Xylem and cell turgor pressure probe measurements in intact roots of glycophytes: transpiration induces a change in the radial and cellular reflection coefficients

Xylem probe measurements in the roots of intact plants of wheat and barley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCl or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflection coefficients (σ_r) depended on the transpiration rate. Under very low transpiration conditions (darkness and high relative humidity), σ_r assumed values of the order of about 0·2–0·4. The σ_r values of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For transpiring plants (light intensities at least 10 μmol m^?2 s^?1; relative humidity 20–40%) the response was nearly 1:1, corresponding to radial reflection coefficients of σ_r= 1. Further increase of the light intensity to about 400 μmol m^?2 s^?1 resulted in a slight but significant decrease of the σ_r values to about 0·8. Similar measurements on maize roots confirmed our previous results (Zhu et al. 1995, Plant, Cell and Environment 18, 906–912) that, in intact transpiring plants at low light intensities of about 10 μmol m^?2 s^?1 and at relative humidities of 20–40% as well as in excised roots, the xylem pressure response was much less than expected from the external osmotic pressure (σ_r values 0·3–0·5). In contrast to wheat and barley, very high light intensities (about 700 μmol m^?2 s^?1) were needed to shift the radial reflection coefficients of maize roots to values of about 0·9. Osmotically induced xylem pressure changes were apparently linked to changes in turgor pressure in the root cortical parenchyma cells, as shown by simultaneous measurements of xylem and cell turgor pressure. In analogy to the σ_r values of the respective glycophytes, the σ_c values of the root cortical cells of wheat and barley were close to unity, whereas σ_c for maize was significantly smaller (about 0·7) under laboratory conditions. When the light intensity was increased up to about 700 μmol m^?2 s^?1 the cellular reflection coefficient of maize roots increased to about 0·95. In contrast to the σ_r values, the σ_c values of the three species investigated remained almost unchanged when the leaves were exposed to darkness and humidified air or when the roots were cut. The transpiration-dependent (species-specific) pattern of the cellular and radial reflection coefficients of the root compartment of the three glycophytes apparently resulted from (flow-dependent) concentration-polarization and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21–31). The far-reaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradients under transpiring conditions, and for the non-linear flow-force relationships in roots found by other investigators are discussed.     

High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long-Distance Water Transport

The rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below-atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg^?1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (?3 to ?6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage- and/or protein-related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h^?1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity-independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity-dependent) convectional flows. Density-driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.     

Intra- and inter-plant variation in xylem cavitation in Betula occidentalis

A modified version of a method that uses positive air pressures to determine the complete cavitation response of a single axis is presented. Application of the method to Betula occidentalis Hook, gave a cavitation response indistinguishable from that obtained by dehydration, thus verifying the technique and providing additional evidence that cavitation under tension occurs by air entry through interconduit pits. Incidentally, this also verified pressure-bomb estimates of xylem tension and confirmed the existence of large (i.e. >0·4 MPa) tensions in xylem, which have been questioned in recent pressure-probe studies. The air injection method was used to investigate variation within and amongst individuals of B. occidentalis. Within an individual, the average cavitation tension increased from 0·66±0·27 MPa in roots (3·9 to 10·7 mm diameter), to 1·17±0·10 MPa in trunks (12 to 16 mm diameter), to 1·36±0·04 MPa in twigs (3·9 to 5 mm diameter). Cavitation tension was negatively correlated with the hydraulically weighted mean of the vessel diameter, and was negatively correlated with the conductance of the xylem per xylem area. Native cavitation was within the range predicted from the measured cavitation response and in situ maximum xylem tensions: roots were significantly cavitated compared with minimal cavitation in trunks and twigs. Leaf turgor pressure declined to zero at the xylem tensions predicted to initiate cavitation in petiole xylem (1·5 MPa). Amongst individuals within B. occidentalis, average cavitation tension in the main axis varied from 0·90 to 1·90 MPa and showed no correlation with vessel diameter. The main axes of juveniles (2–3 years old) had significantly narrower vessel diameters than those of adults, but there was no difference in the average cavitation tension. However, juvenile xylem retained hydraulic conductance to a much higher xylem tension (3·25 MPa) than did adult xylem (2·25 MPa), which could facilitate drought survival during establishment.     

Comparative ecophysiology of CAM and C3 bromeliads. IV. Plant water relations

Abstract An investigation was carried out into the water relations of CAM and C₃ bromeliads in their natural habitat during the dry season in Trinidad. Measurements were made of xylem tension with the pressure chamber and of cell-sap osmotic pressure and titratable acidity on crushed leaf samples. A steady-state CO₂ and H₂O-vapour porometer was also used so that changes in leaf water relations during individual day-night cycles could be directly related to gas-exchange patterns in situ. Xylem tension changed in parallel with transpiration rate and in general reached its maximum value in CAM bromeliads at night and in C₃ bromeliads during the day. In addition, large nocturnal increases in cell-sap osmotic pressure and titratable acidity (ΔH⁺) typically occurred in the CAM bromeliads. The C₃-CAM intermediate Guzmania monostachia showed slight nocturnal acidification, but had higher values of xylem tension during the day. Very high values of AH+ were observed in the CAM species when the tanks of the epiphytic bromeliads contained water: Aechmea nudicaulis showed a mean maximum ΔH⁺ of 474 mol m^?3, the highest value so far observed for CAM plants. On some nights dew formed on the leaf surfaces of the epiphytes, partially curtailing gas exchange and leading to a marked decrease in xylem tension in both C₃ and CAM species. Between-site comparisons were also made for a wide range of habitats from arid coastal scrub to montane rain forest. Compared with values characteristic of other life-forms, xylem tension and cell-sap osmotic pressure were low for all bromeliads, and did not differ significantly in co-occurring CAM and C₃ bromeliads. Mean maximum xylem tension (10 species in total) ranged from 0.29 M Pa at the montane sites to 0.67 MPa at the most arid site, and mean minimum osmotic pressure (17 species) from 0.51 to 0.97 MPa. At the arid sites the bromeliads were exclusively CAM species, two of which (Aechmea aquilega and Bromelia plumieri) grew terrestrially in the undergrowth of the coastal scrub. Xylem tension in these species was low enough to indicate that they must be functionally independent of the substratum during the dry season. In the wetter part of Trinidad, no between-site differences in leaf water relations were found along an altitudinal gradient in the Northern Mountain Range; seasonal differences in this area were also small. Overall, leaf water relations and gas exchange in the bromeliads were strongly affected both by short-term changes in water availability and by longer-term climatic differences in the various regions of the island.     

Comparative measurements of the xylem pressure ofNicotiana plants by means of the pressure bomb and pressure probe

Determination of the pressure in the water-conducting vessels of intactNicotiana rustica L. plants showed that the pressure probe technique gave less-negative values than the Scholander-bomb method. Even though absolute values of the order of −0.1 MPa could be directly recorded in the xylem by means of the pressure probe, pressures between zero and atmospheric were also frequently found. The data obtained by the pressure probe for excised leaves showed that the Scholander bomb apparently did not read the actual tension in the xylem vessles ofNicotiana plants. The possibility that the pressure probe gave false readings was excluded by several experimental controls. In addition, cavitation and leaks either during the insertion of the microcapillary of the pressure probe, or else during the measurements were easily recognized when they occurred because of the sudden increase of the absolute xylem tension to that of water vapour or to atmospheric, respectively. Tension values of the same order could also be measured by means of the pressure probe in the xylem vessels of pieces of stem cut from leaves and roots under water and clamped at both ends. The magnitude of the absolute tension depended on the osmolarity of the bathing solution which was adjusted by addition of appropriate concentrations of polyethylene glycol. Partial and uniform pressurisation of plant tissues or organs, or of entire plants (by means of the Scholander bomb or of a hyperbaric chamber, respectively) and simultaneous recording of the xylem tension using the pressure probe showed that a 1∶1 response in xylem pressure only occurred under a few circumstances. A 1∶1 response required that the xylem vessels were in direct contact with an external water reservoir and/or that the tissue was (pre-)infiltrated with water. Corresponding pressure-probe measurements in isolated vascular bundles ofPlantago major L. orP. lanceolata L. plants attached to a Hepp-type osmometer indicated that the magnitude of the tension in the xylem vessels was determined by the external osmotic pressure of the reservoir. These and other experiments, as well as analysis of the data using classical thermodynamics, indicated that the turgor and the internal osmotic pressure of the accessory cells along the xylem vessels play an important role in the maintenance of a constant xylem tension. This conclusion is consistent with the cohesion theory. In agreement with the literature (P.E. Weatherley, 1976, Philos. Trans. R. Soc. London Ser. B23, 435–444; 1982, Encyclopedia of plant physiology, vol. 12B, 79-109), it was found that the tension in the xylem of intact plants under normal and elevated ambient pressure (as measured with the pressure probe) under quasi-stationary conditions was independent of the transpiration rate over a large range, indicating that the conductance of the flow path must be flow-dependent.     

Xylem pressure response in maize roots subjected to osmotic stress: determination of radial reflection coefficients by use of the xylem pressure probe

The absolute pressure in conducting xylem vessels of roots of 2-week-old, slowly transpiring intact maize plants (bathed in nutrition medium) was determined to be +0·024 ± 0·044 MPa using the xylem pressure probe. When the roots were subjected to osmotic stress (NaCI, KCI or sucrose), the xylem pressure decreased immediately and became more negative. However, the response of xylem pressure to osmotic stress was considerably attenuated, indicating that the radial reflection coefficients, σ₁₃ of the maize root for these solutes were rather low (between 0·2 and 0·4 depending on the concentration of the osmoticum). The low values of a, may be caused (partly) by unstirred layer effects. In repeated osmoticum/nutrition regimes a complex pattern of changes in xylem pressure was observed which was apparently linked to the interplay between transpiration and (passive and/or active) solute loading of the xylem. These processes were not observed when the roots were subjected to osmotic stress after excision. In this case, a biphasic response was observed comparable to that found for excised roots using the root pressure probe.     

Hydraulic properties of living late metaxylem and interactions between transpiration and xylem pressure in maize

A new approach to study dynamic interactions between transpiration and xylem pressure in intact plants is presented. Pressure probe measurements were preformed in living (immature) late metaxylem of maize roots rather than in adjacent mature xylem. This eliminated technical limitations related to the measurement of negative pressures. Water relations of single cells showed that turgor and volumetric elastic modulus were significantly larger in living metaxylem than in cortical cells; hydraulic conductivity was similar in both types of root cells. Increasing transpiration induced an immediate decrease of xylem pressure, and vice versa. Turgor in the living metaxylem could be continuously recorded for more than 1 h. The relationship between xylem pressure and transpiration yielded a root hydraulic resistance of 1.3 x 10⁹ MPa s m^-3. Control experiments indicated that the response of living xylem in the positive pressure range essentially paralleled that of mature root xylem in the negative range. In mature xylem, pressures as low as -0.55 MPa were recorded for short periods (several minutes). Several tests verified that the pressure probe was in contact with mature xylem during the measurements of tensions. The results demonstrate convincingly that transpiration generates an effective driving force for water uptake in roots, a central feature of the cohesion theory.Key words: Hydraulic conductivity, negative pressure, root development, turgor, water transport, Zea mays.      

Simultaneous measurement of water flow velocity and solute transport in xylem and phloem of adult plants of Ricinus communis over a daily time course by nuclear magnetic resonance spectrometry

A new method for simultaneously quantifying rates of flow in xylem and phloem using the FLASH imaging capabilities of nuclear magnetic resonance (NMR) spectrometry was applied in this study. The method has a time resolution of up to 4 min (for the xylem) and was used to measure the velocity of flows in phloem and xylem for periods of several hours to days. For the first time, diurnal time course measurements of flow velocities and apparent volume flows in phloem and xylem in the hypocotyl of 40‐d‐old Ricinus communis L were obtained. Additional data on gas exchange and the chemical composition of leaves, xylem and phloem sap were used to assess the role of leaves as sinks for xylem sap and sources for phloem. The velocity in the phloem (0·250 ± 0·004 mm s^?1) was constant over a full day and not notably affected by the light/dark cycle. Sucrose was loaded into the phloem and transported at night, owing to degradation of starch accumulated during the day. Concentrations of solutes in the phloem were generally less during the night than during the day but varied little within either the day or night. In contrast to the phloem, flow velocities in the xylem were about 1·6‐fold higher in the light (0·401 ± 0·004 mm s^?1) than in the dark (0·255 ± 0·003 mm s^?1) and volume flow varied commensurately. Larger delays were observed in changes to xylem flow velocity with variation in light than in gas exchange. The relative rates of solute transport during day and night were estimated on the basis of relative flow and solute concentrations in xylem and phloem. In general, changes in relative flow rates were compensated for by changes in solute concentration during the daily light/dark cycle. However, the major solutes (K⁺, NO₃^?) varied appreciably in relative concentrations. Hence the regulation of loading into transport systems seems to be more important to the overall process of solute transport than do changes in mass flow. Due to transport behaviour, the chemical composition of leaves varied during the day only with regard to starch and soluble carbohydrates.     

Long-term xylem pressure measurements in the lianaTetrastigma voinierianum by means of the xylem pressure probe

Diurnal changes of xylem pressure in the lianaTetrastigma voinierianum have been measured under greenhouse conditions by means of the recently developed xylem pressure probe. During the early morning hours, tensions in the vessels developed more or less rapidly with time, depending on light intensity. On sunny days, absolute negative pressures down to about -0.4 MPa (atmospheric = 0.1 MPa) were recorded around noon in petiolar or stem xylem vessels, whereas on rainy or cloudy days the xylem pressure remained in the positive sub-atmospheric or slightly negative pressure range. Towards the evening the tension in the vessels always decreased, i.e. the xylem pressure shifted to about atmospheric, or even above-atmospheric, values during the night. Simultaneous xylem pressure recordings at heights of 1 and 5 m frequently yielded either no gradient in tension at all, or far less than expected from the Cohesion Theory. Occasionally, tension gradients were even opposite to those predicted by this theory. Stem-toleaves pressure gradients in accord with the Cohesion Theory were recorded only when tension had been developed during sunny days in the upper branches of the liana, because increases in tension were not immediately propagated to the xylem of the leaves at ground level, as would be expected from a strictly coupled hydraulic system. Parallel recordings of the xylem tension using the pressure chamber yielded rather variable values ranging from 0.1 to 1 MPa; diurnal pressure changes could not be detected at all. The data are discussed on the basis of the equation for the chemical activity of water. They strongly suggest that the xylem tension induced by transpiration is not the sole force for water ascent. Other forces, such as osmotic pressure or convectional and interfacial forces, which to a remarkable extent have already been postulated for decades, seem to be equally important.Abbreviation R.H. relative humidity The authors are very grateful to Professor D. Fürnkranz, Institut für Botanik der Universität Salzburg, for his interest and help with the greenhouse facility, to Walter Gigerl for expert technical assistance, to Heike Schneider and Notburga Gierlinger for the petiolestaining experiments. This work was supported by a grant of the Deutsche Forschungsgemeinschaft to U.Z. (NMR-Graduiertenkolleg Ha 1232/8-1).     

Xylem water transport: is the available evidence consistent with the cohesion theory?

Since its introduction in the late 19th century, the so-called cohesion theory has become widely accepted as explaining the mechanism of the ascent of sap. According to the cohesion theory, the minimum standing vertical xylem tension gradient should be 0·01 MPa m⁻¹. When transpiration is occurring, frictional resistances are expected to make this gradient considerably steeper. The results of numerous pressure chamber measurements reported in the literature are generally regarded as corroborating the cohesion theory. Nevertheless, several reports of pressure chamber measurements in tall trees appear to be incompatible with predictions of the cohesion theory. Furthermore, the pressure chamber is an indirect method for inferring xylem pressure, which, until recently, has not been validated by comparison against a direct method. The xylem pressure probe provides a means of testing the validity of the pressure chamber and other indirect techniques for estimating xylem pressure. We discuss here the results of concurrent measurements made with the pressure chamber and the xylem pressure probe, particularly recent measurements made at the top of a tall tropical tree during the rainy season. These measurements indicate that the pressure chamber often substantially overestimates the tension previously existing in the xylem, especially in the partially dehydrated tissue of droughted plants. We also discuss other evidence obtained from classical and recent approaches for studying water transport. We conclude that the available evidence derived from a wide range of independent approaches warrants a critical reappraisal of tension-driven water transport as the exclusive mechanism of long-distance water transport in plants.     

Diurnal changes in xylem pressure and mesophyll cell turgor pressure of the lianaTetrastigma voinierianum: The role of cell turgor in long-distance water transport

Summary Long-term xylem pressure measurements were performed on the lianaTetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the cell turgor and the osmotic pressure of leaf cells at heights of 1 m and 5 m (partly also at a height of 9.5 m) were recorded. The results showed that tensions (and height-varying tension gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost 1 1 decrease in the cell turgor pressure of the mesophyll cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressure towards more positive values correlated with an increase in the cell turgor pressure (ratio of about 1 1). The cell osmotic pressure remained nearly constant during the day and was about 0.75–0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the turgor pressure primarily determines the corresponding pressure in the vessels (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and cell turgor pressure. From the xylem, cell turgor, and cell osmotic pressure data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35–0.45 MPa; this value was apparently not subject to diurnal changes. Considering that the xylem pressure is determined by the turgor pressure (and vice versa), the xylem pressure of the liana could not drop to — in agreement with the experimental results — less than -0.4 MPa, because this pressure corresponds to zero turgor pressure.     

A critical comparison of the pressure-probe and pressure-chamber techniques for estimating leaf-ceil turgor pressure in Kalanchoë daigremontiana

The aim of the present study was to test the accuracy of the pressure-chamber technique as a method for estimating leaf-cell turgor pressures. To this end, pressure-probe measurements of cell turgor pressure (P^cell) were made on mesophyll cells of intact, attached leaves of Kalanchoë daigremontiana. Immediately following these measurements, leaves were excised and placed in a pressure chamber for the determination of balance pressure (P^bal). Cell-sap osmotic pressure (?^cell) and xylem-sap osmotic pressure (?^xyl) were also measured, and an average cell turgor pressure calculated as P^cell=?^cell–?^xyl–P^bal. The apparent value of P^bal was positively correlated with the rate of increase of chamber pressure, and there was also a time-dependent increase associated with water loss. On expressing sap from the xylem, ?^xyl fell to a plateau value that was positively correlated with ?^cell. Correcting for these effects yielded estimates of P^bal and ?^xyl at the time of leaf excision. On average, the values of P^cell obtained with the two techniques agreed to within ±002 MPa (errors are approximate 95% confidence limits). If ?^xyl were ignored, however, the calculated turgor pressures would exceed the measured values by an average of 0.074 ± 0.012MPa, or 48% at the mean measured pressure of 0.155 MPa. We conclude that the pressure-chamber technique allows a good estimate to be made of turgor pressure in mesophyll cells of K. daigremontiana, provided that ?^xyl is included in the determination. The 1:1 relationship between the measured and calculated turgor pressures also implies that the weighted-average reflection coefficient for the mesophyll cell membranes is close to unity.     

Canopy transpiration in a chronosequence of Central Siberian pine forests

Tree transpiration was measured in 28, 67, 204 and 383‐y‐old uniform stands and in a multicohort stand (140–430 y) of Pinus sylvestris ssp. sibirica Lebed. in Central Siberia during August 1995. In addition transpiration of three codominant trees was monitored for two years in a 130‐y‐old stand. All stands established after fire. Leaf area index (LAI) ranged between 0.6 (28‐y‐old stand) and 1.6 for stands older than 67‐y. Stand xylem area at 1.3 m height increased from 4 cm² m^?2 (28‐y) to 11.5 cm² m^?2 (67‐y) and decreased again to 7 cm² m^?2 in old stands. Above‐ground living biomass increased from 1.5 kg dry weight m^?2 (28‐y) to 14 kg dry weight m^?2 (383‐y). Day‐to‐day variation of tree transpiration in summer was dependent on net radiation, vapour pressure deficit, and soil water stress. Tree‐to‐tree variation of xylem flux was small and increased with heterogeneity in canopy structure. Maximum rates of xylem flux density followed the course of net radiation from mid April when a constant level of maximum rates was reached until mid September when low temperatures and light strongly reduced flux density. Maximum sap flux density (60 g m^?2 s^?1) and canopy transpiration (1.5 mm d^?1) were reached in the 67‐y stand. Average canopy transpiration of all age classes was 0.72 ± 0.3 mm d^?1. Canopy transpiration (E) was not correlated with LAI but related to stand sapwood area SA (E = ? 0.02 + 1.15SA R²) which was determined by stand density and tree sapwood area.     

Xylem Flow and its Driving Forces in a Tropical Liana: Concomitant Flow-Sensitive NMR Imaging and Pressure Probe Measurements

Abstract: Flow-sensitive NMR imaging and pressure probe techniques were used for measuring xylem water flow and its driving forces (i.e., xylem pressure as well as cell turgor and osmotic pressure gradients) in a tropical liana, Epipremnum aureum. Selection of tall specimens allowed continuous and simultaneous measurements of all parameters at various distances from the root under diurnally changing environmental conditions. Well hydrated plants exhibited exactly linearly correlated dynamic changes in xylem tension and flow velocity. Concomitant multiple-probe insertions along the plant shoot revealed xylem and turgor pressure gradients with changing magnitudes due to environmental changes and plant orientation (upright, apex-down, or horizontal). The data suggest that in upright and - to a lesser extent - in horizontal plants the transpirational water loss by the cells towards the apex during the day is not fully compensated by water uptake through the night. Thus, longitudinal cellular osmotic pressure gradients exist. Due to the tight hydraulic coupling of the xylem and the tissue cells these gradients represent (besides the transpiration-induced tension in the xylem) an additional tension component for anti-gravitational water movement from the roots through the vessels to the apex.     

A novel approach to the in situ measurement of oxygen concentrations in the sapwood of woody plants

A novel technique for the physico‐chemical analysis of xylem sap by underwater access to the sapwood of trees is described. In situ measurements of dissolved oxygen in the sapwood are performed by combining this technique with a novel optical method for oxygen detection. In early spring, the oxygen concentration of the sapwood of Betula pendula was in the range of 80–230 µmol O₂ L^?1, corresponding to an oxygen deficit of 40–75% of air saturation. Oxygen concentration maxima and minima occurred early in the morning and in the afternoon, respectively, whereas xylem sap temperatures showed the reverse pattern. In the sapwood, hypoxia increased from the beginning of bud break until frondescence, when a deficit of 86% of air saturation marked the upper limit of oxygen depletion. There seemed to be no relationship between daily variations of oxygen concentration and xylem sap pressure. In summer, sap flow was a major determinant for the diurnal variation of dissolved oxygen concentration. Oxygen supply to the sapwood was determined by both radial influx into the trunk through intercellular gas spaces and transport of dissolved oxygen via xylem sap flow. Radial influx seemed to be favoured during night‐time, when the trunk was warmer than ambient air. During daytime, the hypoxia of the sapwood rose and increased sharply in the evening, when sap flow velocity approximated zero. High temperature in the sapwood enhanced the respiratory oxygen consumption of the wood parenchyma while the supply of dissolved oxygen via the transpiration stream became ineffective.     

The canopy water relations of old-growth Douglas-fir trees

 We investigated whole tree water relations in 56–65 m tall, old-growth Pseudotsuga menziesii trees within the Wind River Canopy Crane site, Carson, Washington, USA. We measured at predawn and solar noon the vertical gradients in xylem pressure potential using a pressure chamber. On an Abies amabilis sapling located in the understory at the base of one of the study trees, predawn and solar noon xylem pressure potentials were also measured. Xylem pressure potential data were measured from late June through early September 1996 on foliage sampled from 1 to 64.5 m. Over this height gradient, predawn water potentials ranged from –0.23 to –1.10 MPa. Solar noon values showed an even greater range (from –0.44 to –2.51 MPa). At predawn, the water potential gradient approached the theoretical hydrostatic gradient (–0.0105 vs –0.010 MPa m^–1). The gradient at solar noon was steeper (–0.0331 MPa m^–1). Instantaneous stomatal conductances were not greatly different between young, sapling-sized and old-growth trees [0.094±0.033 (SD) vs 0.086±0.045 cm s^–1, respectively]. Stomata of both size classes of trees appeared very sensitive to increasing vapor pressure deficits. A comparison of stable carbon isotope values from the old-growth and sapling-sized trees indicated lower stomatal conductances in the old-growth. This study provides sound documentation regarding the utility of the cohesion theory in the interpretation of water potential gradients. This study also emphasizes inherent differences between sapling-sized and tall, old-growth trees. Received: 10 January 1998 / Accepted: 12 October 1998     

树木树液上升机理研究进展

水分在植物体内的运输一直是很多植物生理生态学家所关注的一个重要问题。介绍了内聚力学说的基本假设和其存在争议,总结了近年来这一研究领域的几个热点问题,主要包括：（1）木质部栓塞及其恢复机理;（2）木质部压力探针和压力室法测定的木质部张力值不一致的现象及其可能原因;（3）补偿压学说;（4）不同界面层张力以及输水管道的毛细作用力、薄壁细胞膨压和木质部渗透压、逆向蒸腾等在树木汁液上升中的贡献;（5）最近发现的存在于木质部导管伴胞和韧皮部薄壁细胞等质膜中的水孔蛋白在植物水分运输中的调控作用等。这些方面在解释树木的树液上升中都起着重要的作用。     

Calcium ion supplementation increases brook trout Salvelinus fontinalis spermatozoa activation at the end of the spawning season

The role of environmental ion composition and osmolality in calcium ion (Ca²⁺) signalling of spermatozoa activation over the course of the spawning period of brook trout Salvelinus fontinalis was investigated. Motility at the end of spawning was low (mean ± s.d. of 5 ± 2% motile spermatozoa with curvilinear velocity of 25 ± 8 µm s^?1). Addition of 10 mM Ca²⁺ to the activation medium resulted in values similar to those recorded during the middle of the spawning period (mean ± s.d. of 95 ± 6% motile spermatozoa with curvilinear velocity of 130 ± 15 µm s^?1).     

Abscisic acid introduced into the transpiration stream accumulates in the guard-cell apoplast and causes stomatal closure

Petioles of water‐sufficient intact Vicia faba L. plants were infused with 1 µm abscisic acid (ABA) to simulate the import of root‐source ABA. This protocol permitted quantitative ABA delivery, up to 300 pmol ABA over 60 min, to the leaf without ambiguities associated with perturbations in plant–water status. The ABA concentrations in whole‐leaf samples and in apoplastic sap increased with the amount infused; ABA degradation was not detected. The ABA concentration in apoplastic sap was consistent with uptake of imported ABA into the leaf symplast, but this interpretation is qualified. Our focus was quantitative cellular compartmentation of imported ABA in guard cells. Unlike when leaves are stressed, the guard‐cell symplast ABA content did not increase because of ABA infusion (P = 0·48; 3·0 ± 0·5 versus 4·0 ± 1·2 fg guard‐cell‐pair^?1). However, the guard‐cell apoplast ABA content increased linearly (R² = 0·98) from ?0·2 ± 0·5 to 3·1 ± 1·3 fg guard‐cell‐pair^?1 (≈ 3·1 µm ) and was inversely related to leaf conductance (R² = 0·82). Apparently, xylem ABA accumulates in the guard‐cell wall as a result of evaporation of the apoplast solution. This mechanism provides for integrating transpiration rate and ABA concentration in the xylem solution.