Abstract
Sweet orange improvement by conventional breeding techniques has been inhibited by the complex biology of sweet orange, e.g. large plant size, extended juvenility, and the presence of nucellar embryos in the seed. Common sweet orange cultivars widely grown today probably originated as selections from chance seedlings or from naturally occurring mutations. Therefore, we are pursuing alternative methods to improve sweet orange, and one such method is to take advantage of a phenomenon called somaclonal variation, which is defined as genetic variation that is either uncovered or induced by a tissue culture process. This approach to cultivar improvement is attractive for sweet orange because of potential to identify superior clones with positive genetic changes while maintaining sweet orange integrity. Targeted traits for improvement include altered maturity dates, increased soluble solids, and improved color. We are currently evaluating approximately 2000 trees of 'Hamlin' and 'Valencia' sweet orange clones in the field, including the following four populations of each variety: organogenic (regenerated via adventitious shoot buds); embryogenic (regenerated from secondary embryogenic callus via somatic embryogenesis); protoplast-derived (regenerated via somatic embryogenesis); and nucellar seedlings as a control. Significant stable variation has been observed for the following general tree characteristics: canopy size/shape; leaf size/shape; ploidy level; and juvenility/ thorniness/vigor. Fruit characteristics showing significant variation include brix, acid, ratio, color (fruit/juice), maturity date, size, rind thickness, and juice content. Of particular interest are clones of 'Hamlin' showing improved color, and clones of 'Valencia' showing significantly earlier maturity.
