Sugars accumulated in fruits are synthesized in leaves and translocated into the fruits. Quantitative measurement of phloem sap flow would enhance our understanding of the phenomenon and would facilitate efforts to ensure the production of high-quality fruits. The influence of girdling, which cuts off phloem, on sap flow rate determined using the heat balance method, was investigated in citrus. The present study aimed to measure the flow rates in phloem and xylem sap separately and to investigate the effect of gauge temperature on gauge output accuracy. When the gauge temperature was kept constant, the flow rate was consistent with the amount of water transpired from the branch. Immediately after girdling, the sap flow rate increased in a non-bearing branch, but decreased in a bearing branch. We conclude that the phloem sap flowed basipetally in the non-bearing branch and acropetally toward the fruits in the bearing branch. Regression lines for the sap flow before and after girdling were almost parallel, and the difference in the intercept of the two regression lines can be regarded as the flow rate change caused by girdling. Our results indicate that it is possible to determine separate flow rates for phloem and xylem sap by combining the heat balance method with girdling.
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