Effects of Dehydration and Rehydration on Photosynthesis of Detached Leaves of the Resurrective Plant Boea hygrometrica

Fluorescence induction kinetics was used to investigate the effects of dehydration and rehydration on photosynthesis of detached leaves of the desiccation-tolerant, resurrective plant Boea hygrometrica (Bunge) R. Br. In comparison with the desiccation-intolerant plant Chirita heterotricha Merr., the PSⅡphotochemical activity of Boea hygrometrica was characterized by a faster decline during dehydration and a much higher capacity of recovery during rehydration. By means of native PAGE, it was further shown that the thylakoid pigment-protein complexes of Boea hygrometrica were highly stable during dehydration and rehydration. These features may contribute to the extreme desiccation resistance of photosynthesis apparatus of resurrective plant Boea hygrometrica.     

Differences in physiological adaptation of <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> Friv. leaves and roots during dehydration–rehydration cycle

The ecophysiological responses of the homoiochlorophyllous desiccation-tolerant (HDT) plant Haberlea rhodopensis showed that this plant could tolerate water deficit and both leaves and roots had high ability to survive severe desiccation. The changes and correlation between CO₂ assimilation, stomatal conductance, contents of photosynthetic pigments, root respiration and specific leaf area during dehydration–rehydration cycle were investigated. The physiological activity of leaves and roots were examined in fully hydrated (control) plants and during 72 h of dehydration, as well as following 96 h of rehydration every 6 and 24 h. After 6 h of dehydration, the stomatal conductance declined and the intercellular CO₂ concentration increased. The reduction in CO₂ assimilation rate was observed after 54 h of dehydration. There was a good correlation between the root respiration and water content. Our results showed that the plasticity of adaptation in leaves and roots were different during extreme water conditions. Roots were more sensitive and reacted faster to water stress than leaves, but their activity rapidly recovered due to immediate and efficient utilization of periodic water supply.     

Comparison of sucrose metabolism during the rehydration of desiccation-tolerant and desiccation-sensitve leaf material of Sporobolus stapfianus

Sucrose accumulated during dehydration is a major potential energy source for metabolic activity during rehydration. The objective of the present study was to investigate aspects of leaf sucrose metabolism during the rehydration of desiccation-tolerant Sporobolus stapfianus Gandoger (Poaceae) over a 10-day period. Comparison was then made to sucrose metabolism during the rehydration of both desiccation-tolerant excised leaf material (dehydrated attached to the parent plant) and desiccation-sensitive leaf material (dehydrated detached from the parent plant to prevent the induction of tolerance) over a 48-h period. The pattern of sugar mobilization and glycolytic enzyme activity during the rehydration of the desiccation-tolerant excised leaves was similar to that in leaves attached to the parent plants. Significant breakdown of sucrose was not apparent in the initial phase of rehydration, suggesting the utilization of alternate substrates for respiratory activity. The desiccation-tolerant excised tissues provided a suitable control to compare the metabolism of rehydrating desiccation-sensitive material. In contrast to the tolerant tissues, sucrose breakdown in the sensitive leaves commenced immediately after watering and the accumulation in hexose sugars was inversely proportionate to the decrease in sucrose content. Hexokinase (EC 2.7.1.1), PFK (ATP phosphofructokinase, EC 2.1.7.11), aldolase (EC 4.1.2.13), enolase (EC 4.2.1.11), and PK (pyruvate kinase, EC 2.7.1.40) activity levels were significantly lower in the desiccation-sensitive material during rehydration.     

Glycerolipid analysis during desiccation and recovery of the resurrection plant Xerophyta humilis (Bak) Dur and Schinz

Xerophyta humilis is a poikilochlorophyllous monocot resurrection plant used as a model to study vegetative desiccation tolerance. Dehydration imposes tension and ultimate loss of integrity of membranes in desiccation sensitive species. We investigated the predominant molecular species of glycerolipids present in root and leaf tissues, using multiple reaction monitoring mass spectrometry, and then analysed changes therein during dehydration and subsequent rehydration of whole plants. The presence of fatty acids with long carbon chains and with odd numbers of carbons were detected and confirmed by gas chromatography. Dehydration of both leaves and roots resulted in an increase in species containing polyunsaturated fatty acids and a decrease in disaturated species. Upon rehydration, lipid saturation was reversed, with this being initiated immediately upon watering in roots but only 12–24 hr later in leaves. Relative levels of species with short‐chained odd‐numbered saturated fatty acids decreased during dehydration and increased during rehydration, whereas the reverse trend was observed for long‐chained fatty acids. X. humilis has a unique lipid composition, this report being one of the few to demonstrate the presence of odd‐numbered fatty acids in plant phosphoglycerolipids.     

Desiccation-tolerant plants in dry environments

The majority of terrestrial plants are unable to survive in very dry environments. However, a small group of plants, called ‘resurrection’ plants, are extremely desiccation-tolerant and are capable of losing more than 90% of the cellular water in vegetative tissues. Resurrection plants can remain dried in an anabiotic state for several years and, upon rehydration, are able to resume normal growth and metabolism within 24 h. Vegetative desiccation tolerance is thought to have evolved independently several times within the plant kingdom from mechanisms that allow reproductive organs to survive air-dryness. Resurrection plants synthesise a range of compounds, either constitutively or in response to dehydration, that protect various components of the cell wall from damage during desiccation and/or rehydration. These include sugars and late embryogenesis abundant (LEA) proteins that are thought to act as osmoprotectants, and free radical-scavenging enzymes that limit the oxidative damage during dehydration. Changes in the cell wall composition during drying reduce the mechanical damage caused by the loss of water and the subsequent shrinking of the vacuole. These include an increase in expansin or cell wall-loosening activity during desiccation that enhances wall flexibility and promotes folding.     

Effects of exogenous abscisic acid on leaf carbohydrate metabolism during cucumber seedling dehydration

Cucumber (Cucumis sativus L.) seeds were pretreated with exogenous abscisic acid (ABA) prior to germination. After germination, seedlings with three leaves were exposed to gradual dehydration. The effects of ABA on photosynthetic rate (Pn), daily water loss (WL) and water utilization efficiency (WUE) during dehydration were investigated, in addition to the variation of carbohydrates in leaves. ABA improved the Pn, WL and WUE of cucumber seedlings during dehydration. After rehydration, the seedlings pretreated with ABA showed a higher recovery in Pn, WL and WUE, as compared to those without an ABA pretreatment. Subsequent to dehydration, concentration of stachyose, raffinose, sucrose, glucose, and fructose increased in seedlings pretreated with ABA. Dehydration altered the proportions of the sugars in the total carbohydrates, and accelerated the accumulation of stachyose, raffinose and sucrose. After rehydration, carbohydrate concentrations of seedlings pretreated with ABA recovered to levels observed prior to dehydration. These results demonstrated that pretreatment of seeds with exogenous ABA enhanced carbohydrate tolerance to dehydration of cucumber seedlings.     

Sequence of occurring damages in yeast plasma membrane during dehydration and rehydration: mechanisms of cell death

Yeasts are often exposed to variations in osmotic pressure in their natural environments or in their substrates when used in fermentation industries. Such changes may lead to cell death or activity loss. Although the involvement of the plasma membrane is strongly suspected, the mechanism remains unclear. Here, the integrity and functionality of the yeast plasma membrane at different levels of dehydration and rehydration during an osmotic treatment were assessed using various fluorescent dyes. Flow cytometry and confocal microscopy of cells stained with oxonol, propidium iodide, and lucifer yellow were used to study changes in membrane polarization, permeabilization, and endocytosis, respectively. Cell volume contraction, reversible depolarization, permeabilization, and endovesicle formation were successively observed with increasing levels of osmotic pressure during dehydration. The maximum survival rate was also detected at a specific rehydration level, of 20 MPa, above which cells were strongly permeabilized. Thus, we show that the two steps of an osmotic treatment, dehydration and rehydration, are both involved in the induction of cell death. Permeabilization of the plasma membranes is the critical event related to cell death. It may result from lipidic phase transitions in the membrane and from variations in the area-to-volume ratio during the osmotic treatment.     

Sequence of occurring damages in yeast plasma membrane during dehydration and rehydration: Mechanisms of cell death

Yeasts are often exposed to variations in osmotic pressure in their natural environments or in their substrates when used in fermentation industries. Such changes may lead to cell death or activity loss. Although the involvement of the plasma membrane is strongly suspected, the mechanism remains unclear. Here, the integrity and functionality of the yeast plasma membrane at different levels of dehydration and rehydration during an osmotic treatment were assessed using various fluorescent dyes. Flow cytometry and confocal microscopy of cells stained with oxonol, propidium iodide, and lucifer yellow were used to study changes in membrane polarization, permeabilization, and endocytosis, respectively. Cell volume contraction, reversible depolarization, permeabilization, and endovesicle formation were successively observed with increasing levels of osmotic pressure during dehydration. The maximum survival rate was also detected at a specific rehydration level, of 20 MPa, above which cells were strongly permeabilized. Thus, we show that the two steps of an osmotic treatment, dehydration and rehydration, are both involved in the induction of cell death. Permeabilization of the plasma membranes is the critical event related to cell death. It may result from lipidic phase transitions in the membrane and from variations in the area-to-volume ratio during the osmotic treatment.     

Composition and desiccation-induced alterations of the cell wall in the resurrection plant Craterostigma wilmsii

Resurrection plants have the unique capacity to revive from an air-dried state. In order to tolerate desiccation they have to overcome a number of stresses, mechanical stress being one. In leaves of the Craterostigma species, an extensive shrinkage occurs during drying as well as a considerable cell wall folding. Our previous microscopically analysis using immunocytochemistry on the resurrection plant Craterostigma wilmsii , has shown an increase in labelling of xyloglucan and unesterified pectins in the cell wall during drying. In this study, we have undertaken a biochemical approach to separate, quantify and characterize major cell wall polysaccharides in fully hydrated and dry leaves of C. wilmsii . Our results show that the overall cell wall composition of C. wilmsii leaves was similar to that of other dicotyledonous plants with respect to the pectin content. However, the structure of the hemicellulosic polysaccharide xyloglucan was characterized to be XXGG-type. The data also demonstrate marked changes in the hemicellulosic wall fraction from dry plants compared to hydrated ones. The most conspicuous change was a decrease in glucose content in the hemicellulosic fraction of dry plants. In addition, xyloglucan from the cell wall of dry leaves was relatively more substituted with galactose than in hydrated walls. Together these findings show that dehydration induces significant alteration of polysaccharide content and structure in the cell wall of C. wilmsii , which in turn might be involved in the modulation of the mechanical properties of the wall during dehydration.     

Achievement of rapid osmotic dehydration at specific temperatures could maintain high <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> viability

Various methods have been tried to prevent cell mortality during dehydration, but the reasons why microorganisms die when submitted to dehydration and rehydration are not well understood. The aim of this study was to further investigate the reasons for yeast mortality during dehydration. Osmotic dehydration and rehydration of Saccharomyces cerevisiae W303-1A were performed at different temperatures. Two different approaches were used: isothermic treatments (dehydration and rehydration at the same temperature), and cyclic treatments (dehydration at an experimental temperature and rehydration at 25 degrees C), with significant differences in viability found between the different treatments. Dehydration at lower and higher temperatures gave higher viability results. These experiments allowed us to propose a hypothesis that relates mortality to a high water flow through an unstable membrane during phase transition.     

Reversible cellular and metabolic changes induced by dehydration in desiccation-tolerant wheat seedling shoots

Wheat seedlings obtained after 2 or 3 days of seed germination in darkness at 20°C (i.e. with a 0.5–0.7 cm long coleoptile) were still viable after drying in darkness in ambient conditions which reduced the shoot moisture content to about 0.30 g H₂O g^?1 dry mass (DM). Coleoptile and primary leaf growth resumed upon rehydration, but primary roots died and new roots regenerated. In the present work we have investigated whether desiccation tolerance of the shoot (coleoptile and primary leaf combined) was related to some reversible cellular or metabolic changes induced by dehydration. Non‐dehydrated shoots were high in moisture content (4.0–5.0 g H₂O g^?1 DM) and exhibited an active metabolism as indicated by a high energy charge (EC = 0.85) and cells with well developed mitochondria, endoplasmic reticulum, polysomes and Golgi bodies. Dehydration induced changes in cell membrane properties since it reduced in vivo capacity of the shoot to convert 1‐aminocyclopropane 1‐carboxylic acid (ACC) to ethylene (i.e. ACC oxidase activity). This effect was already observed at 4–5 h of dehydration, namely when shoot moisture content dropped down below about 3.0 g H₂O g^?1 DM, and ACC‐dependent ethylene production became almost nil when shoot moisture content reached 1.0 g H₂O g^?1 DM. Dehydration also resulted in decreases in ATP and non‐adenylic triphosphate nucleotide (NTP) contents down to 1–2% of their initial values, and in EC value to 0.20. Concomitant with water loss, sucrose content of the shoot increased and was maximal (about 330 mg g^?1 DM, namely three‐fold that of non‐dehydrated organs) after 2 days of drying. Upon rehydration, shoots regained their original moisture content within 3 days, during which they progressively recovered apparent normal metabolism. Reversal of extensive dehydration‐associated cell wall folding occurred between 2 and 3 days of rehydration, when the ultrastructure of coleoptile and primary leaf cells also provided evidence of intensive autophagic activity, indicative of the removal of damaged cell components. Concomitantly, apparently undamaged organelles and endomembranes persisted in the cytoplasm. Restoration of 60–70% of ACC oxidase activity and 80–90% of EC value occurred within 48 and 18 h, respectively. However, the values of the ATP/ADP and NTP/ATP ratios remained lower than in control non‐dehydrated shoots, indicating that not all metabolic deterioration induced by dehydration was completely repaired. Differences in relationships between shoot moisture content and ACC‐oxidase activity or energy metabolism during dehydration and upon rehydration, and cell ultrastruture analyses suggest that desiccation tolerance of wheat seedling shoot is related to mechanisms involved in the maintenance of cell structure during water loss and the cell capacity to repair the dehydration damage.     

Histochemistry, ultrastructure and possible significance of dead parenchyma cells with specialized walls in the leaf and rhizome of Sansevieria

Abstract. The internal parenchyma of the leaf and rhizome in 36 species of Sansevieria is made of dead cells and living cells arranged in a regular pattern. Intercellular spaces are lacking. The walls of dead cells consist of an inner amorphous layer positive to the fluorescence test for callose, a middle suberin-like layer and an outer fibrillar layer. In about half of the species examined, the inner layer forms distinctive thickenings. Detached leaves of Sansevieria lose water very slowly, and are able to recover it quickly. The pattern of leaf dehydration appears to be related to leaf morphology, whereas no relation is evident between the pattern of leaf rehydration and leaf morphology. Neither leaf dehydration nor leaf rehydration pattern is affected by the presence of wall thickenings in the dead parenchyma cells. The fresh weight per unit volume of both turgid and droughted leaves is nearly 1, denoting that the dead cells are filled with water and do not undergo substantial cavitation during drought. The data indicate that the dead parenchyma cells of Sansevieria are a specialized water-storing system.     

Protein dynamics in thylakoids of the desiccation-tolerant plant Boea hygroscopica during dehydration and rehydration

Plants of Boea hygroscopica F. Muell were dehydrated to 9% relative water content (RWC) by withholding water for 26 d, and afterward the plants were rehydrated. Leaves were taken from control plants after 7, 12, and 26 d from the beginning of dehydration, and after 6 and 48 h from rehydration. The RWC decreased by 80% during dehydration, but the leaves regained RWC with rehydration. Dehydrated plants showed lesser amounts of proteins, lipids, and chlorophyll, all of which increased following rewatering. The lipid-to-protein ratio, which decreased during dehydration, returned to control level after 48 h of rehydration. Thylakoid lipids were more unsaturated when RWC reached the value of 9%. EPR measurements of spin-labeled proteins showed the presence of three different groups of proteins with different mobility in thylakoid membranes. The rotational correlation time of groups 1 and 2 increased with dehydration and decreased upon rehydration, whereas group 3 showed little changes. Desiccation did not cause thylakoid swelling or breakage, but the membrane system assemblage showed changes in thylakoid stacking. After 48 h of rehydration the membrane system recovered completely the organization of the fully hydrated state, showing several well-defined and regularly distributed grana.     

Physiological and Biochemical Responses of Yarrowia lipolytica to Dehydration Induced by Air-Drying and Freezing

Organisms that can withstand anhydrobiosis possess the unique ability to temporarily and reversibly suspend their metabolism for the periods when they live in a dehydrated state. However, the mechanisms underlying the cell’s ability to tolerate dehydration are far from being fully understood. The objective of this study was to highlight, for the first time, the cellular damage to Yarrowia lipolytica as a result of dehydration induced by drying/rehydration and freezing/thawing. Cellular response was evaluated through cell cultivability determined by plate counts, esterase activity and membrane integrity assessed by flow cytometry, and the biochemical composition of cells as determined by FT-IR spectroscopy. The effects of the harvesting time (in the log or stationary phase) and of the addition of a protective molecule, trehalose, were investigated. All freshly harvested cells exhibited esterase activity and no alteration of membrane integrity. Cells freshly harvested in the stationary phase presented spectral contributions suggesting lower nucleic acid content and thicker cell walls, as well as longer lipid chains than cells harvested in the log phase. Moreover, it was found that drying/rehydration induced cell plasma membrane permeabilization, loss of esterase activity with concomitant protein denaturation, wall damage and oxidation of nucleic acids. Plasma membrane permeabilization and loss of esterase activity could be reduced by harvesting in the stationary phase and/or with trehalose addition. Protein denaturation and wall damage could be reduced by harvesting in the stationary phase. In addition, it was shown that measurements of loss of membrane integrity and preservation of esterase activity were suitable indicators of loss and preservation of cultivability, respectively. Conversely, no clear effect of freezing/thawing could be observed, probably because of the favorable operating conditions applied. These results give insights into Y. lipolytica mechanisms of cellular response to dehydration and provide a basis to better understand its ability to tolerate anhydrobiosis.     

Protection mechanisms against excess light in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa

Mechanisms of avoidance and protection against light damage were studied in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa.In C. wilmsii, a combination of both physical and chemical changes appeared to afford protection against free radical damage. During dehydration leaves curled inwards, and the abaxial surface became exposed to light. The tissue became purple/brown in colour, this coinciding with a three-fold increase in anthocyanin content and a 30% decline in chlorophyll content. Thus light-chlorophyll interactions are progressively reduced as chlorophyll became masked by anthocyanins in abaxial layers and shaded in the adaxial layers. Ascorbate peroxidase (AP) activity increased during this process but declined when the leaf was desiccated (5% RWC). During rehydration leaves uncurled and the potential for normal light-chlorophyll interaction was possible before full hydration had occurred. Superoxide dismutase (SOD) and glutathione reductase (GR) activities increased markedly during this stage, possibly affording free radical protection until full hydration and metabolic recovery had occurred.In contrast, the leaves of X. viscosa did not curl, but light-chlorophyll interactions were minimised by the loss of chlorophyll and dismantling of thylakoid membranes. During dehydration, free radical protection was afforded by a four-fold increase in anthocyanin content and increased activities of AP, GR and SOD. These declined during rehydration. It is suggested that potential free radical damage may be avoided by the persistence of anthocyanins during the period of thylakoid membrane re-assembly and full chlorophyll restitution which only occurred once the leaves were fully rehydrated.     

两种不同生境苦苣苔科植物的复苏特性及其对水分的光合和生理响应

复苏植物可以耐受极度干旱的环境,脱水至10％相对水分含量后仍然可以复苏.苦苣苔科植物包含有较多复苏植物,不同类群的复苏机理可能存在差异.该文选择分布在亚热带和温带石灰岩地区的锈色蛛毛苣苔(Paraboea rufescens)和心叶马铃苣苔(Oreocharis cordatula)两种苦苣苔科植物,并对这两个物种的叶...     

Effects of dehydration and rehydration on the intravascular space in horses

• 1.1. The resistance of sub-tropical horses, and desert-dwelling horses to 72 hr dehydration/24 hr rehydration was investigated via changes in red cell parameters and plasma protein concentration.
• 2.2. Red cell count, haemoglobin and haematocrit increased up to 48 hr dehydration. Between 48 and 72 hr dehydration these parameters decreased, implying a fluid shift onto the intravascular space from the interstitium/hindgut. Most parameters had regained baseline values by 24 hr rehydration.
• 3.3. Mean cell volume, mean cell haemoglobin, mean cell haemoglobin concentration and total plasma protein were not significantly different between breeds at, or between most stages of hydration.
• 4.4. Protection of plasma volume during dehydration/rehydration was aided by maintaining intravascular protein (especially albumin) levels. Red cells were transiently dehydrated and overhydrated but resisted osmolysis.