The Limits to Xylem Embolism Recovery in Pinus sylvestris L.

In this study we test the hypothesis that, when water supplyis under tension, reversal of cavitation can occur as long aswater continuity is maintained in the vicinity of tracheids.The experiments were conducted on young branches, 7–8mm diameter, of Pinus sylvestris L., freshly collected and allowedto lose water on the bench after being debarked. During dehydration,the volumetric fractions of water (V_w) and gas (V_s) changedsteadily as relative water content () declined. Meanwhile, ultrasonicemissions (UAE) started after a threshold = 90% was reachedand were maximal at = 75%. Before and after dehydration, branchsegments were connected to water-filled tubing and placed from0.2 to 3.6 m above a water source and water inflow and outflowwere recorded. These distances provided a source of water ata potential of –2.0 to –36kPa. We considered thatthe segment water potential would be a function of the surfacetension across the water meniscii at the ends of the embolizedtracheids. Thus, water potentials calculated from tracheid dimensionswould be as low as –43 kPa. Water inflow to segments declinedwhen the distance from the source was increased or the segmentswere very dehydrated. Increasing the distance above the watersource would be expected to increase the water potential differencebut to reduce water uptake. The most dehydrated segments absorbedwater faster at the beginning of the refilling period (2h),but at the end of 16h, was lower and V_g larger than in lessembolized tissue. Recovery of water flow followed a similartrend, and was lowest when embolisms increased. For a narrowrange of , hydraulic conductance was reduced sharply, indicatingthat wide tracheids were still gas-filled. Thus, the numberof tracheids remaining embolized increased when the source waterpotential was low and there was severe embolism. We concludethat embolism can be reversed in P. sylvestris at a rate dependingon the water potential of the source, severity of embolism andhydraulic conductivity. Key words: Pinus sylvestris L., cavitation induction-recovery, embolized tracheids, water content and embolism, matrix potential, capillary, hydraulic conductance