Changes In Osmotic Potential Research Articles

Gas exchange, water relations and osmotic adjustment in Phillyrea latifolia grown at various salinity concentrations.

Leaf gas exchange, water relations and osmotic adjustment were studied in hydroponically grown Phillyrea latifolia L. plants exposed to 5 weeks of salinity stress (0, 80, 160, 240 and 320 mM NaCl) followed by 5 weeks of treatment with half-strength Hoagland solution. Whole-plant relative growth rate and root/shoot and lateral/structural root ratios were also evaluated. Net CO2 assimilation rate, stomatal conductance and transpiration rate were markedly decreased by all of the salt treatments. Growth was also strongly depressed by all salt treatments, especially lateral root growth. Leaf water potential decreased soon after salinity stress was imposed, whereas there was a lag of several weeks before leaf osmotic potential decreased in response to the salt treatments. After 5 weeks of salinization, leaf turgor of salt-treated plants was similar to that of controls. Although Na+ + Cl- contributed little to the salt-induced changes in osmotic potential at full turgor (Psi(piFT)), the contributions of K+, mannitol (Man) and glucose (Glc) to Psi(piFT) markedly increased as external salinity increased. Salt accumulation was negligible in the youngest leaves, which mostly accumulated soluble carbohydrates and K+; in contrast, old leaves served as storage sinks for Na+ and Cl-. Photosynthetic performance of salt-treated plants fully recovered once salt was leached from the root zone, with the recovery rate depending on the severity of the salt stress previously experienced by the plants. Recovery of gas exchange occurred even though the leaves still had a salt load similar to that detected in leaves at the end of the 5-week salinity period, and had markedly lower concentrations of K+ and soluble carbohydrates than control leaves. We conclude that salt-induced water stress primarily controlled gas exchange of salt-treated P. latifolia leaves, whereas the salt load in the leaves did not cause irreversible damage to the photosynthetic apparatus.

Pollen treatment in high osmotic potential: a simple tool for in vitro preservation and manipulation of viability in gametophytic populations

A method for in vitro preservation and manipulation of pollen viability based on simple changes in osmotic potential (sucrose concentrations) in culture media was developed using tobacco pollen as the experimental model. High osmotic potentials were capable of reversibly inhibiting pollen germination, preserving its viability at room temperature for long periods, as assessed by subsequent incubation in germination medium. When pollen was pre-germinated for different periods and subsequently incubated in 80 % sucrose medium (inhibiting medium) there was a progressive decrease in its viability, which was a trend best described by a quadratic regression line. Nevertheless, very small variations in pollen-tube lengths and viable pollen grains more resistant to the pre-germination step were detected by this procedure. Consequences and potential applications of these findings were discussed for use in analyses of variability, selection in gametophytic populations and pollen storage.

Concurrent Monitoring of Oxygen Evolution and Chlorophyll Fluorescence in Peeled Leaves with a Liquid-phase Oxygen Electrode

Concurrent monitoring of photosynthetic rate and chlorophyll fluorescence is one of the useful methods to understand the chemical energy balance in photosynthesis. The liquid-phase oxygen electrode is widely employed for determining photosynthetic rate by monitoring oxygen evolution from leaf segments in reaction solution. If it is possible to measure the emittance of chlorophyll fluorescence concurrently with oxygen evolution rate in leaf by the oxygen electrode method, it may become a valid technique for studying the energy flow in photosynthesis of leaf in various physical and chemical solution conditions. We set up a system for concurrent measurement of gross oxygen evolution rate (Og) and chlorophyll fluorescence by partially remodeling a conventional type of liquid-phase oxygen electrode. A piece of 0.785 cm2 leaf with the abaxial epidermis peeled off was fixed in solution of the electrode, and Og and quantum yield of PSII (Fe) were concurrently measured. By the addition of ATP to solution Fe was temporarily decreased, whereas Og was unchanged. Change in osmotic potential from ?0.3MPa to ?1.5 MPa did not affected Og in a leaf, but Fe was gradually reduced with a decrease in the osmotic potential of solution. It may be expected that the concurrent measurement by this remodeled electrode is widely used for analysis of energy balance in photosynthesis and energy utilization efficiency in crop leaves.

Seasonal patterns of tissue water relations in three Mediterranean shrubs co‐occurring at a natural CO2 spring

ABSTRACTSeasonal changes in tissue water relations of Erica arborea L., Myrtus communis L. and Juniperus communis L., grown in a Mediterranean environment, were analysed under field conditions over a 12 month period by comparing plants grown in the proximity of a natural CO2 spring (about 700 μmol mol−1 atmospheric CO2 concentration, [CO2]) with plants in ambient conditions. Tissue water relations varied in response to changes in water availability, but the seasonal course of tissue water relations parameters was also related to ontogeny. Tissue water relations of these co‐occurring shrubs were not alike. Osmotic potentials and saturated mass/dry mass ratio were lowest during peak drought stress periods. Diurnal changes in osmotic potential at the point of turgor loss were least early in the season, maximal in mid‐season, and decreased again in autumn. Turgor potentials decreased as drought progressed and were highest in late fall and mid‐winter. Symplastic water fraction was highest in mid‐spring for E. arborea and M. communis and decreased during the summer, while the opposite was observed for J. communis. Common to all species, under elevated [CO2], was an increase of turgor pressure, particularly during the summer months. Other parameters showed species‐specific responses to long‐term elevated [CO2]. In particular, exposure to elevated [CO2] increased osmotic potentials in E. arborea under drought, while the opposite was the case for J. communis. Site differences in predawn to midday shifts were not strong in any of the species. Differences in tissue water relations suggest that the coexistence of these shrubs in the same environment with similar water availability are partially based on differential water relations strategies and water use patterns. Regardless of the mechanisms, growth of these shrubs in elevated [CO2] may be either less, similarly or more affected by drought stress than plants in ambient [CO2] depending on the species and season.

Frost De-acclimation of Barley (Hordeum vulgare L.) and Meadow Fescue (Festuca pratensis Huds.). Relationship between Soluble Carbohydrate Content and Resistance to Frost and the Fungal PathogenBipolaris sorokiniana (Sacc.) Shoem.

Plants of barley (Hordeum vulgare) and meadow fescue (Festuca pratensis), two species differing in their susceptibility to the fungal pathogen Bipolaris sorokiniana, were cold-acclimated. Changes in frost resistance, susceptibility to B. sorokiniana, osmotic potential, water content, and composition of low molecular weight sugars were studied in leaves over a period of 14d during subsequent de-acclimation. Cold acclimation promoted resistance to frost and the fungal pathogen in both species. Plants subjected to de-acclimation lost their ability to withstand frost after about 24h, but retained enhanced levels of resistance to the pathogen. This effect lasted longer in the less pathogen-resistant barley than in meadow fescue. Changes in osmotic potential observed after cold-acclimation and during dehardening were correlated with changes in frost resistance, but not with changes in susceptibility to the pathogen. The same applied to changes in the low molecular weight sugar content in leaves of meadow fescue. In barley, cold-acclimation induced alterations in osmotic potential and the subsequent increase in frost resistance was not correlated with variation in soluble carbohydrate contents. The results indicate that cold-acclimation induced changes in leaf water potential and that soluble sugar content is not involved directly in the increased resistance to the pathogen which was observed after cold acclimation.

A 69 bp fragment in the pyrroline-5-carboxylate reductase promoter of Arabidopsis thaliana activates minimal CaMV 35S promoter in a tissue-specific manner

The Arabidopsis thaliana gene that encodes pyrroline-5-carboxylate reductase ( At-P5R), the last enzyme in proline biosynthesis in A. thaliana, is developmentally regulated and is highly expressed in cells that divide rapidly or undergo changes in osmotic potential. A 69 bp region (P69; −120 to −51) has previously been identified in a 5′ deletion analysis of the At-P5R promoter to be necessary for the basal expression. Here, the essential role of P69 for tissue-specific expression of At-P5R is demonstrated by loss- and gain-of-function experiments.

Differentiation between osmotic injury and chloride toxicity of rice seedlings grown under saline conditions1

Rice (Oryza sativa, L.) is most sensitive to salinity injury as a seedling. Information describing the effects of salinity on the seedling rice plant is limited. Therefore, the current study was conducted to determine if salinity injury to rice seedlings is the result of toxic levels of chloride, or if a change in osmotic potential interferes with the plant's ability to absorb water and nutrients. Rice was grown in controlled environmental conditions through the seedling growth stage. In hydroponic culture with a 50% Hoagland's solution, the rice was exposed to osmotic tensions of 0, 50, 100, 200, 400, 800, or 1,600 kPa imposed by either CaCl2 or polyethylene glycol (PEG). The rice was grown for 1 wk following imposition of the osmotic treatments. Visual ratings were then obtained and plants were harvested for dry matter determination and Cl‐ concentration. Shoot weights were lowered for PEG treatments compared to CaCl2. Root weights differed little between CaCl2 and PEG treatments. Root weights decreased with increasing osmotic tension whether decreased by CaCl2 or PEG. Salinity related damage to leaves and roots was worse for PEG than for CaCl2 treatments. Not surprisingly, uptake of Cl‐ tended to increase with increasing rates of CaCl2, but not with PEG. These data suggest that rice sensitivity to salinity is the result of changes in the osmotic potential gradient rather than to chloride toxicity.

Mechanisms Involved in Diurnal Changes of Osmotic Potential in Grapevines under Drought Conditions

Summary The aim of this study was the quantitative determination of the contribution of various possible mechanisms involved in diurnal changes of osmotic potential in grapevine leaves ( Vitis vinifera L., cv. Victoria ) under water stress conditions. Results of this study revealed that dehydration accounted for 36 % of the diurnal changes in osmotic potential in stressed plants. The relative contribution of changes in non-osmotic volume was negative. Net solute accumulation accounted for 73 % of the diurnal changes, indicating the occurrence of an active osmotic adjustment during the day in stressed plants. There was evidence that compounds other than sugars contributed to diurnal osmotic adjustment in stressed grapevines.

Cell water balance of white button mushrooms ( Agaricus bisporus) during its post-harvest lifetime studied by quantitative magnetic resonance imaging

A combination of quantitative water density and T 2 MRI and changes therein observed after infiltration with ‘invisible’ Gd-DTPA solution was used to study cell water balances, cell water potentials and cell integrity. This method was applied to reveal the evolution and mechanism of redistribution of water in harvested mushrooms. Even when mushrooms did not lose water during the storage period, a redistribution of water was observed from stipe to cap and gills. When the storage condition resulted in a net loss of water, the stipe lost more water than the cap. The water density in the gill increased, probably due to development of spores. Deterioration effects (i.e. leakage of cells, decrease in osmotic water potential) were found in the outer stipe. They were not found in the cap, even at prolonged storage at 293 K and R.H.=70%. The changes in osmotic potential were partly accounted for by changes in the mannitol concentration. Changes in membrane permeability were also indicated. Cells in the cap had a constant low membrane (water) permeability. They developed a decreasing osmotic potential (more negative), whereas the osmotic potential in the outer stipe increased, together with the permeability of cells.

Diurnal and seasonal osmotic potential changes in Lotus creticus creticus plants grown under saline stress

Lotus creticus creticus plants growing in a greenhouse were exposed to 0, 70 and 140 mM NaCl for 4 months (September–December). Salinity caused a reduction in total dry weight of Lotus plants treated with 140 mM NaCl, whereas no significant effects on growth were observed with 70 mM NaCl. Predawn leaf water potential and predawn leaf osmotic potential showed constant values in control plants during all the experiment, whereas a decrease of both parameters was observed between September and October for the saline treatments. The relative contribution of passive (dehydration) versus active mechanisms (osmotic adjustment) involved in seasonal leaf osmotic potential changes were determined. Seasonal decreases of the osmotic potential at full turgor in the treated plants showed the capacity for osmotic adjustment by accumulation of Na + and Cl −, because the accumulation of organic solutes due to salts was not consistent. In plants treated with 70 mM NaCl, the seasonal changes of the osmotic potential were produced by net solute accumulation, because the dehydration contribution was negligible. In plants treated with 140 mM NaCl, the seasonal changes of the osmotic potential were originated by ions accumulation, but also by tissues dehydration. At the end of the salinization period (December), a possible diurnal adaptation in water relations was also considered. The dehydration was the major mechanism involved in diurnal changes of leaf osmotic potential, and only at the highest salinity level some diurnal osmotic adjustment could be observed. In conclusion, the osmotic adjustment in Lotus might be a beneficial trait when the plants are treated with moderate levels of salinity (70 mM NaCl). At higher salinity (140 mM NaCl), the high absorption and accumulation of ions caused important toxic effects and induced leaf tissue dehydration.

The function of guard cells does not require an intact array of cortical microtubules

cellulose microfibrils. These studies have found that cortical microtubules are needed to produce a pair of kidney The development of stomatal guard cells is known to shaped cells surrounding an aperture, as well as to direct require cortical microtubules; however, it is not known the asymmetric deposition of cellulose microfibrils in the if microtubules are also required by mature guard cells cell wall, which enables the stoma to open and close in for stomatal function. To study the role of microtubules response to changes in osmotic potential (Palevitz, 1981; in guard cell function, epidermal peels of Vicia faba Sack, 1987). Once they mature, guard cells change shape were subjected to conditions known to open or close to regulate the stomatal aperture. It is reasonable to ask stomata in the presence or absence of microtubule whether these changes in shape require, or aVect, the inhibitors. To verify the action of the inhibitors, micro- cortical microtubules. But even though the role of microtubules in appropriately treated epidermal peels were tubules in guard cell development was recognized as localized by cryofixation followed by freeze substitu- important long ago, there has been little assessment of tion and embedding in butyl-methyl methacrylate. the role of cortical microtubules in guard cell function. Mature guard cells had a radial array of microtubules, The possible reasons why the role of microtubules in focused toward the thick cell wall of the pore, and the guard cell function has been little studied are both pracappearance of this array was the same for stomata tical and conceptual. Practically, attempts to localize remaining closed in darkness or induced to open by microtubules in mature stomata may have been frustrated light. Treatment of epidermal peels with 1 mM colchic- by the thick cell wall, which hinders chemical fixation. ine for 1 h depolymerized nearly all cortical microtub- Conceptually, microtubules are considered to orient micules. Measurements of stomatal aperture showed that rofibrils and specify cell shape; once guard cells reach neither 1 mM colchicine nor 20 mM taxol affected any maturity, their structure is plausibly suYcient to allow of the responses tested: remaining closed in the dark, many rounds of opening and closing. However, microtubopening in response to light or fusicoccin, and closing ules have roles beyond shaping cells: the cytoskeleton, in response to calcium and darkness. We conclude including microtubules, participates in signal transducthat intact microtubule arrays are not invariably tion. For example, in animal cells, actin filaments terminrequired for guard cell function. ate in special regions of the plasma membrane enriched in tyrosine receptor-kinases (Mochly-Rosen, 1995). For

Developmental regulation of pyrroline-5-carboxylate reductase gene expression in Arabidopsis.

At-P5R, a gene encoding the last enzyme of the proline (Pro) biosynthetic pathway in Arabidopsis thaliana, is developmentally regulated. To characterize the cis elements responsible for this developmental regulation, a series of 5' deletions of the At-P5R promoter were transcriptionally fused to a beta-glucuronidase (GUS)-coding region and transformed into Arabidopsis. The complete promoter of At-P5R directs strong GUS activity in root tips, the shoot meristem, guard cells, hydathodes, pollen grains, ovules, and developing seeds, all of which contain rapidly dividing cells and/or are undergoing changes in osmotic potential. This expression pattern is consistent with the function of Pro as an energy, nitrogen, and carbon source and as an osmoticum in response to dehydration. Promoters longer than 212 base pairs (bp) showed the same expression pattern, whereas those shorter than 143 bp did not direct any detectable GUS activity in any organs. This suggests that a 69-bp promoter region located between -212 and -143 bp is necessary to establish the tissue-specific expression of At-P5R during development. The Pro content measured in different organs suggests that, in addition to transcriptional control of the biosynthetic pathway, the transport of Pro may play a role in its distribution within Arabidopsis. Several aspects of the relationship between Pro metabolism and plant physiology are discussed.

Location of a gene regulating cold-induced carbohydrate production on chromosome 5A of wheat

Major changes in osmotic potential during cold acclimation are due to changes in sugar concentration, and there is a good correlation between sugar content and frost tolerance. The objective of the present study was to localize a gene(s) responsible for carbohydrate accumulation during cold acclimation on chromosome 5A of wheat using recombinant lines developed from the cross between the substitution lines Chinese Spring (Cheyenne 5A) and CS(Triticum spelta 5A). Previously, major genes influencing frost resistance (Fr1) and vernalization requirement (Vrn1) had been localized on the long arm of that chromosome. The T. spelta 5A chromosome carrying the Fr1 (frost-sensitive) allele for frost tolerance and the Vrn1 (spring-habit) allele for vernalization requirement did not have a major effect on the sucrose and fructan contents in the Chinese Spring background. On the other hand, the presence of Cheyenne alleles for vernalization requirement, vrn1, and frost tolerance, fr1, significantly increased sugar concentrations. A recombinant line thought to exhibit recombination between the Vrn1 and Fr1 loci suggested that the gene regulating sucrose accumulation was closely associated with, or else represented a pleiotropic effect of, Vrn1, but was separable from the Fr1 locus.

Changes in water relations for leaves exposed to a climate-warming manipulation in the Rocky Mountains of Colorado

Relative water content (RWC) and water potential were compared for leaves of several plant species exposed to a warming manipulation at the Rocky Mountain Biological Laboratory, near Crested Butte, Colorado, USA, to test the hypothesis that species-specific changes in water relations parameters will occur in response to future increases in planetary air temperatures. Leaves of Artemisia tridentata, Erigeron speciosus, Festuca thurberi, Helianthella quinquinervis, Potentilla fruticosa, Potentilla gracilis and Rhodiola integrifolia were collected from plants growing in situ in control and infrared (IR)-heated (22 W m −2) plots in a meadow near the upper elevational distribution limit for A. tridentata. For six of the seven herbaceous species, RWC was not significantly different from that for A. tridentata (0.903 ± 0.019 on control plots and 0.846 ± 0.031 from heated plots). However, RWC was 0.644 ± 0.04 and 0.596 ± 0.029 for F. thurberi on control and heated plots, respectively. Water potential (ψ) varied from −1.04 MPa for A. tridentata (control plots) to −4.83 MPa for leaves of F. thurberi (heated plots); ψ for the other species ranged from −1.08 MPa ( R. integrifolia on control plots) to −2.62 MPa ( P. gracilis on heated plots). To characterize further the responses of water relations parameters to the IR heating treatment, pressure-volume isotherms were generated for A. tridentata and P. gracilis. Both species exhibited effects of the IR heating treatment on saturated osmotic potential and the relative symplastic water content. Based on changes in osmotic potential and relative water content, an increase in osmotically active solutes was exhibited for leaves collected from plants under heaters. Species-specific patterns of RWC and water potential (as well as effects on cellular water relations) may influence the ability of plant species to cope with changes in soil water content that are expected to occur with global warming for montane ecosystems in the western USA.

Physiological Responses of Intsia bijuga Trees to Drought Stress

Intsia bijuga is an important native tree on Guam, and is being promoted as an urban forestry tree. Container-grown I. bijuga trees were subjected to a drying cycle (50% of mean water loss replaced daily) to determine physiological responses to drought. Early to mid-morning gas exchange began to decline compared to well-watered plants on day 12, and quickly declined thereafter. Net CO2 assimilation (A) was close to zero by day 29 and became negative by day 36. Chlorophyll fluorescence of drought-stressed trees was not different from that of well-watered trees on numerous days of measurement throughout the drying cycle. Unlike leaflets of well-watered trees, leaflets of the stressed trees exhibited heliotropic movement to avoid direct exposure to the sun. The stressed trees did not respond with any change in osmotic potential of leaflets following rehydration. Carbon dioxide-use efficiency of stressed trees was reduced to 66% of well-watered trees. The most profound response following rewatering (day 37) was leaf shedding. All trees shed some leaves, and 33% of the trees shed the entire canopy. On the trees that retained some leaves, A returned to that of the control trees by day 13 of recovery.

Characterization of the expression of a desiccation-responsive rd29 gene of Arabidopsis thaliana and analysis of its promoter in transgenic plants

We characterized the expression of genes that correspond to a cDNA clone, RD29, which is induced by desiccation, cold and high-salt conditions in Arabidopsis thaliana. Northern analysis of desiccation-induced expression revealed a two-step induction process. Early induction occurs within 20 min and secondary induction occurs 3 h after the start of desiccation. Exogenous abscisic acid (ABA) induces RD29 mRNA within 3 h. Two genes corresponding to RD29, rd29A and rd29B, are located in tandem in an 8 kb region of the Arabidopsis genome and encode hydrophilic proteins. Desiccation induces rd29A mRNA with two-step kinetics, while rd29B is induced only 3 h after the start of desiccation. The expression of both genes is stimulated about 3 h after application of ABA. It appears that rd29A has at least two cis-acting elements, one involved in the ABA-associated response to desiccation and the other induced by changes in osmotic potential. The beta-glucuronidase (GUS) reporter gene driven by the rd29A promoter was induced at significant levels by desiccation, cold, high-salt conditions and ABA in both transgenic Arabidopsis and tobacco. Histochemical analysis of GUS activity revealed that the rd29A promoter functions in almost all the organs and tissues of vegetative plants during water deficiency.

The Influence of Nitrogen (15NO3) Supply to Chicory (Cichorium intybusL.) Plants During Forcing on the Uptake and Remobilization of N Reserves for Chicon Growth

Tuberized tap roots of Witloof chicory {Cichorium intybus L.) were forced by placing in a dark chamber in a hydroponic system under high RH to produce an etiolated bud, the chicon. Plants were fed nutrient solutions with two NO3 concentrations of 1-5 or 18 mol m~3 NO3, or demineralized water. The nutrient solutions were labelled with 2% atom excess 13N. Although the chicon biomass increased with increasing NO3 concentration in the nutrient solution, the chicon dry weight remained unchanged. The increased chicon biomass was, therefore, due to more water in the chicon. The N in the chicon originated from either an endogenous source, the root, and/or an exogenous source, the nutrient solution. Organic N reserves remobilization and transfer to the chicon were not been affected by NO3 supply. At the end of the forcing period 75% of the root N had been remobilized. Differences in the amount of N in the chicons of the three treatments were due to the uptake of exogenous N. The flux of exogenous nitrogen to the chicon in high NO3-plants was 2- to 6-fold higher than in the low NO3-plants and, at the end of the forcing period, exogenous nitrogen contributed 30% of total chicon N in high NO3-plants and 10% in low NO3-plants. Net uptake of NO3 by chicory plants during the forcing process was a function of N influx and N efflux. The increase in N influx was accompanied by an increase in exogenous N flux to the chicon and probably a shift in root and/or chicon osmotic potential which increased water flux to the chicon. Since NO3 did not accumulate in either the chicon or the root, it is proposed that osmotic solutes, such as organic acids and amino acids may be involved in osmotic potential changes in chicory during the forcing process.

Effects of chloride and sodium on gas exchange parameters and water relations of Citrus plants

Gas exchange parameters, water relations and Na+/Cl‐ content were measured on leaves of one‐year‐old sweet orange (Citrus sinensis [L.] Osbeck cv. Hamlin) seedlings grown at increasing levels of salinity. Different salts (NaCl, KCl and NaNO3) were used to separate the effects of Cl− and Na+ on the investigated parameters. The chloride salts reduced plant dry weight and increased defoliation. Accumulation of Cl− in the leaf tissue caused a sharp reduction in photosynthesis and stomatal conductance. By contrast, these parameters were not affected by leaf Na+ concentrations of up to 478 mM in the tissue water. Leaf water potentials reached values near −1.8 MPa at high NaCl and KCl supplies. This reduction was offset by a decrease in the osmotic potential so that turgor was maintained at or above control values. The changes in osmotic potential were closely correlated with changes in leaf proline concentrations. Addition of Ca2+ (as calcium acetate) increased growth and halved defoliation of salt stressed plants. Furthermore, calcium acetate decreased the concentration of Cl− and Na+ in the leaves, and increased photosynthesis and stomatal conductance. Calcium acetate also counteracted the reductions in leaf water and osmotic potentials induced by salinity. In addition, calcium acetate inhibited the accumulation of proline in the leaves which affected the reduction in osmotic potential. These results indicate that adverse effects of salinity in Citrus leaves are caused by accumulation of chloride.

Changes in water relations and in some physiological functions of bean under very light osmotic shock induced by polyethylene glycol

Bean plantlets (Phaseolus vulgaris L. cv. Topcrop) were stressed at the age of 16–18 days by gradual (2–8%) or abrupt addition of 6% (w/v) polyethylene glycol Mw 6000 (PEG 6000) to Hoagland solution. Leaf conductance, photosynthesis, internal CO2 partial pressure (Ci), relative water content (RWC), water content/dry weight (H2O/DW), apoplastic PEG concentrations and weight of leaves, stems and roots were determined. Leaf conductance, photosynthesis and Ci were determined on non‐detached primary leaves, and leaf potentials (water, osmotic and turgor potentials) were investigated in freshly detached (non‐rehydrated) primary leaves, both in treated and control plants; RWC and osmotic potential were also assessed at the null turgor point. Low PEG 6000 concentrations induced early and evident decrease in leaf conductance and photosynthesis, whereas Ci decreased only moderately and tended to recover during advanced stress. There were moderate though significant decreases in RWC and H2O/DW, no change or increases in water potential, no significant changes in osmotic potential and a moderate but significant increase in turgor potential. Even when referred to null turgor point, RWC significantly decreased and osmotic potential was unchanged. It was concluded that apoplastic PEG 6000 accumulation at evaporating sites would account for the early decrease in conductance which would also justify the unchanged or the prevalent increase in water potential and turgor potential. The subsequent PEG diffusion and concentration in the leaf apoplastic water would have induced the RWC and H2O/DW decrease and the final turgor flexion documented.

Embryo Water Status and Development of the Recalcitrant SpeciesQuercus roburL: Determination of Water Relations Parameters by Pressure-Volume Analysis

The water status and water content of embryos of Quercus robur have been determined during development. Using psychrometers, changes in osmotic potential at full imbibition and the proportion of water at zero reciprocal water potential, R0, were estimated from curves of embryo relative water content and reciprocal water potential. The specific moisture content of embryos decreased during development, with a concomitant increase in the osmotic potential. However, R0 also increased such that, during the last four weeks of development, there was little net change in the content of osmoticum per dry weight. Evidence is presented that starch stored in the cotyledon may be the major site of matrix-bound water, R0. Isolated embryonic axes had a slightly higher osmotic potential than the whole embryo but the R0 was much lower. The differences between whole embryo and axis water relations parameters emphasize the need to take account of R0 when discussing seed water status and survival. The results are considered in relation to current models of seed water status and recalcitrant behaviour.