Breeding has been very successful in generating cultivars that in favorable environments, and together with large use of fertilizer and chemical control of weeds, pest and diseases, have increased agricultural production several fold. Today the environmental impact of high input agriculture in more favorable environments causes growing concern. By contrast, the impact of breeding in marginal environments has been elusive. The paper discusses evidence showing that the use of breeding principles developed for, and successfully applied, in favorable environments may be the main reason for the lack of breeding progress in marginal environments. Very little breeding work has actually been done in marginal environments, although the theory of correlated responses to selection indicates that selection conducted in good environments or in well-managed experiment stations is not expected to be very efficient when genotype by environment interactions of a cross-over type exist. The assumptions that heritability is higher under good conditions and that there is a carry-over effect of high yield potential are not supported by experimental evidence. If the target environment is below the cross-over point, selection has to be conducted for specific adaptation to that environment. The concept of wide adaptation has more a geographical than an environmental meaning, and it reduces genetic diversity and increases genetic vulnerability. Eventually the issue of genetic heterogeneity versus genetic uniformity is discussed in relation to specific adaptation to marginal environments.

Specific adaptation and breeding for marginal conditions

Upland rice, produced by smallholder farmers, is the lowest-yielding rice production system. Drought stress is the most severe abiotic constraint in upland rice. Improving productivity of rice in the upland ecosystem is essential to meet rice food security needs of impoverished upland communities. Breeding drought-resistant upland rice is therefore an increasingly important goal. Numerous secondary characters have been suggested to help plant breeders in their selections. Most of these traits are not used in selection, as they are not practical for selection purposes, exhibit low heritability, or are not highly correlated with grain yield. The use of managed drought stress, where drought stress can be imposed at specific periods, has been shown to increase the heritability of yield under stress to values similar to those obtained for yield in well-watered conditions. It has now been demonstrated that drought-tolerant upland rice can be bred by directly selecting for yield in stress environments. The use of molecular markers to perform selection may eventually provide plant breeders with more efficient selection methods. To date, many quantitative trait loci (QTL) for drought resistance have been identified in rice, but few are suitable for use in marker-assisted selection. However, large-effect drought resistance QTL have now been identified and may enable effective use of marker-assisted selection for drought resistance. Copyright © 2008 Society of Chemical Industry

Breeding upland rice for drought resistance

Varietal selection is particularly difficult for very heterogeneous environments where farmers have a range of preferences. To address these issues in Rwanda, local bean experts, generally women, are invited to the research station to assess cultivars and to select those they prefer for their plots. Farmers use observed yield as only one of several criteria to predict varietal performance in their more stressed home environments. The varieties they choose are often higher yielding on-farm and are retained longer by farmers than those selected by the breeders. The results of early farmer involvement also compare favourably with those of the standard breeding programme in terms of enhanced genetic diversity and reduced research costs. There are few pre-conditions for developing such a client-driven breeding programme.Papel del agricultor en la seleccion de variedades para una estacion inaestigacion

Rethinking the farmer's role in plant breeding: local bean experts and on-station selection in Rwanda

It is widely recognized that conventional plant breeding has been more beneficial to farmers in high-potential environments or those who can profitably modify their environment to suit new cultivars, than to the poorest farmers who cannot afford to modify their environment through the application of additional inputs and cannot risk the replacement of their traditional, well known and reliable varieties. As a consequence, low yields, crop failures, malnutrition, famine, and eventually poverty still affect a large proportion of humanity. Participatory plant breeding (PPB) is seen by several scientists as a way to overcome the limitations of conventional breeding by offering farmers the possibility to choose, in their own environment, which varieties suit better their needs and conditions. PPB exploits the potential gains of breeding for specific adaptation through decentralized selection, defined as selection in the target environment, and is the ultimate conceptual consequence of a positive interpretation of genotype × environment interactions. The paper describes a model of PPB developed by The International Center for Agricultural Research in the Dry Areas and used successfully in several countries in West Asia and North Africa. Genetic variability is generated by breeders, selection is conducted jointly by breeders, farmers, and extension specialists in a number of target environments, and the best selections are used in further cycles of recombination and selection. Technically, the process is similar to conventional breeding, with three main differences. Testing and selection take place on-farm rather than on-station, key decisions are taken jointly by farmers and the breeder, and the process can be independently implemented at a large number of locations. The model also incorporates seed production. Farmers handle the initial phases, multiplying promising breeding material in village-based seed production systems. The PPB model is flexible; it can generate populations, pure lines, and eventually mixtures of pure lines in self-pollinated crops; as well as hybrids, populations, and synthetics in cross-pollinated crops. PPB has several advantages. New varieties reach the release phase much faster than in conventional breeding, and are better suited to farmers’ needs and willingness to invest in inputs and management. Release and seed multiplication activities concentrate on varieties known to be farmer-acceptable. These advantages are particularly relevant to developing countries where large investments in plant breeding have not yielded returns, and many “improved” varieties developed through conventional breeding are not adopted by farmers. PPB also ensures that biodiversity is maintained or increased because different varieties are selected at different locations. In addition to the economical benefits, participatory research has a number of psychological, moral, and ethical benefits, which are the consequence of a progressive empowerment of the farmers’ communities; these benefits affect sectors of their life beyond the agricultural aspects. In conclusion, PPB, as a case of demand driven research, gives voice to farmers, including those who have been traditionally the most marginalized such as the women, and elevates local knowledge to the role of science.

Decentralized-participatory plant breeding: an example of demand driven research

An estimated 60% of common bean (Phaseolus vulgaris L.) production worldwide is at risk of drought. A breeding program was developed at the International Center of Tropical Agriculture (CIAT) to create drought resistant breeding lines with varietal potential in the small red, small black, cream (mulatinho) and cream-striped (carioca) grain classes. Breeding populations were created from triple or double crosses. Field screening under terminal drought was performed at Palmira, Colombia in the dry season in F 2 , F 3:5 , and F 6:8 generations over two cycles of recurrent selection in the small red and small black classes, and one cycle in the mulatinho and carioca classes. Drought resistant lines yielded significantly more than commercial check cultivars under drought in all color classes. Some outyielded the respective checks by 15 to 25% (depending on color class and trial) in one or more of three favorable environments, or in the combined analysis across favorable environments, and were also earlier to mature. Drought resistant lines presented up to 36% greater yield d -1 in favorable environments. Some also expressed superior yields in a phosphorus-limited environment. Thus, selection for drought resistance has improved yield potential and plant efficiency across different environments. It is suggested that selection under drought stress reveals genes that correct inefficiencies inherited from the wild Phaseolus vulgaris, and are key to yield improvement of common bean.

Selection for Drought Resistance in Common Bean Also Improves Yield in Phosphorus Limited and Favorable Environments

Regressions of yields of cultivars upon means of sets of cultivars over diverse environments are often used as measures of stability/adaptability. Prolonged selection for performance in environments of high yield potential has generally led to unconscious selection for high regressions. If adaptation to poor environments is required (as it often is in Third World agriculture), common sense suggests that low regressions could be exploited for the purpose. Simulations show that systematic selection in the poor environment is required, not merely trials of potential cultivars after selection in a good environment. In effect, systematic exploitation of a GE interactions effect is proposed. The effects are large enough to reduce correlated responses in different environments to zero. Orderly experimental studies are needed but not available. What information there is does not disagree with the theory developed here.

N. W. Simmonds

Papers

Selection for local adaptation in a plant breeding programme.