In rice, extreme maximum temperature is particularly important during flowering which usually lasts for 2-3 weeks. Exposure to high temperature for a few hours can greatly reduce pollen viability and therefore cause yield loss (
Mirza, 2011;
Raghunath and Beena, 2021). In comparison, breeding rice cultivars resistant to salinity and drought has gotten more attention than breeding types resistant to high temperatures. Only region-specific breeding initiatives have attempted to address rice’s high temperature tolerance since a thorough study conducted in the early 1980s, with limited success. (
Mackill, 1981;
Mackill et al., 1982; Mackill and Ni, 2001). The periods of high temperature negatively affected the sexual reproduction in rice
(Zakaria et al., 2022). Hence, there is an urgent need to address high temperature induced yield losses in rice to face a changing climate.
Heat tolerance is influenced by several genes rather than a single gene (
Mackill, 1981;
Maestri et al., 2002). According to Mackill and Ni (2001), several genes have an impact on the recessive genetic regulation of excessive pollen shedding in rice.
Yoshida et al., (1981), in contrast, noted that the majority of the genetic variation related to pollen shedding is additive. Their results showed significant broad sense and narrow sense heritabilities of 76 and 71%, respectively, while finding a high correlation between spikelet fertility and pollen shedding.
Evaluation of germplasm lines under control (S1) and high temperature conditions (S2 and S3)
Each growth phase of the rice plant is primarily defined by its cultivars, although the plant’s growth environment also influences the source-sink dynamics of the plant as a whole. The grain yield per unit area is used to evaluate the performance of various cultivars in rice. This is a significant trait, but other indicators, such as stability and efficiency, are also becoming more significant in considering climate change and the impending energy and water shortages
(Saikia et al., 2022).
Heat tolerance index
Heat tolerance indices were calculated under stress with relation to performance under control and presented in Table 2. A358 recorded minimum value for heat tolerant index of 0.02. The germplasm line A424 recorded higher HTI values of 40.21 under stress condition. Among the germplasm lines mean value recorded of 6.07 and it was significantly exceeded by sixty nine genotypes.
Heat susceptibility index
Heat susceptibility index were calculated for stress condition and presented in Table 2. A358 (0.004) recorded the minimum HSI. A424 recorded lower HSI values of 2.88. One sixty nine genotypes exceeded the general mean (0.93) significantly. Out of 293 genotypes, sixty nine genotypes significantly exceeded for both HTI and HS I (Fig 1).
Every year, the temperature rises and experiences significant changes, which pose a huge challenge to agricultural productivity. Only genotypes that are physiologically efficient or resistant can be used to produce crops under such conditions
(Saikia et al., 2022). Hence it may be possible to combine single plant yield and heat susceptibility index by specific breeding programme for high temperature tolerance in rice.
Correlation among yield components
The nature and extent of association that existed between the single plant yield and other yield component traits and also the association among the high temperature stress components were studied through correlation analysis. Character correlation studies enable clarify the strength and scope of character association in a crop. The inter relationship of component characters of yield provided the information about the consequences of selection for simultaneous improvement of desirable characters under selection.
The present study indicated that the single plant yield had highly significant and positive association with the traits
viz., panicle exertion, number of total tillers per plant, number of productive tillers, panicle length, number of filled grains per panicle and total dry matter production (Table 3).
Nor et al. (2014) for number of productive tillers and number of total tillers per plant
Rao et al. (2014) for panicle length and for total dry matter production was reported by
Venkanna et al., (2014). Spikelet sterility and heat susceptibility index had negative and significant correlation with single plant yield. Rice plants at the reproductive stage, including the processes of panicle initiation, male and female gametophyte development, anthesis, pollination and fertilization, are more susceptible to heat stress than at the vegetative stage
(Xu et al., 2021). Hence it may be possible to combine single plant yield and heat susceptibility index by specific breeding programme for high temperature tolerance in rice. The information on the correlation among the yield components shows the nature and extent of relationship with each other. This will help in the simultaneous improvement of high temperature tolerance traits along with single plant yield in the breeding programmes.
Number of productive tillers had significant and positive correlation with total dry matter production. Panicle length, number of filled grains per panicle, days to maturity, hundred grain weight, grain breadth and heat susceptible index had significant and negative correlation.
Lakshmi et al. (2014) reported that the correlation between number of productive tillers per plant and panicle length was negatively correlated. Grain breadth recorded significant and negative correlation with grain length breadth ratio. Similar findings were reported by
Venkanna et al. (2014) and
Kiran et al. (2016). Spikelet sterility had negative and significant association with grain breadth.
= Genotypic variance of the trait ‘x’.
= Genotypic variance of the trait ‘y’.
