Responses of nighttime sap flow to atmospheric and soil dryness and its potential roles for shrubs on the Loess Plateau of China

Aims 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. Methods 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 (Psi(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 halfhourly and daily time scales, as well as between E-night, E-day and Psi(leaf). Important Findings 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 shortterm effect of E-night on the standing water balance, E-night had eco-physiologically significance to the plants. The discrepancy in Psi(leaf) between sunset and the following day's predawn (Delta Psi) 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 Delta Psi/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-night to 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.