Analyses on water use characteristics of Salix psammophila based on sap flow and leaf water potential

ObjectiveIn arid and semi-arid areas, water transfer process of plants is regulated by a set of effective mechanisms. Understanding the mechanisms of responses of xerophytic plants to soil drought is important for predicting the structure and functioning of dryland ecosystems under changing climate.

MethodThe stem sap flow of Salix psammophila was monitored continuously using five heat balance sensor, and the leaf water potential of the plant was measured at predawn and midday in nineteen sunny days during 1 May to 10 October, 2017. The photosynthetic active radiation above canopy, air temperature, air relative humidity, and soil moisture content were monitored simultaneously.

ResultThe results were that the sap flux density and soil water potential were highly correlated in short term (May to June, July to September). The sap flux density and the water potential drop (Ψ_L−Ψ_S) were positively correlated during the growing season. Leaf transpiration rate increased with leaf water potential and vapor pressure deficit (VPD), respectively, saturating at −3.7 MPa and 1.9 kPa, leaf conductance for water vapour increased positively with VPD, saturating at 0.9 kPa, then decreasing with these variables when greater than their respective threshold. The vulnerability curve was “r” shape for Salix psammophila. The water potential, at which 50% of hydraulic conductivity was lost as a result of xylem embolism (P₅₀), was 0.72 MPa. The whole-branch hydraulic conductance per unit basal sapwood cross-sectional area (K_s) increased with soil water potential (Ψ_S) and leaf conductance (g_L). The correlation between g_L and Ψ_S was low because of the influence of VPD on g_L. The relative sensitivity of stomata and plant hydraulic conductance to declining soil water potentials (ϭ) was 1.035.

ConclusionThe results show that as water stress develops, Salix psammophila controls the water loss by reducing the K_s and certain level of xylem embolism does not induce the closure of the stomata. This mechanisms may be advantageous in terms of maximizing transpiration and assimilation rates. It has obvious theoretical significance to understand the water use characteristics of plants in arid areas, and lays a foundation for further study on the mechanism of the compensation for hydraulic limitation of Salix psammophila.