Genetic and physiological analyses to determine traits for resistance to salt stress
Citrus trees are classified as being among the fruit trees most susceptible to salt stress. However, a marked diversity prevails for this trait within this family: Poncirus trifoliate (L.) Raf. is known to be highly susceptible, while Citrus reshni Hort. ex Tan (Cleopatra mandarin) is one of the most tolerant genotypes. The usual strategy to improve the resistance of rootstocks is based on hybridization between parents with appropriate complementary traits. Another means of acquiring salt stress tolerance in rootstocks is linked to tetraploidy by chromosomal duplication. A genetic and physiological analysis of the salt stress tolerance of any new genotype is therefore necessary for programmes for the selection of better adapted varieties. Studies combining genetic techniques (genome mapping) and physiological approaches linked to diversity among citrus plant species were thus performed so as to enable future correlations between the salt stress tolerance phenotypes observed and the expression of their respective genomes.
Research protocol and measurement parameters
An F2 population was generated by pollinating an F1 hybrid (Citrus reshni x Poncirus trifoliate). Study of the segregation of 135 microsatellite markers and four candidate genes enabled the establishment of 15 linkage groups. Most of the markers (57%) displayed non-mendelian segregation, probably due to a malfunction during the pairing of chromosomes at the time of meiosis in the hybrid parent. In parallel, 22 genotypes representing citrus diversity were subjected to salt stress in order to measure their degree of tolerance. Several physiological parameters such as growth rate, chlorophyll content, total content in phenolic compounds, the yield of photosystem electron transport, stomatal conductance and the rate of photosynthesis, were also measured.
Stress tolerance is dependent on complex mechanisms
Different physiological behaviours relative to salt stress tolerance were observed as a function of the citrus species studied, suggesting the existence of different mechanisms underlying this tolerance. Citrons proved to be the most susceptible, while all mandarins and grapefruits were tolerant. Numerous genotypes related to citrons displayed symptoms of chlorosis, accumulations of chloride and sodium ions in the leaves and changes to physiological parameters. Specific tolerance profiles were associated with a maintenance of photosynthesis, even though lower stomatal conductance values were recorded. At the same time, plant growth was maintained, with low accumulations of chloride and sodium ions. At the end of the study, grapefruits displayed leaf drop followed by re-growth, which we interpreted as an adaptive response (a novel behaviour not previously described in citrus).
Agronomic interest of zygotic or tetraploid rootstocks
Studies performed by CIRAD showed that tetraploid rootstocks enabled an increase in the salt stress and water stress tolerance properties of citrus plants. It was therefore interesting to characterise the impact of zygotic or tetraploid rootstocks on the yields and quality of the graft relative to tree physiology. The results obtained suggested that hybrid rootstocks, probably resulting from the self-fertilisation of Poncirus trifoliata, had relatively little effect on mean yields, unlike tetraploid rootstocks arising from chromosomal duplication in the embryos of P. trifoliate, which considerably reduced production while fruit quality was not affected. Analysis of the physiology of a clementine grafted onto two diploid rootstocks, and onto their tetraploid rootstocks, suggested that the poorer growth of tetraploid rootstock/clementine combinations was notably due to more limited photosynthesis. However, these rootstocks enabled a better adaptation of photosynthesis to environmental conditions. Combinations of a tetraploid rootstock and clementine graft planted at high density may therefore ensure better water use efficiency and improved salt stress tolerance, so that irrigation can be limited without affecting yields and fruit quality. The results obtained so far will now be applied to the selection of new rootstocks for cultivation in high-salinity soils.
Scientific leaders:
François LURO -
Sajjad HUSSAIN
INRA
UR1103 GEQA - Génétique et Ecophysiologie de la Qualité des Agrumes
20230 SAN GIULIANO
Raphaël MORILLON
CIRAD- Equipe APMV - Amélioration des Plantes à Multiplication Végétative,
Très Grande Unité « Amélioration Génétique et Amélioration des Plantes ».
Centro de Genomica, Instituto Valenciano de Investigaciones Agrarias (IVIA),
46113 - Moncada – Valencia – España
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