The sugar and acidity contents, or those of their components (sugars and organic acids), are directly involved in flavour perception and in monitoring fruit ripening. However, so-called secondary metabolites, such as carotenoids, phenolic compounds and aromatic compounds, also take part in flavour development and in the consumer’s visual and olfactory appreciation of citrus fruits. Certain organic acids, carotenoids, phenol compounds or aromatic compounds can affect health. Citrus fruits are known to be rich in vitamin C, carotenoids (such as beta-cryptoxantin and licopene), and in certain phenolic compounds (such as phlorin, hesperidin or diosmin), which have health benefits.  Other compounds are little appreciated, such as naringin, a bittering agent, or even toxic compounds like certain furanocoumarins. Studies on the presence or absence, or variable quantities of these different compounds have revealed a structuring of the diversity of different Citrus species similar to that revealed by neutral genetic markers (Fanciullino et al., 2006; Luro et al. 2011, Durand-Hulak et al., 2015; Dugrand-Judek et al, 2015) thereby showing that allopatric evolution has contributed to a major differentiation of the primary and secondary metabolite profiles. Nonetheless, intraspecific diversity is sometimes high, leading to varietal identification/utilization or a by‑product (Luro et al. 2012; Fanciulino et al., 2008; Sutour et al. 2015, 2016; Paoli et al. 2016). Genotype x environment interactions are analysed to study heritability and define the most suitable conditions for the expression of quality traits of the primary and secondary metabolisms (Dhuique-Mayer et al., 2009; Oustric et al. 2015; Khefifi et al. 2016) or to assess the impact of restrictive conditions, such as limited carbon resources (Antoine et al. 2015). Some trials testing rootstock/scion interactions showed that using tetraploid rootstocks (Hussain et al. 2012) did not affect quality. Some studies on the heredity of these components from segregating populations of diploid genotypes, or by association genetics, are being developed in order to define some early selection markers for breeding programmes. Characterizing some genes of interest of these metabolic pathways is also one of the team’s objectives (Munakata et al, 2014). 

References cited

• Antoine S., Pailly O., Gibon Y., Luro F., Santini J., Giannettini J., Berti L. (2015) Short and long term effects of carbohydrate limitation on sugar and organic acid accumulation during mandarin fruit growth. Journal of the Science of Food and Agriculture DOI 10.1002/jsfa.7594
• Dhuique-Mayer C., Fanciullino A.L., Dubois C., Ollitrault P. (2009) Effect of genotype and environment on citrus juice carotenoid content. Journal of Agricultural and Food Chemistry, 57 (19) : p. 9160-9168.
• Dugrand-Judek, A., Olry, A., Hehn, A., Costantino, G., Ollitrault, P., Froelicher, Y., & Bourgaud, F. (2015) The Distribution of Coumarins and Furanocoumarins in Citrus Species Closely Matches Citrus Phylogeny and Reflects the Organization of Biosynthetic Pathways. PloS one, 10(11), e0142757.
• Durand-Hulak, M., Dugrand, A., Duval, T., Bidel, L. P., Jay-Allemand, C., Froelicher, Y., Bourgaud F., Fanciullino, A. L. (2015) Mapping the genetic and tissular diversity of 64 phenolic compounds in Citrus species using a UPLC–MS approach. Annals of botany, 115(5), 861-877.
• Fanciullino A.L., Dhuique-Mayer C., Luro F., Casanova J., Morillon R. and Ollitrault P. (2006) Carotenoid diversity in cultivated Citrus is highly influenced by genetic factors. J. Agric. Food. Chem. 54 (12), 4397 -4406.
• Fanciullino A.L., Cercos M., Dhuique-Mayer C., Froelicher Y., Talon M., Ollitrault P., Morillon R. (2008) Changes in carotenoid content and biosynthetic gene expression in juice sacs of four orange varieties (Citrus sinensis) differing in flesh fruit color. Journal of Agricultural and Food Chemistry, 56 (10) : p. 3628-3638.
• Hussain S., Curk F., Dhuique-Mayer C.,Urban L.,  Ollitrault P.,   Luro F.,  Morillon R. (2012) Autotetraploid trifoliate orange (Poncirus trifoliata) rootstocks do not impact clementine quality but reduce fruit yields and highly modify rootstock/scion physiology. Scientia Horticulturae 134:100-107
• Khefifi, H., Tadeo, F.R., Selmane, R., Ben Mimoun, M., Morillon, R. and Luro, F. (2016) Effect of environment on citrus fruit abscission and maturation. Acta Hortic. 1119, 59-64.
• Luro F. Gatto J., Costantino G. and Pailly O. (2011) Analysis of genetic diversity in Citrus. °Pant Genetic Resources, doi:10.1017/S1479262111000189.
• Luro F., Venturini N., Costantino G., Paolini J., Ollitrault P. and Costa J. (2012) Genetic and chemical diversity of citron (Citrus medica L.) based on nuclear and cytoplasmic markers and leaf essential oil composition. Phytochemistry 77:186-196.
• Munakata, R., Inoue, T., Koeduka, T., Karamat, F., Olry, A., Sugiyama, A., Takanashi K., Dugrand A., Froelicher Y., Tanaka R., Uto Y., Hori H., Azuma J., Hehn A., Bourgaud F., Uto, Y. (2014) Molecular cloning and characterization of a geranyl diphosphate-specific aromatic prenyltransferase from lemon. Plant physiology, 166(1), 80-90
• Oustric Julie, Sandrine Antoine, Jean Giannettini, Yves Gibon, François Luro, Liliane Berti and Jérémie Santini (2015) Biochemical characterization of the primary metabolism and antioxidant defence systems of acidic and acidless citrus genotypes during the major stages of fruit growth. Acta Physiologiae Plantarum 37(11) :1-13 doi:10.1007/s11738-015-1982-9
• Paoli Mathieu, Dominique de Rocca Serra, Félix Tomi, François Luro  and Antoine Bighelli (2016) Chemical composition of the leaf essential oil of grapefruits (Citrus paradisi Macf.) in relation with the genetic origin. Journal of Essential Oil Research. DOI: 10.1080/10412905.2016.1140090
• Sutour Sylvain, Pascale Bradesi, François Luro, Joseph Casanova and Félix Tomi (2015) Germacra-1(10),5-dien-4a-ol in Fortunella sp. leaf oils. FFJ 30(6):445-450. DOI:10.1002/ffj.3258.
• Sutour Sylvain, François Luro, Pascale Bradesi, Joseph Casanova and Félix Tomi (2016) Chemical composition of the fruit oils of five Fortunella species grown in the same pedoclimatic conditions in Corsica (France) NPC 11 (2): 259-262.