The Limits to Xylem Embolism Recovery inPinus sylvestrisL.

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Abstract
In this study we test the hypothesis that, when water supply is under tension, reversal of cavitation can occur as long as water continuity is maintained in the vicinity of tracheids. The experiments were conducted on young branches, 7–8 mm diameter, of Pinus sylvestris L., freshly collected and allowed to lose water on the bench after being debarked. During dehydration, the volumetric fractions of water (Vw) and gas (Vs) changed steadily as relative water content (θ) declined. Meanwhile, ultrasonic emissions (UAE) started after a threshold θ = 90% was reached and were maximal at θ = 75%. Before and after dehydration, branch segments were connected to water-filled tubing and placed from 0.2 to 3.6 m above a water source and water inflow and outflow were recorded. These distances provided a source of water at a potential of −2.0 to −36kPa. We considered that the segment water potential would be a function of the surface tension across the water meniscii at the ends of the embolized tracheids. Thus, water potentials calculated from tracheid dimensions would be as low as −43 kPa. Water inflow to segments declined when the distance from the source was increased or the segments were very dehydrated. Increasing the distance above the water source would be expected to increase the water potential difference but to reduce water uptake. The most dehydrated segments absorbed water faster at the beginning of the refilling period (≃2h), but at the end of 16h, θ was lower and Vg larger than in less embolized tissue. Recovery of water flow followed a similar trend, and was lowest when embolisms increased. For a narrow range of θ, hydraulic conductance was reduced sharply, indicating that wide tracheids were still gas-filled. Thus, the number of tracheids remaining embolized increased when the source water potential was low and there was severe embolism. We conclude that embolism can be reversed in P. sylvestris at a rate depending on the water potential of the source, severity of embolism and hydraulic conductivity.

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