Abstract: The discovery and subsequent introgression of the SUB1A-1 gene into rice resulted in remarkable success in tolerating complete submergence for about two weeks. However, the tolerance of Sub1-introgressed cultivars is still inferior to that of FR13A, the donor of the gene. To investigate this discrepancy, we examined the submergence tolerance potential of FR13A and several superior landraces that show significantly better tolerance than the Sub1-introgressed cultivar Swarna Sub1 under prolonged submergence. We found that several landraces (AC42088, AC42087, and AC1303) and FR13A carry functional alleles of both SUB1 and SNORKEL genes, along with a dominant allele of SD1. Genotypes carrying the SD1 allele exhibited greater initial plant height, and the combined presence of both SUB1 and SNORKEL genes may confer better survivability by allowing moderate shoot elongation under prolonged submergence, particularly beyond two weeks. Interestingly, OsSUB1A-1 expression was not suppressed despite high expression of SNORKEL genes in the landraces and FR13A. These genotypes also had relatively thicker leaf gas films and delayed depletion of those films under water. Higher epicuticular wax concentration in these genotypes contributed to prolonged persistence of the gas film and improved maintenance of chlorophyll in inundated leaf tissues. Scanning electron microscopy revealed a thick layer of deposition on the leaf surfaces of these landraces and FR13A. Higher transcript abundance of wax biosynthesis genes and a strong, positive, significant correlation between these transcripts and leaf wax concentration suggest the presence of very long-chain fatty acids that may delay loss of leaf hydrophobicity in these landraces. Overall, this study suggests that SUB1A-mediated quiescence, supplemented by moderate elongation via SNORKEL genes, could be beneficial for survival under prolonged submergence beyond two weeks.

Key words: epicuticular wax, flooding, hypoxia, leaf gas film, leaf hydrophobicity, SNORKEL