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Analysis of heterosis (over better parent) exhibited that significant positive heterosis was observed in the cross Cocorit 71 x A-9-30-1 in both the normal and late planting dates and in the cross HI 8062 x JNK4W-128 in late planting (Table 3). The present results were in conformity with the findings of many wheat workers (Randhawa and Minhas 1977; Gautam and Jam 1985; Prasad et al. 1998). Significant positive heterosis is not desirable in durum wheat because F1 hybrid showed longer grain filling period than its parents. Component analysis of heterosis revealed that in the cross Cocorit 71 x A-9-30-1, dominance x dominance x dominance (z) followed by additive x additive x dominance (x), dominance x dominance (l), additive x additive (i) and additive x dominance x dominance (y) in normal sowing, whereas, in late sowing, additive x dominance x dominance (y) followed by additive x additive x additive (w), additive x dominance (j) and additive (d) contributed towards significant positive heterosis. In the cross HI 8062 x JNK-4W-128 most of the gene effects contributed towards significant positive heterosis, however, maximum contribution was made by dominance x dominance x dominance (z) followed by additive x additive x dominance (x) in the late sown environment. Absence of significant heterosis in remaining cases could be explained due to the internal cancellation of heterosis components. Significant negative inbreeding depression was recorded in the cross Cocorit 71 x A-9-30-1 in normal sowing, which is not desirable because F2 had longer grain filling period than the F1. Significant positive inbreeding depression was observed in the cross HI 8062 x JNK4W-128 in normal sowing because better homeostatic power, due to segregational variation in F2 or favourable gene dispersion in this generation can make the F2 cross superior to F1 for grain filling period (Table 3).

Thus, the results of the present investigation exhibited that epistasis was an integral component of the genetic architecture of grain filling period in durum wheat and hence detection, estimation and consideration of this component are important for the formulation of breeding program. As a consequence of higher magnitude of interactions particularly of trigenic type, the non-fixable gene effects were higher than the fixable indicated the major role of non-additive gene effects. Naturally, the successful breeding methods will be the ones, which can mop-up the genes to form superior gene constellations interacting in a favorable manner. Some forms of recurrent selection namely, diallel selective mating (Jensen 1970) or biparental mating in early segregating generations (Joshi and Dhawan 1966) might prove to be effective alternative approach. The restricted recurrent selection by the way of intermating the most desirable segregants followed by selection (Joshi 1979) might also be a useful breeding strategy for the exploitation of both additive as well as non-additive type of gene actions. These breeding approaches could be helpful in developing durum wheat populations, which upon selection will result in shorter grain filling period as well as higher economic yield in durum wheat varieties under different sowing environments through development of new plant type. Furthermore, as the duplicate type of epistasis was observed in the crosses HI 8062 x JNK-4W-128 under both the normal and late sown conditions and Raj 911 x DWL 5002 under normal sown environments, so the selection intensity should be mild in the earlier and intense in the later generations to achieve the desirable improvement in this trait in durum wheat. The study also showed that inheritance is highly affected by environment, hence an appropriate choice of the environment should be made in such a way that grain filling period show relatively simple inheritance for further tangible advancement of this trait in durum wheat.

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