Abstract
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 (Kl) via precise control over PLC under variable ΔΨ (water potential differential between soil and leaf) conditions to maintain the −m/b constant (−m: the sensitivity of stomatal conductance to VPD; b: reference stomatal conductance at 1.0 kPa VPD), where VPD is vapor pressure deficit. We used Populus euphratica, a phreatophyte species distributed in the desert of Northwestern China, to test the hypothesis. Field measurements of VPD, stomatal conductance (gs), gs responses to VPD, mid-day minimum leaf water potential (Ψlmin), 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/b ratio was almost constant (=0.6) across all the sites; 2) the average Ψ50 (the xylem water potential with 50% loss of hydraulic conductivity) was −1.63 MPa, and mid-day PLC ranged from 62 to 83%; 3) there were tight correlations between Ψ50 and wood density/leaf specific hydraulic conductivity (kl) and between specific hydraulic conductance sensitivity to water potential [d(ks)/dln(−Ψ)] and specific hydraulic conductivity (ks). A modified hydraulic model was applied to investigate the relationship between gs and VPD under variable ΔΨ and Kl conditions. It was concluded that P. euphratica was able to control PLC in order to maintain a relatively constant −m/b under different site conditions. This study demonstrated that branchlet hydraulic architecture and stomatal response to VPD were well coordinated in order to maintain relatively water homeostasis of P. euphratica in the desert. Model simulations could explain the wide variations of PLC across and within woody species that are often observed in the field.
Highlights
A global convergence has been demonstrated in the relationship between drought-induced embolism and daily minimum xylem water potential (Choat et al, 2012; Choat et al, 2018)
The stomatal regulation of xylem pressure is a function of vapor pressure deficit (VPD), leaf specific hydraulic conductance (Kl), soil water potential (Ys), and leaf water potential (Yl) according to the following simplified hydraulic model (Oren et al, 1999; Landsberg et al, 2017): gl = Kl · ð1=VPDÞ · ðYs − YlÞ
Hukin et al (2005) have reported that P. euphratica seedlings have a Y50 for xylem cavitation of only −0.7 MPa, while the field-grown P. euphratica trees in our study had an average Y50 of −1.63 MPa across the four sites. These results suggest that the seedlings of P. euphratica may be much more vulnerable to xylem cavitation than trees at a later developmental stage
Summary
A global convergence has been demonstrated in the relationship between drought-induced embolism and daily minimum xylem water potential (Choat et al, 2012; Choat et al, 2018). The stomatal regulation of xylem pressure is a function of vapor pressure deficit (VPD), leaf specific hydraulic conductance (Kl), soil water potential (Ys), and leaf water potential (Yl) (see Table 1 for the definitions of major acronyms/symbols in the present study) according to the following simplified hydraulic model (Oren et al, 1999; Landsberg et al, 2017): gl = Kl · ð1=VPDÞ · ðYs − YlÞ (1). The above models predict that if Kl decreases due to xylem cavitation, the −m/b ratio will need to increase because a more sensitive stomatal response is required to keep transpiration and DY (=YS – Yl, i.e., water potential difference between soil and leaf) relatively constant (Oren et al, 1999; Landsberg et al, 2017). We hypothesize that the stomatal response to VPD and xylem response to water potential are functionally converged to maintain a functional coherence and integrity of the hydraulic system of the tree
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