Maize is grown on approximately 150 million ha worldwide, of which ~100 million ha is in developing countries, though the latter account for less than half of total production. Over the last 100 years, maize has been the major commercial success of cereals. Adaptation of the crop in the main production areas of the USA is now expanding further into the more marginal western areas (Mason et al., 2008), while production and adoption of hybrids has also begun to increase more rapidly in tropical regions, particularly in Brazil, Argentina, India, Thailand, Vietnam and parts of China. Campos et al. (2006) demonstrated that the major gains in productivity since the mid-1950s have been in an increased tolerance to stress. At low plant densities (< 20,000 plants/ha), modern hybrids have little advantage over mid-20th century hybrids. However, at higher commercial densities and under conditions of drought (Campos et al., 2006) and heat (Mason et al., 2008) modern hybrids are better able to initiate and establish productive ears and grains. Lee and Tollenaar (2007) concluded that the success of maize breeding was realized through continuous and simultaneous improvements in the maintenance of the 'source' of assimilate supply, mainly through increased stay-green (under conditions including stress), and in the 'sink' as described below.
Maize is relatively well adapted to high temperature and also shows good transpiration efficiency because of the C4 characteristic of concentrating CO2 to bypass the oxygenase activity of Rubisco. However, maize shows large genetic variation in the relative timing of male and female flowering, commonly referred to as the anthesis-silking interval (ASI). Since expression of longer ASI is associated with significantly larger relative investment in tassel biomass over ear biomass (and under extreme stress may result in total ear barrenness while not preventing pollen dissemination), delayed ASI leads to reduced kernel set under drought and a number of other stresses (Edmeades et al., 2000). Based on this, ASI was used as the main physiological selection criterion to make genetic gains under drought in CIMMYT's maize stress breeding programme (Edmeades et al., 2000). Building on ASI-improved germplasm and the concept of selection under well-managed stress environments, Banziger and colleagues (2006) began a maize breeding programme in CIMMYT's sub-Saharan Africa operation in 1997 that has resulted in dozens of new hybrids which, on average, outperformed standard checks (checks are well-adapted elite cultivars) across a broad range of environments at over 40 locations across eastern and southern Africa - and by as much as 100% under severe stress.
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