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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 threshold solute concentration for bubble formation via gas segregation on xylem water tension (A), freezing velocity (B), and conduit diameter (C). Xylem water tension determines the concentration at which bubble formation becomes likely, but whether that concentration is obtained during freezing depends on freezing velocity and conduit size (Pe; see Figure 2). (B,C) Show that slightly higher concentrations are needed for bubble formation become likely in small than in large conduits (Eq. 9) and if the time allowed for nucleation to take place is short (high freezing velocity, see Eq. 18). Here the initial gas concentration was set to saturation and C* values presented concentrations above saturation.
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Figure 4: Dependence of the threshold solute concentration for bubble formation via gas segregation on xylem water tension (A), freezing velocity (B), and conduit diameter (C). Xylem water tension determines the concentration at which bubble formation becomes likely, but whether that concentration is obtained during freezing depends on freezing velocity and conduit size (Pe; see Figure 2). (B,C) Show that slightly higher concentrations are needed for bubble formation become likely in small than in large conduits (Eq. 9) and if the time allowed for nucleation to take place is short (high freezing velocity, see Eq. 18). Here the initial gas concentration was set to saturation and C* values presented concentrations above saturation.

Mentions: Bubble formation via segregation of gases differs from the other three mechanisms in that this mechanism does not require high xylem water tensions. The nucleation rate depends on the gas concentration in the freezing conduit (Eqs 1–3, 6, and 7); the higher the concentration the more likely bubbles are to form. Therefore, bubbles are most likely to form right at the water–ice interface inside the freezing conduit (Figure 1). The concentration at which bubble formation becomes likely depends on xylem water tension (Figure 4A) but is much lower than the solute concentration required for producing high enough xylem water tensions for homogenous and heterogeneous nucleation. At xylem water tension 0.1 MPa it is of the order of magnitude 1000 times the saturation concentration, but drops quickly with increasing xylem water tension, and at 5 MPa tension only 20 times the saturation concentration is required. For each xylem water tension, the supersaturation at which bubble formation becomes likely depends slightly on the freezing velocity and conduit diameter (Figures 4B,C). The slight sensitivity to these parameters stems from the time dependence of the bubble formation probability (Eq. 9), and is similar to the effect conduit size has on nucleation events in general; the larger the conduit or the slower the freezing velocity, the more there is time for a successful bubble formation event to occur. But whether the required gas concentration is obtained during freezing depends strongly on both conduit diameter and freezing velocity (Figures 1 and 2). Note that the dependence of nucleation on freezing velocity (Figure 4B) is opposite to the dependence of the increase in the concentration of segregated gas on freezing velocity. In the case of segregated gases the higher the freezing velocity is the higher the concentration and the more likely bubble formation occurs.


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 threshold solute concentration for bubble formation via gas segregation on xylem water tension (A), freezing velocity (B), and conduit diameter (C). Xylem water tension determines the concentration at which bubble formation becomes likely, but whether that concentration is obtained during freezing depends on freezing velocity and conduit size (Pe; see Figure 2). (B,C) Show that slightly higher concentrations are needed for bubble formation become likely in small than in large conduits (Eq. 9) and if the time allowed for nucleation to take place is short (high freezing velocity, see Eq. 18). Here the initial gas concentration was set to saturation and C* values presented concentrations above saturation.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 4: Dependence of the threshold solute concentration for bubble formation via gas segregation on xylem water tension (A), freezing velocity (B), and conduit diameter (C). Xylem water tension determines the concentration at which bubble formation becomes likely, but whether that concentration is obtained during freezing depends on freezing velocity and conduit size (Pe; see Figure 2). (B,C) Show that slightly higher concentrations are needed for bubble formation become likely in small than in large conduits (Eq. 9) and if the time allowed for nucleation to take place is short (high freezing velocity, see Eq. 18). Here the initial gas concentration was set to saturation and C* values presented concentrations above saturation.
Mentions: Bubble formation via segregation of gases differs from the other three mechanisms in that this mechanism does not require high xylem water tensions. The nucleation rate depends on the gas concentration in the freezing conduit (Eqs 1–3, 6, and 7); the higher the concentration the more likely bubbles are to form. Therefore, bubbles are most likely to form right at the water–ice interface inside the freezing conduit (Figure 1). The concentration at which bubble formation becomes likely depends on xylem water tension (Figure 4A) but is much lower than the solute concentration required for producing high enough xylem water tensions for homogenous and heterogeneous nucleation. At xylem water tension 0.1 MPa it is of the order of magnitude 1000 times the saturation concentration, but drops quickly with increasing xylem water tension, and at 5 MPa tension only 20 times the saturation concentration is required. For each xylem water tension, the supersaturation at which bubble formation becomes likely depends slightly on the freezing velocity and conduit diameter (Figures 4B,C). The slight sensitivity to these parameters stems from the time dependence of the bubble formation probability (Eq. 9), and is similar to the effect conduit size has on nucleation events in general; the larger the conduit or the slower the freezing velocity, the more there is time for a successful bubble formation event to occur. But whether the required gas concentration is obtained during freezing depends strongly on both conduit diameter and freezing velocity (Figures 1 and 2). Note that the dependence of nucleation on freezing velocity (Figure 4B) is opposite to the dependence of the increase in the concentration of segregated gas on freezing velocity. In the case of segregated gases the higher the freezing velocity is the higher the concentration and the more likely bubble formation occurs.

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