Evaluation of stress tolerance in IR64 rice near-isogenic lines carrying SUB1 and DRO1 genes

Flooding and drought significantly reduce rice yield in rainfed environments. Rice varieties that tolerate complete inundation for up to two weeks carry the SUB1A gene, while those enduring deeper water conditions for weeks or months have the SK1 and SK2 genes. Conversely, the DRO1 gene, responsible for deep-rooting, helps in water acquisition under drought. In this study, we compared the growth of NIL-SUB1DRO1 a novel rice genotype with an IR64 background regarding its dual tolerance to submergence and drought. Additionally, we assessed its recovery capacity after exposure to stress. Sixteen and thirteen-days old seedlings of three genotypes (Experiment 1-1 and 2-1) and fourteen-days old seedlings of the two genotypes (Experiment 1-2 and 2-2) were tested under submergence and drought stress in a greenhouse experimental condition respectively. Seedlings were submerged for 10 and 7 days and then allowed to recover for 10 and 7 days respectively. In the drought experiment, seedlings underwent 29 days of drought (Experiment 2-1) and 18 days of drought, followed by 10 days of rewatering (Experiment 2-2). Growth parameters were measured before and after treatment, 4 days after submergence in experiment 1-1 and after the recovery periods. Submergence and drought adversely affected growth and performance. Shoot elongation in submerged plants was reduced by 29.2% for NIL-SUB1DRO1 compared to IR64. Accelerated shoot elongation of IR64 negatively affected its recovery. Chlorophyll content and maximum fluorescence of IR64 were significantly lower than other genotypes after 10 and 7 days of complete submergence. Ten days after recovery in experiment 1-1 the Chlorophyll content and maximum fluorescence of IR64 were not significantly different compared to other genotypes. Seven days after of recovery in experiment 1-2, NIL-SUB1DRO1 had significantly higher chlorophyll content and maximum fluorescence than IR64. After 29 days of drought the tiller number and leaf area of IR64 was lower than other genotypes (Experiment 2-1), while in Experiment 2-2 during drought treatment and recovery, NIL-SUB1DRO1 had greater relative water content, leaf water potential, leaf area, SPAD value, dry weights of shoots and roots, root length, surface area and volume compared to IR64. stomatal conductance of IR64 was higher than NIL-SUB1DRO1 during drought, leading to greater water loss and reduced growth during recovery. NIL-SUB1DRO1 absorbed and retained water more effectively under dry conditions. NIL-SUB1DRO1 and NIL-SUB1 is tolerant to submergence and NIL-SUB1DRO1 and NIL-DRO1 to drought, with no negative effects from combining these genes in modern rice varieties for rainfed lowlands.