Evaluation of indices of tolerance and susceptibility to stress and GDD changes in some rice cultivars in response to heat stress

Rice (Oryza sativa.L) is one of the most important grains in the world, which as one of the major food sources, provides food to more than three billion people in the world. In the near future, there are several challenges to achieve higher yields in rice plants, and one of the most basic of these challenges is increasing the average temperature of the earth's surface. Increasing the temperature indirectly reduces production by increasing water demand in areas with limited water and reducing the area under cultivation. Therefore, plant physiologists' understanding of plant physiological responses to high temperatures, as well as the selection of adaptation strategies, is an effective and very important role in rice research. In line with international research, research on introducing a diverse range of heat-tolerant rice cultivars on the one hand and finding effective physiological traits and mechanisms in tolerance or resistance of common rice cultivars in the province to this stress can play a valuable role in inactivation, modulation of negative effects or greater efficiency of the heat and light stress situation arising in the coming years. In other words, making management decisions regarding the production and use of improved cultivars, introducing native tolerant or heat-resistant cultivars and possible required changes in the temporal and spatial patterns of cultivation of cultivars, can be a significant help in combating heat stress. Therefore, this experiment was conducted to evaluate heat stress on grain yield and to study stress tolerance indices in studied cultivars in the Khuzestan region.

In order to evaluate the stress tolerance and susceptibility indices of rice plant (Oryza sativa L.) in response to heat stress, an experiment with two factors of planting date and cultivar on the basis of split plots in a randomized complete blocks design with three replications in two years 2017 and 2018 were carried out in the south of Khuzestan province. In order to apply high temperature stress (heat stress) in field conditions, three planting dates of May 15, June 5 and June 25 were selected as the main plots. At each planting date, seven rice cultivars including Anbouri, Champa, Daniel, Pollen, Hamr, and Hoveyzeh (native cultivars) and N22 (International cultivar) were cultivated as sub-plots. Stress tolerance indices included GWHSI SSI, STI, TOL.

The results showed that at the level of planting date, the highest grain yield was on 5 June with a yield of 5737 kg.ha-1, which was 41.7 more than the first planting date. The highest grain yield was obtained among the cultivars related to Hoveyzeh with an average of 5606 kg.ha-1, which was 34.3% and 29.6% higher than Champa and Anbori, respectively. In evaluating stress tolerance indices, N22 and Hoveyzeh cultivars had the highest SSI and STI, Hammer and Hoveyzeh cultivars showed the highest STI and N22 and Hammer cultivars showed the lowest TOL. The lowest (SSI) was obtained in Hoveyzeh cultivar and then in Hamr and N22 cultivars. The lowest heat stress tolerance was also observed in Champa cultivar. Also, the highest STI index and the lowest TOL index were obtained in Hamr, N22 and Hoveyzeh cultivars. The lowest GDD in all three planting dates was in Hoveyzeh cultivar and the highest one was in sensitive cultivars to heat stress (Anburi and Champa).

In general, heat-tolerant cultivar N22, Daniel and local cultivars Hoveyzeh and Hamr were heat-tolerant cultivar and Geredeh Ramhormoz cultivar and Anbouri and Champa (native cultivars) were identified as heat-sensitive.