New insights into bordered pit structure and cavitation resistance in angiosperms and conifers
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Cavitation resistance in conifers
Under functional, water-filled conditions where the xylem pressure (Px) is below zero, the conifer pit membrane is centrally located in the pit chamber allowing water to move through the margo unimpeded. Should one tracheid become air-filled (Px = 0), the pressure difference across the menisci in the margo pores will be sufficient to deflect the membrane towards the adjoining functional tracheid where Px < 0. This way, the torus is appressed against the pit aperture, thereby isolating the water-filled tracheid from its dysfunctional neighbor. Cavitation presumably occurs when Px becomes negative enough to cause the torus to slip from its sealing position, allowing air to enter the water-filled tracheid. Indeed, the sealing action of the torus over the pit aperture may be one reason why only a weak correlation between pit area and cavitation resistance has been observed in conifer stems (Pittermann et al., 2006).
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