Growth and Photosynthetic Responses of Spinach to Salinity: Implications of K⁺ Nutrition for Salt Tolerance

Abstract

To compare the effects of K⁺ under high and low salinity, spinach plants (Spinacia oleracea) were grown in nutrient solutions containing either 50 mM NaCl (low salinity) or 250 mM NaCl (high salinity), with a diurnal regime of 10 h light (~300 μmol photons m^-2 s^-1, 23°C) and 14 h dark (15°C). At each level of salinity, the nutrient KCl concentration was 0.01, 0.1, 1 or 10 mM.

The plant and shoot biomass was greater at low salinity than high salinity and increased with the logarithmic increase in nominal K⁺ concentrations supplied to the roots. Plant and shoot growth were related to the K⁺ uptake into the leaves, with leaves having a higher K⁺ content under low salinity than high salinity. Variation of the K⁺ content in the leaves, induced by the combinations of nutrient KCl concentrations with high or low salinity, were accompanied by changes in the photosynthetic capacity at light- and CO₂-saturation per unit leaf area; there was a greater decrease in photosynthetic capacity with decreasing K⁺ supply to the roots under high salinity than under low salinity. The photosynthetic capacity was in turn highly correlated with the contents of cytochrome f and ATP synthase per unit leaf area. Under conditions of high salinity and low K⁺ supply, a reduction in the quantum yield of oxygen evolution also occurred, due to malfunction of photosystem II and, apparently, an increased proportion of light absorbed by non-photosynthetic tissue. The decreases in photosynthetic capacity and quantum yield partly account for the lower plant and shoot biomass at high salinity and low nutrient KCl concentrations.

Our results suggest strongly that there are higher K⁺ requirements for shoot growth under high than low salinity conditions, and that high concentrations of Na⁺ in the leaves may help to maintain turgor, but cannot substitute for adequate K⁺ levels in the leaves, presumably because K⁺ is specifically required for protein synthesis. Increasing the K⁺ supply at the roots can ameliorate reductions in plant and shoot biomass imposed by an increase in salinity.