Genetic Improvement of Yield

Crop improvement is usually targeted at improving yields, whether by husbandry or genetic improvement. Breeding improvements are generally aimed at intrinsic increases in yield potential or at the alleviation of either biotic (Brown, 2002) or abiotic (Takeda and Matsuoka, 2008) stresses. As an example, major improvements in inherent yield potential of wheat have occurred over the last 50 years, with the major impact being the introduction of dwarfing genes (the 'Green Revolution'), which enhanced harvest index and avoided the impacts of lodging. Subsequently, in recent years potential yield increments for commercial wheats have been less dramatic although significant.

Yield potentials in breeders' trials of any crop seldom translate fully to actual production: crops are grown in many varied environments with differential biotic/abiotic stress pressures. At the global level conditions are often suboptimal for crop growth, but even in intensely managed ideal crop environments, growers seldom achieve expected yields. The major biotic stresses are pathogens and pests, and progress in introducing durable resistance varies between crops. Transgenic biotechnological approaches, for example Bt in maize and cotton or herbicide resistance can have a major impact on pest resistance, but such approaches remain contentious (Lemaux, 2009).

Yield improvements are selected by breeders and adopted by producers and are usually in conjunction with enhanced pathogen/pest resistance or may even be a direct consequence of this trait. Yield improvements per se will be focused on specific yield components such as tillering, grain number, size, etc. or may be developmental attributes such as early establishment, phenology, post-anthesis canopy longevity, etc.

It is possible that currently theoretical yield maxima are being reached and will not be exceeded unless major breakthroughs in photosynthetic efficiency can be introduced (Long et al., 2006b). As noted above, the key targets are improvements in Rubisco function (Parry et al., 2007) or the introduction of C4 photosynthesis into rice (Hibberd et al., 2008) and other major C3 grain crops. In the latter case, up to a 50% increase in photosynthetic efficiency could be achieved, and given the global dominance of rice as an essential food, as well as meeting increased demand, a net result would undoubtedly be decreased inputs. However, such a strategy is complex, requiring the introduction of multiple traits into the C3 plant and, as observed earlier, at present remains at the initial stages of development in rice and is a long-term option.

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