Somatic Hybridization of Citrus with Wild Relatives for Germplasm Enhancement and Cultivar Development

Abstract

The techniques employed in the somatic hybridization of Citrus sinensis cv. Hamlin with Citropsis gilletiana, a distant wild relative native to Africa, are described. This species is immune to Phytophthora citrophthora and resistant to the nematode Radopholus citrophilus, making its hybrid an excellent candidate for use as a rootstock. Protoplasts were isolated from sterilized juvenile leaves or non-embryogenic callus initiated from juvenile stem sections. About 50% of embryos from callus derived by fusion from the leaf protoplasts produced shoots in the presence of gibberellic acid and coumarin. Some embryos developed both roots and shoots. The remaining majority of shoots formed functional roots on half-strength Murashige & Tucker basal medium with activated charcoal. Somatic hybrid embryo recovery was greater from fusions involving Citropsis leaf protoplasts than callus protoplasts. Regenerated plants had an intermediate trifoliate leaf shape, were vigorous and appeared uniform. The allotetraploid chromosome number was 36.