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Comparative study of daytime and nighttime sap flow of Populus euphratica

Zhao, Chun Yan, Si, Jian Hua, Feng, Qi, Yu, Teng Fei, Li, Pei Du
Plant growth regulation 2017 v.82 no.2 pp. 353-362
Populus euphratica, air temperature, autumn, environmental factors, forest trees, growing season, hysteresis, photosynthetically active radiation, regression analysis, riparian forests, sap flow, soil water, spring, stomatal conductance, summer, vapor pressure, variance, wind speed
In order to quantify and characterize the variance in desert riparian forest tree sap flow, we measured the sap flow from Populus euphratica and compared the daytime and nighttime patterns and responses to environmental variables. Results showed that daytime sap flow velocity was significantly higher (P < 0.05). Daytime and nighttime mean sap flow velocities were 7.65 and 4.01 cm h⁻¹ in spring, 21.38 and 9.60 cm h⁻¹ in summer, and 11.04 and 5.21 cm h⁻¹ in autumn, respectively. Moreover, results indicated that the stoma remained partially open (15% minimum) throughout the night, providing sufficient evidence for the existance of nighttime transpiration. The vapor pressure deficit (VPD), stomatal conductance (Cs), photosynthetically active radiation (PAR), air temperature (Ta), wind speed (WS), and soil moisture (θ) all had significant positive effects on P. euphratica sap flow velocity (P < 0.05). Furthermore, the relationship between daytime sap flow velocity and VPD showed clockwise hysteresis, while the relationship between nighttime sap flow velocity and VPD showed counter-clockwise hysteresis. It was evident that PAR and VPD were the key factors impacting daytime sap flow velocity, while Cs and θ were the key factors impacting nighttime sap flow velocity. Furthermore, linear regression results showed that daytime sap flow had a significant positive effect on nighttime sap flow throughout the growing season (P < 0.05).