Mind the bubbles: achieving stable measurements of maximum hydraulic conductivity through woody plant samples

The maximum specific hydraulic conductivity (k(max)) of a plant sample is a measure of the ability of a plants' vascular system to transport water and dissolved nutrients under optimum conditions. Precise measurements of k(max) are needed in comparative studies of hydraulic conductivity, as well as for measuring the formation and repair of xylem embolisms. Unstable measurements of k(max) are a common problem when measuring woody plant samples and it is commonly observed that k(max) declines from initially high values, especially when positive water pressure is used to flush out embolisms. This study was designed to test five hypotheses that could potentially explain declines in k(max) under positive pressure: (i) non-steady-state flow; (ii) swelling of pectin hydrogels in inter-vessel pit membranes; (iii) nucleation and coalescence of bubbles at constrictions in the xylem; (iv) physiological wounding responses; and (v) passive wounding responses, such as clogging of the xylem by debris. Prehydrated woody stems from Laurus nobilis (Lauraceae) and Encelia farinosa (Asteraceae) collected from plants grown in the Fullerton Arboretum in Southern California, were used to test these hypotheses using a xylem embolism meter (XYL'EM). Treatments included simultaneous measurements of stem inflow and outflow, enzyme inhibitors, stem-debarking, low water temperatures, different water degassing techniques, and varied concentrations of calcium, potassium, magnesium, and copper salts in aqueous measurement solutions. Stable measurements of k(max) were observed at concentrations of calcium, potassium, and magnesium salts high enough to suppress bubble coalescence, as well as with deionized water that was degassed using a membrane contactor under strong vacuum. Bubble formation and coalescence under positive pressure in the xylem therefore appear to be the main cause for declining k(max) values. Our findings suggest that degassing of water is essential for achieving stable and precise measurements of k(max) through woody plant samples. For complete rehydration of woody samples, incubation in water under vacuum for 24 h is suggested as a reliable technique that avoids bubble problems associated with flushing under high positive pressure.

Water uptake by seminal and adventitious roots in relation to whole-plant water flow in barley (Hordeum vulgare L.)

Prior to an assessment of the role of aquaporins in root water uptake, the main path of water movement in different types of root and driving forces during day and night need to be known. In the present study on hydroponically grown barley (Hordeum vulgare L.) the two main root types of 14- to 17-d-old plants were analysed for hydraulic conductivity in dependence of the main driving force (hydrostatic, osmotic). Seminal roots contributed 92% and adventitious roots 8% to plant water uptake. The lower contribution of adventitious compared with seminal roots was associated with a smaller surface area and number of roots per plant and a lower axial hydraulic conductance, and occurred despite a less-developed endodermis. The radial hydraulic conductivity of the two types of root was similar and depended little on the prevailing driving force, suggesting that water uptake occurred along a pathway that involved crossing of membrane(s). Exudation experiments showed that osmotic forces were sufficient to support night-time transpiration, yet transpiration experiments and cuticle permeance data questioned the significance of osmotic forces. During the day, 90% of water uptake was driven by a tension of about -0.15 MPa.

Gating of aqùaporins by light and reactive oxygen species in leaf parenchyma cells of the midrib of Zea mays

Changes of the water permeability aqùaporin (AQP) activity of leaf cells were investigated in response to different light regimes (low versus high). Using a cell pressure probe, hydraulic properties (half-time of water exchange, T(1/2) infinity 1/water permeability) of parenchyma cells in the midrib tissue of maize (Zea mays L.) leaves have been measured. A new perfusion technique was applied to excised leaves to keep turgor constant and to modify the environment around cells by perfusing solutions using a pressure chamber. In response to low light (LL) of 200 micromol m(-2) s(-1), T(1/2) decreased during the perfusion of a control solution of 0.5 mM CaCl(2) by a factor of two. This was in line with earlier results from leaf cells of intact maize plants at a constant turgor. In contrast, high light (HL) at intensities of 800 micromol m(-2) s(-1) and 1800 micromol m(-2) s(-1) increased the T(1/2) in two-thirds of cells by factors of 14 and 35, respectively. The effects of HL on T(1/2) were similar to those caused by H(2)O(2) treatment in the presence of Fe(2+), which produced *OH (Fenton reaction; reversible oxidative gating of aquaporins). Treatments with 20 mM H(2)O(2) following Fe(2+) pre-treatments increased the T(1/2) by a factor of 30. Those increased T(1/2) values could be partly recovered, either when the perfusion solution was changed back to the control solution or when LL was applied. 3mM of the antioxidant glutathione also reversed the effects of HL. The data suggest that HL could induce reactive oxygen species (ROS) such as *OH, and they affected water relations. The results provide evidence that the varying light climate adjusts water flow at the cell level; that is, water flow is maximized at a certain light intensity and then reduced again by HL. Light effects are discussed in terms of an oxidative gating of aquaporins by ROS.

Tradeoffs between hydraulic and mechanical stress responses of mature Norway spruce trunk wood

We tested the effects of growth characteristics and basic density on hydraulic and mechanical properties of mature Norway spruce (Picea abies (L.) Karst.) wood from six 24-year-old clones, grown on two sites in southern Sweden differing in water availability. Hydraulic parameters assessed were specific hydraulic conductivity at full saturation (ks100) and vulnerability to cavitation (Psi50), mechanical parameters included bending strength (sigma b), modulus of elasticity (MOE), compression strength (sigma a) and Young's modulus (E). Basic density, diameter at breast height, tree height, and hydraulic and mechanical parameters varied considerably among clones. Clonal means of hydraulic and mechanical properties were strongly related to basic density and to growth parameters across sites, especially to diameter at breast height. Compared with stem wood of slower growing clones, stem wood of rapidly growing clones had significantly lower basic density, lower sigma b, MOE, sigma a and E, was more vulnerable to cavitation, but had higher ks100. Basic density was negatively correlated to Psi50 and ks100. We therefore found a tradeoff between Psi50 and ks100. Clones with high basic density had significantly lower hydraulic vulnerability, but also lower hydraulic conductivity at full saturation and thus less rapid growth than clones with low basic density. This tradeoff involved a negative relationship between Psi50 and sigma b as well as MOE, and between ks100 and sigma b, MOE and sigma a. Basic density and Psi50 showed no site-specific differences, but tree height, diameter at breast height, ks100 and mechanical strength and stiffness were significantly lower at the drier site. Basic density had no influence on the site-dependent differences in hydraulic and mechanical properties, but was strongly negatively related to diameter at breast height. Selecting for growth may thus lead not only to a reduction in mechanical strength and stiffness but also to a reduction in hydraulic safety.

Hydraulic disruption and passive migration by a bacterial pathogen in oak tree xylem

Xylella fastidiosa (Xf) is a xylem-limited bacterial pathogen that causes leaf scorch symptoms in numerous plant species in urban, agricultural, and natural ecosystems worldwide. The exact mechanism of hydraulic disruption and systemic colonization of xylem by Xf remains elusive across all host plants. To understand both processes better, the functional and structural characteristics of xylem in different organs of both healthy and Xf-infected trees of several Quercus species were studied. Hydraulic conductivity (K(s)) in Xf-infected petioles of Q. palustris and Q. rubra decreased significantly compared with healthy trees as the season progressed and plummeted to zero with the onset of scorch symptoms. Prior to the onset of symptoms, embolism was as much as 3.7 times higher in Xf-infected petioles compared with healthy controls and preceded significant reductions in K(s). Embolism likely resulted from pit membrane degradation during colonization of new petiole xylem and triggered the process that eventually led to vessel occlusion. Pit membrane porosity was studied using the following four methods to determine if a pathway exists in the xylem network of woody stems that allows for passive Xf migration: (i) calculations based on vulnerability to cavitation data, (ii) scanning electron micrographs, (iii) microsphere injections, and (iv) air seeding thresholds on individual vessels. All four methods consistently demonstrated that large pit membrane pores (i.e. greater than the diameter of individual Xf) occur frequently throughout the secondary stem xylem in several Quercus species. These large pores probably facilitate systemic colonization of the secondary xylem network and contribute to the high susceptibility to bacterial leaf scorch exhibited among these species.

Hydraulic and mechanical properties of young Norway spruce clones related to growth and wood structure

Stem segments of eight five-year-old Norway spruce (Picea abies (L.) Karst.) clones differing in growth characteristics were tested for maximum specific hydraulic conductivity (k(s100)), vulnerability to cavitation and behavior under mechanical stress. The vulnerability of the clones to cavitation was assessed by measuring the applied air pressure required to cause 12 and 50% loss of conductivity (Psi(12), Psi(50)) and the percent loss of conductivity at 4 MPa applied air pressure (PLC(4MPa)). The bending strength and stiffness and the axial compression strength and stiffness of the same stem segments were measured to characterize wood mechanical properties. Growth ring width, wood density, latewood percentage, lumen diameter, cell wall thickness, tracheid length and pit dimensions of earlywood cells, spiral grain and microfibril angles were examined to identify structure-function relationships. High k(s100) was strongly and positively related to spiral grain angle, which corresponded positively to tracheid length and pit dimensions. Spiral grain may reduce flow resistance of the bordered pits of the first earlywood tracheids, which are characterized by rounded tips and an equal distribution of pits along the entire length. Wood density was unrelated to hydraulic vulnerability parameters. Traits associated with higher hydraulic vulnerability were long tracheids, high latewood percentage and thick earlywood cell walls. The positive relationship between earlywood cell wall thickness and vulnerability to cavitation suggest that air seeding through the margo of bordered pits may occur in earlywood. There was a positive phenotypic and genotypic relationship between k(s100) and PLC(4MPa), and both parameters were positively related to tree growth rate. Variability in mechanical properties depended mostly on wood density, but also on the amount of compression wood. Accordingly, hydraulic conductivity and mechanical strength or stiffness showed no tradeoff.

Water relations of the pine exine

BACKGROUND AND AIMS

Water adhesion forces, water absorption capacity and permeability of the pine exine were investigated to consider a possible function of sporopollenin coatings in the control of water transport.

METHODS

The experiments were carried out with sporopollenin capsules obtained from pine pollen consisting of an empty central capsule and two sacci. Changes in the concentration of excluded dextran molecules in the medium were analysed to quantify water absorption by purified exine fragments and the osmotic volume flow out of the intact central capsule.

KEY RESULTS

The contact angle of sporopollenin to water is higher than the one to ethanol and lower than the one to n-heptane. The water-filled pore space in pine sporopollenin amounts to only 20.6 % of the matrix volume. A monosaccharide was excluded from 15 % and a trisaccharide from about 38 % of this space. Shrinkage of the central capsule induced by permeable osmotica was transient, whereas that induced by sodium polyacrylate (2100 g mol(-1)) was stable. Values obtained for the hydraulic conductance L(P) of the exine (0.39-0.48 microm s(-1) MPa(-1)) are comparable in size to those of biomembranes. Sodium sulfate solutions induced a significant osmotic flow through the exine (reflection coefficient at least 0.6). The exine around the central capsule can be ruptured by equilibration of its lumen with a concentrated electrolyte solution and subsequent transfer to water. The denatured protoplast along with the intact intine was ejected when pollen grains were subjected to this osmotic shock treatment.

CONCLUSIONS

The pine exine is easily wetted with water and does not represent a significant barrier to water exchange either liquid or gaseous. Through osmotic burst, it can be separated from the intine. The effect of salts and small solute molecules on water fluxes may be functionally significant for rehydration upon pollination.

Variation in xylem structure and function in stems and roots of trees to 20 m depth

•  To assess hydraulic architecture and limitations to water transport across whole trees, we compared xylem anatomy, vulnerability to cavitation (Ψ₅₀ ) and specific hydraulic conductivity (K_s ) of stems, shallow roots and deep roots (from caves to 20 m depth) for four species: Juniperus ashei, Bumelia lanuginosa, Quercus fusiformis and Quercus sinuata. •  Mean, maximum and hydraulically weighted (D_h ) conduit diameters and K_s were largest in deep roots, intermediate in shallow roots, and smallest in stems (P < 0.05 for each). Mean vessel diameters of deep roots were 2.1-4.2-fold greater than in stems, and K_s was seven to 38 times larger in the deep roots. •  Ψ₅₀ also increased from stems to roots with depth, as much as 24-fold from stems to deep roots in B. lanuginosa. For all species together, Ψ₅₀ was positively correlated with both D_h and K_s , suggesting a potential trade-off exists between conducting efficiency and safety. •  The anatomical and hydraulic differences documented here suggest that the structure of deep roots minimizes flow resistance and maximizes deep water uptake.

The role of aquaporins in root water uptake

The capacity of roots to take up water is determined in part by the resistance of living tissues to radial water flow. Both the apoplastic and cell-to-cell paths mediate water transport in these tissues but the contribution of cell membranes to the latter path has long been difficult to estimate. Aquaporins are water channel proteins that are expressed in various membrane compartments of plant cells, including the plasma and vacuolar membranes. Plant aquaporins are encoded by a large multigene family, with 35 members in Arabidopsis thaliana, and many of these aquaporins show a cell-specific expression pattern in the root. Mercury acts as an efficient blocker of most aquaporins and has been used to demonstrate the significant contribution of water channels to overall root water transport. Aquaporin-rich membranes may be needed to facilitate intense water flow across root tissues and may represent critical points where an efficient and spatially restricted control of water uptake can be exerted. Roots, in particular, show a remarkable capacity to alter their water permeability over the short term (i.e. in a few hours to less than 2-3 d) in response to many stimuli, such as day/night cycles, nutrient deficiency or stress. Recent data suggest that these rapid changes can be mostly accounted for by changes in cell membrane permeability and are mediated by aquaporins. Although the processes that allow perception of environmental changes by root cells and subsequent aquaporin regulation are nearly unknown, the study of root aquaporins provides an interesting model to understand the regulation of water transport in plants and sheds light on the basic mechanisms of water uptake by roots.

Compression wood has little impact on the water relations of Douglas-fir (Pseudotsuga menziesii) seedlings despite a large effect on shoot hydraulic properties

• Compression wood has been shown to reduce stem permeability, but it is not known to what extent it affects leaf-level processes. Here, we report whole-plant hydraulic properties of Douglas-fir (Pseudotsuga menziesii) seedlings induced to form varying amounts of compression wood. • Seedlings were grown under three bending treatments to assess the impact of compression wood on hydraulic properties, including stomatal conductance (g_s ), above-ground shoot conductance (K_l(abg) ), and both specific and leaf area-specific conductivity (k_s and k_l , respectively). • K_l(abg) was significantly lower (50% reduction) in severely bent seedlings than in controls. Similarly, both k_s and k_l of the main axis were significantly reduced (by 52% and 46%, respectively) in severely bent seedlings relative to controls. Seedlings in the moderate bending treatments had k_s and k_l that were intermediate between controls and severe bending. • Despite clear differences in above-ground shoot hydraulic properties, severely bent seedlings maintained the same water potentials as controls and had similar diurnal patterns of g_s . This suggests that when the entire soil-plant-atmosphere continuum is considered, even a severe reduction in stem k_s caused by compression wood has little impact on leaf-level processes.

Regulation of microvascular permeability by vascular endothelial growth factors

Generation of new blood vessels from pre-existing vasculature (angiogenesis) is accompanied in almost all states by increased vascular permeability. This is true in physiological as well as pathological angiogenesis, but is more marked during disease states. Physiological angiogenesis occurs during tissue growth and repair in adult tissues, as well as during development. Pathological angiogenesis is seen in a wide variety of diseases, which include all the major causes of mortality in the west: heart disease, cancer, stroke, vascular disease and diabetes. Angiogenesis is regulated by vascular growth factors, particularly the vascular endothelial growth factor family of proteins (VEGF). These act on two specific receptors in the vascular system (VEGF-R1 and 2) to stimulate new vessel growth. VEGFs also directly stimulate increased vascular permeability to water and large-molecular-weight proteins. We have shown that VEGFs increase vascular permeability in mesenteric microvessels by stimulation of tyrosine auto-phosphorylation of VEGF-R2 on endothelial cells, and subsequent activation of phospholipase C (PLC). This in turn causes increased production of diacylglycerol (DAG) that results in influx of calcium across the plasma membrane through store-independent cation channels. We have proposed that this influx is through DAG-mediated TRP channels. It is not known how this results in increased vascular permeability in endothelial cells in vivo. It has been shown, however, that VEGF can stimulate formation of a variety of pathways through the endothelial cell, including transcellular gaps, vesiculovacuolar organelle formation, and fenestrations. A hypothesis is outlined that suggests that these all may be part of the same process.

Hydraulic conductivity and anatomy along lateral roots of cacti: changes with soil water status

To help understand root function for desert plants at different levels of water availability, cellular and water-conducting properties were examined for young lateral roots of Ferocactus acanthodes (Lem.) Britton & Rose and Opuntia ficus-indica (L.) Miller. Hydraulic conductivity (L_p ), radial conductivity (L_R ), and axial conductance (K_h ) were determined for root segments in wet soil, during drought, and after rewetting. All three parameters increased with distance from the root tip under wet conditions and during drought for both species, with larger increases occurring under wet conditions. L_p decreased with drought duration up to 35 d for both species, due to dehydration of cortical cells, suberization of periderm, and embolism of xylem vessels; L_p and L_B decreased more for O. ficus-indica, and K_h decreased more for F. acanthodes. The increase in K_h with distance from the root tip and the decrease with duration of drought were associated with the changes in the number of conducting xylem vessels. After 7 d of rewetting, new secondary lateral roots developed, enhancing water uptake, especially for O. ficus-indica. Rewetting also caused water uptake for primary lateral roots to return to approximately the original value under wet conditions, mainly because decreases in emboli led to recovery of axial flow.