General Combining Ability and Specific Combining Ability Analysis for Terminal Heat Tolerance in Wheat ( Triticum Aestivum L . )

Developing bread wheat genotypes for terminal heat tolerance is a critical objective for future breeding approaches. The line x tester mating analysis is one of the best approaches to demonstrate the appropriateness of the bread wheat genotypes for selection breeding programs. For this purpose, nine genotypes viz. T.J-83, NIA Sarrang, Khirman, SKD-1, Sehar-2006, Sarsabaz, AS-2002, NIA-Amber, and Nifa Barsat were used in this research. The experiment was planned in a factorial design with two treatments (normal and heat stress) at Botanical Garden Farm, Sindh Agriculture University Tandojam, during 2019-2020. The results depicted that at terminal heat stress, the genotypes were significantly affected by yield and physiological traits at late sowing. During the initial screening, the female parents, T.J-83, Sarsabaz, and Nifa Barsat executed very well under heat stress conditions for nearly all the yield and morphological traits. Similarly,the male parents such as Khirman and SKD-1 also performed well under heat stress conditions for all traits compare to the female parents. Furthermore, through genetic analysis, the mean effects of General Combining Ability (GCA) and Specific Combining Ability (SCA) were significant for all the characters signifying that additive and non-additive variances are important. Further, in heat-stress environments, the GCA was dominant for most characters in contrast to SCA variations. Hence, in this study, under both normal and heat stress conditions, Khirman and SKD-1 proved to be better general combiners for various characters. Therefore, these genotypes are recommended as vigor parents for hybridization and selection programs as emerging terminal heat stress tolerant genotypes


Introduction
Triticum aestivum (L.), is a member of the Poaceae family, is widely regarded as the principal cereal staple crop of many nations. It is grown under both irrigated and rain-fed conditions, is classified as a domesticated, self-pollinating crop, and has played a significant role in the development of numerous diverse domesticated wheat varieties (Ijaz et al., 2015). Wheat is a staple food of a huge human population globally consumed, processed and refined (Bhutto et al., 2021). Bread wheat provides approximately 70% of calories and 80% of protein (Ahmed et al., 2022). During 2020-21, t h e cultivation of wheat decreased 2.1 percent as 8,976 thousand hectares against last years's sown of 9,168 thousand hectares. The production of wheat declined to 26.394 million tonnes (3.9 percent) compared to 27.464 million tonnes production of last year. Wheat production declined due to decline in area sown, shortfall in irrigation water and drought conditions at sowing, less fertilizers offtake and heat wave in March/April, Wheat crop recorded high prodcuction of 27.293 million tonnes showing an increase of 8.1 percent over 25.248 million tonnes production of last year (GoP, 2021(GoP, -2022. For a wheat breeding effort to be successful, there must be evidence of General Combining Ability (GCA), Specific Combining Ability (SCA), and gene action in the breeding material. the line-tester analysis technique Kempthorne first proposed (1957) who described the technique as one of the effective strategies for evaluating the effects of combining ability while choosing desired parents and crosses for pedigree manipulation (Jain and Sastry, 2012). While specialized combining ability is an assignment tool that aids in predicting non-additive type gene actions, general combining ability is a tool to aid in evaluating the additive type of gene effects. Also, it is believed that additive gene actions or complementing episttic gene interactions are reliable and unfixable, whereas non-additive gene actions are not (Iqbal et al., 2017). To produce effective genotypes against heat stress, a breeding programme must develop heat tolerant and high yielding genotypes (Moustafa et al., 2021). One of the crucial biometrical methods to evaluate the impacts of general (GCA) and specialised (SCA) combining abilities and to identify the genes responsible for different features is the diallel mating design (Saleem et al., 2020). The best parents and their cross-combinations for producing superior offspring when breeding for heat tolerance can be found using the GCA and SCA effects .
Selection is based on morphological and physiological characteristics related to heat stress performance . Tolerance metabolism has been observed in chlorophyll, leaf senescence, photosynthetic rate and temerature (Moustafa et al., 2021). The relationship between morphophysiological traits with heat tolerance is vital for selecting suitable genotypes against heat stress Poudel et al., 2021). Keeping in view the foregoing, the main goal of the current study is to find general and specific combining ability, among varietal yield in F1 hybrid genotypes, based on morphophysiological features, in order to generate a new productive, heat-tolerant and high yielding variety. The current study used the line x tester approach to assess wheat genotypes for heat stress resistance at the terminal stage based on morpho-physiological features. Determine the combining capacities (GCA and SCA) of parental genotypes and F1 hybrids under heat stress was the goal of our investigation.

Materials and Methods
The present research was carried out at the Experimental Field, Department of Plant Breeding & Genetics, Sindh Agriculture University, Tandojam.
The experiment was laid-out in two successive Rabi season's 2019 and 2020. The plant material used in this study comprised of seven diverse wheat genotypes, including four tolerant and three susceptible to heat stress. The tolerant genotypes were selected based on their superior performance under heat stress, while the susceptible ones were selected from commercial varieties commonly grown in the region. The design used was Factorial Design (RCBD) with three replications. In this context, the experimental materials were evaluated in two sowing dates i.e. normal planting (25 th November) and late planting (25 th December). The experimental details are as under. The investigation was conducted at Botanical Garden, Department of Plant Breeding and Genetics, Sindh Agriculture University, Tando Jam. Meteorological condition: The main meteorological factors such as weekly distribution of rainfall, minimum and maximum temperature and relative humidity recorded through meteorological observatory of Nuclear Institute of Agriculture (NIA), for the period of investigation are presented in graphically for the year 2019-20 in Fig.1.

Fig. 1. Meteorological data of wheat crop during 2019-20
Temperature during the experimental period: The temperature during the entire wheat season was found to be variable and wide range of differences observed for all the traits under both sowing dates ( Fig.1). High temperature of 44 0 C persisted on 27 th October during the third week of November. The temperatures from December till March were relatively favorable for wheat crop planted in normal condition (November 20 th ), while low and high temperature during anthesis and grain formation stage were not suitable as crop planted in late December (20 th ) (Fig.1). Moderately high temperatures (20°C-25°C) were recorded during the 2nd week of February (15 days after anthesis) until the 4th week of February. The temperature increased in March until the 20th, reaching 29°C. Disastrous temperatures were observed in the end of March and the month of April (>40°C). Morphological parameters: The morphological characters studied were days to 75% heading, days to 75% maturity, plant height (cm), number of productive tillers, spike length spike -1 (cm), number of spikelets spike -1, number of spikelets spike -1 , grains yield plant -1 (g), and seed index (weight of 1000 grains in g) Statistical analysis: Line x tester analysis: The experimental data obtained on 13 characters were subjected to analysis of variance (Gomez and Gomez, 1984). After testing the significance among the treatments and crosses, line x tester analysis for the estimation of combining ability was estimated.For the estimation of general and specific combining ability variances as outlined by Kempthorne (1957) was followed. Statistix 8.1 software was used for analysis.

Results
Wheat is a major crop consumed by half of the world population (Bhutto et al., 2021). During flowering and booting stages of wheat crops were more affected by heat stress (Yang et al., 2016;Mirosavljević et al., 2021). Hence, the present study was to work on lines and testers using general combining and specific combining abilities among varietal yield in F1 hybrid genotypes. Analysis of variance: The analysis of variance specified that heat stress treatments caused substantial effects on days to 75% heading, days to 75% maturity, plant height, number of productive tillers plant -1 , spike length 1 , number of spikelets spike -1 , grains weight plant -1 , grain weight spike -1 , grain yield plant -1 , seed index , biological yield plant -1 , harvest index, flag leaf area , relative water content, chlorophyll content, cell membrane stability and stomatal conductance (Tables-1). The F1 hybrid cultivars also significantly varied in their performances for all the yield characters. The genetic variation of heat stress treatments x genotypes were observed for significant with the majority of the characters including days to 75% heading, days to 75% maturity , number of productive tillers plant -1 , plant height, spike length, number of spikelets spike -1 , grain yield plant -1 , biological yield plant -1 , harvest index, seed index , relative water content, flag leaf area, chlorophyll content, cell membrane stability and stomatol conductance indicating that varities behaved differntly under heat stress circumstances (Table 1).

Discussion
For heat stress-tolerant, the General Combining Ability (GCA) and Specific Combining Ability (SCA) are important brreding techniques. In the current study, mean squares for GCA and SCA were significant for morphological and yield traits in the stress and non-stress treatments, demonstrating that parents responded differentially to the heat stress conditions for several traits. Whereas, Xu et al., 2022 studied significant results for five parental lines, our result showed that parent, WH-1139 (-1.167) exhibited highest significant negative GCA effect for days to 75% heading (Table1). Similarly, noteworthy outcomes for the line x tester cross for grain yield in wheat have been reported by Mirosavljević et al., 2021 (Triticum aestivum L.). Certain parental lines during the current study shown favourable GCA effects (Table 1). However, few lines considered to be highly desirable exhibit significant negative GCA effects. Similarly, Tabassum et al., (2017) claimed that PBW681 may be regarded as a good general combiner for dwarf-ness due to their respective strong positive GCA impacts. However, certain parental lines showed significant positive GCA benefits, whilst others showed significant negative GCA effects (Table 1). According to Hassan et al., (2021) and Deniz et al., (2015) reported that the number of tillers among the lines showed positive effects on heat tolerance. Bhalerao et al., 2020. after the GCA demonstrated significantly beneficial impacts on the number of tillers among the lines (1.62). Our results showed that spike length is one of the major yield-contributing traits ( Table 1).
The current findings on the parental lines' showed a significant beneficial GCA impact against the number of spikelets spike -1 . Hassan et al. (2021) and Bhalerao et al., 2020 (Table 2), the number of grains spike -1 exhibited strong favorable GCA effects ( Table  2). As a result, the conclusions of the aforementioned authors are closely supported by the current observations, which show considerable GCA impacts against the number of grains spike -1. . Positive GCA values indicate prospective parents since grain weight spike -1 is a desired yield subsidizing in our research. A strong combiner for the trait was found in the line Sarsabaz. Hassan et al. (2021) noted substantial favorable GCA impacts against lines HTSBWON-15-0029 (0.19g), HTSBWON-15-0040 (2.20g), HTSBWON-15-0079 (0.81g), and Faisalabad-08 (0.18g) in the grains yield plant -1 study ( Table 2). The results of the present study are in close confirmation with the earlier observations made by Kapoor et al., 2011. Bhalerao 2020 stated that parents presented different degrees and magnitudes of GCA effects under heat stress for the trait. Among the lines, AKW-1071 (1.64%), AKAW-4927 (1.19%), PDKV-Sardar (0.49%), and AKAW-4627 (0.3%) were recognized as good general combiners for 1000 grains weight in the ( Table 2). The harvest index for GCA effects was reported similar to the effects of GCA against the yield by Hassan et al., 2021 (Table 3).
Additionally, Mirosavljević et al., 2021Mirosavljević et al., et al. (2017 reported similar findings regarding the SCA effects for days to 75% heading ( Table-4). Moreover, Kamara et al., 2021 investigated the impact of SCA on the number of days till heading under a late sowing date. The crosses P2 x P6 (-0.84), P1 x P4 (-0.76), and P4 x P6 (-0.59) demonstrated SCA effects in early days to 50% heading due to late sowing in the P3 x P5 and P4 x P5 crossings, respectively, displayed substantial negative SCA effects in a decrease of days to heading earlier (Table 4). However, Bhalerao et al., 2020 reported findings that were equivalent to those of this investigation, including negative SCA effects.The present study's findings about the impact of SCA on the reduction of days to 75% maturity against F1 hybrids are consistent with those of the aforementioned researchers for the trait (Table 4). Kamara et al. (2021) reported SCA impacts on plant height and features associated with bread wheat in both hot and normal conditions. Only five of the sixteen crosses under investigation P1 x P3, P2 x P5, P3 x P4, P3 x P6, and P3 x P exhibited highly significant negative SCA effects. Other crosses showed substantial and non-significant negative SCA effects, including P4 x P5, P2 x P6, and P4 x P6, as well as P1 x P2 and P2 x P6. Bhalerao et al. (2020) had similar research documented SCA's impact on contributing characteristics in wheat during heat stress (Table 4). Both highly s Hassan et al (2021) counted significant positive SCA effects for the number of productive tillers plant -1 in crosses that varied from 0.28 to 2.72 under heat stress condition. Whereas, Bhalerao et al., 2020 reported significant and nosignificant positive SCA effects, for number of productive tillers. Majority of the crosses showed positive SCA effects with number of tillers plant -1 those ranged from 0.027 to 2.439 tillers plant -1 ( Table-4). These results are consistent with those from other studies that found SCA effects for the same trait, including Hassan et al., 2021;Bhalerao et al., 2020;and Tabassum et al., (2017) (Table-4).
Yet, the computation of combining ability estimations in heat-stressed bread wheat yielded equal outcomes in their F1. The crosses demonstrated notable favorable SCA effects for the spikelet with the most spikelets, spile -1 . Tabassum et al., (2017); Bhalerao et al., (2020); Hassan et al., (2021) who counted the same crosses under SCA effects as indicated in this work noticed comparable results (Table 5.). The number of grains spike -1 the crosses was reported by Hassan et al., 2021,having substantial positive SCA effects. Out of eighteen hybrids investigated, nine responded positively to SCA effects regarding the grain weight plant -1 ( Table 5). The hybrid's performance revealed that in grain yield plant -1 , the SCA effects were less in normal condition. Bhalerao et al. (2020) found that grain yield plant -1 had a substantially favorable SCA effect in comparison to the crosses. Moreover, (Hassan et al., 2021) revealed outcomes that were comparable to those shown for crosses in this investigation. Naturally all these could be grown further as heat stress tolerant hybrids. Similar study was conducted by Hassan et al., 2021 to know the SCA effects on 1000 grain weight agaisnt crosses under heat stress condition. Furthermore that the present results are also in close agreement to the findings of (Bhalerao et al., 2020;Tabassum et al., 2017).
In the F1 hybrids, the SCA effects were observed against the biological yield plant -1 during current investigation (Table 6). Moreover, at same time the authors also recorded significant higher positive and negative SCA effects against biological yield plant -1 ( Table 6) (Table 6). However, some variable SCA effects against the crosses for harvest index were demonstrated by Tabassumm et al. (2017). Furthermore, the other hybrids remained positive SCA effects under heat-stress    Table 5. The effects of Specific Combining Ability (SCA) on F1 hybrids of bread wheat for Number of spikelets spike -1 , grain spike -1 , grain weight spike -1 , and grain yield plant -1 and seed index grown under non-heat stress and heat stress conditions. SCA = Specific Combining Ability, cm = Centimeter, SE = Standard Error, -1 = Per (each) and % = Percentage

Conclusions
Keeping in view the overall results of the present investigation regarding yield, morphological and physiological characters of the parents and F1 hybrids shown by lines x tester mating design under terminal heat stress are concluded. The morphological and yield haracters were significantly affected by the terminal heat stress and the importance of treatment x parent relations exhibited that parents performed inconsistency through-out non-heat stress condition, however some were observed more heat stress tolerant than the other. The bread wheat parental lines , T.J-83, NIFA-Barsat and Sarsabaz performed very well under heat stress condition for almost all the yield and morphological parameters while the second heat stress tolerant category parents were SKD-1 and Khirman. The Combining Ability (GCA) and Specific Combinng Ability (SCA) were significantly higher for almost yield and morphological. Hence, under both non-heat stress and heat stress conditions revealed that T.J.-83, NIFA-Barsat, Sarsabaz, SKD-1, and Khirman were identified as good common combiners; as a result, these parents could be used in the future for selection and hybridization programmes under terminal heat stress tolerance. Among the eighteen F1 hybrids examined, the crosses, T.J-83 x NIFA-Barsat,  T.J-83 x SKD-1, Sarsabaz x Khirman, Nifa Barsat Barsat x SKD-1, Sarsabaz x SKD-1 and NIFA x Khirman acknowledged to be good specific combiners for majority of the yield and morphological characters, therefore these hybrids should be considered as potential F1 hybrids for crossbreed in terminal heat stress condition.