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 environ-ments,E_(night) is important in the regulation of plant water’s physi-ology.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 con-tent(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 grow-ing season(June to August 2015).VPD,SWC,leaf water potential(Ψleaf)and stomatal conductance(G_(s))were measured at the same time.Regression analyses were conducted to determine the rela-tionships 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) andΨleaf.Important Findings The mean values of E_(night) and E_(day) were higher,but E_(night)/E_(daily) val-ues 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 eco-physiologically significance to the plants.The discrepancy inΨleaf 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 ofΔΨ/E_(night).E_(night) had positive relation-ship with predawn Gs.The increased Gs 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 infor-mation for our understanding of the reactions to climate changes among species in semi-arid areas.