Abstract
With a view to developing drought tolerant variety a study was carried out during Transplanted Aman seasons of 2009 to 2012 with 259 rice genotypes, BRRI, Gazipur to assess their diversity under drought stress. Morpho-physiological and root traits viz. root length, total root number, root weight, total cumulative root length, cumulative root length up to 30cm depth, cumulative root length below 30cm depth, root shoot ratio and deep root shoot ratio showed negative and significant correlation with drought tolerance ability. About 62% of the variability in drought tolerance was contributed by the studied characters and deep root weight (1.487) showed maximum direct effect on drought tolerance score followed by total cumulative root length (0.984), shoot weight (0.313) and root shoot ratio (0.279). The genotypes showed wider genetic diversity and twelve local genotypes and nine IRRI genotypes showed more resist to drought. Some tolerant and susceptible genotypes [IR74371-70-1-1(BRRI dhan56), BR7873-5(Nils)-51-HR6 (BRRI dhan57), Batraj, BR11, Muktahar and Bunadhan] were grown under drought stress for different durations (5, 10 & 15 days) at reproductive (PI, booting and flowering) phase and their response to the drought stresses varied significantly. Relative water content (RWC), leaf area and leaf dry matter significantly reduced due to drought in all the genotypes. After re-watering, all the genotypes recovered leaf area at drought in PI stage for 5 days but the recovery was low or drastically reduced with the severity of drought both in booting and flowering stages. Dry matter accumulation, panicle dry weight, etc. were high in the genotypes (IR74371-70-1-1 and Batraj) compared to the others. Water deficit at PI and booting stage significantly delayed flowering and maturity. Irrespective of genotypes and stage of plant growth, drought stress significantly reduced panicle exertion and spikelet number per plant and increased sterility percentage. The relative yield loss (RYL) was less than 50% in IR74371-70-1-1 and Batraj even at 10 days drought stress while the other genotypes had RYL about 50% even at 5 days of drought stress and the RYL increased up to 95% for 15 days of drought stress. A reduction in maximum quantum yield (Fv/Fm) of PSII in IR74371-70-1-1 and Batraj was significantly lower (7 to 33% and 9 to 37%, respectively) at 15 days of drought compared to other genotypes even at 5 days (24 to 54%) of drought. The rate of net CO₂ assimilation (Pn), stomatal conductance (gs) and transpiration (E) decreased with increasing drought intensity and the decrement was significantly slower in IR74371-70-1-1 and Batraj than the other genotypes. Proline accumulation was about 22-fold higher in IR74371-70-1-1 followed by 19.5-fold in Batraj for drought for 15 days at PI stage over control while proline accumulation was less than 7-fold in the other genotypes. Similarly the soluble sugar accumulation was increased by 207% in IR74371-70-1-1 followed by 191% in Batraj under the same condition. Drought stress caused a sharp increase in Glutathione S- transferase (GST) and Glyoxalase I (Gly I) activities in IR74371-70-1-1 and Batraj indicating their high antioxidant and detoxifying ability under stress. The genotypes (IR74371-70-1-1 and Batraj) had significantly high RWC, low spikelet sterility, high grain yield, high proline & soluble sugars accumulation, enhanced water use efficiency, maximum quantum yield and high photosynthesis rate, and high activities of GST and Gly I enzymes compared to the other genotypes for imposing drought at different durations and stages, and those traits enabled them for the better physiological functions and adaptation under drought. Thus, those characters could be treated as important traits in rice for developing drought tolerant cultivars through breeding practices. Finally the research leads to the conclusion that the genotypes (IR74371-70-1-1 and Batraj) appear to be drought resistant and Batraj can be used as a donor to develop drought tolerant variety through breeding practices.