Sap Flow at Night in the Stems of Ailanthus Altissima and Its Determinants

Abstract

【Background and objective】 Water ascent in plants is driven by hydraulic gradient regulated by stomatal opening and closure as well as soil water content; it is generally assumed to be approximately zero at night. However, recent findings reveal that stomata could partly open and sap continues to flow at night, making stems function as capacitors regulating transpiration in daytime. The purpose of this paper is to investigate the diurnal change in sap flow and its determinants. 【Method】 We take Ailanthus altissima as the model plant and measure sap flow through its stems, as well as variation in stem diameter, using a diffusion probe (TDP) from June to August in an urban area. We also measure environmental factors which we anticipate to affect root water uptake by the tree. 【Result】 Sap flow does occur at night; it contributes 12.1% to the total daily sap flow, of which 65% is from 20:00 to 00:00 (T1) and 35% from 00:00 to 05:00 (T2). The night sap flow rate is positively correlated to the vapor pressure deficit and wind speed, but the correlation is more significant in T2 (P<0.01) than in T1, and greater in sunny days than in rainy days. It is also found that sap flow at night is positively correlated with canopy width (P<0.01) and stem diameter at the breast height (P<0.05), and that it increases linearly with daily variation in stem diameter (R2=0.672). Stems expand more radially in T1 than in T2. 【Conclusion】 Night sap flow results in radial expansion of the stem of A. altissima, which is impacted jointly by environmental factors and plant traits.

Keywords

• ailanthus altissima
• night sap flow
• night transpiration
• night rehydration
• radial micro-variation in the stem