Abstract
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 (Citrus sinensis) grafted on trifoliate orange (Poncirus trifoliata) plants, we analyzed the B concentration in leaves and roots, B-sucrose complex in the phloem sap after foliar application of 10B, girdling, and/or shading treatments. Results indicated that 10B concentration 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 concentration in roots. LC-MS analysis revealed that foliar 10B-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 10B-sucrose complex was found in the phloem sap of foliar 10B-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.
Highlights
Boron (B) is a micronutrient that plays a pivotal role in cell wall stability (Chormova et al, 2014), photosynthesis (Wang et al, 2015), and carbon metabolism (Mishra and Heckathorn, 2016) in the plant
The 10Bpro in leaves (Figure 1B) or roots (Figure 1C) of both T2 and treatment 3 (T3) plants were significantly higher than T1 plants
As for the 10Bdff, it was significantly higher in the leaves of T3 plants than in the leaves of T2 plants (Figure 1D)
Summary
Boron (B) is a micronutrient that plays a pivotal role in cell wall stability (Chormova et al, 2014), photosynthesis (Wang et al, 2015), and carbon metabolism (Mishra and Heckathorn, 2016) in the plant. B deficiency inhibits plant growth, hinders leaf expansion, causes leaf chlorosis or shoot tip dieback, deforms leaf, flower, or fruit, decreases yield and fruit quality, limits root elongation (Wang et al, 2015). Preventing boron deficiency is critical to maintain crop yield and quality, and its success depends on the understanding of B transport and distribution mechanisms. Plants absorb B mainly by roots from the soil in the form of boric acid. The occurrence of B deficiency is mainly caused by the low level of soluble B in the soil and/or low B utilization by plants (Wang et al, 2015).
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