Foliar spraying magnesium promotes soluble sugar accumulation by inducing the activities of sucrose biosynthesis and transport in citrus fruits

Accumulation of soluble sugars promotes the formation of fruit nutritional quality and flavor while proper application of Magnesium (Mg) can increase the sugar content in citrus fruits. However, the underlying mechanism is still unclear. In the present study by using ‘Huagan No.2′ (Citrus reticulate cv. Huagan No.2) as research material, foliar spraying of MgSO4 with two or three times was performed during the fruit expansion period. Results indicated that foliar spraying of Mg significantly increased the total soluble solids (TSS), as well as the contents of sucrose, fructose and glucose in fruits; moreover, the activities of sucrose metabolizing enzymes including sucrose synthase (SS), sucrose phosphate synthesis (SPS) and acid invertase (AI), as well as the transcript levels of SS2, SPS1–3 and vacuolar acid invertase gene (VINV) were significantly increased in the Mg-treated fruits. In addition, the transcript levels of the sucrose transport-related gene (SUT3) and the vacuolar pyrophosphate proton pump type I genes (CsVPP-1 and CsVPP-2) in the fruits were also significantly increased by Mg treatment. These results were further verified by soaking the fruit-bearing branches in 0.5% MgSO4 solution. In conclusion, foliar spraying Mg with two or three times promotes soluble sugar accumulation in fruits through enhancing the sink strength by inducing the activity of sucrose synthase via inducing SS2 transcript level, and improving sucrose transport ability by inducing transcript levels of SUT3, CsVPP-1, CsVPP-2 and VINV.