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Discussion

Since 1980s, genetic studies on resistance in wheat to scab or Fusarium head blight have been reported in China (Jiang and Wu 1989; Lin et al. 1992) and abroad (Tomasovic 1989; Snijders 1990a, 1990b; Ittu et al. 1997). Most researches indicated that the resistance was quantitatively inherited and affected largely by additive gene effects although the results obtained were not completely consistent and even contradictory (Buerstmayr et al. 1997; Ban and Suenaga 1997). In this study, the fact that mean numbers of diseased spikelets of F1 and F2, for most combinations, were less than the mid-parent values indicated that the dominance effects existed. The effect of parents versus crosses tests the mean deviation of the hybrids from their midparent value. It reflects average heterosis contributed by all parents in the crosses and is attributable entirely to non-additive effects (Snijders 1990b). The 'parent versus F1' effect was highly significant but the 'parent versus F2' effect was not significant. This means that there were dominance effects in F1's, predominantly in the direction of resistance. Average degree of dominance showed that there was dominance or overdominance in F1 and F2. The midparent heterosis of F1 may mainly lie in superdominance effects.

Combining ability analysis for scab resistance in wheat was done mostly with F1 crosses following Griffing's methods in the earlier studies. F2 populations were included in very few experiments (Snijders 1990b). For hybrid breeding, information about F1's is undoubtedly suitable. For breeding pure lines, however, F2's should be more valuable than F1's. Because selection of superior genotypes is conducted in F2 population and progenies rather than F1 crosses, higher average resistance of F2 population means greater opportunity of obtaining resistant individuals. This study indicated that SCA effect was more important than GCA among F1 generations, while among F2 generations GCA had nearly the same effect as SCA. The additive effects of resistance genes for F2's played a more important role than those for F1's in the inheritance of the resistance to scab in wheat. In breeding resistant pure lines, therefore, parent selection based on combining ability analysis for F2's should be more reliable than for F1's.

Heritability estimated showed that the expression of resistance to scab was mainly controlled by genetic factors. There was a significant correlation between F1's and F2's for generation means and for specific combining ability. However, correlations between parent means or mid-parent values and the averages of F1's or F2's and between parent means and combining ability effects were not statistically significant. It is concluded that the inheritance of resistance in wheat to scab is complex. The performance of offsprings in resistance to scab was not absolutely expected by the resistance levels of parents. The interaction between parents should be considered.

The cultivar Sumai 3 has been proved to have high and stable resistance to scab and been widely used in combining ability analysis and breeding program for the resistance. Its general combining ability based on F1's was high and obviously increased the resistance of hybrids (Jiang and Wu 1989). However, its undesirable agronomic traits have negative effects on the offsprings. So far, no cultivars derived from Sumai 3 have been applied in production because of their lacking desired agronomic traits and high-yielding capacity. In the present study, Sumai 3 did not have good GCA effect for F2's although it possessed higher resistance and GCA value of 6.07% for F1's in decreasing diseased spikelets of offsprings. Relatively, the recurrent selection strain W14 had not only the highest resistance but also greatest GCA effects both in F1's and F2's. Clearly, it. should be an excellent parent for breeding for resistance to scab in wheat. The recurrent selection strain TFSLO37 had positive GCA effects on performance of resistance to scab in F1's and F2's. As compared with Sumai 3, W14 and TFSL037 have improved agronomic traits such as shorter plant height, higher biomass, grain yield, harvest index and 1000-grain weight, and more grains per spike (Jiang and Wu 1996). It is suggested they could be used as excellent resistant parents in breeding program for scab resistance. It is also demonstrated that the development of a gene pool through recurrent selection integrated with conventional breeding methods using the dominant male-sterile gene Ta1 (ms2) is entirely feasible and effective in developing excellent scab-resistant germplasm of wheat (Jiang et al. 1994; Jiang and Wu 1996). Among the maternal parents, Yangmai 5 and Changjiang 8853 had higher resistance and general combining ability. In practice, some good lines with resistance and desired agronomic characters have been obtained from the cross Yangmai 5/W14.

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