Cell Sap Osmolality Research Articles

Overexpression of Arabidopsis nucleolar GTP-binding 1 (NOG1) proteins confers drought tolerance in rice.

As a major adverse environmental factor in most parts of the world, drought causes substantial crop yield losses. Rice (Oryza sativa) is one of the staple foods for more than one-half of the world's population. Rice plants are sensitive to even mild drought stress and need almost twice the amount of water compared to wheat (Triticum aestivum) or maize (Zea mays). Arabidopsis (Arabidopsis thaliana) small GTPase Nucleolar GTP-binding protein 1 (AtNOG1) plays a role in biotic stress tolerance. Here, we created transgenic rice lines constitutively overexpressing AtNOG1-1 or AtNOG1-2. We also developed rice RNA interference (RNAi) lines that show downregulation of OsNOG1. AtNOG1-1 and AtNOG1-2 overexpressors showed enhanced drought tolerance without compromising grain yield, whereas OsNOG1-RNAi was more susceptible to drought when compared to wild-type plants. Analysis of physiological parameters showed increased cell sap osmolality, relative water content, and abscisic acid (ABA) level, but decreased leaf water loss in AtNOG1-1 or AtNOG1-2 overexpressor lines compared to the control. We found upregulation of several genes involved in ABA and jasmonic acid (JA) signaling, stomata regulation, osmotic potential maintenance, stress protection, and disease resistance in AtNOG1-1 and AtNOG1-2 overexpressor lines compared to the control. We elucidated the role of NOG1-2 and NOG1-1 in regulation of silica body formation around stomata to prevent transpirational water loss. These results provide an avenue to confer drought tolerance in rice.

Mechanisms of frost resistance in Arabidopsis thaliana.

Freezing resistance strategies vary in Arabidopsis depending on origin. Southern accessions may avoid or tolerate freezing, while northern onesare always tolerant and reduce the proportion of freezable tissue water during acclimation. Survival of sub-zero temperatures can be achieved by either avoiding or tolerating extracellular ice formation. Conflicting evidence has been presented showing that detached leaves of Arabidopsis thaliana are either freeze avoiding or tolerant. Here, we used three different natural Arabidopsis accessions from different habitats to investigate the frost resistance strategy of whole plants in soil. Plants were cooled to fixed temperatures or just held at their individual ice nucleation temperature for different time intervals. Tissue damage of whole plants was compared to the standard lethal temperature determined for detached leaves with external ice nucleation. While all detached leaves survived freezing when ice nucleation was externally initiated at mild sub-zero temperatures, whole plants of the southern accession behaved as freeze avoiding in the non-acclimated state. The northern accessions and all cold acclimated plants were freezing tolerant, but the duration of the freezing event affected tissue damage. Because this pointed to cell dehydration as mechanism of damage, the proportion of freezable water in leaves and osmolality of cell sap was determined. Indeed, the freezing tolerant accession Rsch had a lower proportion of freezable water and higher cell sap osmolality compared to the sensitive accession C24 in the cold acclimated state.

Dynamic regulation of the root hydraulic conductivity of barley plants in response to salinity/osmotic stress.

Salinity stress significantly reduces the root hydraulic conductivity (Lpr) of several plant species including barley (Hordeum vulgare). Here we characterized changes in the Lpr of barley plants in response to salinity/osmotic stress in detail using a pressure chamber. Salt-tolerant and intermediate barley cultivars, K305 and Haruna-nijyo, but not a salt-sensitive cultivar, I743, exhibited characteristic time-dependent Lpr changes induced by 100 mM NaCl. An identical response was evoked by isotonic sorbitol, indicating that this phenomenon was triggered by osmotic imbalances. Further examination of this mechanism using barley cv. Haruna-nijyo plants in combination with the use of various inhibitors suggested that various cellular processes such as protein phosphorylation/dephosphorylation and membrane internalization appear to be involved. Interestingly, the three above-mentioned barley cultivars did not exhibit a remarkable difference in root cell sap osmolality under hypertonic conditions, in contrast to the case of Lpr. The possible biological significance of the regulation of Lpr in barley plants upon salinity/osmotic stress is discussed.

Osmotic stress decreases PIP aquaporin transcripts in barley roots but H2O2 is not involved in this process.

Previous reports indicate that salt stress reduces the root hydraulic conductance and the expression of plasmamembrane-type aquaporins (PIPs). As a molecular mechanism for this phenomenon, the present study found evidence that the osmotic component, but probably not an ion-specific component, decreases PIP transcripts. Eight of ten PIP transcripts were reduced to less than half by 360 mM mannitol treatment for 12 h in comparison with control samples. A large decrease of HvPIP2;1 protein was also recorded. This reduction of both transcripts and proteins of HvPIP2s should be physiologically effective for preventing or reducing dehydration at an initial phase of severe salt/osmotic stress. Root cell sap osmolality increased from 278 to 372 mOsm 24 h after 360 mM mannitol treatment. These steps can secure survival and growth recovery with water reabsorption in barley. Our data also suggest that H2O2 seems not to be the main cause of osmotic stress-induced transcriptional down-regulation within the concentrations (20-500 μM) and time periods (24 h) examined, although H2O2 was previously proposed to be involved in the mechanisms of salinity/osmotic tolerance.

Osmotic and ionic effects of NaCl and Na 2SO 4 salinity on Phragmites australis

Osmotic and ion-specific effects of NaCl and Na 2SO 4 on Phragmites australis (Cav.) Trin ex. Steud. were investigated in a laboratory experiment by examining effects of iso-osmotic solutions of NaCl and Na 2SO 4 on growth, osmolality of cell sap, proline content, elemental composition and gas exchange. Plants were supplied with a control standard nutrient solution ( Ψ = −0.09 MPa) or solutions of NaCl or Na 2SO 4 at water potentials of −0.50, −1.09 or −1.74 MPa. Salt treatments increased root concentrations of Na and S or Cl, whereas P. australis had efficient mechanisms for exclusion of Na and S and partly Cl ions from the leaves. Incomplete exclusion of Cl from the leaves may affect aboveground biomass production, which was significantly more reduced by NaCl than Na 2SO 4. Stomatal conductance was negatively influenced by decreasing water potentials caused by NaCl or Na 2SO 4, implying that a non-significant photosynthetic depression observed in plants grown at −1.74 MPa was mainly due to osmotically induced stomatal closure. This was supported by decreasing internal CO 2 concentrations. Saline conditions increased the intrinsic water use efficiency and did not alter photosynthetic parameters derived from light response curves, supporting the assumption of a well-functioning CO 2 utilization in salt stressed plants. The leaf proline concentration increased equally in NaCl and Na 2SO 4-treated plants, and may play an important role as a compatible organic solute. P. australis possesses a range of mechanisms conferring tolerance to both NaCl and Na 2SO 4 stress and except in terms of growth the phytotoxicity of NaCl and Na 2SO 4 are comparable.

Participation of Plant Hormones in Growth Resumption of Wheat Shoots Following Short-Term NaCl Treatment

The effects of sodium-chloride salinity on the leaf elongation rate, transpiration rate, cell sap osmolality, and phytohormone content in 7-day-old shoots of durum wheat (Triticum durum L.) were studied. Leaf growth was suppressed under the salinity stress and resumed 1 h after NaCl removal. The resumption of leaf growth coincided with a decrease in the transpiration rate due to the rapid ABA accumulation in the differentiation leaf zone. The increased IAA concentration in the growing leaf zone promoted the formation of the attraction signal. The authors concluded that the changes in phytohormonal status in wheat plants occurred already following short-term (up to 1 h) salinity and were directed to the maintenance of plant growth under these conditions.

139 Flower Senescence in Hibiscus syriacus L.

This experiment was undertaken to characterize the physiological changes taking place during the petal senescence of Hibiscus syriacus. Five distinctive developmental stages were chronologically suggested. Flower bud dry weight increased almost linearly from Stage I to Stage IV at a rate of ≈15 mg/day. Fresh weight and fresh/dry weight ratio increased much more rapidly between Stage III and Stage IV than during the early stage of development. It showed that petal expansion was partially due to an increased water uptake. The highest osmolality (411 mmol) was found in the fully open flowers. During the subsequent senescence and collapse of the flower, from Stage IV to Stage V, there were a rapid loss of fresh and dry weight and the fall of fresh/dry weight ratio, corresponding to the wilting that characterizes early senescence. A rise in cell sap osmolality coincided with the increase in soluble sugar content and fresh/dry weight ratio, and with the expansion of Hibiscus syriacus petal. Therefore, buds at Stage III, where they are under physiological maturity, might be appropriate to harvest. Hibiscus syriacus flowers showed a small but respiratory peak at Stage IV. The maximum rate of respiration was obtained with fully open flowers (Stage IV), whereas ethylene production remained extremely low until the petals started to open. Ethylene production, ACC synthase, and ACC content increased as the fresh weight of the flowers started to decline. At Stage V, there were a loss of petal fresh weight and a considerable increase in ethylene production (9 nL/g per h). The results of the present study have shown that petal tissue at Stage IV, presenescent stage, was characterized by the increase of soluble sugar and fresh weight, which might be expected to lead to petal expansion and limit turgidity. ABA and the stomata on petal might promote the disorganization.

Osmolality and δ13C of Leaf Tissues of Mangrove Species from Environments of Contrasting Rainfall and Salinity

Neotropical species of the generaRhizophora, LagunculariaandAvicenniagrow in environments of variable salinity and flooding stress. Species ofRhizophorapredominate in riverine and low-energy coastal fringe environments with continuous water movement, whileLaguncularia racemosa, and particularlyAvicennia germinans, grow in areas with stagnating water.Avicennia germinansappears to have the largest range of salinity tolerance.The osmotic characteristics ofRhizophoraspp.,L. racemosaandA. germinansin riverine and coastal environments of north-eastern Venezuela are described and correlated with edaphic and climatic factors. Mature, fully-exposed leaves were collected in humid riverine sites (San Juan River Estuary, Monagas and Sucre States), and seasonally dry coastal fringe habitats (the Unare Coastal Lagoon, and the Chimana Islands off-shore Puerto de La Cruz, Anzoátegui State). Cleaned leaf samples were frozen until measurement of leaf dimensions, chlorophyll, phosphorus and nitrogen contents, δ13C, osmolality of cell sap, and cell sap content of Na, K and Cl. Results indicate: (1) in all species, leaf sap osmolality is highly and positively correlated with interstitial water salinity, and negatively correlated with leaf area; (2) nitrogen and phosphorus contents of leaves are generally lower in dry areas, but average values are not significantly different. Therefore, it appears that nutrient deficiency is not a main factor determining variations in community structure. Nitrogen content per unit dry weight is, in general, twice as high inA. germinanscompared toRhizophoraspecies andL. racemosa; (3) cell sap osmolality is mostly explained by the concentration of Na and Cl; (4) osmolality of riverine plants (957–1253mmol kg−1) is lower than that of coastal plants (1558–1761mmol kg−1); and (5) δ13C values are more negative in riverine (−27·4 to −28·1‰) than in coastal plants (−25·4 to −27·2‰), indicating a higher water-use efficiency in the latter. Coastal plants have a higher water-use efficiency but their growth is inhibited by salinity stress.

Influence of electrolytes on growth, phototropism, nutation and surface potential in etiolated cucumber seedlings.

A variety of electrolytes (10-30 mol m-3) increased the relative growth rate of etiolated cucumber (Cucumis sativus L. cv. Burpee's Pickler) hypocotyls by 20-50% relative to water-only controls. The nonelectrolyte mannitol inhibited growth by 10%. All salts tested were effective, regardless of chemical composition or valence. Measurements of cell-sap osmolality ruled out an osmotic mechanism for the growth stimulation by electrolytes. This, and the nonspecificity of the response, indicate that an electrical property of the solutions was responsible for their growth-stimulating activity. Measurements of surface electrical potential supported this reasoning. Treatment with electrolytes also enhanced nutation and altered the pattern of phototropic curvature development. A novel analytical method for quantitating these effects on growth was developed. The evidence indicates that electrolytes influence an electrophysiological parameter that is involved in the control of cell expansion and the coordination of growth underlying tropisms and nutations.

Solutes Contributing to Osmotic Potential in Young versus Mature Leaves of Cacao Seedlings

Summary The concentrations of sugars, inorganic ions and amino acids were measured at apical and basal leaf positions within both the new (expanding) leaf flush and previous (mature) flush of 8-month-old cacao ( Theobroma cacao L.) seedlings. The osmotic potential of leaves of the previous flush was approximately 0.1 MPa lower (more negative) than that measured in the new flush. No differences were observed, however, for leaf water potential between leaves of the new flush and previous flush. Fructose and glucose were the predominant sugars in cell sap of apical leaves of the new flush, with sucrose accounting for only 11 % of the total concentration (mM L -1 ) of sugars measured. The relative proportions of monosaccharides and sucrose were markedly different in basal leaves of the new flush, however, with sucrose being the predominant sugar and glucose plus fructose being present at less than one-fourth the concentration measured in apical leaves. Sucrose was the predominant sugar in leaves of the previous flush, and concentrations in younger and older leaves were not different. Potassium made the largest single contribution to the osmolality of cell sap, accounting for 38 to 61 % of the total solutes measured. The contribution of amino acids to total osmolality was small, relative to that of sugars or K + . Alanine, asparagine (+ aspartate), γ-aminobutyrate, glutamine (+ glutamate), and serine together accounted for more than 90 % of the osmolality generated by 14 amino acids analyzed. Sugars, inorganic ions and amino acids measured analytically together accounted for 79 to 84 % of the osmolality of expressed sap as measured by vapor pressure osmometry.

Flexibility in CO2 — Fixation Pathway of a Highland Kalanchoë, Kalanchoë petitiana A. Rich.

AbstractPhotosynthetic flexibility and water use efficiency of Kalanchoë petitiana A. Rich., a facultative CAM plant endemic to the highlands of Ethiopia, were investigated to determine the physiological determinants for the ecological success of the plant. Both field measurements of δ13C and greenhouse gas exchange studies showed a shift from C3 photosynthesis to CAM as leaves aged or at the onset of water stress. Recycling of CO2 was observed in developing leaves without concomitant net CO2 uptake. Accumulation of malate was positively correlated with increased cell sap osmolality and improved daily water use efficiency. The importance of flexibility in carbon uptake pathway and of recycling CO2 for the ecological success of the plant is discussed.

Water Relations of Growing Maize Coleoptiles

Water relations of growing segments of maize (Zea mays L.) coleoptiles were investigated with osmotic methods using either mannitol (MAN) or polyethylene glycol 6000 (PEG) as external osmotica. Segments were incubated in MAN or PEG solutions at 0 to - 15 bar water potential (Psi(o)) and the effects were compared on elongation growth, osmotic shrinkage, cell sap osmolality (OC), and osmotic pressure (pi(i)). The nonpenetrating osmoticum PEG affects pi(i) in agreement with Boyle-Mariotte's law, i.e. the segments behave in principle as ideal osmometers. There is no osmotic adjustment in the Psi(o) range permitting growth (0 to -5 bar) nor in the Psi(o) range inducing osmotic shrinkage (-5 to -10 bar). Promoting growth by auxin (IAA) has no effect on the osmotic behavior of the tissue toward PEG. In contrast to PEG, MAN produces an apparent increase in pi(i) accompanied by anomalous effects on segment elongation and shrinkage leading to a lower value for Psi(o) which establishes a growth rate of zero and to an apparent recovery from osmotic shrinkage after 2 hours of incubation. These effects can be quantitatively attributed to uptake of MAN into the tissue. MAN is taken up into the apoplastic space and the symplast as revealed by a large temperature-dependent component of MAN uptake. It is concluded that MAN, in contrast to PEG, is unsuitable as an extemal osmoticum for the quantitative determination of water relations of growing maize coleoptiles.

Improvement of regeneration of Lycopersicon pennellii protoplasts by decreasing ethylene production

Lycopersicon pennellii shoots, cultured in vitro for more than a year (type I plants) produced few viable protoplasts in contrast to shoots cultured in vitro for less than five months (type II plants). Ethylene production of both plant types was compared. The low viability of plant type I protoplasts could be correlated with high ethylene production and an increased cell sap osmolality. The ethylene action inhibitor silver thiosulphate improved protoplast yield and viability, especially when using donor tissue, germinated and cultured on medium containing silver thiosulphate (type III plants). Moreover, the choice of cell wall degrading enzymes influenced protoplast viability, since ethylene release was significantly lower using Cellulase R 10 than Cellulysin. All improvements together resulted in an efficient protocol for the isolation and regeneration of Lycopersicon pennellii protoplasts.

On the mechanism of trap closure of Venus flytrap (Dionaea muscipula Ellis)

The rapid trap closure of Dionaea muscinula Ellis has been explained by either a loss of turgor pressure of the upper epidermis, which should thus become flexible, or by a sudden acid-induced wall loosening of the motor cells. According to our experiments both explanations are doubtful. Objections against the turgor mechanism come from the determination by extracellular measurements from the upper epidermis of action-potential amplitudes before and after trap closure. Neither time course nor amplitude of the action potentials are altered by trap closure. In contrast a rise in the apoplastic concentration of K(+) or Na(+), which are the only ions present in the trap in osmotically significant concentrations, from 1 to 10 mM reduces the action-potential amplitudes by 25% and 15%, respectively. Furthermore, after trap closure the upper epidermal cells retain a considerable cell sap osmolality of 0.41 mol·kg(-1) which equals that of the mesophyll cells as determined by incipient plasmolysis. A sudden cell-wall acidification causing movement is improbable since an acidification of the apoplast from pH 6 to pH 4 reduces action-potential amplitudes by 33% whereas the amplitudes measured extracellylarly from the mesophyll and lower epidermis remain unchanged by trap closure. In addition, buffering the apoplast at pH 6 does not prevent movement in traps which have been incised several times from the margin to the midrib to facilitate buffer diffusion into the mesophyll. Even an alkalinization of cell walls of plasmolysed leaf segments to pH 9 does not prevent considerable extensions of the mesophyll and subsequent movement of the specimens during deplasmolysis.These experiments make it very likely that the mesophyll cells are already extensible but are kept compressed in the open trap, thus developing tissue tension. The mechanism which prevents their extension as long as the trap is open can so far only be explained for traps which have been paralysed by a long-term incubation in 1 mM La(3+). Leaf strips taken from stimulated, closed traps, comprising the lower epidermis and some mesophyll, prove to be highly extensible if they are stretched perpendicular to the midrib on a constant-load extensiometer. By contrast, strips taken from the lower side of paralysed traps are as rigid as those from the upper side of both stimulated and paralysed traps. From observations of semithin cross sections in a polarizing microscope, it is concluded that the extensibilities of these tissue strips are mainly determined by the cell walls of the upper epidermis plus a layer of adjacent mesophyll and by the lower epidermis, respectively, since these are the only cell walls with a preferential microfibril orientation in the direction of the applied stress.

Crassulacean acid metabolism in tropical dicotyledonous trees of the genusClusia

The performance of crassulacean acid metabolism (CAM) by dicotyledonous trees of the genusClusia sampled at three sites in the state of Falcon in northern Venezuela is characterized.Clusia leaves have a somewhat succulent appearance. Unlike leaves of many other CAM plants, which are uniformly built up of very large isodiametric cells, there are distinct layers of palisade and spongy mesophyll, with individual cells being smaller. There is no specialized water storage tissue. δ13C values indicate thatC. multiflora in the elfin-cloud forest on top of Cerro Santa Ana, at ∼800 m altitude, performs C3 photosynthesis (δ13 −27.1‰). However,C. rosea in the tall cloud forest on Cerro Santa Ana (∼600m altitude), andC. rosea andC. alata in the dry forest on Serrania San Luis (∼900 m altitude) perform CAM (δ13C −14.1 to −19.2‰). InC. alta andC. rosea there were large day-night changes in the levels of malic and citric acids ranging from 63 to 240 mmol 1−1 for malid acid and from 35 to 112 mmol 1−1 for citric acid. The sum of the changes in malate and citrate levels accounts for the changes of titratable protons measured. With a day-night change of titratable protons of 768 mmol 1−1 in one of the analyses,C. rosea showed the highest value yet encountered in a CAM plant. Oscillations of free sugars (fructose, glucose, sucrose) and of starch were also analysed in the CAM performingClusia species. Carbon skeletons of the precursors involved in nocturnal malate and citrate synthesis largely derive from free sugars and not from polyglucan. Unlike some other CAM plants, there is no clear and quantitative correlation between day-night changes of organic acid levels and cell sap osmolality.

Ethylene-Induced Lateral Expansion in Etiolated Pea Stems

Treatment of etiolated pea (Pisum sativum L.) internode tissue with ethylene gas inhibits elongation and induces lateral expansion. Precise kinetics of the induction of this altered mode of growth of excised internode segments were recorded using a double laser optical monitoring device. Inhibition of elongation and promotion of lateral expansion began after about 1 hour of treatment and achieved a maximum by 3 hours. Similar induction kinetics were observed after treating internodes with colchicine and 2,6-dichlorobenzonitrile, an inhibitor of cellulose synthesis. In sealed flask experiments, ethylene had no detectable effect on incorporation of label from [(14)C]glucose into any of the classical pectin, hemicellulose, or cellulose wall fractions. Ethylene inhibited fresh weight increase (total cell expansion) of both excised internode segments (in sealed flasks) and intact seedlings. Ethylene treatment resulted in an increase in cell sap osmolality in those tissues (intact and excised) which are inhibited by the gas. A model for ethylene-induced inhibition of elongation and induction of lateral expansion is presented.