Osmotic Adjustment and the Development of Freezing Resistance in Fragaria virginiana

Cold temperature acclimation in strawberry (Fragaria virginiana) leaves apparently involves the alteration of cellular osmotic properties. Alterations in leaf osmotic potential were closely correlated with alterations in soluble carbohydrate content of the leaf tissue and changing temperatures. Leaf starch content was inversely related to soluble carbohydrate levels, suggesting that starch is a partial source of osmoticum during osmotic adjustment associated with cold temperature stress. Free amino acid changes were more closely linked to senescence and growth processes while changes in ion content suggested a rapid mobilization of solutes at the onset of freezing temperatures. This was supported by changes in whole plant gradients in leaf osmotic potential before and after exposure to freezing temperatures. In terms of freezing resistance and the role of osmotic adjustment in the development of resistance, it was found that of all leaves undergoing osmotic adjustment only the younger leaves survived, suggesting an age-dependent component to freezing resistance in leaves. Freezing resistance appears to involve alterations in several cellular properties that act in concert to confer a hardy state of the tissue. Although osmotic adjustment may be an important component of the final combination of cellular properties, this study indicates that solute accumulation does not function alone to confer freezing resistance.     

Leaf age effects on solute accumulation in water-stressed grapevines

The effects of leaf age on water relations, organic solute, and total ion accumulation were studied in mature and immature leaves of two-year-old grapevines (Vitis vinifera L., cv. Savatiano) grown under water stress conditions. Osmotic potential at full turgor decreased significantly in leaves of stressed plants, irrespective of leaf age, indicating the occurrence of an active osmotic adjustment. The apoplastic water fraction (A) increased during leaf ontogeny in both control and stressed plants. However, the values of A were lower in stressed plants. Starch concentration decreased significantly in both mature and immature leaves during the drought cycle, while the relative proportion of monosaccharides and sucrose was markedly different in immature leaves compared to mature. The accumulation of total inorganic ions, induced by drought, was also age dependent, increasing significantly with leaf age, while there were no significant differences in total amino acids content. Inorganic ions and carbohydrates seem to be the major component of osmotic adjustment in mature and immature grapevine leaves, respectively.     

Contribution of osmotic adjustment to the dehydration tolerance of water-stressed pigeon pea (Cajanus cajan (L.) millsp.) leaves

Abstract Water-stressed pigeonpea leaves have high levels of osmotic adjustment at low leaf water potentials. The possible contribution of this adjustment of dehydration tolerance of leaves was examined in plants grown in a controlled environment. Osmotic adjustment was varied by withholding water from plants growing in differing amounts of soil, which resulted in different rates of decline of leaf water potential. The level of osmotic adjustment was inversely related to leaf water potential in all treatments. In addition, at any particular water potential, plants that had experienced a rapid development of stress exhibited less osmotic adjustment than plants that experienced a slower development of stress. Leaves with different levels of osmotic adjustment died at water potentials between –3.4 and –6.3 MPa, but all leaves died at a similar relative water content (32%). Consequently, leaves died when relative water content reached a lethal value, rather than when a lethal leaf water potential was reached. Osmotic adjustment delayed the time and lowered the leaf water potential when the lethal relative water content occurred, because it helped maintain higher relative water contents at low leaf water potentials. The consequences of osmotic adjustment for leaf survival in water-stressed pigeonpea are discussed.     

Influence of Osmotic Adjustment on Leaf Rolling and Tissue Death in Rice (Oryza sativa L.)

Osmotic adjustment, measured by the lowering of the osmotic potential at full turgor, and its influence on leaf rolling and leaf death was assessed in the lowland rice (Oryza sativa L.) cultivar IR36 in both the greenhouse and field. The degree of osmotic adjustment varied with the degree and duration of stress, but was usually 0.5 to 0.6 megapascal (maximally 0.8 to 0.9 megapascal) under severe stress conditions. In leaves in which osmotic adjustment was 0.5 to 0.6 megapascal, leaf rolling and leaf death occurred at lower leaf water potentials in adjusted than in nonadjusted leaves. We conclude that osmotic adjustment aids in the drought resistance of rice by delaying leaf rolling, thereby maintaining gas exchange, and by delaying leaf death.     

Rapid Osmotic Adjustment in Detached Wheat Leaves

Osmotic adjustment is induced in detached wheat leaves by rapiddrying to a relative water content below 0·65, followedby re-saturation. Quantitatively, the response to this treatmentis comparable to the maximum of adjustment obtained with pottedplants at the same developmental stage. Low temperatures duringdrying and re-saturation of the leaves reduce the adjustmentresponse. We conclude that drought stress serves as a triggeronly, while the metabolic events lowering the osmotic potentialare favoured by high or intermediate water contents. Triticum durum L., durum wheat, rapid dehydration, osmotic adjustment, pressure-volume curves     

Osmotic Adjustment in Cotton (Gossypium hirsutum L.) Leaves and Roots in Response to Water Stress

The relative magnitude of adjustment in osmotic potential (ψ_s) of water-stressed cotton (Gossypium hirsutum L.) leaves and roots was studied using plants raised in pots of sand and grown in a growth chamber. One and three water-stress preconditioning cycles were imposed by withholding water, and the subsequent adjustment in solute potential upon relief of the stress and complete rehydration was monitored with thermocouple psychrometers. Both leaves and roots exhibited a substantial adjustment in ψ_s in response to water stress with the former exhibiting the larger absolute adjustment. The osmotic adjustment of leaves was 0.41 megapascal compared to 0.19 megapascal in the roots. The roots, however, exhibited much larger percentage osmotic adjustments of 46 and 63% in the one and three stress cycles, respectively, compared to 22 and 40% in the leaves in similar stress cycles. The osmotically adjusted condition of leaves and roots decreased after relief of the single cycle stress to about half the initial value within 3 days, and to the well-watered control level within 6 days. In contrast, increasing the number of water-stress preconditioning cycles resulted in significant percentage osmotic adjustment still being present after 6 days in roots but not in the leaves. The decrease in ψ_s of leaves persisted longer in field-grown cotton plants compared to plants of the same age grown in the growth chamber. The advantage of decreased ψ_s in leaves and roots of water-stressed cotton plants was associated with the maintenance of turgor during periods of decreasing water potentials.     

Effects of repeated application of water stress on water status and growth of wheat

The effects on water status and growth of controlled cycles of water stress applied at various stages of development were studied on a semi-dwarf spring wheat (Triticum aestivum L.). The plants were grown in controlled environment chambers of the Duke University Phytetron at 24/18°C with a 12-h photo-period at about 600 μE m^?2 s^?1. Groups of plants were subjected to severe water stress by withholding irrigation, beginning at the 7th leaf, early anthesis, or early dough stages of development. A second cycle started 9 to 13 days after termination of the first cycle and maintained until the flag leaf water potential reached –25 bars at each of the growth stages. The lower leaves showed sign of wilting as indicated by curling in the first drying cycle at –7 bars and in the second cycle at –9 bars of leaf water potential during all stages of growth. Although these leaves recovered completely upon rewatering, onset of senescence was accelerated by three days in stressed plants. A preliminary drying cycle did not increase the ability of the plants to withstand subsequent stress because of severity of stress. Water stress of –25 bars at all three stages of growth reduced seed yield. The reduction was greater when a second stress cycle was also applied. Stress applied during early anthesis stage produced the smallest and the least number of seeds. The lack of osmotic adjustment probably was due to very rapid and severe development of water 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.     

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.     

Seasonal Changes in Water Potential and Turgor Maintenance in Sorghum and Maize under Water Stress

Leaf water (Ψ) and solute (ψ) potential were measured in field sorghum and maize under well irrigated (I) and dryland (NI) conditions throughout a season. Despite decreases in ψ due to slow soil water depletion and to apparent increases in liquid phase plant resistance, midday leaf turgor (ψ_p) in the NI sorghum was maintained at similar levels as in the I treatment throughout the season due to concomitant decreases in ψ_s. Osmotic adjustment was also observed in maize, although ψ_p was significantly lower in the NI treatment as compared to I during the final stages of grain filling. A seasonal shift in the ψ vs. relative water content relation of NI sorghum leaves was observed, more water being retained by the older leaf at any particular ψ. The major factor for turgor maintenance was a net increase in solutes per unit of tissue. The role played by increases in the proportion of tissue volume occupied by cell wall was also evaluated. No stomatal closure due to water stress was found in NI sorghum even though leaf ψ reached —20 bars late in the season. Under similar conditions, stomata closed at —14 to —16 bars in younger plants where water stress was made to develop much faster.     

Cell Membrane Stability and Leaf Water Relations as Affected by Potassium Nutrition of Water-Stressed Maize

A field experiment was conducted to investigate the effect ofK nutrition under water stress conditions on cell membrane stabilitymeasured by the polyethylene glycol test, plant growth, internalplant water relations and solute and mineral concentrationsin maize (Zea mays L.). Water-stressed plants showed greateradaptation to water deficits at higher K levels. Cell membranestability increased, leaf water potential and osmotic potentialdecreased, turgor potential increased and stomatal resistancedecreased with increasing K nutrition. Osmotic adjustment wasevident and it may have been influenced by increased K⁺ concentrationsin leaf tissues with increasing K nutrition. Higher leaf thicknessand higher leaf water content were observed at higher K levels.Results suggested that higher supplies of K nutrition may increaseplant production during periods of water stress. Key words: Zea mays L., cell membrane stability, leaf water potential, osmotic adjustment, osmotic potential, potassium nutrition, water stress     

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     

Drought resistance of mycorrhizal pepper plants independent of leaf P concentration - response in gas exchange and water relations

Pepper ( Capsicum annuum L.) plants with and without the VA-mycorrhizal fungus Ghmus deserticola Trappe. Bloss and Menge (VAM and NVAM. respectively), were drought acclimated by four drought cycles (DA) or kept well watered (NDA). All plants were then subjected to an additional drought followed by a 3-day irrigation recovery period. Measurements of water relations, gas exchange and carbohydrates were made at selected intervals throughout the drought cycles and recovery. To equalize growth and avoid higher P in VAM plants. NVAM plants received higher P fertilization. Consequently, similar transpirational surface and shoot mass were achieved in all treatments, but NVAM had a higher tissue P concentration than VAM plants. Plants that were either VAM or DA, but especially the VAM-DA plants, tended to be high in net photosynthetic flux (A), A per unit of tissue P concentration (A/P), stomatal conductance (g) or leaf turgor (Ψ_p) during high environmental stress or recovery from stress. During this time, NVAM-NDA plants had low A. A/P and leaf chlorophyll, but high soluble carbohydrate concentrations in their leaves. All VAM and DA plants had some osmotic adjustment compared to the NVAM-NDA plants, but VAM-DA plants had the most. Osmotic adjustment was not due to accumulation of soluble carbohydrate. The high turgor, A and g in the VAM-DA plants during and following environmental stress indicated superior drought resistance of these plants; however, osmotic adjustment was only apparent during recovery and cannot account for the observed drought resistance during environmental stress. Drought resistance of VAM-DA plants was not attributable to high leaf P concentration or confounded by differences in plant transpirational surface.     

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     

Osmotic adjustment in the mycelial ascomyceteNeocosmospora vasinfecta (E. F. Smith)

Osmotic adjustment in the ascomyceteNeocosmospora vasinfecta was investigated by determining intramycelial water, mycelial solutes, and total mycelial osmolality. Major organic and inorganic solutes as well as proline and glycine betaine were determined under conditions of osmotic stress and shock, imposed by 0.5M KCl. Comparison to glucose as a nonelectrolytic osmoticum was also made. Results quantitatively implicated the polyhydric alcohols as the osmotic adjusters. Changes in amino acids were due to growth and were not osmoregulatory in nature. The osmoticum was not utilized for osmotic adjustment. The growth ofN. vasinfecta in the presence of KCl indicated that this organism is moderately sensitive to osmotic stress.     

Osmotic adjustment to water stress in pearl millet (Pennisetum americanum [L.] Leeke) under field conditions

Abstract. Osmotic adjustment, a mechanism whereby plants maintain positive turgor despite low water potential (ψ), was investigated in pearl millet ( Pennisetum americanum [L.] Leeke) in three types of field experiment at Hyderabad, India:

• (1)

  Osmotic adjustment during the growing season was evaluated by comparing solute potential (ψ_s) of leaves taken at midday from irrigated and droughted plots and allowed to rehydrate in the laboratory. The degree of seasonal adjustment was also estimated by comparing observed values of ψ_s in the field with those expected if ψ_s decreased solely in proportion to water loss. Both types of assessment indicated the maximum seasonal adjustment to be about 0.2 MPa. The cultivars BJ 104 and Serere 39 differed in their capacity to adjust osmotically over the season; Serere 39 was least able to osmoregulate.
• (2)

  Measurements of diurnal variations in ψ and ψ_s in BJ 104 revealed osmotic adjustment during the afternoon hours. At a given value of ψ, turgor (ψ_p) was about 0.1 MPa higher in irrigated, and over 0.2 MPa higher in droughted plants, in the afternoon, than in the morning.
• (3)

  Osmotic adjustment of different leaves within the canopy was investigated. Upper leaves had lower ψ than basal leaves. Differences in ψ were matched by gradients in ψ_s, so that turgor was similar for all leaf layers.

     

Seasonal Changes in the Cytokinin Content of Ginkgo biloba Leaves

Young growth-chamber-grown cotton plants were subjected to a series of eight periods of soil water stress, which served as a preconditioning treatment. After preconditioning, water was withheld and changes in the stomatal resistance and leaf water potential were determined and compared with similar well watered control plants. The stomatal response of stress preconditioned plants adjusted such that the diffusion resistance of the lower surface of the leaf did not reach a value greater than 20 s cm^?1 until the leaf water potential dropped 14 bars below that required to reach the same resistance on previously unstressed plants. The resistance—leaf water potential relation for the adaxial surface was unaltered by the preconditioning treatment. Adjustment of the osmotic potential of the guard cells on the abaxial surface provides at least a partial explanation of this change in response. The lack of adjustment of stomatal response on the adaxial surface of the leaves was correlated with a lack of adjustment in osmotic potential of guard cells on that surface.     

Role of proline and leaf expansion rate in the recovery of stressed white clover leaves with increased phosphorus concentration

Osmotic adjustment (OA) and increased cell-wall extensibility required for expansive leaf growth are well defined components of adaptation to water stress in dry soil, which might interact with soil phosphorus (P) concentration and defoliation frequency for intensively grazed white clover in legume-based pastures. Experiments were conducted with frequently and infrequently defoliated mini-swards of white clover growing in dry soil with low and high P concentrations. The higher yielding high-P plants were able to dry the soil to greater soil water suctions; their leaves had lower water potential values, yet they showed fewer water stress symptoms and underwent a more complete recovery from the water stress symptoms on rewatering, than the low-P plants. High- P plants had greater OA, proline concentration and leaf expansion rate. On the other hand, low-P plants showed an increased osmotic concentration when there was no change in the total solute content per unit of leaf d. wt, indicating more loss of water from the leaf tissue. The key measures that appeared to be directly associated with plant recovery over a short period following water stress were increased proline concentration and leaf expansion rate, probably resulting from increased cell-wall extensibility rather than increased production of cells for the high-P plants.     

OSMOTIC ADJUSTMENT OF PLANTS TO SALINE MEDIA. I. STEADY STATE

Bernstein , Leon . (U. S. Salinity Lab., Riverside, Calif.) Osmotic adjustment of plants to saline media. I. Steady state. Amer. Jour. Bot. 48(10): 909–918. Illus. 1961.—The osmotic pressure (OP) of roots, as well as the OP of aboveground parts of cotton and pepper plants, increase pari passu with increases in the OP of the medium over as wide a range of salinity as would permit any growth. New plasmolytic and cryoscopic techniques yield comparable OP values for roots when dilution of cell sap by the free-space solution and by endosmosis during rinsing of roots are taken into account. Since OP differentials between plant parts and root media are maintained, turgor does not decrease and growth inhibition by salinity cannot be attributed to water stress in the sense of lowered plant turgor. Some alternative mechanisms for osmotic inhibition of growth are considered and the osmotic adjustment process itself is indicated as a likely limiting factor for growth under saline conditions. Osmotic adjustment of cells to salinity depends in part on increased accumulation of ions and also on substitution of monovalent for polyvalent ions. The relative importance of increased salt accumulation, ionic substitution and increased non-electrolyte concentration varies with organ (leaf or root), leaf age and salinity level.     

Growth and water relations of cultured tomato cells after adjustment to low external water potentials

Cultured cells of tomato, Lycopersicon esculentum Mill. cv VFNT-cherry, have been selected for resistance to water stress (low water potential) imposed by the addition of polyethylene glycol to the culture medium. The ability of nonselected cells to grow in media with low water potentials changes dramatically with the age of the cells (with respect to days following inoculation) whereas there is little effect of the age of selected cells on growth over the same media water potentials. The increased resistance of selected cells has limited stability in the absence of stress, indicating that resistance is established by a slow reversible adaptive process.

Increased resistance (growth) in the presence of water stress appears to result from considerable osmotic adjustment by the cells. Growth cycle-dependent changes in resistance of nonselected cells are correlated with osmotic potential changes which are associated with the normal cell growth pattern in culture. Lowered osmotic potential is maintained by selected cells throughout the entire growth cycle and may explain the growth cycle independence of growth of selected cells on polyethylene glycol-containing media. Osmotic adjustment of resistant cells at stationary phase can be as much as 40 bar. Turgor is maintained by resistant cells (as high as 21 bar) in media with low water potentials at least partly at the expense of cell expansion.