Comparison between two methods of generating pressure—volume curves

Abstract. Pressure—volume (P—V) curves were generated on roots and shoots of coastal Douglas fir [ Pseudotsuga menziesii (Mirb.) Franco] seedlings using two procedures. In the first (Method A), samples were dehydrated inside a pressure chamber. Exuded stem sap was collected and weighed at successive pressure increases to derive the P—V curve. In the second method (Method B). excised samples were allowed to dry outside the pressure chamber by evapotranspiration. They were weighed periodically to determine sap loss and their corresponding balance pressures were determined in a pressure chamber in order to derive the P—V curve.
Estimates of volume averaged osmotic potential at full turgor and water potential at zero turgor which were derived graphically from the P—V curves, were different for each method. In general, estimates were more negative in Method A, by as much as 1.5 MPa in one case. Also, Method B did not record an osmotic adjustment in seedlings which were subjected to severe water stress while Method A did.     

A critical re-examination of pressure-volume analysis of conifer shoots: comparison of three procedures for generating PV curves on shoots of Pinus resinosa Ait. seedlings

Prediction of water relations attributes for red pine (Pinusresinosa Ait.) derived from pressure-volume (PV) curves varieddepending on which of three methods was used. The sap expressionmethod entailed the enclosure of a shoot in a pressure chamberand expression of xylem sap by applying a constant selectedpressure until sap flow ceased, at which point xylem water potentialand shoot weight were measured. A sap expression PV curve wasformed by aggregating pairs of water potential-weight measurements,each pair supplied by one of 25 shoots. The repeat pressurizationmethod involved repeatedly measuring xylem water potential andshoot weight on a single shoot drying on a laboratory bench.Repeat pressurization PV curves were constructed from data providedby a single shoot. The composite method utilized single measurementsof xylem water potential and shoot weight on 25-30 differentshoots ranging in relative water content from about 1.0 to 0.5achieved by bench drying. Composite PV curves were constructedfrom aggregate data supplied by a population of shoots. Therewas close agreement in all PV attributes generated using repeatpressurization and sap expression methods. In contrast, withthe composite PV method, there was a fundamental differencein the slope of the linear region of the PV curves, causingosmotic potentials at full turgor and turgor loss to be morenegative, and relative water content at turgor loss to be lowerand symplast fraction to be higher. Comparison of compositeand repeat pressurization PV curves over the same ranges inwater content did not eliminate differences in derived waterrelations attributes. Differences in water potential isothermsrelated to the PV procedures used suggest that prolongedor repeatedexposure to gas at high pressure may introduce errors in theestimation of water relations attributes. Key words: Pinus resinosa, pressure chamber, pressure volume, tissue water relations     

Leaf illumination and root cooling inhibit bean leaf expansion by decreasing turgor pressure

Phaseolus vulgaris plants with expanding primary leaves weresubjected to dark-light or light-dark transition at a root temperatureof 25 °C, or to root cooling to 10 °C. Illuminationor darkening caused rapid changes in water flux through theplants and in epidermal turgor pressure when analysed by pressureprobe. However, these were not concurrent with variations inbulk leaf water potential and turgor pressure as determinedby the pressure chamber method. In addition, the turgor pressureof epidermis measured with the pressure probe was invariably0.05 to 0.15 MPa lower than that measured in bulk tissue withthe pressure chamber. Cooling roots to 10°C induced waterstress and wilting. Both techniques indicated a decrease ofturgor pressure, but a 20-30 min lag was observed with the pressurechamber. Due to stomatal closure and decreased transpiration,root-cooled plants regained cell turgor after 5-7 h of cooling,but bulk tissue and epidermal turgor (as well as leaf growthrate) remained significantly lower than control levels. Thesefindings indicate that changes in turgor pressure as the resultof hydraulic signalling are sufficient to explain the rapidchanges in growth rate following illumination or cooling reportedin earlier work (Sattin et al 1990). They also indicate thatdata obtained by use of the pressure chamber must be treatedwith caution. Key words: Phaseolus vulgaris, expansion growth, water relations, hydraulic signalling, pressure probe, pressure chamber     

Graphical evaluation and partitioning of turgor responses to drought in leaves of durum wheat

The relationship between relative water content (R) and turgor potential (_p) may be derived from pressure-volume (PV) curves and analyzed in various ways. Fifty PV curves were measured with the pressure chamber on leaves of durum wheat (Triticum durum L.). The plots of _p versus R were highly variable and could not be adequately described by a single mathematical function. The area below the curve was therefore determined by means of an area meter. This procedure gave the integral of turgor from full saturation to the turgor-loss point. Responses to drought treatment could thus be quantified and partitioned into effects of osmotic adjustment and elastic adjustment. These two adjustment responses, which are probably of different metabolic origin, together improve turgor maintenance in durum wheat considerably.Abbreviations and symbols PV pressure-volume - R relative water content - T_i turgor integral between full saturation and turgor-loss point - _p turgor (pressure) potential     

Pressure probe and isopiestic psychrometer measure similar turgor

Turgor measured with a miniature pressure probe was compared to that measured with an isopiestic thermocouple psychrometer in mature regions of soybean (Glycine max [L.] Merr.) stems. The probe measured turgor directly in cells of intact stems whereas the psychrometer measured the water potential and osmotic potential of excised stem segments and turgor was calculated by difference. When care was taken to prevent dehydration when working with the pressure probe, and diffusive resistance and dilution errors with the psychrometer, both methods gave similar values of turgor whether the plants were dehydrating or rehydrating. This finding, together with the previously demonstrated similarity in turgor measured with the isopiestic psychrometer and a pressure chamber, indicates that the pressure probe provides accurate measurements of turgor despite the need to penetrate the cell. On the other hand, it suggests that as long as precautions are taken to obtain accurate values for the water potential and osmotic potential, turgor can be determined by isopiestic psychrometry in tissues not accessible to the pressure probe for physical reasons.     

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.     

Tissue water relations and leaf growth of tropical corn cultivars under water deficits

Abstract This study reports on the effect of water deficit on the tissue water relations and leaf growth of six corn cultivars, growing in glasshouse conditions, in order to understand growth responses to drought of tropical corn. A mild water-stress treatment was imposed slowly; plants reached a minimum pre-dawn leaf water potential of about –1.5 MPa by day 12 after watering was withheld. Analysis of the water relation characteristics of growing leaves using the pressure–volume technique demonstrated that under water deficits all the cultivars changed their moisture-release curves compared with irrigated plants. Osmotic potential at full turgor was lowered in water-stressed plants of all the genotypes and the degree of such change was between 0.34 MPa and 0.58 MPa. Thus, turgor pressure was lost at a lower water potential in water-stressed plants than in irrigated plants of all the varieties. Volumetric elastic moduli were also increased under water deficits and the increase ranged between 10% and 141% among the cultivars. In all the genotypes, the stress imposed led to a reduction of leaf area and dry matter accumulation. Leaf expansion was very sensitive to low turgor pressure and it ceased when turgor reached 0.2 MPa. Thus, varieties able to maintain a higher degree of turgor pressure (i.e. by osmotic adjustment) under water deficits may be able to prolong leaf growth.     

Effects of manipulation of water and nitrogen regime on the water relations of the desert shrub Larrea tridentata

Summary Water and nitrogen regimes of Larrea tridentata shrubs growing in the field were manipulated during an annual cycle. Patterns of leaf water status, leaf water relations characteristics, and stomatal behavior were followed concurrently. Large variations in leaf water status in both irrigated and nonirrigated individuals were observed. Predawn and midday leaf water potentials of nonirrigated shrubs were lowest except when measurements had been preceded by significant rainfall. Despite the large seasonal variation in leaf water status, reasonably constant, high levels of turgor were maintained. Pressure-volume curve analysis suggested that changes in the bulk leaf osmotic potential at full turgor were small and that nearly all of the turgor adjustment was due to tissue elastic adjustment. The increase in tissue elasticity with increasing water deficit manifested itself as a decrease in the relative water content at zero turgor and as a decrease in the tissue bulk elastic modulus. Because of large hydration-induced displacement in the osmotic potential and relative water content at zero turgor, it was necessary to use shoots in their natural state of hydration for pressure-volume curve determinations. Large diurnal and seasonal differences in maximum stomatal conductance were observed, but could not easily be attributed to variations in leaf water potential or leaf water relations characteristics such as the turgor loss point. The single factor which seemed to account for most of the diurnal and seasonal differences in maximum stomatal conductance between individual shrubs was an index of soil/root/ shoot hydraulic resistance. Daily maximum stomatal conductance was found to decrease with increasing soil/root/ shoot hydraulic resistance. This pattern was most consistent if the hydraulic resistance calculation was based on an estimate of total canopy transpiration rather than the more commonly used transpiration per unit leaf area. The reasons for this are discussed. It is suggested that while stomatal aperture necessarily represents a major physical resistance controlling transpiration, plant hydraulic resistance may represent the functional resistance through its effects on stomatal aperture.     

Chloroplast response to low leaf water potentials: I. Role of turgor

The effect of decreases in turgor on chloroplast activity was studied by measuring the photochemical activity of intact sunflower (Helianthus annuus L. cv. Russian Mammoth) leaves having low water potentials. Leaf turgor, calculated from leaf water potential and osmotic potential, was found to be affected by the dilution of cell contents by water in the cell walls, when osmotic potentials were measured with a thermocouple psychrometer. After the correction of measurements of leaf osmotic potential, both the thermocouple psychrometer and a pressure chamber indicated that turgor became zero in sunflower leaves at leaf water potentials of −10 bars. Since most of the loss in photochemical activity occurred at water potentials below −10 bars, it was concluded that turgor had little effect on the photochemical activity of the leaves.     

Water Potential-Water Content Relationships In Apple Leaves

Three methods for determining the relationship between xylempressure potential as measured in a pressure chamber (an estimateof leaf water potential) and leaf relative water content werecompared for apple leaves. A range of leaf water contents wasobtained either by sampling leaves in the field at differenttimes of day and on days with differing evaporative demand,or by allowing evaporation from excised leaves in the laboratory,or by expressing sap by overpressurization in a pressure chamber.The first two methods gave very similar results, but the lasttended to give rather lower water potentials at any given watercontent. A possible explanation for these results and theirimplications for the estimation of osmotic potentials usingpressure-volume curves are discussed. Some osmotic adjustmentwas observed in trees droughted for 3 months, with estimatedosmotic potentials, both at full turgor and zero turgor, beingnearly 0.3 MPa lower than in irrigated controls.     

Water Content-Potential Relationship in Soya Bean: Changes in Component Potentials for Mature and Immature Leaves under Field Conditions

Changes in turgor and osmotic potentials of soya bean leaves(Glycine max.) with changes in water content were measured throughouta season using the pressure-volume technique. Two distinct reponsesto water loss were found. When water was expressed from leavesin the pressure chamber their osmotic behavior was describedby a concentration effect based on the osmotic volume. The osmoticfraction of the total water content averaged 0·72 and0·84 for mature and immature leaves, respectively. Thechanges in turgor pressure in the chamber were described bya volumetric modulus of elasticity which increased linearlywith turgor pressure. The changes in total potential at highturgor pressures were almost exclusively due to changes in turgordue to the high modulus (high tissue rigidity) in that range.Responses were different, however, for leaves drying in thefield. For these, the osmotic changes were always large anddominated by solute adjustment. Diurnal changes in osmotic potentialwere as much as 5 bars (500 kPa), or around 50 per cent, andwere about the same magnitude as the changes in turgor pressurefor both mature and immature leaves. The elastic modulus atthe time of sampling showed the normal turgor dependence forimmature leaves but for mature leaves the initial modulus wasapparently constant at about 180 bars. The different behaviourin the pressure bomb and the field is interpreted in terms ofa rate dependence for turgor and osmotic response to water loss.     

Comparison between pressure-volume and dewpoint-hygrometry techniques for determining the water relations characteristics of grass and legume leaves

Summary The water relations characteristics of three grass species (Panicum maximum var. trichoglume, Cenchrus ciliaris, Heteropogon contortus), and a legume (Macroptilium atropurpureum) grown in the field were measured using both a modified pressure/volume technique with pressure bomb measurements on single leaves and a dewpoint hygrometry technique applied to fresh and to frozen and thawed leaf discs.The two techniques agreed well in the estimates of osmotic potential at full turgor and the water potential at zero turgor. However, for parameters such as the relative water content at zero turgor, bound water and bulk modulus of elasticity there was a poor correlation between the estimates from the two methods. The pressure/volume technique gave less variable results and is more convenient for field use than the hygrometry technique. The determination of the modulus of elasticity from various functions relating pressure potential to relative water content is discussed.     

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.     

Synthesis and movement of abscisic Acid in water-stressed cotton leaves

Synthesis and movement of abscisic acid (ABA) into the apoplast of water-stressed cotton (Gossypium hirsutum L.) leaves were examined using pressure dehydration techniques. The exudates of leaves dehydrated in a pressure chamber contained ABA. The level of ABA in the exudates was insensitive to the leaf water potential when dehydration occurred over a 3-hour period. When leaves were rapidly dehydrated in the pressure chamber and held at a balance pressure coincident with the point of zero turgor, ABA accumulated in the leaf tissue and then in the apoplast, but only after 2 to 3 hours of zero turgor. Slow dehydration of leaves by equilibration over varying mannitol concentrations resulted in some accumulation of ABA prior to the point of zero turgor, but ABA accumulated in the tissue and apoplast most rapidly after the onset of zero turgor.     

Leaf Elongation in Relation to Leaf Water Potential in Soybean

Leaf water potential, turgor pressure, and leaf elongation ratewere measured in soybeans growing in controlled environmentchambers, greenhouses, and outdoors. Plants in chambers hadthe highest water potentials and turgor pressures, and plantsoutdoors the lowest. In all three environments there was a linearrelationship between elongation rate and turgor pressure. Leavesof plants in drier environments required less turgor for elongation,and showed a greater increase in elongation rate per unit increasein turgor. Elongation rates over a 72 h period were equal inthe three environments. Leaves reached the largest final sizein the greenhouse (intermediate in water potential). Epidermalcells were larger in chamber- and greenhouse-grown leaves thanin leaves of plants grown outdoors. The number of epidermalcells per leaf was greater in the greenhouse and outdoors thanin the chamber. Leaf elongation characteristics of greenhouseplants were duplicated by mildly stressing chamber plants, andleaf elongation characteristics of field plants were duplicatedby more severely stressing chamber plants. Leaves of mildlystressed chamber plants also reached a larger final size thanleaves of more severely stressed chamber plants, or leaves ofcontrol plants in the chamber. Water stress in the chamber increasedthe number of epidermal cells per leaf. More severe water stressin the chamber reduced epidermal cell size. Based on the waterstress experiments it is concluded that the differences in plantwater status in the chamber, greenhouse, and field caused differencesin elongation characteristics, and were responsible for thedifferences in leaf size.     

An Instrument for Non-Destructive Measurement of the Pressure Potential (Turgor) of Leaf Cells

An instrument is described which permits the non-destructivemeasurement of the mean pressure potential (turgor)of leaf laminacells. Calibration shows that the instrument gives a voltageoutput which is linearly related to mean pressure potentialof living leaf cells as determined with a pressure chamber.Measurements may be made very quickly in the field or controlledenvironment with a resolution of at least 50 k Pa.     

立地条件和树龄对刺槐和小叶杨叶水力性状及抗旱性的影响

以形成黄土高原“小老树”的2种典型树种刺槐和小叶杨为对象,研究了立地条件(沟谷台地和沟间坡地)和树龄对两种树木叶水力学性质和抗旱性的影响,探讨“小老树”形成的水力生理机制.结果表明:水分较好的沟谷台地上生长的两种树木的叶最大水力导度(Kmax)明显大于水分较差的沟间坡地,叶水力脆弱性(P50)也较高;随树龄增加,两种树木的Kmax明显下降,但P50差异不大.台地上生长的两种树木的叶表皮导度和PV曲线参数(膨压损失点时的相对含水量RWCtlp、膨压损失点时的水势ψtlp饱和含水量时的渗透势ψsat)均大于 坡地;随树龄增加,两种树木的叶表皮导度显著下降,PV曲线参数出现不同程度的下降.两种树木Kmax与ψtlp呈显著正相关,P50与PV曲线参数之间存在一定的相关性,表明Kmax与抗旱性之间存在一种权衡关系,P50是反映两种树木的抗旱性特征之一.     

Leaf hygrometer v. pressure chamber: a comparison of pressure–volume curve data obtained on single leaves by alternating measurements

Abstract. Data for the construction of pressure-volume curves were obtained by measuring water potentials of detached leaves repeatedly and alternately, with a pressure chamber and a leaf hygrometer. Good agreement between the parameters of the two resulting curves was observed. Regression lines on values after the loss of turgor were always more negative for the thermocouple data, with a maximum difference for the osmotic potential at full saturation of 0.25 MPa in Triticum and a minimum of 0.01 MPa in Populus. Neither the slopes of the regression lines nor the intercepts with the axes were statistically different. We see no reason for using one of these two unrelated methods as a standard against which the other is calibrated. Implications for the theory of pressure-volume curves are discussed.     

Surface potentials and hydraulic signals in wheat leaves following localized wounding by heat

Abstract. Localized burning of a leaf causes a rapid change in apoplastic electrical potential throughout the shoot of wheat seedlings ('variation potential'). It also causes marked increases in turgor pressure in epidermal cells of adjoining leaves. These turgor increases indicate rapid propagation throughout the seedling, of a hydraulic pressure wave from the site of wounding. Evidence is presented that this pressure wave is caused by relief of xylem tension, by water released from damaged cells in the wounded region. It is demonstrated that, in the absence of wounding, pressure waves imposed at the tip of one leaf can travel to neighbouring leaves, and can there induce change in apoplastic electrical potential similar to a 'variation potential'. This indicates that the hydraulic event produced by wounding is the signal responsible for systemic induction of the 'variation potential'. This signal has been termed 'Ricca's factor'. It is suggested that arrival of the hydraulic wave alters leaf water potential and thereby induces stomatal activity. Leaf surface potential may be dominated by electrogenic ion pumping or flux at stomatal cells, and the 'variation potential' may therefore be a reflection of stomatal activity induced by the hydraulic signal.     

Critical Water Potential for Stomatal Closure in Sitka Spruce

Steady state rates of net photosynthesis and stomatal conductance at high water potentials were measured under controlled conditions in a leaf chamber on Sitka spruce [Picea sitchensis (Bong.) Carr.] shoots detached from the forest canopy or on seedlings. The water supply to the seedlings was terminated by excision and the shoot water potential (or critical water potential) and osmotic potential at the onset of stomatal closure measured. The turgor potential was calculated. The initial osmotic potential before insertion of the shoot into the chamber was also measured. Shoot water potential and osmotic potential at stomatal closure, and initial osmotic potential were significantly higher (less negative) in foliage from the lowest level in the canopy compared with foliage in the upper canopy, and higher in shoots of seedlings transferred to low light than in those at high light. Critical water potential also varied with season, being higher in July than in October and November. In all except one instance, turgor potential at the onset of stomatal closure was negative, possibly because of dilution of the cell sap by the extracellular water during the estimate of osmotic potential. Over all the experiments variation in critical water potential was correlated with variation in critical osmotic potential and, to a lesser extent, the initial osmotic potential. However, turgor potential at the critical potential varied from +0.6 to -4.6 bar. This suggests that difference in turgor between the guard cells and subsidiary cells, which controls stomatal aperture, is only loosely coupled with the bulk leaf turgor and hence that bulk leaf turgor is not a good index of the turbor relations of the guard cells.