Introduction. Sweet orange (Citrus sinensis L

Sweet orange (Citrus sinensis L. Osbeck) is the most horticulturally important and widely grown Citrus species in Florida and worldwide. Approximately 90% of the Florida crop and more than 30% of the world crop are used for processing. Frozen concentrate orange juice has historically been the primary product of the Florida industry, but more recently there has been a strong shift to fresh pasteurized not-from- concentrate (NFC) product. Strong interna- tional competition in the concentrate market and a demand for increased quality in juice products has fuelled this change. The development of higher quality oranges with expanded maturity dates will facilitate this change and increase the competitive ability of the Florida industry. No true sweet orange cultivars have been produced by conventional breeding techniques due to its complex biology including large plant size, extended juvenility and nucellar poly- embryony. Commonly grown sweet orange cultivars probably originated by the selec-

tion of chance seedlings well adapted to a particular area or from a mutation in a par- ticular cultivar or seedling (Nishiura, 1965; Hodgson, 1968). Alternative methods are therefore required to develop improved sweet oranges, and we have been investi- gating somaclonal variation for improving sweet orange.

The term ‘somaclonal variation’ was introduced by Larkin and Skowcroft (1981) to defi ne genetic variation that is present in plants regenerated from tissue cultures, either uncovered or induced by a tissue cul- ture process. Reports of significant somaclonal variation have been primarily from solanaceous or cereal crops, affecting a wide range of traits including plant height, overall growth habit, fl ower, fruit and leaf morphology, juvenility, maturity date, disease resistance, yield and bio- chemical characteristics. Long-term studies in woody perennial fruit crops until now have been lacking. For the past 17 years, we have been investigating the use of tissue culture methods to produce genetic varia- tion in ‘Hamlin’ and ‘Valencia’ sweet

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oranges, the two most important cultivars in Florida.

Sweet orange is well suited for studies of somaclonal variation due to its biology and excellent performance in tissue cul- ture. Sweet orange has a natural tendency to form adventive somatic embryos from nucellar tissue, and many studies of somatic embryogenesis have been reported (Rangan et al., 1968; Button and Bornman, 1971; Kochba et al., 1972). Since the fi rst report by Vardi et al. (1982), numerous sweet orange plants have been regenerated via somatic embryogenesis from protoplasts isolated from embryogenic callus or sus- pension cultures. Sweet orange also responds quite well organogenically, as adventitious buds can be induced effi- ciently from non-meristematic juvenile nucellar seedling explants (Grinblat, 1972; Barlass and Skene, 1982). This chapter will discuss ongoing research that shows how somaclonal variation can be used effec- tively as a tool to develop improved sweet oranges, with focus on the development of improved ‘Hamlin’ and ‘Valencia’ clones for processing (Grosser et al., 1997).