Abstract: Daily and seasonal variability in the radial distribution of xylem sap flow velocity in trunks of pear trees were investigated by the heat pulse method. The results showed that sap flow velocity at different depths beneath the cambium exhibited one-, two-peak diurnal patterns in sunny, cloudy and light rainy days during the fully growing season, whereas diurnal sap flow velocity at different depths became irregular under different weather conditions at the end of growing season. The ratio of radial range of variation at different aspects, sap flow velocity in maximum, average and daily range of variation of the same depth in sunny, cloudy, light rain and moderate rain to storm days were 6:4:3:2 and 5:4:3:2, respectively, whereas minimum sap velocity was relatively similar under different weather conditions. The seasonal trends of daytime radial variation at the same aspect followed similar patterns, and showed three periods clearly- from May to July, August to September and October. Sap flow velocity at the same depth in August to September and October were about 50%–60%, 33%–50% of that from May to July. The ratio of radial range of variation from May to July and from August to September and October was 5:2:1. The seasonal patterns of nighttime radial variation were similar to the corresponding daytime where radial variation was lower compared with the daytime. The ratio of nighttime to daytime sap flow velocity became higher and higher with the extension of growing season. Throughout the season, nighttime sap flow velocity and radial range of variation were 43%±1% and 29%±7% of those of the daytime, respectively. Diurnal radial profile could be grouped into four types: high–low–low, low–high–low, high–low–high and low–high–low–high, which the maximum sap flow velocity mostly occurred in the position of 0.12–0.29 from the cambium.

Key words: heat pulse method, sap flow velocity, radial distribution, pear trees