Vascular plants transport water under negative pressure without constantly creating gas bubbles that would disable their hydraulic systems. Attempts to replicate this feat in artificial systems almost invariably result in bubble formation, except under highly controlled conditions with pure water and only hydrophilic surfaces present. In theory, conditions in the xylem should favor bubble nucleation even more: there are millions of conduits with at least some hydrophobic surfaces, and xylem sap is saturated or sometimes supersaturated with atmospheric gas and may contain surface-active molecules that can lower surface tension. So how do plants transport water under negative pressure? Here, we show that angiosperm xylem contains abundant hydrophobic surfaces as well as insoluble lipid surfactants, including phospholipids, and proteins, a composition similar to pulmonary surfactants. Lipid surfactants were found in xylem sap and as nanoparticles under transmission electron microscopy in pores of intervessel pit membranes and deposited on vessel wall surfaces. Nanoparticles observed in xylem sap via nanoparticle-tracking analysis included surfactant-coated nanobubbles when examined by freeze-fracture electron microscopy. Based on their fracture behavior, this technique is able to distinguish between dense-core particles, liquid-filled, bilayer-coated vesicles/liposomes, and gas-filled bubbles. Xylem surfactants showed strong surface activity that reduces surface tension to low values when concentrated as they are in pit membrane pores. We hypothesize that xylem surfactants support water transport under negative pressure as explained by the cohesion-tension theory by coating hydrophobic surfaces and nanobubbles, thereby keeping the latter below the critical size at which bubbles would expand to form embolisms.

Wood density plays a key role in ecological strategies and life history variation in woody plants, but little is known about its anatomical basis in shrubs. We quantified the relationships between wood density, anatomy, and climate in 61 shrub species from eight field sites along latitudinal belts between 31° and 35° in North and South America. Measurements included cell dimensions, transverse areas of each xylem cell type and percentage contact between different cell types and vessels. Wood density was more significantly correlated with precipitation and aridity than with temperature. High wood density was achieved through reductions in cell size and increases in the proportion of wall relative to lumen. Wood density was independent of vessel traits, suggesting that this trait does not impose conduction limitations in shrubs. The proportion of fibers in direct contact with vessels decreased with and was independent of wood density, indicating that the number of fiber-vessel contacts does not explain the previously observed correlation between wood density and implosion resistance. Axial and radial parenchyma each had a significant but opposite association with wood density. Fiber size and wall thickness link wood density, life history, and ecological strategies by controlling the proportion of carbon invested per unit stem volume.

Pit membranes in bordered pits between neighbouring vessels play a major role in the entry of air-water menisci from an embolised vessel into a water-filled vessel (i.e., air-seeding). Here, we investigate intervessel pit membrane thickness (TPM) and embolism resistance (P50, i.e., the water potential corresponding to 50% loss of hydraulic conductivity) across a broad range of woody angiosperm species. Data on TPM and double intervessel wall thickness (TVW) were compiled based on electron and light microscopy. Fresh material that was directly fixated for transmission electron microscopy (TEM) was investigated for 71 species, while non-fresh samples were frozen, stored in alcohol, or air dried prior to TEM preparation for an additional 60 species. TPM and P50 were based on novel observations and literature. A strong correlation between TPM and P50 was found for measurements based on freshly fixated material (r = 0.78, P >0.01, n = 37), and between TPM and TVW (r = 0.79, P >0.01, n = 59), while a slightly weaker relationship occurred between TVW and P50 (r = 0.40, P >0.01, n = 34). However, non-fresh samples showed no correlation between TPM and P50, and between TPM and TVW. Intervessel pit membranes in non-fresh samples were c.28% thinner and more electron dense than fresh samples. Our findings demonstrate that TPM measured on freshly fixated material provides one of the strongest wood anatomical correlates of droughtinduced embolism resistance in angiosperms. Assuming that cellulose microfibrils show an equal spatial density, TPM is suggested to affect the length and the shape of intervessel pit membrane pores, but not the actual pore size. Moreover, the shrinking effect observed for TPM after dehydration and frost is associated with an increase in microfibril density and porosity, which may provide a functional explanation for embolism fatigue.

Both engineered hydraulic systems and plant hydraulic systems are protected against failure by resistance, reparability, and redundancy. A basic rule of reliability engineering is that the level of independent redundancy should increase with increasing risk of fatal system failure. Here we show that hydraulic systems of plants function as predicted by this engineering rule. Hydraulic systems of shrubs sampled along two transcontinental aridity gradients changed with increasing aridity from highly integrated to independently redundant modular designs. Shrubs in humid environments tend to be hydraulically integrated, with single, round basal stems, whereas dryland shrubs typically have modular hydraulic systems and multiple, segmented basal stems. Modularity is achieved anatomically at the vessel-network scale or developmentally at the whole-plant scale through asymmetric secondary growth, which results in a semiclonal or clonal shrub growth form that appears to be ubiquitous in global deserts.plant hydraulic systems ͉ wood anatomy ͉ hydraulic redundancy ͉ xylem structure and function I n engineering terms, the hydraulic system of a plant is a negative-pressure flow system. This type of hydraulic system, whether natural or man-made, is prone to fail when air bubbles (emboli) are introduced, because under strong negative pressure a single embolism can lead to breakage of the water column unless the air bubble is isolated in a branch or pipe. Both drought and freezing can cause embolisms in plants (1).Drought-induced embolisms form under negative pressure, when air is pulled into a water-filled conduit from adjacent air-filled spaces or cells, a process known as ''air seeding.'' This common, even daily, event (2-4) can lead to complete failure of the hydraulic system if runaway embolism occurs (5). Two of the three attributes by which plants' negative-pressure flow systems can be protected against failure, resistance and reparability, have been subjects of active research during the last decade (2-4, 6-10). The third attribute, redundancy, has received much less attention as an important drought adaptation but is emerging as a focus of research (11)(12)(13)(14). Attributes of redundancy in hydraulic systems of vessel-bearing angiosperms include the numbers of vessels (14), the vessel network topology (12), the number and sizes of pits between adjacent vessels (13,15,16), and the division of whole plants into independent hydraulic units (17).A basic rule of reliability engineering states that the level of independent redundancy should increase with increasing risk of fatal system failure (18); hydraulic engineers routinely increase the safety of man-made pressure-flow systems by designing them to be redundant (19). Redundancy in hydraulic systems (Fig. 1) can vary from a high degree of inter-connectedness (i.e., integrated redundancy) to complete, independent compartmentation (i.e., modular redundancy). In a negative-pressure flow system, integrated redundancy allows alternate water transport pathways around blockage...

Plant xylem is a unique evolutionary invention: It functions as a negative pressure hydraulic system that can move vast quantities of water and solutes over distances longer than 100 m. No other kind of organism transports liquids under negative pressure, and the most successful attempts to build an artificial negative pressure hydraulic system succeeded only to move a small amount of water over a few centimeters in a single microchannel (Wheeler and Stroock, 2008). That experiment proved that it is possible to move water under negative pressure, but it left the question unanswered how plants can do this so effectively and over such long distances. Somehow plants manage to move large volumes of water in heterogeneous channels that range in diameter from nanometers to a few hundred micrometers, that is, in nanofluidic and microfluidic systems where water moves very close to surfaces of other phases, both solid and gas (Eijkel and van den Berg, 2005;Squires and Quake, 2005). Plants move all this water efficiently along these phase surfaces without constantly creating gas bubbles in the system . What do we actually know about these surfaces in xylem conduits (i.e., vessels and tracheids)? INVITED SPECIAL ARTICLE PREMISE OF THE STUDY:Xylem sap in angiosperms moves under negative pressure in conduits and cell wall pores that are nanometers to micrometers in diameter, so sap is always very close to surfaces. Surfaces matter for water transport because hydrophobic ones favor nucleation of bubbles, and surface chemistry can have strong effects on flow. Vessel walls contain cellulose, hemicellulose, lignin, pectins, proteins, and possibly lipids, but what is the nature of the inner, lumen-facing surface that is in contact with sap?METHODS: Vessel lumen surfaces of five angiosperms from different lineages were examined via transmission electron microscopy and confocal and fluorescence microscopy, using fluorophores and autofluorescence to detect cell wall components. Elemental composition was studied by energy-dispersive X-ray spectroscopy, and treatments with phospholipase C (PLC) were used to test for phospholipids.KEY RESULTS: Vessel surfaces consisted mainly of lignin, with strong cellulose signals confined to pit membranes. Proteins were found mainly in inter-vessel pits and pectins only on outer rims of pit membranes and in vessel-parenchyma pits. Continuous layers of lipids were detected on most vessel surfaces and on most pit membranes and were shown by PLC treatment to consist at least partly of phospholipids. CONCLUSIONS:Vessel surfaces appear to be wettable because lignin is not strongly hydrophobic and a coating with amphiphilic lipids would render any surface hydrophilic. New questions arise about these lipids and their possible origins from living xylem cells, especially about their effects on surface tension, surface bubble nucleation, and pit membrane function.