Corticular photosynthesis drives bark water uptake to refill embolized vessels in dehydrated branches of Salix matsudana

It is well known that xylem embolism can be repaired by bark water uptake and that the sugar required for embolism refilling can be provided by corticular photosynthesis. However, the relationship between corticular photosynthesis and embolism repair by bark water uptake is still poorly understood. In this study, the role of corticular photosynthesis in embolism repair was assessed using Salix matsudana branch segments dehydrated to ?1.9 MPa (P₅₀, water potential at 50% loss of conductivity). The results indicated that corticular photosynthesis significantly promoted water uptake and nonstructural carbohydrate (NSC) accumulation in the bark and xylem during soaking, thereby effectively enhancing the refilling of the embolized vessels and the recovery of hydraulic conductivity. Furthermore, the influence of the extent of dehydration on the embolism refilling enhanced by corticular photosynthesis was investigated. The enhanced refilling effects were much higher in the mildly dehydrated (?1.5 MPa) and moderately dehydrated (?1.9 MPa) branch segments than in the severely dehydrated (?2.2 MPa) branch segments. This study provides evidence that corticular photosynthesis plays a crucial role in xylem embolism repair by bark water uptake for mildly and moderately dehydrated branches.     

Foliar water uptake: a common water acquisition strategy for plants of the redwood forest

Evaluations of plant water use in ecosystems around the world reveal a shared capacity by many different species to absorb rain, dew, or fog water directly into their leaves or plant crowns. This mode of water uptake provides an important water subsidy that relieves foliar water stress. Our study provides the first comparative evaluation of foliar uptake capacity among the dominant plant taxa from the coast redwood ecosystem of California where crown-wetting events by summertime fog frequently occur during an otherwise drought-prone season. Previous research demonstrated that the dominant overstory tree species, Sequoia sempervirens, takes up fog water by both its roots (via drip from the crown to the soil) and directly through its leaf surfaces. The present study adds to these early findings and shows that 80% of the dominant species from the redwood forest exhibit this foliar uptake water acquisition strategy. The plants studied include canopy trees, understory ferns, and shrubs. Our results also show that foliar uptake provides direct hydration to leaves, increasing leaf water content by 2–11%. In addition, 60% of redwood forest species investigated demonstrate nocturnal stomatal conductance to water vapor. Such findings indicate that even species unable to absorb water directly into their foliage may still receive indirect benefits from nocturnal leaf wetting through suppressed transpiration. For these species, leaf-wetting events enhance the efficacy of nighttime re-equilibration with available soil water and therefore also increase pre-dawn leaf water potentials.     

Foggy days and dry nights determine crown‐level water balance in a seasonal tropical montane cloud forest

The ecophysiology of tropical montane cloud forest (TMCF) trees is influenced by crown‐level microclimate factors including regular mist/fog water inputs, and large variations in evaporative demand, which in turn can significantly impact water balance. We investigated the effect of such microclimatic factors on canopy ecophysiology and branch‐level water balance in the dry season of a seasonal TMCF in Veracruz, Mexico, by quantifying both water inputs (via foliar uptake, FU) and outputs (day‐ and night‐time transpiration, NT). Measurements of sap flow, stomatal conductance, leaf water potential and pressure–volume relations were obtained in Quercus lanceifolia, a canopy‐dominant tree species. Our results indicate that FU occurred 34% of the time and led to the recovery of 9% (24 ± 9.1 L) of all the dry‐season water transpired from individual branches. Capacity for FU was independently verified for seven additional common tree species. NT accounted for approximately 17% (46 L) of dry‐season water loss. There was a strong correlation between FU and the duration of leaf wetness events (fog and/or rain), as well as between NT and the night‐time vapour pressure deficit. Our results show the clear importance of fog and NT for the canopy water relations of Q. lanceifolia.     

Estimation of hydraulic conductance within field-grown apricot using sap flow measurements

Using the heat pulse and other techniques, the hydraulic architecture of apricot trees was mapped out. The flows (overall flow, flow across the four main branches) and forces (water potential differences between xylem and leaves) measured allowed us to quantify hydraulic conductance of branches and of the root/soil resistance. The experiment was carried out in a commercial orchard of 11-year-old apricot trees (Prunus armeniaca L., cv. Búlida, on Real Fino apricot rootstock) during 1 week (October 27–November 3, 1998). Three representative trees with a cylindrical trunk divided into four main branches of different sizes, orientation and local microclimate were chosen for the experiment. Sap flow was measured throughout the experimental period. Twelve sets of heat-pulse probes were used, one for each main branch. The diurnal course of the environmental conditions, the fraction of the area irradiated and leaf water relations were also considered in each main branch. The relationships between leaf water potential, xylem water potential and transpiration were established for different branches and also for the total plant. Using the slopes of these regressions, total plant conductance, the hydraulic conductance of the stem and root pathway, the hydraulic conductance of the canopy and the hydraulic conductance of each branch were estimated. Our findings show that the root conductance and the canopy hydraulic conductance are similar in magnitude. Leaf hydraulic conductance per leaf area unit was similar for each of the four branch orientations, indicating that, while the light microclimate has a dominant influence on transpiration, in this case it had little effect on the hydraulic properties of the canopy.     

The contribution of fog to the water relations of Sequoia sempervirens (D. Don): foliar uptake and prevention of dehydration

Fog is a defining feature of the coastal California redwood forest and fog inputs via canopy drip in summer can constitute 30% or more of the total water input each year. A great deal of occult precipitation (fog and light rain) is retained in redwood canopies, which have some of the largest leaf area indices known (Westman & Whittaker, Journal of Ecology 63, 493–520, 1975). An investigation was carried out to determine whether some fraction of intercepted fog water might be directly absorbed through leaf surfaces and if so, the importance of this to the water relations physiology of coast redwood, Sequoia sempervirens. An array of complimentary techniques were adopted to demonstrate that fog is absorbed directly by S. sempervirens foliage. Xylem sap transport reversed direction during heavy fog, with instantaneous flow rates in the direction of the soil peaking at approximately 5–7% of maximum transpiration rate. Isotopic analyses showed that up to 6% of a leaf's water content could be traced to a previous night's fog deposition, but this amount varied considerably depending on the age and water status of the leaves. Old leaves, which appear most able to absorb fog water were able to absorb distilled water when fully submersed at an average rate of 0.90 mmol m² s^?1, or about 80% of transpiration rates measured at the leaf level in the field. Sequoia sempervirens has poor stomatal control in response to a drying atmosphere, with rates of water loss on very dry nights up to 40% of midday summer values and rates above 10% being extremely common. Owing to this profligate water use behaviour of S. sempervirens, it appears that fog has a greater role in suppressing water loss from leaves, and thereby ameliorating daily water stress, than in providing supplemental water to foliar tissues per se. Although direct foliar absorption from fog inputs represents only a small fraction of the water used each day, fog's in reducing transpiration and rehydrating leaf tissues during the most active growth periods in summer may allow for greater seasonal carbon fixation and thus contribute to the very fast growth rates and great size of this species.     

Seasonal changes in hydraulic conductance, xylem embolism and leaf area in Eucalyptus tetrodonta and Eucalyptus miniata saplings in a north Australian savanna

Eucalypt saplings in north Australian savannas commonly die back, sometimes to ground level, during the 5 months of the long dry season. Water potentials are lower in saplings than large trees during the dry season, and we hypothesized that low water potentials may lead to high levels of xylem embolism and consequent death of branches and whole shoots. As the dry season progressed, hydraulic conductance of terminal branches decreased by 50% in Eucalyptus tetrodonta but not in Eucalyptus miniata saplings. Hydraulic conductance per leaf area decreased seasonally by 34% in E. tetrodonta branches. These decreases may be associated with the loss of leaves recorded from E. tetrodonta but not E. miniata branches. We modelled the effect of sequential loss of parallel resistors, representing petioles on a branch. This showed there is a non-linear decrease in flow as basal resistors are lost, which can lead to a decrease in mean flow per resistor due to increased mean path-length. Thus the observed loss of basal leaves, together with xylem embolism, probably contributed to the seasonal loss of hydraulic conductance in E. tetrodonta saplings. Loss of hydraulic conductance due to xylem embolism was generally low ( < 15%) in both species, and the seasonal increase in embolism could not fully account for the decline in hydraulic conductance of E. tetrodonta branches. There was little evidence that branch and shoot death was caused by these levels of embolism. Developing an embolism vulnerability curve for species with long vessels is problematic and this issue is discussed.     

Light-dependent maintenance of hydraulic function in mangrove branches: do xylary chloroplasts play a role in embolism repair?

? To clarify the role of branch photosynthesis in tree functioning, the presence and function of chloroplasts in branch xylem tissue were studied in a diverse range of mangrove species growing in Australia. ? The presence of xylary chloroplasts was observed via chlorophyll fluorescence of transverse sections. Paired, attached branches were selected to study the effects of covering branches with aluminium foil on the gas exchange characteristics of leaves and the hydraulic conductivity of branches. ? Xylary chloroplasts occurred in all species, but were differently distributed among living cell types in the xylem. Covering stems altered the gas exchange characteristics of leaves, such that water-use efficiency was greater in exposed leaves of covered than of uncovered branches. ? Leaf-specific hydraulic conductivity of stems was lower in covered than in uncovered branches, implicating stem photosynthesis in the maintenance of hydraulic function. Given their proximity to xylem vessels, we suggest that xylary chloroplasts may play a role in light-dependent repair of embolized xylem vessels.     

EVIDENCE FOR XYLEM CONSTRICTIONS IN THE PRIMARY VASCULATURE OF PSILOPHYTON DAWSONII,AN EMSIAN TRIMEROPHYTE

Serial peels through the branch junctions of Psilophyton dawsonii were examined in an attempt to determine if “hydraulic constrictions” (i.e., a localized decrease in mean diameter of xylem elements) sensu Zimmermann (1983) had occurred during bifurcation. Based on the mean diameters of primary xylem tracheids measured acropetally and basipetally to branch junctions, evidence for xylem constrictions (= localized decrease in mean tracheid diameter) was found within the basalmost portions of four out of six minor axes examined near their attachment to major axes (anisotomous junctions). Xylem constrictions were not detected in branch junctions between axes of equal girth (isotomous junctions). Xylem constrictions were detected within the base of five out of seven junctions between fertile and main axes. The mean diameters of tracheids of the branch trace near its origin are larger both basipetally and acropetally from the constriction. There is no evidence for a localized decrease in the mean diameter of the xylem strand in the region of a constriction. Therefore, xylem constrictions are not the result of epidogenesis. Based on the production of “hydraulic constrictions” in extant plants, which serve to localize the formation of embolisms in lateral branches during water stress, it is speculated that P. dawsonii could protect the vasculature of major axes by a similar anatomical feature.     

Distribution of vessel size,vessel density and xylem conducting efficiency within a crown of silver birch (<Emphasis Type="Italic">Betula pendula</Emphasis>)

Spatial patterns in vessel diameter, vessel density and xylem conducting efficiency within a crown were examined in closed-canopy trees of silver birch (Betula pendula). The variation in anatomical and hydraulic characteristics of branches was considered from three perspectives: vertically within a crown (lower, middle and upper crown), radially along main branches (proximal, middle and distal part), and with respect to branch orders (first-, second- and third-order branches). Hydraulically weighted mean diameter of vessels (D _h) and theoretical specific conductivity of the xylem (k _t) exhibited no vertical trend within the tree crown, whereas leaf-specific conductivity of the xylem (LSC_t) decreased acropetally. Variation in LSC_t was governed by sapwood area to leaf area ratio (Huber value) rather than by changes in xylem anatomy. The acropetal increase in soil-to-leaf conductance (G _T) within the birch canopy is attributable to longer path length within the lower-crown branches and higher hydraulic resistance of the shade leaves. D _h, k _t and LSC_t decreased, while vessel density (VD) and relative area of vessel lumina (VA) increased distally along main branches. A strong negative relationship between vessel diameter and VD implies a trade-off between hydraulic efficiency and mechanical stability of xylem. D _h and VD combined explained 85.4% of the total variation of k _t in the regression model applied to the whole data set. Xylem in fast-growing branches (primary branches) had greater area of vessel lumina per unit cross-sectional area of sapwood, resulting in a positive relationship between branch radial growth rate and k _t. D _h, k _t and LSC_t decreased, whereas VD increased with increasing branch order. This pattern promotes the hydraulic dominance of primary branches over the secondary branches and their dominance over tertiary branches. In this way crown architecture contributes to preferential water flow along the main axes, potentially providing better water supply for the branch apical bud and foliage located in the outer, better-insolated part of the crown.     

Natural and experimentally altered hydraulic architecture of branch junctions in Acer saccharum Marsh. and Quercus velutina Lam. trees

 The functional xylem anatomy and the hydraulic conductivity of intact and treated branch junctions of the diffuse-porous sugar maple (Acer saccharum Marsh.) were compared to those of the ring-porous black oak (Quercus velutina Lam.). Maple shoots possessed greater growth intensity than those of oak. The extensive growth of the maple trees resulted in about a two-fold increase in xylem production in the maple branches. Branches were altered by removing a patch of bark from the base of a branch (near a junction) leaving a bridge of bark on the upper or lower side of the branch. The experimentally treated branch junctions revealed that, in oak, most (up to 92%) of the water flows in the lower side of a branch, where most of the large vessels occurred. In maple, most of the conductive tissue was observed to form in the upper side of the branches, which was equally or more conductive than the lower side. A treatment of longitudinal, parallel scratches in the bark-bridge, which reduced earlywood vessel width, substantially decreased conductivity (to only 15%) in oak, but had no effect on conductivity in maple. In maple, such wounding stimulated more wood formation and increased conductivity. In both trees, a narrow bridge at the junction induced more wood formation and higher conductivity in the branch. The mechanisms controlling wood formation and water flow in branch junctions of ring- and diffuse-porous trees are discussed. Received: 14 February 1996 / Accepted: 27 May 1996     

Mechanism of freeze-induced embolism in Fagus sylvatica L.

 The mechanism of freeze stress-induced embolism in Fagus sylvatica L. branches was analyzed under controlled conditions. Excised branches were exposed to successive freeze-thaw cycles in temperature controlled chambers. Thermocouples were placed on the bark to detect sap freezing exotherms. The degree of xylem embolism was estimated after each cycle by the loss of hydraulic conductivity. After one freeze-thaw cycle the degree of embolism was found to decrease with xylem specific hydraulic conductivity, small apical shoots being more susceptible to embolism. Exotherms revealed that distal shoots were freezing first and exuded sap as a result of water expansion. The lower water content in apical shoots upon freezing probably induced higher sap tensions which promoted air bubble expansion and vessel cavitation preferentially near the apices. When the decrease in water content was experimentally prevented, embolism developed to a lesser extent. The higher vulnerability of shoot apices may protect the rest of the branch from winter damage. Received: 29 May 1998 / Accepted: 15 August 1998     

Basal reiteration improves the hydraulic functional status of mature Cinnamomum camphora trees

We compared soil-to-leaf hydraulic conductance (G _T), hydraulic conductivity and water-relations characteristics of leaves between reiterated axes (produced by sprouting of suppressed buds) and sequential axes (produced by elongation of terminal buds) on the same branch to investigate how basal reiteration affected the hydraulic architecture of mature Cinnamomum camphora (L.) Sieb. trees. Given similar light conditions, G _T was higher for leaves on reiterated shoots than for those on sequential shoots. However, where leaves on sequential shoots received more light, G _T was similar to that of leaves on reiterated shoots, suggesting that some compensatory mechanism worked to increase hydraulic conductance to the more distal sequential shoots, which have higher potential for carbon gain. Both xylem- and leaf-specific conductivities were higher for reiterated than sequential shoots. Pressure–volume measurements indicated that leaves on reiterated shoots were more vulnerable to water stress, suggesting that they developed under favorable water status. Because basal reiteration occurs on lower-order branch axes, reiterated shoots have better connectivity to higher conducting xylem and this may contribute to favorable water status. As trees grow larger, hydraulic pathlength and hydraulic resistance both increase as numbers of branch junctions and nodes increase. Our results suggest that basal reiteration improves the hydraulic functional status of mature C. camphora trees by shortening the hydraulic pathway and increasing hydraulic conductance to transpiring leaves.     

摘叶造成的碳限制对刺槐碳素分配和水力学特性的影响

以3年生刺槐（Robinia pseudoacacia Linn.）为研究对象,通过对其进行连续3次摘叶造成严重碳限制,检测摘叶后刺槐的生物量分配、叶片形态和不同部位的非结构性碳（NSC）浓度,同时检测其根压和根系导水率、枝条水势和导水率损失值（PLC）及茎的抗栓塞能力,研究摘叶造成的碳限制对刺槐碳素分配和水力学特性的影响。结果显示,摘叶显著降低了刺槐不同部位的生物量,其中细根生物量降低程度最大;摘叶还造成了刺槐不同部位NSC浓度显著降低,茎韧皮部、茎木质部、根韧皮部和根木质部的NSC浓度分别为对照的29.6%、20.2%、10.2%和8.7%,且根部NSC的降低程度显著高于茎;碳限制显著降低了刺槐苗木的根压和根系导水率,增加了枝条凌晨和正午的PLC,降低了其抗栓塞能力。研究结果表明摘叶造成的碳限制改变了刺槐的碳素分配模式,限制了碳素向根的分配,抑制细根的发生,进而限制根的水分吸收能力,加重枝条栓塞程度,同时还会导致枝条抗栓塞能力下降,从而降低植物水分输导的安全性。     

Polystichum munitum (Dryopteridaceae) varies geographically in its capacity to absorb fog water by foliar uptake within the redwood forest ecosystem

? Premise of the study: Fog provides a critical water resource to plants around the world. In the redwood forest ecosystem of northern California, plants depend on fog absorbed through foliar uptake to stay hydrated during the rainless summer. In this study, we identified regions within the redwood ecosystem where the fern Polystichum munitum canopy most effectively absorbs fog drip that reaches the forest floor. ? Methods: We measured the foliar uptake capacity of P. munitum fronds at seven sites along 700 km of the redwood forest ecosystem. We quantified the canopy cover of P. munitum at each site and estimated how much water the fern canopy can acquire aboveground through fog interception and absorption. ? Key results: Throughout the ecosystem, nocturnal foliar uptake increased the leaf water content of P. munitum by 7.2%, and we estimated that the P. munitum canopy can absorb 5 ± 3% (mean ± SE) of intercepted fog precipitation. Strikingly, P. munitum had the highest foliar uptake capacity in the center of the ecosystem and may absorb 10% more of the fog its canopy intercepts in this region relative to other regions studied. Conversely, P. munitum had no foliar uptake capacity in the southern end of the ecosystem. ? Conclusions: This study shows the first evidence that foliar uptake varies within species at the landscape scale. Our findings suggest that the P. munitum at the southern tip of the redwood ecosystem may suffer most from low summertime water availability because it had no potential to acquire fog as an aboveground water subsidy.     

Gradients and dynamics of inner bark and needle osmotic potentials in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst)

Preconditions of phloem transport in conifers are relatively unknown. We studied the variation of needle and inner bark axial osmotic gradients and xylem water potential in Scots pine and Norway spruce by measuring needle and inner bark osmolality in saplings and mature trees over several periods within a growing season. The needle and inner bark osmolality was strongly related to xylem water potential in all studied trees. Sugar concentrations were measured in Scots pine, and they had similar dynamics to inner bark osmolality. The sucrose quantity remained fairly constant over time and position, whereas the other sugars exhibited a larger change with time and position. A small osmotic gradient existed from branch to stem base under pre‐dawn conditions, and the osmotic gradient between upper stem and stem base was close to zero. The turgor in branches was significantly driven by xylem water potential, and the turgor loss point in branches was relatively close to daily minimum needle water potentials typically reported for Scots pine. Our results imply that xylem water potential considerably impacts the turgor pressure gradient driving phloem transport and that gravitation has a relatively large role in phloem transport in the stems of mature Scots pine trees.     

How important is woody tissue photosynthesis in poplar during drought stress?

Key message

Woody tissue photosynthesis might play a key role in maintaining plant carbon economy and hydraulic function under unfavourable conditions such as drought stress.

Abstract

Within trees, a portion of respired CO₂ is assimilated by bark and woody tissue photosynthesis, but its physiological role remains unclear, in particular under unfavour able conditions like drought stress. We hypothesised that woody tissue photosynthesis will contribute to overall tree carbon gain both under sufficient water supply and during drought, and plays a role in maintaining the hydraulic function. We subjected half of the trees to a stem and branch light-exclusion treatment to prevent bark and woody tissue photosynthesis. Then, we measured leaf gas exchange and stem growth in Populus deltoides x nigra ‘Monviso’ trees both under well-watered and dry conditions. We additionally measured cavitation using acoustic emission in detached control and light-excluded branches to illustrate the role of woody tissue photosynthesis in xylem embolism repair. Under well-watered conditions, light exclusion resulted in reduced stem growth relative to control trees by 30 %. In response to drought, stem shrinkage of light-excluded trees was more pronounced as compared to control trees. During drought stress also maximum photosynthesis and transpiration rate tended to decrease more rapidly in light-excluded trees compared to control trees. Leaf fall in light-excluded branches together with the larger number of acoustic emissions in control branches indicates that in the latter more xylem vessels were still hydraulically functional under drought. Therefore, our study highlights that photosynthesis at branch and stem level might be a key factor in the resilience of trees to drought stress by maintaining both the plant carbon economy and hydraulic function.

     

Are leaves ‘freewheelin'? Testing for a Wheeler‐type effect in leaf xylem hydraulic decline

A recent study found that cutting shoots under water while xylem was under tension (which has been the standard protocol for the past few decades) could produce artefactual embolisms inside the xylem, overestimating hydraulic vulnerability relative to shoots cut under water after relaxing xylem tension (Wheeler et al. 2013). That study also raised the possibility that such a ‘Wheeler effect’ might occur in studies of leaf hydraulic vulnerability. We tested for such an effect for four species by applying a modified vacuum pump method to leaves with minor veins severed, to construct leaf xylem hydraulic vulnerability curves. We tested for an impact on leaf xylem hydraulic conductance (K_x) of cutting the petiole and minor veins under water for dehydrated leaves with xylem under tension compared with dehydrated leaves after previously relaxing xylem tension. Our results showed no significant ‘cutting artefact’ for leaf xylem. The lack of an effect for leaves could not be explained by narrower or shorter xylem conduits, and may be due to lesser mechanical stress imposed when cutting leaf petioles, and/or to rapid refilling of emboli in petioles. These findings provide the first validation of previous measurements of leaf hydraulic vulnerability against this potential artefact.     

Impact of soil texture and water availability on the hydraulic control of plant and grape-berry development

Aims

All components of the soil-plant-atmosphere (s-p-a) continuum are known to control berry quality in grapevine (Vitis vinifera L.) via ecophysiological interactions between water uptake by roots and water loss by leaves. The scope of the present work was to explore how the main hydraulic components of grapevine influence fruit quality through changes in liquid- and gas-phase hydraulic conductance.

Methods

To reach our objectives, determinations of shoot growth, berry size and sugar content, leaf gas exchange, predawn leaf water potential (as a proxy of soil water potential), midday stem water potential and leaf water potential were performed in conjunction with anatomical measurements of shoot xylem. All measurements were conducted in two different cultivars (Cabernet franc and Merlot) and on three different soil types (clayey, gravelly, and sandy).

Results

Shoot xylem morphometric characteristics and whole-plant hydraulic conductance were influenced by cultivar and soil type. Differences in leaf gas exchange parameters and water potentials were determined by soil type significantly more than by cultivar. Between the two extremes (gravelly soil imposing drought conditions and sandy soil with easily accessible water) the clayey soil expressed an intermediate plant water consumption and highest sugar accumulation in berry.

Conclusions

Hydraulic and non hydraulic limitations to vine/berry interactions supported the conclusion that water availability in the soil overrides differences due to cultivar in determining the productive potential of the vineyard. Non hydraulic stomatal control was expected to be an important component on plants grown on the clayey soil, which experienced a moderate water stress. Possible links between hydraulic traits and berry development and quality are discussed.     

Hydraulic flow characteristics in the lignotuberous mallee Eucalyptus behriana F. Muell. in the field

Abstract The lignotuberous mallee Eucalyptus behriana F. Muell, had much lower predawn leaf water potentials (not higher than – 1.2MPa) than other eucalypts (as high as – 0.2MPa), even after extended rain. This led to the expectation that the lignotuber of E. behriana might have specific hydraulic characteristics. Keeping the soil around partially defoliated mallces for several days underwater did not raise the water status above the maximum leaf water potential observed under natural conditions. Digging a plant out and placing its roots in water after removal of the soil rapidly increased the water status to a level consistant with other eucalypts. This indicated that the major impedance to water uptake was a component of the soil rather than in the roots or in the lignotuber. Some of the individual mallces had only two major stems or branches. One stem or branch was kept covered throughout the experiments to prevent transpiration. The other stem was subjected to a variety of different conditions in order to modify water loss from it. The transpiring branch affected the water status of the non-transpiring plant parts. Hydraulic resistances in the shoot and root/lignotuber were determined from differences in the leaf water potential of covered and uncovered branches, at high water flow rates through the plant. Resistances in branches, including the liquid phase component of the leaf, were significantly larger than in root or lignotuber. The total plant hydraulic resistance of E. behriana was similar to that of other eucalypts, such as E. pauciflora Sieb. ex Spreng. or E. delegatensis R. T. Bak., even though its growth form was different and its natural leaf water potentials were much lower. An osmotic adjustment at the leaf level was observed in the mallee, keeping its bulk leaf turgor in the same range as compared to the other eucalypt species.     

Hydraulic properties and photosynthetic rates in co-occurring lianas and trees in a seasonal tropical rainforest in southwestern China

In this study, we examined wood anatomy, hydraulic properties, photosynthetic rate, and water status and osmotic regulation in three liana species and three tree species co-occurring in a seasonal tropical rain forest. Our results showed that the three liana species had larger vessel diameter, lower sapwood density, and consequently higher branch sapwood specific hydraulic conductivity (K _S) than the three tree species. Across species, K _S was positively correlated with leaf nitrogen concentration and maximum net CO₂ assimilation rate. However, it was also positively correlated with xylem water potential at 50% loss of hydraulic conductivity, indicating a trade-off between hydraulic efficiency and safety. Compared to the tree species, the liana species had higher predawn leaf water potential and lower osmotic adjustment in the dry season. The combination of more efficient water transport, higher photosynthetic rates, and their ability to access to more reliable water source at deeper soil layers in the dry season in the lianas should contribute to their fast growth.