Physiological Traits Associated with Tolerance to Stagnant Flooding in Rice

High rainfall usually causes flooding, adversely affecting rice production in developing countries. Recently, varieties have been developed that overcome losses caused by complete submergence by introgression of the SUB1 gene into popular varieties. However, Sub1 varieties do not perform well under long-term partial stagnant flooding (SF; 20-50 cm) that predominates in large areas of rainfed lowlands. A set of selected genotypes, including two Sub1 varieties were subjected to SF with water depth increased by 10 cm weekly after transplanting until it reaches 55 cm, then maintained through maturity. Water depth in the control plots was kept at 5 cm. The study aims to understand the bases for variation in SF tolerance observed among selected rice genotypes. IRRI 119 and IRRI 154 had higher survival and yield than PSBRc18-Sub1 and Swarna-Sub1 under SF. Swarna-Sub1 did not survive, whereas PSBRc-Sub1 performed poorly with low survival and low yield. Stagnant flooding increased shoot elongation but decreased tiller number and dry matter accumulation. The contribution to shoot elongation by the leaf sheath was 59% while that of the leaf blade was 41%. Concentrations of soluble sugars in leaves and roots decreased but those in the stem increased. Correlations of length of leaf blade at day 7 of treatment with survival at maturity (r = 0.77**) and with yield (r = 0.86**) were high. Stem dry weight per plant measured 7 d after imposition of SF correlated significantly with survival (r = 0.70**) and with yield (r =0.54**). Root soluble sugar concentration had highest correlation with survival (r = 0.60** ) and with yield (r = 0.60**) at day 28 of the treatment. These traits could be further verified and used to identify quantitative trait loci associated with tolerance to SF for breeding new cultivars tolerant to prolonged partial SF.