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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 the critical radius (rc) of a gas bubble on xylem water tension at different solute concentrations (C*). Bubbles larger than rc at a certain tension will grow spontaneously and smaller will redissolve to the solution. Similarly, during thawing, smaller bubbles than rc are more likely to redissolve, and the higher the xylem water tension or gas concentration the smaller the bubble has to be to redissolve. This means that while bubble formation is more likely at high freezing velocities (high Pe leads to high C*), the small bubbles are more likely to redissolve during thawing than bubbles that may form during slow freezing.
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Figure 6: Dependence of the critical radius (rc) of a gas bubble on xylem water tension at different solute concentrations (C*). Bubbles larger than rc at a certain tension will grow spontaneously and smaller will redissolve to the solution. Similarly, during thawing, smaller bubbles than rc are more likely to redissolve, and the higher the xylem water tension or gas concentration the smaller the bubble has to be to redissolve. This means that while bubble formation is more likely at high freezing velocities (high Pe leads to high C*), the small bubbles are more likely to redissolve during thawing than bubbles that may form during slow freezing.

Mentions: High freezing velocity also results in smaller but more numerous bubbles because the higher the gas concentration, the lower the energy barrier for nucleation (Eqs 2, 3, 6, and 7), the smaller the critical size bubble, and the higher the nucleation rate. Also, during fast freezing there will be less time for bubbles to grow before enclosed in ice. Therefore, one could expect smaller but more numerous bubbles trapped in conduits freezing rapidly than in conduits freezing slowly. Narrow conduits observed containing more and smaller bubbles than wide conduits (see Ewers, 1985) may thus be due to differences in the freezing velocity. Similarly, the size of the bubbles formed decreases with increasing xylem water tension as presented by Pittermann and Sperry (2006). These small bubbles, even if numerous, are more likely to redissolve to the liquid during thawing. Dissolving bubbles to the liquid is a reverse process to bubble formation, and therefore the ease with which bubbles dissolve depends also on xylem water tension (see also Mayr and Sperry, 2010). From Eq. 3 we can calculate the pressure dependence of the minimum size of bubbles that will not dissolve to the liquid. If there are no dissolved gases in the liquid (C* = 0; Figure 6), all bubbles smaller than 0.1 μm will dissolve under tensions of 0–1 MPa, and only slightly positive pressures would be needed to force bubbles, filling even the largest conduits (d ∼ 10−4 m), to redissolve. On the other hand, if xylem tension is high or there are already dissolved gasses present in the liquid, dissolving becomes increasingly difficult and avoiding bubbles altogether consequently more important. It is worth noting that if freezing is fast enough and the bubbles have little time to grow, these minuscule bubbles, either trapped in the ice or in the eutectic domains between the crystals, would be invisible even with a scanning electron microscope (SEM), but during thawing, if the xylem tension was high enough, capable of causing embolization (Figure 6).


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 the critical radius (rc) of a gas bubble on xylem water tension at different solute concentrations (C*). Bubbles larger than rc at a certain tension will grow spontaneously and smaller will redissolve to the solution. Similarly, during thawing, smaller bubbles than rc are more likely to redissolve, and the higher the xylem water tension or gas concentration the smaller the bubble has to be to redissolve. This means that while bubble formation is more likely at high freezing velocities (high Pe leads to high C*), the small bubbles are more likely to redissolve during thawing than bubbles that may form during slow freezing.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 6: Dependence of the critical radius (rc) of a gas bubble on xylem water tension at different solute concentrations (C*). Bubbles larger than rc at a certain tension will grow spontaneously and smaller will redissolve to the solution. Similarly, during thawing, smaller bubbles than rc are more likely to redissolve, and the higher the xylem water tension or gas concentration the smaller the bubble has to be to redissolve. This means that while bubble formation is more likely at high freezing velocities (high Pe leads to high C*), the small bubbles are more likely to redissolve during thawing than bubbles that may form during slow freezing.
Mentions: High freezing velocity also results in smaller but more numerous bubbles because the higher the gas concentration, the lower the energy barrier for nucleation (Eqs 2, 3, 6, and 7), the smaller the critical size bubble, and the higher the nucleation rate. Also, during fast freezing there will be less time for bubbles to grow before enclosed in ice. Therefore, one could expect smaller but more numerous bubbles trapped in conduits freezing rapidly than in conduits freezing slowly. Narrow conduits observed containing more and smaller bubbles than wide conduits (see Ewers, 1985) may thus be due to differences in the freezing velocity. Similarly, the size of the bubbles formed decreases with increasing xylem water tension as presented by Pittermann and Sperry (2006). These small bubbles, even if numerous, are more likely to redissolve to the liquid during thawing. Dissolving bubbles to the liquid is a reverse process to bubble formation, and therefore the ease with which bubbles dissolve depends also on xylem water tension (see also Mayr and Sperry, 2010). From Eq. 3 we can calculate the pressure dependence of the minimum size of bubbles that will not dissolve to the liquid. If there are no dissolved gases in the liquid (C* = 0; Figure 6), all bubbles smaller than 0.1 μm will dissolve under tensions of 0–1 MPa, and only slightly positive pressures would be needed to force bubbles, filling even the largest conduits (d ∼ 10−4 m), to redissolve. On the other hand, if xylem tension is high or there are already dissolved gasses present in the liquid, dissolving becomes increasingly difficult and avoiding bubbles altogether consequently more important. It is worth noting that if freezing is fast enough and the bubbles have little time to grow, these minuscule bubbles, either trapped in the ice or in the eutectic domains between the crystals, would be invisible even with a scanning electron microscope (SEM), but during thawing, if the xylem tension was high enough, capable of causing embolization (Figure 6).

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