NEW CITRUS ROOTSTOCKS VIA PROTOPLAST FUSION

Citrus rootstock improvement is challenging because so many traits must be packaged into any successful new hybrid rootstock. In Florida, these include: resistance/ tolerance to blight, citrus tristeza virus, Phytophthora spp., nematodes, Diaprepes (Sugarcane root weevil), and in some cases salinity; adaptation to challenging soils; tree size control; nucellar embryony for seed propagation; good nursery performance; and the ability to consistently produce high yields of quality fruit. No existing rootstock meets all of these criteria. During the past 15 years, we have produced somatic hybrid plants from more than 120 parental combinations, including more than 70 for rootstock improvement (Fig. 1A). Many of these have been propagated and entered in replicated commercial rootstock trials. Our primary strategy has been to produce allotetraploid somatic hybrids that combine diploid rootstocks with complementary traits. A second strategy has been to combine citrus with sexually incompatible or difficult to hybridize related species exhibiting important traits. Production of such hybrids and performance in field trials will be discussed. Smaller trees are desirable to facilitate harvesting and to maximize cold protection. Tree size and yield efficiency data from somatic hybrid field trials indicate that somatic hybrid rootstocks have excellent potential for use in high density plantings (due to polyploidy). Several somatic hybrid rootstocks are performing well in field trials, and some of these are producing adequate nucellar seed for standard nursery propagation. Presently, our somatic hybridization program is focused on the resynthesis of an improved sour orange by combining selected superior mandarin and pummelo parents, and production of more widely-adapted hybrids with a diluted trifoliate orange contribution. Fertile somatic hybrid trees are also providing new opportunities for citrus rootstock improvement by breeding and selection at the tetraploid level.

Download Adobe Acrobat Reader (free software to read PDF files)