Somatic Embryogenesis in Tropical Fruit Trees

Abstract

Food and shelter are two basic needs of life. Since the earliest times, humans have enjoyed eating fresh fruits, as indicated by the frequent mention of fruits throughout the recorded history. The cultivation of fruits such as datepalm and pomegranate, has been dated back to 7000 to 3500 BC, respectively (Singh, 1985). In today’s world fruit play a vital role in our diet due to its nutritional importance (Nagy et al., 1990). Many fruit species are trees and with a few exceptions, e.g. banana, pineapple and strawberry, are woody. Genetic improvement of fruit crops by conventional plant breeding achieved through the selection of superior genotypes from genetically variable population derived from sexual recombination and seedling propagation (Bose, 1985). Tree architecture, floral morphology, prolonged juvenile period and irregularities in bearing habit limits the improvement of fruit trees through traditional breeding methods (Nijjar, 1977). Since most important horticultural characteristic, conferred by complexes of genes, the genetic integrity of many important fruit tree cultivars has been maintained by means of vegetative propagation. Using the conventional methods of vegetative propagation such as, cutting, layering, and grafting, substantial number of plants are produced. However, many tropical fruits and spice trees (e.g. clove) can not be propagated asexually (Litz, 1984b). Due to the increasing significance of tropical fruits for food and export income (Underhill, 1993), there has been a recent expansion of efforts and resources devoted to tropical fruit cultivar improvement. It is almost imposible to keep pace with the increasing demand of tropical fruits only through the conventional methods of plant propagation. For improvement of tropical and subtropical fruits by somatic cell genetics, a highly efficient regeneration system is a prerequisite (Litz and Jaiswal, 1991). Many programs are attempting to integrate emerging biotechnologies with conventional breeding methods to facilitate and expedite the fruit tree cultivars improvement (Litz, 1997). These include in vitro culture and micropropagation (Grosser, 1994), production of homozygous parental lines by chromosome doubling of haploids (Zhang and Lespinasse, 1992), regeneration of interspecific and intergeneric hybrids for root stock development (Gmitter et al., 1992; Ochatt et al., 1992), production of artificial seeds (Akhtar and Jaiswal, 1994; Bajaj, 1995a, b, Gray, 1987a, c; Redenbaugh, 1993), in vitro selection of somaclonal variation having enhanced diseased resistance (Hammerschlag, 1992; Jain et al., 1997), genetic transformation for specific horticultural traits (Gradner, 1993; Oliveira et al., 1996)