2018 ASHS Annual Conference

Rootstock is important for commercial citrus production in Florida and in other citrus production areas around the world. Although the beneficial influence of rootstock on stress and disease tolerance and productivity is clearly documented, the biochemical interaction of rootstock with the scion in a grafted tree has not been much investigated in citrus. In this study, we applied untargeted gas chromatography-time of flight mass spectrometry (GC-TOF MS) to investigate the metabolic influence of four rootstock varieties with different genetic backgrounds on two-year old field-grown ‘Cara Cara’ navel orange (C. sinensis) trees. The rootstocks included were ‘Cleopatra’ mandarin (C. reticulata), ‘Ridge Pineapple’ sweet orange (C. sinensis), sour orange (C. aurantium), and ‘Swingle’ citrumelo (C. paradisi x Poncirus trifoliata). In total, we detected 500 unique metabolites in leaves and roots; of these, 147 were identified by their chemical structure. The concentrations of 48 metabolites varied significantly among roots from the four different rootstocks. Of the chemically known metabolites, allantoic acid and gamma tocopherol were among the most discriminating compounds. Whereas allantoic acid is implicated in the transport of nitrogen from roots to leaves in other tree species, tocopherols are important plant antioxidants and essential for the proper functioning of carbohydrate metabolism. Discrimination of root samples was mainly due to significantly higher concentrations of these compounds in ‘Swingle’ compared with the other three rootstocks. In contrast, ‘Swingle’ exhibited lowest concentrations of conduritol-beta epoxide, an important inhibitor of beta glucosidases, enzymes that are involved in the chemical defense against pests and pathogens. Other metabolites were found in highest concentrations in ‘Cleopatra’ roots, notably raffinose and myo-inositol. Both compounds are intermediates of the raffinose family oligosaccharides biosynthetic pathway and important osmoprotectants of plant cells. The most dramatic differences between rootstocks were found for the large group of chemically unknown metabolites, suggesting their involvement in important metabolic pathways. Several metabolites identified in this study may be suitable for use as biochemical markers for rootstock-specific traits. Rootstock was also found to influence the metabolic profiles of leaves in the navel orange scion. In total, 226 metabolites differed significantly in concentrations among leaves depending on the rootstock. Similar to roots, conduritol-beta-epoxide and myo-inositol were among the most discriminating metabolites. This demonstrates that rootstock variety can influence the metabolic profile of leaves in a grafted tree. However, the majority of root metabolites that discriminated most between rootstocks did not display the same rootstock-specific discrimination in the leaves.