Impact of Scion/Rootstock Reciprocal Effects on Metabolomics of Fruit Juice and Phloem Sap in Grafted Citrus reticulata

Rootstock has a significant impact on plant growth and development, including fruit maturation. However, the existence of a mutual interaction between scion and rootstock is often neglected. To explore the origin of different fruit quality traits in citrus, we studied the reciprocal interaction between scion and rootstock of nine combinations; three mandarin varieties (`Merav`, `Michal’, `Orra Shani’) grafted on three different rootstocks (SB-812, Sour Orange, Volkameriana Lemon). We profiled phloem sap composition in the scion and in the rootstock using liquid chromatography-mass spectrometry (LC-MS). Moreover, we analyzed the metabolic profile of juice via gas chromatography mass spectrometry (GC-MS) and LC-MS . Quality traits of fruit and their physico-chemical characteristics were also evaluated. For all three cultivars, rootstock was found to affect fruit yield and biochemical fruit quality parameters, such as TSS and acidity level. Most chemical compounds in the juice were related to the scion variety, but Sour Orange rootstock significantly affected fruit level of asparagine and quinic acid. Moreover, juice flavonoid content, e.g. levels of rhoifolin and diosmin, was pronouncedly modulated by the rootstock. Based on phloem sap analysis, there was a reciprocal effect between the rootstock and the scion on sap metabolite composition. Rootstock was found to significantly affect the metabolic profile of scion sap, and specifically the content of sugars and amino acids. Rootstock sap composition differed between the different combinations, indicating a significant modulation of root metabolism by the shoot, mainly for fructose, the abundantly present amino acids proline, aspartate, and GABA. Our work suggests a reciprocal relationship between the scion and roots in a grafted tree, whereby a mutual alteration in the major carbon and nitrogen metabolism is underway in a sort of acclimation process in the forced connection between two different genotypes.