Day-night changes in leaf water relations associated with the rhythm of crassulacean acid metabolism in Kalanchoë daigremontiana

A study was made of the day-night changes under controlled environmental conditions in the bulk-leaf water relations of Kalanchoë daigremontiana, a plant showing Crassulacean acid metabolism. In addition to nocturnal stomatal opening and net CO₂ uptake, the leaves of well-watered plants showed high rates of gas exchange during the whole of the second part of the light period. Measurements with the pressure chamber showed that xylem tension increased during the night and then decreased towards a minimum at about midday; a significant increase in xylem tension was also seen in the late afternoon. Cell-sap osmotic pressure paralleled leaf malate content and was maximum at dawn and minimum at dusk. The relationship between these two variables indicated that the nocturnally synthesized malate was apparently behaving as an ideal osmoticum. To estimate bulk-leaf turgor pressure, values for water potential were derived by correcting the pressurechamber readings for the osmotic pressure of the xylem sap. This itself was found to depend on the malate content of the leaves. Bulk-leaf turgor pressure changed rhythmically during the day-night cycle; turgor was low during the late afternoon and for most of the night, but increased quickly to a maximum of 0.20 MPa around midday. In water-stressed plants, where net CO₂ uptake was restricted to the dark period, there was also an increase in bulk-leaf turgor pressure at the start of the light period, but of reduced magnitude. Such changes in turgor pressure are likely to be of considerable ecological importance for the water economy of crassulacean-acid-metabolism plants growing in their natural habitats.Abbreviation and symbols CAM Crassulacean acid metabolism - P turgor pressure - osmotic pressure - water potential Dedicated to Professor Dr. H. Ziegler on the occasion of his 60th birthday     

What are the driving forces for water lifting in the xylem conduit?

After Renner had shown convincingly in 1925 that the transpirational water loss generates tensions larger than 0.1 MPa (i.e. negative pressures) in the xylem of cut leafy twigs the Cohesion Theory proposed by Böhm, Askenasy, Dixon and Joly at the end of the 19th century was immediately accepted by plant physiologists. Introduction of the pressure chamber technique by Scholander et al. in 1965 enforced the general belief that tension is the only driving force for water lifting although substantial criticism regarding the technique and/or the Cohesion Theory was published by several authors. As typical for scientific disciplines, the advent of minimal‐ and non‐invasive techniques in the last decade as well as the development of a new, reliable method for xylem sap sampling have challenged this view. Today, xylem pressure gradients, potentials, ion concentrations and volume flows as well as cell turgor pressure gradients can be monitored online in intact transpiring higher plants, and within a given physiological context by using the pressure probe technique and high‐resolution NMR imaging techniques, respectively. Application of the pressure probe technique to transpiring plants has shown that negative absolute pressures (down to ? 0.6 MPa) and pressure gradients can exist temporarily in the xylem conduit, but that the magnitude and (occasionally) direction of gradients contrasts frequently the belief that tension is the only driving force. This seems to be particularly the case for plants faced with problems of height, drought, freezing and salinity as well as with cavitation of the tensile water. Reviewing the current data base shows that other forces come into operation when exclusively tension fails to lift water against gravity due to environmental conditions. Possible candidates are longitudinal cellular and xylem osmotic pressure gradients, axial potential gradients in the vessels as well as gel‐ and gas bubble‐supported interfacial gradients. The multiforce theory overcomes the problem of the Cohesion Theory that life on earth depends on water being in a highly metastable state.     

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.     

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.     

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。     

Transpiration Induces Radial Turgor Pressure Gradients in Wheat and Maize Roots

Previous studies have shown both the presence and the absence of radial turgor and osmotic pressure gradients across the cortex of roots. In this work, gradients were sought in the roots of wheat (Triticum aestivum) and maize (Zea mays) under conditions in which transpiration flux across the root was varied This was done by altering the relative humidity above the plant, by excising the root, or by using plants in which the leaves were too young to transpire. Roots of different ages (4-65 d) were studied and radial profiles at different distances from the tip (5-30 mm) were measured. In both species, gradients of turgor and osmotic pressure (increasing inward) were found under transpiring conditions but not when transpiration was inhibited. The presence of radial turgor and osmotic pressure gradients, and the behavior of the gradient when transpiration is interrupted, indicate that active membrane transport or radial solvent drag may play an important role in the distribution of solutes across the root cortex in transpiring plants. Contrary to the conventional view, the flow of water and solutes across the symplastic pathway through the plasmodesmata cannot be inwardly directed under transpiring conditions.     

Direct and indirect measurements of Phloem turgor pressure in white ash

Direct determinations and indirect calculations of phloem turgor pressure were compared in white ash (Fraxinus americana L.). Direct measurements of trunk phloem turgor were made using a modified Hammel-type phloem needle connected to a pressure transducer. Turgor at the site of the direct measurements was calculated from the osmotic potential of the phloem sap and from the water potential of the xylem. It was assumed that the water potentials of the phloem and xylem were close to equilibrium at any one trunk location, at least under certain conditions. The water potential of the xylem was determined from the osmotic potential of xylem sap and from the xylem tension of previously bagged leaves, measured with a pressure chamber. The xylem tension of bagged leaves on a branch adjacent to the site of the direct measurements was considered equivalent to the xylem tension of the trunk at that point. While both the direct and indirect measurements of phloem turgor showed clear diurnal changes, the directly measured pressures were consistently lower than the calculated values. It is not clear at present whether the discrepancy between the two values lies primarily in the calculated or in the measured pressures, and thus, the results from both methods as described here must be regarded as estimates of true phloem turgor.     

Water relations in silver birch during springtime: How is sap pressurised?

• Positive sap pressures are produced in the xylem of birch trees in boreal conditions during the time between the thawing of the soil and bud break. During this period, xylem embolisms accumulated during wintertime are refilled with water. The mechanism for xylem sap pressurization and its environmental drivers are not well known.
• We measured xylem sap flow, xylem sap pressure, xylem sap osmotic concentration, xylem and whole stem diameter changes, and stem and root non‐structural carbohydrate concentrations, along with meteorological conditions at two sites in Finland during and after the sap pressurisation period.
• The diurnal dynamics of xylem sap pressure and sap flow during the sap pressurisation period varied, but were more often opposite to the diurnal pattern after bud burst, i.e. sap pressure increased and sap flow rate mostly decreased when temperature increased. Net conversion of soluble sugars to starch in the stem and roots occurred during the sap pressurisation period. Xylem sap osmotic pressure was small in comparison to total sap pressure, and it did not follow changes in environmental conditions or tree water relations.
• Based on these findings, we suggest that xylem sap pressurisation and embolism refilling occur gradually over a few weeks through water transfer from parenchyma cells to xylem vessels during daytime, and then the parenchyma are refilled mostly during nighttime by water uptake from soil. Possible drivers for water transfer from parenchyma cells to vessels are discussed. Also the functioning of thermal dissipation probes in conditions of changing stem water content is discussed.

     

Water Flow Across the Sieve-Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. II. Phloem in the Stem

Confirming a previous analysis by Lang (1974), it is concludedthat in tree trunks, phloem turgor and turgor gradients maybe estimated from osmotic pressure and osmotic-pressure gradients,respectively. The present analysis is an improvement becauseit is based on observed osmotic-pressure gradients rather thansupposed turgor gradients, and allowance is made for sucroseunloading and gradients of external water potential. It is concludedthat the rate of sucrose unloading in tree trunks must be lessthan 50 nmol m^–2 S m^–1. In small plants, higherrates of unloading (100 nmol m m^–2 S m^–1) and steeperconcentration gradients will lead to larger errors, but turgorpressures can still be estimated with acceptable accuracy. Oneshould be more cautious when considering turgor gradients insmall plants, although it seems likely that reasonable estimateswill still be obtained. Assuming plasmodesmatal transport throughan unconstricted cytoplasmic annulus, it is concluded that thesieve elements and their associated cells will sustain verysimilar turgor and osmotic pressures. Convection and diffusioncan both contribute significantly to plasmodesmatal sucroseunloading. Similarly, the plasmodesmatal volume flux will reflecta combination of pressure flow and osmosis. Water fluxes acrossthe sieve element plasmalemma and through the plasmodesmatacan be in opposite directions. It may be possible to assessthe extent of hydraulic coupling between the sieve elementsand their associated cells from studies of phloem water relations Phloem, turgor, osmotic pressure, plasmodesmata, phloem unloading, Munch hypothesis     

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.     

Gradients of turgor, osmotic pressure, and water potential in the cortex of the hypocotyl of growing ricinus seedlings : effects of the supply of water from the xylem and of solutes from the Phloem

To evaluate the possible role of solute transport during extension growth, water and solute relations of cortex cells of the growing hypocotyl of 5-day-old castor bean seedlings (Ricinus communis L.) were determined using the cell pressure probe. Because the osmotic pressure of individual cells (πⁱ) was also determined, the water potential (ψ) could be evaluated as well at the cell level. In the rapidly growing part of the hypocotyl of well-watered plants, turgor increased from 0.37 megapascal in the outer to 1.04 megapascal in the inner cortex. Thus, there were steep gradients of turgor of up to 0.7 megapascal (7 bar) over a distance of only 470 micrometer. In the more basal and rather mature region, gradients were less pronounced. Because cell turgor ≈ πⁱ and ψ ≈ 0 across the cortex, there were also no gradients of ψ across the tissue. Gradients of cell turgor and πⁱ increased when the endosperm was removed from the cotyledons, allowing for a better water supply. They were reduced by increasing the osmotic pressure of the root medium or by cutting off the cotyledons or the entire hook. If the root was excised to interrupt the main source for water, effects became more pronounced. Gradients completely disappeared and turgor fell to 0.3 megapascal in all layers within 1.5 hours. When excised hypocotyls were infiltrated with 0.5 millimolar CaCl₂ solution under pressure via the cut surface, gradients in turgor could be restored or even increased. When turgor was measured in individual cortical cells while pressurizing the xylem, rapid responses were recorded and changes of turgor exceeded that of applied pressure. Gradients could also be reestablished in excised hypocotyls by abrading the cuticle, allowing for a water supply from the wet environment. The steep gradients of turgor and osmotic pressure suggest a considerable supply of osmotic solutes from the phloem to the growing tissue. On the basis of a new theoretical approach, the data are discussed in terms of a coupling between water and solute flows and of a compartmentation of water and solutes, both of which affect water status and extension growth.     

Water storage and osmotic pressure influences on the water relations of a dicotyledonous desert succulent

Abstract Water storage and nocturnal increases in osmotic pressure affect the water relations of the desert succulent Ferocactus acanthodes, which was studied using an electrical circuit analog based on the anatomy and morphology of a representative individual. Transpiration rates and osmotic pressures over a 24-h period were used as input variables. The model predicted water potential, turgor pressure and water flow for various tissues. Plant capacitances, storage resistances and nocturnal increases in osmotic pressure were varied to determine their role in the water relations of this dicotyledonous succulent. Water coming from storage tissues contributed about one-third of the water transpired at night: the majority of this water came from the nonphotosynthetic, water storage parenchyma of the stem. Time lags of 4 h were predicted between maximum transpiration and maximum water uptake from the soil. Varying the capacitance of the plant caused proportional changes in osmotically driven water movement but changes in storage resistance had only minor effects. Turgor pressure in the chlorenchyma depended on osmotic pressure, but was fairly insensitive to doubling or halving of the capacitance or storage resistance of the plant. Water uptake from the soil was only slightly affected by osmotic pressure changes in the chlorenchyma. For this stem succulent, the movement of water from the chlorenchyma to the xylem and the internal redistribution of water among stem tissues were dominated by nocturnal changes in chlorenchyma osmotic pressure, not by transpiration.     

Cell water potential,osmotic potential,and turgor in the epidermis and mesophyll of transpiring leaves

Water potential, osmotic potential and turgor measurements obtained by using a cell pressure probe together with a nanoliter osmometer were compared with measurements obtained with an isopiestic psychrometer. Both types of measurements were conducted in the mature region of Tradescantia virginiana L. leaves under non-transpiring conditions in the dark, and gave similar values of all potentials. This finding indicates that the pressure probe and the osmometer provide accurate measurements of turgor, osmotic potentials and water potentials. Because the pressure probe does not require long equilibration times and can measure turgor of single cells in intact plants, the pressure probe together with the osmometer was used to determine in-situ cell water potentials, osmotic potentials and turgor of epidermal and mesophyll cells of transpiring leaves as functions of stomatal aperture and xylem water potential. When the xylem water potential was-0.1 MPa, the stomatal aperture was at its maximum, but turgor of both epidermal and mesophyll cells was relatively low. As the xylem water potential decreased, the stomatal aperture became gradually smaller, whereas turgor of both epidermal and mesophyll cells first increased and afterward decreased. Water potentials of the mesophyll cells were always lower than those of the epidermal cells. These findings indicate that evaporation of water is mainly occurring from mesophyll cells and that peristomatal transpiration could be less important than it has been proposed previously, although peristomatal transpiration may be directly related to regulation of turgor in the guard cells.     

植物生理学研究中的压力探针技术

压力探针技术是一种用来测定微系统中压力大小和变化的新技术。其最初被设计用于直接测定巨型藻类的细胞膨压。随着操作装置的进一步微型化和精密化, 后来被应用于测定普通高等植物细胞膨压及其它水分关系参数。该技术的发展建立在一系列相应的流体物理学理论基础上。通过这些物理学公式的计算, 该技术能测定跨细胞膜或器官的水分运输速度以及它们的水力学导度; 测定溶液中水分和溶质的相对运输速度以及它们之间的相互影响; 还可以测定细胞壁的刚性等。目前压力探针技术已成为植物生理学和生态学领域研究中的多用途技术。它可以在细胞水平上原位测定水分及溶质跨膜运输及分布情况, 这对于阐明水通道功能具有极其重要的意义。此外, 木质部压力探针技术是目前唯一可以直接测定导管或管胞中负压的工具。该技术还可以用于单细胞汁液的样品采集, 结合微电极技术测定导管或其它细胞中的pH值、离子浓度以及细胞膜电位。本文重点介绍该技术使用的基本原理和相应的理论基础, 并详细地描述了操作过程中的技术和技巧。     

The mechanisms of refilling of xylem conduits and bleeding of tall birch during spring

Seasonal variations in osmolality and components of xylem sap in tall birch trees were determined using several techniques. Xylem sap was extracted from branch and trunk sections of 58 trees using the very rapid gas bubble-based jet-discharge method. The 5-cm long wood pieces were taken at short intervals over the entire tree height. The data show that large biphasic osmolality gradients temporarily exist within the conducting xylem conduits during leaf emergence (up to 272 mosmol x kg(-1) at the apex). These gradients (arising mainly from glucose and fructose) were clearly held within the xylem conduit as demonstrated by (1)H NMR imaging of intact twigs. Refilling experiments with benzene, sucrose infusion, electron and light microscopy, as well as (1)H NMR chemical shift microimaging provided evidence that the xylem of birch represents a compartment confined by solute-reflecting barriers (radial: lipid linings/lipid bodies; axial: presumably air-filled spaces). These features allow transformation of osmolality gradients into osmotic pressure gradients. Refilling of the xylem occurs by a dual mechanism: from the base (by root pressure) and from the top (by hydrostatic pressure generated by xylem-bound osmotic pressure). The generation of osmotic pressure gradients was accompanied by bleeding. Bleeding could be observed at a height of up to 21 m. Bleeding rates measured at a given height decreased exponentially with time. Evidence is presented that the driving force for bleeding is the weight of the static water columns above the bleeding point. The pressure exerted by the water columns and the bleeding volume depend on the water-filling status of (communicating) vessels.     

Hydraulic propagation of pressure along immature and mature xylem vessels of roots of Zea mays measured by pressure-probe techniques

Turgor pressure was measured in cortical cells and in xylem elements of excised roots and roots of intact plants of Zea mays L. by means of a cell pressure probe. Turgor of living and hence not fully differentiated late metaxylem (range 0.6–0.8 MPa) was consistently higher than turgor of cortical cells (range 0.4–0.6 MPa) at positions between 40 and 180 mm behind the root tip. Closer to the tip, no turgor difference between the cortex and the stele was measured. The turgor difference indicated that late-metaxylem elements may function as nutrient-storage compartments within the stele. Excised roots were attached to the root pressure probe to precisely manipulate the xylem water potential. Root excision did not affect turgor of cortical cells for at least 8 h. Using the cell pressure probe, the propagation of a hydrostatic pressure change effected by the root pressure probe was recorded in mature and immature xylem elements at various positions along the root. Within seconds, the pressure change propagated along both early and late metaxylems. The half-times of the kinetics, however, were about five times smaller for the early metaxylem, indicating they are likely the major pathway of longitudinal water flow. The hydraulic signal dissipated from the source of the pressure application (cut end of the root) to the tip of the root, presumably because of radial water movement along the root axis. The results demonstrate that the water status of the growth zone and other positions apical to 20 mm is mainly uncoupled from changes of the xylem water potential in the rest of the plant.Abbreviations and Symbols CPP cell pressure probe - EMX early metaxylem - LMX Late metaxylem - P_c cell turgor - P_r root pressure - RPP root pressure probe - t_1/2,c half-time of water exchange across a single cell - t_1/2 half-time of water exchange across multiple cells We thank Antony Matista for his expert assistance in the construction and modification of instruments. The work was supported by grant DCB8802033 from the National Science Foundation and grant 91-37100-6671 from USDA, and by the award of a Feodor Lynen-Fellowship from the Alexander von Humboldt-Foundation (Germany) to J.F.     

Thermodynamic analysis of the interaction of the xylem water and phloem sugar solution and its significance for the cohesion theory

The cohesion theory explains water transport in trees by the evaporation of water in the leaves (transpiration), which in turn generates the tension required for sap ascent, i.e. the flow of pure water from the soil through the root system and the non-living cells of the tree (xylem tracheids) up to the leaves. Only a small part of this water flow entering the leaves is used in photosynthesis to produce sugar solution, which is transported from the leaves through the living cells (phloem) to everywhere in the tree where it is needed and used. The phloem sieves are connected to the xylem tracheids by water transparent membranes, which means that the upflow of pure water and downflow of sugar solution interact with each other, causing the osmotic pressure in the sugar solution (Münch model). In this paper we analyse this interaction with a thermodynamic approach and we show that some open questions in the cohesion theory can then perhaps be better understood. For example, why under a quite high tension the water can flow in the xylem mostly without any notable cavitation, and how the suction force itself depends on the cavitation. Minimizing Gibbs energy of the system of xylem and phloem, we derive extended vapor pressure and osmotic pressure equations, which include gas bubbles in the xylem conduits as well as the cellulose-air-water interface term. With the aid of the vapor pressure equation derived here, we estimate the suction force that the cavitation controlled by the phloem sugar solution can generate at high moisture contents. We also estimate the suction force that the transpiration can generate by moisture gradient at low moisture contents. From the general osmotic pressure equation we derive an equation for calculating the degree of cavitation with different sugar solution concentrations and we show the conditions under which the cavitation in the xylem is totally avoided. Using recent field measurement results for a Scotch pine, the theory is demonstrated by showing its predictions for possible amounts of cavitation or embolism from morning hours to late afternoon.     

Do Woody Plants Operate Near the Point of Catastrophic Xylem Dysfunction Caused by Dynamic Water Stress? : Answers from a Model

We discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate `runaway embolism,' whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-physiology, and evolution of woody plants.     

Influence of Xylem Water Potential on Leaf Elongation and Osmotic Adjustment of Wheat and Lupin

The leaf elongation rate and osmotic pressure at full turgorof wheat (Triticum aestivum L.) and lupin (Lupinus cosentiniiGuss.) were measured in well watered plants, in plants thatwere allowed to dry the soil slowly over 7 d, and in plantsin which the water potential of the leaf xylem was maintainedhigh by applying pressure to the roots during the drying cycle.Maintenance of high xylem water potentials failed to preventa reduction in the rate of leaf elongation as the soil dried,while the osmotic pressure at full turgor and the degree ofosmotic adjustment increased as the soil water content decreased.The rate of leaf elongation was reduced more and the degreeof osmotic adjustment was higher in leaves with high xylem waterpotentials than in those in which leaf xylem potentials wereallowed to decrease as soil water content decreased. Osmoticadjustment was linearly correlated with the reduction in leafelongation rate in both wheat and lupin. Key words: Osmotic adjustment, leaf elongation, turgor regulation     

三角叶滨藜根吸水特点与其抗盐性的关系

采用压力室和冰点渗透压计测定了三角叶滨藜在不同浓度NaCl的根系环境溶液中根木质部的压力势和伤流液的渗透势,并利用原子吸收分光光度计测定了植株和伤流液以及环境溶液中Na 含量。结果表明:随着根环境溶液NaCl浓度的增加,三角叶滨藜植株和木质部伤流液中Na 含量虽呈上升趋势,但根系的过滤系数和体内Na 相对累积量逐渐降低,说明三角叶滨藜根细胞对盐分有很强的过滤作用;木质部伤流液的渗透势随着环境溶液渗透势的降低而降低,但根木质部溶液的水势则逐渐高出根外环境溶液的渗透势;表明三角叶滨藜能够利用较低的木质部负压来抵抗根外溶液的低渗透势而反渗透吸水,并利用根细胞对盐分的过滤作用来避免从环境摄取过量的盐分。