Extent Of Dehydration Research Articles

Denaturation Temperatures of Dentin Matrices. I. Effect of Demineralization and Dehydration

The denaturation temperature (T d) of dentin collagen in mineralized versus demineralized teeth was examined as a function of dentin age and the extent of dehydration. Using differential scanning calorimetry, T d of mineralized dentin was shown to be between 160°C to 186°C, depending on whether it was from young or old dentin that was hydrated or dehydrated, respectively. Demineralized dentin exhibited a T d of 65.6°C that increased with dehydration to 176°C. The presence of apatite crystallites or interpeptide bonding increased the T d of demineralized matrices. Interpeptide hydrogen bonding seems to stabilize collagen to thermal challenge. Water breaks interpeptide hydrogen bonds making collagen more susceptible to thermal denaturation. Rises in intracanal temperature are unlikely to cause extensive denaturation of mineralized root dentin walls. However, hydrated or partially dehydrated root canal walls that have been partially demineralized with chelating agents or mild acids may be susceptible to thermal denaturation.

Microphysical modeling of southern polar dehydration during the 1998 winter and comparison with POAM III observations

Stratospheric dehydration and high aerosol extinctions are examined for the 1998 Antarctic winter using the Integrated Microphysics and Aerosol Chemistry on Trajectories (IMPACT) model and data obtained by the Polar Ozone and Aerosol Measurement (POAM) III instrument. The model is applied to individual air parcels which are advected along 3‐D trajectories using the United Kingdom Meteorological Office (UKMO) global wind and temperature fields. Model results are compared to water vapor and aerosol extinction measurements obtained with the POAM instrument. Results suggest that the water vapor mixing ratio at the end of the season is predicted with reasonable accuracy. However, dehydration occurs more rapidly in the simulation than is indicated by the POAM data. In addition to dehydration results, the frequency of high aerosol extinction measurements is examined for all model runs and compared to POAM data. The aerosol extinction comparisons are consistent with the assumption that heterogeneous nitric acid trihydrate (NAT) freezing occurs in approximately 1% of all particles. Various model parameters influencing ice cloud microphysics are altered to examine their effects on both the water vapor mixing ratio and high aerosol extinction events. While a reduction in the ice accommodation coefficient and an increase in the ice nucleation barrier both improve the agreement in the water vapor mixing ratio, the agreement in aerosol extinction is worsened. Extinction comparisons suggest that the model results are consistent with either high or low NAT‐ice lattice compatibility factors, although intermediate values agree poorly with POAM data. The extent of dehydration is highly dependent on temperature; therefore, an uncertainty as small as ±1 K in the UKMO temperature fields may significantly change the model results.

Plant vitrification solution 2 lowers water content and alters freezing behavior in shoot tips during cryoprotection

Plant shoot tips do not survive exposure to liquid nitrogen temperatures without cryoprotective treatments. Some cryoprotectant solutions, such as plant vitrification solution 2 (PVS2), dehydrate cells and decrease lethal ice formation, but the extent of dehydration and the effect on water freezing properties are not known. We examined the effect of a PVS2 cryoprotection protocol on the water content and phase behavior of mint and garlic shoot tips using differential scanning calorimetry. The temperature and enthalpy of water melting transitions in unprotected and recovering shoot tips were comparable to dilute aqueous solutions. Exposure to PVS2 changed the behavior of water in shoot tips: enthalpy of melting transitions decreased to about 40 J g H 2O −1 (compared to 333 J g H 2O −1 for pure H 2O), amount of unfrozen water increased to ∼ 0.7 g H 2O g dry mass −1 (compared to ∼0.4 g H 2O g dry mass −1 for unprotected shoot tips), and a glass transition ( T g) at −115 °C was apparent. Evaporative drying at room temperature was slower in PVS2-treated shoot tips compared to shoot tips receiving no cryoprotection treatments. We quantified the extent that ethylene glycol and dimethyl sulfoxide components permeate into shoot tips and replace some of the water. Since T g in PVS2-treated shoot tips occurs at −115 °C, mechanisms other than glass formation prevent freezing at temperatures between 0 and −115 °C. Protection is likely a result of controlled dehydration or altered thermal properties of intracellular water. A comparison of thermodynamic measurements for cryoprotection solutions in diverse plant systems will identify efficacy among cryopreservation protocols.

Hydration energies of Thallium(I) halides—a novel simulation methodology

A simple simulation methodology to estimate the hydration energies and surface potentials of thallium halides is proposed, incorporating random distribution of molecules, nearest neighbor distances and hydration numbers. The extent of dehydration during each step and the corresponding variation in the hydration numbers are evaluated, assuming the validity of hard spheres. The correlation of the computed hydration energies with molecular sizes, lattice energies, crystal structures and feasibility of complexation reactions is discussed.

Effects of Exercise in the Heat on Thermoregulation of Japanese and Malaysian Males

The effect of low-intensity exercise in the heat on thermoregulation and certain biochemical changes in temperate and tropical subjects under poorly and well-hydrated states was examined. Two VO2max matched groups of subjects consisting of 8 Japanese (JS) and 8 Malaysians (MS) participated in this study under two conditions: poorly-hydrated (no water was given) and well-hydrated (3 mL x Kg(-1) body weight of water was provided at onset of exercise, and the 15th, 35th and 55th min of exercise). The experimental room in both countries was adjusted to a constant level (Ta: 31.6+/-0.03 degrees C, rh: 72.3+/-0.13%). Subjects spent an initial 10 min rest, 60 min of cycling at 40% VO2max and then 40 min recovery in the experimental room. Rectal temperatures (Tre) skin temperatures (Tsk), heart rate (HR), heat-activated sweat glands density (HASG), local sweat rate (M sw-back) and percent dehydration were recorded during the test. Blood samples were analysed for plasma glucose and lactate levels.The extent of dehydration was significantly higher in the combined groups of JS (1.43+/-0.08%) compared to MS (1.15+/-0.05%). During exercise M sw-back was significantly higher in JS compared to MS in the well-hydrated condition. The HASG was significantly more in JS compared to MS at rest and recovery. Tre was higher in MS during the test. Tsk was significantly higher starting at the 5th min of exercise until the end of the recovery period in MS compared to JS. In conclusion, tropical natives have lower M sw-back associated with higher Tsk and Tre during the rest, exercise and recovery periods. However, temperate natives have higher M sw-back and lower Tsk and Tre during experiments in a hot environment. This phenomenon occurs in both poorly-hydrated and well-hydrated states with low intensity exercise. The differences in M sw-back, Tsk and Tre are probably due to a setting of the core temperature at a higher level and enhancement of dry heat loss, which occurred during passive heat exposure.

A simple simulation methodology for estimation of dehydration energies and surface potentials of concentrated NaCl solutions.

A novel simulation procedure for estimating the dehydration energies of NaCl solutions spanning a wide range of concentrations, which incorporates ionic and molecular sizes, ion-pair formation, etc., is proposed on geometric and phenomenological considerations. The extent of dehydration during each movement of the hydrated molecule is evaluated using the expected and actual displacement of the species and mean nearest-neighbor distances. The interdependence between the size of the simulation box, number of molecules, and electrolyte concentration is pointed out.

Study of Type-A Zeolites. Part 2: Effect of Dehydration on the Effective Aperture Dimension

A new insight into the role played by water molecules in the crystalline framework of type-A zeolites is demonstrated. The effect of dehydration on the effective free aperture dimension, D f , is studied by utilizing temperature-programmed decapsulation of He and Ne. The interplay between the various known mechanisms governing D f is directly sensed by the decapsulation behavior of differently sized inert atoms. The results are qualitatively interpreted using pure dimensional considerations, revealing the occurrence of a strict relation between the content of water molecules and the redistribution of zeolitic cations in determining D f . Water content is shown to have a strong effect on the blocking state of the O 8 and O 6 zeolitic windows, which in turn, governs gas accessibility to the a and β cages. It is proposed that D f is regulated via a subtle balance between two coupled principal mechanisms. One involves direct lattice adjustments, where two opposite sub-mechanisms seem to play competitive roles. Removal of water molecules by simultaneous heating and pumping enlarges D f , while hydroxyl groups elimination from within the zeolitic channels results in their partial collapse, thus reducing D f . The second mechanism involves the regulation of aperture blocking via relocations of the counterions initiated by increasing vacancies due to removal of water molecules. While dehydration continuously contracts the O 6 apertures, a two-stage effect is observed for the wider O 8 windows. Upon dehydration at large water contents, O 8 apertures are reduced; then, beyond a critical extent of dehydration, the net effect is reversed, widening the O 8 windows.

Effects of sugars on the kinetics of drying and on the survival of partially dehydrated larvae of Anopheles mosquitoes

The ability to cryopreserve a stage of Anopheles mosquitoes would facilitate the development of strains incapable of transmitting malaria. Cryopreservation requires that the freezable water in cell systems be removed or rendered incapable of undergoing ice formation. The present study was concerned with the rate at which water is removed from lst instar larvae of Anopheles gambiae by air-drying, with the extent of dehydration that the larvae will tolerate, and with the effect of trehalose and sucrose on both drying kinetics and survival. Eighty-one percent of the larvae are water. Air-drying removes 90% of that water in ~20 min. Survivals after partial dehydration are highest if the larvae are rehydrated in 1/2× isotonic saline (0.13 osm); they are poorest if rehydrated in water or 0.13 osm sucrose. In the former, about 34% survive the removal of half the water, but next to none survive the loss of >70% initial water. Prior exposure to 0.2 M trehalose for as little as 1 min slows the drying rate and increases the tolerance of the larvae to dehydration. With 30-min exposure, 88% survive the loss of 50% of their water and 63% survive the loss of 75%. Protection is abolished with 0.4 M trehalose. The results are similar with sucrose. It is substantially reduced if sugar-exposed larvae are briefly washed with water prior to drying. The protection appears not to be related to the decreased drying rate. Rather it appears related, by an unknown mechanism, to the presence of sugar on the outer surface of the larvae.

Ship-in-bottle synthesis of anionic Rh carbonyls in faujasites

Direct carbonylation of [Rh(NH 3) 5Cl] 2+ ion exchanged in sodium and potassium faujasites yields carbonyls, the composition of which depends on the extent of dehydration before carbonylation, and on the carbonylation temperature. Polynuclear anionic Rh carbonyls are formed at 25–100°C, if the water amount is sufficient. Greenish color of the species created at 75–100°C in NaX, KX and KY points to the [Rh 6(CO) 15] 2− complex; at 50°C a white carbonyl is formed in NaX which structure seems to differ from the green complex in CO configuration. The anionic character of this species also follows from the lower wavenumber of terminal CO ligands in X zeolites and in KY compared to polynuclear neutral Rh carbonyl formed in less basic NaY. Carbonylation at 70–100°C after dehydration or carbonylation at 160–200°C lead to the formation of [Rh(CO) 2] +. This carbonyl reacts to the polynuclear complexes at 25–100°C after readsorption of water vapors. Individual carbonyl types once formed can be interchanged by varying the temperature and water amount. In addition to the FTIR spectroscopy, UV/VIS measurements and mass spectrometric analysis of ammonia released during the thermal decomposition of carbonyls were also employed.

Hydration effects of Ni(2+) binding to synthetic polynucleotides with regularly alternating purine-pyrimidine sequences.

High precision ultrasonic and densimetric techniques have been used to study the interaction of Ni(2+)ions with right-handed poly[d(G-C)].poly[d(G-C)], poly-[d(A-C)].poly[d(G-T)] and poly[d(A-T)].poly[d(A-T)] in 5 mM CsCl, 0.2 mM HEPES, pH 7.5 at 20 degrees C. From these measurements the changes in the apparent molar volume and the apparent molar adiabatic compressibility due to the interaction have been obtained. The volume effects of the binding, calculated per mole of Ni(2+)ions, range from 11.7 to 23.9 cm(3)mol(-1)and the compressibility effects range from 19.3 x 10(-4)to 43.1 x 10(-4)cm(3)mol(-1)bar(-1). These data are interpreted in terms of dehydration of the polynucleotides and Ni(2+)ions, i.e. the release of water molecules from the hydration shells of the molecules. An increase in G+C content gives an increase in volume and compressibility effects, indicating a rise in the extent of dehydration. The dehydration effects of Ni(2+)binding to poly[d(G-C)].poly[d(G-C)] are approximately twice those of poly[d(A-T)].poly[d(A-T)]. The volume and compressibility effects of Ni(2+)-EDTA complex formation have also been measured and used as a model system for quantitative estimation. These values revealed that Ni(2+)ions can coordinate two atomic groups of poly[d(G-C)]. poly[d(G-C)], while in the case of the Ni(2+)-poly[d(A-T)]. poly[d(A-T)] complex volume and compressibility effects correspond to one direct or two indirect (through water) contacts.

Modeling of Adsorption Properties of Zeolites: Correlation with the Structure

The adsorption of N2 and CO in Na X-zeolites has been studied for different framework structures and extraframework cation distributions. To this aim, the cation-molecule system modeling one site has been embedded in a set of external point charges which simulate the zeolite environment of the site and has been treated quantum chemically, using a method based on density functional theory. This procedure has been applied to the 64 cationic sites accessible for adsorption in a crystal unit cell of an ideal X-zeolite with a Si/Al ratio equal to 1. These calculations have shown that only a few cations are favorable for initial adsorption and that those cations are always of type III(III ′). Their efficiency depends both on the framework geometry and on their location in the supercages. The analysis of the quantum chemical results in terms of a classical description involving electrostatic and induction interaction energies with the framework has led to the conclusion that the direction of the electric field vector created by the zeolite in the supercages is an important factor determining the zeolite adsorption properties. Zeolites are aluminosilicate materials that are widely used for size and shape catalysis, hydrocarbon conversion, and sorbing processes. Their physicochemical properties are based on the ability of these open crystalline structures to enclose charged and neutral species within cavities. Their properties are thus strongly related to the structure of the framework and also to the distribution among cages and channels of the cations that are associated with the Al centers. Indeed, those extraframework cations are the active sites for adsorption processes, whereas their neighboring oxygens are involved in base catalysis. Adsorption and desorption studies of small probe molecules like N2 and CO are used to provide information about cations and basic sites. Those experiments show that the adsorption of molecules within the cages varies with the number and nature of occupied cationic sites. 1-3 However, there is presently no real understanding of how adsorption properties of zeolites depend on the cation distribution. The major difficulty encountered in the study of such a correlation arises from the incomplete information provided by experimental structure determinations about cationic sites. First, very few structures are known for low-silica zeolites, i.e. for those including a large number of cations. 4 Moreover, the studies concerned with zeolites containing a single type of cations are even more scarce, due to the difficulty of full cationic exchange. Crystal and powder structure determinations provide positions for the cations associated with the probabilities of occupying the various sites, which, very often, do not sum up to the total number of cations. Cation positions vary also with their chemical nature and with the extent of dehydration. Lithium cations are hardly detectable by X-ray diffraction, and sodium cations may not be well differentiated from water molecules. Finally, experimental conditions of preparation and dehydration may induce differences in the solids, leading to different structures and thus different adsorption properties. For all these reasons, we think that modeling may be an appropriate tool to analyze the relations between adsorption properties and zeolite structures. Although a large variety of modeling techniques have already been used for the prediction and analysis of zeolite structures and properties, only a very few studies have been devoted to systems including cations other than protons. During the last decade, the main effort has been devoted to H-zeolites, mainly due to their widespread applications in acid catalysis. 5-7 Only recently have some dynamical studies of Na and Ca A-zeolite, 8-10 Na Y-zeolite, 11 and Na and K offretite 12 proposed detailed structures including cation positions. In recent papers, we have shown that the nature of the cation and its location in the framework are essential factors for N2 and O2 adsorption. 13,14 In these studies, it was shown that short-range interactions between incoming molecules and

Plasma Spray Deposition and Engineering Applications of Magnesium Hydroxide-polyamide Composite Coatings

SummaryMagnesium hydroxide is shown to dehydrate endothermically to magnesium oxide over a similar temperature range to that of the thermal degradation of polyamide. The extent of dehydration in plasma spraying is shown to be a function of particle size. However the research indicates that the use of magnesium hydroxide as a filler in plasma sprayed polyamide composite coatings does not significantly increase the quality of the matrix by suppressing its degradation. A high content of unreacted magnesium hydroxide can be obtained in plasma sprayed polyamide composites by control of the feedstock particle size. The practical implications of the results for potential engineering applications of the composite are discussed in terms of wear resistance and fire retardance.

Surface, Acidic and Catalytic Properties of Silica—Alumina Catalysts in Relation to Their Chemical Composition

Silica–alumina catalysts of varying chemical compositions were prepared under controlled conditions, employing the coprecipitation technique. The surface properties were determined from the adsorption of nitrogen at 77 K. The acidity was investigated by determining the adsorption of pyridine at 423 K and by following its thermal desorption up to 773 K. The conversion of 2-propanol at 523 K and the cracking of cumene at 693 K were investigated using the pulse microcatalytic technique. The surface area decreased and the mean pore volume increased with increasing alumina content. The amount of acid present and its surface density increased while the relative acid strength decreased with an increase in the alumina content. The extent of dehydration of 2-propanol increased with increasing surface acid density and proved to be insensitive to the acid strength. The cracking of cumene appears to depend on the surface area, the amount of acid present and the acid strength of the catalyst, although its dependence on these parameters is of a relative complex nature.

Effects of dehydration on the electron transport of Chlorella. An in vivo fluorescence study

Chlorella was used to study the effects of dehydration on photosynthetic activities. The use of unicellular green algae assured that the extent of dehydration was uniform throughout the whole cell population during the course of desiccation. Changes in the activities of the cells were monitored by measurements of fluorescence induction kinetics. It was found that inhibition of most of the photosynthetic activities started at a similar level of cellular water content. They included CO2 fixation, photochemical activity of Photosystem II and electron transport through Photosystem I. The blockage of electron flow through Photosystem I was complete and the whole transition occurred within a relative short time of dehydration. On the other hand, the suppression of Photosystem II activity was incomplete and the transition took a longer time of dehydration. Upon rehydration, the inhibition of Photosystem II activity was fully reversible when samples were in the middle of the transition, but was not thereafter. The electron transport through Photosystem I was also reversible during the transition, but was only partially afterward.

The biological reality of the interlacunar network in the embryonic, cartilaginous, skeleton: a thiazine dye/absolute ethanol/LR White resin protocol for visualizing the network with minimal tissue shrinkage.

Third toe phalanges of chicks aged 8-13 days in ovo and 7-day post-natal rat femoral growth plate were examined to determine whether the interlacunar network (IN), a structure with no lipoprotein membrane component or cytoplasmic organelles, is a genuine component of young growth cartilage. In chick phalanges dehydrated by 70% (v/v) ethanol and LR White resin, variable metachromatic staining of the interlacunar network by toluidine blue and red staining by picro-Sirius red indicate the presence of glycosaminoglycans and collagen. The network in phalanges dehydrated by 80% (v/v) ethanol appears little different; however, the network is much less widely detectable in phalanges dehydrated by 90% (v/v) ethanol and, after dehydration by absolute ethanol, is almost completely undetectable. In contrast, when the young cartilage is permeated by a thiazine dye such as toluidine blue, using a solution of dye in the aldehyde fixative, the network is widely detectable, following dehydration by absolute ethanol, both in chick phalanges and in rat growth plate. Comparison of projected areas shows that the extent to which whole chick feet are found to have shrunk, by the time that they are photographed under LR White resin, is determined principally by the extent of dehydration, by 70% (v/v) or absolute ethanol; post-shrinkage areas are 33% or 35% of areas measured in buffer for 70% (v/v) ethanol/LR White resin and 71% or 75% for absolute ethanol/LR White resin (the higher value in each is for the toluidine blue treatment). The network is thus present in radically shrunk tissue, but, significantly, is also fully represented in tissue shrunk by only a conventional margin and is therefore not produced as an artefact by exceptional tissue shrinkage as has been suggested.

Hysteresis in the Binary Exchange of Cations on 2:1 Clay Minerals: A Critical Review

AbstractThe binary exchange of cations on clays and soils is generally regarded as a thermodynamically reversible process. The literature on soil chemistry and geochemistry, however, abounds with reports on cation exchange reactions that appear to have only limited reversibility, i.e., that exhibit hysteresis. A satisfactory explanation of this phenomenon is still lacking, even though a number of mechanisms have been advocated, e.g., charge or site heterogeneity at the surface, differential hydration of cations, dehydration of the exchanger, crystalline swelling hysteresis, and inaccessibility of sites caused by domain or quasi-crystal formation. In the present article, the relevant literature is reviewed and analyzed critically. On the basis of available evidence, it is shown that exchangeable cations can be classified into three groups, defined in such a way that hysteresis has, in the literature, generally not been observed when exchange reactions involved cations belonging to the same group, but has often been found when the reactions involved cations from different groups. Furthermore, it is argued that none of the five mechanisms mentioned can, in and of itself, account fully for the observed exchange hysteresis. A conceptual model is proposed that combines elements of these five mechanisms and is able to describe, at least qualitatively, the effects of factors such as clay type, electrolyte concentration, and extent of dehydration.

Hydration of high-water nonionic soft lenses during hydrogen peroxide disinfection

With the recent introduction of a number of high-water nonionic (Group II) materials, it is germane to consider the influence of hydrogen peroxide on the stability of these hydrogels using high-water ionic (Group IV) lenses as a basis for comparison. We measured the water content (hydration) of nine Group II lens types and two Group IV lens types during three different protocols using two-step hydrogen peroxide (Omnicare, Allergan)—brief: 20 minute disinfection followed by 10 minute neutralization; overnight: 9 hour disinfection followed by 20 minute neutralization; and extended: 1 week disinfection followed by 20 minute neutralization. The water content of each lens was measured with a refractometer before disinfection, after disinfection, and after neutralization. The Group II lenses showed a consistent decrease in mean percentage water content after disinfection; however, the extent of dehydration was unlikely to pose clinically significant problems. In addition, the Group II lenses exhibited a greater tendency to return to baseline hydration levels following 20 minute neutralization compared to ]0 minute neutralization despite the longer periods in hydrogen peroxide. The Group IV lenses were less stable—hydration changes were of greater magnitude and were less consistent. We concluded that although two-step hydrogen peroxide lens care is acceptable with Group II lenses it may not be appropriate for use with Group IV lenses.

Effects of changes in plasma volume on fatal rhabdomyolysis in the rat induced by glycerol injections.

Rhabdomyolysis can be fatal in both experimental animals and man, but very little is known of the factors causing increased mortality in rhabdomyolysis. The aims of this study were to create an animal model of fatal rhabdomyolysis in rats by a glycerol injection into the leg muscle, and to elucidate some of the factors affecting mortality as a result of rhabdomyolysis formation. In this study, two factors which can result in increased mortality in rats as a result of glycerol injection, were examined. These factors include varying doses of 50% glycerol (0.5-2 ml/100 g) and various stages of dehydration prior to glycerol injection. Dehydration was induced by 1: chronic dehydration, in which the rats underwent water deprivation for a period of 24 to 72 hours prior to injection of glycerol; 2: acute dehydration, by the induction of either diuresis, by injecting sucrose (200-600 mg/100 g) to the femoral vein, or hemorrhage (0.7-2.1 ml/100 g). The results demonstrate that the mortality rate in rats increased in all three models of dehydration as the dose of glycerol injected to the rats increased (above a dose of 0.75 ml/100 g) and as the extent of dehydration increased. Use of a blood substitute before or after glycerol injection in order to compensate for the loss of body fluids did not increase the survival rate of the glycerol-injected rats. In contradistinction, rats treated with non-lethal doses of glycerol exhibited substantial resistance to a second lethal dose of glycerol, injected two weeks following the first injection.

Survival of intact Drosophila eggs at various stages of embryonic development as a function of the extent of dehydration and of intraembryonic freezing

Survival of intact Drosophila eggs at various stages of embryonic development as a function of the extent of dehydration and of intraembryonic freezing

Suboptimal nitrogen status sensitizes the photosynthetic apparatus in willow leaves to long term but not short term water stress

The response to drought was compared for willow plants of optimal leaf nitrogen content (100 N) and those of 86% of this content (86 N). Gas exchange measurements revealed that the carboxylation efficiency (CE) of photosynthesis was more sensitive to drought than the photosynthetic capacity in both N regimes. Since the leaf content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was found to be much more resistant it is suggested that a decreased specific activity of Rubisco underlies the decreased CE. Although the rate of water consumption was the same for 86 N and 100 N plants the photosynthetic apparatus responded much more rapidly in the 86 N leaves. This increased sensitivity of 86 N leaves was not due to accelerated senescence as judged by comparison with parallel plants subjected to discontinued fertilization; the two categories of treatments resulted in the same loss of leaf nitrogen and Rubisco but drought induced a much more rapid photosynthetic depression. In contrast to the drought situation, 86 N and 100 N plants behaved similarly when compared under short term water stress. First, when single attached leaves were exposed to a sudden drop in air humidity the capacity of CO2 uptake in both N regimes decreased about 20% over 10 min while the leaf water potential remained high. Second, in freely transpiring leaf discs cut from 86 N and 100 N leaves the same relationship between capacity of O2 evolution and extent of dehydration was observed. The possible mechanisms underlying the increased susceptibility of 86 N leaves to drought is discussed; the water status of the roots not the leaves is suggested to be the determining factor.