The study of some physiological responses to drought stress in three Iranian rice cultivars

Introduction Water scarcity causes rice yield loss more than 30% under drought stress at field conditions. Hence, employing drought tolerant cultivars in this condition is an effective way to overcome such a challenge. Therefore, in current research, we have used three Iranian rice cultivars named Amol3, Sang-tarom and Neda, which is originated from a cross between Amol3 and Sang-tarom, in order to investigate responses of the selected rice cultivars to drought stress. In this study, several important physiological characteristics were evaluated under drought stress during a growth period. Therefore, present study was aimed to achieve some information about mechanisms and responses to drought stress and to use them in identifying tolerant cultivars in drought stress situation as well as employ them in rice breeding program.   Materials and Methods  The rice seedlings of three genotypes were tested at greenhouse under three levels of water treatments as: control (FTSW=1.0, Fraction of Transpirable Soil Water), mild drought stress (FTSW=0.5) and severe drought stress (FTSW=0.2) in four replications in 28±2ºC in day and 25± 2º in night and in 14 L/10 D light period in a factorial arrangement of treatments in randomized complete block design in PVC pipes with 1-meter-long and 10 cm in diameter. The root length, stomatal aperture size, ABA content in leaves and wet and dry weights of panicles were measured as essential features in these cultivars in both drought stress levels and in control treatment. For data analysis SPSS and SAS software was applied.   Results and discussion  In this study, we found that in rice cultivar Neda, showed a significant reduction (P< 0.05) in ABA level under mild and severe drought stress treatments, while rice cultivar Amol3 had a significant reduction in ABA level only at severe drought stress compared to control. We also observed a non-significant increasing trend in ABA level in response to drought stress levels in Sang-tarom cultivar. The amount of ABA reduction was higher in Neda compared to Amol3. In all three cultivars root growth was increased but in different way in drought stress treatments compared to control. Stomatal aperture size reduced significantly in Neda under severe drought stress, under both two drought stress levels in Amol3 compared to control. No significant difference was indicated in stomatal aperture size of Sang-tarom at both drought stress treatments compared to control. Differences in stomatal aperture size reduction were not significant among cultivars. Wet and dry weights of panicles were also affected by drought stress treatments in three cultivars. There was no significant reduction in wet and dry weights of panicles in Neda at mild drought stress level compared to control, but highly significant reduction was observed for two characteristics of wet and dry weights of panicles in Amol3 and Sang-tarom at both drought stress levels as compared to control treatment. In all three cultivars, decreasing of wet and dry weights of panicles was significant under severe drought stress compared to control.   Conclusion   In Neda, no significant reduction was observed in stomatal aperture size while the maximum dry and fresh weights of panicles was found in Neda under mild drought stress condition, which could be a remarkable sign among the cultivars in long-term drought stress. A total of 30% reduction was observed for grain yield of Neda while the yield loss for Amol3 and san-tarom were 58% and 50%, respectively. Reduction in ABA level in leaf, root length stimulation and stomatal aperture size regulation could be assigned to the behaviors in highly tolerant cultivars under drought stress condition. Neda showed more similar responses to Amol3, one of its parent, than to Sang-tarom regarding increasing in root length and reduction in ABA level in leaf in response to drought stress. A good correlation between root length and reduction in ABA level or redistribution of ABA in root for better hydraulic conductivity and also preventing ethylene action in drought stress that needs to be more studied.