Genetic basis of heterosis for grain yield and salinity tolerant traits in rice through diallel analysis

Introduction

Rice (Oryza sativa L.) a member of the Poaceae family, is a self-pollinating crop with a diploid chromosome number of 2n=24. Its floral structure consists of six stamens arranged in a circular pattern around a central pistil. As a staple food, rice is essential for the diet of most people in Asia. The Oryza genus includes 24 species, of which only two are domesticated: O. sativa (Asian rice) and O. glaberrima (African rice). While African rice is native to western Africa, Asian rice originated in southern and eastern Asia. Traditionally, O. sativa has been divided into two subspecies: japonica and indica. However, recent genetic research has identified five distinct groups within species: indica, aus, tropical japonica, temperate japonica and aromatic (Garris et al., 2005) [1] . Rice is cultivated across a wide array of environments, including irrigated fields, rainfed lowlands, uplands and flood prone areas. Asia referred to as the "rice bowl" of the world accounts for over 90 per cent of global rice production and consumption (Tyagi et al., 2004) [2] . Rice kernels are naturally free of cholesterol and sodium, making them an important source of calories worldwide. One cup (186 g) of cooked rice provides approximately 218 calories, 4.5 g protein, 45.8 g carbohydrates, 3.5 g fiber and 1.6 g fat. Rice also supplies essential nutrients like iron, manganese, magnesium and B vitamins (Cervoni, 2024) [3] . Globally, during 2024-25, rice was grown on 168.36 million hectares, producing 532.87 million metric tonnes (Anon., 2025a) [4] . In India, third advance estimates projected production at 119.93 million tonnes (Anon., 2025b) [5] . In Gujarat, rice covered 0.88625 million hectares, yielding 2.13 million tonnes at a productivity rate of 2403.27 kg ha⁻¹ (Anon., 2025c) [6] . India has the largest area under rice cultivation, while China remains the top producer. In India, rice is a staple for nearly 65 per cent of the population, contributing approximately 42 per cent to total food grain production and 45 per cent to cereal output. This critical crop supports a large portion of the global population and also serves as a major employment source, especially in developing nations (Muthayya et al., 2014) [7] . Nearly 3.7 billion people globally depend on rice. Yet, rapid population growth, deteriorating soil fertility and limited quality irrigation water are driving cultivation into marginal areas increasingly affected by abiotic stress. In India, around 11 per cent of land suffers from salinity, with 1.5 million hectares of irrigated land salinized each year (Gregorio et al., 2003) [8] . Soil salinity is a major environmental constraint to agriculture, leading to widespread land degradation in coastal and inland regions. This presents a significant challenge to food security and sustainable farming. Improving the salt tolerance of major crops is one of the most practical strategies to utilize underexploited salt affected areas. Despite extensive research on salinity, only limited success has been achieved in developing salt tolerant varieties for farmers. Given rice's status as a staple crop, breeding salt tolerant cultivars is essential for enhancing global food security. However, salinity tolerance in rice is a complex trait, regulated by multiple physiological and biochemical factors. Thus, there is a need for ongoing research and the development of high yielding varieties for saline environments (Singh et al., 2008) [9] . Heterosis in rice was first reported in 1926 and was commercially exploited in China. It remains a powerful tool for enhancing yield and agronomic traits. Parent selection is crucial in this process. Earlier, selection was based on general performance, but now combining ability analysis is preferred. It helps not only in selecting parents but also in understanding gene actions controlling trait expression, enabling breeders to design efficient programs for quick genetic gains. Diallel cross analysis is a robust and comprehensive genetic breeding methodology that offers valuable insights into the genetic architecture of rice breeding populations. By systematically analysing genetic components, inheritance patterns and the combining abilities of parental lines, this technique allows breeders to identify superior genetic combinations and formulate efficient breeding strategies. A major advantage of diallel analysis is its capacity to simultaneously evaluate genetic variance, types of gene action and hybrid potential within a single generation. This integrated approach provides a holistic understanding of quantitative trait inheritance and supports the strategic selection of parental lines with high genetic potential for the development of superior rice cultivars. Such knowledge is particularly significant for devising targeted breeding strategies aimed at enhancing crop performance in coastal and salt affected soils, thereby improving rice's adaptability and tolerance to salinity.

Materials and Methods

Experimental site

The current study was conducted at the Coastal Soil Salinity Research Station farm at Navsari Agricultural University in Danti, Umbharat, during the Kharif season of 2024-25.

Plant materials

The experimental material comprised Eight genotypes (TNR-1, GNR-3, GR-17, IR-55179, NVSR-6531, NVSR-6526, GR-19 and CSR 103 10 2), which were crossed using 8 8 Half diallel mating design. The experimental evaluation included 28 F1 hybrids derived from the crosses, along with their eight parental genotypes and Standard check GR-19 (P7).

Layout

The experiment was conducted using a Randomized Block Design (RBD) with three replications. Each plot representing the parents and F₁ hybrids consisted of a single row comprising 14 plants. A spacing of 20 × 15 cm was maintained for raising the crop. Observations were recorded from five randomly selected plants in each plot. All recommended agronomic practices were followed to ensure the growth of a healthy and uniform crop stand.

Studied traits

Data were recorded for a range of morphological, physiological and yield-related traits to assess the performance of the genotypes and their hybrids. The traits evaluated included days to 50% flowering, plant height (cm), panicle length (cm), number of effective tillers per plant, number of grains per panicle, L/B ratio, grain yield per plant (g), straw yield per plant (g), 100-grain weight (g), K⁺/Na⁺ ratio in the shoot, proline content (µg g⁻¹ FW) and chlorophyll fluorescence.

Data analysis Analysis of Variance for Experimental Design

The statistical techniques described by Snedecor and Cochran (1967) and subsequently developed by Panse and Sukhatme (1978) [10] were used to examine the data gathered for each attribute in both parental and hybrid lines. An analysis of variance (ANOVA) was performed for the experimental setting using a Randomised Block Design (RBD). Using a fixed-effect model for the study, this method was utilised to determine the significance of genotype differences for all evaluated attributes.

Estimation of Heterosis

It is thought to be more useful and significant to evaluate heterosis in relation to the superior parent and the standard check. Thus, two important criteria were used in this study to assess heterosis: heterosis over the better parent (Heterobeltiosis), as suggested by Fonseca and Patterson (1968) [11] , and heterosis over the standard check (Standard heterosis), as described by Meredith and Bridge (1972) [12] . Following are the formulae used to calculate these parameters as well as the significance tests for them.

Results and Discussion

Analysis of Variance

The analysis of variance (ANOVA) conducted to assess differences between parents and hybrids across 12 traits is summarized in Table 1. The findings showed statistically significant mean squares for genotypes in all examined traits, suggesting substantial genetic variability among the selected genotypes. The analysis of variance revealed significant differences among the genotypes for all studied traits, confirming the presence of substantial genetic variability. Parents showed high significance for most traits, while hybrids exhibited strong significance for all characters except chlorophyll fluorescence, indicating considerable variation within hybrid combinations. The significant differences observed between parents and hybrids for most traits further demonstrated the expression of heterosis, particularly for grain yield per plant and its related components. Overall, the results confirm that the genetic materials used in this study possessed sufficient diversity to support meaningful selection and improvement.

Mean performance

For any new hybrids to be acceptable, mean performance will be the foremost criteria for selecting the genotypes. The data on mean performance of parents and hybrids along with range for different characters are presented in Table 2. On the basis of mean performance, parents TNR 1, NVSR 6531 and IR 55179 and hybrids GNR 3 × GR 19, NVSR 6526 × GR 19, NVSR 6531 × GR 19, GR 17 × GR 19 and NVSR 6531 × CSR 103-10-2 exhibited the maximum grain yield per plant. These hybrids were also found to be outstanding in respect of component traits like plant height (cm) panicle length (cm), effective tillers per plant, grains per panicle, straw yield per plant (g), 100-grain weight (g) and proline content. In respect to salinity tolerance, parent GR 19 and hybrid GNR 3 × NVSR 6526 depicted the maximum K + / Na + ratio in shoot. For proline content and chlorophyll fluorescence, parents IR 55179 and hybrid NVSR 6526 × GR 19 showed excellent performance.

Heterosis Days to 50 per cent flowering

Early flowering is important for any crop. So, the parent of particular hybrids which flowered earlier was considered as better parent and accordingly heterotic effects were estimated. Heterobeltiosis values ranged between -2.11 per cent (NVSR 6526 × GR 19) to 10.57 per cent (GR 17 × GR 19). Standard heterosis values for days to 50 per cent flowering varied from -5.28 per cent (GR 17 × CSR 103-10-2) to 8.45 per cent (NVSR 6531 × GR 19). No hybrids depicted significant negative heterobeltiosis and standard heterosis for this trait. Similar findings were earlier reported by Singh (2012) [13] , Devi et al. (2017) [14], Vange et al. (2020) [15] , Rasheed et al. (2021) [16] and Gupta et al. (2024) [17] .

Positive heterobeltiosis (HB) and standard heterosis (SH) estimates are desirable for plant height. The values of heterobeltiosis for plant height varied from -32.75 per cent (NVSR 6526 × CSR 103-10-2) to 37.80 per cent (GR 17 × GR 19). Out of all crosses generated, 16 were found significant out, of which 2, were positive. Best significant cross combinations over better parent for plant height were GR 17 × GR 19 (37.80%) and TNR 1 × NVSR 6531 (15.11%). Standard heterosis for plant height ranged between -11.59 per cent (GR 17 × CSR 103-10-2) to 43.21 per cent (TNR 1 × NVSR 6531). Out of 28 hybrids, 10 were significantly positive. The best three hybrid combinations were TNR 1 × NVSR 6531 (43.21%), GR 17 × GR 19 (37.80%) and TNR 1 × GR 17 (32.84%) (Table 3.). The results are in agreement with the findings Rasheed et al. (2021) [16] , Rahman et al. (2022) [18] , Salem et al. (2022) [19] , Gupta et al. (2024) [17] and Singh et al. (2025) [20] for heterobeltiosis and standard heterosis in negative as well as in positive direction. [16] , Rahman et al. (2022) [18] and Salem et al. (2022) [19] .

Effective tillers per plant

Effective tillers per plant is a most important yield contributing trait in rice, hence positive values for both better parent heterosis and standard heterosis are desirable. The estimates of heterobeltiosis for effective tillers per plant varied from -51.59 per cent (GNR 3 × NVSR 6531) to 94.90 per cent (TNR 1 × NVSR 6531). Out of 28 crosses developed, 17 were significant for better parent heterosis, of which 14 were positive. The best three hybrid combinations were TNR 1 × NVSR 6531 (94.90%), NVSR 6531 × CSR 103-10-2 (89.81%) and GNR 3 × GR 19 (88.31%). Standard heterosis for effective tillers per plant varied from -50.65 per cent (GNR 3 × NVSR 6531) to 98.70 per cent (TNR 1 × NVSR 6531). 17 crosses among 28 crosses had significant positive standard heterosis. Among these, three best hybrids with significant positive standard heterosis includes TNR 1 × NVSR 6531 (98.70%) followed by NVSR 6531 × CSR 103-10-2 (93.51%) and GNR 3 × GR 19 (88.31%) (Table 3.). The results are in concordance for heterobeltiosis and standard heterosis with the findings of Shukla et al. (2020) [21] and Gupta et al. (2024) [17] . [16] , Rahman et al. (2022) [18] and Salem et al. (2022) [19] . However, Singh et al. (2025) [20] reported heterobeltosis in negative direction. , Rahman et al. (2022) [18] , Gupta et al. (2024) [17] and Singh et al. (2025) [20] .

Grains per panicle

L/B ratio

For the character straw yield per plant, positive heterobeltiosis (HB) and standard heterosis (SH) estimates are desirable. The estimates of heterobeltiosis in straw yield per plant varied from -47.29 per cent (GNR 3 × NVSR 6531) to 101.87 per cent (GNR 3 × GR 19). In all, 10 hybrids out of 28 hybrids exhibited significant positive better parent heterosis. The highest better parent heterosis was observed in hybrid GNR 3 × GR 19 (101.87%) followed by TNR 1 × GR 17 (85.11%) and NVSR 6531 × GR 19 (83.02%). Standard heterosis for straw yield per plant ranged from -26.17 per cent (GNR 3 × GR 17) to 192.06 per cent (NVSR 6531 × GR 19). Out of 28 crosses developed, 19 were significantly positive. The cross combination NVSR 6531 × GR 19 (192.06%) expressed the highest significant positive standard heterosis which was followed by TNR 1 × GR 17 (167.29%) and TNR 1 × NVSR 6531 (142.41%) (Table 4.). The results were found in agreement with Ray et al. (2021) [22] .

Significant positive results in better parent heterosis and standard heterosis are desirable for the trait. The heterosis over better parent varied from -35.39 per cent (GR 17 × GR 19) to 7.56 per cent (TNR 1 × NVSR 6526). Out of 28 cross combinations, no hybrids depicted significant positive heterobeltiosis for this trait. The standard heterosis ranged from -9.67 per cent (IR 55179 × GR 19) to 37.62 per cent (GNR 3 × GR 17) over check GR 19. Out of 28 hybrids, 13 hybrids exhibited significant positive standard heterosis for this trait. The three best hybrid combinations were GNR 3 × GR 17 (37.62%), GR 17 × NVSR 6526 (36.61%) and GR 17 × IR 55179 (32.39%) (Table 4.). Significant positive estimates of heterosis over better parent and standard check for 100-grain weight were reported by Salem et al. (2022) [19] , Gupta et al. (2024) [17] and Singh et al. (2025) [20] . [23] .

Proline content (µg g [23] . 4.). Significant estimates of heterobeltiosis and standard heterosis in both positive and negative direction for chlorophyll fluorescence Patel (2021) [23] and Saini et al. (2023) [24] .

Chlorophyll fluorescence

Acknowledgement

AcknowledgementI gratefully acknowledge and express my sincere gratitude to my research guide, Dr. H. K. Joshi and the esteemed advisory committee for their invaluable guidance, encouragement and constructive suggestions throughout this study. I am thankful to Navsari Agricultural University authorities, faculty members and staff for providing essential facilities and support. I owe my deepest gratitude to my parents, family members, relatives, and friends for their unwavering moral support and encouragement.

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