Growth characteristics and yield of contrasting rice genotypes under long-term stagnant flooding

Context: Stagnant flooding (SF, water depth of 25-50 cm for few weeks to months) is one of the major limiting factors for rice production in humid rainfed areas and is becoming more serious with climate change. Research question: Evaluating growth and yield of rice under SF is essential to develop effective phenotyping approaches for breeding SF-tolerant genotypes to sustain yield gains and ensure food security in affected areas. To date, little attention has been paid for assessing responses of diverse rice genotypes under long-term stagnant flooding in natural fields. Methods: Field experiments were conducted in wet season of 2015 (WS 2015) and dry season of 2016 (DS 2016). Here we evaluated six rice genotypes contrasting in tolerance of complete submergence and SF - Apo, Swarna, Ciherang are sensitive to both submergence and SF, Swarna-Sub1 and Ciherang-Sub1 are sensitive to SF but tolerant of submergence, and IRRI154 is tolerant of SF but sensitive to submergence. Growth, yield and yield attributes were evaluated under water depths of 0 (T1), 5 (T2, control), 30 (T3) and 50 cm (T4) from 14 d after transplanting to maturity. Results: Growth duration of all rice genotypes was prolonged under SF, mainly during vegetative stage. Survival rate, tiller number, crop growth rate and leaf area index (LAI) decreased substantially under SF and with increasing depth of floodwater. However, plant height and specific leaf weight (SLW) remarkably increased under SF. Grain yield decreased progressively with increasing water depth, by 31.6% and 43.3% in the WS 2015, and by 42.9% and 56.8% in the DS 2016, respectively, under T3 and T4. The decline in yield under SF was associated with a significant decrease in number panicles per unit area, spikelets per panicle, grain filling percentage, biomass and harvest index (HI). IRRI154, the SF-tolerant genotype, produced higher grain yield under SF because of its higher survival rate, number of panicles per unit area, and biomass. Sub1 genotypes, SwarnaSub1 and Ciherang-Sub1 had lower grain yield than their non- Sub1 types Swarna and Ciherang under SF. Overall grain yield correlated positively with panicle number, total number of spikelets, crop growth rate, survival, biomass, and HI, but negatively with plant height. Conclusions: Stagnant flooding significantly reduces growth and yield of rice. Both plant height and SLW increased under SF probably as adaptive strategies to increase survival and photosynthetic carbon assimilation per unit leaf area. Greater LAI is more important than thicker leaves under SF. Grain yield decreased progressively with increasing water depth, which was attributed to significant decrease in several yield attributes under SF. The higher survival rate, number of panicles, and biomass production contributed to the highest grain yield of SF-tolerant genotype under SF. Grain yield of the SUB1 genotype was lower than that of their respective nonSUB1 genotype under SF, suggesting that SUB1 should be introduced into varieties that are inherently tall or can sufficiently elongate under SF.