Constraints on hydraulic efficiency in conifer xylem

The conifers are comprised of seven families whose members are globally dispersed, but most dominant in the Northern Hemisphere. Their wood is homogenous, being comprised primarily of unicellular tracheids that function in support of the canopy, and water transport. Conifers are susceptible to drought and freezing-induced cavitation whereby air enters the functional tracheids, expands and blocks transport in that conduit. This research evaluated the hydraulic and structural trade-offs associated with resistance to freeze-thaw and drought-induced cavitation in conifer wood.; When a sap-filled tracheid freezes, the air is forced out of the ice and forms bubbles. Upon thawing, these bubbles may redissolve, or they may expand and nucleate cavitation. Larger conduits contain more water, thereby producing larger bubbles that are more likely to expand, while the opposite is true of smaller conduits. Current data showed that conifers whose mean tracheid diameter exceeded 30 mum were highly vulnerable to freeze-thaw embolism at a xylem pressure of -0.5 MPa. This is because the diameter-dependence of freeze-thaw embolism is a consequence of the radial pattern of freezing. The tracheid diameter required for freeze-thaw cavitation decreased at more negative xylem pressures, down to 2 mum at -8 MPa, which was consistent with the theory of bubble expansion described above. Species with narrow conduit diameters are 'safe' against freeze-thaw cavitation, but at the cost of reduced hydraulic efficiency.; The 'safety vs. efficiency' trade-off was also apparent with respect to drought-induced cavitation. Tracheid efficiency decreased with greater cavitation resistance, and this was largely due to smaller tracheid diameters. A reduction in tracheid diameter caused an increase in wood density and wall thickness to span ratios, in order to reinforce the tracheid against the stresses caused by negative pressure. An increase in these strength parameters led to a reduction in tracheid efficiency, rather than any feature associated with the pit membrane. This was a contrast to the limits imposed by cavitation on vessel efficiency in angiosperms. A remarkable finding was that the tracheid torus-margo pit allowed conifers to achieve hydraulic efficiencies matching those of angiosperms bearing equivalent vessel diameters. Torus-margo pitting allows the conifers to effectively compete with angiosperms.