Turgor Maintenance by Osmoregulation inBrassica napusandB. junceaunder Field Conditions

Indian mustard (Brassica juncea(L) Czernjacw) maintains higherleaf turgor than canola (B. napusL.) under water deficits andthis is related to the greater yield of mustard under theseconditions. The work reported in this paper was designed tostudy the way mustard maintains this turgor advantage. It wasbased on three field experiments that each used at least twocultivars or lines of each species. The leaf water potentialat which leaves reached zero turgor was consistently lower inmustard than in canola (up to 1.1 MPa lower). This differencearose from a greater rate of decline in leaf osmotic potentialwith declining water potential in mustard rather than from anydifference in the osmotic potential at full turgor. Calculationsof solute accumulation showed that mustard had a greater capacityto osmoregulate than canola, with this capacity being the basisfor its advantage in turgor maintenance. Other differences inplant water relations were consistent with the differences inosmoregulation, with the predicted relative water content ofleaves at an osmotic potential of -2.5 MPa being 0.43 for canolaand 0.61 for mustard. Mustard's greater capacity to accumulatesolutes is concluded to be a major factor in its greater yieldunder water deficits. Brassica napusL.; Brassica juncea(L) Czernjacw; Indian mustard; canola; water deficit; plant water relations; osmoregulation; osmotic adjustment; turgor     

Growth Rate and Water Relations of Citrus Leaf Flushes

Growth rates of seasonal leaf flushes of ‘Valencia’orange [Citrus sinensis (L.) Osbeck] were measured and waterrelations characteristics of young (new) and over-wintered (old)citrus leaves were compared. New flush leaves had lower specificleaf weights and lower midday leaf water potentials than comparablyexposed old leaves. Spring and summer flush new leaves had higherosmotic potentials than old leaves. These differences becamenon-significant as the new leaves matured. During summer conditions,water-stressed new leaves reached zero turgor and stomatal conductancealso began to decrease in them at higher leaf water potentialsthan in old leaves. Old leaves were capable of maintaining openstomata at lower leaf water potentials. Opened flowers and newflush leaves lost more water, on a dry weight basis, than flowerbuds, fruit or mature leaves. The results illustrate differencesin leaf water potential and stomatal conductance which can beattributed to the maintenance of leaf turgor by decreases inleaf osmotic potentials as leaves mature. These changes in citrusleaf water relations are especially important since water stressresulting from high water loss rates of new tissues could reduceflowering and fruit set. Citrus sinensis (L.) Osbeck, orange, Citrus paradisi Macf., grapefruit, growth rate, leaf water relations, osmotic potential, water potential, stomatal conductance     

The Influence of Resaturation Method and Tissue 3Quercus alba L. Seedlings

Parker, W. C. and Pallardy, S. G. 1987. The influence of resaturationmethod and tissue type on pressure-volume analysis of Quercusalba L. seedlings.—J. exp. Bot. 38: 535–549. The effect of resaturation method and amount of woody tissueon pressure-volume analysis was investigated using materialcollected from Quercus alba L. seedlings. Leaves excised fromwell-irrigated, intact plants had lower initial xylem pressurepotentials than did leaves resaturated by two artificial methods.Differential capacity for tissue rehydration among the threemethods was linked to shifts in the relative position of pressure-volumecurves, and differences in the osmotic potential and relativewater content at which turgor loss occurred were observed. Pressure-volumecurves from leaves resaturated by all three methods contained‘plateaus’ near full turgor, where xylem pressurepotential declined only slightly with relative water content.These plateaus were apparently associated with apoplastic waterthat accumulated in intercellular spaces of the leaf near fullturgidity, and acted to buffer changes in leaf xylem pressurepotential as tissues dehydrated. The presence of this waterhas implications for derived water relations parameter estimates.Pressure-volume curves for excised shoots also exhibited plateaus,but the relationship between xylem pressure potential and relativewater content over this region was steeper than was found forleaves. Shoot osmotic potentials were somewhat lower than thosefor leaves. The slope of the linear portion of shoot pressure-volumecurves was more shallow than for single leaves, a response associatedwith comparatively lower values of the symplastic water fractionin shoots. Key words: Pressure-volume curve, tissue-water relations, elasticity     

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.     

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     

Changes in water relations and in some physiological functions of bean under very light osmotic shock induced by polyethylene glycol

Bean plantlets ( Phaseolus vulgaris L. cv. Topcrop) were stressed at the age of 16–18 days by gradual (2–8%) or abrupt addition of 6% (w/v) polyethylene glycol Mw 6000 (PEG 6000) to Hoagland solution. Leaf conductance, photosynthesis, internal CO₂ partial pressure (C_i), relative water content (RWC), water content/dry weight (H₂O/DW), apoplastic PEG concentrations and weight of leaves, stems and roots were determined. Leaf conductance, photosynthesis and C_i were determined on non-detached primary leaves, and leaf potentials (water, osmotic and turgor potentials) were investigated in freshly detached (non-rehydrated) primary leaves, both in treated and control plants; RWC and osmotic potential were also assessed at the null turgor point. Low PEG 6000 concentrations induced early and evident decrease in leaf conductance and photosynthesis, whereas C_i decreased only moderately and tended to recover during advanced stress. There were moderate though significant decreases in RWC and H₂O/DW, no change or increases in water potential, no significant changes in osmotic potential and a moderate but significant increase in turgor potential. Even when referred to null turgor point, RWC significantly decreased and osmotic potential was unchanged. It was concluded that apoplastic PEG 6000 accumulation at evaporating sites would account for the early decrease in conductance which would also justify the unchanged or the prevalent increase in water potential and turgor potential. The subsequent PEG diffusion and concentration in the leaf apoplastic water would have induced the RWC and H₂O/DW decrease and the final turgor flexion documented.     

Evaluation of a Non-Destructive Method for Measuring Turgor Pressure in Helianthus

The portable instrument described by Heathcote, Etherington,and Woodward (1979) for the non-destructive measurement of turgorpressure was evaluated in Helianthus annuus and Helianthus paradoxus.A good correlation was obtained between turgor pressure measuredwith the instrument and turgor pressure estimated by the pressure-volumetechnique for individual leaves allowed to dry after excision;however, variation in both the intercept and slope of the relationshipoccurred between leaves. Consequently, there was no correlationbetween the output of the instrument for individual leaves andthe turgor pressure of the same leaves estimated by conventionalmethods. Moreover, for a given leaf, the instrument had onlya limited ability to detect temporal variation in turgor pressurewhen compared with turgor pressure calculated from measuredvalues of leaf water potential and leaf osmotic potential. Theinstrument's output was influenced by its proximity to majorveins and by leaf thickness. We conclude that variability inleaf thickness and the presence of large veins limits its usefulnessfor measurement of turgor pressure in Helianthus. Key words: Leaf thickness, Turgormeter, Turgor pressure, Helianthus     

A comparison of the water relations characteristics of Helianthus annuus and Helianthus petiolaris when subjected to water deficits

The effect of water deficits on the water relations and stomatal responses of Helianthus annuus and Helianthus petiolaris were compared in plants growing in the glasshouse under controlled conditions. Unirrigated plants of both genotypes were subjected to two different stress rates in which predawn leaf water potentials declined steadily at either 0.15 MPa day^?1 or 0.50 MPa day^?1. In both genotypes water stress induced a gradual and similar decrease in leaf conductance from 1.6 to 0.3 cm s^?1 as water potential decreased from-0.5 to-2.0 MPa. The relationship between leaf conductance and leaf water potential was not affected by the rate of stress development. Development of predawn leaf water potentials of-1.3 MPa had no significant effect on the relative water content at zero turgor, the apoplastic water content or the volumetric elastic modulus of whole leaves in either species, but decreased the osmotic potential at full turgor and zero turgor by 0.22 MPa and decreased the turgid weight: dry weight ratio from 10.6 to 8.4 in H. annuus, but not in H. petiolaris. In H. annuus leaves expanded during stress development, changes in the osmotic potential at full turgor induced by water deficits did not disappear on rewatering.     

Effects of Water Stress and Hardening on the Internal Water Relations and Osmotic Constituents of Cotton Leaves

Experiments were designed to test the hypothesis that the internal water relations of leaves are altered when cotton plants (Gossypium hirsutum L.‘Acala SJ-2′) are conditioned by several cycles of water stress. Preliminary experiments suggested that plants so conditioned are less sensitive to water deficits and that the change might be partly explained by an accumulation of solutes or by structural alterations attendant on development under conditions of water stress. Leaves of preconditioned plants maintained turgor to lower values of water potential than did leaves of well-watered plants. Accompanying this change was a lower osmotic potential at any given leaf water content in preconditioned plants. Tissue analysis of several osmotically active solutes indicated that soluble sugars and malate accumulate to about the same levels (dry-weight basis) in both conditioned and unconditioned plants exposed to stress. These accumulations could not account for the turgor change. Analysis of the data on relative water content indicated that the leaves of conditioned plants had less water per unit dry weight than did leaves of controls. This change accounts for a substantial fraction of the difference between the osmotic potential of conditioned and control plants. The results of a simple model suggest that structural changes may play a significant role in explaining differences in the responses of conditioned and control plants to water stress.     

Osmotic adjustment in leaves of sorghum in response to water deficits

The relationships among the total water potential, osmotic potential, turgor potential, and relative water content were determined for leaves of sorghum (Sorghum bicolor [L.] Moench cvs. `RS 610' and `Shallu') with three different histories of water stress. Plants were adequately watered (control), or the soil was allowed to dry slowly until the predawn leaf water potential reached either −0.4 megapascal (MPa) (treatment A) or −1.6 MPa (treatment B). Severe soil and plant water deficits developed sooner after cessation of watering in `Shallu' than in `RS 610', but no significant differences in osmotic adjustment or tissue water relations were observed between the two cultivars. In both cultivars, the stress treatments altered the relationship between leaf water potential and relative water content, resulting in the previously stressed plants maintaining higher tissue water contents than control plants at the same leaf water potential. The osmotic potential at full turgor in the control sorghum was −0.7 MPa: stress pretreatment significantly lowered the osmotic potential to −1.1 and −1.6 MPa in stress treatments A and B, respectively. As a result of this osmotic adjustment, leaf turgor potentials at a given value of leaf water potential exceeded those of the control plants by 0.15 to 0.30 MPa in treatment A and by 0.5 to 0.65 MPa in treatment B. However, zero turgor potential occurred at approximately the same value of relative water content (94%) irrespective of previous stress history. From the relationship between turgor potential and relative water content there was an approximate doubling of the volumetric elastic modulus, i.e. a halving of tissue elasticity, as a result of stress preconditioning. The influence of stress preconditioning on the moisture release curve is discussed.     

Nitrate Accumulation and its Relation to Leaf Elongation in Spinach Leaves

The leaf elongation rate (LER) of spinach leaves during theday was twice that during the night when grown at a photon fluxdensity of 145 µmol m^–2 s^–1. All leaves showedthe same LER-pattern over 24 h. Due to low turgor, LER was lowin the afternoon and in the first hours of the night until wateruptake restored full turgor. Osmotic potential remained constantdue to increased nitrate uptake and starch degradation in thisperiod. LER increased to high rates in the second part of thenight and in the morning. The lower rate in the dark comparedto the light was not caused by the lower night temperatures,as increased photon flux density during growth resulted in equalrates in the light and the dark. Increased relative humiditydecreased LER and afternoon rates were most sensitive to waterstress. A ‘low light’ night period did not changeLER-pattern during the night or on the following day. We concludethat nitrate is not an obligatory osmoticum during the nightand can be exchanged for organic osmotica without decreasingLER. During the night the turgor is first restored by increasingwater uptake, nitrate uptake and starch degradation. This resultedin increased leaf fresh weight in this period. Thereafter, elongationincreased by simultaneous uptake of nitrate and water. Nitrateconcentration was, therefore, constant in the older leaves.In the younger leaves nitrate concentration increased to replacesoluble carbohydrates. The vacuoles of the old leaves were filledwith nitrate before those of the young leaves. Key words: Spinacia oleracea L., nitrate accumulation, osmotic potential, organic acids     

Differences in Osmoregulation in Brassica species

Brassica carinata L (cv Carinata-2) and Brassica napus L (cvHNS-3) were tested for osmoregulation under three sets of environmentsOsmoregulation was found to vary markedly between two species,with the cv Carinata-2 having a greater degree of osmoregulationthan the cv HNS-3 Furthermore, the differences in osmoregulationwere related to leaf diffusive conductance and grain yield inBrassica species Thus, it has potential use as a selection criterionin Brassicas Brassica carinata L, Brassica napus L, osmoregulation, relative water content, leaf water potential, turgor potential, osmotic potential, leaf temperature, leaf diffusive conductance     

Effects of Sodium Chloride on Water Status and Growth of Sugar Beet

The effects of sodium chloride on the water status, growth,and physiology of sugar beet subjected to a range of soil waterpotentials were studied under controlled conditions. Sodiumchloride increased plant dry weight and the area, thickness,and succulence of the leaves. It increased the water capacityof the plant, mainly the shoot, but there was no evidence thatit altered the relationships between leaf relative water contentand the leaf water, osmotic, and turgor potentials or changedthe way stomatal conductance and photosynthesis responded todecreasing leaf water potential. The greater leaf expansionin sodium-treated plants is thought to be the consequence ofadjustments made by leaf cells to accommodate changes in ionsand water in a way that minimizes change in water and turgorpotentials. It is also suggested that the greater water capacityof treated plants buffers them against deleterious changes inleaf relative water content and water potential under conditionsof moderate stress.     

Osmotic relations of the drought-tolerant shrub Artemisia tridentata in response to water stress

Turgor maintenance, solute content and recovery from water stress were examined in the drought-tolerant shrub Artemisia tridentata. Predawn water potentials of shrubs receiving supplemental water remained above ?2 MPa throughout summer, while predawn water potentials of untreated shrubs decreased to ?5 MPa. Osmotic potentials decreased in conjunction with water potentials maintaining turgor pressures above 0 MPa. The decreases in osmotic potentials were not the result of osmotic adjustment (i.e. solute accumulation). Leaf solute contents decreased during drought, but leaf water volumes decreased more than 75% from spring to summer, thereby passively concentrating solutes within the leaves. The maintenance of positive turgor pressures despite decreases in leaf water volumes is consistent with other studies of species with elastic cell walls. Inorganic ion, organic acid, and carbohydrate contents of leaves declined during drought. The only solutes accumulating in leaves of A. tridentata with water stress were proline and a cyclitol, both considered compatible solutes. Total and osmotic potentials recovered rapidly following rewatering of shrubs; solute contents did not change except for a decrease in proline. Maintaining turgor through the passive concentration of solutes may be advantageous compared to synthesis of new solutes for osmotic adjustment in arid environments.     

Roles of leaf water potential and soil-to-leaf hydraulic conductance in water use by understorey woody plants

Diurnal changes of leaf water potential and stomatal conductance were measured for 12 deciduous shrubs and tree saplings in the understorey of a temperate forest. Sunflecks raised the leaf temperature by 4°C, and vapor pressure deficit to 2 kPa. Although the duration of the sunflecks was only 17% of daytime, the photon flux density (PFD) of sunflecks was 52% of total PFD on a sunny summer day. Leaf osmotic potential at full turgor decreased in summer, except in some species that have low osmotic potential in the spring. Plants that endured low leaf water potential had rigid cell walls and low osmotic potential at full turgor. These plants did not have lower relative water content and turgor potential than plants with higher leaf water potential. There were three different responses to an increase in transpiration rate: (i) plants had low leaf water potential and slightly increased soil-to-leaf hydraulic conductance; (ii) plants decreased leaf water potential and increased the hydraulic conductance; and (iii) plants had high leaf water potential and largely increased the hydraulic conductance.     

Leaf Water Relations, Osmotic Adjustment, Cell Membrane Stability, Epicuticular Wax Load and Growth as Affected by Increasing Water Deficits in Sorghum

A field experiment was conducted with a non-irrigated waterstress treatment and an irrigated control using four sorghum(Sorghum bicolor L. Moench) cultivars. We investigated the effectsof water deficits on leaf water relations, osmotic adjustment,stomatal conductance, cuticular conductance, cell membrane stability(CMS) measured by the polyethylene glycol (PEG) test, epicuticularwax load (EWL), cytoplasmic lipid content, solute concentrationin cell sap, and growth. Osmotic adjustment was observed under water deficit conditions.Lower osmotic potential enabled plants to maintain turgor anddecreased the sensitivity of turgor-dependent processes. Sugarand K were identified as the major solutes contributing to osmoticpotential in sorghum. Sugar and K concentrations in cell sapincreased by 37·4% and 27%, respectively, under waterdeficit conditions in favour of decreasing osmotic potential.Stomatal conductance and cuticular conductance were lower inthe non-irrigated plants. A wide range in CMS among four cultivarswas observed. CMS increased with increasing water deficits.EWL increased on leaves of water deficient plants and was positivelycorrelated with cuticular conductance and CMS. Membrane phospholipidcontent increased in water-stressed plants. CMS as measured by the PEG test, was influenced by EWL, cuticularthickness, and osmotic concentration of leaf tissues. The cultivarswhich maintained higher CMS, higher EWL, lower cuticular conductance,higher turgor and higher osmotic adjustment under water deficitconditions were identified as drought tolerant. Key words: Sorghum bicolor, cell membrane stability, leaf water relationsosmotic adjustment, water stress     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant II. Water relations, stomatal conductance, abscisic acid content in leaves and xylem sap of plants subjected to water deficiency

The response of w-1, a wilty sunflower (Helianthus annuus L.)mutant, to water stress is described in comparison with thecontrol line (W-1). Detached leaves of w-1 strongly dehydratedduring the first 30 min without significant changes in leafconductance, whereas W-1 responded rapidly to water loss byreducing stomatal aperture. After 2 h stress ABA increased slightlyin w-1, while W-1 leaves showed a 20-fold increase. When waterstress was imposed to potted plants by water withholding, w-1quickly dehydrated, and lost turgor, while W-1 maintained positiveturgor values for a longer period. Wild-type plants respondedto small changes in leaf water potential by accumulating ABAand by closing stomata, whereas in the mutant significant changesin ABA content and in stomatal conductance were found only atvery low water potentials. In another experiment in which waterwas withheld under high relative humidity, when soil water contentstarted to decrease W-1 rapidly closed stomata in the absenceof any change in leaf water status and the reduction in conductancewas paralleled by a rise in xylem sap ABA concentration. Bycontrast the mutant started to accumulate ABA in the xylem sapand to close stomata when soil water content and leaf waterpotential were dramatically reduced. The low endogenous ABAlevels and the inability to synthesize the hormone rapidly eitherin the leaves or in the roots seem to be responsible for thehigh sensitivity of w-1 to water stress. Key words: ABA, Helianthus annuus L, water relations, stomatal conductance, drought, wilty mutant     

Mineral nutrition and the water relations of plants

Summary When young tomato plants were transferred from nutrient solution to mineral-free water, reductions in transpiration, water content of the shoots and stomatal aperture were not accompanied by a reduction in the relative water content or an increase in the suction pressure of the leaves. The relative water content of the leaves was increased and the suction pressure was little affected.Following transfer of the plants to mineral-free water, the mineral content of the shoots and the osmotic pressure of expressed leaf sap were reduced. It was concluded that mineral salts were necessary for maintaining the osmotic pressure of the leaf cell sap and that this was achieved, at least in part, by maintaining the mineral concentration of the sap. The amount of water that could be taken up by leaves and their turgor pressure were related to the osmotic pressure of the sap and calculations of turgor pressure showed that it was less in the leaves of plants with their roots in mineral-free water than in the leaves of plants in nutrient solution.Evidence was obtained that in leaflets detached from plants with their roots in mineral-free water, stomatal closure could occur at a higher water content than in leaflets detached from plants in nutrient solution, indicating a further role of minerals in leaf water relations. It is suggested that this role may be related to the properties of the cell walls.     

Leaf water relations and maintenance of gas exchange in coffee cultivars grown in drying soil

Plant water status, leaf tissue pressure-volume relationships, and photosynthetic gas exchange were monitored in five coffee (Coffea arabica L.) cultivars growing in drying soil in the field. There were large differences among cultivars in the rates at which leaf water potential (Ψ_L) and gas exchange activity declined when irrigation was discontinued. Pressure-volume curve analysis indicated that increased leaf water deficits in droughted plants led to reductions in bulk leaf elasticity, osmotic potential, and in the Ψ_L at which turgor loss occurred. Adjustments in Ψ_L at zero turgor were not sufficient to prevent loss or near loss of turgor in three of five cultivars at the lowest values of midday Ψ_L attained. Maintenance of protoplasmic volume was more pronounced than maintenance of turgor as soil drying progressed. Changes in assimilation and stomatal conductance were largely independent of changes in bulk leaf turgor, but were associated with changes in relative symplast volume. It is suggested that osmotic and elastic adjustment contributed to maintenance of gas exchange in droughted coffee leaves probably through their effects on symplast volume rather than turgor.     

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.