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• Institut national de la recherche agronomique de Tunisie (INRAT), Tunisia
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The Mediterranean, where production is primarily geared towards supplying the European fresh fruit market, is the world's second citrus production basin, after Brazil. However, there are fears that yields are set to drop due to the spread of the tristeza virus, which will soon rule out the use of the rootstock traditionally used in citrus growing, the bitter orange tree. Future rootstocks will therefore have to be tolerant of the virus, and also of three current or potential types of stress: increased soil salinity due to irrigation, lack of water, and iron deficiencies due to calcareous soils. The EU CIBEWU project, led by six research organizations working in the Mediterranean, including CIRAD, which is project coordinator, focused on what determines tolerance of these types of abiotic stress. It has just been completed.
The project concentrated particularly on making better use of citrus genetic resources. The researchers developed standard protocols for identifying physiological and molecular indicators of tolerance, based on rootstocks whose performance was already known. They were thus able to measure the effects of stress on photosynthesis, transpiration and saline ion flow to the leaves. They then used those indicators to determine the threshold values that determine whether or not rootstocks are tolerant of the types of stress studied.
It is also vital to study the genome when developing new varieties. The research done in this field served to identify candidate genes as regards tolerance of salt, water and iron stress, and genetic markers. These crucial results opened the way for early varietal selection.
Lastly, the researchers obtained some ten new somatic hybrids. Through cell fusion, somatic hybridization makes it possible to cumulate all the genes of both parents. Using two diploid parents with 18 chromosomes each results in tetraploid progenies with 36 chromosomes. In particular, four Citrus x Poncirus intergeneric combinations were carried out to combine the disease tolerance of Poncirus with the abiotic stress tolerance of Citrus . Moreover, by studying the impact of ploidy on salt and water stress tolerance, the researchers revealed that tetraploids were more tolerant. The results suggest that diploids and tetraploids regulate the synthesis of abscissic acid - the key hormone in water and salt stress response as it triggers the closure of the stomata, hence avoiding water loss - in different ways.
Thanks to the project, two regional networks have been set up. The first is due to assess the new rootstocks at various sites, while the other should serve to improve the management and use of citrus genetic resources, particularly in Turkey, Tunisia and Morocco.
The rootstocks obtained under the project are currently being assessed in the laboratory. This phase, which is due to take two years, will be followed by agronomic trials in the field. It takes at least ten years between the creation of a new hybrid and its commercial availability once it has been validated in agronomic terms.

Literature

Saleh B., Allario T., Dambier D., Ollitrault P., Morillon R., 2008. Tetraploid citrus rootstocks are more tolerant to salt stress than diploid. C. R. Biologies, Académie des Sciences, 331: 703-710.
Terol J, Naranjo M.A., Ollitrault P., Talon M., 2008. Development of genomic resources for Citrus clementina : characterization of three deep-coverage BAC libraries and analysis of 46,000 BAC end sequences. BMC Genomics 9:423.
Ollitrault P., Guo W., Jude Grosser W., 2007. Recent advances and evolving strategies in citrus somatic hybridization. Citrus Genetics, Breeding and Biotechnology. I. Khan Edit. CABI pub., 235-260.