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CHAPTER 25 RECENT ADVANCES IN BREEDING CITRUS FOR DROUGHT AND SALINE STRESS TOLERANCE
GOZAL BEN-HAYYIM1 AND GLORIA A. MOORE2 1
Institute of Plant Sciences, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel 2 Department of Horticultural Sciences, Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, USA Abstract:
1.
Citrus is a major world horticultural commodity, and most of its world-wide production depends on irrigation, which is inevitably associated with the deterioration of water quality from run-off or ground water. Citrus, like most fruit trees, is relatively salt sensitive. The deleterious effects of salt stress lead to reduction in fruit yield and quality. In recent years, only a few relatively salt-tolerant rootstocks have been obtained through selection and conventional breeding, due to a rather limited existing genetic pool and the long period of time required for experiments. Attempts to regenerate salt-tolerant citrus plants via in vitro production of salt-tolerant callus or mutagenesis have been rather limited and as of yet not in use. Therefore, efforts should be invested to identify traits/genes that have a key role in tolerance to salt in order to speed up the process and to enlarge these genetic resources. QTL analyses revealed that response to salt in citrus is a multigenic trait, as has been shown in other species, but some genes probably exist that have a major impact on salt tolerance and (or) mineral accumulation. Several robust EST databases now exist and are growing, the first microarray chips have been manufactured, and an initial genome sequencing effort is underway. These tools should allow citrus physiologists, biochemists, and geneticists to make much more rapid progress in understanding salt and water stress in the future and to design strategies to ameliorate their effects
INTRODUCTION
Citrus is a major world horticultural commodity, and most of its world-wide production depends on irrigation for economic production (Shalhevet and Levy, 1990). Irrigation is inevitably associated with the deterioration of water quality from run-off or ground water especially due to increases in soluble salts. Poor water quality unavoidably leads to increased soil salinity (Levy and Syversten, 2004). 627 M.A. Jenks et al. (eds.), Advances in Molecular Breeding Toward Drought and Salt Tolerant Crops, 627–642. © 2007 Springer.
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BEN-HAYYIM AND MOORE
Citrus, like most fruit trees, is relatively salt sensitive (Bernstein, 1969). The deleterious effects of salt stress lead to reduction in fruit yield and quality. The common citrus rootstocks differ in their tolerance to salinity (Bernstein, 1969; Wutscher, 1979) and citrus trees can withstand relatively moderate salinity levels depending on the climate, scion cultivar, rootstock, and irrigation-fertilizer management. The response of different citrus species to different salt(s) further brings differential responses, when fruit quality is concerned. In young trees, salt damage is usually manifested as leaf burn and defoliation, which is associated with accumulation of toxic levels (Na+ and/or Cl− ) in leaf cells. In many studies Cl− exclusion from leaves served as a reliable criterion for salt tolerance leading to a decreasing order of salinity tolerance in rootstocks: Cleopatra mandarin > Sour orange > Sweet orange = Swingle citrumelo > Rough lemon > Poncirus trifoliata (Chapman, 1968; Newcomb, 1978). Differences in salt tolerance have also been shown to depend on the nature of the citrus scions (Cooper et al., 1952). Unraveling the mechanisms by which plants adapt to sustain salt stress, might provide an indication to plant breeders and biotechnologists as to how to proceed further in crop improvement. Present day scion-rootstock combinations represent outcomes of human selection over the last 1500 years, with an especially intense selection pressure during the last century, enabling citriculture in environments far removed from source habitats of citrus species. While citrus is commonly grown in regions where the salinity of the irrigation water is relatively low, e.g. in Australia, typically
