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 mechanisms of foliar B transport from leaves to roots are still unclear. In this study, performed with 1-year-old "Newhall" navel orange () grafted on trifoliate orange () plants, we analyzed the B concentration in leaves and roots, B-sucrose complex in the phloem sap after foliar application of B, girdling, and/or shading treatments. Results indicated that B concentration was significantly increased in roots after foliar B 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 B concentration in roots. LC-MS analysis revealed that foliar B-treated plants had higher concentration 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 the B-sucrose complex was found in the phloem sap of foliar B-treated plants compared to the control plants. Taken together, it is concluded that foliar B can be long distance transported from leaves to roots via phloem, at least by forming a B-sucrose complex in citrus plants.