Stomatal Sensitivity to Vapor Pressure Deficit and the Loss of Hydraulic Conductivity Are Coordinated inPopulus euphratica, a Desert Phreatophyte Species

There are considerable variations in the percentage loss of hydraulic conductivity (PLC) at mid-day minimum water potential among and within species, but the underpinning mechanism(s) are poorly understood. This study tested the hypothesis that plants can regulate leaf specific hydraulic conductance (K-l)viaprecise control overPLCunder variable Delta psi(water potential differential between soil and leaf) conditions to maintain the-m/bconstant (-m: the sensitivity of stomatal conductance toVPD;b: reference stomatal conductance at 1.0 kPaVPD), whereVPDis vapor pressure deficit. We usedPopulus euphratica, a phreatophyte species distributed in the desert of Northwestern China, to test the hypothesis. Field measurements ofVPD, stomatal conductance (g(s)),g(s)responses toVPD, mid-day minimum leaf water potential (psi(lmin)), and branch hydraulic architecture were taken in late June at four sites along the downstream of Tarim River at the north edge of the Taklamakan desert. We have found that: 1) the-m/bratio was almost constant (=0.6) across all the sites; 2) the average psi(50)(the xylem water potential with 50% loss of hydraulic conductivity) was-1.63 MPa, and mid-dayPLCranged from 62 to 83%; 3) there were tight correlations between psi(50)and wood density/leaf specific hydraulic conductivity (k(l)) and between specific hydraulic conductance sensitivity to water potential [d(k(s))/dln(-psi)] and specific hydraulic conductivity (k(s)). A modified hydraulic model was applied to investigate the relationship betweeng(s)andVPDunder variable Delta psi andK(l)conditions. It was concluded thatP. euphraticawas able to controlPLCin order to maintain a relatively constant-m/bunder different site conditions. This study demonstrated that branchlet hydraulic architecture and stomatal response toVPDwere well coordinated in order to maintain relatively water homeostasis ofP. euphraticain the desert. Model simulations could explain the wide variations ofPLCacross and within woody species that are often observed in the field.