Osmotic adjustment in leaves ofLycopersicon esculentum andL. pennellii in response to saline water irrigation

Two tomato species (Lycopersicon esculentum andL. pennellii) were grown under unheated plastic greenhouse and irrigated with 0 or 140 mM NaCl. Salinity induces a more important reduction in predawn leaf water potential (ψ_pd) inL. esculentum than inL. pennellii. In both species the osmotic adjustment was achieved by active solute accumulation. The leaf water potential at turgor loss point (ψ_tlp) seemed to be controlled by leaf osmotic potential (ψ_os). The results revealed the existence of limits to the accumulation of osmotic solutes in leaf tissues and the existence of an ontogenetic effect on the solute accumulation. In both species, but essentially inL. pennellii the inorganic solutes contribution especially Na⁺ and Cl^? accumulation to ψ_os was higher than the organic solutes. Therefore, wild species save energy more markedly.     

Osmotic adjustment in indoor grown cassava in response to water stress

The water content-water potential relation in stressed and unstressed cassava ( Man-ihot species) was examined to ascertain (i) the magnitude of osmotic adjustment in response to water stress and (ii) the mechanisms of such adjustments.
Water stress resulted in a displacement of the water content-potential relation such that at any leaf water potential the water content was higher in the stressed plants. The osmotic potentials of turgid leaves (100% relative water content) were -0.97 and -1.00 MPa in the unstressed cultivars CMC 9 and MCOL 113 respectively. In the stressed plants, the values were-1.13 MPa (CMC 9) and-1.14 MPa (MCOL 113). The 0.14 to 0.16 MPa osmotic potential difference between the stressed and unstressed plants suggests that a stress-induced osmotic adjustment occurred in both cultivars. The biiSk volumetric elastic moduli at turgor pressures above 0.10 MPa were 9.84 MPa (CMC 9) and 13.58 MPa (MCOL 113) in the unstressed plants. Tbe higher values found in the stressed plants, 14.56 MPa in CMC 9 and 16.91 MPa in MCOL 113, suggest a stress-induced decrease in cell wall elasticity. Hence, the observed shift in the wafer content-potential relations in the cassava involved both an osmotic adjustment and a decrease in cell wall elasticity. Increasing the number of stress cycles per plant did not cause a further displacement of the water content-potential curves.     

Osmotic adjustment in sorghum: I. Mechanisms of diurnal osmotic potential changes

Osmotic adjustment, defined as a lowering of osmotic potential (ψ_π) due to net solute accumulation in response to water stress, has been considered to be a beneficial drought tolerance mechanism in some crop species. The objective of this experiment was to determine the relative contribution of passive versus active mechanisms involved in diurnal ψ_π changes in sorghum (Sorghum bicolor L. Moench) leaf tissue in response to water stress. A single sorghum hybrid (cv AT×623 × RT×430) was grown in the field under variable water supplies. Water potential, ψ_π, and relative water content were measured diurnally on expanding and the uppermost fully expanded leaves before flowering and on fully expanded leaves during the grain-filling period. Diurnal changes in total osmotic potential (Δψ_π) in response to water stress was 1.1 megapascals before flowering and 1.4 megapascals during grain filling in comparison with 0.53 megapascal under well-watered conditions. Under water-stressed conditions, passive concentration of solutes associated with dehydration accounted for 50% (0.55 megapascal) of the diurnal Δψ_π before flowering and 47% (0.66 megapascal) of the change during grain filling. Net solute accumulation accounted for 42% (0.46 megapascal) of the diurnal Δψ_π before flowering and 45% (0.63 megapascal) of the change during grain filling in water-stressed leaves. The relative contribution of changes in nonosmotic volume (decreased turgid weight/dry weight) to diurnal Δψ_π was less than 8% at either growth stages. Water stress did not affect leaf tissue elasticity or partitioning of water between the symplasm and apoplasm.     

Stomatal response to plant water deficits

A steady state model of stomatal response to plant water deficit was developed. It simulates typical stomatal resistance-leaf water potential functions; that is, stomatal resistance remains relatively constant until leaf water potential decreases to a threshold value, at which point stomatal resistance increases abruptly. The model shows that differences in the elastic properties of guard cells and surrounding epidermal cells determine the shape of the stomatal resistance-leaf water potential function. The model also simulates the effects of seasonal osmoregulation. This relatively slow accumulation of osmotically active solutes within plant cells reduces the threshold leaf water potential value. By contrast, the relatively rapid diurnal accumulation of osmotically active photosynthates in non-guard cells raises the threshold leaf water potential. This effect can occur if guard cells have a mechanism such as a sugar-starch conversion which prevents osmotically-active photosynthates from accumulating in their vacuoles.     

Inheritance of osmotic adjustment to water stress in three grain sorghum crosses

Water stress is one of the major constraints to the grain yield of sorghum in tropical and sub-tropical areas of the world. Osmotic adjustment has been widely proposed as a plant attribute that confers adaptation to water stress. The inheritance of osmotic adjustment to water stress was investigated in a series of generations derived from the three possible bi-parental crosses between two inbred sorghum lines with a high capacity for osmotic adjustment (Tx2813 and TAM422; high-OA lines) and one with a low capacity (QL27; low-OA line). Broad-sense heritability on a single-plant basis was generally found to be high. Analysis of segregation ratios by the mixture method of clustering identified two independent major genes for high osmotic adjustment. The line Tx2813 possessed a recessive gene which is given the symbol oa1; the line TAM422 possessed an additive gene which is given the symbol OA2. There was some evidence that there may be other minor genes which influence the expression of osmotic adjustment in these crosses as two putative transgressive segregants, with higher osmotic adjustment than the parents, were identified from the cross between Tx2813 and TAM422. Populations of recombinant inbred lines were developed and characterised for osmotic adjustment for two of the crosses (QL27 x TAM422, low-OA x high-OA; Tx2813 x TAM422, high-oal x high-OA2). These will be used to conduct experiments which test hypotheses about the contribution of the high-osmotic-adjustment genes to the grain yield of sorghum under a range of water-stress conditions.     

Diurnal growth trends, water potential, and osmotic adjustment of maize and sorghum leaves in the field

The daily cycle of leaf elongation rate, water potential, and solute potential of maize and sorghum, as well as temperature, were monitored in the field. Major climatic features were high radiation and a minimum air temperature of about 12 C. Leaf elongation of both crops was slowest at night, presumably because of low temperature. Peak elongation rates were in daytime when leaf water potential (Ψ) was low. Solute potential also decreased during daylight, thus permitting the maintenance of appreciable turgor pressure, a critical parameter for cell expansion.     

Osmotic adjustment in Spirulina platensis

Unilateral irradiation of maize (Zea mays L.) seedlings results in a fluence-rate gradient, and hence below saturation, a gradient of the far-red-absorbing form of phytochrome (Pfr). The Pfr-gradients established by blue, red and far-red light were spectrophotometrically measured in the mesocotyl. Based on these Pfr-gradients and the fluence-response curves of phytochrome photoconversion the fluence-rate gradients were calculated. The fluence-rate gradient in the blue (460 nm) was steeper than that in the red (665 nm), which in turn was steeper than that in the far-red light (725 nm). The fluence-rate ratios front to rear were 1:0.06 (460 nm), 1:0.2 (665 nm), and 1:0.33 (725 nm). The assumption that phytochrome-mediated phototropism of maize mesocotyls is caused by local phytochrome-mediated growth inhibition was tested in the following manner. Firstly, the Pfr response curve for growth inhibition was calculated; these calculations were based on measurements of Pfr-gradients and data from red-light-induced phototropism. Secondly, the Pfr response curve for growth inhibition was used as a basis for calculating fluence-response curves for blue-and far-red-light-induced phototropism. Finally, these calculated results were compared with experimental data. It was concluded that the threshold for phytochrome-mediated phototropism of maize mesocotyls reflects the apparent photoconversion cross section of phytochrome whereas the maximal inducable curvature depends on the steepness of the light (Pfr) gradient across the mesocotyl.Abbreviations Pfr far-red-absorbing form of phytochrome - Ptot total phytochrome - Fr far-red light     

Osmotic adjustment in marine yeast

The effect of environmental salinity on cell growth, and onthe composition and accumulation of compatible solutes, or osmotica,of five yeast strains (Aureobasidium pullulans, Candida sp.,Cryptococcus albidus var. albidus, Debaryomyces hansenii andRhodotorula rubra) was compared. All these yeast were isolatedfrom manne environments, but were able to grow in the absenceof salt and should therefore be considered as halotolerant strains.According to their specific cell growth rates at different saltconcentrations, these strains vary in their capacity to osmoticallyadjust to modifications in external salinity. Candida sp. appearsto be the most sensitive since the maximum salt concentrationat which it can grow is 1.54 mol 1^-1 NaCl; however, it showedthe highest specific cell growth in the range of 0 to 1.54 mol1^-1 NaCl. Aureobasidium pullulans, on the other hand, showedthe lowest specific growth rate, but the highest halotolerancerange from 0 to 5.13 mol 1^-1 NaCl. Debaryomyces hansenii, incontrast, showed higher specific growth at this salinity rangeCryptococcus albidus var. albidus and Rhodotorula rubra showedsimilar specific cell growth rate values and halotolerance between0 and 2.45 mol 1^-1 NaCl. The protein and carbohydrate contentof the biomass of the different yeast cells, as a result ofexternal salinity vanation, remained practically constant. Themost important effects of the increase in salt concentrationin the culture medium were the reduction of cell volume andthe accumulation of low-molecular-weight metabolites (LMWM).which appear to act as osmoregulators. Glycerol was found asthe major compatible solute in the different marine yeasts studiedherein with a total contribution of 64–96% of the internalcell osmolarity. Other LMWM, like carbohydrates and amino acids,contributed to a lesser extent to compensate for the rise inosmotic pressure promoted by the salinity of the external environment.     

Osmotic adjustment in tobacco plantlets

UsingNicotiana tabacum L. plantlets cultivatedin vitro as a model system it was proved that osmotic adjustment may be caused by a decrease in water potential of substrate as well as by a decrease in air humidity.     

Osmotic adjustment as a mechanism of dehydration postponement in chickpea (Cicer arietinum L.) leaves

The objectives of this study were to test the existence of osmotic adjustment in a field-grown chickpea (Cicer arietinum L.) and to reproduce it in controlled conditions for a more complete study. In a first experiment, carried out in the field with the cultivar Casoar, we described two types of drought stress that a field-grown chickpea could experience during flowering in our conditions. They were characterized with soil and plant water status. Osmotic adjustment was taking place when the stress increased progressively. This evidence was obtained with the measurement of plant water potential and relative water content during a drying-rewatering cycle. In a second experiment, carried out in pots with rain shelter, with cultivars Casoar and Sombrero, we reproduced this particular type of drought stress, on the basis of soil water potential. Measurement of plant water status was based on water, osmotic, and turgor potentials, and relative water content. It showed that chickpea is able to realize osmotic adjustment during a controlled drying-rewatering cycle limited in intensity and duration. The analysis of a broad range of solutes (nitrate, sucrose, glucose, proline, malic acid and six other organic acids) gave a good explanation of the measured reduction of osmotic potential. Organic acids accounted for most of this reduction: 97% for Casoar and 96% for Sombrero. Malic acid, which represented about half of these acids, and malonic acid significantly accumulated during the drought stress. They explained 78.2% (for Casoar) and 75.8% (for Sombrero) of the reduction of osmotic potential. Cultivar Sombrero was the only one able to accumulate some sucrose.     

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.     

Osmotic adjustment of Zymomonas mobilis to concentrated glucose solutions

Summary The physiological basis of the exceptionally high sugar tolerance of Zymomonas mobilis was investigated. Determinations of the internal metabolite concentrations of Z. mobilis showed that an increase in the extracellular glucose concentration was accompanied by a parallel rise in the intracellular glucose concentration, bringing about an almost complete osmotic balance between internal and external space. Studies of glucose transport confirmed that Z. mobilis has a facilitated diffusion system which enables a rapid equilibration between internal and external glucose concentrations. Studies using the non-metabolisable sugars maltose (impermeable) and xylose (permeable) revealed that these sugars were able to alter the osmotic pressure on the cytoplasmic membrane resulting in volume changes.Dedicated to Professor R. K. Finn on the occasion of his 70th birthday     

不同甘蔗品种叶片气孔对水分胁迫的响应

干旱是甘蔗面临最主要的环境胁迫之一,为了解不同甘蔗品种在干旱胁迫时的气孔响应,该研究以F172、GT21、YT93/159和 YL6四个抗旱性有显著差异的甘蔗品种为材料,采用桶栽,在伸长期进行四种不同程度的干旱胁迫(不浇水)处理：土壤持水量在①65％~70％为轻度干旱;②45％~50％为中度干旱;③25％~30％为重度干旱;④以土壤含水量为75％为对照(CK).检测不同品种不同处理甘蔗的叶片相对持水量变化,并利用扫描电镜技术观察甘蔗叶片下表皮气孔特性.结果表明：在干旱胁迫下,四个甘蔗品种叶片气孔导度急剧下降,重度干旱时耐旱性强的 F172和 GT21的气孔导度低于耐旱性弱的 YT93/159和 YL6的;复水后3 d,F172和 GT21的气孔导度上升至82．07和88．85 mmol·m-2·s-1,而 YT93/159和 YL6的仅有18．88和33．08 mmol·m-2·s-1.干旱还导致气孔下陷、闭合,气孔器的长、宽明显减小,且品种间气孔器长度变化差异显著;干旱胁迫下气孔密度增大,尤以耐旱性最强的 F172在重度干旱时达到显著差异.重度干旱时 F172与GT21的气孔闭合百分比是 YT93/159和 YL6近3~4倍.在水分胁迫下,叶片相对含水量降低,但 F172和GT21在重度干旱时仍可以保持相对较高的含水量,其它两个品种相对较低,尤以 YT93/159的最低.在复水后叶片含水量都有所恢复.这些研究结果表明不同甘蔗品种抗旱能力与叶片气孔特性和含水量密切相关.     

Osmotic adjustment in cyanobacteria from hypersaline environments

The intracellular concentrations of the monovalent inorganic cations K⁺ and Na⁺, low molecular weight carbohydrates and quaternary ammonium compounds have been determined for 4 strains of cyanobacteria (Aphanothece halophytica, Coccochloris elabens, Dactylococcopsis salina and Synechocystis DUN52) originally isolated from hypersaline habitats (i.e. habitats with a salinity greater than that of seawater) over a range of external salt concentration (from 50% to 400% seawater). Intracellular cation levels (Na⁺ and K⁺) were determined to be within the range 80–320 mmol · dm^-3 (cell volume), showing only minor changes in response to salinity. Intracellular carbohydrates were found to comprise a negligible component of the intracellular osmotic potential [at 2–19 mmol · dm^-3 (cell volume)], throughout the salinity range. Quaternary ammonium compounds, however, were recorded in osmotically significant quantities [up to 1,640 mmol · dm^-3 (cell volume)] in these strains, showing major variation in response to salinity. Thus Synechocystis DUN 52 showed an increase in quaternary ammonium compounds in the oder of 1,200 mmol · dm^-3 between 50% and 400% seawater medium, accounting for a significant proportion of the change in external osmotic potential.Examination of intact cells and cell extracts using ¹³C and ¹H nuclear magnetic resonance (NMR) spectroscopy confirmed the presence of the quaternary ammonium compound glycine betaine as the major osmoticum in the 4 strains; no other compounds were detected during NMR assays. These results suggest a common mechanism of osmotic adjustment, involving quaternary ammonium compounds, in cyanobacteria from hypersaline environments.     

Stomatal and photosynthetic responses of olive (Olea europaea L.) leaves to water deficits

The leaf gas exchange of mature olive trees (Olea europaea L.) was characterized over a wide range of water deficits in the field during 1998, in Cordoba, Spain. Leaf photosynthesis (A) and stomatal conductance (g_l) responded diurnally and seasonally to variations in tree water status and evaporative demand. In the absence of water stress, A and g_l were generally high during autumn and low in days of high vapour pressure deficits (VPD). Leaf A varied between 0 and 2 µmol m^?2 s^?1 under severe water deficits that lowered the stem water potential (Ψ_x) to ?8·0 MPa, but recovered rapidly following rehydration. Transpiration efficiency (TE) was curvilinearly related to VPD and not influenced by water deficits except in cases of severe water stress, where low TE values were observed at Ψ_x below ?4 MPa. Three models of leaf conductance were calibrated and validated with the experimental data; two were based on the model proposed by Leuning (L) and the other was derived from the widely used Jarvis (J) model. The L models performed better than the J model in two validation tests. The scatter of the predictions and the limited accuracy of all three models suggest that, in addition to the physiological and environmental variables considered, there are additional endogenous factors influencing the g_l of olive leaves.     

Osmotic adjustment in three succulent species of Zygophyllaceae

Three species, Zygophyllum album L., Z. coccineum L. and Z. simplex L., from family Zygophyllaceae were collected from two locations in Egypt to study their response to environmental conditions. Organic solutes (amino acids, soluble proteins and soluble sugars) and inorganic solutes (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^?, PO₄^3? and SO₄^2?) were estimated to study their role in osmotic adjustment under the effect of drought and salinity. The study showed that Z. coccineum is most tolerant for drought and salinity than Z. simplex. Z. coccineum was dependent on soluble proteins and soluble sugars, to increase its content of bound water, to undergo water deficit in desert. Z. simplex accumulated inorganic solutes more than Z. coccineum and less organic solutes. Amino acids content increased in Z. coccineum and Z. simplex survived in saline conditions to play a role in osmotic adjustment. Under the effect of salinity, all the studied species showed a tendency and high capacity to accumulate inorganic solutes. The main inorganic salutes were Ca²⁺, Mg²⁺ and Cl^?. The role of Na⁺ was less than Ca²⁺ and Mg²⁺. Z. album and Z. simplex preferred Mg²⁺ more than Z. coccineum which preferred Ca²⁺.     

Osmotic adjustment by intact isolated chloroplasts in response to osmotic stress and its effect on photosynthesis and chloroplast volume

Spinach leaf chloroplasts isolated in isotonic media (330 millimolar sorbitol, −1.0 megapascals osmotic potential) had optimum rates of photosynthesis when assayed at −1.0 megapascals. When chloroplasts were isolated in hypertonic media (720 millimolar sorbitol, −2.0 megapascals osmotic potential) the optimum osmotic potential for photosynthesis was shifted to −1.8 megapascals and the chloroplasts had higher rates of CO₂-dependent O₂ evolution than chloroplasts isolated in 330 millimolar sorbitol when both were assayed at high solute concentrations.

Transfer of chloroplasts isolated in 330 millimolar sorbitol to 720 millimolar sorbitol resulted in decreased chloroplast volume but this shrinkage was only transient and the chloroplasts subsequently swelled so that within 2 to 3 minutes at 20°C the chloroplast volume had returned to near the original value. Thus, actual steady state chloroplast volume was not decreased in hypertonic media. In isotonic media, there was a slow but significant uptake of sorbitol by chloroplasts (10 to 20 micromoles per milligram chlorophyll per hour at 20°C). Transfer of chloroplasts from 330 millimolar sorbitol to 720 millimolar sorbitol resulted in rapid uptake of sorbitol (up to 280 micromoles per milligram chlorophyll per hour at 20°C) and after 5 minutes the concentration of sorbitol inside the chloroplasts exceeded 500 millimolar. This uptake of sorbitol resulted in a significant underestimation of chloroplast volume unless [¹⁴C]sorbitol was added just prior to centrifuging the chloroplasts through silicone oil. Sudden exposure to osmotic stress apparently induced a transient change in the permeability of the chloroplast envelope since addition of [¹⁴C]sorbitol 3 minutes after transfer to hypertonic media (when chloroplast volume had returned to normal) did not result in rapid uptake of labeled sorbitol.

It is concluded that chloroplasts can osmotically adjust in vitro by uptake of solutes which do not normally penetrate the chloroplast envelope, resulting in a restoration of normal chloroplast volume and partially preventing the inhibition of photosynthesis by high solute concentrations. The results indicate the importance of matching the osmotic potential of isolation media to that of the tissue, particularly in studies of stress physiology.

     

Water deficits and reproduction in maize : response of the reproductive tissue to water deficits at anthesis and mid-grain fill

Reproductive development in maize (Zea mays L.) is vulnerable to plant water deficits during anthesis but becomes less sensitive as reproduction progresses. To determine whether changes in tissue water status correlated with the change in sensitivity, we examined the water potential (Ψ_w), osmotic potential (Ψ_s), and turgor of reproductive tissues during a short-term water deficit imposed at anthesis or mid-grain fill. Plants were grown in controlled environments in soil. At anthesis, leaf, husk, silk, and ovary Ψ_w of control plants was similar (−0.5 to −0.65 megapascal) at midday. When water was withheld, Ψ_w decreased to −1.75, −1.3, −1.2, and −1.0 megapascal in these tissues. Net water uptake by the ovaries was inhibited, but final dry weight, solute content, and total extractable carbohydrates were similar to the controls. At mid-grain fill, leaf, husk, grain, and embryo Ψ_w of control plants were −0.55, −0.35, −0.75, and −0.80 megapascal at midday. When water was withheld, leaf and husk Ψ_w decreased to −2.4 and −1.4 megapascal within 6 days. However, grain and embryo Ψ_w remained within 0.15 megapascal of control values. The grain continued to accumulate dry matter despite a net loss of water and a reduction in total solute content. These results indicate that the response of the reproductive tissues to plant water deficits varies with stage of grain development. The maintenance of a favorable water status only after grain filling is under way may explain, at least in part, the high sensitivity to plant water deficits early in reproductive development and the decrease in sensitivity as reproduction progresses.