Markers of ontogenical variation in mandarin trees

Breeders are constantly seeking for new varieties of edible species with distinctive characteristics that could be able to follow consumption trends. In the case of citrus, particularly mandarins, the absence of seed, easy peeling, big fruit with good flavor, as well as early maturity and remarkable nutritional properties are desirable characteristics. The major difficulty lies on the biology of the citrus tree, because the final set of organoleptic characteristics and nutritionally valuable substances in the fruit are only detectable when the tree is fully developed. This period takes 10 to 20 years since the hybridization was performed(1). The biosynthetic origin of the compounds responsible for the flavor characteristics is diverse and although the genetics involved can be unequivocally identified, the mechanism of expression of these genes has not yet been completely elucidated. A possible approach is to study the metabolome, a group of secondary metabolites, not only responsible for the adaptation of the organism to a generally hostile environment but also for the organoleptic and nutraceutical characteristics of the fruit. Polyphenols and limonoids fulfill vital functions in plants, but also contribute to combat chronic diseases from the human health point of view (type II diabetes, heart diseases and various types of cancer) given their high antioxidant capacity(2,3,4). Limonoids are responsible for the characteristic bitter taste in citrus fruits, anti-cancer, hypocholesterolemia and antiviral properties have been reported to them(5,6). The study of the evolution in the content of these metabolites in two ontogenical plant stages (productive and non-productive) and in two different organs, would allow an early characterization of the new hybrid.
In this work a targeted metabolomic study of the profile of polyphenols and limonoids was performed by HPLC-(ESI)-MS/MS in 4 commercial cultivars (Satsuma, Page, Ellendale and Willow leaf) as well as in 3 hybrids at their productive stage, harvested during 2015 and 2016 in two plant organs: the edible part of the fruit (pulp) and the leaf. Furthermore, leaves from the same parental varieties were evaluated at their non-productive stage. Non-productive leaves showed the highest content of polyphenols and limonoids. The most significant differences between non-productive leaves and those from adult trees, was the presence of Naringin, Limonin, Naringenin, and Quercitrin in the first ones, whereas Hesperidin was the most abundant in the latter. The proportion of each flavonoid was very different between varieties, being Naringin, Luteolin and Nobiletin the flavonoids that allowed to differentiate between cultivars at non non-productive stage, whereas, different proportions of Tangeretin and Nobiletin were markers for productive trees. The principal component analysis (PCA) of the profiles of leaf and pulp of the studied varieties separated both groups. The data of pulp was grouped together, showing no major differences between cultivars, while the leaf yielded a more disperse pattern. The pulp showed a rather homogenous composition, where Hesperidin and Narirutin dominated the profiles studied. It seems that the change in the prevalence of Narigin to Hesperidin and Nomilin to Limonin are clues to understand the evolution of the non-productive to productive stages of mandarin trees in the four commercial varieties.

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