Hydraulic resistance of pits and pit membranes - the cost of safety
Impacts of pit function on whole-plant hydraulics
Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids) that are essential components in the water-transport system of higher plants. The pit membrane, which lies in the center of each pit, allows water to pass between xylem conduits but limits the spread of embolism and vascular pathogens in the xylem. Averaged across a wide range of species, pits account for > 50% of total xylem hydraulic resistance, indicating that they are an important factor in the overall hydraulic efficiency of plants. The structure of pits varies dramatically across species, with large differences evident in the porosity and thickness of pit membranes. Because greater porosity reduces hydraulic resistance but increases vulnerability to embolism, differences in pit structure are expected to correlate with trade-offs between efficiency and safety of water transport. However, trade-offs in hydraulic function are influenced both by pit-level differences in structure (e.g. average porosity of pit membranes) and by tissue-level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connects vessels. In this review we address the impact of variation in pit structure on water transport in plants from the level of individual pits to the whole plant.
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The ability of plants to draw water from the soil and transport it to the leaf surface at a rate that allows a net positive carbon gain depends on a highly compartmentalized, extensively redundant system. Because water is transported through the xylem under negative pressure (tension), it is exceptionally vulnerable to dysfunction, and it is essential that gas voids (embolism) or vascular pathogens can be isolated within a conduit while water transport continues in adjacent conduits (Tyree & Sperry, 1989). The functionality of the xylem network depends to a large degree on bordered pits that connect adjacent conduits and the finely porous pit membranes, which prevent the movement of gas and pathogens between conduits (Zimmermann & Brown, 1971). Thus bordered pits act as safety valves in the hydraulic system of plants.
However, the protective benefit provided by pit membranes comes at a functional cost to the plant. Although the relatively wide lumens of vessels and tracheids provide a low-resistance pathway through the xylem, the length of conduits is finite, and water must pass between many thousands of conduits in order to move from the roots to the canopy (Tyree & Ewers, 1991). Therefore water will encounter two principal resistances as it moves through the xylem: the resistance along the lumen, and the resistance imposed by pits (Comstock & Sperry, 2000). Here, the narrow pores of pit membranes are expected to result in significant additional resistance. Recent research in this area has shown that pit membranes can account for 50% or more of the total hydraulic resistance in the xylem (Schulte & Gibson, 1988; Pittermann et al., 2005; Sperry et al., 2005; Choat et al., 2006). Therefore pits occupy a crucial role in the water-transport system of plants. The hydraulic function of xylem tissue in any plant organ, whether root, stem or leaf, cannot be understood without taking into account the influence of bordered pit structure on the balance of safety and efficiency in vascular transport.
II. The basic structures
1. Pit connections between xylem conduits
Xylem conduits are connected through openings in the lignified secondary walls known as pits (Fig. 1). Pits in conductive elements are characteristically bordered: they possess an overarching secondary cell wall with a narrow aperture that flares into a wider pit chamber. This architecture maximizes both structural support provided by secondary walls and the surface area available for transfer of water between conduits (Carlquist, 2001). The primary walls and intervening middle lamella of two opposing cells make up the pit membrane, which lies in the center of the pit pair. The term ‘membrane’ may create confusion, given that there is no lipid bilayer membrane associated with pits of mature xylem vessels.
The water-transport system in the xylem of all tracheophytes consists of cells that are dead at maturity. In the final stages of development, xylem conduits undergo programmed cell death with the release of autolytic enzymes that digest the protoplast. Before cell death, the wall of each element is remodeled from within by some combination of reinforcement by secondary thickening and hydrolysis of the primary cell wall at its interface to other elements (Barnett, 1981; O’Brien, 1981). In the xylem of gymnosperms, water transport is facilitated by single cells known as tracheids. In nearly all angiosperms and some pteridophytes, conduits are composed of multiple dead cells, in which at least one end of each vessel element has a perforation plate, a section of primary wall that is digested in the later stages of maturation (White, 1961; Butterfield, 1995; Nakashima et al., 2000). Because vessels are multicellular in nature, they can attain much greater lengths than tracheids. For further information on developmental and evolutionary aspects of pit structure, see Sperry (2003), Catesson (1983), and Chaffey et al. (1997).
2. Pit membrane structure and composition
Understanding the structure of pit membranes is critical to understanding their functional behavior in relation to both the safety and efficiency of water conduction. One of the most conspicuous differences in pit membrane structure across taxa can be observed between gymnosperm and angiosperm species. Most gymnosperms possess visually striking ‘torus–margo’ pit membranes, which are divided into two spatially distinct regions (Fig. 1f). The outer margo region is porous, allowing for low-resistance transfer of water between tracheids, while the center of the pit membrane is thickened into an impermeable torus. In contrast, pit membranes of most vessel-bearing angiosperms are almost exclusively ‘homogeneous’, having a relatively even deposition of microfibrils, with no differentiation between regions of the membrane. Torus–margo membranes have also been recorded in some angiosperm species (Ohtani & Ishida, 1978; Dute & Rushing, 1990; Dute et al., 1996; Jansen et al., 2004b; Jansen et al., 2007), but are relatively rare and often occur only in the narrow latewood vessels.
Examining the fine structure of the pit membrane is challenging because of their inherently delicate nature. Classical transmission electron microscope (TEM) and scanning electron microscope (SEM) studies showed that homogeneous pit membranes in angiosperms consist of a number of layers of microfibrils, with the orientation of microfibrils differing between layers (Schmid, 1965; Schmid & Machado, 1968; Catesson, 1983; Sano, 2005). In the outermost layers the orientation is random, while in inner layers the orientation is parallel. This can be explained in terms of cell growth, with the early-formed layers stretched by growth and the late-forming layers randomly arrayed after cell expansion is complete (Schmid & Machado, 1968).
Pit membranes appear to be similar in composition to primary cell walls of living cells, with a tightly woven meshwork of cellulose microfibrils and hemicelluloses in a matrix of pectin polysaccharides (Thomas & Nicholas, 1968; Bauch & Berndt, 1973; Boyce et al., 2004). Evidence suggests that some degree of hydrolysis of the pit membranes takes place during the final stages of maturation in xylem conduits, but the variation and extent of this are not well understood. Some earlier papers state that noncellulosic components of the membrane (pectins and hemicelluloses) are removed during maturation of the conduit (O’Brien & Thimann, 1967; O’Brien, 1970; Butterfield & Meylan, 1982). Evidence from physiological studies suggests that pectin is still present in intervessel pit membranes after hydrolysis, and plays an important role in regulating vascular resistance through hydrogel behavior (section V) (Zwieniecki et al., 2001b; Boyce et al., 2004). However, there is no direct evidence that pectins are present in mature angiosperm pit membranes. In contrast, there is clear evidence that pectins occur in pit membranes of gymnosperms (especially in the torus: Bauch et al., 1972; Hafren et al., 2000; Coleman et al., 2004). Guglielmino et al. (1997), using ionic microscopy and immunological tools, illustrated that pectins occur in unlignified, differentiating cell walls of vessels. Further mystery surrounds the fact that the primary cell wall material of perforation plates is digested (Murmanis, 1978), while pit membranes remain intact, and the mechanisms preventing complete hydrolysis in pit membranes remain an open question (Butterfield & Meylan, 1982; Butterfield, 1995).
3. Variation in porosity and thickness of pit membranes
The density and arrangement of microfibrils, together with the level of hydrolysis, determine the strength and porosity of a pit membrane. The diameter of pores strongly influences both the hydraulic resistance of the membrane and its ability to limit the spread of embolism between conduits (section III). There are many estimations of pit membrane porosity; in general, they can be divided into observations using electron microscopy and measurements made by gas penetration or particle perfusion. Electron microscopy offers the advantage of detailed observation of membrane structure, but results must be viewed with caution because of the chemical treatments and dehydrations that accompany sample preparation (Shane et al., 2000; Thorsch, 2000; Jansen et al., in press). Particle or gas perfusions allow measurements on hydrated membranes in intact conduits, but do not provide details on the structure of individual membranes.
Conifers Early SEMs of pit membranes made using carbon replicas showed large pores in the margo region of conifer pit membranes (Frey-Wyssling et al., 1956; Sachs, 1963; Liese, 1965; Fengel, 1966). The existence of large pores in the margo region was confirmed by particle perfusion experiments of Liese & Johann (1954), who showed that margo pores are commonly up to 200 nm in diameter. Electron micrographs reveal great morphological variation in the pit membranes of gymnosperms in terms of the porosity of the margo, thickness of margo strands and prominence of the torus (Liese, 1965; Tsoumis, 1965; Bauch et al., 1972). These differences are apparent both between species and within individuals between earlywood and latewood tracheids (Petty, 1972; Sano et al., 1999; Domec et al., 2006). However, there have been few attempts to quantify the differences in margo strength and porosity across species.
Angiosperms In homogeneous membranes, pores are often not observable using electron microscopy (Wheeler, 1981, 1983; Shane et al., 2000; Choat et al., 2003). Early SEM studies emphasized the uniform nature of intervessel pit membranes across angiosperm species (Schmid, 1965; Schmid & Machado, 1968). However, more recent studies utilizing field-emission SEM and TEM indicate that there is considerable variation in the structure of angiosperm pit membranes (Sano, 2004; Sano, 2005; Jansen et al., 2007; Schmitz et al., 2007). Choat et al. (unpublished) demonstrated that the thickness of pit membranes varied by almost an order of magnitude (70–500 nm) across 14 hardwood species (Fig. 2) and that, while pores were not resolvable in membranes of some species, openings of up to 200 nm were visible in others. Although SEM data on porosity must be viewed with caution because of the risk of artifacts from sample preparation, the observed differences in porosity are so striking that it is difficult to believe porosity under natural conditions does not follow the same pattern (e.g. Salix alba and Laurus nobilis; Fig. 3). Observations by particle perfusion techniques give maximum pit membrane porosities of 5–420 nm for a range of angiosperm species (Cronshaw, 1960; Murmanis & Chudnoff, 1979; Van Alfen et al., 1983; Jarbeau et al., 1995; Shane et al., 2000; Choat et al., 2003, 2004), although it should be stressed that these values are usually < 100 nm. The distinction between maximum and average pore diameter is an important one, and is discussed further in section III. Studies using perfusions of colloidal gold indicate that the average pore size of homogeneous membranes is around 5 nm, although maximum pore sizes may be much greater than this (Choat et al., 2004). This distinction is particularly significant when considering vulnerability to embolism and trade-offs in the safety and efficiency of xylem tissue.
Ferns and vesselless angiosperms Few anatomists have paid special attention to the structure of pit membranes in pteridophytes, basal angiosperms or vesselless angiosperms. Carlquist and Schneider have focused on the presence of ‘porous or web-like pit membrane remnants’ in perforation plates and pits of several families, including monocots and ferns (Carlquist, 1992; Carlquist & Schneider, 2001, 2007). They interpreted these remnants as precursors to the disappearance of pit membranes, and thus a significant stage in vessel element evolution. Studies of vesselless angiosperms have also presented SEM evidence of large pores in the pit membranes (Feild et al., 2000; Hacke et al., 2007). This is thought to lead to the formation of ‘cryptic vessels’ composed of a number of tracheids connected through very porous pit membranes.
4. New methods for probing pit membrane structure
More recently, methods such as atomic force microscopy (AFM), cryoSEM and environmental SEM have allowed observations of hydrated pit membranes. Pesacreta et al. (2005) employed AFM to compare the structure of dried and hydrated membranes of the angiosperm species Sapium sebiferum. Hydrated pit membranes had a noncellulosic layer and more loosely arranged microfibrils than dried membranes, which had a compacted appearance. Using cryoSEM, Shane et al. (2000) demonstrated that even partial drying of pit membranes in maize roots caused tearing and disruption compared with frozen, hydrated pit membranes. However, this is not the case in all species, because many studies have shown intact pit membranes in dried material (Choat et al., 2003; Sano, 2004; Sano, 2005). Thus, while it is clear that electron microscopy introduced some artifacts to pit membrane structure, the extent of these artifacts and their implications for interpretations of pit membrane function remain to be established.
III. Safety of transport – pits as safety valves
Plants routinely face xylem tensions great enough to allow a spontaneous transition from liquid to gaseous water, known as cavitation (Scholander et al., 1965; Tyree & Sperry, 1989; Holbrook et al., 1995). The resultant embolism renders xylem conduits dysfunctional and increases hydraulic resistance in the long-distance transport pathway. This problem is exacerbated by environmental stresses such as drought, freezing and salinity, which can cause lethal embolism, resulting in branch dieback or complete death of the plant (Rood et al., 2000; Davis et al., 2002; Vilagrosa et al., 2003; Stuart et al., 2007). It is therefore of paramount importance for plants to avoid extensive cavitation and embolism. The presence of pit membranes prevents the spread of embolism and pathogens through the xylem network; presumably this is the selective pressure that has favored maintaining these structures after autolysis of other cell wall components. Recent work has offered new insights into how the wide variation in pit structure influences vulnerability to embolism and the ability of plants to maintain vascular function in the face of environmental stress.
1. The air-seeding hypothesis
The air-seeding hypothesis provides a theoretical link between the structure of pit membranes and their function in limiting the spread of embolism (Zimmermann, 1983; Sperry & Tyree, 1988; Tyree & Sperry, 1989). Under this hypothesis, the movement of gas between conduits is limited by high surface tension of water and the nanometer-scale pores of a wetted pit membrane. When a gas interface is drawn to a pit membrane, it will break into many small menisci at the membrane pores. Gas will be prevented from moving into adjacent vessels as long as the diameters of the menisci exceed those of the pores. The pressure difference (ΔP) between the embolized and functional vessels will increase as xylem water potential declines. As ΔP across pit membranes increases, the diameter of the menisci (Dm) will shrink toward the diameter of the pit membrane pores (Dp). When Dm < Dp a gas bubble is drawn through a pit membrane pore and into an adjacent vessel. The magnitude of ΔP required to cause air seeding through a pore of a given Dp can be calculate from:
where γ is the surface tension of water (0.072 N m−1 at 20°C) and θ is the contact angle at the air–water–membrane interface. The contact angle is usually assumed to be 0, because the membrane surface is coated with hydrophilic pectin polysaccharides (Tyree & Zimmermann, 2002). If the contact angle were > 0 then the ΔP required for gas penetration would decrease for a given pore diameter. An interesting theoretical analysis of the consequences of variation in contact angle is given by Meyra et al. (2007). It is also likely that the situation becomes more complex for thicker pit membranes in which pores have a winding, tortuous path. In this case the role of geometry and surface chemistry may be of greater importance.
The air-seeding hypothesis directly ties the diameter of pit membrane pores to the chance of embolism spreading throughout the xylem. Importantly, air seeding will always occur at the largest pore that connects two vessels, therefore the air-seeding threshold between vessels depends on the maximum rather than the average diameter of pit membrane pores; only one large pore in the thousands of pit membranes connecting adjacent vessels is required for air seeding to occur.
2. Conifers and torus–margo pit membranes
Nowhere is the pit membrane's function as a safety valve more clearly illustrated than in torus–margo pits (Bailey, 1913; Pittermann et al., 2005). Under hydrated conditions, the pit membrane rests in the middle of the pit chamber and water flows through the large pores of the margo. These large-diameter pores would be predicted to air seed at very low pressure differences. Instead, the pressure difference between an embolized and water-filled tracheid deflects the membrane across the pit chamber, and the torus plugs the pit aperture, preventing gas from moving into the water-filled conduit (Bailey, 1913; Liese & Bauch, 1967). The pit membrane is then said to be ‘aspirated’, sealing the embolized tracheid off from adjacent conduits.
As the pressure difference increases, further stretching or even tearing of the margo may occur, allowing the torus to be displaced from the pit aperture, and a gas bubble to move into the adjacent water-filled tracheid, rendering it dysfunctional (Sperry & Tyree, 1990; Hacke et al., 2004). The pressure difference required to displace the torus depends on the strength and stiffness of the margo meshwork; theoretical considerations suggest that some plastic deformation of microfibrils in the margo must occur during aspiration (Petty, 1972; Hacke et al., 2004). Pit membranes with more densely woven margos (more or thicker strands) are expected to be more resistant to displacement, although this relationship is still to be confirmed (Pittermann et al., 2006). It is unclear whether aspiration of pits is a permanent process, a problem of relevance to the manufacture of treated softwood timber (Krahmer & Côté, 1963; Thomas & Kringstad, 1971). In some species, it appears that tracheids are never refilled (Utsumi et al., 2003). Refilling has been observed in other conifer species, indicating that aspiration can be reversed (Sobrado et al., 1992; Sperry & Sullivan, 1992).
3. Angiosperm pit membranes and air seeding
The homogeneous pit membranes of angiosperms have no torus to plug the pit aperture. However, the pore sizes in homogeneous membranes are smaller than those in the margo region of conifer pit membranes (Schmid & Machado, 1968; Jarbeau et al., 1995; Shane et al., 2000; Choat et al., 2003). Physically, the pores of homogeneous pit membranes are narrow enough to prevent air seeding until xylem tensions cross a critical threshold. As the level of water stress increases, air seeding through these pores becomes more likely. The relationship between increasing xylem tension and the spread of embolism through the xylem is quantified for a species or genotype by ‘vulnerability curves’ (Tyree & Sperry, 1989) (Fig. 3). Vulnerability curves can be viewed as the response of populations of vessels, with some vessels becoming embolized at very low xylem tensions and others being very resistant to gas penetration. This suggests that there is a distribution of pit membrane pore sizes between vessels, with vulnerable vessels connected by large pores and resistant vessels connected by small pores.
Attempts to relate the air-seeding threshold of a species to observable differences in pit membrane structure have shown ambiguous results. Embolism resistance of a species is often standardized by the xylem tension at which 50% of hydraulic conductivity is lost (P50). The P50 value should correlate with the maximum pore diameter typical of connections between vessels within the xylem. While some studies have shown a relationship between average pore diameter and P50 (Jarbeau et al., 1995), other studies have failed to detect pores of a diameter sufficient to allow air seeding at realistic tensions. Choat et al. (2003) demonstrated that there was no relationship between P50 values of four hardwood species and the average pore diameter measured with either SEM or with perfusions of gold colloids. The species ranged in P50 from 1.4 to 5.1 MPa, equivalent to pore diameters of 190–50 nm; however, the average pore diameter was between 5 and 20 nm in diameter for all species. Similar results were obtained by Shane et al. (2000) in an examination of pit membranes in maize roots. Incidentally, a pore of 20 nm diameter would allow gas penetration at a ΔP of 14 MPa, a xylem tension so extreme it is seldom seen in the plant kingdom!
The disparity between vulnerability to embolism and the structure of pit membranes can be explained in a number of ways. First, it might be that cavitation is not caused by air seeding, but occurs inside vessels as a result of homogeneous nucleation in the bulk liquid phase or by adhesion failure at hydrophobic surfaces, although this seems unlikely given the large body of indirect evidence from vulnerability curves supporting the air-seeding hypothesis (Sperry & Tyree, 1988; Cochard et al., 1992; Tyree et al., 1994; Pockman et al., 1995). Second, increased contact angles could result in a smaller Dp for a given ΔP across the pit membrane (Meyra et al., 2007), although more detailed observations of pit membrane surface chemistry and pore geometry are required for this to be assessed properly (Zwieniecki & Holbrook, 2000). Third, it is the maximum, not the average pit membrane pore diameter, that is important in air seeding, so a large discrepancy between the average and maximum pore diameter for pairs of conduits would account for the observed differences. Thus, if the relatively large pores responsible for air seeding are rare, occurring in only a few of the many thousands of membranes that connect adjacent vessels, it would be very difficult to detect them via electron microscopy or particle perfusion experiments. A fourth explanation relates to deflection and deformation of membranes during the process of air seeding, which may stretch existing pores (Choat et al., 2004).
4. Pit membrane deflection and vestured pits
In conifer pits, pressure differences between tracheids are known to deflect the membrane to the point where the torus seals the aperture. In homogenous pit membranes, the membrane is deflected without sealing the aperture (Thomas, 1972) (Fig. 4). In this case, membrane porosity may be increased either by rupture or by reversible stretching of existing pores (Sperry & Hacke, 2004). Choat et al. (2004) demonstrated that pit membranes of Fraxinus americana increased in porosity with increasing pressure difference across the membrane, suggesting that membrane stretching could increase vulnerability to embolism. This is consistent with studies indicating that pit membranes become ‘fatigued’ after successive cycles of embolism and repair, resulting in temporary increases in vulnerability to embolism (Hacke et al., 2001). These experiments suggest that the increased porosity does not result from permanent rupture of the membranes, because increases in vulnerability to gas penetration are reversible (Stiller & Sperry, 2002; Choat et al., 2004).
The idea that membrane porosity may increase because of deflection is supported by evidence from an anatomical feature of angiosperm pits known as vestures (Jansen et al., 1998). Vestures are outgrowths from the pit chamber that surround the pit membrane (Bailey, 1913) (Fig. 4d). Wood anatomists have speculated that they perform a protective function by preventing excessive deflection and rupture caused by pressure differences across the membrane (Zweypfenning, 1978; Jansen et al., 2003). Consistent with this, the distribution of species and genera with vestured pits is skewed towards hotter, drier environments, suggesting that vestures provide some adaptive advantage in water stress-prone environments (Jansen et al., 2004a). (Choat et al., 2004) demonstrated that, in single vessels, a species with vestured pits, Sophora japonica, showed little increase in pit membrane porosity with increasing pressure difference across pit membranes, in contrast with F. americana, a species that lacks vestures in its earlywood. Therefore anatomical, ecological and physiological evidence now support the theory that vestured pits represent an adaptive feature that reduces the risk of damage to pit membranes and the spread of embolism during periods of water stress. However, it is clear that further physiological data are required to confirm this hypothesis.
It must also be noted that vestured pits are not required for existence in water stress-prone environments. Many species growing in xeric regions with a high resistance to embolism lack vestured pits. Other pit characteristics, such as the size, thickness and strength of pit membranes, should influence the air-seeding threshold (Zimmermann, 1983). In addition, differences in embolism resistance are not necessarily linked to differences in structure that occur at the pit level. Tissue level changes, such as differences in vessel density, vessel size (diameter and length) and overlap area of pit membranes between vessels, can also heavily influence vulnerability to embolism and hydraulic resistance of the xylem and will be discussed further in section IV.
5. Spatial variation in pit function within a tree
Although direct measurements of variation in pit membrane characteristics within a tree are rather limited, considerable indirect evidence suggests that pit membrane properties differ depending on where and when they were produced (cambium or meristem age and position), as well as shifts in pit membrane structure and function with aging of the xylem.
In conifers, the situation is more complex. Drying experiments with conifer wood show that many of the pits in narrow latewood tracheids remain unaspirated after wider earlywood pits have aspirated (Petty, 1972). Consistent with this, Domec & Gartner (2002) reported that earlywood tracheids of Douglas fir were less vulnerable to cavitation than latewood tracheids. This was related to observed differences in pit membrane structure, with latewood pit membranes having smaller tori and less flexible margo, preventing them from sealing the pit aperture effectively (Domec et al., 2006). However, dye perfusion studies show that a notable fraction of the latewood remains conductive after all the earlywood tracheids are embolized (Sperry & Tyree, 1990). This suggests changes in pit characteristics across the latewood, but the theory remains to be tested.
Experiments examining differences in vulnerability to embolism of xylem across growth rings within a branch have shown that the threshold pressure for air seeding declines in older wood (Sperry et al., 1991; Melcher et al., 2003). Sperry et al. (1991) demonstrated that differences in the air-seeding threshold were linked qualitatively to changes in pit membrane structure in Populus tremuloides. Changes in pit membrane structure in older conduits may represent the first step toward permanent dysfunction and heartwood formation (Wheeler, 1981; Sperry et al., 1991; Sano & Nakada, 1998).
Variation between different heights and organs Vulnerability to embolism also varies with height and organ within a single plant (Alder et al., 1996; Tsuda & Tyree, 1997; Domec & Gartner, 2002; Choat et al., 2005; Burgess et al., 2006). Zimmermann (1983) proposed that the vascular system of plants might be structured in such a way that distal organs were preferentially sacrificed in times of water stress in order to preserve the main trunk. One corollary of this ‘segmentation hypothesis’ is that either pit structure, conduit structure, or both should change between different organs or at different heights.
In conifers, resistance to embolism generally increases with height, and the wood of branches is more resistant to embolism than trunk wood or the roots (Domec & Gartner, 2002; Burgess et al., 2006; Domec et al., 2006; Pittermann et al., 2006). Mayr et al. (2003) showed that leader shoots of Norway spruce growing at the alpine timberline were more resistant than side branches to embolism. In this case, increased resistance to embolism was correlated with smaller pit apertures. Measurements on very tall conifers show a large gradient in hydrostatic pressure required to move water to the top of the crown, and a correlated increase in resistance to embolism. In Douglas fir, differences in vulnerability to embolism between the roots, trunk and branches were correlated with differences in pit structure; torus size and margo pore size both decreased with height. Calculations based on pit dimensions showed that pit membranes in the roots and trunk would deflect and air seed at lower pressure differences than those in branches (Domec et al., 2006). Thus pit structure appears to be of great importance in facilitating the growth of very tall trees.
IV. Hydraulic resistance of pits and pit membranes – the cost of safety
It is clear that the fine porosity of pit membranes is vital in preventing the spread of vascular pathogens and embolism throughout the xylem. However, because of the powerful relationship between pore diameter and conductivity (Vogel, 1994), the increased safety afforded by pit membranes comes at a substantial cost in terms of increased hydraulic resistance (Box 1). It has long been recognized that the hydraulic conductivity of wood is less than the theoretical conductivity calculated from the diameter of xylem conduits (Ewart, 1906; Riedl, 1937; Münch, 1943). Zimmermann & Brown (1971) asserted that limitations in vessel length, and therefore an increase in the number of pit crossings, was the most important factor in the reduction of hydraulic conductivity from its theoretical maximum. More recent studies have indicated that pit resistance can play a major role in determining the efficiency of water transport through the xylem, accounting for > 50% of the total xylem hydraulic resistance in many species (Schulte & Gibson, 1988; Sperry et al., 2005; Wheeler et al., 2005; Choat et al., 2006; Pittermann et al., 2006). As such, it was hypothesized that a trade-off should occur between hydraulic efficiency of the xylem and the safety from embolism, both of which are strongly influenced by pit membrane porosity. Next, we will next examine the potential for trade-offs at the individual pit level: how much do changes in pit geometry, and pit membrane porosity and thickness influence the relationship between vulnerability to embolism and hydraulic conductivity?
Table Box 1 . Conductance and resistance, conductivity and resistivity
Hydraulic conductance (kh) is the volume flow rate of water (F, m3 s−1) divided by the pressure difference (ΔP, MPa) and has units of m3 s−1 MPa−1. Hydraulic conductivity (Kh) is kh normalized to the length (L, m) of the stem or root and has units of m4 s−1 MPa−1.
Hydraulic resistance (Rh) is the reciprocal of kh and has units of MPa s m−3. Hydraulic resistivity (rh) is the reciprocal of conductivity with units of MPa s m−4. Resistivity is sometimes normalized to conduit length and cross-section area with units of MPa s m−2.
For pits, we normalize resistance to surface area (m2) of pitted wall to give the area-specific resistance (rp) in units of MPa s m−1.
Resistances are additive in series and are therefore useful for understanding how much different components of a system contribute to the overall resistance. In this section we use both resistances and conductance as convenient for the matter being discussed.
1. The influence of pit-level hydraulic parameters
Measurement of hydraulic characteristics at the individual pit level is extremely difficult because of the small size of pits. Likewise, mathematical modeling of pit membrane hydraulics is inherently difficult because of their spatially complex nature. In homogeneous pit membranes, pores are not likely to be straight paths through the membrane, but rather tortuous pathways through the interstices of multiple microfibril layers (Schmid & Machado, 1968). Mathematical models of angiosperm pit membrane hydraulic resistance based on pore sizes predicted from vulnerability to embolism of each species yielded values from 0.1–30.0 MPa s m−1 for rp of homogeneous membranes (Sperry & Hacke, 2004) and 0.14–0.50 MPa s m−1 for species with torus–margo membranes (Hacke et al., 2004). However, empirical measurements of rp from a variety of methods are generally much greater than modeled values (Fig. 5a).
The largest body of data for rp in homogeneous pit membranes comes from the work of Wheeler et al. (2005) and Hacke et al. (2006), who estimated rp by subtracting lumen resistance from total resistance. These experiments yielded values of 30–2040 MPa s m−1 across 29 angiosperm species. Slightly higher values (2.56–5.32 × 103 MPa s m−1) were obtained by Choat et al. (2006) by measurements on individual vessels in two ring-porous species. Schulte & Gibson (1988) estimated rp in stems and petioles of several species by measuring hydraulic resistance before and after pit membranes had been dissolved with cellulase. These measurements gave a relatively low range of rp between 1.0 and 28.8 MPa s m−1. The only value available for roots comes from Peterson & Steudle (1993), who estimated an rp of 8.33 × 104 MPa s m−1 from root pressure probe measurements of pit membranes in maize. Thus values of rp have shown a large range both within a single study and between studies using different techniques.
2. Torus–margo vs homogeneous pit membranes
One of the most obvious differences in pit membrane hydraulic characteristics across species is between homogeneous membranes and torus–margo membranes. As with homogeneous pit membranes, the resistance of torus–margo membranes estimated from physical and mathematical models falls at the low end of values obtained empirically. Using physical models, Lancashire & Ennos (2002) calculated the rp of torus–margo pits of Tsuga canadensis at 0.4 MPa s m−1, which would contribute 29% of total resistance for a ‘typical’ tracheid. Mathematical models of torus–margo pits predicted very similar values (0.14–0.5 MPa s m−1) for a range of gymnosperm species (Hacke et al., 2004). However, empirical measurements indicated that rp values were much higher than this, ranging from 0.2–20 MPa s m−1, with a mean of 5.7 MPa s m−1 (Pittermann et al., 2006). Once again, the difference in modeled and measured values could be attributed to the complex geometry of the flow pathway through pit membranes. However, measured values of rp for torus–margo membranes are still far below measured values for angiosperm membranes (Pittermann et al., 2005). The much lower resistance of torus–margo pits results from the very porous margo region, and provides gymnosperms with a large advantage in pit hydraulic efficiency over species with homogeneous pit membranes. Pittermann et al. (2005) proposed that the development of torus–margo membranes in gymnosperms may be analogous to the development of vessels in angiosperms (analogous to the development of perforation plates). Thus, while the evolution of vessels in angiosperms species greatly increased efficiency by reducing the frequency of pit crossings, the torus–margo pit membrane provides an increase in efficiency in the xylem tissue of gymnosperms, which is composed entirely of much shorter tracheids (Pittermann et al., 2005).
3. Influence of pit chamber and pit membrane dimensions
The dimensions of the pit canal and pit chamber are quite variable: species with thick secondary conduit walls have much longer pit canals than those with thin walls. However, for both homogeneous and torus–margo membranes, the measured pit resistances are much larger than resistances calculated for the pit canal. When resistance is calculated on the individual pit level (Rind), the pit canal resistance typically represents < 5% of total resistance (Gibson et al., 1985; Wheeler et al., 2005; Choat et al., 2006; Pittermann et al., 2006). For instance, the resistance of a single pit of F. americana is 4.26 × 1014 MPa s m−3 compared with calculated resistance of a pit canal of 3.68 × 1011 MPa s m−3, more than 1000 times smaller (Choat et al., 2006). Therefore it appears that the hydraulic efficiency of pits is governed primarily by the structure of the pit membrane. This makes sense given the very small-diameter pores found in pit membranes compared with the size of the pit aperture and pit canal. The hydraulic resistance of individual pits is therefore dictated primarily by the average porosity and thickness of the membrane. The findings of Choat et al. (2006) show that rp appears to scale with pit membrane thickness; however, data were obtained for only two species, and a larger data set is required to test this hypothesis. Careful comparison of anatomical observations and measurement of rp will be required to elucidate how pit-level changes in structure influence hydraulic characteristics.
4. The balance of pit and lumen resistance
Thus far we have focused on structural and functional variation at the pit level. However, when examining functional relationships at the individual pit level, the impact of tissue-level differences in structure are neglected; the total xylem hydraulic resistance (Rtot) is determined by the dimensions and arrangement of conduits as well as the area-specific resistance of pits (Fig. 6). The total resistance can be broken into separate terms for lumen and pit resistances, which are additive in series by Ohm's law analogy. Given the length (L) and diameter (D) of a conduit, the resistance of the lumen (Rlum) can be calculated using the Hagen–Poiseuille equation:
where η is the viscosity of water at 20°C (1.002 × 10−9 MPa s). Thus Rlum is particularly sensitive to changes in the diameter of conduits, with resistance decreasing with the fourth power of diameter. The Hagen–Poiseuille equation has been shown to provide a close approximation of the resistance in open vessels with simple perforation plates (Zwieniecki et al., 2001a). However, irregularities in the shape and sculpturing of vessels such as scalariform perforation plates and tapering at vessel ends may contribute to the often substantial deviations of empirical measurements from theoretical estimates (Schulte et al., 1989; Lewis & Boose, 1995).
The resistance of pits (Rpit) is determined by rp and the surface area of overlap between vessels (Apit):
The total resistance is then calculated as:
Thus Rlum will increase with increasing conduit length; however, the total resistivity (Rtot/L) should decrease because there will be fewer end-wall crossings per unit length. For Rpit, resistance will decrease with thinner, more porous pit membranes (low rp) and as the surface area of pit membrane connecting vessels increases. Thus xylem tissue with a high degree of connectivity between vessels will minimize the resistance caused by pit membranes.
The contribution of pit resistance to total resistivity has been estimated in a range of angiosperm, conifer and fern species using a variety of techniques (Fig. 5). From these experiments, it appears that the contribution of pit resistance is highly variable across species, ranging from 12–91% of total xylem resistivity. Because of this variation, the principal methods used to measure pit resistance bear further discussion. The most common method of estimating the contribution of pits to xylem resistivity is simple subtraction: Rlum is estimated from conduit diameters using the Poiseuille equation, the resistance through the xylem is measured empirically, and the difference is ascribed to the pit membranes (Sperry et al., 2005; Wheeler et al., 2005; Hacke et al., 2006; Pittermann et al., 2006). This technique should provide robust estimates of tissue-level parameters provided that resistance components other than pit and lumen resistance (irregularity of vessel form) are not a large factor. This method also requires working through many averages to scale down to the pit-level parameters. Another technique involves the digestion of pit membranes by perfusing cellulase through xylem tissue (Calkin et al., 1986; Schulte & Gibson, 1988). This technique offers the advantage that all components of resistance are taken into account, that is, anything additional to theoretical lumen resistance. Thus the true pit resistivity can be estimated as long as all pit membranes are dissolved properly. Finally, measurements on individual vessels provide a direct estimate of pit resistance, but require scaling up through many averages to arrive at tissue-level parameters (Zwieniecki et al., 2001a; Choat et al., 2006).
Although the contribution of pits to total resistivity varies considerably, the average across species is 58% for angiosperms and 64% for conifers (Fig. 5b). The largest data sets for cross-species comparisons of pit resistance using a standard technique come from Hacke et al. (2006); Pittermann et al. (2006) for angiosperms and conifers, respectively. These studies both indicate that there is a significant linear correlation between lumen and pit resistivity: species with high lumen resistivity also tend to have high pit resistivity. The fact that pit resistivity does not become dominant in species with wide vessels suggests that the tendency of wide vessels to be longer may allow lumen resistivity to keep pace with pit resistivity (Fig. 7; Schulte & Gibson, 1988; Lancashire & Ennos, 2002).
Lancashire & Ennos (2002) demonstrated that tracheid resistance per length normalized by cross-sectional area is minimized when diameter increases with 2/3 power of the length if the density of pitting (pits per area) remains constant (Fig. 7). Thus a greater proportional increase in tracheid length relative to diameter across species could conserve the proportion of resistance contributed by pit membranes. At these optimal dimensions, pits should account for 67% of the total resistivity. This is very close to the species average for conifers of 64% found by Pittermann et al. (2006). The model of Lancashire & Ennos (2002) also suggests that the optimization of conduit dimensions places an upper limit on tracheid diameter because of the developmental constraints on tracheid length: tracheids are unicellular and appear to be limited to < 10 mm long by the cambial initials.
In angiosperms, the length of vessels is not constrained by unicellularity. Vessels are also largely free from biomechanical constraints because fibers bear much of the mechanical stress of self-support in angiosperms. In this case, xylem resistivity could be minimized by attaining vessel lengths that saturate resistivity at the Poiseuille limit, where pit resistivity becomes negligible (Fig. 7b). Pit resistivity could also be decreased by having the maximum possible overlap area between vessels. The fact that vessels have not reached these saturating lengths in plants is suggestive of a powerful constraint on vessel dimensions and connectivity. Because water is transported through the xylem under tension, it is constantly vulnerable to dysfunction. A vascular system in which there is little partitioning between vessels is also most vulnerable to dysfunction via embolism or pathogens. Recent studies have shed further light on how scaling in conduit dimensions and connectivity influences the balance between safety and efficiency in xylem structure (Choat et al., 2005; Wheeler et al., 2005; Pittermann et al., 2006; Loepfe et al., 2007).
5. Trade-offs in safety and efficiency: pit level vs tissue level
The air-seeding hypothesis states that vulnerability to water stress-induced cavitation should be related to the porosity of intervessel pit membranes. Because of the large proportion of hydraulic resistance attributed to pit membranes, one would expect a tight trade-off in the porosity of pit membranes driven by the competing requirements to reduce hydraulic resistance and to reduce the potential for air seeding between vessels. Therefore species that are vulnerable to embolism should have more porous membranes and lower pit hydraulic resistance (Sperry & Hacke, 2004). However, this relationship relies on a link between the average diameter of pores in the pit membrane, which will dictate pit membrane resistance, and the air-seeding threshold of the vessel. In fact, some studies have shown that the average porosity of pit membranes does not appear to be linked to the air-seeding threshold (Choat et al., 2003, 2004). Instead, vulnerability to embolism may be related to rare, large pores that occurred in only a few membranes. Because hydraulic resistance would be less affected by the presence of a few large pores, no trade-off would be expected between vulnerability to embolism and hydraulic resistance of pits.
Wheeler et al. (2005) provided empirical evidence for this theory, showing that rp was not correlated with average cavitation pressure across 16 vessel-bearing species. Instead, vulnerability to embolism was correlated with the average area of pit membrane per vessel. Wheeler et al. (2005) hypothesized that the chance of having a large pore between two vessels increases stochastically with increasing pit membrane area per vessel. In contrast, the average porosity, and therefore rp, would not necessarily change with increasing area. This theory, the ‘pit-area hypothesis’, was later extended to 29 vessel-bearing species with a wide range of xylem anatomies (Hacke et al., 2006).
Hydraulic resistance at the tissue level is strongly influenced by the area of pit membrane between two vessels: as the area of overlap increases, pit resistance falls (equation 4). Because the area of overlap tends to be larger in vessels of greater size (diameter and length), this provides a basis for the relationship between vessel diameter and vulnerability to embolism observed within a single plant (Lo Gullo & Salleo, 1991; Hargrave et al., 1994), and also for the weak relationship between vessel diameter and vulnerability to embolism across species (Tyree & Zimmermann, 2002). However, it is important to emphasize that the relationship between vessel size and vulnerability is not sufficient to make predictions across species: a species with narrow vessels may still be vulnerable to embolism if the overlap between vessels is large. Another, simpler link between vessel dimensions and vulnerability to embolism is that species with larger vessels will suffer a greater proportional loss of conductivity for each vessel lost. Therefore increases in conduit dimensions may worsen both the probability of air seeding between vessels, because of the increased pit area, and the consequences of each air-seeding event.
One prediction of the pit-area hypothesis is that pit membrane porosity is ‘generic’ among vessel-bearing species (Sperry et al., 2006). However, recent anatomical studies have revealed significant variation in the thickness and porosity of pit membranes (Sano, 2005; Jansen et al., 2007; Schmitz et al., 2007; Choat et al., unpublished). The order of magnitude variation in pit membrane thickness observed could translate to a significant difference in rp, although changes in resistance could be less than proportional to thickness in very thin pit membranes (see Loudon & McCulloh, 1999). Additionally, while it is not certain that pore sizes observed with SEM represent the pores of hydrated membranes, it is easy to imagine that differences in porosity may be indicative of the propensity for large pores to develop under natural conditions. Thus species with thinner membranes probably have lower hydraulic resistance and a greater chance of developing a large pore or rupture for a given membrane area. This raises the question as to how much variation in pit structure may influence the safety vs efficiency trade-off. Although pit area is well correlated with cavitation pressure, there is a large range of cavitation pressures for a given average pit area (see Fig. 7 in Hacke et al., 2006). This spread of values for a given pit area may be caused by differences in pit membrane structure: for a given average pit area, a species with thinner or more porous membranes will have a lower cavitation threshold.
It is worth noting that, mechanistically, both the intrinsic characteristics of pit membranes and the pit area between conduits must play a role in susceptibility to embolism, so the pit-area and pit-resistance hypotheses are not mutually exclusive alternatives. Instead, the question is to what extent variation in vulnerability across taxa can be attributed to variation in pit-level vs tissue-level properties. These hypotheses can be seen as addressing the underlying question of why or how different species achieve different vulnerabilities to cavitation. The results of Wheeler et al. (2005); Hacke et al. (2006) indicate that, in general, the selective pressures that act to match the vulnerability of angiosperm species to their environment have been satisfied mostly by shifts in pit area, rather than microscopic pit structure. However, further measurements that provide finer detail of pit membrane function and the connectivity between vessels are necessary to confirm this.
In gymnosperms, it appears that the average pit area is a less important factor (Pittermann et al., 2006; Sperry et al., 2006). Instead, Pittermann et al. (2006) showed a correlation between pit hydraulic resistance and cavitation pressure for northern hemisphere conifers, although the relationship was obscured when southern hemisphere conifers were included. Domec et al. (2006) found a strong relationship between P50 and pit resistance within a single Douglas fir tree. These results indicate that the trade-off between safety and efficiency is much stronger at the pit level in margo–torus membranes than in homogeneous membranes. A denser margo meshwork would provide greater protection against stretching and rupture of the pit membrane, but would also increase pit hydraulic resistance. It is clear that the relationship between margo porosity and cavitation pressure is complicated by other factors, such as depth of pit chamber (distance that the margo is stretched), pit aperture diameter and thickness of microfibril strands. Further anatomical work is required to confirm the importance of various factors involved in this relationship.
6. Three-dimensional analysis of xylem network
Thus far, the vast majority of analyses and models have examined xylem structure in two dimensions. While these analyses are useful in understanding partitioning of the xylem resistances, they overlook the complex and convoluted nature of many xylem networks (Zimmermann & Tomlinson, 1966; Burggraaf, 1972; Kitin et al., 2004) and the effects of the three-dimensional arrangement of xylem conduits on the transport of water and the propagation of embolism (Loepfe et al., 2007). In a simulation of a 3D xylem vessel network, Loepfe et al. (2007) found that hydraulic conductivity was lower than would be predicted from the sum of pit and lumen resistance. This suggests that some network-level properties are also contributing to the overall resistance of the vascular system, consistent with the results of Calkin et al. (1986), who found that conductance of the fern Pteris vittata was still only 69% of theoretical conductance for lumens and pit cavities even after pit membranes were dissolved. The discrepancy could have arisen from the only partial degradation of some pit membranes, but also from the effects of vessel topology. Consistent with the pit-area hypothesis, Loepfe et al. (2007) nominated connectivity of the system (over area of pits) as one of the most important factors determining vascular efficiency and safety. The influence of connectivity can also be seen in work on the sectoriality of plants showing that plants with highly sectored xylem are more tolerant to drought (Zanne et al., 2006). The topology of vessels is also important in terms of the propagation of embolism: vessels with large pit pores will not air seed unless neighboring vessels are air-filled.
V. Impacts of pit function on whole-plant hydraulics
The intrinsic properties of pit membranes arise at a scale of tens of nanometers, yet their effects on hydraulics are mediated by every level of vascular organization in the plant. Many of the allometric variables controlling the connection between pit properties and whole-plant function vary in concert throughout the plant: conduit diameter and length, for example, tend to be smaller further down the transpiration stream (Becker et al., 2003). An accurate picture of how pit structure and function affect the hydraulic behavior of a whole plant requires some consideration of the complexity of the xylem and of within-plant variation in structure.
The efforts of Zimmermann (1983) to understand whole-plant hydraulic function have inspired a wealth of research on the variation in xylem structure throughout plants, what he termed ‘hydraulic architecture’. Most work on hydraulic architecture has relied on anatomical measurements of conduit dimensions, which allow an estimate of the theoretical resistance of an organ but provide no direct information about how the resistance contribution of pits might vary throughout a plant, nor of how the dynamic between efficiency of transport and containment of embolism might shift in different parts of the plant.
1. Whole-plant transport models
Fractal branching models were suggested as a way of characterizing the large number of functionally significant parameters that covary along the transpiration stream (West et al., 1999), often according to power laws (Weitz et al., 2006). Becker et al. (2003) showed that it was possible to incorporate a resistive contribution from pits into a fractal branching model in several ways while leaving many of the predictions of the model intact, although empirical data on how the contribution of pits might vary throughout the plant are lacking. Other frameworks for understanding within-plant variation in xylem structure have been proposed that do not involve self-similar (fractal) branching, such as Murray's law, which predicts a preservation of the sum of conduit radii cubed before and after junctions, but makes no claims about what happens to conduit number (McCulloh et al., 2003). For the purpose of understanding whole-plant hydraulics, any model that accurately describes the changes in xylem structure along the transpiration stream must take into account empirical evidence about possible changes in the importance of pit resistance along the hydraulic pathway.
Lancashire & Ennos (2002) produced a scheme (length proportional to radius to the 3/2) by which the proportion of resistance residing in end walls would be constant with respect to conduit size (Fig. 7), in which case the contribution of end resistance can easily be accommodated in a whole-plant transport model by multiplying the resistance of each axis by a constant factor. However, there are not yet sufficient data to indicate whether the proportion of resistance accounted for by pits is constant throughout individual plants, nor whether a 3/2-power allometric scaling between conduit length and diameter holds within individuals.
2. Ion-mediated changes in pit hydraulic resistance
In bench-top measurements, hydraulic conductivity of angiosperm xylem increases with the ionic concentration of perfusing fluid (Zimmermann, 1983; Van Ieperen et al., 2000; Zwieniecki et al., 2001b). When a segment is perfused with deionized water, the conductivity begins a long and steady decline. The addition of ionic salts such as K+ or Ca2+ to the perfusate produces an immediate increase in conductivity. Zimmermann (1983) suggested that the drop in conductance with time could be related to small air bubbles or swelling of pit membranes; he noted that it could be reversed with potassium chloride solution. Zwieniecki et al. (2001b) provided strong evidence that ion-mediated changes in conductance were associated with pits, and proposed that the pectin polysaccharide matrix of pit membranes exhibited a swelling and shrink (hydrogel) behavior in response to ionic concentration of the xylem sap or perfusing fluid. The change in conductivity is rapid and reversible, and can alter hydraulic conductivity up 2.5-fold, although the magnitude of the response varies between taxa (Zwieniecki et al., 2001b; Boyce et al., 2004). No such effect has been observed in conifers (Zimmermann, 1983), perhaps because the large pores of the margo do not change significantly in size with shrinking and swelling of hydrogels.
The hydrogel response provides a mechanism by which distribution of water through the xylem network may be modified in response to short-term environmental fluctuations experienced by the plant, perhaps allowing more water to be taken up from roots in more nutrient-rich regions of soil, for example. This would be of particular benefit for large trees, allowing for a dynamic response to local variation in canopy microclimate (Zwieniecki et al., 2004; Choat et al., 2006). However, the challenge remains to demonstrate that the hydrogel response plays a significant role in planta. Some researchers have suggested that changes in the concentration of a single ionic species would have little effect against the background of other mono- and divalent cations found in the xylem, and further, that Ca2+ may have an inhibitory effect on the action of K+ ions (Van Ieperen, 2007). Future efforts should focus on careful measurement of temporal changes in xylem sap properties (ionic concentration, pH, osmostic pressure) in relation to microenvironment. Variation in xylem sap ionic concentration must then be linked with changes in the hydraulic conductance of the xylem in planta, utilizing techniques that allow the measurement of pressure gradients and flow rates within a transpiring plant.
VI. Future directions
• Anatomical observations of hydrated pit membranes, using new methods such as AFM and deep etching coupled with immunolabelling methods, should provide information on the cell wall ultrastructure and chemical composition at the molecular level. Further work on the composition and surface chemistry of pit membranes is also required to understand their interaction with gas–liquid interfaces and xylem pathogens as well as ion-mediated changes in rp.
• We need to know much more about pit development and try to identify the cytological, biochemical and molecular markers for pit differentiation. Which genes encode these developmental processes? Linking genetics with pit characteristics will help us to understand to the evolution of functional traits such as vulnerability to embolism.
• Three-dimensional mapping of wood structure by X-ray-computed tomography scanning or serial sectioning is essential to understanding pit and xylem function in the context of a complex vascular network. This is especially important in determining the overlap areas and true lengths of vessels.
• Future work should incorporate more direct measurement of pit hydraulic characteristics across species. These measurements should be combined with careful anatomical observations to tease apart the roles of pit- and tissue-level factors involved in safety–efficiency trade-offs.
• Measurement of pit structure and function in different organs and height positions within a tree should establish whether the contribution of pits to total vascular resistance changes along the plant axis, as well as the relationship of pit resistance to conduit allometry.
• This work should culminate in our understanding of how pit structure and function influence whole-plant water transport and leaf gas exchange. Ultimately, this will allow us to understand the adaptive value of changes in pit and pit membrane structure and the role of bordered pits in the evolution of vascular plants.
We thank Jarmila Pittermann and Jim Wheeler for valuable discussions and feedback. We also thank Stephanie Stuart for editorial assistance during the preparation of the manuscript.