IN VITRO BREEDING FACILITATES CONVENTIONAL BREEDING FOR SCION AND ROOTSTOCK IMPROVEMENT IN CITRUS
The application of protoplast fusion technology in citrus improvement has resulted in the regeneration of somatic hybrid plants from more than 500 parental combinations, and many of these have been evaluated in field trials. In rare cases, a new somatic hybrid may have direct utility as an improved scion or rootstock cultivar; however, the most important application of somatic hybridization is the building of novel germplasm as a source of elite breeding parents for various types of conventional crosses. Somatic hybridization is beginning to have a major impact on our scion improvement program by the generation of superior allotetraploid breeding parents for use in interploid crosses to generate seedless triploids. Seedlessness is a primary breeding objective for new fresh fruit citrus cultivars, and hundreds of triploid hybrids have been produced using somatic hybrids as the tetraploid parent. Triploid progeny from such crosses are beginning to fruit, and superior seedless selections are being made for mandarin, pummelo/grapefruit, and acid fruit (lemon/lime) improvement; examples of each will be provided. Characterization and selection of parents for interploid crosses is also impacting other important traits including thorniness, disease resistance and cold-hardiness. Successful somatic hybridization in citrus rootstock improvement has allowed for the creation of a rootstock breeding program at the tetraploid level that achieves maximum genetic diversity in zygotic progeny and has great potential for tree size control via polyploidy. Tree size control has gained importance as a means of reducing harvesting costs, maximizing the efficiency of modern cold protection methodology, and facilitating the adaptation of new fruit production systems that feature high density plantings and open hydroponics. Rootstock breeding at the tetraploid level facilitates the packaging of the many required traits for an improved rootstock into individual hybrids, including wide adaptation to diverse soil and environmental conditions, ability to consistently produce high yields of quality fruit, and especially disease and insect resistance. Recent progress and successful examples of these applications will be discussed. Continued feedback from field trials of both scion and rootstock somatic hybrids and their progeny is facilitating the identification of superior breeding parents and cross combinations, thereby accelerating the development of improved citrus cultivars.
Grosser, J.W., Calović, M. and Gmitter Jr., F.G. (2012). IN VITRO BREEDING FACILITATES CONVENTIONAL BREEDING FOR SCION AND ROOTSTOCK IMPROVEMENT IN CITRUS. Acta Hortic. 961, 27-34
citrus, disease resistance, scion, seedless fruit, somatic hybridization, rootstock