Abstract: To clarify the effects of environmental factors on transpiration process of apple trees in rain-fed ecosystem, the dynamics of sap flow in apple trees from Loess Plateau area in western Shanxi Province of China were monitored using the thermal diffusion technique. Meanwhile, environmental factors including meteorological elements and soil moisture content were measured. The results showed that both net solar radiation (R_n) and atmospheric vapor deficit (VPD) were closely correlated with sap flow rate (J_s), as the main ones among all the measured environmental factors. At both hourly and daily scales, the cumulative variances of the three principal components of the environmental factor were above 86%. The first principal component contained VPD and R_n, and explained the variance of over 52% (at hour scale) and 63% (at daily scale), which could be classified as the evaporation demand index (EDI) and as the key comprehensive environmental varia-bles affecting tree sap flow. The second principal component mainly included soil moisture content (SWC) and other factors, which were referred to the soil water and heat supply index. The third principal component mainly included wind speed and other factors, and could be classified into the hydrothermal dissipation index. At the scale of hourly or daily, the response of J_s to EDI showed a significant exponential growth relationship. At the hourly time scale, J_s of apple trees could be accurately modelled based on the first principal component EDI (R²=0.72). At the daily scale, J_s of apple trees could be better modelled based on potential evapotranspiration (ET₀) (R²=0.88). Our results were of great significance for clarifying the responses of water transport in apple tree to environmental factors, estimating water consumption of apple tree based on meteorological factors, as well as directing orchard water management.

Key words: net solar radiation, vapor pressure deficit, sap flow rate, thermal dissipation method, evaporative demand index