PRODUCTION OF TETRAPLOID SOMATIC HYBRID BREEDING PARENTS FOR USE IN LEMON CULTIVAR IMPROVEMENT

Allotetraploid somatic hybrid plants of 'Hamlin' sweet orange (Citrus sinensis L. Osbeck) + 'Femminello' lemon (C. limon L. Burm. f.), and Milam lemon (purported hybrid of C. jambhiri Lush) + 'Femminello' lemon were regenerated via somatic embryogenesis following protoplast fusion. 'Hamlin' and Milam protoplasts were isolated from undeveloped ovule-derived embryogenic callus cultures and fused using a polyethylene glycol method with seedling leaf-derived protoplasts of 'Femminello' lemon. Somatic hybrids were identified on the basis of leaf morphology, root-tip cell chromosome number, and electrophoretic analyses of phosphoglucose isomerase, phosphoglucose mutase, and 6-phosphogluconate dehydrogenase leaf isozymes. The somatic hybrids will be used in interploid crosses with lemon in an effort to generate seedless triploid lemon types with improved tolerance to mal secco disease.