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Sap flow is an important parameter for orchard irrigation scheduling. According to seasonal variation of trunk diameter and leaf area index (LAI), the whole growth stage of apple tree was divided into stage I (rapid leaf and trunk growing) and stage II (rapid fruit enlarging and maturing). Trunk sap flow (SF) of apple tree irrigated with 100% evapotranspiration (ET) was measured using the compensation heat-pulse method, and meteorological variables, soil water content (theta). LAI, maximum daily stern shrinkage (MDS) and leaf water potential (phi(1)) were monitored during the whole stages from 2008 to 2010 in an apple orchard of arid region of northwest China. The relationships between SF and affecting factors were also analyzed. The results show that LAI increased rapidly at stage I. and SF was mainly determined by vapor pressure deficit (VPD), LAI and reference evapotranspiration (ET0). However, LAI was relatively stable at stage II, and SF was affected by VPD. ET0 and theta. Under the condition of border irrigation with 100% ET, tree SF could be calculated using VPD, ET0 and LAI at stage I, and VPD, ET0 and theta at stage II. Moreover, SF had significantly negative linear relationship with phi(1) and exponential relationship with MDS. Therefore, it is beneficial for more accurate modeling of truck sap flow and orchard water management under border irrigation by analyzing major factors affecting sap flow during two stages of apple tree. (C) 2011 Elsevier B.V. All rights reserved.