Salt tolerance in Okra: Ion relations and gas exchange characteristics

Influence of sodium chloride (NaCl) on growth, ion, accumulation, ion uptake, ion utilization, water relations, plant pigments, and photosynthetic characteristics of two varieties of okra (Hibiscus esculentus L.) was assessed under glasshouse conditions. Twenty-eight-day-old plants of two varieties (Posa Sawni and Sabz Bhindi) of H. esculentus were subjected to 0 or 100 mol m(-3) NaCl in full strength Hoagland's nutrient solution in sand culture for 30 days. Percent fresh and dry weights of roots and shoots, and mean shoot length and leaf area were significantly higher in Sabz Bhindi than those of Posa Sawni under saline conditions, but the varieties did not differ in concentrations of Na+, Cl-, K+, and Ca2+, potassium/sodium (K/Na) and calcium/sodium (Ca/Na) ratios, and K+ vs. Na+, and Ca2+ vs. Na+ selectivities in both shoots and roots. Uptake of Na+ and Cl- increased, whereas that of K+ and Ca2+ decreased with the addition of salt to the growth medium, but the difference between the varieties with respect to ion uptake was not significant. Utilization efficiency of all the four ions declined considerably under salt stress, but the varieties differed significantly only at the control treatment. The high biomass producing cv. Sabz Bhindi maintained relatively high leaf turgor potential as compared to that of low biomass producing cv. Posa Sawni under salt stress. Chlorophyll a increased in both varieties, but it was higher in Sabz Bhindi than in Posa Sawni. Sabz Bhindi had significantly higher net CO2 assimilation rate, transpiration, stomatal conductance, and water use efficiency under both non-saline and saline conditions than those in Posa Sawni. In conclusion, none of the ion relation parameters was found to be helpful in discriminating between the two H. esculentus varieties differing in salt tolerance. But the high biomass production of Sabz Bhindi and hence its high salt tolerance was found to be associated with its relatively higher photosynthetic rate, transpiration, stomatal conductance, and water use efficiency.