The experiment was conducted to estimate variability, correlation, and genetic parameters of hybrids and F2 generation based on morphological traits. Additionally, heterosis for F1 hybrids was estimated to determine their superiority compared to their parents. However, to estimate the reduced fitness, inbreeding depression of the F2 generation was done.
The estimation of correlation among the studied morphological characters bears great importance in identifying the key characters that can be exploited for increased yield. Grain yield plant− 1 was significantly and positively correlated with the number of tillers hill− 1, number of effective tillers hill− 1, flagleaf length, panicle length, grains panicle− 1, filled grains panicle− 1, and grain yield panicle− 1 (Table 3). This finding showed that increased grain yield plant− 1 is the result of the increased number of tillers and effective tillers plant− 1 (Table 3, Tiwari et al. 2019). On the contrary, a significant negative correlation was found for days to first flowering with grain yield plant− 1 (Table 3, Mishu et al. 2015).
The majority of characters has complex inheritance pattern and are significantly influenced by multiple genes acting with several environmental factors; as a result, the study of phenotypic and genotypic coefficient of variation is very effective in determining the probability for selection-based improvement. There was an environmental effect on phenotype which is suggested by a relatively higher phenotypic coefficient of variation than the genotypic coefficient of variation (Table 6).
Grain yield plant− 1 had the highest genotypic coefficient of variation (GCV) and phenotypic coefficient of variation (PCV) values in hybrid, followed by grain yield panicle− 1 and filled grains panicle− 1, which indicated that selection could be done from these traits (Table 6, Islam et al. 2019; Kumari and Parmar 2020). On the other hand, days to first flowering recorded lower values of PCV and GCV, indicating a narrow genetic base for these traits (Table 6, Shivapriya 2000). The F2 segregants exhibited higher PCV and GCV values for grains panicle− 1, filled grains panicle− 1, grain yield panicle− 1, and grain yield plant− 1, recommending the scope for choosing genotypes with the aforementioned traits (Table 6, Ratnakar et al. 2012; Kiran et al. 2012). Contrarily, plant height showed lower GCV and PCV, suggesting less effect of this trait for improvement (Shivapriya 2000).
To determine the amount of heritable variation, only GCV and PCV alone cannot provide a concise and meaningful result. The combination of heritability and genetic advance is more relevant in forecasting how selection affects phenotypic expression. Knowledge about heritability is a prerequisite for any breeding program since it can help to pick superior genotypes from a diverse genetic pool (Singh et al. 2011). However, the only knowledge of heritability is insufficient for a successful selection. Genetic advance if studied in addition to heritability might be more useful (Vanisree et al. 2013). Therefore, to forecast the effect of selection to choose the best individual, heritability estimates (above 60%) combined with genetic advance (above 20%) would be more fruitful than heritability alone (Johnson et al. 1955; Islam et al. 2019).
The present study revealed high estimates of heritability coupled with high genetic advance (20%) for all the traits, excluding days to first flowering and plant height of hybrids (Table 6, Kahani et al. 2015; Abebe et al. 2017). In F2 generation, high heritability with high genetic gain over the percentage mean for grains panicle− 1, filled grains panicle− 1, grain yield panicle− 1, and grain yield plant− 1 concluded that these characters were governed by additive gene action (Table 6). The traits having low heritability and genetic advance would not be useful for selection (Rahman et al. 2016).
Since increasing yield is the main objective of rice breeding, highly significant positive heterosis estimates are preferred for yield and features that contribute to yield, such as the number of tillers hill− 1, panicle length, flagleaf length, grains panicle− 1, and filled grains panicle− 1 (Shukla et al 2020). On the contrary, negative heterosis is preferable for flowering and maturation to produce short-duration variety because hybrids or crosses are more likely to achieve early maturity with early flowering than those with positive heterosis. Both crosses had significant and negative relative heterosis and heterobeltiosis in days to first flowering which indicates the scope of the extensive use of heterosis for earliness (Gokulakrishnan 2018).
Positive heterosis is considered undesirable for plant height because the newly produced hybrids are more prone to lodge. In the present investigation, significant positive heterosis over mid and better parent for plant height was obtained for both the crosses (Table 7). However, to develop dwarf plant types, Sari et al. (2019) and Shukla et al. (2020) emphasized the importance of negative heterosis for plant height. One of the most important yield-determining factors is the tillering habit of a genotype. A high degree of heterosis was observed in both crosses for the number of tillers hill− 1 and number of effective tillers hill− 1 manifesting superior performance of hybrids (Table 7, Xalxo et al. 2018). Panicle length is another main attribute of the higher yield of rice. Positive and significant heterosis was recorded for panicle length in both crosses (Table 7, Shukla et al. 2020). The number of grains panicle− 1 and number of filled grains panicle− 1 showed higher estimates of positive significant heterosis, which are two crucial factors for increased yield in rice (Table 7, Thakor et al. 2018 and Sari et al. 2019). Both crosses showed the highest positive heterosis for grain yield plant− 1 which is the most desirable trait for plant breeders who want to select superior genotypes over the existing ones. (Table 7, Devi et al. 2017). Negative heterosis over better parent was recorded for the weight of thousand seeds as it was desired to obtain fine rice grains since fine grains belong to less weight than coarse grains (Table 7, Shukla et al. 2020).
However, significant negative inbreeding depression was recorded for plant height, which revealed that the F2 segregants had larger plant heights than hybrids (Table 7, Asati and Yadab 2020). A high magnitude of positive and significant inbreeding depression was recorded for days to first flowering, number of tillers hill− 1, number of effective tillers hill− 1, flagleaf length, and grain yield panicle− 1 in F1 concluding their reduced performance (Table 7, Ismaeel et al. 2019). On the other hand, F1′ showed positive significant inbreeding depression in days to first flowering, number of tillers hill− 1, number of effective tillers hill− 1, grains panicle− 1, filled grains panicle− 1, grain yield panicle− 1, grain yield plant− 1, straw yield plant− 1, and weight of thousand seeds which indicates their decreased output comparing to their prior generation (Table 7, Ismaeel et al. 2019).
According to Abdel-Moneam et al. (2016), the inheritance of yield and yield components is regulated by the non-additive type of gene action. Since increased yield is the ultimate aim in plant breeding, it is recommended to give more emphasis on yield-contributing traits, as there is no separate gene for yield.