Individual and additive stress impacts of Na⁺ and Cl‾ on proline metabolism and nitrosative responses in rice

It is well-established that plants accumulate high concentrations of sodium (Na⁺) and chloride (Cl‾) ions when subjected to salinity stress. However, little is known about individual or relative toxic impacts of these ions and whether they exert additive impacts under NaCl. Most of the investigations have historically been directed to decode Na⁺-toxicity, and as a result deeper understandings about Cl‾-toxicity are lacking. In this study, the extent to which sodium and chloride ions contribute in inducing nitrosative responses and proline metabolism is shown in two rice cultivars, one tolerant (Panvel-3) and one sensitive (Sahyadri-3). Equimolar (100 mM) concentrations of Na⁺, Cl‾ and NaCl (EC ≈ 10 dSm^-1) reduced biomass production in both the cultivars in following manner NaCl > Na⁺>Cl‾. Na⁺ and NaCl treatments displaced K⁺, however, the tolerant cultivar maintained low Na⁺/K⁺ levels. Hyper-accumulation of Na⁺ may apparently be attributed for the reduced plant growth and biomass accumulation, and higher lipid-peroxidation. Nitric oxide and nitrate reductase were more responsive to NaCl followed by Na⁺ and Cl‾, respectively. The expression patterns of key-genes involved in proline biosynthesis and degradation confirmed the involvement of proline in better performance of salt tolerant cultivar under stresses, with higher responsiveness to NaCl and then Na⁺ and Cl‾ treatments. Principal component analysis revealed correlations in proline metabolism and nitrosative responses under ionic stresses and confirmed the closeness of NaCl and Na⁺ stresses. Overall, amongst the individual ions, Na⁺ induced higher toxicity than Cl‾ and both these ions exerted additive stress impacts under NaCl treatment.