The Meaning of Matric Potential

The commonly used equation, = P - + , which describes thepartitioning of plant water potential, , into components ofhydrostatic pressure, P, osmotic pressure, , and matric potential,, is misleading. The term , which is supposed to show the influenceof a solid phase on , is zero if a consistent definition ofpressure is used in the standard thermodynamic derivation. However,it can be usefully defined by = + _D, where _D is the osmoticpressure of the equilibrium dialysate of the system. The practicaland theoretical significance of this definition is discussed.     

Water Uptake and Germination of Leguminous Seeds in Soils of Changing Matric and Osmotic Water Potential

Water uptake and germination rate of chickpea and pea seedswere compared under changing water potentials in sand and soilaggregate columns and osmotic solutions. The final water uptake and germination were the same in allcases for a given water potential, but the rates were lowerfor seeds planted in sand columns, probably due to mechanicalconstraints imposed on the swelling seed by the dense sand,since the capillary conductivity, and the diffusivity to waterof the sand were very high. The area of the seed in contact with soil is not of importanceif soil aggregates are small as compared to the seeds but increasesin importance when the seeds and the soil aggregates are ofthe same size and at low water potentials.     

Effect of Excision on Leaf Water Potential

Improvements of thermocouple hygrometric techniques for in situleaf water potential measurement in the field now allows forcontinuous monitoring of water potential in response to an externalperturbation, such as leaf excision. Using Citrus jambhiri plants,measured leaf water potentials of completely excised leaf portionsimmediately increased when the petiole was excised or incisionswere made either transverse or parallel to the midrib. Incisionsparallel to the midrib were on the side nearest the hygrometeror opposite it if preceded by a petiole excision. Midrib incisionswere 100–150 mm long with the nearest cut edge being 20–50mm from the hygrometer cavity. All excisions were such thatleaf tissue was removed from the leaf with water potential onone of the leaf portions being measured continuously prior toand after excising. The peak increase in measured water potentialof the excised leaf portions ranged between 20 kPa and 80 kPabut averaged 50 kPa. In uncovered leaves, particularly underfield conditions with the associated high evaporative demand,measured leaf water potential declined rapidly after the initialincrease. The increase in measured water potential immediatelyfollowing various types of excision was confirmed for dark andlight conditions (laboratory and field respectively) using bothpsychrometric and dewpoint modes and occurred for secondaryexcisions, but to a lesser extent. Discovery of this phenomenonimplies that water potential measured on detached leaves maynot always represent accurately in situ leaf water potential. Key words: Leaf water potential, Thermocouple hygrometers, Leaf excision effects     

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.     

Matric Bound Water of Water Tissue from Succulents

Matric bound water was measured as water retained by frozen and thawed tissue after desorption on a pressure membrane filter under 20 bars nitrogen gas pressure. Central water-storage tissue and peripheral chlorenchyma from leaves or stems of 15 taxonomically diverse non-halophytic succulent species were investigated. Matric bound water as a per cent of the dry weight averaged higher in water storage than in chlorenchyma tissue but lower than values reported for many mesophytic leaves. Matric bound water as a proportion of the total water held, however, was lower in water tissues. Osmotic potentials were generally high (solute contents low). It is concluded that matric or osmotic forces cannot account, in any unique way, for the high water content of water tissues. This appears to depend, instead, on the enormous ability of the thin-walled cells to take up available water and expand.     

The Relationship Between Leaf Thickness and Plant Water Potential

Leaf thickness was continuously measured in a wide range ofenvironments using a new type of displacement transducer whichis easy to set-up and automatically compensates for the effectsof temperature. Simultaneous measurements were made of waterpotential using either a psychrometer attached to the leaf petioleor a leaf pressure chamber. Thickness of leaves was a sensitiveindicator of plant water status but calibrations against anindependent method were necessary in every plant for accurateestimates of water potential. The relationship between leafthickness changes and water potential, measured in detachedleaves, was usually curvilinear and was strongly influencedby leaf age, stress history and, in young leaves, by the effectsof leaf growth. Leaf thickness growth was absent in mature cabbageleaves. Key words: Leaf thickness, plant water potential, psychrometer     

Diurnal Changes in Citrus Leaf Thickness, Leaf Water Potential and Leaf to Air Temperature Difference

Diurnal changes in leaf water potential and leaf thickness ofwell-watered citrus trees were found to be highly correlated.Midday decreases in leaf thickness of about 30–35 µm reflected midday decreases in leaf water potential of about1.1–1.3 MPa from predawn values. Leaf water potentialwas also correlated with changes in leaf-to-air temperaturedifference and ambient vapour pressure deficit. Leaf thicknessas well as leaf to air temperature difference could possiblybe used to monitor leaf water status continuously as an indicatorof citrus tree water stress.     

Leaf Water Potential Response to Transpiration by Citrus

This paper reports on further studies of a model for interpreting leaf water potential data for Citrus. Experimental data confirmed the assumption that the ratio of vapor pressure deficit to leaf diffusion resistance adequately estimates transpiration when leaf-to-air temperature differences are small. Data collected diurnally indicated that the relationship between leaf water potential and transpiration followed a sequence of steady states without hysteresis. No difference in water transport characteristics was found for Valencia orange on three rootstocks in well-watered soil, but the two rootstocks Cleopatra mandarin and Rangpur gave slightly greater leaf water stress in Valencia orange leaves than‘Troyer’ citrange rootstock at high transpiration rates under mild soil water deficits. In laboratory studies, previously unstressed seedlings had higher leaf water potentials than field trees at equivalent transpiration rates. After several drying cycles, however, leaf water potentials were similar to those observed in the field.     

Water Potential and Leaf Elongation in Radiata Pine and Wheat

The rate of leaf elongation in radiata pine (Pinus radiata) and wheat seedlings was closely related to the osmotic potential of the rooting solution. Sudden stress application and removal treatments caused immediate changes in the leaf elongation rate and a new steady-state rate independent of the old was quickly established. The osmotic potential in the active elongation zones of a leaf differed from that of the remainder of the leaf and was considered more appropriate for deriving turgor pressure to be related to leaf elongation. Shifts in elongation rate were associated with corresponding shifts in total leaf water potential in both species and with corresponding shifts in turgor pressure in wheat. There was a linear correlation between the pitch of the helical thickenings in the protoxylem of radiata pine needles and their elongation rate. A record of elongation rate is, therefore, preserved in the needle. Developmental aspects of the deposition of the helix are discussed.     

Leaf Water Potential and Control of Water Loss in Droughted Sitka Spruce Seedlings

Sitka spruce seedlings were subjected to drought in experimentsin a growthroom, a greenhouse, and out of doors. The plantswere grown in a double chamber with the bulk of the roots inthe upper part where they dried out the soil when water waswithheld. A few new roots penetrated into the lower part inwhich the soil remained moist. The double chamber system enabledthe plant to attain a high water psotential by night and theshoot was only periodically under mild water stress. Measurementswere made on soil water potential (_solt), leaf water potential(₁), transpiration (E), and stomatal conductance (k_s). As soildecreased over a period of 4.5 d, E and k_s decreased progressively.The decline in E and k_s which indicated stomatal closure, occurredat a higher ₁ than has been reported for Sitka spruce. The behaviourof the stomata appeared to be modified by conditions at theroot, and it is proposed that differences in the response to₁,depend on Whether the latter is reduced by resistances in thexylem between root and leaf, as is known to occur in large treesin moist soil, or by stresses at the root itself.     

The Relationship between Transpiration, Root Water Uptake, and Leaf Water Potential

The effect of changing the transpiration rate on leaf waterpotential and water balance has been examined to show if permeabilityof the plant (predominantly the roots) is constant or varieswith the transpiration rate. Measurements of leaf effectivethickness, water potential, transpiration, and uptake of waterby roots were made on sunflower, barley, and maize plants grownin solution culture and subjected to a range of atmosphericconditions and root treatments: cooling, low osmotic potential,and removal of part of the root system. Leaf water potential changed little under a wide range of atmosphericconditions and rates of water flux in the three species, sothat the root permeability to water increases as the rate oftranspiration, and therefore flow across the root surface, increases.Equality between uptake and loss of water and thereby maintenanceof constant leaf water potential is assisted by stomatal changes,which appear to be in response to conditions at or in the rootrather than a direct response to changes in bulk leaf waterpotential.     

Environmental Effects on Transpiration and Leaf Water Potential in Avocado

A field study was conducted to determine how atmospheric and edaphic conditions influenced the water relations of avocado trees (Persea americana Mill. cv. Bacon). With high and low levels of incident photosynthetically active radiation (PAR, 400–700 nm wave length), and either wet or dry soil, leaf conductance decreased as the absolute humidity difference from leaf to air increased. For any water stress treatment, conductance was higher at high PAR than at low PAR. Both conductance and transpiration were higher in well-watered trees than in stressed trees, and in prestressed trees levels were intermediate to unstressed and stressed trees. A model for water flux through the soil-plant-atmosphere continuum was used to examine the relationship of leaf xylem pressure potential to transpiration in well-watered trees and in trees stressed by dry soil. There was a close linkage between leaf xylem pressure potential and transpiration in unstressed and previously stressed trees with high or low PAR, i.e. similar potentials occurred with equivalent transpiration regardless of previous treatment or time of day. In stressed trees, xylem pressure potential was lower than in unstressed trees both during the day and night, and at a given transpiration rate the potential was lower after 1400 h than before that time. The model indicated that in stressed trees xylem pressure potential was uncoupled from transpiration, presumably because of altered resistance in the soil-root portion of the transport system.     

Responses of Soybean Leaf Angle, Photosynthesis and Stomatal Conductance to Leaf and Soil Water Potential

The hypothesis that soil water potential (_s) is better correlatedto heliotropic leaf orientation, photosaturated photosyntheticCO₂ assimilation and stomatal conductance during periods oflimited water availability than is bulk leaf water potential(₁) was examined in greenhouse-grown soybean (Glycine max) plants,submitted to a progressive drought. Paired plants were exposedto either 1000 or 100 µmol m^–2 s^–1 photonflux densities (PFD) for 45–60 mins. The higher irradianceinduced short-term decreases in ₁, due to increased transpiration,while _l in the plant exposed to low PFD did not decrease. Thesechanges in ₁ occurred independently of changes in soil waterstatus. Concurrent to the light treatments, a single attachedleaf from each of the two plants was isolated from the restof the plant by shading, and the pulvinus of its terminal leafletwas exposed to a perpendicular PFD of 500 µmol m–²S^–1. Leaf movement of this leaflet was recorded in responseto this light, until a stable leaflet angle was achieved. Valuesof _s and _l (before and after light treatment), and photosaturatedrates of photosynthesis and stomatal conductance, were thenmeasured on these leaves. Leaflet angle and gas exchange werebetter correlated with _s (r² = 0.50, 0.50 and 0.57 for angle,photosynthesis and conductance, respectively) than with _l especiallywhen _l was the result of short-term, high-light induced changesin leaf water status (r² = 0.36, 0.32 and 0.49, for the sameparameters). Leaflet angle was also correlated with stomatalconductance (r² = 0.61) and photosynthetic rate (r² = 0.60),suggesting a close association between leaf orientation, leafmetabolism and soil water availability. Glycine max (L.) Merr. cv. Essex, soybean, heliotropism, water potential, photosynthesis, stomatal conductance, solar tracking     

Psychrometric Field Measurement of Water Potential Changes following Leaf Excision

In situ measurement of sudden leaf water potential changes has not been performed under field conditions. A laboratory investigation involving the measurement of leaf water potential prior to and 2 to 200 minutes after excision of citrus leaves (Citrus jambhiri) showed good linear correlation (r = 0.99) between in situ leaf psychrometer and Scholander pressure chamber measurements. Following this, a field investigation was conducted which involved psychrometric measurement prior to petiole excision and 1 minute after excision. Simultaneous pressure chamber measurements were performed on neighboring leaves prior to the time of excision and then on the psychrometer leaf about 2 minutes after excision. These data indicate that within the first 2 minutes after excision, psychrometer and pressure chamber measurements were linearly correlated (r = 0.97). Under high evaporative demand conditions, the rate of water potential decrease was between 250 and 700 kilopascals in the first minute after excision. These results show that the thermocouple psychrometer can be used as a dynamic and nondestructive field technique for monitoring leaf water potential.     

Exclusion Errors in Pressure Chamber Estimates of Leaf Water Potential

The common practice of having between 8 and 20 mm of plant samplesexcluded from pressure chambers during determinations of xylemsap pressure potential can produce ‘exclusion’ errorsas large as –1300 kg–1. These may be minimized bysimple improvements in methodology which also facilitate operationand permit determinations with samples of different size, shape,and rigidity.     

Measurement of Leaf Water Potential Using the J14 Press

The diurnal variation in leaf water potential of wheat underfield conditions is determined by means of the Scholander pressurechamber and the J14 press. Experimental work was carried outon several days during both the vegetative and reproductivegrowth stages. An exponential relationship (v = 4.09 e^0.05x),with coefficient of determination r² = 0.81, is found to existbetween the J14 press and Scholander pressure chamber measurements.Both the Scholander pressure chamber and J14 press appear capableof following hourly variations in leaf water potential provided8 and 16 leaves per sample are used for the two instrumentsrespectively.     

Leaf Water Potential and the Rooting of Cuttings under Mist and Polythene

The relationship between leaf water potential and rooting was investigated in cuttings of Rhododendron (Hardy Hybrid) ‘Mrs. W. Agnew’, Ceanothus thyrsiflorus Esch. and Hebe (Garden Hybrid) ‘Caledonia’ (of the ‘Mrs. Winder’ group), during propagation under mist and polythene. Water potentials well below –10 bars frequently occurred and low mean water potentials related to poor rooting. Propagation under polythene gave better results than mist in the lower radiation conditions of winter but the reverse occurred under high radiation conditions. Treatment of cuttings with a poly-vinyl resin antitranspirant coating temporarily increased leaf water potentials in the consistently humid conditions under polythene but not under mist. The coating peeled and lost its effectiveness within 6 weeks. Multiple regression analysis showed that much of the variation in water potential in cuttings under mist could be accounted for by inclusion of three variables, viz. current day's radiation, number of days from insertion of the cuttings and either the previous day's leaf water potential or radiation. Under polythene the influence of yesterday's water potential was relatively unimportant, probably because cuttings were able to take up water overnight from condensation on the under surface of the polythene. For optimal rooting, propagation procedure must ensure that high leaf water potentials are maintained, but conventional methods do not always achieve this.     

水稻叶片水势的QTL定位与候选基因分析

为探究叶片水势(LWP)相关基因在水稻(Oryza sativa)抗旱中的作用及其遗传机制,以热研2号(Nekken2)和华占(HZ)为亲本以及构建的120个重组自交系(RILs)群体为实验材料,对水稻分蘖期叶片水势进行检测,并利用前期基于高通量测序构建的分子遗传连锁图谱进行数量性状基因座(QTL)分析。结果表明,共检...     

Influence of Soil Water Matric Potential and Hydraulic Conductivity on the Germination of Rape (Brassica napus L.)

The influence of matric potential and hydraulic conductivityon the water absorption and germination of rape (Brassica napusL.) was investigated by comparative studies in three seed-soilwater systems. In all systems the rate of germination and totalgermination were substantially retarded with decreasing matricpotential. Hydraulic conductivity of the soil system was demonstratedto be a limiting factor in the germination process. The effectsof matric potential and hydraulic conductivity on water absorptionfollowed closely those demonstrated for germination. The influenceof matric potential was shown to be comparable to that of osmoticpotential provided consideration was given to the hydraulicproperties of the soil-seed system.     

A Vapour-pressure Instrument for the Measurement of Leaf and Soil Water Potential

A previously described vapour-pressure osmometer has been adaptedfor the measurement of depression of water potential (DPD) insamples of leaf tissue and soil. A pile of about 30 leaf annuliare contained in an air-tight metal capsule through the lidof which passes a pipette. A drop of water hangs from the tipof the pipette into the central cavity of the pile of leaf annuli.The drop is drawn bath into the pipette from time to time andthe rate of evaporation from it measured directly. The water-potentialdepression is linearly related to the rate of evaporation. Theinstrument is calibrated empincally. A period of a hours isnecessary to attain vapour equilibnum before readings are takenThe readings then occupy less than 30 minutes. For the calibration data, using filter paper moistened withNaCl solution, the 5 percent inverse tolerance limits for singlereadings of water-potential depression were ±0.26 atm.With leaf material the errors appear to be about the same, althoughrelease of cell sap at the cut edges of the disks may causean overestimation by as much as 0.5 atm. with turgid leaf material.Lining the capsule with leaf strips gave erroneous results.Using a suitably modified capsule the instrument can be usedfor soil-water potential measurement. Equilibration takes only30 minutes so that a reading can be completed within an hour.