Effect of nitrogen management on improving resilience of submergence tolerant (Sub1) rice varieties in flood-prone rainfed lowlands

Flash flood causing submergence adversely affects rice production in vast areas of rainfed lowlands of South and Southeast Asia. Incorporation of tolerance into conventional/modern high yielding varieties has been made by introgression of the Sub1 locus through molecular assisted backcrossing, and a few varieties have been released and made available to farmers for commercial cultivation in India. In the present study, we investigated the response of split dose N applications to further enhance the survival and yield potential of two Sub1 introgressed varieties i.e. Sambha Mahsuri-Sub1 and BR11-Sub1, during the wet seasons of 2019 and 2020. Experiments were conducted in outdoor "ponds" at the Crop Physiology research farm in A.N.D.U.A&T, Kumarganj, Ayodhya, India. Thirty days (d) old seedlings were transplanted, and 28-days after transplanting, plants were completely submerged for 18-days. Full doses of phosphorus and potassium @40 kg ha-1 P2 O 5 -K2 O (recommended dose of fertilizer) through single super phosphate and muriate of potash, respectively were applied as basal, whereas nitrogen @120kg ha-1 N through Urea was applied in four split doses. Plant survival was recorded at 0-, 5- and 20-days after desubmergence (AS) to study the recovery dynamics of the Sub1 varieties. Flood water characteristics were measured throughout the experimental period. Results of the study revealed that 1/4th N applied as basal and remaining N in three equal splits at 5‑, 20- and 40- d, significantly enhanced the plant survival than those plants were grown without N or ½ dose N applied as basal before submergence. The split N application significantly influenced the pre- and post-submergence shoot dry biomass plant-1, tiller number plant-1, total chlorophyll and soluble sugar concentrations and grain yield plant-1. Whereas, in submergence-prone ecologies, farmers usually apply the full dose of N fertilizer after transplanting and no additional dose of N fertilizer applied after de-submergence. Present study clearly indicates that Sub1 rice varieties need considerable amounts of N for (faster) recovery after de-submergence because most of the applied N as basal before flooding is replenished from soil. Our results can help to develop better N application scheduling for submerged rice crops, and increase the stress tolerance of the newly developed submergence tolerant rice varieties.