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Freeze/Thaw-induced embolism: probability of critical bubble formation depends on speed of ice formation.

Sevanto S, Holbrook NM, Ball MC - Front Plant Sci (2012)

Bottom Line: Our results confirm the common assumption that bubble formation during freezing is most likely due to gas segregation by ice.Therefore, bubble formation probability depends on these variables.Avoidance of bubble formation during freezing could thus be one piece of the explanation why xylem conduit size of temperate and boreal zone trees varies quite systematically.

View Article: PubMed Central - PubMed

Affiliation: Department of Organismic and Evolutionary Biology, Harvard University Cambridge, MA, USA.

ABSTRACT
Bubble formation in the conduits of woody plants sets a challenge for uninterrupted water transportation from the soil up to the canopy. Freezing and thawing of stems has been shown to increase the number of air-filled (embolized) conduits, especially in trees with large conduit diameters. Despite numerous experimental studies, the mechanisms leading to bubble formation during freezing have not been addressed theoretically. We used classical nucleation theory and fluid mechanics to show which mechanisms are most likely to be responsible for bubble formation during freezing and what parameters determine the likelihood of the process. Our results confirm the common assumption that bubble formation during freezing is most likely due to gas segregation by ice. If xylem conduit walls are not permeable to the salts expelled by ice during the freezing process, osmotic pressures high enough for air seeding could be created. The build-up rate of segregated solutes in front of the ice-water interface depends equally on conduit diameter and freezing velocity. Therefore, bubble formation probability depends on these variables. The dependence of bubble formation probability on freezing velocity means that the experimental results obtained for cavitation threshold conduit diameters during freeze/thaw cycles depend on the experimental setup; namely sample size and cooling rate. The velocity dependence also suggests that to avoid bubble formation during freezing trees should have narrow conduits where freezing is likely to be fast (e.g., branches or outermost layer of the xylem). Avoidance of bubble formation during freezing could thus be one piece of the explanation why xylem conduit size of temperate and boreal zone trees varies quite systematically.

No MeSH data available.


Related in: MedlinePlus

Dependence of nucleation rate (J) and critical radius (rc insert) on xylem water tension for homogenous and heterogeneous vaporization of water in a conduit 50 μm in diameter and 20 mm in length. Homogenous nucleation rate at xylem tensions < 100 MPa is so low, and the critical radius so large, that bubble formation is unlikely. The energy barrier for nucleation (Eq. 2) has to decrease by a factor (F; Eq. 8) of 10 for heterogeneous vaporization of water to become likely at xylem water tensions below −50 MPa. The horizontal line shows the nucleation rate at which bubble formation becomes likely in conduits of this size (Eq. 9).
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Figure 3: Dependence of nucleation rate (J) and critical radius (rc insert) on xylem water tension for homogenous and heterogeneous vaporization of water in a conduit 50 μm in diameter and 20 mm in length. Homogenous nucleation rate at xylem tensions < 100 MPa is so low, and the critical radius so large, that bubble formation is unlikely. The energy barrier for nucleation (Eq. 2) has to decrease by a factor (F; Eq. 8) of 10 for heterogeneous vaporization of water to become likely at xylem water tensions below −50 MPa. The horizontal line shows the nucleation rate at which bubble formation becomes likely in conduits of this size (Eq. 9).

Mentions: Based on our calculations (Eqs 1–5) and supported by previous studies (see e.g., Tyree and Sperry, 1989; Hölttä et al., 2002), homogenous bubble nucleation in the xylem by vaporization of water is unlikely. The tension in the xylem has to exceed 100 MPa, for homogenous nucleation rate to reach 1/s, which is the nucleation rate at which bubble formation becomes likely in typical xylem conduits (Eq. 9; Figure 3). This is an order of magnitude larger than any reported xylem tensions. The nucleation rate increases as the critical radius of the formed bubble decreases (Figure 3 insert), and therefore, in general, where nucleation is fast lots of small bubbles will form. At a nucleation rate of 1/s the critical radius of a nucleus is ∼1 nm (Figure 3 insert), which is a typical size for the critical cluster initiation in phase transition processes (e.g., Kulmala et al., 2007).


Freeze/Thaw-induced embolism: probability of critical bubble formation depends on speed of ice formation.

Sevanto S, Holbrook NM, Ball MC - Front Plant Sci (2012)

Dependence of nucleation rate (J) and critical radius (rc insert) on xylem water tension for homogenous and heterogeneous vaporization of water in a conduit 50 μm in diameter and 20 mm in length. Homogenous nucleation rate at xylem tensions < 100 MPa is so low, and the critical radius so large, that bubble formation is unlikely. The energy barrier for nucleation (Eq. 2) has to decrease by a factor (F; Eq. 8) of 10 for heterogeneous vaporization of water to become likely at xylem water tensions below −50 MPa. The horizontal line shows the nucleation rate at which bubble formation becomes likely in conduits of this size (Eq. 9).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3368182&req=5

Figure 3: Dependence of nucleation rate (J) and critical radius (rc insert) on xylem water tension for homogenous and heterogeneous vaporization of water in a conduit 50 μm in diameter and 20 mm in length. Homogenous nucleation rate at xylem tensions < 100 MPa is so low, and the critical radius so large, that bubble formation is unlikely. The energy barrier for nucleation (Eq. 2) has to decrease by a factor (F; Eq. 8) of 10 for heterogeneous vaporization of water to become likely at xylem water tensions below −50 MPa. The horizontal line shows the nucleation rate at which bubble formation becomes likely in conduits of this size (Eq. 9).
Mentions: Based on our calculations (Eqs 1–5) and supported by previous studies (see e.g., Tyree and Sperry, 1989; Hölttä et al., 2002), homogenous bubble nucleation in the xylem by vaporization of water is unlikely. The tension in the xylem has to exceed 100 MPa, for homogenous nucleation rate to reach 1/s, which is the nucleation rate at which bubble formation becomes likely in typical xylem conduits (Eq. 9; Figure 3). This is an order of magnitude larger than any reported xylem tensions. The nucleation rate increases as the critical radius of the formed bubble decreases (Figure 3 insert), and therefore, in general, where nucleation is fast lots of small bubbles will form. At a nucleation rate of 1/s the critical radius of a nucleus is ∼1 nm (Figure 3 insert), which is a typical size for the critical cluster initiation in phase transition processes (e.g., Kulmala et al., 2007).

Bottom Line: Our results confirm the common assumption that bubble formation during freezing is most likely due to gas segregation by ice.Therefore, bubble formation probability depends on these variables.Avoidance of bubble formation during freezing could thus be one piece of the explanation why xylem conduit size of temperate and boreal zone trees varies quite systematically.

View Article: PubMed Central - PubMed

Affiliation: Department of Organismic and Evolutionary Biology, Harvard University Cambridge, MA, USA.

ABSTRACT
Bubble formation in the conduits of woody plants sets a challenge for uninterrupted water transportation from the soil up to the canopy. Freezing and thawing of stems has been shown to increase the number of air-filled (embolized) conduits, especially in trees with large conduit diameters. Despite numerous experimental studies, the mechanisms leading to bubble formation during freezing have not been addressed theoretically. We used classical nucleation theory and fluid mechanics to show which mechanisms are most likely to be responsible for bubble formation during freezing and what parameters determine the likelihood of the process. Our results confirm the common assumption that bubble formation during freezing is most likely due to gas segregation by ice. If xylem conduit walls are not permeable to the salts expelled by ice during the freezing process, osmotic pressures high enough for air seeding could be created. The build-up rate of segregated solutes in front of the ice-water interface depends equally on conduit diameter and freezing velocity. Therefore, bubble formation probability depends on these variables. The dependence of bubble formation probability on freezing velocity means that the experimental results obtained for cavitation threshold conduit diameters during freeze/thaw cycles depend on the experimental setup; namely sample size and cooling rate. The velocity dependence also suggests that to avoid bubble formation during freezing trees should have narrow conduits where freezing is likely to be fast (e.g., branches or outermost layer of the xylem). Avoidance of bubble formation during freezing could thus be one piece of the explanation why xylem conduit size of temperate and boreal zone trees varies quite systematically.

No MeSH data available.


Related in: MedlinePlus