Cutting xylem under tension or supersaturated with gas can generate PLC and the appearance of rapid recovery from embolism

Abstract

We investigated the common assumption that severing stems and petioles under water preserves the hydraulic continuity in the xylem conduits opened by the cut when the xylem is under tension. In red maple and white ash, higher percent loss of conductivity (PLC) in the afternoon occurred when the measurement segment was excised under water at native xylem tensions, but not when xylem tensions were relaxed prior to sample excision. Bench drying vulnerability curves in which measurement samples were excised at native versus relaxed tensions showed a dramatic effect of cutting under tension in red maple, a moderate effect in sugar maple, and no effect in paper birch. We also found that air injection of cut branches (red and sugar maple) at pressures of 0.1 and 1.0 MPa resulted in PLC greater than predicted from vulnerability curves for samples cut 2 min after depressurization, with PLC returning to expected levels for samples cut after 75 min. These results suggest that sampling methods can generate PLC patterns indicative of repair under tension by inducing a degree of embolism that is itself a function of xylem tensions or supersaturation of dissolved gases (air injection) at the moment of sample excision. Implications for assessing vulnerability to cavitation and levels of embolism under field conditions are discussed.

Reversible Deformation of Transfusion Tracheids in Taxus baccata Is Associated with a Reversible Decrease in Leaf Hydraulic Conductance

“…Steady-state stem vulnerability curves for which samples were excised at native versus relaxed tensions showed no significant difference, suggesting no effect of cutting under tension (Wheeler et al, 2013) in this species (Fig. 4A).…”

“…The potential for cutting under tension to depress xylem conductance (Wheeler et al, 2013) was assessed in big stems by constructing a second vulnerability curve in which xylem tensions were relaxed prior to excision of the measurement segment. For tension relaxation, bench-dried branches were recut under water and allowed to rehydrate to near zero (approximately 20.3 MPa) prior to sample excision.…”

Predicting plant vulnerability to drought in biodiverse regions using functional traits

“…The remaining branch was placed under water and cut into an unbranched segment. Several cuts were made at each end to ensure that the xylem tension relaxed (63). Native flow rate was measured by connecting each stem to tubing attached to a pressure head containing KCl solution (20 mM).…”

Hydraulic failure and repair are not routine in trees

“…Diurnal cycles of cavitation and repair have been reported, even in well watered plants, suggesting that plants routinely face cavitation and recover easily from it. Writing in Plant Cell and Environment, Wheeler et al (2013) provided compelling new experimental evidence that trees are much more resistant to cavitation than previously thought, demonstrating that this change of paradigm should now be reconsidered.…”

“…However, through a new series of experiments, Wheeler et al (2013) have shifted the burden of proof. They have identified methodological issues resulting in underestimations of the level of stress required to induce substantial cavitation in trees.…”