Refilling of Embolized Vessels in Young Stems of Laurel. Do We Need a New Paradigm?

Recovery of hydraulic conductivity after the induction of embolisms was studied in woody stems of laurel (Laurus nobilis). Previous experiments confirming the recovery of hydraulic conductivity when xylem pressure potential was less than −1 MPa were repeated, and new experiments were done to investigate the changes in solute composition in xylem vessels during refilling. Xylem sap collected by perfusion of excised stem segments showed elevated levels of several ions during refilling. Stem segments were frozen in liquid N₂ to view refilling vessels using cryoscanning electron microscopy. Vessels could be found in all three states of presumed refilling: (a) mostly water with a little air, (b) mostly air with a little water, or (c) water droplets extruding from vessel pits adjacent to living cells. Radiographic probe microanalysis of refilling vessels revealed nondetectable levels of dissolved solutes. Results are discussed in terms of proposed mechanisms of refilling in vessels while surrounding vessels were at a xylem pressure potential of less than −1 MPa. We have concluded that none of the existing paradigms explains the results.     

Cutting stems before relaxing xylem tension induces artefacts in Vitis coignetiae,as evidenced by magnetic resonance imaging

It was recently reported that cutting artefacts occur in some species when branches under tension are cut, even under water. We used non‐destructive magnetic resonance imaging (MRI) to investigate the change in xylem water distribution at the cellular level in Vitis coignetiae standing stems before and after relaxing tension. Less than 3% of vessels were cavitated when stems under tension were cut under water at a position shorter than the maximum vessel length (MVL) from the MRI point, in three of four plants. The vessel contents remained at their original status, and cutting artefact vessel cavitation declined to <1% when stems were cut at a position farther than the MVL from the MRI point. Water infiltration into the originally cavitated vessels after cutting the stem, i.e. vessel refilling, was found in <1% of vessels independent of cutting position on three of nine plants. The results indicate that both vessel cavitation and refilling occur in xylem tissue under tension following stem cutting, but its frequency is quite small, and artefacts can be minimized altogether if the distance between the monitoring position and the cutting point is longer than the MVL.     

Easy Come,Easy Go: Capillary Forces Enable Rapid Refilling of Embolized Primary Xylem Vessels

Protoxylem plays an important role in the hydraulic function of vascular systems of both herbaceous and woody plants, but relatively little is known about the processes underlying the maintenance of protoxylem function in long-lived tissues. In this study, embolism repair was investigated in relation to xylem structure in two cushion plant species, Azorella macquariensis and Colobanthus muscoides, in which vascular water transport depends on protoxylem. Their protoxylem vessels consisted of a primary wall with helical thickenings that effectively formed a pit channel, with the primary wall being the pit channel membrane. Stem protoxylem was organized such that the pit channel membranes connected vessels with paratracheal parenchyma or other protoxylem vessels and were not exposed directly to air spaces. Embolism was experimentally induced in excised vascular tissue and detached shoots by exposing them briefly to air. When water was resupplied, embolized vessels refilled within tens of seconds (excised tissue) to a few minutes (detached shoots) with water sourced from either adjacent parenchyma or water-filled vessels. Refilling occurred in two phases: (1) water refilled xylem pit channels, simplifying bubble shape to a rod with two menisci; and (2) the bubble contracted as the resorption front advanced, dissolving air along the way. Physical properties of the protoxylem vessels (namely pit channel membrane porosity, hydrophilic walls, vessel dimensions, and helical thickenings) promoted rapid refilling of embolized conduits independent of root pressure. These results have implications for the maintenance of vascular function in both herbaceous and woody species, because protoxylem plays a major role in the hydraulic systems of leaves, elongating stems, and roots.There is a pressing need to understand how plants manage the maintenance of water transport from roots through leaves under changing environmental conditions (Allen et al., 2010; Choat et al., 2012). The problem arises because water is transported through the xylem under tension (i.e. under negative absolute pressure). As tension increases, conduits become increasingly vulnerable to cavitation, which causes the conduits to lose their ability to transport water. Conduits can become embolized during normal diurnal function as a result of tensions induced by transpiration and in response to environmental conditions such as drought or freezing stress (Zimmermann and Tyree, 2002). Vulnerability to cavitation and embolism formation suggests that plants have mechanisms to regain lost hydraulic capacity, either through the formation of new conduits or by refilling embolized ones.The vulnerability of conduits to embolisms and the capacity for repair are related to the structural diversity of xylem tissue (Zwieniecki and Holbrook, 2009; Lens et al., 2011; Cai et al., 2014). In vascular plants, the classification of xylem tissues depends on the meristem that produced them (Evert and Eichhorn, 2006). Primary xylem is produced by apical meristems and includes both protoxylem and metaxylem conduits, which are distinguished by their wall structure and the timing of their development. Protoxylem matures during organ elongation, which results in loss of function due to stretching in some tissues and species, while in many others, functionality is maintained throughout the life of the organ. In contrast, metaxylem matures in elongated tissue. In herbaceous plants, primary xylem is the major hydraulic system of the roots, stems, and leaves. In woody plants, the primary xylem remains the main hydraulic system of the leaves, while the radial growth of stems occurs through the activity of a vascular cambium, which produces secondary xylem with only metaxylem conduits. As a woody plant grows, the secondary xylem (and hence the metaxylem) thus becomes of increasing importance to stem hydraulic function. However, protoxylem remains an integral component of the plant hydraulic system due to its function in leaves and elongating stems and roots.As discussed in a recent review (Brodersen and McElrone, 2013), refilling of embolized vessels has been shown to depend on the generation of positive pressure by roots in many monocots, herbaceous plants, and a few woody species. However, many species lack root pressure; thus, attention has focused on so-called novel refilling, which involves adjacent living cells in the repair of embolized metaxylem or secondary xylem in stems of mature plants. Novel refilling has been studied with a variety of methods to visualize temporal variation in the presence and subsequent absence of embolized vessels, including cryo-scanning electron microscopy (Cryo-SEM; Canny, 1997; McCully et al., 2014), double staining (Zwieniecki and Holbrook, 1998; Zwieniecki et al., 2000), NMR imaging (Holbrook et al., 2001; Zwieniecki et al., 2013), and high-resolution x-ray computed tomography (Lee and Kim, 2008; Brodersen et al., 2010; Kim and Lee, 2010; Lee et al., 2013; Suuronen et al., 2013). These observations, in combination with other measurements, led to a working hypothesis of an osmotically driven repair mechanism in which sugars pumped into embolized vessels by adjacent paratracheal parenchyma provide the osmotic pressure difference that refills the vessel (Nardini et al., 2011).Little is known about embolism and its repair in protoxylem, which has structural features that make it potentially more vulnerable to embolism than metaxylem in the same plant or tissue (Choat et al., 2005). These include a greater exposed area of the primary cell wall with annular or helical thickenings instead of secondary walls. This could enhance stretching of the primary wall when large pressure differences develop between functional and embolized vessels, thereby decreasing the pressure required for air seeding of bubbles (Choat et al., 2004). Choat et al. (2005) suggested that greater vulnerability of protoxylem to embolism might underpin the roles of petioles, leaves, and small stems in the hydraulic segmentation hypothesis of Zimmermann (1983), in which sacrifice of the most easily replaceable tissues protects the function of the main structure of a plant during water stress. If ease of protoxylem embolism were to contribute to the function of hydraulic fuses during mild water stress, then ease of refilling would be required to rapidly reset the system.This study focuses on embolism repair in two distantly related, vascular species, Azorella macquariensis (Apiaceae) and Colobanthus muscoides (Caryophyllaceae), that depend exclusively on protoxylem for vascular water transport. Both species form cushions, with the former being an endemic, keystone species in the alpine zone of subantarctic Macquarie Island and the latter being a regional endemic that plays a major role in rocky coastal areas often within the supralittoral zone (Selkirk et al., 1990; Orchard, 1993). Both species are of ecological interest, because the subantarctic region is under increasing threat from climate change (Adams, 2009). Specifically, the climate on Macquarie Island is progressively changing from one that is perpetually wet and misty to one with increased exposure to periodic drying (Bergstrom et al., 2015). Dieback of alpine vegetation was first observed in 2008, and by 2010, extensive and unprecedented decline of A. macquariensis led to its listing as critically endangered (Bricher et al., 2013).In this study, protoxylem structure was studied in relation to embolism repair. Refilling of gas-filled vessels was compared between excised tissue and that in intact, detached shoots. The results showed that the physical properties of the protoxylem facilitated refilling by capillary forces and that rapid refilling in detached shoots supplied with water occurred without root pressure.     

Vulnerability to cavitation differs between current‐year and older xylem: non‐destructive observation with a compact magnetic resonance imaging system of two deciduous diffuse‐porous species

Development of xylem embolism during water stress in two diffuse‐porous hardwoods, Katsura (Cercidiphyllum japonicum) and Japanese white birch (Betula platyphylla var. japonica), was observed non‐destructively under a compact magnetic resonance imaging (MRI) system in addition to conventional quantitation of hydraulic vulnerability to cavitation from excised stem segments. Distribution of white and dark areas in MR images corresponded well to the distribution of water‐filled/embolized vessels observed by cryo‐scanning electron microscopy in both species. Water‐filled vessels were observed in MR images as white areas in Katsura and as white dots in Japanese white birch, respectively, and embolisms could be detected as a change to dark areas. The increase in the relative embolized area (REA: %) in the cross‐sectional area of total xylem during water stress, which was estimated from the binarized MR images, was consistent with the hydraulic vulnerability curves of these species. From the non‐destructive MRI observations, cavitation induced by water stress was shown to develop earlier in 1‐ or 2‐year‐old xylem than in the current‐year xylem in both species; that is, the vulnerability to cavitation differs between vessels in the current‐year xylem and those in older annual rings.     

Dynamic changes in hydraulic conductivity in petioles of two savanna tree species: factors and mechanisms contributing to the refilling of embolized vessels

Diel variation in specific hydraulic conductivity (k_s) was recorded in petioles of two savanna tree species, Schefflera macrocarpa and Caryocar brasiliense, from central Brazil. These two species have compound leaves with long petioles (10–30 cm). In both species, petiole k_s decreased sharply with increasing transpiration rates and declining leaf water potentials (ψ_L) during the morning. Petiole k_s increased during the afternoon while the plants were still transpiring and the water in the non‐embolized vessels was still under tension. Dye experiments confirmed that in both species diel variation in k_s was associated with embolism formation and repair. When transpiration was prevented in individual leaves, their petiole k_s and water potential remained close to their maximum values during the day. When minimum daily ψ_L on selected branches was experimentally lowered by 0.2–0.6 MPa, the rate of k_s recovery during the afternoon was slower in comparison with control branches. Several field manipulations were performed to identify potential mechanisms involved in the refilling of embolized petiole vessels. Removal of the cortex or longitudinal incisions in the cortex prevented afternoon recovery of k_s and refilling of embolized vessels. When distilled water was added to petiole surfaces that had been abraded to partially remove the cuticle, k_s increased sharply during the morning and early afternoon. Evidence of starch to sugar conversion in the starch sheath cells surrounding the vascular bundles of the petioles was observed during periods of rapid transpiration when the abundance of starch granules in the starch sheath cells surrounding the vascular bundles decreased. Consistent with this, petiole sugar content was highest in the early afternoon. The most parsimonious explanation of the field observations and the experimental results was that an increase in osmotically active solutes in cells outside the vascular bundles at around midday leads to water uptake by these cells. However, the concurrent increase in tissue volume is partially constrained by the cortex, resulting in a transient pressure imbalance that may drive radial water movement in the direction of the embolized vessels, thereby refilling them and restoring water flow. This study thus presents evidence that embolism formation and repair are two distinct phenomena controlled by different variables. The degree of embolism is a function of tension, and the rate of refilling a function of internal pressure imbalances.     

Measurement of vulnerability to water stress‐induced cavitation in grapevine: a comparison of four techniques applied to a long‐vesseled species

Among woody plants, grapevines are often described as highly vulnerable to water‐stress induced cavitation with emboli forming at slight tensions. However, we found native embolism never exceeded 30% despite low xylem water potentials (Ψ_x) for stems of field grown vines. The discrepancy between native embolism measurements and those of previous reports led us to assess vulnerability curve generation using four separate methods and alterations (i.e. segment length and with/without flushing to remove embolism prior to measurement) of each. Centrifuge, dehydration and air‐injection methods, which rely on measurement of percentage loss of hydraulic conductivity (PLC) in detached stems, were compared against non‐invasive monitoring of xylem cavitation with nuclear magnetic resonance (NMR) imaging. Short segment air‐injection and flushed centrifuge stems reached >90 PLC at Ψ_x of‐0.5 and ?1.5 MPa, respectively, whereas dehydration and long‐segment air‐injection measurements indicated no significant embolism at Ψ_x > ?2.0 MPa. Observations from NMR agreed with the dehydration and long segment air‐injection methods, showing the majority of vessels were still water‐filled at Ψ_x > ?1.5 MPa. Our findings show V. vinifera stems are far less vulnerable to water stress‐induced cavitation than previously reported, and dehydration and long segment air‐injection techniques are more appropriate for long‐vesseled species and organs.     

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.     

A survey of root pressures in vines of a tropical lowland forest

Pre-dawn xylem pressures were measured with bubble manometers attached near the stem bases of 32 species of vines on Barro Colorado Island, Panama, to determine if pressures were sufficient to allow for possible refilling of embolized vessels. Of 29 dicotyledonous species 26 exhibited only negative xylem pressures, even pre-dawn during the wet season. In contrast, three members of the Dilleniaceae exhibited positive pre-dawn xylem pressures, with a maximum of 64 kPa in Doliocarpusmajor. A pressure of 64 kPa is sufficient to push water to a height of 6.4 m against gravity, but the specimens reached heights of 18 m. Thus, in all 29 dicotyledons examined, the xylem pressures were not sufficient to refill embolized vessels in the upper stems. In contrast, two of the smaller, non-dicotyledonous vines, the climbing fern Lygodiumvenustrum and the viny bamboo Rhipidocladumracemiflorum, had xylem pressures sufficient to push water to the apex of the plants. Therefore, a root pressure mechanism to reverse embolisms in stem xylem could apply to some but not to most of the climbing plants that were studied. Received: 18 March 1996 / Accepted: 24 October 1996     

Functional lifespans of xylem vessels: Development,hydraulic function,and post‐function of vessels in several species of woody plants

Premise of the Study

Xylem vessels transition through different stages during their functional lifespan, including expansion and development of vessel elements, transition to vessel hydraulic functionality, and eventual transition to post‐functionality. We used information on vessel development and function to develop a model of vessel lifespan for woody plants.

Methods

We examined vessel functional lifespan using repeated anatomical sampling throughout the growing season, combined with active‐xylem staining to evaluate vessel hydraulic transport functionality. These data were combined with a literature review. The transitions between vessel functional lifespans for several species are illustrated, including grapevine (Vitis vinifera L., Vitaceae), English oak (Quercus robur L., Fagaceae), American chestnut [Castanea dentata (Marshall) Borkh.; Fagaceae], and several arid and semi‐arid shrub species.

Key Results

In intact woody plants, development and maturation of vessel elements may be gradual. Once hydraulically functional, vessel elements connect to form a vessel network that is responsible for bulk hydraulic flow through the xylem. Vessels become nonfunctional due to the formation of gas emboli. In some species and under some conditions, vessel functionality of embolized conduits may be restored through refilling. Blockages, such as tyloses, gels, or gums, indicate permanent losses in hydraulic functional capacity; however, there may be some interesting exceptions to permanent loss of functionality for gel‐based blockages.

Conclusions

The gradual development and maturation of vessel elements in woody plants, variation in the onset of functionality between different populations of vessels throughout the growing season, and differences in the timing of vessel transitions to post‐functionality are important aspects of plant hydraulic function.     

Relax and refill: xylem rehydration prior to hydraulic measurements favours embolism repair in stems and generates artificially low PLC values

Diurnal changes in percentage loss of hydraulic conductivity (PLC), with recorded values being higher at midday than on the following morning, have been interpreted as evidence for the occurrence of cycles of xylem conduits' embolism and repair. Recent reports have suggested that diurnal PLC changes might arise as a consequence of an experimental artefact, that is, air entry into xylem conduits upon cutting stems, even if under water, while under substantial tension generated by transpiration. Rehydration procedures prior to hydraulic measurements have been recommended to avoid this artefact. In the present study, we show that xylem rehydration prior to hydraulic measurements might favour xylem refilling and embolism repair, thus leading to PLC values erroneously lower than those actually experienced by transpiring plants. When xylem tension relaxation procedures were performed on stems where refilling mechanisms had been previously inhibited by mechanical (girdling) or chemical (orthovanadate) treatment, PLC values measured in stems cut under native tension were the same as those measured after sample rehydration/relaxation. Our data call for renewed attention to the procedures of sample collection in the field and transport to the laboratory, and suggest that girdling might be a recommendable treatment prior to sample collection for PLC measurements.     

Polar auxin transport is implicated in vessel differentiation and spatial patterning during secondary growth in Populus

Premise of the Study

Dimensions and spatial distribution of vessels are critically important features of woody stems, allowing for adaptation to different environments through their effects on hydraulic efficiency and vulnerability to embolism. Although our understanding of vessel development is poor, basipetal transport of auxin through the cambial zone may play an important role.

Methods

Stems of Populus tremula ×alba were treated with the auxin transport inhibitor N‐1‐naphthylphthalamic acid (NPA) in a longitudinal strip along the length of the lower stem. Vessel lumen diameter, circularity, and length; xylem growth; tension wood area; and hydraulic conductivity before and after a high pressure flush were determined on both NPA‐treated and control plants.

Key Results

NPA‐treated stems formed aberrant vessels that were short, small in diameter, highly clustered, and angular in cross section, whereas xylem formed on the untreated side of the stem contained typical vessels that were similar to those of controls. NPA‐treated stems had reduced specific conductivity relative to controls, but this difference was eliminated by the high‐pressure flush. The control treatment (lanolin + dimethyl sulfoxide) reduced xylem growth and increased tension wood formation, but never produced the aberrant vessel patterning seen in NPA‐treated stems.

Conclusions

These results are consistent with a model of vessel development in which basipetal polar auxin transport through the xylem‐side cambial derivatives is required for proper expansion and patterning of vessels and demonstrate that reduced auxin transport can produce stems with altered stem hydraulic properties.     

New evidence for a role of vessel-associated cells and phloem in the rapid xylem refilling of cavitated stems of Laurus nobilis L.

Xylem recovery from embolism was studied in Laurus nobilis L. stems that were induced to cavitate by combining negative xylem pressure potentials (P_X = ?1.1 MPa) with positive air pressures (P_C) applied using a pressure collar. Xylem refilling was measured by recording the percentage loss of hydraulic conductance (PLC) with respect to the maximum 2 min, 20 min and 15 h after pressure release. Sodium orthovanadate (an inhibitor of many ATP‐ases) strongly inhibited xylem refilling while fusicoccin (a stimulator of the plasma membrane H⁺‐ATPase) promoted complete embolism reversal. So, the refilling process was interpreted to result from energy‐dependent mechanisms. Stem girdling induced progressively larger inhibition to refilling the nearer to the embolized stem segment phloem was removed. The starch content of wood parenchyma was estimated as percentages of ray and vasicentric cells with high starch content with respect to the total, before and after stem embolism was induced. A closely linear positive relationship was found to exist between recovery from PLC and starch hydrolysis. This, was especially evident in vasicentric cells. A mechanism for xylem refilling based upon starch to sugar conversion and transport into embolized conduits, assisted by phloem pressure‐driven radial mass flow is proposed.     

Patterns of Water Movement in Intact and Excised Stems of Fraxinus americana and Acer saccharum Seedlings

Patterns of water movement in intact and excised stems of Fraxinusamericana L. and Acer saccharum Marsh. seedlings were delineatedwith periodic acid-reduced basic fuchsin staining techniqueand microscopy. In intact Fraxinus stems water movement occurredprimarily in the large early-wood vessels of the current xylemincrement except in the current shoot where the ring porouscharacter was not developed and water movement occurred in largevessels scattered throughout the xylem. In intact Acer stemswater movement occurred in large vessels in the current annualring and in the outer two-thirds of the prior-year annual ring.The pattern of water movement in excised stems under suctionwas similar to the pattern in stems of intact transpiring Fraxinusplants, whereas in Acer a larger portion of the cross-sectionalarea of stems was used when suction was applied to excised stems.Relative conductivity was similar for the two species as a resultof conduction in Fraxinus in a small number of large-diametervessels and in Acer in a large number of small-diameter vessels.     

Mechanisms of xylem embolism repair in woody plants: Research progress and questions

To maintain long-distance water transport in woody plants is critical for their survival, growth and development. Water under tension is in a metastable state and prone to cavitation and embolism, which leads to loss of hydraulic conductance, reduced productivity, and eventually plant death. In face to water stress-induced cavitation, plants either reduce frequency of embolism occurrence through cavitation resistance with specialized anatomical struc- ture, or/and form a metabolically active embolism repair mechanism. For the xylem embolism and repair, however, there are controversies regarding the occurring frequency, conditions and underlying mechanisms. In this review paper, we first examined the process, temporal dynamics and frequency of xylem embolism and repair. Then, we summarized hypotheses for the mechanisms of the novel refilling in xylem embolism repair, including the osmotic hypothesis, the reverse osmotic hypothesis, the phloem-driven refilling hypothesis, and the phloem unloading hypothesis. We further compared differences in xylem embolism and repair between conifers and angiosperms tree species, and examined the trade-offs between cavitation resistance and xylem recovery performance. Finally, we proposed four priorities in future research in this field: (1) to improve measuring technology of xylem embolism; (2) to test hypotheses for the mechanisms of the novel refilling in xylem embolism repair and the signal triggering xylem refilling; (3) to explore species-specific trait differences related to xylem embolism and repair and their underlying trade-off relationships; and (4) to enhance studies on the relationship between the involvement of carbon metabolism and aquaporins expression in xylem embolism and repair.     

Rapid hydraulic recovery in Eucalyptus pauciflora after drought: linkages between stem hydraulics and leaf gas exchange

In woody plants, photosynthetic capacity is closely linked to rates at which the plant hydraulic system can supply water to the leaf surface. Drought‐induced embolism can cause sharp declines in xylem hydraulic conductivity that coincide with stomatal closure and reduced photosynthesis. Recovery of photosynthetic capacity after drought is dependent on restored xylem function, although few data exist to elucidate this coordination. We examined the dynamics of leaf gas exchange and xylem function in Eucalyptus pauciflora seedlings exposed to a cycle of severe water stress and recovery after re‐watering. Stomatal closure and leaf turgor loss occurred at water potentials that delayed the extensive spread of embolism through the stem xylem. Stem hydraulic conductance recovered to control levels within 6 h after re‐watering despite a severe drought treatment, suggesting an active mechanism embolism repair. However, stomatal conductance did not recover after 10 d of re‐watering, effecting tighter control of transpiration post drought. The dynamics of recovery suggest that a combination of hydraulic and non‐hydraulic factors influenced stomatal behaviour post drought.     

Patterns of PIP gene expression in Populus trichocarpa during recovery from xylem embolism suggest a major role for the PIP1 aquaporin subfamily as moderators of refilling process

Embolism and the refilling of xylem vessels are intrinsic to the ability of plants to handle the transport of water under tension. Although the formation of an embolized vessel is an abiotic process, refilling against the pressure gradient requires biological activity to provide both the energy and the water needed to restore xylem transport capacity. Here, we present an analysis of the dynamics of embolism and refilling in Populus trichocarpa and follow temporal dynamics of co‐occurring changes in expression level of aquaporins. Under mesic conditions, we found that the percent loss of conductance (PLC) varied diurnally by as much as 20%, suggesting a continuous embolism/refilling cycle. An increase in water stress tilted the balance between the two processes and increased the PLC to as much as 80%. Subsequent re‐watering resulted in the reversal of water stress and recovery of PLC to pre‐stress levels. Stem parenchyma cells responded to drought stress with considerable up‐regulation of the PIP1 subfamily of water channels but not the PIP2 subfamily. Even more significant was the finding that PoptrPIP1.1 and PoptrPIP1.3 genes were up‐regulated in response to embolism, but not to water stress, and were down‐regulated after embolism removal, suggesting a local ability of plants to sense an embolism presence.     

Thermal dissipation probe measurements of sap flow in the xylem of trees documenting dynamic relations to variable transpiration given by instantaneous weather changes and the activities of a mistletoe xylem parasite

The thermal dissipation probe was described in the early 1930s for the demonstration of a volume and mass flow of sap in the conductive elements of the xylem in trees. It was subsequently developed further and is now widely used in physiological ecology including measurements in the field. Thermal dissipation demonstrates the occurrence of sap flow and allows determination of its velocity. Here we report simultaneous continuous measurements of sap flow using the thermal dissipation technique and of transpiration by infrared gas analysis for diurnal and annual cycles in a deciduous and an evergreen oak tree, Quercus robur L. and Quercus turneri Willd., respectively, in a deciduous and an evergreen conifer, Larix decidua Mill. and Pinus griffithii McClell., respectively, and the host/mistletoe consortium of the deciduous linden Tilia mandschurica Rupr. & Max. and the evergreen Viscum album L. We show (1) that in diurnal cycles sap flow closely follows dynamic changes of the rate of transpiration elicited by daily fluctuations of weather parameters (sunshine, cloudiness, air temperature and humidity), (2) that in annual cycles sap flow reflects autumnal yellowing and shedding of leaves of the deciduous trees. We report for the first time comparative measurements of sap flow towards mistletoe shoots and host branches in a parasite/host consortium. This demonstrates (3) that mistletoes maintain considerably larger sap flow rates in their xylem conduits than the adjacent host branches dragging the transpiration stream of their host towards their own shoots. We also show (4) that even after the deciduous host has shed its leaves and itself does not transpire any more the evergreen mistletoe towards its shoots can maintain the persistence of a continuous sap flow via the stem and branches of the host as long as frost does not prevent that. The work presented underlines the contention that transpiration is the driving force for sap flow with continuous files of water in the xylem. It shows for the first time that mistletoes direct the flow of water through host roots and stems towards its own shoots by not only performing stronger transpiration as it is known from the literature but also by maintaining larger sap flow rates in the xylem conduits of its stems.     

Leaf Conductance as Related to Xylem Water Potential and Carbon Dioxide Concentration in Sitka Spruce

Current year shoots of Sitka spruce [Picea sitchensis Bong. (Carr.)] from the forest canopy were equilibrated in a leaf chamber. The shoots were excised in air, and removed at differing times in order to establish a relationship between stomatal conductance and xylem water potential. The experiment was repeated at five ambient CO₂ concentrations. A second set of excised forest shoots, and shoots excised from 2-year- old nursery seedlings were allowed to evaporate freely in a controlled environment wind tunnel until a constant rate of transpiration was measured, to establish a relationship between cuticular conductance and xylem water potential. Cuticular conductance was estimated to be 0.012 cm s^-1 at high water potential and declined linearly to 0.007 cm s^-1 at ?3.5 MPa. The implication of this decline in the subsequent calculation of stomatal and mesophyll conductance is considered. Stomatal conductance remained constant at water potentials above ?1.4 MPa and was not affected by ambient carbon dioxide concentrations between 20 and 600 cm^-3. At lower water potentials, stomatal conductance declined and approached zero at ?2.5 to ?2.6 MPa. The results suggest that stomatal aperture is not controlled by either ambient or intercellular space carbon dioxide concentration, and that stomatal closure at low water potential is unlikely to be mediated by carbon dioxide.     

A technique for measuring xylem hydraulic conductance under high negative pressures

A technique for measuring hydraulic conductances of excised xylem segments exposed to high negative pressures is described. A centrifugal force is used to generate negative pressures (P) in the sample and to create a positive hydrostatic pressure difference (ΔP) between its two ends. ΔP forces water through the sample at a flow rate (F) determined optically during centrifugation. The sample hydraulic conductance k is derived from F and ΔP. The sample vulnerability curve is given by the dependence of k on P. Results for Cedrus atlantica Manetti and Laurus nobilis L. shoots are given. The technique is appropriate for the analysis of xylem refilling under negative pressure.     

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.