Unexploited germplasm, natural mutations, and selected in vitro techniques for Citrus cultivar improvement

Breeding of Citrus by sexual hybridization has not been an effective strategy for cultivar improvement because of long juvenile stages and widespread apomixis. Investigations of unexploited genetic resources or systems of evaluation may identify methods to increase the rate of genetic advance. One potential germplasm resource, two sources of genetic mutants, and one novel method of in vitro evaluation for disease resistance were examined during this study. The potential value of each project was critically reviewed.; A diverse population of Citrus, known as forbidden fruit, was discovered in Saint Lucia. Historical records, morphological characteristics, and isozyme evidence indicated that this population was closely related to grapefruit. High degrees of zygotic embryony and diversity within the population may make it a useful source of breeding parents.; Significant morphological variation was observed during initial growth among Citrus plants regenerated from callus cultures by organogenesis. Many of the aberrant plants were indistinguishable from controls after a second cycle of vegetative regrowth. The frequency of genetic mutation that could be measured as stable morphological change in young plants was too low to allow efficient mutant recovery.; Fruit sector chimeras were observed among eight Citrus cultivars at a frequency of 0.01 to 0.27 percent in commercial packinghouses. Mutants were recovered from some sectored fruit as autotetraploid seedlings. Several kinds of sectors with potential value as sources of genetic mutations were described.; In vitro response of etiolated seedling shoots to Phytophthora parasitica was measured as stem discoloration. The non-host, or hypersensitive, response observed in Poncirus trifoliata and Severinia buxifolia was clearly distinguished from the susceptible reaction of eight Citrus selections and hybrids. However, two Citrus x Poncirus hybrids with useful levels of field resistance to this pathogen exhibited a highly susceptible reaction in vitro. This technique may prove useful in the study of resistance mechanisms, but probably will not be effective for population screening.