Increases in water potential gradient reduce xylem conductivity in whole plants. Evidence from a low-pressure conductivity method

Abstract

A new method using hydrostatic suctions (less than 0.02 MPa) was used to measure whole-root conductivity (K(r)) in saplings of two angiosperm pioneer trees (Eucalyptus regnans and Toona australis) and two rainforest conifers (Dacrycarpus dacrydioides and Nageia fleurii). The resultant K(r) was combined with measurements of stem and leaf hydraulic conductivity to calculate whole-plant conductivity and to predict leaf water potential (Psi(l)) during transpiration. At normal soil temperatures there was good agreement between measured and predicted Psi(l) during transpiration in all species. Changes in the soil-to-leaf water potential gradient were produced by root chilling, and in three of the four species, changes in Psi(l) corresponded to those expected by the effect of increased water viscosity on K(r). In one species, however, root chilling produced severe plant wilting and a decline in Psi(l) significantly below the predicted value. In this species Psi(l) decreased to a value close to, or below, the Psi(l) at 50% xylem cavitation. It is concluded that decreased whole-plant conductivity in T. australis resulted from a decrease in xylem conductivity due to stress-induced cavitation.

Figure 1

Hydrostatic suction applied to the root stump versus hydraulic flux minus osmotic flux (see Eq. 4) in six specimens of E. regnans with leaf areas ranging from 0.17 to 0.95 m². Regressions have been forced through zero. There is no evidence of non-linearity in the response of flux to applied pressure.

Figure 2

Changes in transpiration and Ψ₁ in response to root chilling in T. australis. Air temperature and humidity were maintained constant during the measurement period, indicating that changes in transpiration were largely due to decreases in stomatal conductance. Note that readings of root temperature, transpiration, and Ψ₁ were made after 20 min of steady-state EF.

Figure 3

Calculated ΔP versus measured Ψ₁ in E. regnans (×), T. australis (▵), D. dacrydioides (□), and N. fleurii (○). Arrows indicate the change in Ψ₁ before and during root chilling in the various individuals. The stippled region of the graph indicates the mean Ψ_cav50% ± se for T. australis. Ψ_cav50% in other species was less than −2 MPa. Note that only in T. australis does predicted ΔP deviate strongly from measured Ψ₁ after root chilling.

Figure 4

There was a highly significant (P < 0.001) correlation between measured Ψ₁ and calculated ΔP in pooled data from un-chilled T. australis and all measurements from E. regnans, D. dacrydioides, and N. fleurii (□). The slope of this regression is not significantly different from −1. Also shown is the change in measured and calculated ΔP in a single specimen of T. australis (⧫) starting at low light (50 μE m⁻² s⁻¹), then chilled roots (5.8°C), and finally low light with roots rewarmed. Root chilling in T. australis caused Ψ₁ to approach Ψ_cav50% (marked with a dashed line).