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

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 (K _l) 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 (g _s), g _s responses to VPD, mid-day minimum leaf water potential (Ψ _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/b ratio was almost constant (=0.6) across all the sites; 2) the average Ψ ₅₀ (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 Ψ ₅₀ and wood density/leaf specific hydraulic conductivity (k _l) and between specific hydraulic conductance sensitivity to water potential [d(k _s)/dln(-Ψ)] and specific hydraulic conductivity (k _s). A modified hydraulic model was applied to investigate the relationship between g _s and VPD under variable ΔΨ and K _l 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.

Keywords: hydraulic model; leaf specific hydraulic conductance; stomatal conductance; water homeostasis; xylem cavitation.

Figure 1

Geological locations of the four studied sites. Site name: A = 31 Tuan, B = 33 Tuan, C = Yingsu, D = Alagan.

Figure 2

Relationships between g_s and VPD (A, B), and the relationship between −m and b (C) of Populus euphratica among the four sites. The measurements were taken by manipulating VPD in late June (A, C), and natural VPD in late June and late July (B). The curves were generated by using the hydraulic model (Equation 1). In (C), red line represents the theoretical line of the expected relationship (−m = 0.6b), black dash lines represent 95% confidence intervals of linear regression between observed −m and b.

Figure 3

PLC as a function of Ψ for the four populations of Populus euphratica (means ± 1 se). The curves were fitted using all data points. The vertical dotted line indicates the position of measured Ψ_min; the horizontal dotted line indicates the percentage loss of hydraulic conductivity at Ψ_min. The number under the horizontal dotted line is the PLC at noon. See Table 1 for the definition of symbols.

Figure 4

Ψ₅₀ as a function of: (A) WD and (B) k_l. k_l as a function of b at the population scale (C). d(k_s)/dln(−Ψ) as a function of: (D) k_s, (E) Ψ₅₀, and (F) b at the population scale of Populus euphratica at the four sites. g_s as a function of: (G) T_r, and (H) Ψ_min at the population scale of Populus euphratica at the four sites. In Figures 4C, F and H , data were shown as mean ± 1 se. For relationships at the population scale, the four points are the means of the four sites.

Figure 5

Ψ_min as a function of Ψ₅₀ for Populus euphratica at the four sites. The four black points are the means of the four sites. The red line is the regression line fitted using measurements for angiosperm species in the world (Choat et al., 2012). The F value and probability for the regression black line of Populus euphratica at the four sites are 14.76 and 0.061, respectively. The green line is the 1:1 regression line between Ψ_min and Ψ₅₀. Errors bars represent ± se.

Figure 6

Semi-contour plots of −m/b as dependent on ΔΨ and K_l. Plots are derived from the hydraulic model (Equation 1). The simulated −m/b at the four studied sites are shown as (Δ). A and B in the figure represent belts with same −m/b but different K_l at noon, C represents belt with higher −m/b than A and B, but its K_l at noon is between those of A and B. Note the unit of K_l is mmol m⁻² MPa⁻¹ s⁻¹.