Determination of the appropriate level of salinity for screening of hydroponically grown rice for salt tolerance

Development of salt tolerant rice varieties has become an urgent priority because of the increase of salinity in rice lands. The objectives of the present experiment were to determine: (a) the appropriate level of salt stress to be imposed for screening rice varieties grown in hydroponics for salt tolerance at Phase I (osmotic stress) and at Phase II (salt ion toxicity) of salt stress development; (b) the point of transition from Phase I to II and (c) the degree of salt tolerance in the Sri Lankan improved rice variety At354 in comparison to the known salt-tolerant rice variety, Pokkali. Seedlings of the two varieties were grown in a plant house in nutrient solutions in a range of salt concentrations, i.e. 1 (Control), 20, 40, 60, 80, 100, 120 and 150 mM NaCl. Area of the youngest fully-expanded leaf at salt stress commencement and at 2-7 day intervals was measured non-destructively. The total plant biomass was measured after 30 days of salt stress. Based on the time courses of relative area of the youngest fully-expanded leaf RLA (i.e. area of the leaf under salt stress as a proportion of that in the control), 100 mM Na+ was determined as the optimum salinity level for varietal screening. At 100 mM Na+, the two phases could be identified clearly, with Phase I (from 0 to 10 days after reaching 100 mM Na+) showing a significantly lower rate of reduction of RLA than Phase 11 (starting after 10 days). The Sri Lankan improved rice variety At354 showed lower rates of reduction of relative plant biomass and RLA and a slower rate of Na+ accumulation in the shoot in comparison to Pokkali with increasing salt stress, demonstrating its greater salt tolerance than that of Pokkali.