Sodium-calcium interactions with growth, water, and photosynthetic parameters in salt-treated beans

Calcium (Ca2+) amelioration of the plant's growth response to salinity depends on genetic factors. In this work, supplemental Ca2+ did not improve growth in Phaseolus vulgaris L. cv. Contender under high-saline conditions and negatively affected several physiological parameters in nonsalinized plants. The response to supplemental Ca2+ was examined using plants grown in 25% modified Hoagland solution at different Na+ : Ca2+ ratios. In control plants (1 mM Ca2+; 1 mM Na+) surplus Ca2+ (4 or 10 mM) was associated with stomatal closure, decrease of hydraulic conductivity, sap flow, leaf specific dry weight, leaf K+ and leaf Mg2+ concentrations, and inhibition of CO assimilation. Leaf water content was enhanced, while water-use efficiency and dry matter were unaffected during the 15 d experimental period. The Ca2+ effect was not cation-specific since similar results were found in plants supplied with high external Mg2+ or with a combination of Ca2+ and Mg2+. Relative to control plants, salinization (50 and 100 mM NaCl) caused a decrease in dry matter, hydraulic conductivity, sap flow, leaf Mg2+ activity, and inhibition of stomatal opening and CO2 assimilation. However, NaCl (50 and 100 mM NaCl) enhanced leaf K+ concentration and water-use efficiency. At 100 mM NaCl, leaf water content also significantly increased. Supplemental Ca2+ had no amelioration effect on the salt-stress response of this bean cultivar. In contrast, the 50 mM-NaCl treatment improved stomatal conductance and CO2-assimilation rate in plants exposed to the highest Ca2+ concentration (10 mM). Phaseolus vulgaris is classified as a very NaCl-sensitive species. The similarities in the effects caused by supplemental Ca2+, supplemental Mg2+, and NaCl salinity suggest that P vulgaris cv. Contender has a high non-ion-specific salt sensitivity. On the other hand, the improvement in gas-exchange parameters in Ca2+-supplemented plants by high NaCl could be the result of specific Na+-triggered responses, such as an increase in the concentration of K+ in the leaves.