Diurnal sap-flow relationship between stem and root 1 on August 12, 2018. SFD recorded by the lower point of the sap-flow sensor (SFD_lower) was multiplied by −1 to represent the absolute value of the reverse sap flow. SFD recorded by the upper point of the sap-flow sensor (SFD_upper) and −SFD_lower versus SFD_stem (A), and −SFD_lower vs. SFD_upper (B). Filled symbols are 0:00–12:00, and open symbols are 12:00–24:00. Note two-fold decrease in SFD_upper compared with wet soil conditions (Figure 5B, blue August 17, 2018), supporting the same level of transpiration (SFD_stem). Hypothetical water-transport mode within root 1 (C). The gray line is the root pith. The sap-flow probe and the measured points were according to their actual positions (Figure 2F). The solid arrows represent the actual water flow which was monitored by sap-flow sensors (positive flow in the upper root section and reverse flow in the lower root section). The dashed arrows represent the presumed water movement