Abstract:

Nocturnal sap flow (E_night) has been observed in a variety of plant species and environmental conditions. In water-limited environments, E_night is important in the regulation of plant water’s physiology. This study was designed to evaluate the way in which E_night (defined as sap flow from 20:30 to 06:00) responded to changes in the atmospheric vapor pressure deficit (VPD) and the soil water content (SWC), and explored its potential physiological significance for different plant species in a semi-arid area.

We selected three shrub species: Vitex negundo L. (VN), Hippophae rhamnoides L. (HR) and Spiraea pubescens Turcz (SP) in the semi-arid Loess Plateau of China. The plots of the three communities dominated by each of three species were on the same hill slope. Half-hourly sap flow density was measured in six to seven sample stems for each species during the main growing season (June to August 2015). VPD, SWC, leaf water potential (Ψ_leaf) and stomatal conductance (G_s) were measured at the same time. Regression analyses were conducted to determine the relationships between E_night, E_day, E_night/E_daily, VPD and SWC at half-hourly and daily time scales, as well as between E_night, E_day and Ψ_leaf.

The mean values of E_night and E_day were higher, but E_night/E_daily values were lower for VN compared to HR and SP. The responses of sap flow density to VPD and SWC varied at different temporal scales. VPD was the dominant factor that affected E_night and E_day at the half-hourly scale for all three species. In contrast, SWC was the key factor that influenced E_day at the daily time scale. The values of E_day and E_night/E_daily correlated negatively with SWC because the effect of SWC was stronger on E_day than on E_night. Although the low fraction of E_night/E_daily (4%–7%) may indicate a minor short-term effect of E_night on the standing water balance, E_night had ecophysiologically significance to the plants. The discrepancy in Ψ_leaf between sunset and the following day’s predawn (ΔΨ) indicated that stem refilling occurred during this period. SP had the higher fraction of recharge water among the three species, as it had the relatively higher value of ΔΨ/E_night. E_night had positive relationship with predawn G_s. The increased G_s facilitated rapid onset of photosynthesis in the early morning. In addition, the positive correlation between E_night and E_day from 06:00 to 10:30 suggested that E_night was beneficial for transpiration in the following morning. The responses of E_nightto VPD and SWC indicated differences in plant adaptation to drought stress, which provides important information for our understanding of the reactions to climate changes among species in semi-arid areas.

Key words: nocturnal sap flow, stem refilling, stomatal conductance, leaf water potential, Loess Plateau