CHAPTER 1: LITERATURE REVIEW
1.4 Grain Yield and Stress Tolerance
Primarily breeding aims at developing cultivars that satisfy farmers’ requirements.
Evans and Fischer (1999) defined grain yield as the grain mass with specific moisture content. “Yield potential” is obtained when a cultivar is grown under non- limiting conditions and in an environment of its adaptation, or in the absence of stress. Therefore, stress tolerance is defined in relation to yield potential. Tollenaar (2002) defined stress tolerance as the ability of cultivars to mitigate the impact of stress. The difference between “yield potential” and “actual yield” reflects the level of stress tolerance of a cultivar. Thus, as the actual yield approaches yield potential, cultivars are regarded as relatively stress tolerant.
1.4.2 Progress in Improving Grain Yield Potential
Appreciable progress has been realised in improving grain yield, especially in temperate maize. Tollenaar and Lee (2002) have reported grain yield potential of 14.5 to 20.9 t/ha in the USA, compared with the actual yield of ±7 t/ha. Other studies
reported that the actual yield was only about a quarter of the yield potential for the USA (Tollenaar, 1983; Tollenaar, 2002). However, Duvick and Cassman (1999) reported attainable yield of 18 t/ha under irrigated conditions in Nebraska (USA). In tropical sub-Saharan Africa, Pingali and Pandey (2001) reported yield potential of 5 t/ha against actual yield of 0.5 t/ha in highland/transitional zones; 7 t/ha versus 2.5 t/ha in mid-altitude/subtropical zones; and 4.5 t/ha versus 0.7 t/ha in tropical lowland environments. In Eastern and Southern Africa, Banziger and Diallo (2002) reported actual yield of 1.3 t/ha for small-scale farmers and 4 to 14 t/ha for the researchers. It appears that conditions for obtaining a high yield are hard to achieve by researchers, let alone for the resource-constrained, small-scale farmers in marginal areas of Southern Africa. The wide gap between actual and yield potential indicates that there is still a huge opportunity for improving grain yield, especially in tropical sub-Saharan Africa.
1.4.3 Stress Tolerance as a Basis for Yield Improvement
Breeding for stress tolerance is not new. There is overwhelming evidence in support of the predominance of stress tolerance in explaining yield improvement in temperate maize. Duvick (1997) reported that improved grain yield potential of the best hybrids in Central Iowa was due to stress tolerance and high yield per plant. Tollenaar and Wu (1999) reported that improved stress tolerance was associated with lower plant- to-plant variability. Tollenaar et al. (1997) reported one case where new hybrids were even more competitive with weeds than the old hybrids. Superiority of hybrids under stress sharply contradicts the opinion that genotypic and phenotypic heterogeneity are positively correlated with yield stability. Tollenaar and Wu (1999) reported that single cross hybrids had better adaptation and stress tolerance than genetically variable open pollinated cultivars and double cross hybrids. Previously, Troyer (1996) reported that the trend of maize evolution in the USA, beginning with open pollinated varieties (OPVs) in the 1930 era, and followed by double cross (1930-1960s) and single cross hybrids (late 1960s), was associated with improved yield and adaptation. Arguably, the more than 20% yield improvement could not be attributed to heterosis alone (Duvick, 1992, 1997; Duvick and Farnham, 1997).
However, an equal effort has not been applied to the development and improvement of OPVs in the USA as breeding emphasis of researchers seems to have shifted towards the hybrids after the 1960s era. Perhaps if they had applied an equal effort to research and development of OPVs, the grain yield potential and yield stability could be more comparable or even higher than that of the hybrids.
Studies by Tollenaar and Wu (1999) have suggested that there are common mechanisms for conferring tolerance to different forms of stress in temperate maize cultivars. This has also been reported in some tropical cultivars that had tolerance to both drought and low soil N (Banziger et al., 2002; Lafitte and Edmeades, 1995), but such studies are still limited. High level of stress tolerance might have resulted in part from selection for improved grain yield stability in many multi-locational trials (Tollenaar and Lee, 2002; Tollenaar, 2002). Furthermore, cultivars have increasingly been tested under conditions representing commercial production (Tollenaar and Lee, 2002). Another explanation for the success in yield improvement is recycling of the best inbreds in pedigree breeding. Duvick (1997) reported that breeders used inbred lines derived from the best hybrids to develop new hybrids. It has also been argued that yield improvement was a result of improved efficient use of resources due to delayed leaf senescence or the “stay green” trait (Duvick, 1997; Tollenaar and Wu, 1999), which improved grain filling.
1.4.4 Gene Action Conditioning Grain Yield
Different types of gene action control grain yield and its associated traits. Betran and Hallauer (1996) reported that additive effects were more important than dominance for grain yield, lodging, and the flowering days in hybrids. Wolf et al. (2000) reported larger dominance than additive effects for grain yield in an F2 of B73 x Mo17. Wolf et al. (2000) reported high level of dominance of 2.44 comparable to 1.28 that had been previously reported by Han and Hallauer (1989) for grain yield. At times the high levels of dominance are a result of an upward bias by linkage disequilibrium in F2 single crosses of inbred lines.
There are different reports regarding contribution of epistasis in conditioning grain yield that has generally been reported to be negligible (Darrah and Hallauer, 1972;
Eta-Ndu and Openshaw, 1999; Hinze and Lamkey, 2003; Lamkey et al., 1995;
Melchinger et al., 1988; Wolf and Hallauer, 1997). However, Wolf and Hallauer (1997) reported significant epistasis effects for ear traits, days to flowering and grain yield. Wolf and Hallauer (1997) suggested that favourable epistasis could have contributed to heterosis in the highly adapted B73 x Mo17 hybrid. Favourable epistasis can be fixed through repeated selfing (Lamkey et al., 1995; Wolf and Hallauer, 1997).
This review shows that additive, dominance and epistasis effects explained high yield in the widely adapted hybrids. However, such studies have been limited to temperate germplasm. A survey of literature revealed that little research has been conducted on gene action in African maize populations. Relevant information about inheritance should be generated from regional maize grown under sub-Saharan African environments. According to Falconer (1981) information from genetic studies is specific to the specific germplasm and the environments tested. Thus, information generated in temperate maize and temperate environments might not have direct application in Southern Africa. In the current study, diverse germplasm drawn from nine major heterotic groups, which are used by breeding programmes in Southern Africa were evaluated for gene action in Southern African environments.