Application of the ideotype concept in breeding for higher yield in the oilseed brassicas

Abstract

Approaches to yield-improvement breeding methodology which utilize information from biometrical genetic studies and from plant physiological studies aimed at developing ideotype models are reviewed here, with special emphasis on yield improvement in the oilseed brassicas. Major difficulties involved in yield-improvement programmes are the accurate selection of parents from which to generate superior breeding populations, and the selection of high-yielding genotypes from early-segregating generations. In both cases, the complex polygenic inheritance of yield and the magnitude of genotype × environment interactions affecting yield greatly reduce the chances of isolating superior genotypes. A wide array of biometrical procedures have been proposed in developing yield-improvement programmes. Some have been utilized successfully. However, their use in analyzing a character such as yield has been generally limited because of the need to conduct analyses with populations sown at much lower densities than commercial crops. Development of the ideotype concept has focussed the attention of plant physiologists on identification of simple morphological characters which have some influence on physiological processes determining the yield of the economic organs. Characters such as leaf inclination and leaf shape, for example, are often simply inherited and can greatly influence crop canopy structure and radiation interception. Such characters could be rapidly modified by selection to increase crop photosynthesis and yield. Ideotype definitions integrate information on these relationships and provide plant breeders with a clear blueprint of the characteristics of a high-yielding cultivar in a specified environment. Research with oilseed Brassica species in Western Australia and Tasmania has identified a number of highly heritable characters which provide a basic framework for the definition of a yield ideotype for spring rape ( B. napus). These include an optimal time of flowering, apetalous flowers and long upright pods to improve light penetration of the dense pod canopy, and pod shattering resistance to allow for direct harvesting of the crop. A major advantage in breeding for this ideotype is that genes for certain characters can be easily introgressed from the related species B. campestris and B. juncea. It is also possible, using microspore cultures, to generate large numbers of B. napus pure lines directly from plants selected in F2 populations on the basis of ideotype characteristics. This would allow for large-scale yield testing of selections within two to three years of making the original crosses.