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

Salt stress tolerance in acidic and sweet mandarins

Mandarin genotypes can be split in two main groups: the acidic and the sweet mandarins. Commonly, acidic are used as rootstocks when sweet mandarins are the varieties that are consumed as fruits. The acidic 'Cleopatra' mandarin (Citrus reshni Hort. ex Tan.) is considered to be the most salt stress tolerant rootstock since it is able to limit the absorption of chloride and sodium at root level. As a consequence, limited amount of toxic ions are translocated from root to leaves. If salt stress tolerance of 'Cleopatra' mandarin and its hybrids has been well documented, little data exist for sweet mandarins. We investigated 16 genotypes representing a large diversity in mandarins subjected to salt stress. Physiological parameters such as stomatal conductance and leaf chloride contents were analyzed. If most of the acidic varieties were tolerant, also some sweet varieties presented traits of tolerance. Taking into account the results of leaf ion contents, we observed that large leaf chloride contents were not always associated with sensitivity. (Résumé d'auteur

Behavior of diploid and tetraploid genotypes of 'Carrizo' citrange under abiotic stresses

Tetraploidy is a spontaneous phenomenon in citrus and some anatomical differences have been described between tetraploid (4x) citrus plants and their corresponding diploids (2x). The choice of a suitable rootstock is often very restricted by a combination of biotic and abiotic stresses. Recent data points out that tetraploidy in citrus induces interesting phenotypes like dwarfing and tolerance to abiotic constraints. In this work we investigated the behavior of 4x and 2x 'Carrizo' citrange (Citrus sinensis X Poncirus trifoliata) subjected to the main abiotic stresses that threat Spanish citrus industry. Seedlings were exposed to salinity, drought and iron deficiency and physiological parameters were measured. Results suggest that 4x plants stressed by drought were able to maintain high leaf water potential more efficiently than 2x, thus gas exchange parameters (transpiration, stomatal conductance, and photosynthesis) were less affected. Under salt stress no difference was found in leaf chloride content neither in gas exchange parameters, but leaf burn was less severe in 4x than in 2x. Concerning iron deficiency, a better tolerance of 4x compared to 2x was supported by higher ferricchelate reductase activity, SPAD index and chlorophyll fluorescence. These preliminary results suggest that 4x 'Carrizo' citrange could be a promising rootstock, having the same general performance than the original 2x, but producing smaller trees more tolerant to abiotic stresses. (Résumé d'auteur

Citrus tetraploid rootstocks are more tolerant to salt stress than diploid

Salt stress has a dramatic impact on the citrus industry by decreasing the growth of trees and fruit yield. We studied at the physiological and molecular level diploid and tetraploid citrus rootstocks when confronted to salt stress. Those tetraploid plants originate from the chromosome duplication in nucellar cells (somatic cells) of the apomictic diploid parent. Diploid and autotetraploid of Poncirus trifoliate (Pomeroy Poncirus trifoliata) and WillowLeaf mandarin (Citrus deliciosa) were studied. The allotetraploid somatic hybrid FLHORAG1 which was obtained by electrofusion between Willow-Leaf mandarin and Pomeroy Poncirus protoplasts was as weil investigated. Poncirus trifoliate is known to be a salt sensitive rootstock. Willow leaf mandarin is not used as a rootstock but was supposed to have a greater property of salt tolerance, similar to other mandarins such as Cleopatra mandarin. The anatomy of the leaf of diploid and tetraploid plants was first investigated. The stomatal area of tetraploid is 1.6 fold higher than the one of diploid plants. Stomatal density was also different between diploid and tetraploid. A 50 mM sodium chloride (NaCl) stress was perforrned for 9 weeks. Salinity caused leaf injuries and leaf fall in diploid of Poncirus while the tetraploid plants were not damaged. Autotetraploids and the allotetraploid FLHORAG1 acted like salt tolerant when compared to diploid plants. Sodium and chloride accumulation were similar for both diploid and tetraploid genotypes suggesting that root exclusion and/or accumulation of toxic ions in vacuoles is not the only way for salt tolerance. In order to characterize the molecular deterrninants mediating the salt tolerance of polyploids we investigated the gene expression profiles by using cDNA-AFLP technique. Contrasted gene expression profiles were detected between diploids and tetraploids as weil as between control and stressed plants. Transcript Derived Fragments (TDFs) from contrasted profiles pattern were sequenced. 14 genes involved in osmotic adjustment, defense and signal transduction were isolated. The expression of sorne of those genes such as genes coding for the choline monooxigenase enzyme and the Group 5 late embryogenesis abundant protein (LEA5) and genes involved in the Abscissic Acid biosynthesis pathway are actually monitored by using Real Time PCR. (Texte intégral

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

Evaluation of fruit quality in a lemon cybrid with mandarin mitochondria

Nutritional and organoleptic qualities of fruit are currently very important objectives for plant breeders. Organic acids, sugars and carotenoids were studied by high liquid chromatography on the pulp of a citrus cybrid. This cybrid named 'WLM + EUR' (Citrus deliciosa Ten.) + (Citrus limon (L.) Burm.), inherited nuclear and chloroplasts genomes of Eureka lemon (Citrus limon (L.) Burm.) plus mitochondria from Willow leaf mandarin (Citrus deliciosa Ten.). In our work, impact of new mitochondria on fruit quality was studied during the maturity period. We observed that the cybrid was different from willow leaf mandarin and close to lemon parent. Organic acids level is slightly increased in the cybrid fruit pulp compared to Eureka lemon. No significant difference is observed in sugars and carotenoids between the cybrid and the lemon. The results confirm that mains genetic information for sugars, organic acids and carotenoids biosynthesis are contained in the nucleus. Cybridisation should be used in citrus as a strategy to breed specifie traits associated with mitochondrial genomes such as male sterility without affecting the main organoleptic and nutritional qualities. (Texte intégral

Genetic structure and phylogeny of the 'true citrus fruit trees' group (Citrinae, Rutaceae)

Despite considerable differences in morphology, the general representing 'true citrus fruit trees' are sexually compatible, but their phylogenetic relationships remain unclear. Most of the important commercial species of Citrus are believed to be of interspecific origin. By studying SNP and indel polymorphisms of 27 nuclear genes on 32 genotypes of Citrus and relatives of Citrus, the phylogenetic relationship between 'true citrus fruit trees' was clarified. Additionally, four mitochondrial (mtDNA) and five chloroplastic (cpDNA) SSRs markers were analysed for the citrus species to specify the maternal phylogeny of Citrus secondary species. A total of 16238 kb of DNA was sequenced for each genotype, and 1156 SNPs and 58 indels were identified. Nuclear phylogenetic analysis revealed that C. reticulata, Poncirus, and Fortunella form a clade that is clearly differentiated from the clade that includes three other basic taxa of cultivated citrus (C. maxima, C. medica and C. micrantha). Clymenia is included in a third strong clade with Eremocitrus and Microcitrus. The nuclear phylogeny of Citrus and its sexually compatible relatives was consistent with the geographic origins of these genera. The origin of Citrus secondary species was analysed both at nuclear and cytoplasmic level and the conclusions mostly agree with previous hypotheses. The SNPs and indels identified at the nuclear level from a relatively large discovery panel will be useful for systematic characterisation of citrus germplasm, both at intra- and inter-specific level. (Résumé d'auteur

Physiological and molecular responses of diploid and tetraploid Carrizo Citrange under water Stress : S02P14

In citrus, the use rootstock promotes productivity, improves fruit quality and may confer resistance or tolerance to biotic and abiotic stress. 'Carrizo' citrange (Citrus sinensis [L.] Osbeck × Poncirus trifoliata [L.] Raf), is one of the most popular rootstock in the Mediterranean basin. It is sensitive to drought and salt stress but confers tolerance to Tristeza virus, and promotes very good fruit quality. Previous studies have shown that doubled diploid (4x) 'Rangpur' lime (Citrus limonia, Osbeck) seedlings are more tolerant to water deficit than their respective diploid (2x). In the present work, we have characterized the water deficit tolerance in 2x and 4x 'Carrizo' citrange seedlings. Water deficit was applied for 35 days, followed by irrigation. Several physiological parameters were measured periodically during the experiment and samples were collected to investigate i) the activity of enzymes involved in detoxification processes, ii) the expression analysis of candidate genes involved in ABA biosynthesis, as well as iii) ABA and H2O2 production. Doubled diploid 'Carrizo' citrange seedlings were showed to be more drought tolerant than 2x. Water deficit caused a greater reduction in photosynthetic rates and stomatal conductance in 2x compared to 4x. Also higher ABA and H2O2 production were induced in 2x when compared to 4x. The better tolerance of 4x seedlings is discussed to the light of candidate genes expression analysis and activities of enzymes of detoxification. Work supported by CNPq and CAPES. (Texte intégral