Foliar Supplied Boron Can Be Transported to Roots as a Boron-Sucrose Complex via Phloem in Citrus Trees

Although foliar boron (B) fertilization is regarded as an efficient way to remedy B deficiency, the mechanism of foliar B transport from leaves to roots, is still unclear. In this study, by using one-year-old ‘Newhall’ navel orange (Citrus sinensis) / trifoliate orange (Poncirus trifoliata) plants as research materials, we analyzed the B contents in leaves and roots, B-sucrose complex in the plant phloem sap after foliar application of 10B, girdling, and/or shading treatments. Results indicated that 10B content was significantly increased in roots after foliar 10B treatment. On the other hand, both girdling the scion stem and shading over the plants with a black plastic net significantly reduced the B and 10B contents in roots. LC-MS analysis revealed that foliar 10B-treated plants had higher contents of sucrose and some sugar alcohols in the phloem sap as compared to foliar water-treated plants. Combining with the analysis in the artificial mixture of B and sucrose, a higher peak intensity of 10B-sucrose complex was found in the phloem sap of foliar 10B-treated plants against to the control plants. Taken together, foliar B can be transported a long distance from leaves to roots via phloem at least by forming a B-sucrose complex in citrus plants.