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Concluding Remarks






Signifi cant somaclonal variation in whole plant characteristics can be found in both ‘Valencia’ and ‘Hamlin’ sweet oranges, with ‘Valencia’ showing more profound and useful variation. Genetically stable varia- tion for useful traits can be obtained. We have identifi ed early and late maturing somaclones of ‘Valencia’ with superior quality that should facilitate the processing industry, particularly NFC production. Sensory testing has been added as another tool for evaluating commercial potential of specifi c clones, and clones with preferred flavour are being identified. Improved somaclones of ‘Valencia’ with fresh market potential have been identifi ed, including seedless clones with different maturity dates, and large-fruited clones with a more melting fl esh. These clones should be eval- uated in a Mediterranean environment. Improved somaclones of ‘Hamlin’ have been selected for better juice colour and increased soluble solids. The next step is to determine if the improved traits are expressed in trees propagated from the orig- inal selections, and if these selected somaclones yield adequately. Second-gen- eration trees of selected clones have been propagated on various rootstocks and planted in the three representative citricul- ture areas of Florida. Results obtained so far indicate that the generation and evaluation of somaclonal variation in sweet orange is a viable method for developing improved sweet oranges both for processing and for the fresh market. New cultivars from this should facilitate NFC production by pro- viding juice of higher quality throughout an expanded season. Somaclonal variation can certainly be exploited as a tool to expand the maturity dates (and associated market window) and reduce the seed content of commercially important citrus cultivars.

Studies to determine the potential causes of somaclonal variation revealed that polyploid and aneuploid cells are pro- duced consistently in cultured citrus cells. However, their percentages remain rela- tively constant over increased time in cul-


 


 

ture. Also, the percentages of cells with altered chromosome numbers decrease as cultures proceed through the plant regener- ation process. These facts indicate that more normal diploid cells are more com- petitive in culture, and that the plant regen- eration process selects against cells with radical genetic changes. This suggests that long-term cultures may accumulate minor genetic changes (i.e. mutations/movement of transposons (Kubis et al., 2003) or cyto- logical aberrations such as inversions or translocations) that do not significantly affect the mitotic index or interfere with the plant regeneration process, thereby result- ing in regenerated plants with the observed minor but signifi cant variation.


 






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