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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 Pe (A) and conduit diameter (B) at which bubble formation becomes likely on xylem water tension. The larger the xylem water tension the smaller Pe is required (A). Pe depends on freezing velocity, conduit diameter, and diffusivity of solutes (Eq. 12) and therefore at the same xylem water tension a different threshold conduit diameters are obtained at different freezing velocities (B).
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Figure 5: Dependence of Pe (A) and conduit diameter (B) at which bubble formation becomes likely on xylem water tension. The larger the xylem water tension the smaller Pe is required (A). Pe depends on freezing velocity, conduit diameter, and diffusivity of solutes (Eq. 12) and therefore at the same xylem water tension a different threshold conduit diameters are obtained at different freezing velocities (B).

Mentions: Because the bubble formation probability depends on accumulation of gases and the concentration at which bubble formation becomes likely depends on xylem water tension (Figure 4A), for each xylem water tension there is a threshold Pe at which cavitation starts to occur (Figure 5A). The threshold Pe decreases with increasing tension meaning that the diameter of the conduits likely to cavitate (cavitation threshold diameter) and/or the freezing velocity at which cavitation begins decreases with increasing tension. Also, several cavitation threshold diameters exist at the same xylem tension, and they depend on the freezing velocity; the higher the freezing velocity the smaller the threshold diameter (Figure 5B). This means that for the same sample, one can obtain a higher loss of conductivity when freezing the sample fast than if it freezes slowly. The freezing velocity will depend on the size and structure of the sample (heat capacity, thermal conductivity) as well as the temperature difference between the sample and outside air (or other cooling fluid). Inside the xylem, the freezing velocity of conduits of different size may also vary because of these reasons. Therefore, even if large conduits are theoretically more susceptible to bubble formation during freezing, some variation is this pattern may occur (see e.g., Mayr and Sperry, 2010 and references therein).


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 Pe (A) and conduit diameter (B) at which bubble formation becomes likely on xylem water tension. The larger the xylem water tension the smaller Pe is required (A). Pe depends on freezing velocity, conduit diameter, and diffusivity of solutes (Eq. 12) and therefore at the same xylem water tension a different threshold conduit diameters are obtained at different freezing velocities (B).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 5: Dependence of Pe (A) and conduit diameter (B) at which bubble formation becomes likely on xylem water tension. The larger the xylem water tension the smaller Pe is required (A). Pe depends on freezing velocity, conduit diameter, and diffusivity of solutes (Eq. 12) and therefore at the same xylem water tension a different threshold conduit diameters are obtained at different freezing velocities (B).
Mentions: Because the bubble formation probability depends on accumulation of gases and the concentration at which bubble formation becomes likely depends on xylem water tension (Figure 4A), for each xylem water tension there is a threshold Pe at which cavitation starts to occur (Figure 5A). The threshold Pe decreases with increasing tension meaning that the diameter of the conduits likely to cavitate (cavitation threshold diameter) and/or the freezing velocity at which cavitation begins decreases with increasing tension. Also, several cavitation threshold diameters exist at the same xylem tension, and they depend on the freezing velocity; the higher the freezing velocity the smaller the threshold diameter (Figure 5B). This means that for the same sample, one can obtain a higher loss of conductivity when freezing the sample fast than if it freezes slowly. The freezing velocity will depend on the size and structure of the sample (heat capacity, thermal conductivity) as well as the temperature difference between the sample and outside air (or other cooling fluid). Inside the xylem, the freezing velocity of conduits of different size may also vary because of these reasons. Therefore, even if large conduits are theoretically more susceptible to bubble formation during freezing, some variation is this pattern may occur (see e.g., Mayr and Sperry, 2010 and references therein).

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