Effects of submergence time, light and nitrate compounds on the survival of some modern rice cultivars under flooded conditions

Flooding significantly affects rice survival due to fast oxygen depletion, and the possible use of nitrate (NO3-) as an alternate terminal electron acceptor to enable anaerobic metabolism remains little investigated. In particular, the efficiency of different NO3 - compounds in increasing submergence tolerance across different rice cultivars has received little attention. Six rice cultivars-OM4900, OM6976, OM4218, IR50404, OM7347 and IR64Sub1-were tested for their flooding responses during 5-10 and 15-day submergence periods in dark conditions. Additionally, the effects of exogenous NO3 - treatments on seedling survival were evaluated. After 5 days, IR64Sub1, OM7347 and OM4900 exhibited the highest survival rates (86, 83 and 81 %, respectively). Survival sharply decreased after 10 days, with most cultivars falling below 26 %, except OM7347 and IR64Sub1, which were reduced to 38.7 and 30.7 %, respectively. At 15 days, survival was ≤10 % for all lines. Light significantly improved survival (72.91 %) compared with dark conditions (12.31 %), and darkness induced a 142.252 % increase in soluble sugar levels, demonstrating the importance of carbohydrate reserves under hypoxia. Application of 3 mg L-1 calcium nitrate (Ca(NO3)2) increased survival after 10-day submergence by 47.7 % in OM4218, 53 % in OM4900, and 40 % in IR64Sub1, while silver nitrate (AgNO3) stimulated shoot elongation (up to 4.42 cm). Calcium nitrate also increased soluble sugar accumulation to 2.91.5.31 mg g-1 dry weight (DW). Dissolved oxygen measurements showed delayed oxygen depletion under Ca(NO3)2 and elevated oxygen availability with AgNO3. These results highlight the novel role of NO3 - compounds in enhancing rice submergence tolerance and provide a foundation for further elucidation of NO3- mediated mechanisms of anaerobic survival.