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Summary Drought predisposes forest trees to bark beetle‐induced mortality, but the physiological mechanisms remain unclear. While drought‐induced water and carbon limitations have been implicated in defensive failure and tree susceptibility, evidence demonstrating how these factors interact is scarce. We withheld water from mature, potted Pinus edulis and subsequently applied a double‐stem girdle to inhibit carbohydrate transport from the crown and roots. Within this isolated segment we then elicited a defense response by inoculating trees with a bark beetle‐fungal symbiont (Ophiostoma sp.). We quantified local mono‐ and sesquiterpenes (MST), nonstructural carbohydrates (NSC), and pressure potential of the inner bark. Both drought‐stressed and watered trees had similar NSC concentrations just before inoculation and depleted NSC similarly following inoculation, yet MST induction (i.e. increased concentration and altered composition) was constrained only in drought‐stressed trees. Thus, NSC consumption was largely unrelated to de novo MST synthesis. Instead, stoichiometric calculations show that induction originated largely from stored resin. Watered trees experiencing higher pressure potentials consistently induced higher MST concentrations. We demonstrate the importance of preformed resin toward an induced MST response in a semi‐arid conifer where drought‐constraints on defense occurred through biophysical limitations (i.e. reduced turgor hindering resin transport) rather than through substrate limitation. [ABSTRACT FROM AUTHOR]

Summary: Turgor loss point (TLP) is an important proxy for plant drought tolerance, species habitat suitability, and drought‐induced plant mortality risk. Thus, TLP serves as a critical tool for evaluating climate change impacts on plants, making it imperative to develop high‐throughput and in situ methods to measure TLP.We developed hyperspectral pressure–volume curves (PV curves) to estimate TLP using leaf spectral reflectance. We used partial least square regression models to estimate water potential (Ψ) and relative water content (RWC) for two species, Frangula caroliniana and Magnolia grandiflora. RWC and Ψ's model for each species had R2 ≥ 0.7 and %RMSE = 7–10. We constructed PV curves with model estimates and compared the accuracy of directly measured and spectra‐predicted TLP.Our findings indicate that leaf spectral measurements are an alternative method for estimating TLP. F. caroliniana TLP's values were −1.62 ± 0.15 (means ± SD) and −1.62 ± 0.34 MPa for observed and reflectance predicted, respectively (P > 0.05), while M. grandiflora were −1.78 ± 0.34 and −1.66 ± 0.41 MPa (P > 0.05).The estimation of TLP through leaf reflectance‐based PV curves opens a broad range of possibilities for future research aimed at understanding and monitoring plant water relations on a large scale with spectral ecophysiology. [ABSTRACT FROM AUTHOR]

Summary: The high productive potential, heat resilience, and greater water use efficiency of C4 over C3 plants attract considerable interest in the face of global warming and increasing population, but C4 plants are often sensitive to dehydration, questioning the feasibility of their wider adoption.To resolve the primary effect of dehydration from slower from secondary leaf responses originating within leaves to combat stress, we conducted an innovative dehydration experiment. Four crops grown in hydroponics were forced to a rapid yet controlled decrease in leaf water potential by progressively raising roots of out of the solution while measuring leaf gas exchange.We show that, under rapid dehydration, assimilation decreased more steeply in C4 maize and sorghum than in C3 wheat and sunflower. This reduction was due to a rise of nonstomatal limitation at triple the rate in maize and sorghum than in wheat and sunflower.Rapid reductions in assimilation were previously measured in numerous C4 species across both laboratory and natural conditions. Hence, we deduce that high sensitivity to rapid dehydration might stem from the disturbance of an intrinsic aspect of C4 bicellular photosynthesis. We posit that an obstruction to metabolite transport between mesophyll and bundle sheath cells could be the cause. [ABSTRACT FROM AUTHOR]

Summary: Pollen tubes have dynamic tubular vacuoles. Functional loss of AP‐3, a regulator of one vacuolar trafficking route, reduces pollen tube growth. However, the role of canonical Rab5 GTPases that are responsible for two other vacuolar trafficking routes in Arabidopsis pollen tubes is obscure.By using genomic editing, confocal microscopy, pollen tube growth assays, and transmission electron microscopy, we demonstrate that functional loss of canonical Rab5s in Arabidopsis, RHA1 and ARA7, causes the failure of pollen tubes to grow through style and thus impairs male transmission.Functional loss of canonical Rab5s compromises vacuolar trafficking of tonoplast proteins, vacuolar biogenesis, and turgor regulation. However, rha1;ara7 pollen tubes are comparable to those of wild‐type in growing through narrow passages by microfluidic assays.We demonstrate that functional loss of canonical Rab5s compromises endocytic and secretory trafficking at the plasma membrane (PM), whereas the targeting of PM‐associated ATPases is largely unaffected. Despite that, rha1;ara7 pollen tubes contain a reduced cytosolic pH and disrupted actin microfilaments, correlating with the mis‐targeting of vacuolar ATPases (VHA). These results imply a key role of vacuoles in maintaining cytoplasmic proton homeostasis and in pollen tube penetrative growth through style. [ABSTRACT FROM AUTHOR]

Summary: Every existing optimal stomatal model uses photosynthetic carbon assimilation as a proxy for plant evolutionary fitness. However, assimilation and growth are often decoupled, making assimilation less ideal for representing fitness when optimizing stomatal conductance to water vapor and carbon dioxide. Instead, growth should be considered a closer proxy for fitness.We hypothesize stomata have evolved to maximize turgor‐driven growth, instead of assimilation, over entire plants' lifetimes, improving their abilities to compete and reproduce. We develop a stomata model that dynamically maximizes whole‐stem growth following principles from turgor‐driven growth models. Stomata open to assimilate carbohydrates that supply growth and osmotically generate turgor, while stomata close to prevent losses of turgor and growth due to negative water potentials.In steady state, the growth optimization model captures realistic stomatal, growth, and carbohydrate responses to environmental cues, reconciles conflicting interpretations within existing stomatal optimization theories, and explains patterns of carbohydrate storage and xylem conductance observed during and after drought.Our growth optimization hypothesis introduces a new paradigm for stomatal optimization models, elevates the role of whole‐plant carbon use and carbon storage in stomatal functioning, and has the potential to simultaneously predict gross productivity, net productivity, and plant mortality through a single, consistent modeling framework. See also the Commentary on this article by Buckley, 238: 457–460. [ABSTRACT FROM AUTHOR]

Summary: Growth is central to plant morphogenesis. Plant cells are encased in rigid cell walls, and they must overcome physical confinement to grow to specific sizes and shapes. Cell wall tension and turgor pressure are the main mechanical components impacting plant cell growth. Cell wall mechanics has been the focus of most plant biomechanical studies, and turgor pressure was often considered as a constant and largely passive component. Nevertheless, it is increasingly accepted that turgor pressure plays a significant role in plant growth. Numerous theoretical and experimental studies suggest that turgor pressure can be both spatially inhomogeneous and actively modulated during morphogenesis. Here, we revisit the pressure–growth relationship by reviewing recent advances in investigating the interactions between cellular/tissular pressure and growth. [ABSTRACT FROM AUTHOR]

Summary: Evolutionary relationships are likely to play a significant role in shaping plant physiological and structural traits observed in contemporary taxa. We review research on phylogenetic signal and correlated evolution in plant–water relation traits, which play important roles in allowing plants to acquire, use, and conserve water. We found more evidence for a phylogenetic signal in structural traits (e.g. stomatal length and stomatal density) than in physiological traits (e.g. stomatal conductance and water potential at turgor loss). Although water potential at turgor loss is the most‐studied plant–water relation trait in an evolutionary context, it is the only trait consistently found to not have a phylogenetic signal. Correlated evolution was common among traits related to water movement efficiency and hydraulic safety in both leaves and stems. We conclude that evidence for phylogenetic signal varies depending on: the methodology used for its determination, that is, model‐based approaches to determine phylogenetic signal such as Blomberg's K or Pagel's λ vs statistical approaches such as ANOVAs with taxonomic classification as a factor; on the number of taxa studied (size of the phylogeny); and the setting in which plants grow (field vs common garden). More explicitly and consistently considering the role of evolutionary relationships in shaping plant ecophysiology could improve our understanding of how traits compare among species, how traits are coordinated with one another, and how traits vary with the environment. [ABSTRACT FROM AUTHOR]

Summary: In higher plants, photosystems II and I are found in grana stacks and unstacked stroma lamellae, respectively. To connect them, electron carriers negotiate tortuous multi‐media paths and are subject to macromolecular blocking. Why does evolution select an apparently unnecessary, inefficient bipartition? Here we systematically explain this perplexing phenomenon. We propose that grana stacks, acting like bellows in accordions, increase the degree of ultrastructural control on photosynthesis through thylakoid swelling/shrinking induced by osmotic water fluxes. This control coordinates with variations in stomatal conductance and the turgor of guard cells, which act like an accordion's air button. Thylakoid ultrastructural dynamics regulate macromolecular blocking/collision probability, direct diffusional pathlengths, division of function of Cytochrome b6f complex between linear and cyclic electron transport, luminal pH via osmotic water fluxes, and the separation of pH dynamics between granal and lamellar lumens in response to environmental variations. With the two functionally asymmetrical photosystems located distantly from each other, the ultrastructural control, nonphotochemical quenching, and carbon‐reaction feedbacks maximally cooperate to balance electron transport with gas exchange, provide homeostasis in fluctuating light environments, and protect photosystems in drought. Grana stacks represent a dry/high irradiance adaptation of photosynthetic machinery to improve fitness in challenging land environments. Our theory unifies many well‐known but seemingly unconnected phenomena of thylakoid structure and function in higher plants. [ABSTRACT FROM AUTHOR]

Summary: Experimental evidence that nonstomatal limitations to photosynthesis (NSLs) correlate with leaf sugar and/or leaf water status suggests the possibility that stomata adjust to maximise photosynthesis through a trade‐off between leaf CO2 supply and NSLs, potentially involving source–sink interactions. However, the mechanisms regulating NSLs and sink strength, as well as their implications for stomatal control, remain uncertain.We used an analytically solvable model to explore optimal stomatal control under alternative hypotheses for source and sink regulation. We assumed that either leaf sugar concentration or leaf water potential regulates NSLs, and that either phloem turgor pressure or phloem sugar concentration regulates sink phloem unloading.All hypotheses led to realistic stomatal responses to light, CO2 and air humidity, including conservative behaviour for the intercellular‐to‐atmospheric CO2 concentration ratio. Sugar‐regulated and water‐regulated NSLs are distinguished by the presence/absence of a stomatal closure response to changing sink strength. Turgor‐regulated and sugar‐regulated phloem unloading are distinguished by the presence/absence of stomatal closure under drought and avoidance/occurrence of negative phloem turgor. Results from girdling and drought experiments on Pinus sylvestris, Betula pendula, Populus tremula and Picea abies saplings are consistent with optimal stomatal control under sugar‐regulated NSLs and turgor‐regulated unloading.Our analytical results provide a simple representation of stomatal responses to above‐ground and below‐ground environmental factors and sink activity. [ABSTRACT FROM AUTHOR]

Summary: Photosynthetic sensitivity to drought is a fundamental constraint on land‐plant evolution and ecosystem function. However, little is known about how the sensitivity of photosynthesis to nonstomatal limitations varies among species in the context of phylogenetic relationships.Using saplings of 10 Eucalyptus species, we measured maximum CO2‐saturated photosynthesis using A–ci curves at several different leaf water potentials (ψleaf) to quantify mesophyll photosynthetic sensitivity to ψleaf (MPS), a measure of how rapidly nonstomatal limitations to carbon uptake increase with declining ψleaf. MPS was compared to the macroclimatic moisture availability of the species' native habitats, while accounting for phylogenetic relationships.We found that species native to mesic habitats have greater MPS but higher maximum photosynthetic rates during non‐water‐stressed conditions, revealing a trade‐off between maximum photosynthesis and drought sensitivity. Species with lower turgor loss points have lower MPS, indicating coordination among photosynthetic and water‐relations traits.By accounting for phylogenetic relationships among closely related species, we provide the first compelling evidence that MPS in Eucalyptus evolved in an adaptive fashion with climatically determined moisture availability, opening the way for further study of this poorly explored dimension of plant adaptation to drought. [ABSTRACT FROM AUTHOR]

Summary: The effects of climate change on tropical forests will depend on how diverse tropical tree species respond to drought. Current distributions of evergreen and deciduous tree species across local and regional moisture gradients reflect their ability to tolerate drought stress, and might be explained by functional traits.We measured leaf water potential at turgor loss (i.e. 'wilting point'; πtlp), wood density (WD) and leaf mass per area (LMA) on 50 of the most abundant tree species in central Panama. We then tested their ability to explain distributions of evergreen and deciduous species within a 50 ha plot on Barro Colorado Island and across a 70 km rainfall gradient spanning the Isthmus of Panama.Among evergreen trees, species with lower πtlp were associated with drier habitats, with πtlp explaining 28% and 32% of habitat association on local and regional scales, respectively, greatly exceeding the predictive power of WD and LMA. In contrast, πtlp did not predict habitat associations among deciduous species.Across spatial scales, πtlp is a useful indicator of habitat preference for tropical tree species that retain their leaves during periods of water stress, and holds the potential to predict vegetation responses to climate change. [ABSTRACT FROM AUTHOR]

Summary: Under prolonged drought and reduced photosynthesis, plants consume stored nonstructural carbohydrates (NSCs). Stored NSC depletion may impair the regulation of plant water balance, but the underlying mechanisms are poorly understood, and whether such mechanisms are independent of plant water deficit is not known. If so, carbon costs of fungal symbionts could indirectly influence plant drought tolerance through stored NSC depletion.We connected well‐watered Pinus ponderosa seedling pairs via ectomycorrhizal (EM) networks where one seedling was shaded (D) and the other kept illuminated (LD) and compared responses to seedling pairs in full light (L). We measured plant NSCs, osmotic and water potential, and transfer of 13CO2 through EM to explore mechanisms linking stored NSCs to plant water balance regulation and identify potential tradeoffs between plant water retention and EM fungi under carbon‐limiting conditions.NSCs decreased from L to LD to D seedlings. Even without drought, NSC depletion impaired osmoregulation and turgor maintenance, both of which are critical for drought tolerance. Importantly, EM networks propagated NSC depletion and its negative effects on water retention from carbon stressed to nonstressed hosts.We demonstrate that NSC storage depletion influences turgor maintenance independently of plant water deficit and reveal carbon allocation tradeoffs between supporting fungal symbionts and retaining water. [ABSTRACT FROM AUTHOR]

Summary: Although xylem embolism resistance is traditionally considered as static, we hypothesized that in grapevine (Vitis vinifera) leaf xylem becomes more embolism‐resistant over the growing season.We evaluated xylem architecture, turgor loss point (ΨTLP) and water potentials leading to 25% of maximal stomatal conductance (gs25) or 50% embolism in the leaf xylem (P50) in three irrigation treatments and at three time points during the growing season, while separating the effects of leaf age and time of season.Hydraulic traits acclimated over the growing season in a coordinated manner. Without irrigation, ΨTLP, gs25, and P50 decreased between late May and late August by 0.95, 0.77 and 0.71 MPa, respectively. A seasonal shift in P50 occurred even in mature leaves, while irrigation had only a mild effect (< 0.2 MPa) on P50. Vessel size and pit membrane thickness were also seasonally dynamic, providing a plausible explanation for the shift in P50.Our findings provide clear evidence that grapevines can modify their hydraulic traits along a growing season to allow lower xylem water potential, without compromising gas exchange, leaf turgor or xylem integrity. Seasonal changes should be considered when modeling ecosystem vulnerability to drought or comparing datasets acquired at different phenological stages. [ABSTRACT FROM AUTHOR]

Summary: In the stems of terrestrial vascular plants studied to date, the diameter of xylem water‐conducting conduits D widens predictably with distance from the stem tip L approximating D ∝ Lb, with b ≈ 0.2. Because conduit diameter is central for conductance, it is essential to understand the cause of this remarkably pervasive pattern. We give reason to suspect that tip‐to‐base conduit widening is an adaptation, favored by natural selection because widening helps minimize the increase in hydraulic resistance that would otherwise occur as an individual stem grows longer and conductive path length increases. Evidence consistent with adaptation includes optimality models that predict the 0.2 exponent. The fact that this prediction can be made with a simple model of a single capillary, omitting much biological detail, itself makes numerous important predictions, e.g. that pit resistance must scale isometrically with conduit resistance. The idea that tip‐to‐base conduit widening has a nonadaptive cause, with temperature, drought, or turgor limiting the conduit diameters that plants are able to produce, is less consistent with the data than an adaptive explanation. We identify empirical priorities for testing the cause of tip‐to‐base conduit widening and underscore the need to study plant hydraulic systems leaf to root as integrated wholes. [ABSTRACT FROM AUTHOR]

Summary: A valid representation of intra‐annual wood formation processes in global vegetation models is vital for assessing climate change impacts on the forest carbon stock. Yet, wood formation is generally modelled with photosynthesis, despite mounting evidence that cambial activity is rather directly constrained by limiting environmental factors.Here, we apply a state‐of‐the‐art turgor‐driven growth model to simulate 4 yr of hourly stem radial increment from Picea abies (L.) Karst. and Larix decidua Mill. growing along an elevational gradient. For the first time, wood formation observations were used to validate weekly to annual stem radial increment simulations, while environmental measurements were used to assess the climatic constraints on turgor‐driven growth.Model simulations matched the observed timing and dynamics of wood formation. Using the detailed model outputs, we identified a strict environmental regulation on stem growth (air temperature > 2°C and soil water potential > −0.6 MPa). Warmer and drier summers reduced the growth rate as a result of turgor limitation despite warmer temperatures being favourable for cambial activity.These findings suggest that turgor is a central driver of the forest carbon sink and should be considered in next‐generation vegetation models, particularly in the context of global warming and increasing frequency of droughts. [ABSTRACT FROM AUTHOR]

Summary: Plants in nature are constantly exposed to organisms that touch them and wound them. A highly conserved response to these stimuli is a rapid collapse of membrane potential (i.e. a decrease of electrical field strength across membranes). This can be coupled to the production and/or action of jasmonate or ethylene. Here, the various types of electrical signals in plants are discussed in the context of hormone responses. Genetic approaches are revealing genes involved in wound‐induced electrical signalling. These include clade 3 GLUTAMATE RECEPTOR‐LIKE (GLR) genes, Arabidopsis H+‐ATPases (AHAs), RESPIRATORY BURST OXIDASE HOMOLOGUEs (RBOHs), and genes that determine cell wall properties. We briefly review touch‐ and wound‐induced increases in cytosolic Ca2+ concentrations and their temporal relationship to electrical activities. We then look at the questions that need addressing to link mechanostimulation and wound‐induced electrical activity to hormone responses. Utilizing recently published results, we also present a hypothesis for wound‐response leaf‐to‐leaf electrical signalling. This model is based on rapid electro‐osmotic coupling between the phloem and xylem. The model suggests that the depolarization of membranes within the vascular matrix triggered by physical stimuli and/or chemical elicitors is linked to changes in phloem turgor and that this plays vital roles in leaf‐to‐leaf electrical signal propagation. [ABSTRACT FROM AUTHOR]

Summary: Signal coordination in response to changes in water availability remains unclear, as does the role of embolism events in signaling drought stress.Sunflowers were exposed to two drought treatments of varying intensity while simultaneously monitoring changes in stomatal conductance, acoustic emissions (AE), turgor pressure, surface‐level electrical potential, organ‐level water potential and leaf abscisic acid (ABA) concentration. Leaf, stem and root xylem vulnerability to embolism were measured with the single vessel injection technique.In both drought treatments, it was found that AE events and turgor changes preceded the onset of stomatal closure, whereas electrical surface potentials shifted concurrently with stomatal closure. Leaf‐level ABA concentration did not change until after stomata were closed. Roots and petioles were equally vulnerable to drought stress based on the single vessel injection technique. However, anatomical analysis of the xylem indicated that the increased AE events were not a result of xylem embolism formation. Additionally, roots and stems never reached a xylem pressure threshold that would initiate runaway embolism throughout the entire experiment.It is concluded that stomatal closure was not embolism‐driven, but, rather, that onset of stomatal closure was most closely correlated with the hydraulic signal from changes in leaf turgor. [ABSTRACT FROM AUTHOR]

Summary: Maintaining water balance has been a critical constraint shaping the evolution of leaf form and function. However, flowers, which are heterotrophic and relatively short‐lived, may not be constrained by the same physiological and developmental factors.We measured physiological parameters derived from pressure–volume curves for leaves and flowers of 22 species to characterize the diversity of hydraulic traits in flowers and to determine whether flowers are governed by the same constraints as leaves.Compared with leaves, flowers had high saturated water content, which was a strong predictor of hydraulic capacitance in both leaves and flowers. Principal component analysis revealed that flowers occupied a different region of multivariate trait space than leaves and that hydraulic traits are more diverse in flowers than in leaves.Without needing to maintain high rates of transpiration, flowers rely on other hydraulic traits, such as high hydraulic capacitance, to maintain turgor pressure. As a result, instead of employing a metabolically expensive but durable carbon (C)‐based skeleton, flowers may rely predominantly on a metabolically cheaper, hydrostatic skeleton to keep their structures on display for pollinators, which has important implications for both the costs of reproduction and the biomechanical performance of flowers, particularly during drought. See also the Commentary on this article by Olson & Pittermann, 223: 8–10. [ABSTRACT FROM AUTHOR]

ContentsSummary84I.Introduction84II.Stomatal form and biomechanics85III.Stomatal function86IV.Evolution of guard cell ion channels87V.Conclusions88Acknowledgements88Author contributions88References88 Summary: Stomatal pores with apertures that can be adjusted by changes in guard cell turgor have facilitated plant success in dry environments. We explore their evolutionary origins, considering recent findings from bryophytes. Unlike vascular plant stomata, which close to prevent water loss, bryophyte stomata become locked open to promote spore desiccation. We find that the families of ion channels, known to control stomatal movements in angiosperms, are ancient and represented across extant land plants. However, although angiosperm guard cells express specific ion channel genes, none appear specifically expressed in stomata‐bearing moss tissues. Given the evolutionary shift in stomatal function from promotion to prevention of water loss, we postulate that ion channels adopted guard cell‐specific functions after the divergence of bryophytes. [ABSTRACT FROM AUTHOR]

Summary: To grow straight, plants need a motor system that controls posture by generating forces to offset gravity. This motor function in trees was long thought to be only controlled by internal forces induced in wood. Here we provide evidence that bark is involved in the generation of mechanical stresses in several tree species.Saplings of nine tropical species were grown tilted and staked in a shadehouse and the change in curvature of the stem was measured after releasing from the pole and after removing the bark. This first experiment evidenced the contribution of bark in the up‐righting movement of tree stems.Combined mechanical measurements of released strains on adult trees and microstructural observations in both transverse and longitudinal/tangential plane enabled us to identify the mechanism responsible for the development of asymmetric mechanical stress in the bark of stems of these species.This mechanism does not result from cell wall maturation like in wood, or from the direct action of turgor pressure like in unlignified organs, but is the consequence of the interaction between wood radial pressure and a smartly organized trellis structure in the inner bark. See also the Commentary on this article by Poppinga & Speck, 221: 7–9. [ABSTRACT FROM AUTHOR]

The high productive potential, heat resilience, and greater water use efficiency of C 4 over C 3 plants attract considerable interest in the face of global warming and increasing population, but C 4 plants are often sensitive to dehydration, questioning the feasibility of their wider adoption. To resolve the primary effect of dehydration from slower from secondary leaf responses originating within leaves to combat stress, we conducted an innovative dehydration experiment. Four crops grown in hydroponics were forced to a rapid yet controlled decrease in leaf water potential by progressively raising roots of out of the solution while measuring leaf gas exchange. We show that, under rapid dehydration, assimilation decreased more steeply in C 4 maize and sorghum than in C 3 wheat and sunflower. This reduction was due to a rise of nonstomatal limitation at triple the rate in maize and sorghum than in wheat and sunflower. Rapid reductions in assimilation were previously measured in numerous C 4 species across both laboratory and natural conditions. Hence, we deduce that high sensitivity to rapid dehydration might stem from the disturbance of an intrinsic aspect of C 4 bicellular photosynthesis. We posit that an obstruction to metabolite transport between mesophyll and bundle sheath cells could be the cause., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation.)

Summary: Physical forces are involved in the regulation of plant development and morphogenesis by translating mechanical stress into the modification of physiological processes, which, in turn, can affect cellular growth. Pollen tubes respond rapidly to external stimuli and provide an ideal system to study the effect of mechanical cues at the single‐cell level. Here, pollen tubes were exposed to mechanical stress while monitoring the reconfiguration of their growth and recording the generated forces in real‐time. We combined a lab‐on‐a‐chip device with a microelectromechanical systems (MEMS)‐based capacitive force sensor to mimic and quantify the forces that are involved in pollen tube navigation upon confronting mechanical obstacles. Several stages of obstacle avoidance were identified, including force perception, growth adjustment and penetration. We have experimentally determined the perceptive force threshold, which is the force threshold at which the pollen tube reacts to an obstacle, for Lilium longiflorum and Arabidopsis thaliana. In addition, the method we developed provides a way to calculate turgor pressure based on force and optical data. Pollen tubes sense physical barriers and actively adjust their growth behavior to overcome them. Furthermore, our system offers an ideal platform to investigate intracellular activity during force perception and growth adaption in tip growing cells. [ABSTRACT FROM AUTHOR]

Summary: Epiphytes are common in tropical montane cloud forests (TMCFs) and play many important ecological roles, but the degree to which these unique plants will be affected by changes in climate is unknown. We investigated the drought responses of three vascular epiphyte communities bracketing the cloud base during a severe, El Niño‐impacted dry season. Epiphytes were instrumented with sap flow probes in each site. Leaf water potential and pressure–volume curve parameters were also measured before and during the drought. We monitored the canopy microclimate in each site to determine the drivers of sap velocity across the sites. All plants greatly reduced their water use during the drought, but recovery occurred more quickly for plants in the lower and drier sites. Plants in drier sites also exhibited the greatest shifts in the osmotic potential at full saturation and the turgor loss point. Although all individuals survived this intense drought, epiphytes in the cloud forest experienced the slowest recovery, suggesting that plants in the TMCF are particularly sensitive to severe drought. Although vapor pressure deficit was an important driver of sap velocity in the highest elevation site, other factors, such as the volumetric water content of the canopy soil, were more important at lower (and warmer) sites. [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]

The Scholander-Hammel pressure chamber has been used in thousands of papers to measure osmotic pressure, πc, turgor pressure, Pt, and bulk modulus of elasticity, ε, of leaf cells by pressure-volume ( PV) curve analysis. PV analysis has been questioned in the past., In this paper we use micromechanical analysis of leaf cells to examine the impact on PV curve analysis of negative turgor in living cells ( Pt)., Models predict negative Pt (−0.1 to −1.8 MPa) depending on leaf cell size and shape in agreement with experimental values reported by J. J. Oertli. Modeled PV curves have linear regions even when Pt is quite negative, contrary to the arguments of M.T. Tyree. Negative Pt is totally missed by PV curve analysis and results in large errors in derived πc and Pt but smaller errors in ε., A survey of leaf cell sizes vs habitat (arid, temperate, and rainforest), suggests that the majority of published PV curves result in errors of 0.1-1.8 MPa in derived πc and Pt, whereby the error increases with decreasing cell size. We propose that small cell size in leaves is an ecological adaptation that permits plants to endure negative values of water potential with relatively little water loss. [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]

The article focuses on the use of Dynamic Global Vegetation Models (DGVMs) for the simulation of terrestrial carbon (C) pools and fluxes. It discusses the key drivers of plant growth which include availability of soil water, nutrients, carbon dioxide, and temperature. It mentions that wrong representation of plant growth affects the interaction between vegetation and climate.

Currently, phloem transport in plants under field conditions is not well understood. This is largely the result of the lack of techniques suitable for the measurement of the physiological properties of phloem., We present a model that interprets the changes in xylem diameter and live bark thickness and separates the components responsible for such changes. We test the predictions from this model on data from three mature Scots pine trees in Finland. The model separates the live bark thickness variations caused by bark water capacitance from a residual signal interpreted to indicate the turgor changes in the bark., The predictions from the model are consistent with processes related to phloem transport. At the diurnal scale, this signal is related to patterns of photosynthetic activity and phloem loading. At the seasonal scale, bark turgor showed rapid changes during two droughts and after two rainfall events, consistent with physiological predictions. Daily cumulative totals of this turgor term were related to daily cumulative totals of canopy photosynthesis. Finally, the model parameter representing radial hydraulic conductance between phloem and xylem showed a temperature dependence consistent with the temperature-driven changes in water viscosity., We propose that this model has potential for the continuous field monitoring of tree phloem function. [ABSTRACT FROM AUTHOR]

• Ecosystem respiration is known to vary following changes in canopy photosynthesis. However, the timing of this coupling is not well understood. • Here, we summarize the literature on soil and ecosystem respiration where the speed of transfer of photosynthetic sugars from the plant canopy via the phloem to the roots was determined. Estimates of the transfer speed can be grouped according to whether the study employed isotopic or canopy/soil flux-based techniques. These two groups should provide different estimates of transfer times because transport of sucrose molecules, and pressure-concentration waves, in phloem differ. A steady-state and a dynamic photosynthesis/phloem-transport/soil gas diffusion model were employed to interpret our results. • Starch storage and partly soil gas diffusion affected transfer times, but phloem path-length strongly controlled molecule transfer times. Successful modelling required substantially different phloem properties (higher specific conductivity and turgor pressure difference) in tall compared with small plants, which is significant for our understanding of tall trees’ physiology. • Finally, we compared isotopic and flux-based approaches for the determination of the link between canopy photosynthesis and ecosystem respiration. We conclude that isotopic approaches are not well suited to document whether changes in photosynthesis of tall trees can rapidly affect soil respiration. [ABSTRACT FROM AUTHOR]

• Oxaziclomefone [OAC; IUPAC name 3-(1-(3,5-dichlorophenyl)-1-methylethyl)-3,4-dihydro-6-methyl-5-phenyl-2 H-1,3-oxazin-4-one] is a new herbicide that inhibits cell expansion in grass roots. Its effects on cell cultures and mode of action were unknown. In principle, cell expansion could be inhibited by a decrease in either turgor pressure or wall extensibility. • Cell expansion was estimated as settled cell volume; cell division was estimated by cell counting. Membrane permeability to water was measured by a novel method involving simultaneous assay of the efflux of 3H2O and [14C]mannitol from a ‘bed’ of cultured cells. Osmotic potential was measured by depression of freezing point. • OAC inhibited cell expansion in cultures of maize ( Zea mays), spinach ( Spinacia oleracea) and rose ( Rosa sp.), with an ID50 of 5, 30 and 250 n m, respectively. In maize cultures, OAC did not affect cell division for the first 40 h. It did not affect the osmotic potential of cell sap or culture medium, nor did it impede water transport across cell membranes. It did not affect cells’ ability to acidify the apoplast (medium), which may be necessary for ‘acid growth’. • As OAC did not diminish turgor pressure, its ability to inhibit cell expansion must depend on changes in wall extensibility. It could be a valuable tool for studies on cell expansion. [ABSTRACT FROM AUTHOR]

Spatial distribution of cell turgor pressure, cell osmotic pressure and relative elemental growth rate were measured in growing tall fescue leaves (Festuca arundinacea). Cell turgor pressure (measured with a pressure probe) was c. 0.55 MPa in expanding cells but increased steeply (-0.3 MPa) in cells where elongation had stopped. However, cell osmotic pressure (measured with a picolitre osmometer) was almost constant at 0.85 MI'a throughout the leaf. The water potential difference between the growth zone and the mature zone (0,3 MPa) was interpreted as a growth-induced water potential gradient. This and further implications for the mechanism of growth control are discussed. [ABSTRACT FROM AUTHOR]

The effects of potassium deficiency (KD) and all-macronutrient deficiency (MD) on elongation of tap and lateral roots were studied on maritime pine seedlings (Pinus pinaster Ait.) in hydroponic culture. Tap root elongation was unaffected by either of the two deficiencies. By marked contrast, lateral root elongation was strongly reduced. The analyses of cell turgor pressure and relative elemental growth rate (RECK) profile in the growing zone allowed us to determine the effects of the nutrient stresses on cell-wall properties. For both deficiency treatments, elongation rate, (REGR) profile (measured only for control and KD) and turgor pressure in the fastest growing cells were unaffected in the tap root, suggesting that KD and MD did not modify cell-wall properties in the growing zone. In lateral roots, KD shortened the growing zone and significantly reduced REGR. However, turgor pressure remained unaffected in this region. The absence of turgor pressure change suggests that KD reduced elongation of lateral roots by tightening cell walls. In mature cells of the two types of roots, turgor and osmotic pressures tended to be reduced by the nutrient deficiencies, indicating that these parameters were better maintained in the growing cells. Cell turgor and osmotic pressures of control plants were 0.1MPa lower at 30 mm (mature cells) than at 2-4 mm (expanding cells) from the meristem. Moreover, these parameters were 0.1 MPa lower in expanding cells of lateral roots than in those of tap the root. Turgor and osmotic pressures were not homogeneous throughout the root system and were affected differently by the nutrient deficiencies depending on the location in the root system. [ABSTRACT FROM AUTHOR]

A method which determines the null point for stomatal aperture has been used to estimate the apoplastic concentrations of potassium and calcium adjacent to the stomatal guard cells of Commelina communis. These two ions are contributors to important aspects of stomatal physiology : the determination of guard cell turgor (K+) and intracellular signalling (Ca2+) when the guard cells respond to the various stimuli that effect changes in stomatal aperture. We obtained estimates of apoplastic K+ concentrations in the range of 50-75 mol m-3, which are in general agreement with those of Bowling (1987). Ca2+ concentrations appear to be in the region of 0.05 mol m-3 adjacent to the guard cells even though much higher concentrations (up to 4 mol m-3) may be delivered to the leaf in the xylem sap. Thus it is suggested that the gradient in apoplastic Ca2+ may be very large over short distances, and may be strictly controlled by cells within the epidermis. [ABSTRACT FROM AUTHOR]

summary: The dependence of shoot growth and growth form on water availability was studied experimentally in six species of maritime Antarctic moss. Under all conditions the largest growth increments were observed in the hydric species Brachythecium austro‐salebrosum and Drepanocladus uncinatus. The xeric Andreaea depressinervis grew the least. Lateral shoot production varied within and between species. Over 50 % of the biomass produced in D. uncinatus was derived from lateral shoot production, whereas Polytrichum alpestre produced very few lateral shoots and A. depressinervis produced none. Leaf density and leaf size also varied with total water content. In all species growth ceased at total water contents of 100% d. wt or less. However, the total water content at which maximum growth was observed differed between species. Racomitrium austro‐georgicum (mesic/xeric) had the lowest optimum for growth at 370% of d. wt and D. uncinatus (hydric) exhibited maximum growth between 890 and 2300% d. wt. Optimum total water contents for growth were greater than those at full turgor and published optima for net assimilation. Growth and total water content of these Antarctic mosses were similar to those reported for temperate species. [ABSTRACT FROM AUTHOR]

Xylem recovery from cavitation-induced embolism was studied in 1-yr-old twigs of Laurus nobilis L. Cavitation was induced by applying pre-established pressure differentials (ΔP0-1) across the pit membranes of xylem conduits. ΔP0-1 were 1.13, 1.75 and 2.26 MPa, corresponding to about 50, 77 and 100% of the measured leaf water potential at the turgor loss point. ΔP0-1 were obtained either by increasing xylem tensions or by applying positive pressures from outside, or by a combination of the two. The percentage loss of hydraulic conductivity (PLC) did not change, regardless of how the ΔP0-1 were obtained. This confirmed that xylem cavitation was nucleated by microbubbles coming from outside the vessels. Positive pressures, however, amplified (up to 75%) and sped up the xylem refilling (20 min) in comparison with that measured in unpressurized twigs (c.50% in 15 h). Twigs girdled proximally to their pressurized segment 1 min after the desired pressure value had been reached, did not recover from embolism. The later the twigs were girdled with respect to when they were tested for PLC, the higher was their recovery from embolism, suggesting that some messenger was transported in the phloem which stimulated xylem refilling. Indol-3-acetic acid (IAA) applied to the exposed cortex of both pressurized and unpressurized twigs, induced an almost complete recovery from PLC. We hypothesize that the refilling of cavitated xylem might be a result of an auxin-induced increase in the phloem loading with solutes. This would cause radial transport of solutes to cavitated xylem conduits via the rays, thus decreasing their osmotic potential and making them refill. No positive xylem pressure potentials were measured during xylem recovery from PLC. [ABSTRACT FROM AUTHOR]

Leaf extension was stimulated following exposure of three interamerican hybrid poplar clones (Populus trichocarpa × P. deltoides); 'Unal', 'Boelare', and 'Beaupre' and a euramerican clone 'Primo' (Populus nigra × P. deltoides) to elevated CO2 in controlled environment chambers. For all three interamerican clones the evidence suggests that this was the result of increased leaf cell expansion associated with enhanced cell wall extensibility (WEx), measured as tensiometric increases in cell wall plasticity. For the interamerican clone 'Boelare', there was also a significant increase in cell wall elasticity following exposure to elevated CO2 (P ≤ 0.001). The effect of elevated CO2 in stimulating cell wall extensibility was confirmed in a detailed spatial analysis of extensibility made across the lamina of expanding leaves of the clone 'Boelare. For two of the interamerican hybrids, 'Unal' and 'Beaupre', both leaf cell water potential (ψ) and turgor pressure (P) were lower in elevated than in ambient CO2. By contrast, no significant effects on the cell wall properties or leaf water relations for the euramerican hybrid 'Primo' were observed following exposure to elevated CO2, suggesting that the mechanism for increased leaf extension in elevated CO2 differed, depending on clone. The cumulative total length of leaves of 'Boelare' grown in elevated CO2 was significantly increased (P ≤ 0.05) and since leaf number was not significantly increased in any inter-american clone it is hypothesized that final leaf size was stimulated in elevated CO2 for these clones. By contrast, there was no significant effect of CO2 on cumulative total leaf length for the euramerican clone 'Primo', but leaf number was significantly increased by elevated CO2. [ABSTRACT FROM AUTHOR]

The effect of osmotic stress on the regulation of extension of roots of pine seedlings (Pinus pinaster Ait.) was studied. Extension rate (r) was measured as increase in root length, and as local growth rate (LGR) along the growing region. Turgor pressure (P) was measured over the same profile using the cell pressure probe. Correlation of LGR and P allowed changes in cell wall mechanical properties to be followed. In unstressed plants, differences in LGR along the root apex were owing to variations in the mechanical properties of the cell walls since P remained constant at 0.5 MPa along the first 10 mm of the root tip. Contrary to expectation, a moderate water stress of 0.15 MPa stimulated root growth despite a small reduction in P to 0.4 MPa. The walls of the expanding cells became looser, allowing enhanced growth in spite of reduced turgor. Examination of LGR showed that these changes occurred uniformly over the whole elongation zone. The advantage to the plant of this is discussed. Higher stresses (0.45 and 0.66 MPa) decreased cell turgor to a greater degree (&les; 0.3 MPa) and decreased growth rate. At the highest stress (0.66 MPa), P was maximal in cells that were still elongating, resulting in a longitudinal gradient in turgor. In both treatments LGR remained unchanged at the apex of the growing zone. Wall loosening appeared to have occurred in this region. Pine root extension is therefore modulated by changes in cell wall properties, with cell wall loosening resulting in increased growth at moderate stress. It appears that growth under higher stresses is limited by inability to maintain turgor. [ABSTRACT FROM AUTHOR]

The expansion of roots is considered at the level of the single cell. The water relations of cell expansion are discussed. Water entry, solute import and cell wall properties are considered as possible regulatory points. It is argued that root cell expansion can be understood in terms of cell turgor pressure and the physical properties of the cell wall, provided solute supply is not limiting. Various measurements of cell wall properties in roots are presented and the assumptions underlying their measurements are presented. It is concluded that cell wall properties must he measured over short time periods to prevent alterations in wall properties during the experiment. The radial location of the load-hearing layers is discussed and it is concluded that, unlike aerial tissue, growth is limited by the properties of the inner layer of the root cortex. Evidence is presented to show that cell wall properties can change both during development and following turgor perturbation. In general, however, turgor itself is tightly regulated, particularly towards the root tip. A number of environmental situations are presented in which root growth is altered. The mechanism of the alteration is discussed at the single cell level. These 'stresses' include osmotic stress, low temperature and soil compaction. In many cases the alteration of root growth is consistent with changes in the cell wall properties of the growing cells. Severe stress. resulting in near cessation of root cell extension, can result in a change (usually an increase) in turgor pressure. The change in turgor pressure of the cells in the growing zone is smaller than that which would be expected from a continuation of an unstressed solute import rate. This exemplifies both the change in cell wall properties and the tight turgor homeostasis of root tips. The biochemical processes which underlie the modulation of cell wall properties are presented as they are currently understood in roots. [ABSTRACT FROM AUTHOR]

Two cases studies are presented illustrating how the behaviour of plants using crassulacean acid metabolism (CAM) provides adaptation to salinity. Perennial cacti having constitutive CAM show adaptation at the whole-plant level, engaging regulation of stomata, internal CO[sub2]-recycling and root physiology with salt exclusion. They are stress avoiders. Annual plants such as Mesembryanthemum crystallinum, with inducible CAM, are salt includers. They are stress-tolerant and show reactions at an array of levels: (i) regulation of turgor and gas exchange at the whole-plant level; (ii) metabolic adjustments at the cellular level; (iii) adaptive transport proteins at the membrane levels and also (iv) at the macromolecular level; and (v) inductive changes at the gene expression level of the enzyme complement for metabolism (in particular involving glycolysis and malic-acid synthesis with phosphoenolpyruvate carboxylase (PEPC) as the key enzyme, and gluconeogenesis (with pyruvate phosphate dikinase (PPDK) as a key enzyme) and membrane transport (in particular involving the tonoplast ATPase). [ABSTRACT FROM AUTHOR]

The sequence of events leading to wound-healing and resumption of growth are studied by light microscopy including the use of differential interference contrast optics in Vaucheria longicaulis Hoppaugh var. macounii Blum. Wound-healing has been divided into four stages. In stage 1, a wound-healing vesicle is rapidly extruded from the wound site. Loss of turgor pressure, as observed by the appearance of plasmolysis, is seen along the length of the filament. In stage 2, the wound-healing vesicle is sealed-off from the filament. Organelles then accumulate at the barrier created by the sealing-off process. This is followed by chloroplast retraction from and return to the wound site, concurrent with the restoration of turgor pressure in late stage 2. In stage 3 chloroplasts again retract from and return to the wound site, in preparation for stage 4 that finally leads to the resumption of growth on the part of the healed filament. The effect of wound healing on organelle distribution has been studied with the use of selective fluorochromes. The distribution of nuclei, identified by DAPI staining, remains largely unchanged after wounding. Mitochondria and probable lipid bodies (designated RBIs) are identified by staining with DiOC6 and Rhodamine B, respectively. Their presence in the wound-healing vesicle cytoplasm and accumulation along the wound site points to the existence of an efficient mechanism of ATP synthesis in support of the energy requirements associated with the healing process. CTC fluorescence along the wound site and wound vesicle cytoplasm suggests that Ca2+-based exocytosis is important in both wound-healing vesicle expansion and its sealing-off from the filament. [ABSTRACT FROM AUTHOR]

Expansive growth of plant cells is controlled principally by processes that loosen the wall and enable it to expand irreversibly. The central role of wall relaxation for cell expansion is reviewed. The most common methods for assessing the extension properties of plant cell walls ('wall extensibility') are described, categorized and assessed critically. What emerges are three fundamentally different approaches which test growing cells for their ability (a) to enlarge at different values of turgor, (b) to induce wall relaxation, and (c) to deform elastically or plastically in response to an applied tensile force. Analogous methods with isolated walls are similarly reviewed. The results of these different assays are related to the nature of plant cell growth and pertinent biophysical theory. I argue that the 'extensibilities' measured by these assays are fundamentally different from one another and that some are more pertinent to growth than others. [ABSTRACT FROM AUTHOR]

Barley [Hordeum distichum (L.) Lam.] seedlings were infected with urediniospores of rust, Puccinia hordei Otth., 10 d after germination and the Zimmermann micro-pressure probe was used to measure the turgor potential of individual epidermal and palisade cells, during disease development. Turgor potential was measured as a function of time and as a function of distance from the centre of the rust colony. When the centre of a rust colony was within 300 μm of probed cells, palisade cell turgor declined significantly on days 6, 7 and 9 after infection in comparison to control leaf palisade cells. Control leaf palisade cell turgor remained constant with time. On day 4 after infection, epidermal cell turgor of control and infected leaves did not differ. However, by day 6 when the uredinium was visible and approaching sporulation, epidermal cell turgor of infected leaves declined. The reduction was dependent upon the distance from the centre of the colony, thereby creating a gradient of turgor such that the lowest turgors were recorded at the centre of the colony and the highest at the colony boundary. By day 9 after infection, when `green island' development had begun, this gradient was reversed. However, turgors of all epidermal cells in colony areas were significantly less than those in control leaves. These results are discussed in relation to the impact of rust infection upon leaf water relations in barley. [ABSTRACT FROM AUTHOR]

Water tracers have been used to demonstrate the continued functioning of the anther filament during dehiscence in Lycopersicon esculentum Mill. Since Lycopersicon possesses neither nectar nor nectaries dehiscence cannot be related to sugar secretion, as has been demonstrated for other plants. The anthers seem structurally adapted for water conservation and the only transpirational loss of the flower appears to be through the petals. Transpiration from the anthers themselves thus appears not to be involved in regulating dehiscence. Anther opening is preceded by dehydration of the locule and circumstantial evidence indicates it to be an active process, with water being exported through the filaments to the petals along an osmotic gradient generated by starch/sugar interconversion. Measurement of water potential differentials between the petals and anthers points to the mechanism driving this export of water from the anthers. Interestingly, turgor pressure is maintained in the majority of the anther tissues until senescence, remaining remarkably constant against a background of dramatic changes in osmotic potential. These data, combined with the observation that the hydration level of the anthers falls only to 70% at senescence, indicates that dehiscence cannot primarily be a desiccatory process. Some domains within the anther do desiccate, but these are strictly localized. Thus, while hydration levels of living tissues remain independent of the environment, those observed to undergo protoplast degeneration and wall collapse exhibit environmentally-linked hygroscopic absorption. Dehiscence thus emerges as an orchestrated programme of structural and physiological events leading to the desiccation of specific domains of the anthers. These findings are discussed in terms of current models for anther dehiscence in flowering plants. [ABSTRACT FROM AUTHOR]

Leaf growth rate and the turgor pressure of cells in the expanding zone of leaves of Lolium temulentum were measured simultaneously. Growth rate was reduced over a range from 30 μm min to zero by reducing the temperature of the expanding zone from 20 to 2 °C. Turgor pressure remained constant at 0.5 MPa. This implies that growth reduction by low temperature is due to changes in cell wall rheology. Cell membrane hydraulic conductivity (LP) was reduced with temperature as expected, but this was not sufficient to influence growth rate detectably. [ABSTRACT FROM AUTHOR]

Leaf growth of first year seedlings of silver birch (Betula pendula Roth,) and sycamore (Acer pseudoplatanus L.) was measured continuously using linear variable transducers (LVTDs) when shoots were enclosed in small Perspex chambers, in which temperature, vapour pressure deficit (VPD) and photon fluence rate were all controlled. A new method was used to measure the yield turgor (Y) of growing leaves. The VPD of air in the chamber was altered so that transpiration rate was changed. Turgor pressures, measured using thermocouple psychrometers, and growth rates varied as a result of this treatment, and the relationship between leaf growth and turgor pressure (P) and the turgor pressure at which growth ceased (F) could he measured within a very short time period using well-watered plants. The value of Y was constant for illuminated and darkened leaves of both species, but Y was higher for sycamore leaves (Y = 0.250 MPa) compared with the value for birch leaves (Y = 0.071 MPa). Cell wall extensibility (m, MPa[sup-1]h[sup-1]) was calculated from the slope of the line relating turgor pressure to growth. For leaves of birch, there was a significant increase in pu when leaves were illuminated (m, dark = 1.3 x 10[sup-2] MPa[sup-1]h[sup-1]; m, light = 4.6 x 10[sup-2] MPa[sup-1]), similar in magnitude to the increase in wall extensibility measured previously by mechanical analysis of killed leaf tissue (WEX, the plasticity of tissue subjected to a uni-axial stress). No change in was detected when darkened sycamore leaves were illuminated (mn, dark = 4.2 x l0[sup-2] MPa[sup-1] h[sup-1]; m, light = 4.4 x 10[sup-2] MPa[sup-1] h[sup-1]). The results confirm earlier work on the factors controlling the diurnal patterns of leaf growth of these two species and suggest that leaf growth of birch is primarily limited by cell wall loosening hut that the growth of sycamore leaves may be limited by leaf turgor. [ABSTRACT FROM AUTHOR]

Effective inhibition of high temperature-induced leakage of betacyanin from beet root discs occurs at concentrations of about 100 mM K[SUP+], Na[SUP+] or choline[SUP+], about 10 mM inorganic divalent cations in which Mn[SUP2+] ≥ Ca[SUP2+] ≥ Sr[SUP2+] > Ba[SUP2+] > Mg[SUP2+], and about 1 mM, Ln[SUP3+]. Thermal protection is also afforded by a range of sugars and sugar alcohols suggesting that the action of monovalent cation salts, in particular, is at least partially due to an osmotic lowering of cell turgor pressure. Inorganic divalent cation specificity, as well as the ineffectiveness of the organic cations, decamethonium[SUP2+] and tris(ethylendiamine)cobaltic III, suggest that screening of membrane surface charge is not involved in decreasing betacyanin leakage. It is suggested that Ca[SUP2+], and other inorganic multivalent cations, act by binding to membrane components which may produce a decrease in membrane fluidity. [ABSTRACT FROM AUTHOR]

When mycelium of Dendryphiella salina Nicot & Pugh is exposed for 5 h to media containing various concentrations of the very slowly metabolized sugar, L-sorbose, the total ethanol-soluble carbohydrate content increases and there is also a stimulated synthesis of mannitol. Provided L-sorbose is absorbed, the presence of glucose, fructose, mannitol or arabitol in the medium does not prevent these changes which are different from those brought about by the non-metabolizable sugar, 3-O-methyl glucose. It is suggested that the apparent regulation of mycelial ethanol-soluble carbohydrate concentration following uptake of the latter sugar cannot be controlled primarily by turgor. [ABSTRACT FROM AUTHOR]

Field measurements of selected upland and lowland species have demonstrated a decline in specific leaf area with altitude, probably due to changes in both air temperature and wind-speed. Both field and controlled environment experiments have demonstrated that the upland species have lower specific leaf areas than the lowland species, under similar environmental conditions. Leaf growth and turgor of the upland species, Phleum alpinum, are insensitive to changes in wind-speed in the range of 0.2 to 3 m s[SUP-1], while both growth and turgor decline with wind-speed for the lowland P. pratense. Leaf turgor of P. pratense declined with increasing specific leaf area but no such relationship was found for P. alpinum. [ABSTRACT FROM AUTHOR]

The possible role in drought resistance played by leaf water potential, osmotic potential, weight-averaged turgor pressure and bulk modulus of elasticity is studied in mature and young leaves of Senecio candidus and S. ambiguus. Small water losses cause relatively large reductions in leaf water potential in S. candidus without greatly altering its osmotic potential. This occurs because S. candidus develops thicker leaf cell walls with high values of leaf bulk elastic modulus. Recovery from water stress is very rapid in this species. Leaf growth is influenced by the structural pattern of the cell walls in that S. candidus leaves complete their expansion within a shorter period of time than those of S. ambiguus. The possible adaptive significance of changing the leaf bulk modulus of elasticity in mature and young leaves of the two species is discussed. [ABSTRACT FROM AUTHOR]