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Abscission is a developmental process with important implications for agricultural practices. Ethylene has long been considered as a key regulator of the abscission process. The existence of an ethylene-independent abscission pathway, controlled by the complex of INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) peptide and the HAESA (HAE) and HAESA-like2 (HSL2) kinases, has been proposed, based mainly on observations that organ abscission in ethylene-insensitive mutants was delayed but not inhibited. A recent review on plant organ abscission signaling highlighted the IDA–HAE–HSL2 components as the regulators of organ abscission, while the role of auxin and ethylene in this process was hardly addressed. After a careful analysis of the relevant abscission literature, we propose that the IDA–HAE–HSL2 pathway is essential for the final stages of organ abscission, while ethylene plays a major role in its initiation and progression. We discuss the view that the IDA–HAE–HSL2 pathway is ethylene independent, and present recent evidence showing that ethylene activates the IDA–HAE–HSL2 complex. We conclude that the ability of an organ to abscise is tightly linked to cell turgidity in the abscission zone, and suggest that lack of cell turgidity might contribute to the failure of floral organ abscission in the ida mutants. [ABSTRACT FROM AUTHOR]

Background and Aims Turgor-driven plant cell growth depends on cell wall structure and mechanics. Strengthening of cell walls on the basis of an association and interaction with silicon (Si) could lead to improved nutrient uptake and optimized growth and metabolism in rice (Oryza sativa). However, the structural basis and physiological mechanisms of nutrient uptake and metabolism optimization under Si assistance remain obscure. Methods Single-cell level biophysical measurements, including in situ non-invasive micro-testing (NMT) of NH4+ ion fluxes, atomic force microscopy (AFM) of cell walls, and electrolyte leakage and membrane potential, as well as whole-cell proteomics using isobaric tags for relative and absolute quantification (iTRAQ), were performed. Key Results The altered cell wall structure increases the uptake rate of the main nutrient NH4+ in Si-accumulating cells, whereas the rate is only half in Si-deprived counterparts. Conclusions Rigid cell walls enhanced by a wall-bound form of Si as the structural basis stabilize cell membranes. This, in turn, optimizes nutrient uptake of the cells in the same growth phase without any requirement for up-regulation of transmembrane ammonium transporters. Optimization of cellular nutrient acquisition strategies can substantially improve performance in terms of growth, metabolism and stress resistance. [ABSTRACT FROM AUTHOR]

Osmotic adjustment (OA), the accumulation of solutes in higher plant cells in response to water deficits, was first reported more than four decades ago. Since then, variation in OA among genotypes/cultivars in response to drought has been reported in many crop plants, but its role in maintaining growth and yield in water-limited environments has been questioned. The role of OA in the physiological and agronomic adaptation to water stress of crops, the methods of reliably measuring the degree of OA among genotypes or species, the range of OA in many studies, and its impact on grain yield in water-limited environments are reviewed. The genetics of OA has received limited study, and the breeding and selection for high OA has only resulted in the release of one commercial cultivar of wheat as far as is known. The reasons for the limited interest in breeding for the OA trait are discussed. [ABSTRACT FROM AUTHOR]

The shape and function of plant cells are often highly interdependent. The puzzle- shaped cells that appear in the epidermis of many plants are a striking example of a complex cell shape, however their functional benefit has remained elusive. We propose that these intricate forms provide an effective strategy to reduce mechanical stress in the cell wall of the epidermis. When tissue-level growth is isotropic, we hypothesize that lobes emerge at the cellular level to prevent formation of large isodiametric cells that would bulge under the stress produced by turgor pressure. Data from various plant organs and species support the relationship between lobes and growth isotropy, which we test with mutants where growth direction is perturbed. Using simulation models we show that a mechanism actively regulating cellular stress plausibly reproduces the development of epidermal cell shape. Together, our results suggest that mechanical stress is a key driver of cell-shape morphogenesis. [ABSTRACT FROM AUTHOR]

Daytime decreases in temperature-normalised stem CO2 efflux (EA_D) are commonly ascribed to internal transport of respired CO2 (FT) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure and FT, sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO2 fluxes was applied to estimate FT. Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of FT, were reliable predictors of EA_D. After tree manipulation, only turgor pressure was a robust predictor of EA_D. Moreover, FT accounted for < 15% of EA_D. Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of EA_D in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models. [ABSTRACT FROM AUTHOR]

Daytime decreases in temperature-normalised stem CO2 efflux (EA_D) are commonly ascribed to internal transport of respired CO2 (FT) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure and FT, sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO2 fluxes was applied to estimate FT. Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of FT, were reliable predictors of EA_D. After tree manipulation, only turgor pressure was a robust predictor of EA_D. Moreover, FT accounted for < 15% of EA_D. Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of EA_D in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models. [ABSTRACT FROM AUTHOR]

Silicon (Si) is an abundant element which, when supplied to plants, confers increased vigor and resistance to exogenous stresses, as well as enhanced stem mechanical strength. Plant species vary in their ability to take Si up and to accumulate it under the form of silicon dioxide (SiO2) in their tissues: emblematic of this is the example of Poales, among which there is rice, a high Si accumulator. Monocots usually accumulate more Si than dicots; however, the impact that Si has on dicots, notably on economically important dicots, is a subject requiring further study and scientific efforts. In this review, we discuss the impact that Si has on bast fibre-producing plants, because of the potential importance that this element has in sustainable agriculture practices and in light of the great economic value of fibre crops in fostering a bio-economy. We discuss the data already available in the literature, as well as our own research on textile hemp. In particular, we demonstrate the beneficial effect of Si under heavy metal stress, by showing an increase in the leaf fresh weight under growth on Cd 20 μM. Additionally, we propose an effect of Si on bast fibre growth, by suggesting an action on the endogenous phytohormone levels and a mechanical role involved in the resistance to the turgor pressure during elongation. We conclude our survey with a description of the industrial and agricultural uses of Si-enriched plant biomass, where woody fibres are included in the survey. [ABSTRACT FROM AUTHOR]

The article discusses role of leaf turgor loss as a predictor of plant drought response strategies, referencing a research on leaf turgor loss point (TLP) published in this issue. It states that a meta-analysis of TLP and its relationship was conducted to a range of hydraulic traits linked to drought tolerance, as well as to leaf economic trait, and mentions that that species with higher TLP tend to have wider hydraulic safety margins. It notes role of TLP as an integrated and adaptive trait.

Information about water relations within crop canopies is needed to improve our understanding of canopy resource distribution and crop productivity. In this study, we examined the dehydration/rehydration kinetics of different organs of wheat plants using ZIM-probes that continuously monitor water status non-destructively. ZIM-probes were clamped to the flag leaf and penultimate leaf of the same stem to monitor changes in turgor pressure, and a novel stem probe was clamped to the peduncle (just below the spike of the same stem) to monitor changes in stem water status. All organs behaved similarly under well-watered conditions, dehydrating and recovering at the same times of day. When water was withheld, the behaviour diverged, with the leaves showing gradual dehydration and incomplete recovery in leaf turgor pressure during the night, but the stem was affected to a lesser extent. Penultimate leaves were the most severely affected, reaching turgor loss point before the flag leaf. Upon rewatering, turgor pressure recovered but the output patch-pressure of the probes (P[sub p]) oscillated at ~30 min periods in all organs of most plants (n = 4). Oscillations in P[sub p] were attributed to oscillations in stomatal opening and appear to only occur above a threshold light intensity. The mechanisms identified in this study will be beneficial for crop productivity because the flag leaf is the source of most photoassimilates in developing grains, so the plant's ability to maintain flag leaf hydration at the expense of older leaves should moderate the impact of drought on yield. Stomatal oscillations could increase water use efficiency as the plant attempts to rehydrate after drought. [ABSTRACT FROM AUTHOR]

Stomata mediate gas exchange between the inter-cellular spaces of leaves and the atmosphere. CO2 levels in leaves ( Ci) are determined by respiration, photosynthesis, stomatal conductance and atmospheric [ CO2]. [ CO2] in leaves mediates stomatal movements. The role of guard cell photosynthesis in stomatal conductance responses is a matter of debate, and genetic approaches are needed. We have generated transgenic Arabidopsis plants that are chlorophyll-deficient in guard cells only, expressing a constitutively active chlorophyllase in a guard cell specific enhancer trap line. Our data show that more than 90% of guard cells were chlorophyll-deficient. Interestingly, approximately 45% of stomata had an unusual, previously not-described, morphology of thin-shaped chlorophyll-less stomata. Nevertheless, stomatal size, stomatal index, plant morphology, and whole-leaf photosynthetic parameters ( PSII, qP, qN, FV′/ FM′) were comparable with wild-type plants. Time-resolved intact leaf gas-exchange analyses showed a reduction in stomatal conductance and CO2-assimilation rates of the transgenic plants. Normalization of CO2 responses showed that stomata of transgenic plants respond to [ CO2] shifts. Detailed stomatal aperture measurements of normal kidney-shaped stomata, which lack chlorophyll, showed stomatal closing responses to [ CO2] elevation and abscisic acid ( ABA), while thin-shaped stomata were continuously closed. Our present findings show that stomatal movement responses to [ CO2] and ABA are functional in guard cells that lack chlorophyll. These data suggest that guard cell CO2 and ABA signal transduction are not directly modulated by guard cell photosynthesis/electron transport. Moreover, the finding that chlorophyll-less stomata cause a 'deflated' thin-shaped phenotype, suggests that photosynthesis in guard cells is critical for energization and guard cell turgor production. [ABSTRACT FROM AUTHOR]

Previous studies have reported correlation of leaf hydraulic vulnerability with pressure–volume parameters related to cell turgor. This link has been explained on the basis of the effects of turgor on connectivity among cells and tissue structural integrity, which affect leaf water transport. In this study, we tested the hypothesis that osmotic adjustment to water stress would shift the leaf vulnerability curve toward more negative water potential (Ψleaf) by increasing turgor at low Ψleaf. We measured leaf hydraulic conductance (Kleaf), Kleaf vulnerability [50 and 80% loss of Kleaf (P50 and P80); |Ψleaf| at 50 and 80% loss of Kleaf, respectively), bulk leaf water relations, leaf gas exchange and sap flow in two Vitis vinifera cultivars (Tempranillo and Grenache), under two water treatments. We found that P50, P80 and maximum Kleaf decreased seasonally by more than 20% in both cultivars and watering treatments. However, Kleaf at −2 MPa increased threefold, while osmotic potential at full turgor and turgor loss point decreased. Our results indicate that leaf resistance to hydraulic dysfunction is seasonally plastic, and this plasticity may be mediated by osmotic adjustment. [ABSTRACT FROM AUTHOR]

Root elongation and bending require the coordinated expansion of multiple cells of different types. These processes are regulated by the action of hormones that can target distinct cell layers. We use a mathematical model to characterise the influence of the biomechanical properties of individual cell walls on the properties of the whole tissue., Taking a simple constitutive model at the cell scale which characterises cell walls via yield and extensibility parameters, we derive the analogous tissue-level model to describe elongation and bending. To accurately parameterise the model, we take detailed measurements of cell turgor, cell geometries and wall thicknesses., The model demonstrates how cell properties and shapes contribute to tissue-level extensibility and yield. Exploiting the highly organised structure of the elongation zone ( EZ) of the Arabidopsis root, we quantify the contributions of different cell layers, using the measured parameters. We show how distributions of material and geometric properties across the root cross-section contribute to the generation of curvature, and relate the angle of a gravitropic bend to the magnitude and duration of asymmetric wall softening., We quantify the geometric factors which lead to the predominant contribution of the outer cell files in driving root elongation and bending. [ABSTRACT FROM AUTHOR]

Close hydraulic connection between the xylem and the phloem challenges phloem transport during drought. Current theories suggest that the cause of phloem transport failure during drought depends on the hydraulic permeability of phloem conduit walls.Drought challenges plant water uptake and the vascular system. In the xylem it causes embolism that impairs water transport from the soil to the leaves and, if uncontrolled, may even lead to plant mortality via hydraulic failure. What happens in the phloem, however, is less clear because measuring phloem transport is still a significant challenge to plant science. In all vascular plants, phloem and xylem tissues are located next to each other, and there is clear evidence that these tissues exchange water. Therefore, drought should also lead to water shortage in the phloem. In this review, theories used in phloem transport models have been applied to drought conditions, with the goal of shedding light on how phloem transport failure might occur. The review revealed that phloem failure could occur either because of viscosity build-up at the source sites or by a failure to maintain phloem water status and cell turgor. Which one of these dominates depends on the hydraulic permeability of phloem conduit walls. Impermeable walls will lead to viscosity build-up affecting flow rates, while permeable walls make the plant more susceptible to phloem turgor failure. Current empirical evidence suggests that phloem failure resulting from phloem turgor collapse is the more likely mechanism at least in relatively isohydric plants. [ABSTRACT FROM AUTHOR]

Highlights: [•] A generalized form of a strain energy function is proposed. [•] A mathematical relation between turgor pressure and stretch ratio is developed. [•] The developed relation shows good fit with experimental data. [•] Effects of turgor pressure on cell walls of apple and potato tissues are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Stomata respond in a common pattern to various hydraulic perturbations on any part of the &#x2018;soil&#x2013;plant&#x2013;air&#x2019; system: initial transient &#x2018;wrong-way&#x2019; responses and final stationary &#x2018;right-way&#x2019; responses. In order to describe this pattern on the basis of statistical physics, we propose a simple model where turgor pressure of a cell is taken to be a power function of its volume, and obtain results in qualitative agreement with experimental data for responses to a variety of hydraulic perturbations: Firstly, stationary stomatal conductance as a function of the vapor pressure deficit divides into three regimes characterized by sensitivities of the stomatal conductance and the transpiration rate with respect to vapor pressure deficit; secondly, for every hydraulic perturbation, the initial transient &#x2018;wrong-way&#x2019; responses always appear; thirdly, on condition that water is supplied insufficiently, stomatal oscillations are often observed; finally, stomatal responses following leaf excision exhibit, after the initial transient wrong-way responses, slow relaxation to stomatal closing. In particular, comparison of areoles having different numbers of stomata demonstrates that areoles with small numbers of stomata tend to provoke lack of water in the soil as well as in the plant. In addition, our model also describes well dependence of the stomatal conductance on temperature. It may be extended further to describe stomatal responses to other environmental factors such as carbon dioxide, light, and temperature. [ABSTRACT FROM AUTHOR]

The mechano-sensitive channels of plants may sense increases in tension induced by mechanical stimuli, such as touch, wind and turgor pressure, and a gravitational stimulus. Recent studies have identified plant homologues of the bacterial mechano-sensitive channel MscS, which is gated by membrane tension and reduces intracellular osmolality by releasing small osmolytes from bacterial cells. However, the physiological roles of these homologues have not yet been clearly elucidated, and only two of them have been shown to be involved in the protection of osmotically stressed plastids in Arabidopsis thaliana. We identified another group of candidates for mechano-sensitive channels in Arabidopsis, named MCA1 and MCA2, whose homologues are exclusively found in plant genomes. MCA1 and MCA2 are composed of 421 and 416 amino acid residues, respectively, share 73% homology in their amino acid sequences, and are not homologous to any known ion channels or transporters. Our structural study revealed that the N-terminal region (one to 173 amino acids) of both proteins was necessary and sufficient for Ca2+ influx activity. Interestingly, this region had one putative transmembrane segment containing an Asp residue whose substitution mutation abolished this activity. Our physiological study suggested that MCA1 expressed at the root tip was required for sensing the hardness of the agar medium or soil. In addition, MCA1 and MCA2 were shown to be responsible for hypo-osmotic shock-induced increases in [Ca2+]cyt. Thus, both proteins appear to be involved in the process of sensing mechanical stresses. We discussed the possible role of both proteins in sensing mechanical and gravitational stimuli. [ABSTRACT FROM AUTHOR]

The drought tolerance of three economically important apple cultivars, Golden Delicious, Braeburn and Red Delicious, was analysed. The work offers insights into the hydraulics of these high-yield trees and indicates a possible hydraulic limitation of carbon gain. The hydraulic safety and efficiency of branch xylem and leaves were quantified, drought tolerance of living tissues was measured and stomatal regulation, turgor-loss point and osmotic potential at full turgor were analysed. Physiological measurements were correlated with anatomical parameters, such as conduit diameter, cell-wall reinforcement, stomatal density and stomatal pore length. Hydraulic safety differed considerably between the three cultivars with Golden Delicious being significantly less vulnerable to drought-induced embolism than Braeburn and Red Delicious. In Golden Delicious, leaves were less resistant than branch xylem, while in the other cultivars leaves were more resistant than branch xylem. Hydraulic efficiency and xylem anatomical measurements indicate differences in pit properties, which may also be responsible for variations in hydraulic safety. In all three cultivars, full stomatal closure occurred at water potentials where turgor had already been lost and severe loss of hydraulic conductivity as well as damage to living cells had been induced. The consequential negative safety margins pose a risk for hydraulic failure but facilitate carbon gain, which is further improved by the observed high stomatal conductance. Maximal stomatal conductance was clearly seen to be related to stomatal density and size. Based on our results, these three high-yield Malus domestica Borkh. cultivars span a wide range of drought tolerances, appear optimized for maximal carbon gain and, thus, all perform best under well-managed growing conditions. [ABSTRACT FROM AUTHOR]

Leaves can be both a hydraulic bottleneck and a safety valve against hydraulic catastrophic dysfunctions, and thus changes in traits related to water movement in leaves and associated costs may be critical for the success of plant growth. A 4-year fertilization experiment with nitrogen (N) and phosphorus (P) addition was done in a semideciduous Atlantic forest in northeastern Argentina. Saplings of five dominant canopy species were grown in similar gaps inside the forests (five control and five N + P addition plots). Leaf lifespan (LL), leaf mass per unit area (LMA), leaf and stem vulnerability to cavitation, leaf hydraulic conductance (Kleaf_area and Kleaf_mass) and leaf turgor loss point (TLP) were measured in the five species and in both treatments. Leaf lifespan tended to decrease with the addition of fertilizers, and LMA was significantly higher in plants with nutrient addition compared with individuals in control plots. The vulnerability to cavitation of leaves (P50leaf) either increased or decreased with the nutrient treatment depending on the species, but the average P50leaf did not change with nutrient addition. The P50leaf decreased linearly with increasing LMA and LL across species and treatments. These trade-offs have an important functional significance because more expensive (higher LMA) and less vulnerable leaves (lower P50leaf) are retained for a longer period of time. Osmotic potentials at TLP and at full turgor became more negative with decreasing P50leaf regardless of nutrient treatment. The Kleaf on a mass basis was negatively correlated with LMA and LL, indicating that there is a carbon cost associated with increased water transport that is compensated by a longer LL. The vulnerability to cavitation of stems and leaves were similar, particularly in fertilized plants. Leaves in the species studied may not function as safety valves at low water potentials to protect the hydraulic pathway from water stress-induced cavitation. The lack of rainfall seasonality in the subtropical forest studied probably does not act as a selective pressure to enhance hydraulic segmentation between leaves and stems. [ABSTRACT FROM AUTHOR]

* Premise of the study: Cell turgor plays an important role in the mechanical stability of herbaceous plants. This study on petioles of Caladium bicolor 'Candyland' analyzes the correlation between flexural rigidity and cell turgor. The results offer new insights into the underlying form-structure-function relationship and the dependency of mechanical properties from water availability. * Methods: Bending modulus E of petioles is calculated from two-point bending tests, taking into account the tapering mode. The corresponding turgor of parenchyma cells during wilting is investigated by pressure probe tests. * Key results: Wilting petioles show highly significant lower values of E than petioles with sufficient water supply. These differences are also found when comparing well-watered petioles to drought-stressed petioles having parenchyma turgor values in the same range. These results indicate an additional mechanical system sensitive to drought stress. On the basis of analyses of the contribution of different petiolar tissues toward the axial second moment of area and by using experimentally determined and literature values of E for the different tissues, we were able to (1) recalculate E of the intact petiole and to compare it with experimental data and (2) quantitatively estimate the importance of the different tissues for flexural rigidity and E of the petiole. * Conclusions: Our results show that the decrease in flexural rigidity of petioles of Caladium bicolor 'Candyland' during wilting results from (1) a water-loss-induced decrease in mechanical efficiency of collenchyma fibers and (2) turgor loss of parenchyma cells. [ABSTRACT FROM AUTHOR]

By taking advantage of large cell size of Chara corallina, we analyzed the membrane depolarization induced by decreased turgor pressure (Shimmen in J Plant Res 124:639-644, ). In the present study, the response to increased turgor pressure was analyzed. When internodes were incubated in media containing 200 mM dimethyl sulfoxide, their intracellular osmolality gradually increased and reached a steady level after about 3 h. Upon removal of dimethyl sulfoxide, turgor pressure quickly increased. In response to the increase in turgor pressure, the internodes generated a transient membrane depolarization at its nodal end. The refractory period was very long and it took about 2 h for full recovery after the depolarizing response. Involvement of protein synthesis in recovery from refractoriness was suggested, based on experiments using inhibitors. [ABSTRACT FROM AUTHOR]

A cell-growth-based algorithm is presented based on the biomechanics of plant cells in tissues to help explain the typical differences in cellular architecture found between different pome fruit species, cultivars and tissues. The cell was considered as a closed thin-walled structure, maintained in tension by turgor pressure. The cell walls of adjacent cells were modelled as parallel and linearly elastic elements, which obeyed Hooke's law. A Voronoi tessellation was used to generate the initial topology of the cells. Cell expansion then resulted from turgor pressure acting on the yielding cell wall material. To find the sequence positions of each vertex of the cell walls, and thus, the shape of the cells with time, a system of differential equations for the positions and velocities of each vertex were established and solved using a Runge-Kutta fourth and fifth order (ODE45) method. The model was used to generate realistic 2D fruit tissue structures composed of cells of random shapes and sizes, cell walls and intercellular spaces. Comparison was made with fruit tissue micrographs. The virtual tissues can be used for numerical simulation of heat and mass transfer phenomena or mechanical deformation during controlled atmosphere storage of fresh pome fruit. [ABSTRACT FROM AUTHOR]

Plant cell expansion is controlled by a fine-tuned balance between intracellular turgor pressure, cell wall loosening and cell wall biosynthesis. To understand these processes, it is important to gain in-depth knowledge of cell wall mechanics. Pollen tubes are tip-growing cells that provide an ideal system to study mechanical properties at the single cell level. With the available approaches it was not easy to measure important mechanical parameters of pollen tubes, such as the elasticity of the cell wall. We used a cellular force microscope ( CFM) to measure the apparent stiffness of lily pollen tubes. In combination with a mechanical model based on the finite element method ( FEM), this allowed us to calculate turgor pressure and cell wall elasticity, which we found to be around 0.3 MPa and 20-90 MPa, respectively. Furthermore, and in contrast to previous reports, we showed that the difference in stiffness between the pollen tube tip and the shank can be explained solely by the geometry of the pollen tube. CFM, in combination with an FEM-based model, provides a powerful method to evaluate important mechanical parameters of single, growing cells. Our findings indicate that the cell wall of growing pollen tubes has mechanical properties similar to rubber. This suggests that a fully turgid pollen tube is a relatively stiff, yet flexible cell that can react very quickly to obstacles or attractants by adjusting the direction of growth on its way through the female transmitting tissue. [ABSTRACT FROM AUTHOR]

Abstract: It has been well-established that many epiphytic bromeliads of the atmospheric-type morphology, i.e., with leaf surfaces completely covered by large, overlapping, multicellular trichomes, are capable of absorbing water vapor from the atmosphere when air humidity increases. It is much less clear, however, whether this absorption of water vapor can hydrate the living cells of the leaves and, as a consequence, enhance physiological processes in such cells. The goal of this research was to determine if the absorption of atmospheric water vapor by the atmospheric epiphyte Tillandsia usneoides results in an increase in turgor pressure in leaf epidermal cells that subtend the large trichomes, and, by using chlorophyll fluorescence techniques, to determine if the absorption of atmospheric water vapor by leaves of this epiphyte results in increased photosynthetic activity. Results of measurements on living cells of attached leaves of this epiphytic bromeliad, using a pressure probe and of whole-shoot fluorescence imaging analyses clearly illustrated that the turgor pressure of leaf epidermal cells did not increase, and the photosynthetic activity of leaves did not increase, following exposure of the leaves to high humidity air. These results experimentally demonstrate, for the first time, that the absorption of water vapor following increases in atmospheric humidity in atmospheric epiphytic bromeliads is mostly likely a physical phenomenon resulting from hydration of non-living leaf structures, e.g., trichomes, and has no physiological significance for the plant''s living tissues. [Copyright &y& Elsevier]

Abstract: Stem sap flow (Q) and leaf turgor pressure (P c) were measured simultaneously on 4-year-old, 2.4m tall ‘Arbequina’ olive trees in a hedgerow orchard. Measurements were performed on well-watered control trees as well as on 60RDI and 30RDI trees (RDI=regulated deficit irrigation). The 60RDI trees received 59.2% of the crop water needs (ETc), and the 30RDI trees received 29.4% of ETc. P c was determined non-invasively using the magnetic leaf patch clamp pressure probe (ZIM probe). The patch pressure P p measured by the probe is inversely correlated with turgor pressure at P c > ca. 50kPa. P c is coupled with xylem pressure; thus P p yields information about the development of tension in xylem. In the case of the control trees a positive correlation between Q and P p was generally found, i.e. Q increased usually with increasing P p and decreased with decreasing P p, as expected. However, Q peaking did not always coincided with P p peaking at noon. Occasionally, Q peaking preceded or followed P p peaking with a time difference of up to 3h in both cases. Under some circumstances, the onset of Q after sunrise was greatly delayed, even though a pronounced increase of P p was observed. A delayed onset of Q after sunrise resulted in hysteresis phenomena, i.e. the linear increase of Q and P p in the morning hours did not coincide with the corresponding decrease of Q and P p in the afternoon. The development of severe water stress (P c < ca. 50kPa) associated with the increase in the intercellular spaces of the spongy tissue in the leaves resulted in inverted diurnal P p curves, i.e. minimum P p values were recorded at noon and maximum P p values during the night on the 30RDI trees. The effects were reversible as shown by re-watering. By contrast, the magnitude of Q decreased continuously from the turgescent state to the state of severe water stress; maximum Q values were still recorded around noon. The data suggests that short-range tension forces are responsible for water lifting in olive trees and that water uptake from water storage reservoirs must play an important role in the supply of the leaves with water. Furthermore, for setting of irrigation thresholds the finding of shape changes of the P p curves upon severe water stress seems to be a useful indicator. Such shape changes are detected and monitored more sensitively than changes in the magnitude of sap flow rates or of turgor pressure. [Copyright &y& Elsevier]

1. Across plant species, drought tolerance and distributions with respect to water availability are strongly correlated with two physiological traits, the leaf water potential at wilting, that is, turgor loss point (πtlp), and the cell solute potential at full hydration, that is, osmotic potential (πo). We present methods to determine these parameters 30 times more rapidly than the standard pressure-volume (p-v) curve approach, making feasible community-scale studies of plant drought tolerance. 2. We optimized existing methods for measurements of πo using vapour-pressure osmometry of freeze-thawed leaf discs from 30 species growing in two precipitation regimes, and developed the first regression relationships to accurately estimate pressure-volume curve values of both πo and πtlp from osmometer values. 3. The πo determined with the osmometer (πosm) was an excellent predictor of the πo determined from the p-v curve (πpv, r2 = 0·80). Although the correlation of πosm and πpv enabled prediction, the relationship departed from the 1 : 1 line. The discrepancy between the methods could be quantitatively accounted for by known sources of error in osmometer measurements, that is, dilution by the apoplastic water, and solute dissolution from destroyed cell walls. An even stronger prediction of πpv could be made using πosm, leaf density (ρ), and their interaction ( r2 = 0·85, all P < 2 × 10−10). 4. The πosm could also be used to predict πtlp ( r2 = 0·86). Indeed, πosm was a better predictor of πtlp than leaf mass per unit area (LMA; r2 = 0·54), leaf thickness (T; r2 = 0·12), ρ ( r2 = 0·63), and leaf dry matter content (LDMC; r2 = 0·60), which have been previously proposed as drought tolerance indicators. Models combining πosm with LMA, T, ρ, or LDMC or other p-v curve parameters (i.e. elasticity and apoplastic fraction) did not significantly improve prediction of πtlp. 5. This osmometer method enables accurate measurements of drought tolerance traits across a wide range of leaf types and for plants with diverse habitat preferences, with a fraction of effort of previous methods. We expect it to have wide application for predicting species responses to climate variability and for assessing ecological and evolutionary variation in drought tolerance in natural populations and agricultural cultivars. [ABSTRACT FROM AUTHOR]

The main goal of this research was to determine the quality by measuring turgor in flowers stems of chrysanthemums stored in tap water and on distilled water, using the method of relative water content (RWC) and the equipment Wiltmeter®. The flower stems were harvested in a commercial field and placed randomly into unitary containers containing distilled water or tap water. In order to monitor the water status of the stems the follow analyses were done: variation of fresh mass, water absorption rate, water transpiration rate, relative water content and turgor pressure. Data were subjected to analysis of variance by F test, means were compared through the days of vase life, using Tukey test at 5% significance level. For comparison of the RWC and the turgor pressure the Pearson correlation coefficient was calculated. Flower stems kept in tap water maintained turgor significantly better than the ones maintained on distilled water. There is a correlation between turgor pressure and relative water content, which remain higher as the plant tissues are new and well hydrated. The estimate of the turgor pressure was effective and sensitive in evaluating the water status of the tissues of the stems of cut chrysanthemums. [ABSTRACT FROM AUTHOR]

Turgor generates the stress that leads to the expansion of plant cell walls during cellular growth. This has been formalized by the Lockhart equation, which can be derived from the physical laws of the deformation of viscoelastic materials. However, the experimental evidence for such a direct correlation between growth rate and turgor is inconclusive. This has led to challenges of the Lockhart model. We model the oscillatory growth of pollen tubes to investigate this relationship. We couple the Lockhart equation to the dynamical equations for the change in material properties. We find that the correct implementation of the Lockhart equation within a feedback loop leading to low amplitude oscillatory growth predicts that in this system changes in the global turgor do not influence the average growth rate in a linear manner, consistent with experimental observations. An analytic analysis of our model demonstrates in which regime the average growth rate becomes uncorrelated from the turgor pressure. [ABSTRACT FROM AUTHOR]

Uptake of CO2 by the leaf is associated with loss of water. Control of stomatal aperture by volume changes of guard cell pairs optimizes the efficiency of water use. Under water stress, the protein kinase OPEN STOMATA 1 (OST1) activates the guard-cell anion release channel SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1), and thereby triggers stomatal closure. Plants with mutated OST1 and SLAC1 are defective in guard-cell turgor regulation. To study the effect of stomatal movement on leaf turgor using intact leaves of Arabidopsis, we used a new pressure probe to monitor transpiration and turgor pressure simultaneously and non-invasively. This probe permits routine easy access to parameters related to water status and stomatal conductance under physiological conditions using the model plant Arabidopsis thaliana. Long-term leaf turgor pressure recordings over several weeks showed a drop in turgor during the day and recovery at night. Thus pressure changes directly correlated with the degree of plant transpiration. Leaf turgor of wild-type plants responded to CO2, light, humidity, ozone and abscisic acid (ABA) in a guard cell-specific manner. Pressure probe measurements of mutants lacking OST1 and SLAC1 function indicated impairment in stomatal responses to light and humidity. In contrast to wild-type plants, leaves from well-watered ost1 plants exposed to a dry atmosphere wilted after light-induced stomatal opening. Experiments with open stomata mutants indicated that the hydraulic conductance of leaf stomata is higher than that of the root–shoot continuum. Thus leaf turgor appears to rely to a large extent on the anion channel activity of autonomously regulated stomatal guard cells. [ABSTRACT FROM AUTHOR]

High water availability and mechanical stress can induce opposite responses in plants. In arid areas of Northern China the occurrence of high wind and high water availability tend to be negatively correlated. Since turgor pressure is a determinant of the mechanical stability of annuals, it is hypothesised that the effects of mechanical perturbation (MP) on annuals may depend on soil water availability. To test this proposal, we conducted an experiment in which a pioneering annual Corispermum mongolicum was subjected to two levels of MP and water supply, and then determined its growth and mechanical traits. Brushing had no effect on plant height and total biomass, but stimulated leaf and branch production. Water supply affected plant height, basal diameter, total biomass and stem rigidity, but not leaf and branch number, root/shoot ratio or flexibility. With high water availability, brushing stimulated the production of stiffer stems (thicker and with a higher Young’s modulus) and more roots relative to shoot mass, but with low water availability MP induced the opposite response. This shows that both the degree and direction of plant responses to MP depend on the presence of other factors. We discuss how the interactive effects of MP and water availability on growth and mechanical properties may help C. mongolicum to establish in windy and arid environments. [ABSTRACT FROM AUTHOR]

An advanced non-invasive, field-suitable and inexpensive leaf patch clamp pressure probe for online-monitoring of the water relations of intact leaves is described. The probe measures the attenuated output patch clamp pressure, Pp, of a clamped leaf in response to an externally applied input pressure, Pclamp. Pclamp is generated magnetically. Pp is sensed by a pressure sensor integrated into the magnetic clamp. The magnitude of Pp depends on the transfer function, Tf, of the leaf cells. Tf consists of a turgor pressure-independent (related to the compression of the cuticle, cell walls and other structural elements) and a turgor pressure-dependent term. Tf is dimensionless and assumes values between 0 and 1. Theory shows that Tf is a power function of cell turgor pressure Pc. Concomitant Pp and Pc measurements on grapevines confirmed the relationship between Tf and Pc. Pp peaked if Pc approached zero and assumed low values if Pc reached maximum values. The novel probe was successfully tested on leaves of irrigated and non-irrigated grapevines under field conditions. Data show that slight changes in the microclimate and/or water supply (by irrigation or rain) are reflected very sensitively in Pp. [ABSTRACT FROM AUTHOR]

Salt spray is one of many abiotic factors that can influence plant productivity and species composition in coastal ecosystems. However, little is known about how marsh plants respond physiologically to the accumulation of sea aerosols on foliar tissues. In this study, experimental microcosms maintained in controlled greenhouse conditions were used to evaluate how low- (1.7 mg dm-2 day-1, weekly averages) and high- (8.6 mg dm-2 day-1) salt-spray loads would influence plant-water relations in Spartina alterniflora (Loisel.). While no differences in plant performance (e.g., changes in biomass and leaf area) were observed between the treatments and control plants, a number of physiological modifications attributed to salt spray were observed. In general, salt-treated plants underwent significant decreases in water potential (Ψ) and osmotic potential (Ψπ) and increases in leaf conductance (g) and bulk modulus of elasticity (ϵ). It is likely that these physiological responses were used to generate lower Ψ while maintaining osmotic and water homeostasis. That is, by decreasing Ψπ and increasing g and ϵ, more efficient water flow through the soil-plant-atmosphere continuum can be achieved, thus generating lower Ψ without promoting loss of turgor. [ABSTRACT FROM AUTHOR]

Hand sections of root and stem xylem of diverse species of Nymphaea (including Ondinea) were studied with SEM in order to explore the diversity of wall structure within the genus. Lateral walls of root tracheary elements are untextured at the magnifications used, but end wall structure suggests that lysis of pit membranes leaves a cellulosic network with a large reticulum. Stem tracheids have lateral walls that are untextured or have a few prominent fibrils, but end walls have a dense accretion of coarse fibrils. These fibrils form a spongiform or compressed network overlying the pit membranes. In addition, on the lumen side of pit membranes, coarse fibrils cross the pits in an axial direction. These coarse fibrils are connected to pit borders, and they may extend onto wall surfaces of the tracheary elements. These fibrillar accretions are considered here to fulfill criteria as secondary-wall components. Variation within the genus with respect to the fibrillar accretions may occur, but there may be more extensive diversity within any given sample than among species. Root tracheary elements could be considered to be vessel elements because of the extensive removal of wall material from the end-wall pit membranes. Stem tracheary elements qualify as tracheids because, rather than showing lysis of pit-membrane portions, end-wall pit membranes bear fibrillar accretions that are only sparsely porose. The absence of coarse fibrillar accretions on lateral walls of stem tracheids (on tracheid-to-parenchyma interfaces) provides difficulty for any hypothesis that would relate wall thickenings to turgor pressure. The patterns of fibrillar accretion observed in Nymphaea include all patterns thus far reported from stem tracheids in the other genera of the family, and they must be considered to be a characteristic of Nymphaeaceae. [ABSTRACT FROM AUTHOR]

Vitis vinifera L. berries are non-climacteric fruit that exhibit a double sigmoid growth pattern and dynamic changes in gene expression, cell metabolism, and water relations at the onset of ripening. The cell-pressure probe was utilized to examine the relationships of turgor pressure ( P) in mesocarp cells to growth, sugar accumulation, and fruit softening during development. In replications utilizing three different varieties, mesocarp cell P demonstrated a consistent pattern of a relative mid-range P early in development, followed by an increase to a maximum of about 0.35 MPa, and a subsequent rapid decline before ripening to less than 0.1 MPa. Fruit “apparent elastic modulus” ( E, units of MPa), was introduced as a standard measure to describe ripening-related softening. E changed dynamically and synchronously with P during development and in response to water deficits for fruit grown in greenhouse and field conditions. Thus, E and P were positively and linearly related. Sugar accumulation did not increase significantly until P had declined to less than 0.1 MPa. The results suggest that P is an important determinant of fruit softening and that P decreases in conjunction with many of the physiological and gene expression changes that are known to occur at the onset of ripening. [ABSTRACT FROM AUTHOR]

Potassium (K+) is the most abundant ion in the plant cell and is required for a wide array of functions, ranging from the maintenance of electrical potential gradients across cell membranes, to the generation of turgor, to the activation of numerous enzymes. The majority of these functions depend more or less directly upon the activities and regulation of membrane-bound K+ transport proteins, operating over a wide range of K+ concentrations. Here, we review the physiological aspects of potassium transport systems in the plasma membrane, re-examining fundamental problems in the field such as the distinctions between high- and low-affinity transport systems, the interactions between K+ and other ions such as NH4+ and Na+, the regulation of cellular K+ pools, the generation of electrical potentials and the problems involved in measurement of unidirectional K+ fluxes. We place these discussions in the context of recent discoveries in the molecular biology of K+ acquisition and produce an overview of gene families encoding K+ transporters. [ABSTRACT FROM AUTHOR]

A high-precision pressure probe is described which allows non-invasive online-monitoring of the water relations of intact leaves. Real-time recording of the leaf water status occurred by data transfer to an Internet server. The leaf patch clamp pressure probe measures the attenuated pressure, Pp, of a leaf patch in response to a constant clamp pressure, Pclamp. Pp is sensed by a miniaturized silicone pressure sensor integrated into the device. The magnitude of Pp is dictated by the transfer function of the leaf, Tf, which is a function of leaf patch volume and ultimately of cell turgor pressure, Pc, as shown theoretically. The power function Tf=f(Pc) theoretically derived was experimentally confirmed by concomitant Pp and Pc measurements on intact leaflets of the liana Tetrastigma voinierianum under greenhouse conditions. Simultaneous Pp recordings on leaflets up to 10 m height above ground demonstrated that changes in Tf induced by Pc changes due to changes of microclimate and/or of the irrigation regime were sensitively reflected in corresponding changes of Pp. Analysis of the data show that transpirational water loss during the morning hours was associated with a transient rise in turgor pressure gradients within the leaflets. Subsequent recovery of turgescence during the afternoon was much faster than the preceding transpiration-induced water loss if the plants were well irrigated. Our data show the enormous potential of the leaf patch clamp pressure probe for leaf water studies including unravelling of the hydraulic communication between neighbouring leaves and over long distances within tall plants (trees). [ABSTRACT FROM PUBLISHER]

Water relation parameters including elastic modulus (ε), half-times of water exchange (Tw1/2), hydraulic conductivity and turgor pressure (P) were measured in individual root cortical and cotyledon midrib cells in intact figleaf gourd (Cucurbita ficifolia) seedlings, using a cell pressure probe. Transpiration rates (E) of cotyledons were also measured using a steady-state porometer. The seedlings were exposed to low ambient (approximately 10 μmol m−2 s−1) or high supplemental irradiance (approximately 300 μmol m−2 s−1 PPF density) at low (8°C) or warm (22°C) root temperatures. When exposed to low irradiance, all the water relation parameters of cortical cells remained similar at both root temperatures. The exposure of cotyledons to supplemental light at warm root temperatures, however, resulted in a two- to three-fold increase in Tw1/2 values accompanied with the reduced hydraulic conductivity in both root cortical (Lp) and cotyledon midrib cells (Lpc). Low root temperature (LRT) further reduced Lpc and E, whether it was measured under low or high irradiance levels. The reductions of Lp as the result of respective light and LRT treatments were prevented by the application of 1 μM ABA. Midrib cells required higher concentrations of ABA (2 μM) in order to prevent the reduction in Lpc. When the exposure of cotyledons to light was accompanied by LRT, however, ABA proved ineffective in reversing the inhibition of Lp. LRT combined with high irradiance triggered a drastic 10-fold reduction in water permeability of cortical and midrib cells and increased ε and Tw1/2 values. Measurement of E indicated that the increased water demand by the transpiring plants was fulfilled by an increase in the apoplastic pathway as principal water flow route. The importance of water transport regulation by transpiration affecting the hydraulic conductivity of the roots is discussed. [ABSTRACT FROM AUTHOR]

In Vitis vinifera L. berries, the onset of ripening (known as “veraison”) involves loss of turgor (P) in the mesocarp cells. We hypothesized that P loss was associated with an accumulation of apoplastic solutes in mesocarp tissue prior to veraison. Apoplastic sap was extracted from the mesocarp by centrifugation at the appropriate gravity to measure the apoplast solute potential (Ψ) and assay the sap composition. The Ψ was about −0.2 MPa early in development, decreased about 1.0 MPa by veraison, and continued to decrease during ripening to almost −4.0 MPa by the end of berry development. Potassium, malate, tartrate, proline, glucose, fructose, and sucrose were quantified in apoplastic sap. The calculated contribution of these solutes was about 50% of the total Ψ preveraison, but increased to about 75% as fructose and glucose accumulated during ripening. The contribution of the estimated matric potential to apoplast water potential decreased during development and was only 1.5% postveraison. We conclude that high concentrations of solutes accumulated in the mesocarp apoplast prior to veraison, and that P loss was a direct result of decreased Ψ. Because Ψ decreased before veraison, our findings suggest that apoplast solutes play an important role in the events of cellular metabolism that lead to the onset of ripening. [ABSTRACT FROM AUTHOR]

Stomatal regulation of transpiration constrains leaf water potential (ΨL) within species-specific ranges that presumably avoid excessive tension and embolism in the stem xylem upstream. However, the hydraulic resistance of leaves can be highly variable over short time scales, uncoupling tension in the xylem of leaves from that in the stems to which they are attached. We evaluated a suite of leaf and stem functional traits governing water relations in individuals of 11 lowland tropical forest tree species to determine the manner in which the traits were coordinated with stem xylem vulnerability to embolism. Stomatal regulation of ΨL was associated with minimum values of water potential in branches (Ψbr) whose functional significance was similar across species. Minimum values of Ψbr coincided with the bulk sapwood tissue osmotic potential at zero turgor derived from pressure–volume curves and with the transition from a linear to exponential increase in xylem embolism with increasing sapwood water deficits. Branch xylem pressure corresponding to 50% loss of hydraulic conductivity ( P 50) declined linearly with daily minimum Ψbr in a manner that caused the difference between Ψbr and P 50 to increase from 0.4 MPa in the species with the least negative Ψbr to 1.2 MPa in the species with the most negative Ψbr. Both branch P 50 and minimum Ψbr increased linearly with sapwood capacitance ( C) such that the difference between Ψbr and P 50, an estimate of the safety margin for avoiding runaway embolism, decreased with increasing sapwood C. The results implied a trade-off between maximizing water transport and minimizing the risk of xylem embolism, suggesting a prominent role for the buffering effect of C in preserving the integrity of xylem water transport. At the whole-tree level, discharge and recharge of internal C appeared to generate variations in apparent leaf-specific conductance to which stomata respond dynamically. [ABSTRACT FROM AUTHOR]

The physiological mechanisms responsible for reduced extension growth as trees increase in height remain elusive. We evaluated biophysical constraints on leaf expansion in old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Needle elongation rates, plastic and elastic extensibility, bulk leaf water (ΨL) and osmotic (Ψπ) potential, bulk tissue yield threshold and final needle length were characterized along a height gradient in crowns of > 50-m-tall trees during the period between bud break and full expansion (May to June). Although needle length decreased with increasing height, there was no height-related trend in leaf plastic extensibility, which was highest immediately after bud break (2.9%) and declined rapidly to a stable minimum value (0.3%) over a 3-week period during which leaf expansion was completed. There was a significant positive linear relationship between needle elongation rates and plastic extensibility. Yield thresholds were consistently lower at the upper and middle crown sampling heights. The mean yield threshold across all sampling heights was 0.12 ± 0.03 MPa on June 8, rising to 0.34 ± 0.03 MPa on June 15 and 0.45 ± 0.05 MPa on June 24. Bulk leaf Ψπ decreased linearly with increasing height at a rate of 0.004 MPa m-1 during the period of most rapid needle elongation, but the vertical osmotic gradient was not sufficient to fully compensate for the 0.015 MPa m-1 vertical gradient in ΨL, implying that bulk leaf turgor declined at a rate of about 0.011 MPa m-1 increase in height. Although height-dependent reductions in turgor appeared to constrain leaf expansion, it is possible that the impact of reduced turgor was mitigated by delayed phenological development with increasing height, which resulted in an increase with height in the temperature during leaf expansion. [ABSTRACT FROM AUTHOR]

Cortical senescence is an important feature of the roots of a number of graminaceous species because it may contribute to rhizodeposition of carbon and influence the efficiency of nutrient uptake. A major limitation to understanding the physiological control of senescence and its impact on rhizosphere processes has been the lack of reliable techniques for characterizing the progress of senescence along the root. The use of a single-cell pressure probe was evaluated for quantifying cell integrity in different regions of wheat roots. The percentage of locations with turgid cells declined with increasing distance behind the root apex. The decline preceded visible collapse of the cortex but after the loss of nuclear staining in the outer cortex. The percentage of locations with turgid cells was closely associated with root diameter, which suggests that measurements of changes in diameter, made using minirhizotrons under well-watered conditions in the field, could provide estimates of in situ rates of cortical senescence. [ABSTRACT FROM AUTHOR]

Abstract: The effect of turgor on the micromechanical properties of apples (Malus domestica Borkh. cv Jonagored) and its relation to structural parameters of the cells was investigated. Mechanical measurements were carried out using a miniature tensile stage mounted under a microscope. Turgor was manipulated by soaking tissue samples in iso-, hypo- or hypertonic mannitol solutions. Samples were then subjected to tensile and compression tests, during which the deformation of the individual cells was recorded. In both tests, sigmoidal stress–strain curves were found, where stiffness increases with strain. With increasing turgor, strain at maximum stress became significantly (p <0.05) lower while stiffness increased, but no clear effect on failure stress was found. As expected, fresh apples proved to be firmer and stiffer than stored apples, while the influence of turgor was the same. Measurement of initial cell parameters (area, length, width, aspect, roundness) showed that the apples in the experiment were homogeneous, having comparable cell sizes. Manipulation of turgor did not affect initial cell parameters, but the magnitude of cell deformations was clearly influenced. Larger deformations were found in flaccid cells. Changes in length and width were closely related to strain measurements. In conformity with micromechanical parameters, deformations were smaller in stored than in fresh apples. [Copyright &y& Elsevier]

Potassium is an inevitable component of plant life, and potassium channels play a pivotal role in plant growth and development. The role of potassium and of K+ channels in plant cell division and cell-cycle progression, however, has not been determined so far. K+ channel blocker studies with synchronized tobacco BY-2 cells revealed that K+ uptake is required for proper cell-cycle progression during the transition from G1 to S phase. Electrophysiological studies (patch-clamp and voltage-clamp techniques) showed a cell-cycle dependency of K+ channel activities and reduced driving force for K+ uptake in dividing cells. Among the four Shaker-like K+ channel genes expressed in BY-2 cells, NKT1 represents an inwardly rectifying K+ channel that mediates K+ uptake. NKT1 is transcriptionally induced during G1 phase, while transcripts of the outward-rectifier NTORK1 dominate S phase. Elongating BY-2 cells appeared hyperpolarized (−101 ± 11 mV), and had elevated osmotic pressure and approximately twice the turgor pressure when compared with depolarized (−64 ± 8 mV) dividing cells. This indicates that cells have to gain a threshold K+ level to re-enter the cell cycle. Based on these findings, turgor regulation through modulation of K+ channel density in plant cell division and cell-cycle progression is discussed. [ABSTRACT FROM AUTHOR]

Guard cell movement is induced by environmental and hormonal signals that cause changes in turgor through changes in uptake or release of solutes and water. Several transporters mediating these fluxes at the plasma membrane have been characterized; however, less is known about transport at endomembranes. CHX2O, a member of a poorly understood cation/H+ exchanger gene family in Arabidopsis (Arabidopsis thaliana), is preferentially and highly expressed in guard cells as shown by promoter: : β-glucuronidase activity and by whole-genome microarray. Interestingly, three independent homozygous mutants carrying T-DNA insertions in CHX2O showed 35% reduction in light-induced stomatal opening compared to wild-type plants. To test the biochemical function of CHX20, cDNA was expressed in a yeast (Saccharomyces cerevisiae) mutant that lacks Na+(K+)/H+ antiporters (Δnhx1 Δnha1 Δkha1) and plasma membrane Na+ pumps (Δena1-4). Curiously, CHX20 did not enhance tolerance of mutants to moderate Na+ or high K+ stress. Instead, it restored growth of the mutant on medium with low K+ at slightly alkaline pH, but had no effect on growth at acidic pH. Green fluorescent protein-tagged CHX2O expressed in mesophyll protoplasts was localized mainly to membranes of the endosomal system. Furthermore, light-induced stomatal opening of the Arabidopsis mutants was insensitive to external pH and was impaired at high KC1. The results are consistent with the idea that, in exchanging K+ for H+, CHX20 maintains K+ homeostasis and influences pH under certain conditions. Together, these results provide genetic and biochemical evidence that one CHX protein plays a critical role in osmoregulation through K+ fluxes and possibly pH modulation of an active endomembrane system in guard cells. [ABSTRACT FROM AUTHOR]

Variation in osmotic adjustment (OA) among chickpea (Cicer arietinum L.) cultivars has been observed when exposed to terminal drought, but some studies suggest that this benefits yield while others suggest it does not benefit yield in water-limited environments. In the present study, parents differing in OA were crossed and a set of advanced breeding lines (ABLs) developed for yield testing. The variation in OA during podding was measured under terminal drought in the F2, F3, F7, and F8 progeny and in the parents by either rehydrating the leaves before sampling for osmotic potential (OP) or by measuring the relative water content (RWC) and OP on adjacent leaves for the calculation of the OP at full turgor. Yields were measured in the F8 progeny under terminal drought in Australia and India. While differences in OA were measured in the chickpea lines and parents, OA varied from year to year and did not consistently benefit yield when measured in the field under terminal drought. In Australia, differences in OA were not associated with any yield benefit in any year, while in India early flowering resulted in higher yields at three of the four sites, and OA had an inconsistent effect on seed yields. A comparison of OP at full turgor measured after rehydration and from measurements of RWC and OP showed that the rehydration technique underestimated OA. The lack of contribution of OA to yield of chickpea is discussed. [ABSTRACT FROM PUBLISHER]

Given that stomatal movement is ultimately a mechanical process and that stomata are morphologically and mechanically diverse, we explored the influence of stomatal mechanical diversity on leaf gas exchange and considered some of the constraints. Mechanical measurements were conducted on the guard cells of four different species exhibiting different stomatal morphologies, including three variants on the classical ‘kidney’ form and one ‘dumb-bell’ type; this information, together with gas-exchange measurements, was used to model and compare their respective operational characteristics. Based on evidence from scanning electron microscope images of cryo-sectioned leaves that were sampled under full sun and high humidity and from pressure probe measurements of the stomatal aperture versus guard cell turgor relationship at maximum and zero epidermal turgor, it was concluded that maximum stomatal apertures (and maximum leaf diffusive conductance) could not be obtained in at least one of the species (the grass Triticum aestivum) without a substantial reduction in subsidiary cell osmotic (and hence turgor) pressure during stomatal opening to overcome the large mechanical advantage of subsidiary cells. A mechanism for this is proposed, with a corollary being greatly accelerated stomatal opening and closure. Gas-exchange measurements on T. aestivum revealed the capability of very rapid stomatal movements, which may be explained by the unique morphology and mechanics of its dumb-bell-shaped stomata coupled with ‘see-sawing’ of osmotic and turgor pressure between guard and subsidiary cells during stomatal opening or closure. Such properties might underlie the success of grasses. [ABSTRACT FROM AUTHOR]

The article focuses on the general and analytic solutions of the Ortega equation. Ortega equation suggests a simple first-order differential equation related to plant cell extension which considers the elastic deformation of the cell wall. The general solution to the Ortega equation consists of two terms. Such terms include the exponential reduction of the turgor pressure when the cell is deprived of water supply and the additional pressure which results from the presence of the water source.

From a mechanical point of view, plant and hyphal cells are more complex than their animal counterparts because the variety of structural components determining cellular architecture is broader. In addition to cytoskeletal elements and the plasma membrane, the cell wall and turgor pressure equip plant and hyphal cells with structures analogous to an exoskeleton and a hydroskeleton, respectively. To quantify the physical properties of plant and hyphal cells, researchers have developed a plethora of experimental methods. This review provides an overview of experimental approaches that have been used to measure turgor pressure and to determine the mechanical properties of the plant cell wall at the subcellular level. It is completed by a glimpse into the arsenal of techniques that has been used to characterize the physical properties of cytoskeletal elements. These have mostly been used on animal cells, but we hope they will find their way into plant cell research. Finally, assays and tests to measure the generation of forces by cells and subcellular structures are discussed. [ABSTRACT FROM AUTHOR]

The conventional method of measuring plant cell turgor pressure is the pressure probe but applying this method to single cells in suspension culture is technically difficult and requires puncture of the cell wall. Conversely, compression testing by micromanipulation is particularly suited to studies on single cells, and can be used to characterise cell wall mechanical properties, but has not been used to measure turgor pressure. In order to demonstrate that the micromanipulation method can do this, pressure measurements by both methods were compared on single suspension-cultured tomato ( Lycopersicon esculentum vf36) cells and generally were in good agreement. This validates further the micromanipulation method and demonstrates its capability to measure turgor pressure during water loss. It also suggests that it might eventually be used to estimate plant cell hydraulic conductivity. [ABSTRACT FROM AUTHOR]

• Background and Aims New wall deposition usually accompanies plant growth. External osmotica inhibit both processes but wall precursors continue to be synthesized, and exocytosis follows. Consequently, the osmotica appear to act outside of the plasma membrane. Because this implies an action of turgor pressure (P) on the periplasm by unknown mechanisms, the following study was undertaken to determine whether P could act in a way that altered wall deposition and assembly in the periplasm while the cells grow.• Methods Cells of Chara corallina were exposed to P slightly below normal by using a pressure probe while supplying inorganic carbon in light. After labelling, the walls were isolated and the amount of new wall was determined. Similar measurements were made after treatment with osmotica. Chlortetracycline-stimulated exocytosis was determined microscopically. Polysaccharide properties were determined by confocal microscopy and vapour pressure osmometry in an ‘artificial periplasm’ in isolated Chara cell walls, using labelled dextran as an anologue of hemicellulose, and polygalacturonate as pectin.• Key Results Rapid growth and wall deposition occurred at normal P of 0.5 MPa but both processes decreased when P was lowered 0.1 MPa. Inorganic carbon uptake and exocytosis were unaffected. In the artificial periplasm, normal P caused high polysaccharide concentrations and rapid polysaccharide entry into the wall, and gel formation in the pectin. Lowering P decreased entry and gel formation.• Conclusions This is the first indication that normal P of 0.5 MPa can concentrate periplasmic polysaccharides sufficiently to cause cross-linking and gel formation in pectins while simultaneously fostering the entry of large polysaccharides into small interstices in the existing wall. This P-action would thicken the primary wall and form a smooth transition between the new and old structure, suggesting a molecular mechanism of wall deposition and assembly while the wall extends. [ABSTRACT FROM AUTHOR]

To test whether the inhibition of leaf expansion by high evaporative demand is a result of hydraulic processes, we have followed both leaf elongation rate (LER) and cell turgor in leaves of maize plants either normally watered or in water-saturated soil in which hydraulic resistance at the soil–root interface was abolished. Cell turgor was measured in situ with a pressure probe in the elongating zone of the first and sixth leaves, and LERs of the same leaves were measured continuously with transducers or by following displacements of marks along the growing leaves. Both variables displayed spatial variations along the leaf and positively correlated within the elongating zone. Values peaked at mid-distance of this zone, where the response of turgor to evaporative demand was further dissected. High evaporative demand decreased both LER and turgor for at least 5 h, with dose-effect linear relations. This was observed in five genotypes with appreciable differences in turgor maintenance among genotypes. In contrast, the depressing effects of evaporative demand on both turgor and LER disappeared when the soil was saturated, thereby opposing a negligible resistance to water flow at the soil–root interface. These results suggest that the response of LER to evaporative demand has a hydraulic origin, enhanced by the resistance to water flux at the soil–root interface. They also suggest that turgor is not completely maintained under high evaporative demand, and may therefore contribute to the reductions in LER observed in non-saturated soils. [ABSTRACT FROM AUTHOR]