Preselection of promising triploid mandarin varieties in Corsica : S17P13

The selection of seedless mandarin varieties adapted to the European market is one of the main objectives of the CIRAD breeding program. Triploid hybrid creation is the method selected to develop seedless varieties. More than one thousand triploid progenies are currently evaluated at the San Giuliano Research Station in Corsica. There were created by 2x X 2x hybridization exploiting spontaneous 2n gametes. The objective of a first set of crosses with clementine as female parent was to select clementine-like varieties, in order to extend the production period of this crop in Corsica. Seven hybrids were preselected at the end of the first level of evaluation according to the visual and organoleptic fruit characteristics, the period of production and the yield. There are currently under the second level of evaluation to analyse their agronomic behaviour. One of this hybrids is under D.U.S. examination for the grant of Community plant variety rights. The objective of a more recent second set of crosses is to develop late mandarin varieties. Several progenies, with different mandarins as female parent, present interesting fruit characteristics. One of them, with short juvenile period and very high yield, is preselected for the second stage of evaluation. (Texte integral

DNA amplified fingerprinting, a useful tool for determination of genetic origin and diversity analysis in Citrus

We used three short repetitive nucleotide sequences [(GTG)5, (TAC)5, and (GACA)4] either as radiolabeled probes for hybridization with restricted Citrus DNA or as single primers in polymerase chain reaction amplification experiments with total genomic DNA. We tested the ability of the sequences to discriminate between seedlings of zygotic or nuclear origin in the progeny of a Volkamer lemon #Citrus volkameriana# Ten. & Pasq.) tree. The genetic variability within two species [#Citrus sinensis# (L.) Osbeck (sweet oranges) and #Citrus reticulata# Blanco and relatives (mandarins)] was evaluated. DNA amplified figerprinting with single primers was the more successful technique for discriminating between nucellular and zygotic seedlings. Although we were not able to distinguish among 10 cultivars of #C. sinensis#, all 10 #C. reticulata# cultivars tested were distinguishable. However, it still is difficult to identify the putative parents of a hybrid plant when the two parental genomes are closely related. (Résumé d'auteur

Analysis of genetic diversity and population structure of Citrus Germplasm using nuclear (SSRs, INDELs) and mitochondrial markers

Previous studies with molecular markers (ISSR, RAPD, SCAR, AFLP and SSR) have shown that most of the genetic diversity of cultivated Citrus (except C. aurantifolia) comes from the recombination between three main species: C. medica (citron), C. reticulata (mandarin) and C. maxima (pummelo). However, the precise contribution of these basic species to the genome constitution of secondary species (C. sinensis, C. limon, C. aurantium, C. paradisi) and recent hybrids is not known. In this study, 58 nuclear and four mitochondrial markers were used to investigate the genetic diversity among 106 Citrus accessions, representing the three main ancestors groups, secondary species and several hybrids from the 20th century breeding programs. For the nuclear analysis, 50 simple sequence repeats (SSRs) developed from genomic libraries and ESTs databases were used. Moreover, 10 Insertion-Deletion (INDEL) markers were developed from genomic sequences of some primary and secondary metabolites determining the citrus fruit quality (sugars, acids, flavonoids and carotenoids). All the SSR markers and one INDEL are included in a consensus genetic map of clementine and pummelo Chandler and are distributed along the nine linkage groups, representing positively the global genome of Citrus. Genetic diversity statistics were calculated for each SSR and INDEL marker, within the entire population and within and between the different specified Citrus groups. The organizations of the genetic diversity among all the accessions were determined by constructing neighbor-joining trees for the different sets of primers. INDEL markers are less polymorphic than SSRs, display a higher organization of genetic diversity and appear to be better phylogenetic markers to trace the contribution of the three ancestral species. Population structure was studied using the Structure software, version 2.2.3, (http://cbsuapps.tc.cornell.edu/structure) which implements a model-based clustering method for inferring population structure using genotype data. The relative proportion of ancestral taxa genomes in the secondary species and recent hybrids was assigned. Mitochondrial markers revealed a maternal phylogeny of citrus germplasm accessions in agreement with previous studies with chloroplastic markers. This analysis allowed a better understanding of the genetic diversity organization among citrus cultivars, opening the way for a better management of citrus germplasm banks and breeding programs.(Texte intégral

Abscission study during citrus fruit maturation in Corsica: unfavorable environmental conditions for fruit shedding

Citrus fruit development and ripening are complex processes involving physiological and biochemical changes that are under hormonal, nutritional and environmental control. One of the most evident phenomena in late maturation is shedding of ripe fruit. A previous study on sweet orange abscission supposed that fruit shedding was related to the increase of sugars content of pulp juice. To investigate this potential relationship between fruit abscission and internal maturity parameters we investigated the fruit maturity process of 10 mandarin x clementine hybrids and 9 commercial sweet orange varieties with different maturity time. Morphological and biochemical analyses (acidity, total soluble solids, fruit weight thickness of the flavedo, thickness of the peduncle and external color) were carried out on fruit sets of the different varieties during the maturation process, from December to June. The abscission initiation was evaluated by measuring the force required to detach the fruit of the peduncle. Analysis showed that under our local conditions, abscission was generally not expressed even though fruit maturation was evolving. Therefore, we suppose that even though the local conditions have an important effect, abscission of citrus fruit also results from environmental-genotype interactions. Moreover, evolution of abscission and fruit parameters are not correlated, suggesting the independence in the processing of maturation and abscission. (Résumé d'auteur

Physiological analysis of salt stress behaviour of citrus species and genera: low chloride accumulation as an indicator of salt tolerance : S08O07

Tolerant citrus rootstocks are defined as Cl- excluders. However, little is known about the salt tolerance of cultivars used as scions, particularly the tolerance of monoembryonic citrus genotypes. To enhance the genetic resources for generating improved hybrid rootstocks, the evaluation of large samples of citrus species, including both monoembryonic and polyembryonic genotypes, is necessary. In this study, 12 citrus genotypes representing the major Citrus species and all the three genera of the Rutaceae family were subjected to moderate salt stress (75mM) for 12 weeks to characterise their physiological response to salt stress. Various symptoms and physiological parameters were evaluated to characterise their salt sensitivity. These included plant growth (stem diameter), leaf chlorophyll content, leaf flavonoid content, maximum quantum yield of PSII [(Fm-F0)/Fm)], net photosynthesis, stomatal conductance and leaf Na and Cl- contents. The results clearly demonstrated that the most salt sensitive genotypes accumulated high concentrations of Na and Cl- and maintained a fair growth and photosynthetic rate. By contrast, salt-tolerant genotypes accumulated less Na and Cl- and decreased their growth and gas exchange. 'Poncire commun' citron and 'Marumi' kumquat were the most sensitive species, while mandarins, pummelo and 'Australian' sour orange were the most tolerant species. Among the genotypes, 'Engedi' pummelo presented a specific trait for salt tolerance that has not been previously reported. Taken together, the results suggest that low leaf chloride content can be used as an indicator of salt stress tolerance in citrus genotypes. Exploitation of this indicator will enable the improved evaluation of citrus genetic resources and should lead to the identification of new sources of tolerance for rootstock breeding. (Texte intégral

Genetic diversity analysis and population structure of the mandarin germplasm revealed by nuclear SNP markers

Citrus reticulata is considered one of the four main ancestral taxa of cultivated citrus and is widely associated with the mandarin horticultural group. However, several studies suggest introgressions of other taxa and the precise structure of the mandarin group is not known. To analyse the structuration of mandarin germplasm and the potential interspecific introgressions, 208 accessions were genotyped by the KASPar method, with sixty-seven single nucleotide polymorphism (SNP) markers mined in 27 nuclear genes and in clementine BAC-ends. These SNP markers were employed for the study of 208 genotypes comprised of 169 'mandarin-like' genotypes and 39 genotypes belonging to ancestral taxa. Structure software was used to identify groups within the mandarin germplasm. Six mandarin groups were identified, some of them in agreement with the species defined by Tanaka. Many mandarins were located between these mandarin groups, indicating a high level of admixture between the identified parental groups. New insights in the organization of mandarin germplasm and its structure are presented in this work, which will be useful for better management of citrus germplasm collections. (Résumé d'auteur

Diversity of citron (Citrus medica) and phylogenetic analysis of related citron hybrids using molecular markers and essential oil composition

Citrus diversity is known to be essentially supported by three essential basic species, mandarin, pummelo and citron, which are at the origin of quite all the cultivated citrus. Based on studies of phenotype, the citron group (Citrus medica) is supposedly low in genetic diversity compared to mandarins and pummelos. To assess the diversity of the citron group we analyzed the molecular polymorphism of nuclear and cytoplasmic genetic markers and the variation in composition of essential oils from leaves and peels of 24 varieties of citron or apparent hybrids of citron. Our results demonstrate that the citron group has a relatively high allelic diversity, probably due to multiple introductions of different varieties. Self-fertilization was the mode of reproduction which most frequently led to the development of commercial varieties with specific aromas and flavors, such as the cultivar 'Corsican', a putative 'Common Poncire' self-crossed hybrid. However, some varieties appeared to be interspecific hybrids related to citron but only by the male parent. This varietal diversity was probably favored by spreading seeds to extend the cultivation of citron in Mediterranean countries. The chemical diversity does not always correspond to genetic diversity but nevertheless reveals specific profiles associated with some genotypes. (Résumé d'auteur

Comparison of photosynthesis and antioxidant performance of several Citrus and Fortunella species under natural chilling stress : S08P24

Citrus plants originate from southeastern Asia, in a large area with various climates characterized by a broad range of temperatures. Temperature is assumed to be a key factor in Citrus species adaptation and diversification. In a field experiment, the tolerance of the three fundamental Citrus species (C. medica -citron-, C. reticulata -mandarin- and C. maxima -pummelo-) and Fortunella japonica -kumquat- to photooxidative stress caused by seasonal climatic changes was evaluated on adult trees by measuring net photosynthesis (Pnet), stomatal conductance (Gs), maximum photosynthesis (Pmax) and chlorophyll fluorescence (Fv/Fm). In addition, seasonal changes in oxidative status, antioxidant enzymes and antioxidant metabolites were monitored. Mandarin and pummelo appeared to be the most tolerant, showing the lowest down-regulation of photosynthetic parameters, and the lowest accumulation of oxidized compounds associated with efficiency of their antioxidant system. Kumquat showed intermediate behaviour, with a large diminution of photosynthetic parameters and marked accumulation of hydrogen peroxide, whereas the malondialdehyde content remained low, with a strong induction of glutathione synthesis. Finally, citron appeared to be the most sensitive genotype with a marked decrease in photosynthetic performance, the largest accumulation of oxidative parameters, insufficient induction of antioxidant enzymes and down-regulation of ascorbate and glutathione synthesis. (Texte integral

Genetic strategy for identification or genes involved in citrus salt stress tolerance : Status of genome mapping program

Salt stress is one of the most obvious effects of high salinity on Citrus, which is classified among the most sensitive tree crops. Genetic and genomic analysis of tolerant and sensitive plants is a prerequisite for breeding programs and the selection of more adapted varieties to high salinity. The combined approach of genetic mapping and localization of candidate genes has been applied in plant genetics in the past decade with the objective of characterizing and cloning quantitative trait loci (QTLs). The segregation of the tolerancel sensitivity to salt character was initiated on the F2 population resulting from the crossing Cleopatra mandarin X Poncirus trifoliata. Citrus linkage map was derived from the segregation analysis of SSR markers from EST or BAC ends. From many polymorphic primers only few makers have Mendelian segregation. The skewed segregation can be due to abnormal meiosis at the intergeneric level. The excess of homozygous locus was observed only for the marker CiBE2626b. No difference was observed between frequency of alleles from Poncirus and alleles from mandarin in homozygous loci. Half ofF2 population (61 hybrids) has a percentage of homozygous loci lower than or equal to 20%. Other hybrids seem to have proportions of homozygous and heterozygous loci that follow a normal distribution although there is a slight shift of sorne homozygote individuals to represent the expected Gaussian curve (- 0.45 instead of 0.5). We have also studied the segregation and mapped several candidate genes putatively involved in salinity tolerance. We plan to confirm the role played by these genes by gene expression analysis from selected homozygous F2 genotypes under strong salt stress conditions. We will measure the effects of salt stress on physiological traits on the segregating population. (Texte intégral

Characterization of water deficit tolerance of Poncirus trifoliata genotypes related to diversity

Rootstock provides better adaptation to biotic and abiotic constraints. Poncirus and its hybrids are widely used since they are tolerant to Citrus Tristeza Virus. However, when grafted, most of them are considered to be sensitive to salinity and water deficit. Diversity does exist within the Poncirus trifoliata specie but little is known regarding the behaviour variability of genotypes to abiotic constraints. The citrus diversity of 72 P. trifoliata genotypes was investigated using SSR markers. Two mayor genetic groups were clearly identified. Five genotypes, belonging to each genetic group, were then selected to investigate their properties of tolerance to water deficit. Water deficit was applied by withdrawing irrigation for 4 weeks. Physiological parameters such leaf stomatal conductance and quantum yield of PSII electron transport, soil water potential, leaf osmotic pressure and transpiration were estimated. Among genotypes, we observed that some genotypes such as' Rubidoux' were clearly more tolerant to water deficit than others such as 'Pomeroy'. Interestingly, the genotypes that were sensitive belonged to one genetic group and the tolerant ones belonged to the other group. Therefore, it sounds that among the Poncirus genus, it is possible to select genotypes as parent in breeding programs, which are more tolerant to water deficit. (Résumé d'auteur