Introduction. Importance and distribution of citrus

Importance and distribution of citrus

Citrus is the most important fruit crop in the world, with a production of almost 100 Mt in 2001 (Food and Agriculture Organization, 2001). It is grown in more than 100 countries all over the world, mainly in tropical and subtropical areas

© CAB International 2007. Citrus Genetics, Breeding and Biotechnology (ed. I.A. Khan) 329

(~40° latitude on each side of the equator) where favourable soil and climatic condi- tions occur. Major producing countries include Brazil, the USA, China, Spain, Mexico, India, Iran, Italy, Egypt, Argentina, Turkey, Japan, Pakistan, South Africa, Greece, Thailand, Morocco, Israel, Indonesia, Korea and Australia. Citrus fruits are marketed fresh or as processed juice and canned segments.

The general area of origin of citrus is believed to be South-east Asia, including south China, north-eastern India and Burma, though its introduction into cultiva- tion probably started in China. Commercial citrus species and related genera belong to the order Geraniales, family Rutaceae, sub- family Aurantoidea. All rootstocks and varieties used are included in the genus Citrus, except for kumquats (Fortunella spp.) and trifoliate orange (Poncirus trifoli- ata L. Raf.), the latter used exclusively as a rootstock. Commercial citrus fruits fall into several main groups: sweet oranges (C. sinensis (L.) Osb.), mandarins, including satsumas (C. unshiu (Mak.) Marc.) and clementines (C. clementina Hort. ex Tan.), grapefruits (C. paradisi Macf.), pummelos (C. grandis (L.) Osb.), lemons (C. limon (L.) Burm. f.) and limes (C. aurantifolia (Christm.) Swing.). There are other species of relative importance in certain areas, such as sour oranges (C. aurantium L.), citrons (C. medica L.) and bergamots (C. bergamia Risso & Poit.). Some hybrids of commercial interest include citranges (sweet orange ´ trifoliate orange) and citrumelos (grapefruit

´ trifoliate orange), used as rootstocks, and

tangelos (mandarin ´ grapefruit), tangors (mandarin ´ sweet orange) and mandarin hybrids, used as varieties.

Need for genetic improvement

Many different citrus genotypes are com- mercially grown in a wide diversity of soil and climatic conditions; therefore, trees are subjected to important abiotic and biotic stresses that limit the production and, in some instances, the use of certain root-

stocks and varieties. The main abiotic stresses are acid, alkaline and salty soils, fl ooding and drought, freezing and high temperatures. Citrus trees are also affected by many pests and diseases caused by nematodes, fungi, bacteria, spiroplasmas, phytoplasmas, viruses and viroids. Some diseases are spread throughout the world, such as those produced by the fungus Phytophthora sp. or by the citrus tristeza virus (CTV), that preclude the use of certain excellent rootstocks, and severely restrict fruit production and quality of important varieties in some countries. Other diseases are restricted to specifi c geographic areas, such as those caused by the bacterium Xylella fastidiosa in Brazil and Liberobacter asiaticum in most countries of South-east Asia. In both cases, these bacteria are cur- rently devastating millions of trees and there are no means for effi cient control.

At the same time that the citrus indus- try is threatened by important biotic and abiotic stresses, the markets in developed countries demand fruit of increasing qual- ity. In this situation, genetic improvement of citrus is a major priority. However, con- ventional breeding of citrus has important limitations.

Genetic improvement of citrus

Citrus species have a complex reproductive biology. Some important genotypes have total or partial pollen and/or ovule sterility and cannot be used as parents in breeding programmes. There are many cases of cross- and self-incompatibility. Most species are apomictic, which means that adventitious embryos initiate directly from maternal nucellar cells precluding the development of zygotic embryos, and thus the recovery of sexual progeny populations. They have a long juvenile period and most species need at least fi ve years to start fl owering in sub- tropical areas. All these features together with their large plant size, high heterozy- gosity, lack of basic knowledge about how the most important horticultural traits are inherited, and quantitative inheritance of

most characters have greatly impeded genetic improvement of citrus through con- ventional breeding methods.

Although some breeding programmes for citrus improvement started more than 100 years ago (Soost and Cameron, 1975), nowadays most rootstocks are citrus species without any improvement, and most impor- tant varieties have been originated by bud- sport mutations and chance seedlings. Only a few hybrid varieties are economically rel- evant in certain local markets. Probably, the most signifi cant results from any improve- ment programme performed in the world are the hybrids Carrizo and Troyer citranges, originating from a cross made in 1909 that was originally oriented to introduce cold tolerance in edible fruits (Savage and Gardner, 1965). Nowadays, citranges are widely used as rootstocks in countries such as Spain and the USA.

Recently, the development of genetic markers is providing a new potential tool for citrus breeding. Linkage maps have been performed using isozymes, restriction frag- ment length polymorphism (RFLP), random amplified polymorphic DNA (RAPD), sequence-characterized amplifi ed regions (SCAR), amplifi ed fragment length poly- morphism (AFLP), microsatellites (single sequence repeat; SSR) and cleaved ampli- fied polymorphic sequences (CAPS). Although these studies have served to determine the mode of inheritance of these traits and they could be useful for breeding purposes, map-based cloning of the corre- sponding genes is still a long way off. The only exception could be the CTV resistance gene from P. trifoliata. Cloning of this gene is under way in several laboratories (Deng et al., 2001; Yang et al., 2001). Other prom- ising biotechnologies for citrus improve- ment include somatic hybridization (Grosser et al., 2000) and embryo rescue mainly for obtaining triploid (seedless) varieties (Ollitrault et al., 1998).

Genetic transformation may provide an effi cient alternative for citrus improvement, opening the way for the introduction of spe- cifi c traits into known genotypes without altering their genetic elite background.