Analysis of freeze-thaw embolism in conifers. The interaction between cavitation pressure and tracheid size

Abstract

Ice formation in the xylem sap produces air bubbles that under negative xylem pressures may expand and cause embolism in the xylem conduits. We used the centrifuge method to evaluate the relationship between freeze-thaw embolism and conduit diameter across a range of xylem pressures (Px) in the conifers Pinus contorta and Juniperus scopulorum. Vulnerability curves showing loss of conductivity (embolism) with Px down to -8 MPa were generated with versus without superimposing a freeze-thaw treatment. In both species, the freeze-thaw plus water-stress treatment caused more embolism than water stress alone. We estimated the critical conduit diameter (Df) above which a tracheid will embolize due to freezing and thawing and found that it decreased from 35 microm at a Px of -0.5 MPa to 6 microm at -8 MPa. Further analysis showed that the proportionality between diameter of the air bubble nucleating the cavitation and the diameter of the conduit (kL) declined with increasingly negative Px. This suggests that the bubbles causing cavitation are smaller in proportion to tracheid diameter in narrow tracheids than in wider ones. A possible reason for this is that the rate of dissolving increases with bubble pressure, which is inversely proportional to bubble diameter (La Place's law). Hence, smaller bubbles shrink faster than bigger ones. Last, we used the empirical relationship between Px and Df to model the freeze-thaw response in conifer species.

Figure 1.

A simple geometric model for the relationship between bubble size and conduit size. A freezing conduit of diameter D produces centrally located air bubbles with a volume proportional to the volume of a conduit section of length L (the volume proportionality constant is the k in Eq. 2). Tracheids and vessels of similar D (vessel A and tracheid B) have been found to have similar vulnerability to cavitation by freezing and thawing, suggesting similar bubble diameter (D_b; Pittermann and Sperry, 2003). This means that L is similar for the same D and shorter than a tracheid. Species with narrower conduits (e.g. tracheid C) are more resistant to cavitation by freezing and thawing, indicating a link between bubble and conduit diameter. The conduit length per bubble (L) apparently does not increase markedly in narrower conduits; instead, L may be independent of conduit diameter (as shown) or perhaps more likely to decline with smaller D.

Figure 2.

The water-stress and freeze-thaw + water-stress vulnerability curves of P. contorta (A) and J. scopulorum (B). Asterisks denote significantly different means for the same pressure (n = 5–6 at each P_x; mean ± sd; Student's t test, P = 0.05).

Figure 3.

Tracheid diameter frequency distributions measured in P. contorta and J. scopulorum stem wood (n = 5–6; mean ± sd; sd bars may be smaller than symbols).

Figure 4.

The relationship between the critical diameter (D_f) and the freeze-thaw xylem pressure, P_x^* (n = 5–6; mean ± sd). The P_x^* is plotted as the dependent variable for comparison with scaling predicted from Equation 3.

Figure 5.

The kL coefficient plotted as a function of P_x^* (n = 5–6; mean ± sd). The P_x^* is plotted as the dependent variable for comparison with scaling predicted from Equation 3.

Figure 6.

Freeze-thaw vulnerability curves estimated from tracheid diameter distributions and the P_x^* versus D_f scaling in Figure 4. The means of n = 5 to 6 stem diameter distributions ± sd (subsample of distributions in Fig. 3) were used to construct the P. contorta and J. scopulorum curves. A curve for bald cypress (T. distichum) stems (mean tracheid diameter of 40 ± 2.7 μm) is shown for comparison.