We tested the hypothesis that greater cavitation resistance correlates with less total inter-vessel pit area per vessel (the pit area hypothesis) and evaluated a trade-off between cavitation safety and transport efficiency. Fourteen species of diverse growth form (vine, ring- and diffuse-porous tree, shrub) and family affinity were added to published data predominately from the Rosaceae (29 species total). Two types of vulnerability-to-cavitation curves were found. Ring-porous trees and vines showed an abrupt drop in hydraulic conductivity with increasing negative pressure, whereas hydraulic conductivity in diffuse-porous species generally decreased gradually. The ring-porous type curve was not an artifact of the centrifuge method because it was obtained also with the air-injection technique. A safety versus efficiency trade-off was evident when curves were compared across species: for a given pressure, there was a limited range of optimal vulnerability curves. The pit area hypothesis was supported by a strong relationship (r2 = 0.77) between increasing cavitation resistance and diminishing pit membrane area per vessel (A(P)). Small A(P) was associated with small vessel surface area and hence narrow vessel diameter (D) and short vessel length (L)--consistent with an increase in vessel flow resistance with cavitation resistance. This trade-off was amplified at the tissue level by an increase in xylem/vessel area ratio with cavitation resistance. Ring-porous species were more efficient than diffuse-porous species on a vessel basis but not on a xylem basis owing to higher xylem/vessel area ratios in ring-porous anatomy. Across four orders of magnitude, lumen and end-wall resistivities maintained a relatively tight proportionality with a near-optimal mean of 56% of the total vessel resistivity residing in the end-wall. This was consistent with an underlying scaling of L to D(3/2) across species. Pit flow resistance did not increase with cavitation safety, suggesting that cavitation pressure was not related to mean pit membrane porosity.

The hypothesis that greater safety from cavitation by airseeding through inter-vessel pits comes at the cost of less porous pit membranes with greater flow resistance was tested . Sixteen vessel-bearing species were compared: 11 from the Rosaceae, four from other angiosperm families, and one fern. Unexpectedly, there was no relationship between pit resistance (and hence the prevailing membrane porosity) and cavitation pressure. There was, however, an inverse relationship between pit area per vessel and vulnerability to cavitation ( r 2 = = = = 0.75). This suggests that cavitation is caused by the rare largest membrane pore per vessel, the average size of which increases with total pit area per vessel. If safety from cavitation constrains pit membrane surface area, it also limits vessel surface area and the minimum vessel resistivity. This trade-off was consistent with an approximately three-fold increase in vessel resistivity with cavitation pressure dropping from ----0.8 to ----6.6 MPa. The trade-off was compensated for by a reduction in the percentage of vessel wall pitted: from 10-16% in vulnerable species to 2-4% in resistant species. Across species, endwall pitting accounted for 53 ± ± ± ± 3% of the total xylem resistivity. This corresponded to vessels achieving on average 94 ± ± ± ± 2% of their maximum possible conductivity if vessel surface area is constrained.

We investigated the common assumption that severing stems and petioles under water preserves the hydraulic continuity in the xylem conduits opened by the cut when the xylem is under tension. In red maple and white ash, higher percent loss of conductivity (PLC) in the afternoon occurred when the measurement segment was excised under water at native xylem tensions, but not when xylem tensions were relaxed prior to sample excision. Bench drying vulnerability curves in which measurement samples were excised at native versus relaxed tensions showed a dramatic effect of cutting under tension in red maple, a moderate effect in sugar maple, and no effect in paper birch. We also found that air injection of cut branches (red and sugar maple) at pressures of 0.1 and 1.0 MPa resulted in PLC greater than predicted from vulnerability curves for samples cut 2 min after depressurization, with PLC returning to expected levels for samples cut after 75 min. These results suggest that sampling methods can generate PLC patterns indicative of repair under tension by inducing a degree of embolism that is itself a function of xylem tensions or supersaturation of dissolved gases (air injection) at the moment of sample excision. Implications for assessing vulnerability to cavitation and levels of embolism under field conditions are discussed.

Wood structure and function of juvenile wood from 18 conifer species from four conifer families (Araucariaceae, Cupressaceae, Pinaceae and Podocarpaceae) were examined for a trade-off between wood reinforcement and hydraulic efficiency. Wood density and tracheid 'thicknessto-span' ratio were used as anatomical proxies for mechanical properties. The thickness:span represented the ratio of tracheid double wall thickness to lumen diameter. Hydraulic resistivity (R) of tracheids on a cross-sectional area basis (R CA ) increased over 50-fold with increasing density and thickness:span, implying a strength versus efficiency conflict. The conflict arose because density and thickness:span were increased by narrowing tracheid diameter rather than by thickening walls, which may be developmentally difficult. In the Pinaceae and Cupressaceae species, density and thickness:span correlated strongly with protection from drought-induced embolism, suggesting that mechanical strength was required in part to withstand tracheid collapse by negative sap pressure. These species showed a corresponding trade-off between increasing R CA and embolism protection. In contrast, species of Podocarpaceae and Araucariaceae were overbuilt for their embolism protection and were hydraulically inefficient, having greater density, thickness:span and R CA , none of which were correlated with vulnerability to embolism.

The hydraulic resistivity ( R , pressure gradient/flow rate) through end walls of xylem conduits was estimated in seven species of diverse anatomy and affinity including a vesselbearing fern, a tracheid-bearing gymnosperm, and angiosperms with versus without vessels. Conduit lengths were measured with a silicone injection method which was easier and more accurate than the usual paint injection. The R declined linearly with the removal of end walls as stems were shortened from 10 to 0.3 cm. This relationship gave the minimum R with no end walls present, or the lumen resistivity ( R L ). This was indistinguishable from the HagenPoiseuille value. The maximum R with all end walls present gave R C , the resistivity of end wall and lumen in series. Average end-wall resistivity ( R W ) was the difference R C ----R L and the 'wall fraction' was R W / R C . Wall fraction was approximately constant, averaging 0.54 ± ± ± ± 0.07. This suggests that end wall and lumen resistivities are nearly colimiting in vascular plants. Average conduit length was proportional to the diameter squared across species ( r 2 = 0.94). Together with a constant wall fraction, this was consistent with the end wall resistance ( r w , pressure difference/flow rate) being inversely proportional to conduit length. Lower r w in longer conduits is consistent with their having more end wall pits than shorter conduits.

Plant xylem must balance efficient delivery of water to the canopy against protection from air entry into the conduits via air-seeding. We investigated the relationship between tracheid allometry, end wall pitting, safety from air-seeding, and the hydraulic efficiency of conifer wood in order to better understand the trade-offs between effective transport and protection against air entry. Root and stem wood were sampled from conifers belonging to the Pinaceae, Cupressaceae, Podocarpaceae, and Araucariaceae. Hydraulic resistivity of tracheids decreased with increasing tracheid diameter and width, with 64 ± 4% residing in the end wall pitting regardless of tracheid size or phylogenetic affinity. This end-wall percentage was consistent with a near-optimal scaling between tracheid diameter and length that minimized flow resistance for a given tracheid length. There was no evidence that tracheid size and hydraulic efficiency were constrained by the role of the pits in protecting against cavitation by air-seeding. An increase in pit area resistance with safety from cavitation was observed only for species of the northern hemisphere (Pinaceae and Cupressaceae), but this variable was independent of tracheid size, and the increase in pit resistance did not significantly influence tracheid resistance. In contrast to recent work on angiosperm vessels, protection against air-seeding in conifer tracheids appears to be uncoupled from conduit size and conducting efficiency.

The unicellular conifer tracheid should have greater flow resistance per length (resistivity) than the multicellular angiosperm vessel, because its high-resistance end-walls are closer together. However, tracheids and vessels had comparable resistivities for the same diameter, despite tracheids being over 10 times shorter. End-wall pits of tracheids averaged 59 times lower flow resistance on an area basis than vessel pits, owing to the unique torus-margo structure of the conifer pit membrane. The evolution of this membrane was as hydraulically important as that of vessels. Without their specialized pits, conifers would have 38 times the flow resistance, making conifer-dominated ecosystems improbable in an angiosperm world.

Cavitation has long been recognized as a key constraint on the structure and functional integrity of the xylem. Yet, recent results call into question how well we understand cavitation in plants. Here, we consider embolism formation in angiosperms at two scales. The first focuses on how air-seeding occurs at the level of pit membranes, raising the question of whether capillary failure is an appropriate physical model. The second addresses methodological uncertainties that affect our ability to infer the formation of embolism and its reversal in plant stems. Overall, our goal is to open up fresh perspectives on the structure-function relationships of xylem.A central question in the biology of vascular plants is under what conditions the continuity of the liquid phase, essential for the transport of water from soil to leaves, is lost (Tyree and Zimmerman, 2002). Without the highconductance pathway for water movement through the xylem, vascular plants could not sustain the water loss associated with the diffusional uptake of CO 2 from a subsaturated atmosphere. As a result, substantial effort in the field of xylem transport focuses on quantifying vulnerability to cavitation and its impact. Yet, a number of recent studies raise questions regarding how well we understand cavitation in plant stems, pointing to apparently anomalous or inconsistent experimental results that suggest methodological artifacts Ennajeh et al., 2011;Wheeler et al., 2013). Such results provide the motivation for this Update. We recognize that there is currently no consensus on the extent to which any particular experimental approach is subject to a problem; in addition, we note that the implications of such potential artifacts for the estimation of leaf xylem vulnerability involves further methodological considerations beyond our current scope. Our intention with this Update is to clarify the physical basis for a number of potential experimental artifacts relating to angiosperm stem xylem, in the hope that this will be useful for designing experiments that can resolve these issues.In this spirit, we begin with a discussion of how cavitation occurs, sketch an alternative model to meniscal failure for how air seeding across homogenous pit membranes could occur in the absence of discrete pores, and discuss the implications of these two models for the relative importance of probabilistic versus deterministic constraints on air-seeding resistance. We then turn to recent evidence that xylem vulnerability to cavitation in some species may have been overestimated and consider possible physical effects that could lead to biases sensitive to conduit size in the three principal methods of vulnerability estimation (dehydration, air injection, and centrifugation); to the extent that such biases are quantitatively important, these methods cannot be considered independent for long-vesseled species. Experimental approaches that may prove helpful in reconciling some of the divergent results between methods or protocols are proposed. The potential for measureme...