Among digenic interactions, dominance x dominance (1) and among trigenic interactions, dominance x dominance x dominance (z) played significantly greater role in the inheritance of this trait than other interactions. This indicated the greatest significance of interactions purely involving the dominance effects in both the environments. However, their relative magnitude and signs changed frequently with the cross and the environments. In the cross Cocorit 71 x A-9-30-1 and Raj 911 x DWL 5002 under late sown condition, where additive-dominance model was adequate, the additive (d) effect was either higher than or comparable to dominance (h) effect (Table 1). Earlier studies (Saini 1987; Kathiria 1991; Khedar 1998) are in agreement with the present findings.
The results of absolute totals of epistatic effects indicated that the second order interactions were more important than the first and both were higher than the main effects, where 10-parameter model was adequate (Table 2). Similarly the absolute totals of the first order interactions were higher than the main effects, where 6- parameter model was adequate. Thus, it is evident that epistatic interactions were more important than the main effects in all the cases where different epistatic models were appropriately fitted the data. Ilyhchenko (1977), Patel (1981) and Dhindsa (1982) also reported the role of epistasis in controlling the inheritance of this trait in bread wheat, whereas, in durum wheat Ghalib AI-Shalaldeh and Duwayri (1986) observed that this trait was controlled by polygenic gene effects. Duplicate epistasis at two-gene level and at three-gene level was observed only in the dross HI 8062 x JNK-4W-128 in normal and late planting condition, respectively. No conclusion regarding type of epistasis could be drawn in other crosses because (h) was found non-significant. Except in the late sown condition in the crosses Cocorit 71 x A-9-30-1 and Raj 911 x DWL 5002, where additive-dominance model was adequate, the non-fixable gene effects were higher than the fixable gene effects (Table 2). This indicated the requirement of complicated procedures of breeding for their further exploitation. However, if the crosses viz., Cocorit 71 x A-9-30-1 and Raj 911 x DWL 5002 are sown under late sown condition, simple breeding methods will be appropriate to exploit additive gene effects, which could be used successfully to improve this trait. Yadav et al. (1981) and Bariga (1989) also reported that additive effects were more important than non-additive effects in the inheritance of this trait while Hsu and Walton (1970), Jain and Singh (1976), and Prabu and Sharma (1984) reported that non-additive gene effects were predominant in the inheritance of this trait. Such varied results obtained in the present study may be due to difference in the materials used or even from the difference in the environments (sowing time).
Analysis of components of heterosis over better parent revealed that non-allelic interactions were the major source of heterosis. However, significant heterosis and inbreeding depression were not frequently observed in most of the cases (Table 3). The significant heterosis was only observed in the cross HI 8062 x JNK-4W-128 in late sown condition due to dominance (h), dominance x dominance (l), additive x dominance x dominance (y) and dominance x dominance x dominance (z) components of heterosis. Significant inbreeding depression in two cases was observed only due to the dissipation of non-additive dominance effects or epistatic effects involving dominance in F2 generation. Absence of heterosis in the crosses could be explained on the basis of internal cancellation of heterosis components.
As a consequence of higher magnitude of interactions (digenic/trigenic type) in most of the cases, the absolute totals of non-fixable gene effects were recorded higher than the fixable gene effects. Naturally, the successful breeding methods will be the ones, which will exploit the non-additive gene effects. The methods which mop-up the non-additive effects are restricted recurrent selection by the way of intermating the most desirable segregates followed the selection (Joshi 1979) and diallel selective mating (Jensen 1978). Furthermore, as the duplicate type of epistasis was observed in the cross HI 8062 x JNK-4W-128 under both the environments, so as the selection intensity should be mild in the earlier and intense in the later generations for tangible advancement of this trait. Furthermore, an appropriate choice of the environment should be made in such a way that character show relatively simple inheritance, so that flag leaf area could be improved in the crosses Cocorit 71 x A-9-30-1 and Raj 911 x DWL 5002 by simple breeding method (progeny selection) for tangible advancement of grain yield in durum.