Changes In Osmotic Potential Research Articles

Drought responsiveness in six wheat genotypes: identification of stress resistance indicators.

Wheat (Triticum aestivum L.) is among the world's most important staple food crops. In the current climate change scenario, a better understanding of wheat response mechanisms to water stress could help to enhance its productivity in arid ecosystems. In this study, water relations, gas exchange, membrane integrity, agronomic traits and molecular analysis were evaluated in six wheat genotypes (D117, Syndiouk, Tunisian durum7 (Td7), Utique, Mahmoudi AG3 and BT) subjected to drought-stress. For all the studied genotypes, drought stress altered leaf area, chlorophyll content, stomatal density, photosynthetic rate and water-use efficiency, while the relative water content at turgor loss point (RWC0) remained stable. Changes in osmotic potential at turgor loss point (Ψπ0), bulk modulus of elasticity (Ɛmax) and stomatal regulation, differed greatly among the studied genotypes. For the drought-sensitive genotypes AG3 and BT, no significant changes were observed in Ψπ0, whereas the stomatal conductance (gs) and transpiration rate (E) decreased under stress conditions. These two varieties avoided turgor loss during drought treatment through an accurate stomatal control, resulting in a significant reduction in yield components. On the contrary, for Syndiouk, D117, Td7 and Utique genotypes, a solute accumulation and an increase in cell wall rigidity were the main mechanisms developed during drought stress. These mechanisms were efficient in enhancing soil water uptake, limiting leaf water loss and protecting cells membranes against leakage induced by oxidative damages. Furthermore, leaf soluble sugars accumulation was the major component of osmotic adjustment in drought-stressed wheat plants. The transcriptional analysis of genes involved in the final step of the ABA biosynthesis (AAO) and in the synthesis of an aquaporin (PIP2:1) revealed distinct responses to drought stress among the selected genotypes. In the resistant genotypes, PIP2:1 was significantly upregulated whereas in the sensitive ones, its expression showed only a slight induction. Conversely, the sensitive genotypes exhibited higher levels of AAO gene expression compared to the resistant genotypes. Our results suggest that drought tolerance in wheat is regulated by the interaction between the dynamics of leaf water status and stomatal behavior. Based on our findings, Syndiouk, D117, Utique and Td7, could be used in breeding programs for developing high-yielding and drought-tolerant wheat varieties.

Water relations after Ca, B and Si application determine fruit physical quality in relation to aquaporins in Prunus

As climacteric fruits, fleshy Prunus has a short ripening period and short shelf-life after harvest. Therefore, the main problem with these fruits is its physiological parameters related to the maintenance of firmness. In this work, the effects of combined foliar sprays (Ca+B and Ca+Si) on the physical and physiological parameters of prunus fruits were studied. Also, the involvements of aquaporin expression in relation to the maintenance of water and nutrients in the fruits were determined. Foliar treatments were applied to cherry (Prunus avium L.), apricot (Prunus armeniaca), doughtnut peach (Prunus persica L. var platycarpa), and nectarine (Prunus persica L.) trees grown under commercial conditions. The effects of treatments on fresh weight seemed to be dependent on the different species, with increases in apricot by Ca+Si and in nectarines by Ca+B. The firmness in cherries and apricots was enhanced by all the treatments. Also, apricot, doughnut peach and nectarine fruits did not show changes in osmotic potential, although increases in water potential and turgor potential were observed. Increases in some aquaporins only appeared in cherry and nectarine but not in apricot and doughnut peach which is in agreement with the results obtained in the activity of aquaporins in association with the retaining of electrolytes. Therefore, the results of this work indicate that the study of water relations involving aquaporins in mature prunus fleshy fruits could be a key aspect for the maintenance of turgor.

Modeling the effect of salt-affected soil on water balance fluxes and nitrous oxide emission using modified DNDC

Soil salinity restricts plant growth, affects soil water balance and nitrous oxide (N2O) fluxes and can contaminate surface and groundwater. In this study, the Denitrification Decomposition (DNDC) model was modified to couple salt and water balance equations (SALT-DNDC) to investigate the effect of salinity on water balance and N2O fluxes. The model was examined against four growing seasons (2008–11) of observed data from Lethbridge, Alberta, Canada. Then, the model was used to simulate water filled pore space (WFPS), salt concentration and the N2O flux from agricultural soils. The results show that the effects of salinity on WFPS vary in different soil layers. Within shallow soil layers (<20 cm from soil surface) the salt concentration does not affect the average WFPS when initial salt concentrations range from 5 to 20 dS/m. However, in deeper soil layers (>20 cm from soil surface), when the initial salt concentration ranges from 5 to 20 dS/m it could indirectly affect the average WFPS due to changes of osmotic potential and transpiration. When AW is greater than 40%, the average growing season N2O emissions increase to a range of 0.6–1.0 g-N/ha/d at initial salt concentrations (5–20 dS/m) from a range of 0.5–0.7 g-N/ha/d when the salt concentrations is 0 dS/m. The newly developed SALT-DNDC model provides a unique tool to help investigate interactive effects among salt, soil, water, vegetation, and weather conditions on N2O fluxes.

Specific leaf metabolic changes that underlie adjustment of osmotic potential in response to drought by four Quercus species.

Osmotic adjustment is almost ubiquitous as a mechanism of response to drought in many forest species. Recognized as an important mechanism of increasing turgor under water stress, the metabolic basis for osmotic adjustment has been described in only a few species. We set an experiment with four species of the genus Quercus ranked according to drought tolerance and leaf habit from evergreen to broad-leaved deciduous. A cycle of watering deprivation was imposed on seedlings, resulting in well-watered (WW) and water-stressed (WS) treatments, and their water relations were assessed from pressure-volume curves. Leaf predawn water potential (Ψpd) significantly decreased in WS seedlings, which was followed by a drop in leaf osmotic potential at full turgor (Ψπ100). The lowest values of Ψπ100 followed the ranking of decreasing drought tolerance: Quercus ilex L.<Quercus faginea Lam.<Quercus pyrenaica Willd.<Quercus petraea Matt. Liebl. The leaf osmotic potential at the turgor loss point (ΨTLP) followed the same pattern as Ψπ100 across species and treatments. The pool of carbohydrates, some organic acids and cyclitols were the main osmolytes explaining osmotic potential across species, likewise to the osmotic adjustment assessed from the decrease in leaf Ψπ100 between WW and WS seedlings. Amino acids were very responsive to WS, particularly γ-aminobutyric acid in Q. pyrenaica, but made a relatively minor contribution to osmotic potential compared with other groups of compounds. In contrast, the cyclitol proto-quercitol made a prominent contribution to the changes in osmotic potential regardless of watering treatment or species. However, different metabolites, such as quinic acid, played a more important role in osmotic adjustment in Q. ilex, distinguishing it from the other species studied. In conclusion, while osmotic adjustment was present in all four Quercus species, the molecular processes underpinning this response differed according to their phylogenetic history and specific ecology.

INFLUENCE OF NITRATES, PHOSPHATE, SULPHATE AND SALTS OF Ca, Mg AND K, ON HYDROGEL HYDRATATION CAPACITY

ABSTRACT Hydrogel hydration capacity in solution was evaluated at different levels of electrical conductivity (EC) using nitrate, phosphates, sulfates, and salts of calcium, potassium and magnesium. Four sequential experiments were performed using potassium polyacrylate copolymer. The first experiment aimed to determine the lowest hydrogel dose, at which occur maximum deionized water absorption. The test was assembled in a completely randomized design with six hydrogel doses (1, 2, 4, 6, 8 and 10 g L-1) and four replicates. The second experiment consisted of testing the sucrose P.A. doses (0, 1, 2, 4 and 6 %) in order to evaluate the change in osmotic potential of the solution on hydrogel hydration. The third and fourth studies were carried out in a complete randomized design, using a 6 x 3 x 2 factorial scheme with four replicates. For the third experiment, the factors were six levels of EC (0; 0.25; 0.50; 1.00; 1.50 and 2.00 mS cm1) increased by the addition of three anions (NO3-; PO43- and SO42-) along with counter-ions Ca++ and K+. For the fourth experiment, the same six levels of EC were achieved by adding the three cations (Ca++, Mg++ and K+), along with the counter-ions Cl- and SO42-. Hydration capacity of the hydrogel decreases when the ionic concentration of the solution increases and the reduction of the osmotic potential through the addition of sucrose does not interfere in its swelling capability. Anions and cations in the solution negatively influence the process of the hydrogel hydration. The most pronounced effect is for those with higher valence.

Water Relations and Osmolite Accumulation Related to Sugarcane Yield Under Drought Stress in a Tropical Climate

The number of areas subject to drought is likely to increase in response to climate change and will affect the yields of globally important crops such as sugarcane. In this respect, the aim of the present study was to assess the drought tolerance of sugarcane varieties in different phenophases and identify the physiological and biochemical characteristics associated with the lowest yield loss under drought conditions. Six of the most widely cultivated varieties in the region were studied, namely SP79-1011, RB855113, RB92579, RB867515, RB72454 and RB855536. Plant physiological characteristics were assessed during the dry season in the tillering, intense growth and ripening stages, and yield was quantified after 12 months of cultivation. This study shows that different sugarcane varieties are more affected by drought in the intense growth phenophase, since plants showed a greater decline in water and osmotic potential at midday during this period. The varieties most affected by drought in this phenophase were RB855536 and RB855113, which exhibited greater reductions in water and osmotic potential. They also had larger osmoregulator accumulation as a response to drought, but not enough to prevent dehydration, which likely contributed to the lower yield. The RB867515 and RB92579 varieties underwent no significant change in osmotic potential at midday during drought stress in the intense growth phase, despite of low leaf water potential, and little variation in the osmoregulators contents. For these reasons, these two varieties showed greater drought acclimation potentials and were more productive as submitted to drought conditions.

Sweet Cherry Fruit: Ideal Osmometers?

Osmotic water uptake through the skin is an important factor in rain cracking of sweet cherries. The objective was to establish whether a sweet cherry behaves like an ideal osmometer, where: (1) water uptake rates are negatively related to fruit osmotic potential, (2) a change in osmotic potential of the incubation solution results in a proportional change in water uptake rate, (3) the osmotic potential of the incubation solution yielding zero water uptake is numerically equal to the fruit water potential (in the absence of significant fruit turgor), and (4) the fruits' cuticular membrane is permeable only to water. The fruits' average osmotic potential and the rate of water uptake were related only weakly. Surprisingly, incubating a fruit in (a) the expressed juice from fruit of the same batch or (b) an isotonic artificial juice composed of the five major osmolytes of expressed juice or (c) an isotonic glucose solution—all resulted in significant water uptake. Decreasing the osmotic potential of the incubation solution decreased the rate of water uptake, while decreasing it still further resulted in water loss to the incubation solution. Throughout fruit development, the “apparent” fruit water potential was always more negative than the fruits' measured average osmotic potential. Plasmolysis of epidermal cells indicates the skin's osmotic potential was less negative than that of the flesh. When excised flesh discs were incubated in a concentration series of glucose solutions, the apparent water potential of the discs matched the osmotic potential of the expressed juice. Significant penetration of 14C-glucose and 14C-fructose occurred through excised fruit skins. These results indicate a sweet cherry is not an ideal osmometer. This is due in part to the cuticular membrane having a reflection coefficient for glucose and fructose less than unity. As a consequence, glucose and fructose were taken up by the fruit from the incubation solution. Furthermore, the osmotic potential of the expressed fruit juice is not uniform. The osmotic potential of juice taken from the stylar scar region is more negative than that from the pedicel region and that from the flesh more negative than that from the skin.

Mediterranean trees coping with severe drought: Avoidance might not be safe

Plants coexisting in Mediterranean regions differ widely in their sensitivity to drought and in their ability to maintain carbon assimilation and hydraulic function ranging from avoidance to tolerance of stressful periods. We studied the response of three coexisting Mediterranean tree species (Quercus ilex L. spp. Ballota (Desf.) Samp., Quercus faginea Lam. and Pinus nigra ssp. Salzmannii J.F. Arnold) under natural conditions for three years to test their functional strategies in dealing with extremely dry years. Despite P. nigra following a drought-avoiding strategy, it was unable to effectively avoid the hampering effects of a severe drought in the driest year, and experienced significant drops in the efficiency of the hydraulic system, in gas exchange at the leaf level and in tree water use estimated from daily sap flow. In contrast, the two Quercus species showed a more drought-tolerant strategy by maintaining a more profligate water use at the leaf and tree level, with Q. ilex having a larger tolerance threshold to severe drought. The main decrease in carbon assimilation was due to stomatal closure, with a minor effect of photochemistry damage or down regulation in the three species. In the case of the two oaks, seasonal osmoregulation was inferred from changes in the osmotic potential at full turgor (πo), bringing about the lowest leaf osmotic potentials at turgor lost (πtlp) at late summer in both species, and maintenance of leaf turgor under the lowest midday leaf water potential (Ψmid). Seasonal changes in osmotic potentials were observed in the three years, more during the driest year. Plasticity in modulating the tolerance to leaf dehydration as the season advances, especially in response to the intensity of inter-annual summer droughts was higher for the evergreen Q. ilex than in the semi-deciduous Q. faginea. In spite of the intensity of the drought and the associated stress experienced by the three forest tree species, especially by P. nigra, mortality or severe defoliation were not observed after the extreme dry year. This reveals a large operational range of coping with water stress by the three species regardless of the hydric strategy, which is crucial for survival under the increasing drought expected in the most likely climate change scenarios for the region.

Impact of weak water deficit on growth, photosynthetic primary processes and storage processes in pine and spruce seedlings.

We investigated the influence of 40 days of drought on growth, storage processes and primary photosynthetic processes in 3-month-old Scots pine and Norway spruce seedlings growing in perlite culture. Water stress significantly affected seedling water status, whereas absolute dry biomass growth was not substantially influenced. Water stress induced an increase in non-structural carbohydrate content (sugars, sugar alcohols, starch) in the aboveground part of pine seedlings in contrast to spruce seedlings. Due to the relatively low content of sugars and sugar alcohols in seedling organs, their expected contribution to osmotic potential changes was quite low. In contrast to biomass accumulation and storage, photosynthetic primary processes were substantially influenced by water shortage. In spruce seedlings, PSII was more sensitive to water stress than PSI. In particular, electron transport in PSI was stable under water stress despite the substantial decrease of electron transport in PSII. The increase in thermal energy dissipation due to enhancement of non-photochemical quenching (NPQ) was evident in both species under water stress. Simultaneously, the yields of non-regulated energy dissipation in PSII were decreased in pine seedlings under drought. A relationship between growth, photosynthetic activities and storage processes is analysed under weak water deficit.

Changes in osmotic potential, protein content and proline accumulation in response to NaCl salinity and phosphorus in cucumber microshoots grown on proliferation medium

The aim of this work was to explore the contributions of phosphorus in mitigating the salinity impacts on osmotic potential, protein content and proline accumulation of cucumber grown in vitro on proliferation medium. Microshoots were grown on Murashige and Skoog (MS) proliferation media containing 1 mg/l kinetin and supplemented with a combination of NaCl at 0.0, 25, 50, 75, 100 mM with 0.5, 1.0 or 2.0 mM phosphorus using KH2PO4. Leaf osmotic potential was reduced with increasing NaCl concentration and within each of the 50, 75 and 100 mM NaCl concentrations, the incorporation of 2 mM KH2PO4 was needed to significantly increase leaf osmotic potential. Protein content of cucumber microshoots was shown to decrease with NaCl concentration, this reduction was lessened with increase in KH2PO4 level applied. In vitro-induced salinity caused profound proline accumulation in cucumber microshoots and combinations having 2 mM KH2PO4 showed lower proline accumulation especially at the highest two concentrations of NaCl.

Expansion of the dehydrin gene family in the Pinaceae is associated with considerable structural diversity and drought-responsive expression.

Temperatures are expected to increase over the next century in all terrestrial biomes and particularly in boreal forests, where drought-induced mortality has been predicted to rise. Genomics research is helping to develop hypotheses regarding the molecular basis of drought tolerance and recent work proposed that the osmo-protecting dehydrin proteins have undergone a clade-specific expansion in the Pinaceae, a major group of conifer trees. The objectives of this study were to identify all of the putative members of the gene family, trace their evolutionary origin, examine their structural diversity and test for drought-responsive expression. We identified 41 complete dehydrin coding sequences in Picea glauca, which is four times more than most angiosperms studied to date, and more than in pines. Phylogenetic reconstructions indicated that the family has undergone an expansion in conifers, with parallel evolution implicating the sporadic resurgence of certain amino acid sequence motifs, and a major duplication giving rise to a clade specific to the Pinaceae. A variety of plant dehydrin structures were identified with variable numbers of the A-, E-, S- and K-segments and an N-terminal (N1) amino acid motif including assemblages specific to conifers. The expression of several of the spruce dehydrins was tissue preferential under non-stressful conditions or responded to water stress after 7-18 days without watering, reflecting changes in osmotic potential. We found that dehydrins with N1 K2 and N1 AESK2 sequences were the most responsive to the lack of water. Together, the family expansion, drought-responsive expression and structural diversification involving loss and gain of amino acid motifs suggests that subfunctionalization has driven the diversification seen among dehydrin gene duplicates. Our findings clearly indicate that dehydrins represent a large family of candidate genes for drought tolerance in spruces and in other Pinaceae that may underpin adaptability in spatially and temporally variable environments.

Low temperature caused modifications in the arrangement of cell wall pectins due to changes of osmotic potential of cells of maize leaves (Zea mays L.)

The cell wall emerged as one of the important structures in plant stress responses. To investigate the effect of cold on the cell wall properties, the content and localization of pectins and pectin methylesterase (PME) activity, were studied in two maize inbred lines characterized by different sensitivity to cold. Low temperature (14/12 °C) caused a reduction of pectin content and PME activity in leaves of chilling-sensitive maize line, especially after prolonged treatment (28 h and 7 days). Furthermore, immunocytohistological studies, using JIM5 and JIM7 antibodies, revealed a decrease of labeling of both low- and high-methylesterified pectins in this maize line. The osmotic potential, quantified by means of incipient plasmolysis was lower in several types of cells of chilling-sensitive maize line which was correlated with the accumulation of sucrose. These studies present new finding on the effect of cold stress on the cell wall properties in conjunction with changes in the osmotic potential of maize leaf cells.

Drought Stress Memory in Sugar Beet: Mismatch Between Biochemical and Physiological Parameters

In a greenhouse study, we aimed to determine whether a temporary water deficit induces ‘drought memory’ in sugar beet (Beta vulgaris L.), and whether this effect can be quantified by alterations in the fluorescence signature of the leaves. Plants were subjected to three consecutive water deficit phases, each followed by a recovery period, and in each cycle new, fully developed leaves were analyzed. Changes in the photosynthetic performance and pigment fluorescence were recorded with a hand-held fluorescence sensor, a laser-induced fluorescence spectrometer, and a leaf gas exchange analyzer. Parameters such as osmotic potential, proline, and chlorophyll content were used as indicators for biochemical modifications and quantification of stress intensity. In general, the evaluated cultivars showed a similar response pattern to water deficit, although the intensity of the stress-induced modification was not always on the same level in the distinct parameters. The long-term and repeated drought caused a decrease of net photosynthesis, increase of far-red fluorescence, and a decrease of both the ‘Simple Fluorescence Ratio’ and the fluorescence lifetime (LT mean) in the blue spectral region. In the second drought cycle, changes in osmotic potential and proline content were lower, but alterations in photosynthesis and fluorescence were as strong as in the first and third drought cycles. This indicates that even if a drought stress memory might occur, it was not possible to precisely identify it using gas exchange and pigment fluorescence determinations. Irrespective of that, the photosynthesis and chlorophyll fluorescence-based parameters (RF, SFR) clearly indicated with high temporal resolution the response of sugar beet plants to the stress, and their partial recovery.

Dissecting the effects of nitrate, sucrose and osmotic potential on Arabidopsis root and shoot system growth in laboratory assays.

Studying the specific effects of water and nutrients on plant development is difficult because changes in a single component can often trigger multiple response pathways. Such confounding issues are prevalent in commonly used laboratory assays. For example, increasing the nitrate concentration in growth media alters both nitrate availability and osmotic potential. In addition, it was recently shown that a change in the osmotic potential of media alters the plant's ability to take up other nutrients such as sucrose. It can also be difficult to identify the initial target tissue of a particular environmental cue because there are correlated changes in development of many organs. These growth changes may be coordinately regulated, or changes in development of one organ may trigger changes in development of another organ as a secondary effect. All these complexities make analyses of plant responses to environmental factors difficult to interpret. Here, we review the literature on the effects of nitrate, sucrose and water availability on root system growth and discuss the mechanisms underlying these effects. We then present experiments that examine the impact of nitrate, sucrose and water on root and shoot system growth in culture using an approach that holds all variables constant except the one under analysis. We found that while all three factors also alter root system size, changes in sucrose and osmotic potential also altered shoot system size. In contrast, we found that, when osmotic effects are controlled, nitrate specifically inhibits root system growth while having no effect on shoot system growth. This effectively decreases the root : shoot ratio. Alterations in root : shoot ratio have been widely observed in response to nitrogen starvation, where root growth is selectively increased, but the present results suggest that alterations in this ratio can be triggered across a wide spectrum of nitrate concentrations.

A systems approach implicates nuclear receptor targeting in the Atp7b(-/-) mouse model of Wilson's disease.

Wilson's disease (WD) is an inherited disorder of copper metabolism characterized by liver disease and/or neurologic and psychiatric pathology. The disease is a result of mutation in ATP7B, which encodes the ATP7B copper transporting ATPase. Loss of copper transport function by ATP7B results in copper accumulation primarily in the liver, but also in other organs including the brain. Studies in the Atp7b(-/-) mouse model of WD revealed specific transcript and metabolic changes that precede development of liver pathology, most notably downregulation of transcripts in the cholesterol biosynthetic pathway. In order to gain insight into the molecular mechanisms of transcriptomic and metabolic changes, we used a systems approach analysing the pre-symptomatic hepatic nuclear proteome and liver metabolites. We found that ligand-activated nuclear receptors FXR/NR1H4 and GR/NR3C1 and nuclear receptor interacting partners are less abundant in Atp7b(-/-) hepatocyte nuclei, while DNA repair machinery and the nucleus-localized glutathione peroxidase, SelH, are more abundant. Analysis of metabolites revealed an increase in polyol sugar alcohols, indicating a change in osmotic potential that precedes hepatocyte swelling observed later in disease. This work is the first application of quantitative Multidimensional Protein Identification Technology (MuDPIT) to a model of WD to investigate protein-level mechanisms of WD pathology. The systems approach using "shotgun" proteomics and metabolomics in the context of previous transcriptomic data reveals molecular-level mechanisms of WD development and facilitates targeted analysis of hepatocellular copper toxicity.

Effects of K and N Nutrition on Function and Production of Ranunculus asiaticus

Potassium (K) affects a range of physiological processes in the plant and is a key factor controlling crop productivity and yield quality. Little information is available concerning effects of K nutrition on function of cut flower plants. The present study was carried out to investigate the interaction between K and N nutrition, on flower quantity and quality of Ranunculus asiaticus L. The plants were supplied with three levels of K fertigation (60, 120, or 180 mg K L −1) under 50 mg N L −1 application, and at the intermediate level of 120 mg K L −1, to three levels of N applications (50, 100, or 150 mg N L −1). The two lowest K treatments and the lowest N treatment excelled in flower production due to the lower incidence of stem-toppling, a disorder associated with localized Ca deficiencies in rapidly expanding tissues of the flower stem. Detrimental effects in terms of yield quality were apparent already under supply of 180 mg K L −1 and 50 mg N L −1, and were not associated with changes in osmotic potential, relative water content or membrane stability of the plant tissue, or with changes in mineral contents of the leaves other than reduced Ca under high N application. Our results suggest a low nutritional requirement of R. asiaticus L. for K and N, a lack of involvement of tissue water relations in the reduced flower quality under the application of high concentrations of K and N, and an induction of stem toppling under high application of N and K by reduced availability of Ca to the expanding tissue of the flower stem.

Differences in the way potassium chloride and sucrose solutions effect osmotic potential of significance to stomata aperture modulation

Guard cell solution osmotic potential changes resulting in the opening and closing of stomata apertures follow an initial influx of potassium ions, their substitution with sucrose molecules and the subsequent reduction of the latter. To provide an insight into the osmotic mechanism of the changes, the new equation for calculating osmotic pressure, which equates the difference between the energy of pure water across a semi-permeable membrane interface with that of solution water, was used to compare the osmotic properties of KCl and sucrose. For sucrose solutions, the effect of the sucrose molecules in increasing the spacing of the solution water was mainly responsible for osmotic potential; this contrasted with K+ + Cl(-) ions where their spacing effect was only a little higher to that of water held to those ions. At solute concentrations giving an osmotic potential level of -3.0 MPa near that of turgid guard cells, the spacing effect on the potential of the unattached solution water molecules caused by sucrose, but in its theoretical absence, was estimated as -2.203 MPa compared with -1.431 MPa for KCl. In contrast, the potential attributed to water molecules firmly held to the K+ + Cl(-) ions was -1.212 MPa versus zero for sucrose. The potential to keep the sucrose molecules in solution was -0.797 MPa compared with -0.357 MPa for KCl. The findings illustrate that the way KCl effects osmotic pressure is very different to that of sucrose. It is concluded that stomata aperture modulation is closely linked to the osmotic properties of its guard cell solution solutes.

Relationship between Water Content and Osmotic Potential of Lentinula edodes

This study was conducted to understand how osmotic potentials in Lentinula edodes tissues are related to water contents and how they change while a mushroom matures. Water content and osmotic potential of L. edodes mushroom tissues from log cultivation and sawdust cultivation were measured and the relationships were analyzed. Osmotic potentials in the tissues were exponentially proportional to their moisture contents and there were strain differences in the potentials. Strain 290 has lower osmotic potential than strain 302, in the tissues at the same water content. As the mushrooms mature, tissue water content maintained ca 94% in head tissues and ca 90% in gills, but significantly decreased from ca 90% to 82% in the stipe tissues. Osmotic potential changes were similar to the tissue water content changes as the mushrooms mature. While osmotic potentials maintained −0.25 to −0.45 MPa in head and gill tissues, the potentials greatly decreased from −0.65 to −1.33MPa in stipe tissues. Our results show that osmotic potentials in L. edodes tissues are exponentially proportional to tissue water contents, that strains differ in osmotic potential related to water, and that stipe tissues can still have nutritional value when they mature.

Differential effects of aluminium on osmotic potential and sugar accumulation in the root cells of Al-resistant and Al-sensitive wheat.

The changes in osmotic potential and the concentration of osmotic solutes in the cell sap of the root tips exposed to Al were examined in two cultivars of wheat (Triticum aestivum) differing in Al resistance. Root elongation was less influenced by an 8-h exposure to 20 microM or 50 microM Al in Al-resistant cv. Atlas 66 than in Al-sensitive cv. Scout 66. After Al treatment the osmotic potential of the root cells was decreased in Atlas 66 but increased in Scout 66 indicating that the Al treatment osmotically stimulated the driving force for water uptake in Atlas 66 but suppressed it in Scout 66. Al increased the concentration of soluble sugars, the major osmotic solute in the root cells in Atlas 66, but decreased it in Scout 66. Al at both low (5 microM) and high (50 microM) concentrations, also increased the concentration of soluble sugars in the Al-resistant genotype ET8 but a high Al concentration decreased it in Al-sensitive genotype ES8. Enzymatic analyses and thin-layer chromatography revealed that soluble sugars in the root cells of both Atlas 66 and Scout 66 mainly consisted of monosaccharides such as glucose, fructose and a small amount of sucrose. These results suggest that the accumulation of soluble sugars in Al-resistant wheat Atlas 66 keeps the osmotic potential in the root cells low and thus, enables the root cells to take up water and to elongate against the pressure produced by cell wall rigidification under Al stress.

Abscisic acid accumulation and osmotic adjustment in cassava under water deficit

Three cassava ( Manihot esculenta) genotypes were grown in greenhouse conditions and subjected to water deficit treatments to: (a) assess the extent to which abscisic acid (ABA) and osmotically active solutes accumulate in cassava leaves during water deficit and solute contributions to osmotic adjustment (OA), and (b) determine whether cassava leaves substantially increase their content of dehydrin-like polypeptides during water deficit. The concentration of ABA, sugars (sucrose, glucose, and fructose), proline, and potassium salts (K-salts) were determined in mature and expanding leaves. ABA content increased under water deficit, consistent with a putative role in regulating solute accumulation, and thus OA. The extent of osmotic adjustment, although modest, was higher in young folded leaves than in mature leaves. K-salts were the major contributors to total osmolyte concentration in both mature and expanding leaves, accounting for approximately 60% of osmotic potential. The concentration of K-salts increased in response to water stress and was positively correlated with the extent of OA. In contrast, total sugars (sucrose + glucose + fructose) decreased during water deficit, showing a negative correlation with OA. Although the concentration of proline in mature leaves increased in response to water stress its contribution to the total change in osmotic potential was insignificant. Thus, in the current study, cassava used K-salts as its primary osmolyte, and did not substantially adjust its osmotic solute concentration during the 6-day water deficit. Expression of dehydrins was not found in any of the genotypes or leaf stages.