Summary — In this paper we review vulnerability to embolism caused by drought and freezing in six species of oak (Quercus). The xylem pressure potential that induces 50% loss hydraulic conductivity ranges from -2.5 to -6.0 MPa for the species reviewed and correlates with other measures of drought tolerance in oaks. The probability of vessel dysfunction increases with vessel size for both drought-and freezing-induced embolism. The impact of embolism is a reduction in hydraulic conductivity in the vascular system. We conclude that embolism plays little role in the drought tolerance of oaks since drought-induced embolism occurs at more negative water potentials than are known to cause damage (eg, reduced growth) or mortality. Nevertheless

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The Pneumatic method has been introduced to quantify embolism resistance in plant xylem of various organs. Despite striking similarity in vulnerability curves between the Pneumatic and hydraulic methods, a modeling approach is highly needed to demonstrate that xylem embolism resistance can be accurately quantified based on gas diffusion kinetics. A Unit Pipe Pneumatic (UPPn) model was developed to estimate gas diffusion from intact conduits, which were axially interconnected by interconduit pit membranes. The physical laws used included Fick's law for diffusion, Henry's law for gas concentration partitioning between liquid and gas phases at equilibrium, and the ideal gas law. The UPPn model showed that 91% of the extracted gas came from the first two series of embolized, intact conduits, and only 9% from the aqueous phase after 15 s of simulation. Embolism resistance measured with a Pneumatic apparatus was systematically overestimated by 2 to 17%, corresponding to a typical measuring error of 0.11 MPa for P50 (the water potential equivalent to 50% of the maximum amount of gas extracted). Because results from the UPPn model are supported by experimental evidence, there is a good theoretical and experimental basis for applying the pneumatic method to research on embolism resistance of angiosperms.

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