2019 ASHS Annual Conference

Commercial citrus production depends on the proper choice of rootstock. Rootstock influences tolerance of trees to biotic and abiotic stresses and influences horticultural traits, especially tree size, fruit quality, and yield. Grafting of scion on a rootstock results in a tree that is composed of two often genetically quite different units, and therefore influences the metabolic composition and physiology of the composite tree. In addition to rootstock effects on the scion, the scion is also likely to influence the physiology of the rootstock. In this study, we examined the metabolic profiles of eleven different citrus rootstocks grown as seedlings and as grafted trees in combination with ‘Valencia’ (Citrus sinensis) scion. Root metabolic profiles were established in seedlings and in grafted trees, and the influence of rootstock on leaf metabolic profiles was established in the grafted scion. Seedlings and grafted trees were one-year old and grown under controlled and identical conditions in the greenhouse. We used 11 citrus rootstock cultivars including ‘Cleopatra’ mandarin (Citrus reticulata), sour orange (C. aurantium), ‘Ridge’ pineapple (C. sinensis), and 8 hybrids of citrus and trifoliate orange (Poncirus trifoliata). Most of these varieties are widely used in commercial citrus production in Florida. Seedling roots, and roots and leaves of grafted trees were analyzed for their metabolite composition by gas chromatography time-of-flight mass spectrometry (GC-TOF MS). Partial least squares discriminant analysis (PLS-DA) revealed clear metabolic differences among rootstock cultivars in both seedlings and grafted ‘Valencia’ trees. Among the most discriminating root metabolites were hexitol, conduritol beta epoxide, galactinol, and nicotinic acid. Metabolic profiles were clearly associated with the taxonomic relationships among cultivars. Interestingly, in the unifoliate rootstocks, nicotinic acid, salicylic acid, and saccharic acid were found in considerably higher concentrations in the roots of grafted trees compared with seedlings; in contrast, in the trifoliate hybrid rootstocks, concentrations were lower in grafted trees than in seedlings. The reverse was found for quinic acid, allantoic acid, and shikimic acid. Analysis of leaf metabolic profiles in grafted trees showed discrimination of ‘Valencia’ trees based on the rootstock on which they were grafted. Leaf metabolites that discriminated most among rootstocks included beta-gentobiose, citrulline, hexaric acid, and ornithine. This demonstrates that rootstock influences a grafted scion metabolically. The results from this study are valuable for understanding rootstock−scion interactions, and for evaluating the positive and negative traits of rootstock and scion cultivars when they are deployed in graft combinations.