Polar lipids are linked to nanoparticles in xylem sap of temperate angiosperm species

Xylem sap of angiosperm species has been found to include low concentrations of polar lipids and nanoparticles, including surfactant-coated nanobubbles. Although the nanoparticles have been suggested to consist of polar lipids, no attempt has been made to determine if nanoparticle and lipid concentrations are related. Here, we examined concentrations of nanoparticles and lipids in xylem sap and contamination control samples of six temperate angiosperm species with a NanoSight device and based on mass spectrometry. We found (1) that the concentration of nanoparticles and lipids were both diluted when an increasing amount of sap was extracted, (2) that their concentrations were significantly correlated in three species, (3) that their concentrations were affected by vessel anatomy, and (4) that concentrations of nanoparticles and lipids were very low in contamination-control samples. Moreover, there was little seasonal difference, no freezing-thawing effect on nanoparticles, and little seasonal variation in lipid composition. These findings indicate that lipids and nanoparticles are related to each other, and largely do not pass interconduit pit membranes. Further research is needed to examine the formation and stability of nanoparticles in xylem sap in relation to lipid composition, and the complicated interactions among the gas, liquid, and solid phases in xylem conduits.

Anatomical and blue intensity methods to determine wood density converge in contributing to explain different distributions of three palaeotropical pine species

Wood density constitutes an integrative trait of water relations and growth. We compared the recently developed blue intensity (BI) method, which has only rarely been applied to tropical conifers, for determining wood density with anatomical analyses in studying the three rarely investigated palaeotropical pine species Pinus kesiya, P. dalatensis and P. krempfii, which co-occur in South-Central Vietnam, but differ in their distribution areas. For species comparisons, we also calculated the hydraulic conductivity of the xylem with the Hagen-Poiseuille equation and the water potential causing 50% loss of hydraulic conductivity () based on the anatomical analyses. We hypothesized (i) that the BI values are correlated with the cell wall fractions, the calculated hydraulic conductivity and the values; and (ii) that the wider occurrence of P. kesiya, which also can grow at drier sites, is reflected by higher wood density, lower hydraulic conductivity, lower (more negative) values and a smaller variation in the wood anatomical features across the years compared to the other two species. In agreement to our hypotheses, the results of the BI and the anatomical method were closely correlated, especially for sapwood, and P. kesiya exhibited features that are related to the growth at drier sites and to a higher tolerance towards drought: higher wood density and cell wall:lumen area ratios of its smaller xylem conduits, lower calculated hydraulic conductivity and more negative values. The BI method is well suitable for determining the wood density in tropical conifers. As a fast and inexpensive method, it may be used for initial screening woody species for their water transport capacity and drought resistance.

Xylem embolism spread is largely prevented by interconduit pit membranes until the majority of conduits are gas-filled

Xylem embolism resistance varies across species influencing drought tolerance, yet little is known about the determinants of the embolism resistance of an individual conduit. Here we conducted an experiment using the optical vulnerability method to test whether individual conduits have a specific water potential threshold for embolism formation and whether pre-existing embolism in neighbouring conduits alters this threshold. Observations were made on a diverse sample of angiosperm and conifer species through a cycle of dehydration, rehydration and subsequent dehydration to death. Upon rehydration after the formation of embolism, no refilling was observed. When little pre-existing embolism was present, xylem conduits had a conserved, individual, embolism resistance threshold that varied across the population of conduits. The consequence of a variable conduit-specific embolism threshold is that a small degree of pre-existing embolism in the xylem results in an apparently more resistant xylem in a subsequent dehydration, particularly in angiosperms with vessels. While our results suggest that pit membranes separating xylem conduits are critical for maintaining a conserved individual embolism threshold for given conduit when little pre-exisiting embolism is present, as the percentage of embolized conduits increases, gas movement, local pressure differences, and connectivity between conduits increasingly contribute to embolism spread.

The Widened Pipe Model of plant hydraulic evolution

Shaping global water and carbon cycles, plants lift water from roots to leaves through xylem conduits. The importance of xylem water conduction makes it crucial to understand how natural selection deploys conduit diameters within and across plants. Wider conduits transport more water but are likely more vulnerable to conduction-blocking gas embolisms and cost more for a plant to build, a tension necessarily shaping xylem conduit diameters along plant stems. We build on this expectation to present the Widened Pipe Model (WPM) of plant hydraulic evolution, testing it against a global dataset. The WPM predicts that xylem conduits should be narrowest at the stem tips, widening quickly before plateauing toward the stem base. This universal profile emerges from Pareto modeling of a trade-off between just two competing vectors of natural selection: one favoring rapid widening of conduits tip to base, minimizing hydraulic resistance, and another favoring slow widening of conduits, minimizing carbon cost and embolism risk. Our data spanning terrestrial plant orders, life forms, habitats, and sizes conform closely to WPM predictions. The WPM highlights carbon economy as a powerful vector of natural selection shaping plant function. It further implies that factors that cause resistance in plant conductive systems, such as conduit pit membrane resistance, should scale in exact harmony with tip-to-base conduit widening. Furthermore, the WPM implies that alterations in the environments of individual plants should lead to changes in plant height, for example, shedding terminal branches and resprouting at lower height under drier climates, thus achieving narrower and potentially more embolism-resistant conduits.

Perspectives and design considerations of capillary-driven artificial trees for fast dewatering processes

Recent progresses on nanocapillary-driven water transport under metastable conditions have substantiated the potential of artificial trees for dewatering applications in a wide pressure range. This paper presents a comprehensive performance analysis of artificial trees encompassing the principle for negative capillary pressure generation; impacts of structural, compositional, and environmental conditions on dewatering performance; and design considerations. It begins by delineating functionalities of artificial trees for evaporation (leaves), conduction (xylem), and filtration (root) of water, in the analogy to natural trees. The analysis revealed that the magnitude of (negative) capillary pressure in the artificial leaves and xylem must be sufficiently large to overcome the osmotic pressure of feed at the root. The required magnitude can be reduced by increasing the osmotic pressure in the artificial xylem conduits, which reduces the risk of cavitation and subsequent blockage of water transport. However, a severe concentration polarization that can occur in long xylem conduits would negate such compensation effect of xylem osmotic pressure, leading to vapor pressure depression at the artificial leaves and therefore reduced dewatering rates. Enhanced Taylor dispersions by increasing xylem conduit diameters are found to alleviate the concentration polarization, allowing for water flux enhancement directly by increasing leaf-to-root membrane area ratio.

Sperry’s packing rule affects the spatial proximity but not clustering of xylem conduits: the case of Fagus sylvatica L.

Sperry’s packing rule predicts the optimum packing of xylem conduits in woody plants, where the frequency of xylem conduits varies approximately inversely with the square of the conduit radius. However, it is well established that such anatomical disposition does not remain fixed but is subject to a suite of adaptations induced by physiological constraints driven by both ontogenetic development and environmental characteristics. Here we challenge the hypothesis that increasing frequency of xylem conduits, concomitant with the decrease in their lumen area along the xylem pathway, would affect the spatial distribution of vessels inside tree-rings and their aggregation. To this end, we measured the vessels’ anatomical characteristics inside each tree-ring along with a complete radial series taken at different stem heights of Fagus sylvatica L. trees. Point pattern analysis indicated a significant effect of the distance from the tree base and a weak effect of cambial age on the nearest neighbour distance among xylem vessels, suggesting that vessels were closer to each other near the apex, and became progressively more distant toward the base. The spatial pattern of xylem vessels violated the assumption of complete spatial randomness, vessel spatial arrangement followed a uniform distribution at different distances from the tree base. Although there was an increase in the intensity and proximity among vessels, we demonstrated that no patterns of aggregation between vessels were found in sampled F. sylvatica trees. Rather, point pattern profiles clearly highlighted a lack of aggregation of vessels in the face of a regular spatial distribution in the annual growth rings along the stems.

Analysis of Non-Structural Carbohydrates and Xylem Anatomy of Leaf Petioles Offers New Insights in the Drought Response of Two Grapevine Cultivars

In grapevine, the anatomy of xylem conduits and the non-structural carbohydrates (NSCs) content of the associated living parenchyma are expected to influence water transport under water limitation. In fact, both NSC and xylem features play a role in plant recovery from drought stress. We evaluated these traits in petioles of Cabernet Sauvignon (CS) and Syrah (SY) cultivars during water stress (WS) and recovery. In CS, the stress response was associated to NSC consumption, supporting the hypothesis that starch mobilization is related to an increased supply of maltose and sucrose, putatively involved in drought stress responses at the xylem level. In contrast, in SY, the WS-induced increase in the latter soluble NSCs was maintained even 2 days after re-watering, suggesting a different pattern of utilization of NSC resources. Interestingly, the anatomical analysis revealed that conduits are constitutively wider in SY in well-watered (WW) plants, and that water stress led to the production of narrower conduits only in this cultivar.

Inferring the role of pit membranes in solute transport from solute exclusion studies in living conifer stems

The functional role of pits between living and dead cells has been inferred from anatomical studies but amassing physiological evidence has been challenging. Centrifugation methods were used to strip water from xylem conduits, permitting a more quantitative gravimetric determination of the water and solid contents of cell walls than is possible by more traditional methods. Quantitative anatomical evidence was used to evaluate the water volume in xylem conduits and the water content of living cells. Quantitative perfusion of stems with polyethylene glycol of different molecular weight was used to determine the solute-free space. We measured the portioning of water and solute-free space among anatomical components in stems and demonstrated that lignin impeded the free movement of solutes with molecular weight >300. Hence, movement of large solutes from living cells to xylem conduits is necessarily confined to pit structures that permit transmembrane solute transport via primary walls without lignin. The functional role of pits was additionally indicated by combining data in this paper with previous studies of unusual osmotic relationships in woody species that lack pits between dead wood fibers and vessels. The absence of pits, combined with the evidence of exclusion of solutes of molecular weight >300, explains the unexpected osmotic properties.