Differential growth and yield responses of salt-tolerant and susceptible rice cultivars to individual (Na+ and Cl−) and additive stress effects of NaCl

Negative impacts exerted by sodium (Na+) and chloride (Cl−) ions individually as well their possible additive effects (under NaCl) were evaluated on growth and yield reductions in rice, besides investigating whether salt-tolerant genotypes respond differentially than their sensitive counterparts. Though both Na+ and Cl− ions get accumulated in plant tissues under NaCl stress, most research has historically been aimed to decipher harmful effects induced by Na+ ions. Accordingly, physiological and molecular mechanisms involved in Cl− toxicity are not clearly understood in crop plants. To address these issues, 65-day-old plants of two rice cultivars, Panvel-3 (tolerant) and Sahyadri-3 (sensitive) were subjected to Cl−, Na+ and NaCl (each with 100 mM concentration and electrical conductivity of ≈10 dS m−1) stress using soil-based systems. Stress conditions were maintained till harvesting of mature (128-day-old) plants. All three treatments induced substantial antagonistic effects on growth, dry mass, yield components (number of grains per panicle, length, width, thickness and weight of grain, along with the percentage of grains filled) and overall crop yield, with greater impacts under NaCl than its constituent ions. Salinity treatments caused an imbalance in reducing sugars, protein, starch and proline contents, with the greatest magnitude under NaCl. A negative correlation between Cl−/Na+ accumulation and crop yield was witnessed, with higher severity on the sensitive cultivar. The overall magnitude of toxicity was observed highest in NaCl followed by Na+ and Cl−, respectively, suggesting additive effects of constituent ions under NaCl. Both cultivars responded similarly; however, the tolerant cultivar, unlike the sensitive one, kept Na+:K+ ratio <1.0 and accumulated proline in response to salinity treatments used in this study.