Genetic analysis of yield and its attributes in bread wheat (Triticum aestivum L. em. Thell) under irrigated and rainfed conditions

Experiments were conducted to study genetics and combining ability for yield and its attributes under Irrigated (E1) and Rainfed (E2) conditions using F1 hybrids derived from Line X Tester mating design by crossing eleven lines with three testers. Significant differences were observed among all the genotypes for all the traits in both E1 and E2 environments. The σ2gca/σ2sca ratio indicated the predominance of non-additive gene action for all the characters in both environments. Therefore, this component of the variance can be utilized in the breeding program through the exploitation of heterosis and the selection process for identification of superior plant types should be postponed to further generations like F4 or F5. Based on combining ability effect and per se performance in rainfed conditions, parents UP2572 and VL3001 were identified as good general combiners for grain yield per plant. In cross combinations, cross PRL/2*PASTOR//PBW343*2/KUKUNA/3/ROLF07/4/BERKUT//… × WH1142 and UP2572 × WH1142 were identified as the best specific combiners for grain yield per plant under irrigated and rainfed conditions, respectively. So, these identified parents and cross combinations can be exploited for developing water stress tolerant cultivars by identifying transgressive segregants in further breeding cycles.


Introduction
Wheat (Triticum aestivum L. em. Thell) is an allohexaploid (2n = 6x = 42) self-pollinated annual plant having global significance which belongs to the Graminae (Poaceae) family, tribe Triticeae and of the genus Triticum. It is a staple food crop of more than one-third of the world population because it supplies 20% of food calories to the growing population in the world. Wheat grain contains 70% carbohydrates, 22% crude fibres, 12% protein, 12% water, 2% fat, and 1.8% minerals (Noorka et al., 2013). The major wheat-producing countries are China, India, USA, France, Russia, Canada, Australia, Pakistan, etc. Wheat has been described as the 'King of cereals' because of the acreage it occupies, high productivity and world trade of this crop is greater in comparison to all other crops combined. The wheat crop, in the world, occupies ranks first in terms of area (216.00 Mha) and second in terms of production (692.25 Mt) and yield is (28.55 q/ha) (USDA, 2018). India is the second largest producer of wheat after China. An effective breeding strategy needs to be developed for maximum improvement in the genetic yield potential Abstract Experiments were conducted to study genetics and combining ability for yield and its attributes under Irrigated (E1) and Rainfed (E2) conditions using F1 hybrids derived from Line X Tester mating design by crossing eleven lines with three testers. Significant differences were observed among all the genotypes for all the traits in both E1 and E2 environments. The σ 2 gca/σ 2 sca ratio indicated the predominance of non-additive gene action for all the characters in both environments. Therefore, this component of the variance can be utilized in the breeding program through the exploitation of heterosis and the selection process for identification of superior plant types should be postponed to further generations like F4 or F5. Based on combining ability effect and per se performance in rainfed conditions, parents UP2572 and VL3001 were identified as good general combiners for grain yield per plant. In cross combinations, cross PRL/2*PASTOR//PBW343*2/ KUKUNA/3/ROLF07/4/BERKUT//… × WH1142 and UP2572 × WH1142 were identified as the best specific combiners for grain yield per plant under irrigated and rainfed conditions, respectively. So, these identified parents and cross combinations can be exploited for developing water stress tolerant cultivars 135 Page 2 of 9 Vol:. (1234567890) of wheat. To evolve an effective hybridization program in wheat which is a self-pollinated crop, there is a need to develop such a strategy that allows the accumulation of fixable gene effects in a homozygous line. To exploit different types of gene action present in the population, information regarding the magnitude of genetic variance and combining ability for important traits are essential. Combining ability analysis is an important tool for the selection of superior parents because the success of an effective breeding program is depending on the choice of parents and it also provides information regarding the nature and magnitude of gene effects controlling quantitative traits. The line x tester technique, developed by Kempthorne (1957) is used to estimate, in both selfand cross-pollinated field crops, combining ability and gene effects for understanding the nature of gene action involved in the expression of quantitative traits. In the formulation of a system breeding project for rapid improvement, this technique provides a guideline for the selection of superior parents and desirable cross combinations.
In the world, 70% of the wheat is cultivated under rain-fed conditions (Raza et al., 2018), which may increase further due to climate changes. Even the irrigated wheat cultivating areas are predicted to experience water scarcity, therefore the development of wheat varieties having good water-use efficiency and/ or drought tolerance is a priority research area for wheat breeders. Drought stress is one of the main limiting factors for wheat yield in the semi-arid regions of the world. Accordingly, for sustainable production of the wheat crop, it is highly important to identify the wheat cultivars and lines which have a high yield in stress-free conditions as well as acceptable yield in moisture stress conditions. Water deficiency affects various growth and development stages (vegetative, reproductive, and grain development) and has a negative impact on the physiological processes of the plant, consequently affecting yield as well (Mursalova et al., 2015). The relationship between the morphophysiological traits associated with tolerance to rainfed conditions is very much important in the selection criteria for drought tolerance. The effect of water stress depends on the plant growth stage and may affect potential yield and yield components. Drought stress can reduce grain yield, and due to this stress, 17% to 70% loss in average grain yield has been estimated (Nouri-Ganbalani et al., 2009). Morphological traits, such as the number of grains per spike, productive tillers per plant, 1000 grain weight, peduncle length, spike length, plant height, and grain weight per spike influence the tolerance of wheat to moisture stress conditions (Sattar et al., 2018). Therefore, grain yield and its components are two important selection criteria in arid regions.

Material and methods
Eleven lines of wheat, BECARD/KACHU, BOW/ VEE/5/ND/VG9144//KAL/BBB/YACO/4/CHIL/6/ CASKOR/3/…, 92.001E7.32.5/SLVS/5/NS-732/ HER/3/PRL/SARA//TSI/VEE#5/…, FRANCO-LIN#1/BAJ#1, KACHU*2//WHEAR/SOKOLL, P R L / 2 * PA ST O R / / P BW 3 4 3 * 2 / K U K U NA / 3 / ROLF07/4/BERKUT//…, UP2572, VL3001, NW5054, PBW644, C306 were crossed with three testers WH1080, WH1142, HD3086. This crossing material exhibits drought-tolerant wheat varieties suitable for growing in rainfed conditions, viz. C306, VL3001, UP2572, WH1080, WH1142 and PBW644. The number of seeds produced through hybridization in each of the cross combinations is represented in Fig. 1. The F1 seeds of the thirtythree cross along with their parents and two checks HD2967 and PBW660 were evaluated in a randomized complete block design with three replications in irrigated (E1) and rainfed (E2) conditions. Each plot consisted of 2 rows of 1 m length with a row to row and plant to plant distance of 20 cm or 23 cm and 10 cm, respectively. The proposed research work has been conducted in Norman E. Borlaug Crop Research Centre of Govind Ballabh Pant University of Agriculture and Technology, Pantnagar during the Rabi, 2016-2017 and Rabi, 2018-2019. Climate-wise, Pantnagar falls in the humid subtropical zone having the miscellaneous type of soil texture, which is generally 1.0-1.5 m deep. The high-water table, shallow depth, and calcareous nature are the key features of the soil in this area. Only single irrigation was applied at tillering stage, after that no irrigation was applied to keep the experiment under moisture stress. While in irrigated condition, four irrigations were applied during the crown root initiation stage, tillering stage, flowering stage, and the dough stage for proper growth of the wheat genotypes. The water requirement is 450-650 mm in the whole production period of the wheat crop (Tadesse et al., 2017). During this experiment in the whole rabi season, the total rainfall is 75.8 mm (Fig. 2). "The effect of water stress depends on growth stages of wheat as well as duration and intensity of water stress" (Daryanto et al., 2016;Sarto et al., 2017). In the rainfed environment (E2), it is shown from Fig. 2 that there was no rain from the tillering to the booting stage (November to January).
Five plants at random of all genotypes were used to record biometrical observations viz., days to 75% heading, days to maturity, plant height (cm), peduncle length (cm), awn length (cm), tillers per plant, flag leaf area (cm 2 ), spike length (cm), spikelets per spike, grains per spike, grain weight per spike (g), 1000 grain weight (g) and grain yield per plant (g). The mean value of each trait over the replication was used for statistical analysis. The data of each environment was analysed separately and pooled analysis over Number of seeds produced in each cross cpmbination  (17), (32) the environments was also carried out. The recorded data were subjected to analysis of variance and line x tester combining ability analysis studied in the above genotypes as described by Kempthorne (1957).

Results and discussion
The mean performance of the parents evaluated in irrigated and water stress environments differed from one another for all the traits. Under rainfed conditions, the parent's FRANCOLIN#1/BAJ#1, UP2572, VL3001, NW5054, C306, WH1080, and WH1142 increased grain production, but the rest of the parents decreased yield performance under stress conditions. So, these were found to be tolerant to water stress.
Here, the varieties that showed water stress tolerance are released as drought tolerant cultivars. The cross-combination BOW/VEE/5/ND/VG9144//KAL/ BBB/YACO/4/CHIL/6/CASKOR/3/… × WH1080, KACHU*2//WHEAR/SOKOLL × HD3086 and UP2572 × HD3086 showed an increase in the yield performance under rainfed conditions, so they are found to be tolerant to water stress. Whereas, the rest of the parents and crosses showed a reduction in the yield under water stress conditions. The graphical representation of the mean performance for grain yield of all the parents and cross combinations under both environments is presented in Fig. 3.

Analysis of variance
The mean performance and analysis of variance indicated that all the genotypes differ significantly for all the traits in both environments, as well as in pooled analysis. Polled analysis of variance revealed that the environmental differences were statistically   (47) significant for all the traits except awn length and grain weight per spike. The differences due to pooled analysis among the wheat genotypes were statistically significant for plant height, peduncle length, spike length, awn length, spikelets per spike, flag leaf area, 1000 grain weight and it was non-significant for days to 75% heading, days to maturity, tillers per plant, grains per spike, grain weight per spike, grain yield per plant.The mean squares due to genotype x environment (G x E) interactions were significant for all the traits except peduncle length, indicating the major contribution of G X E interaction than a genotypic component in the expression of these traits. The significant mean square due to genotype, environment and G X E for plant height, spike length, spikelets/ spike, flag leaf area and 1000 grain weight suggested the importance of both genotype and environment components for these traits. Therefore, inference of the present study and also the previous work (Ahmed et al., 2007 andAlmeselmani et al., 2011) revealed that the high yielding wheat genotypes with water stress tolerance cannot simply develop by crossing between the water stress tolerant and high yielding genotypes without referring to environmental effect.

Analysis of variance for combining ability
The analysis of variance for combining ability was performed for fifteen yield and their contributing traits in irrigated (E1) and rainfed (E2) conditions. The mean squares due to crosses were partitioned into mean squares due to testers, due to lines and line x tester interaction components. The mean square due to lines was significant for ten characters in E1, days to 75% heading, days to maturity, plant height, spike length, peduncle length, awn length, tillers per plant, 1000 grain weight, grain weight per spike. In E2, ten characters exhibited significant mean squares namely days to 75% heading, plant height, spike length, peduncle length, awn length, spikelets per spike, flag leaf area, 1000 grain weight and grain yield per plant.
Mean squares due to testers were significant for five characters namely plant height, spike length, spikelets per spike, flag leaf area and grains per spike in E1, while in E2 conditions four characters viz. spike length, awn length, spikelets per spike and flag leaf area.
Mean squares due to line x tester interaction were found to be significant for all fifteen characters namely days to 75% heading, days to maturity, plant height, spike length, peduncle length, awn length, tillers per plant, spikelets per spike, flag leaf area, 1000 grain weight, grain weight per spike, grain yield per plant and grains per spike in E1 and twelve characters in E2 namely days to 75% heading, plant height, peduncle length, awn length, spikelets per spike, flag leaf area, 1000 grain weight, grain weight per spike, grains per spike and grain yield per plant.
A higher magnitude of line x tester component than either due to lines or testers indicated the predominant role of non-additive gene action (dominance). The results from the present study revealed that line x tester interactions are significant for most of the traits in both environments. Line x tester interaction is far more important than line or tester alone in deciding the hybrid performance. Similar findings were also reported by Srivastava et al., (2012) and Farooq et al., (2019). The hybrid performance in turn is indicative of diversity among the parental lines and the extent of variability that will be generated in segregating populations, with the possibility of identifying transgressive segregants in subsequent generations.

Variance components of combining ability
In irrigated conditions, the highest general combining ability variance (σ 2 gca) was observed for plant height and peduncle length. The rest of the characters showed a relatively smaller amount of σ 2 gca. However, in the rainfed condition, the highest gca variance was observed for plant height followed by peduncle length and grain yield/plant while the rest of the characters showed a relatively smaller amount of σ 2 gca. In irrigated conditions, maximum variance for specific combing ability (σ 2 sca) was observed for the trait grains per spike followed by grain yield per plant. The estimate of σ 2 gca/σ 2 sca indicated the predominance of non-additive gene action for all the characters. However, in rainfed conditions, maximum σ 2 sca was observed for the trait grains per spike followed by plant height. The estimates of σ 2 gca/σ 2 sca indicate the predominance of non-additive gene action for yield and its contributing characters condition as in E1. Results 135 Page 6 of 9 Vol:. (1234567890) presented by Rajesh et al., (2002), Majeed et al., (2011), Srivastava et al., (2012 and Farooq et al., (2019) also indicated a preponderance of non-additive gene effects in the expression for grain yield and its attributes.
Considering the above results, it may be concluded that there was a predominant role of nonadditive gene action in the inheritance of all the traits in both irrigated and rainfed conditions. The most efficient way for utilizing the non-additive genetic variance is through the exploitation of heterosis and the selection process for identification of superior plant types should be postponed to further generations like F4 or F5. Since it was observed that sca was the predominant contributor to genetic variance, thus, it is suggested that the selection of sca is likely to be the most effective method to exploit hybrid vigor.

Estimates of combining ability effect
In combining ability effects, the general combining ability (gca) effect represents the additive gene action and specific combining ability (sca) effects represent the non-additive gene action interactions. The nonadditive gene effects contribute to the improvement of grain yield in self-pollinated crops by the commercial exploitation of heterosis. In self-pollinated crops, however, the additive x additive type of interaction is also feasible in later generations and can be exploited for the improvement of grain yield and its attributes. The results of gca effect of parents and the sca effect of all cross combinations for the significant traits that exhibit maximum combing ability variances are given in Tables 1 and 2, respectively. Similar studies for combing ability analysis in irrigated and water stress condition has also been reported by Adebayo and Menkir (2015) in maize and Sultan et al., (2016) in barley crop.  Estimates of gca effect and per se performance would help the breeder in selecting the appropriate parents for the exploitation of variation and extracting superior genotypes through recombination breeding (Kuchanur et al., 2013). Water stress influenced the genetic expression of grain yield in this study. Estimation of gca effects and per se performance showed that in irrigated conditions, lines PRL/2*PASTOR// PBW343*2/KUKUNA/3/ROLF07/4/BERKUT//… and PBW644 were identified as good general combiners for grain yield/plant. Whereas, in rainfed conditions, UP2572 and VL3001 were identified as good general combiners for this trait. If comparing testers in irrigated as well as rainfed conditions, WH1142 was identified as a good general combiner for grain yield per plant. Similar studies by combining ability analysis in wheat to identify the best general combiner for grain yield have also been reported by Ahmed et al., (2017) Parveen et al., (2018) and Rajput and Kandalkar (2018). The identified best genotypes based on gca effect and higher mean value are suitable for incorporation of respective traits into their progeny. So, the identified best general combiners under rainfed conditions were VL3001, C306, WH1142, and UP2572 which can be used to develop water stress tolerant progenies in further generations.
The exploitation of hybrids for further breeding cycles in any crop is mostly determined by per se performance of the hybrids and sca effect in the desired direction. It may not be possible all the time that good-performing parents when crossed will produce better hybrids (Jatoi et al., 2012). The hybrids identified based on mean performance and sca effects could be exploited in future breeding cycles to identify useful transgressive segregants. Based on per se performance, cross PBW644 × WH1142 showed the highest grain yield per plant in irrigated conditions, while cross UP2572 × WH1142 in rainfed conditions revealed the highest grain yield. To identify the best cross combinations, the priority is to be the highest mean yield and then a good sca effect (Anand et al., 1999). For grain yield/plant, the best cross combinations identified on the basis of perse performance with good sca effect in irrigated conditions are PRL/2*PASTOR//PBW343*2/ KUKUNA/3/ROLF07/4/BERKUT//… × WH1142 followed by UP2572 × WH1142, while for rainfed conditions, UP2572 × WH1142 followed by VL3001 × WH1080. In the best cross combinations for grain yield at least one of the parents was a good general combiner, indicating that these crosses eventually will produce desirable transgressive segregants. A similar finding has also been reported by Suresh et al. (2013) for combing ability analysis in the rice crop. The best cross combination identified in rainfed conditions can be used in future breeding programs to develop water stress tolerant cultivars.
In nutshell, understanding inheritance for various traits and identification of superior parents are important prerequisites for an effective breeding program. The gca and sca effects are good measures of additive and non-additive gene action, respectively. This information about the nature of gene action in irrigated and water-stressed conditions of the genotypes can help to determine an appropriate breeding strategy for yield improvement in wheat for water-limited environments.