summary In this paper, we have reviewed how the hydraulic design of trees influences the movement of water from roots to leaves. The hydraulic architecture of trees can limit their water relations, gas exchange throughout the crown of trees, the distribution of trees over different habitats and, perhaps, even the maximum height that a particular species can achieve. Parameters of particular importance include: (1) the vulnerability of stems to drought‐induced cavitation events because cavitation reduces the hydraulic conductance of stems, (2) the leaf specific conductivity‐of stems because it determines the pressure gradients and most negative water potentials needed to sustain evaporation from leaves, (3) the water storage capacity of tissues because this might determine the ability of trees to survive long drought periods. All of these parameters are determined by the structure and function of anatomical components of trees. Some of the ecological and physiological trade‐offs of specific structures are discussed.

Possible mechanical and hydraulic costs to increased cavitation resistance were examined among six co-occurring species of chaparral shrubs in southern California. We measured cavitation resistance (xylem pressure at 50% loss of hydraulic conductivity), seasonal low pressure potential (P min ), xylem conductive efficiency (specific conductivity), mechanical strength of stems (modulus of elasticity and modulus of rupture), and xylem density. At the cellular level, we measured vessel and fiber wall thickness and lumen diameter, transverse fiber wall and total lumen area, and estimated vessel implosion resistance using (t/b) h 2 , where t is the thickness of adjoining vessel walls and b is the vessel lumen diameter. Increased cavitation resistance was correlated with increased mechanical strength (r 2 5 0.74 and 0.76 for modulus of elasticity and modulus of rupture, respectively), xylem density (r 2 5 0.88), and P min (r 2 5 0.96). In contrast, cavitation resistance and P min were not correlated with decreased specific conductivity, suggesting no tradeoff between these traits. At the cellular level, increased cavitation resistance was correlated with increased (t/b) h 2 (r 2 5 0.95), increased transverse fiber wall area (r 2 5 0.89), and decreased fiber lumen area (r 2 5 0.76). To our knowledge, the correlation between cavitation resistance and fiber wall area has not been shown previously and suggests a mechanical role for fibers in cavitation resistance. Fiber efficacy in prevention of vessel implosion, defined as inward bending or collapse of vessels, is discussed.Among vascular plants, water is transported through xylem under negative pressure. Xylem must withstand both the mechanical stresses associated with negative pressure as well as the risk of air entering the hydraulic pathway. Failure to do so may lead to cavitation of water columns and blockage of water transport. Failure may occur when gas is pulled into water-filled xylem conduits from gas-filled cells or intercellular spaces through pores in the xylem pit membrane in a process referred to as air-seeding (Zimmermann, 1983;Baas et al., 2004). Failure may also occur when negative pressures overcome the ability of the xylem conduit walls to resist implosion (i.e. inward bending or collapse; Carlquist, 1975;Hacke et al., 2001a;Donaldson, 2002;Cochard et al., 2004;Brodribb and Holbrook, 2005). Implosion may trigger cavitation or, in leaves, may restrict hydraulic transport by reducing conduit diameter (Brodribb and Holbrook, 2005). In addition to negative pressures, freezing can lead to failure when sap in the xylem freezes and the gas dissolved in the sap comes out of solution. This can lead to cavitation upon thawing if the bubbles do not go back into solution but instead expand (Yang and Tyree, 1992). The result of cavitation is embolism or gas blockage, which reduces hydraulic transport and can result in reduced stomatal conductance (Pratt et al., 2005), reduced photosynthesis (Brodribb and Feild, 2000), and dieback of branchlets (Rood et al., 2000;Da...

Summary• Here, hypotheses about stem and root xylem structure and function were assessed by analyzing xylem in nine chaparral Rhamnaceae species.• Traits characterizing xylem transport efficiency and safety, mechanical strength and storage were analyzed using linear regression, principal components analysis and phylogenetic independent contrasts (PICs).• Stems showed a strong, positive correlation between xylem mechanical strength (xylem density and modulus of rupture) and xylem transport safety (resistance to cavitation and estimated vessel implosion resistance), and this was supported by PICs. Like stems, greater root cavitation resistance was correlated with greater vessel implosion resistance; however, unlike stems, root cavitation resistance was not correlated with xylem density and modulus of rupture. Also different from stems, roots displayed a trade-off between xylem transport safety from cavitation and xylem transport efficiency. Both stems and roots showed a trade-off between xylem transport safety and xylem storage of water and nutrients, respectively.• Stems and roots differ in xylem structural and functional relationships, associated with differences in their local environment (air vs soil) and their primary functions.

Resistance to xylem cavitation depends on the size of xylem pit membrane pores and the strength of vessels to resist collapse or, in the case of freezing-induced cavitation, conduit diameter. Altering these traits may impact plant biomechanics or water transport efficiency. The evergreen sclerophyllous shrub species, collectively referred to as chaparral, which dominate much of the mediterranean-type climate region of southern California, have been shown to display high cavitation resistance (pressure potential at 50% loss of hydraulic conductivity; P 50 ). We examined xylem functional and structural traits associated with more negative P 50 in stems of 26 chaparral species. We correlated raw-trait values, without phylogenetic consideration, to examine current relationships between P 50 and these xylem traits. Additionally, correlations were examined using phylogenetic independent contrasts (PICs) to determine whether evolutionary changes in these xylem traits correlate with changes in P 50 . Co-occurring chaparral species widely differ in their P 50 (À0.9 to À11.0 MPa). Species experiencing the most negative seasonal pressure potential (P min ) had the highest resistance to xylem cavitation (lowest P 50 ). Decreased P 50 was associated with increased xylem density, stem mechanical strength (modulus of rupture), and transverse fiber wall area when both raw values and PICs were analyzed. These results support a functional and evolutionary relationship among these xylem traits and cavitation resistance. Chaparral species that do not sprout following fire but instead recruit post-fire from seed had the greatest resistance to cavitation, presumably because they rely on post-fire survival of seedlings during the summer dry period to persist in the landscape. Raw values of hydraulic vessel diameter (d h ), maximum vessel length, and xylem-specific hydraulic conductivity (K s ) were correlated to P 50 ; however, d h , maximum vessel length, and K s were not correlated to P 50 when analyzed using PICs, suggesting that these traits have not undergone correlated evolutionary change. We found no difference in xylem traits between species occurring at freezing vs. nonfreezing sites, although freezing has been shown to affect the survival and distributions of some chaparral species. Stem mechanical strength, fiber properties, and post-fire regeneration type appear to be key factors in the evolution of cavitation resistance among chaparral shrubs.

Summary 1Climate change in South Africa may threaten the sclerophyllous evergreen shrubs of this region. Available data suggest that they are not as tolerant of water stress as chaparral shrubs occurring in climatically similar California, USA. 2 Seventeen species from nine angiosperm families, including both fynbos and succulent karoo species, were studied at a field site in Western Cape Province, South Africa. Minimum seasonal pressure potential ( P min ), xylem specific conductivity ( K s ), stem strength against breakage (modulus of rupture, MOR), xylem density, theoretical vessel implosion resistance ( ) and several fibre and vessel anatomical traits were measured. 3 Species displayed great variability in P min , similar to the range reported for chaparral and karoo shrub species, but in contrast to previous reports for fynbos shrubs. 4 More negative P min was associated with having greater xylem density, MOR and . There was no relationship between P min and traits associated with increased water transport efficiency. 5 Xylem density integrates many xylem traits related to water stress tolerance, including P min , MOR and , as well as percentage fibre wall, parenchyma, vessel area and fibre lumen diameter. 6 Xylem density may be an integral trait for predicting the impact of climate change on evergreen shrubs.

Coniferous trees, dicotyledonous trees, and dicotyledonous lianas (woody vines) form interesting morphological contrasts in their xylem structure and function. Lianas have among the largest (up to 8 metres or more) and widest (up to 500 µm) vessels in the plant kingdom. In conifers the water transport occurs through tracheids, which are relatively inefficient in transport. We can compare disparate growth forms in terms of leaf-specific. conductivity (LSC), which is hydraulic conductivity per surface area of leaves supplied by a stem. LSC is inversely proportional to localised pressure potential gradients. LSC is equal to the Huber value (sapwood area per leaf area supplied) times the specific conductivity (hydraulic conductivity per sapwood area). Lianas are similar to dicot trees and conifers in having hydraulic constrictions (low LSCs) at branch junctions. However, lianas generally have greater LSCs and specific conductivities but lower Huber values than do conifers. Dicot trees are intermediate in these values. The narrow but efficient stems of lianas are possible partly because lianas are not self-supporting; the mechanical requirements are reduced. Secondly, the wide and efficient vessels of lianas remain conductive for much longer than might be expected (two to several years, versus one year for similar wide vessels in dicots). Based upon experiments with glass capillary tubes and with living stem tissue, larger vessels are more susceptible to freezinginduced embolism than are small ones. However, in lianas, root pressures might serve to refill cavitated vessels on a daily or seasonal basis.

SUMMARYThe relationship between conduit (vessel and tracheid) diameter and water-stress-induced air embolism was examined using a double staining technique. Comparisons were made between irrigated control plants at water potentials of -1-3 MPa and water stressed plants at about -8 MPa. Water stress was induced either by natural drought conditions or by laboratory drying of shoots from previously irrigated shrubs. Stem segments were perfused with 0-1 "o basic fuchsin to mark the initially conductive conduits, and. following high pressure perfusion of lOmM citric acid to remove embolisms, with 0-1 % alcian blue to mark the initially embolized conduits. Hydraulic conductance per pressure gradient {k^) was measured before and after embolisms were removed. Diameters of non-embolized and embolized conduits were then measured microscopically in transverse stem sections. In irrigated controls there was little embolism and mean diameters were not significantly different for embolized vs. non-embolized conduits. For both artificially dehydrated and naturally droughted plants there was a 91 "o drop in A,, due to embolism, and the mean diameter of embolized conduits was about 30 //m vs. 21 //m for non-embolized conduits. With increasing conduit diameter there was an increased probability of embolism. Wider conduits may have larger pores in their pit membranes, thus increasing their vulnerability to water-stress-induced embolism. Alternatively, wider conduits may merely have more pits, thus increasing their statistical chances of having a particularly large pore in an air-exposed pit membrane. Narrow vessels and tracheids provide an interwoven auxiliary transport system that appears to be of importance to transport when many of the wider, more efficient conduits become embolized.

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