Individual and additive effects of Na+ and Cl- ions on rice under salinity stress

This study was attempted to assess the extent of toxicity contributed by Na+ and/or Cl- ions individually, besides their possible additive effects under NaCl using physiological and biochemical parameters. Despite the fact that most annual plants accumulate both Na+ and Cl- under saline conditions and each ion deserves equal considerations, most research has been focused on Na+ toxicity. Consequently, Cl- toxicity mechanisms including its accumulation/exclusion in plants are poorly understood. To address these issues, effects of equimolar (100mM) concentrations of Na+, Cl- and NaCl (EC approximate to 10dS m(-1)) were studied on 15-day-old seedlings of two rice cultivars, Panvel-3 (tolerant) and Sahyadri-3 (sensitive), using in vitro cultures. All three treatments induced substantial reductions in germination rate and plant growth with greater impacts under NaCl than Na+ and Cl- separately. Apparently, salt tolerance of Panvel-3 was due to its ability to exclude Na+ and Cl- from its shoots and maintaining low (<1.0) Na+/K+ ratios. Panvel-3 exhibited better vigour and membrane stability indices coupled with lower reactive oxygen species and lipid peroxidation levels, besides stimulated synthesis of proline, glycine betaine and ascorbic acid. Overall, the magnitude of toxicity was observed in NaCl>Na+>Cl- manner. Though Cl- was relatively less toxic than its countercation, its effect cannot be totally diminished.