUE) were measured. The highest total biomass yield was achieved by the Na estimated rate of 0.25 mM, corresponding to a leaf K: Na ratio of 3.41, and having the lowest δ 13 C values. Conversely, the highest Na rate (1.8 mM) induced K deficiency symptoms, lower growth, reduced total dry matter yield, leaf gas exchange, LA, SD and a higher δ 13 C, which presented a trend to an inverse correlation with CO 2 assimilation rate ( A ), WUE and shoot dry matter. Collectively, our results conclude that substitution of 25% of K by Na (0.45 mM of Na) provided significant gains in nutritional status and positive plant physiological responses by increasing WUE, stomatal diffusion, and by augmenting CO 2 uptake efficiency. This nutritional management can therefore be an alternative option to optimize yields and resource use efficiencies in Eucalyptus cultivation.