Considering that five of the seven biblical species are fruit trees, their long standing economic importance and role as important element of food production becomes evident. Genetically, domestication of fruit trees involves a change in the reproductive biology of the plants by shifting from sexual reproduction to vegetative propagation (Zohary and Spiegel Roy 1975). As a rule, cultivated varieties of fruit trees are maintained vegetatively by cuttings, rootings of twigs, or suckers. This is in sharp contrast with the life cycle of their wild relatives, which reproduce from seed. Wild populations maintain themselves through sexual reproduction, are distinctly allogamous, assuring cross-pollination either by self-incompatibility or by dioecy, with high levels of heterozygosity.
In contrast to forest trees, fruit trees and grapevines have undergone thousands of years of domestication and centuries of breeding. Man-made selection led to some improved cultivars, sometimes at the cost of genetic variability (Zohary and Hopf 1993; Maghuly et al. 2005a). Most important fruit crops, in contrast to forest tree species, have been vegetatively propagated as cultivar clones, a factor which has also contributed to the spread of latent viruses and phytoplasmas (Laimer 2003a).
Modern cultivation of fruit trees further faces rapid rotation of orchards and a shorter life-span of individual plants. However, to release a new fruit tree cultivar it takes several years, sometimes even decades.
Given their nutritional and dietetic value, fruit crops contribute considerably to an improved world food production and human nutrition (Table 9.1) (FAOSTAT 2004). Current fruit production faces problems with biotic and abiotic stress factors during production, harvest and storage.
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