Symplastic Water Research Articles

Regulation of Water Permeability of Vacuolar Symplast

Effect of exogenous ABA and an inhibitor of energy metabolism NaN3 on water permeability of the desmotubules and tonoplast as the structural elements of vacuolar symplast ensuring water permeability of this transport system was investigated. The methodological approach based on the use of NMR with magnetic field pulse gradient is described in detail. It was shown that ABA affects water permeability of the vacuolar symplast in the root cells of maize (Zea mays L.) seedlings by temporary increase in water permeability of its membrane (tonoplast) and does not modify water permeability of desmotubules. At the same time, the effect of sodium azide is related to the disturbance of water permeability in the latter, and this evidence is corroborated by the additivity in the effects of the two above-mentioned agents on diffusion decay of spin echo produced by vacuolar symplast water molecules. ABA effect was detected only at high exogenous concentrations (10−4 M). The effect of ABA on water permeability of the tonoplast did not depend on or was weakly related to intracellular concentration of ATP, whereas the open state of desmotubules was ATP-dependent. Observations were made on the role of aquaporins in the ABA influence on tonoplast water permeability and the physiological role of high ABA concentrations.

Impairment of NtAQP1 gene expression in tobacco plants does not affect root colonisation pattern by arbuscular mycorrhizal fungi but decreases their symbiotic efficiency under drought

We investigated in two tobacco (Nicotiana tabacum) plant lines (wildtype or antisense mutant) whether impairment in expression of the plasma membrane aquaporin gene (NtAQP1) affects the arbuscular mycorrhizal (AM) fungal colonisation pattern or the symbiotic efficiency of AM fungi. These two objectives were investigated under well-watered and drought stress conditions. Both plant lines had a similar pattern of root colonisation under well-watered and drought stress conditions. In contrast, under drought stress, AM wildtype plants grew faster than mycorrhizal antisense plants. Plant gas exchange also appeared to depend on the expression of NtAQP1 and parallelled the determined growth increments. The implications of enhanced symplastic water transport via NtAQP1 for the efficiency of the AM symbiosis under drought stress conditions are further discussed.

Viewpoint: Consideration of apoplastic water in plant organs: a reminder.

The importance of apoplastic water was confirmed for the leaves of a range of species by a comparison of tissue solute concentrations determined by the extrapolation of water potential isotherms to 100% relative water content (symplastic solute concentration at full turgor) and concentrations derived more directly from frozen / thawed tissue, where there is dilution of the symplastic water fraction by the apoplastic water fraction. A thermocouple psychrometer was used for both water potential and solute potential measurements. Parallel measurements of the apoplastic water content, estimated by the extrapolation of pressure-volume curves to zero (1 / water potential) with a pressure chamber and measurements based on the dilution method, with a thermocouple psychrometer, showed that the two methods gave similar results. This lends support to the conclusion that water is lost from the symplast and not from the apoplast of leaves when these are subjected to increasing pressure in a pressure chamber. However, where tissues or organs are air-dried the loss of water occurs from both the symplast and apoplast.

Overexpression of a plasma membrane aquaporin in transgenic tobacco improves plant vigor under favorable growth conditions but not under drought or salt stress.

Most of the symplastic water transport in plants occurs via aquaporins, but the extent to which aquaporins contribute to plant water status under favorable growth conditions and abiotic stress is not clear. To address this issue, we constitutively overexpressed the Arabidopsis plasma membrane aquaporin, PIP1b, in transgenic tobacco plants. Under favorable growth conditions, PIP1b overexpression significantly increased plant growth rate, transpiration rate, stomatal density, and photosynthetic efficiency. By contrast, PIP1b overexpression had no beneficial effect under salt stress, whereas during drought stress it had a negative effect, causing faster wilting. Our results suggest that symplastic water transport via plasma membrane aquaporins represents a limiting factor for plant growth and vigor under favorable conditions and that even fully irrigated plants face limited water transportation. By contrast, enhanced symplastic water transport via plasma membrane aquaporins may not have any beneficial effect under salt stress, and it has a deleterious effect during drought stress.

Water relations of temperate mistletoes on various hosts.

The daily field water relations and gas exchange of the temperate mistletoes Ileostylus micranthus (Hook.f.) Tiegh. and Tupeia antarctica Cham. et Schlecht. on various hosts were examined seasonally in Dunedin, New Zealand during 1996-1998. Mistletoes commonly have higher transpiration rates (E) than their hosts, and this is generally cited as the reason why mistletoes develop lower water potentials (ψ) than their hosts. The mistletoe-host pairs that we examined showed no significant overall differences in E and stomatal conductance (g), and we used them to test the hypothesis that lowered ψ in mistletoes result from higher E. Despite the lack of differences in E and g, osmotic potentials, predawn and daily minimum ψ (ψmin) were significantly more negative in mistletoes, although differences between host and mistletoe ψ were less on hosts with low osmotic potentials and ψ. Mistletoes maintained lower ψ than their hosts both when unshaded and under artificial shading, had lower ψ than hosts at equal E, but had shoot hydraulic resistances similar to that of their hosts. E and ψ of hosts and mistletoes tended to be coordinated only in summer, when hosts were most water-stressed. Mistletoes maintained higher relative water contents at turgor loss, symplastic water contents, and bulk moduli of elasticity (ε) than their hosts. We conclude that the lower ψ in these temperate mistletoes are a consequence of greater mistletoe E only when host ψ are low, but are otherwise maintained by greater succulence and higher ε than in their hosts.

Space and time dependence of temperature and freezing in evergreen leaves.

Infrared video thermography was used to study space and time dependence of freezing in intact, attached leaves of snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings. Freezing initiated in the midvein and spread through the apoplast at 10 mm s-1. Freezing of apoplastic water was detected by a local, rapid increase in temperature, and was followed by a slower increase in leaf temperature to the equilibrium freezing temperature as symplastic water moved from cells to extracellular sites of ice formation. The duration of freezing varied with position, leaf thickness and water content. Most of the cellular water in the leaf tip and margins froze quickly, while freezing was slower near the petiole and midvein. Regions that had frozen more rapidly then began to cool more rapidly, producing steep gradients in leaf temperatures and hence also freeze-induced dehydration. Thus, spatial variation in physical properties of leaves could affect the distribution of minimum leaf temperatures, and hence, the distribution and extent of damage due to freeze-induced dehydration. These results are consistent with patterns of freezing damage in autumn when the duration of freezing may be insufficient for the whole leaf to freeze before sunrise, and may explain the general observation of increased leaf water content and thickness with altitude.

Effects of Organic Farming Practices on Photosynthesis, Transpiration and Water Relations, and Their Contributions to Fruit Yield and the Incidence of Leaf-Scorch in Pear Trees

Summary This study involved a comparison of two pear (Pyrus communis L. cv. Nijuseki) orchards in the South Nagano area that were managed according to either chemical farming (CF) or organic farming (OF) practices. Leaf-scorch was not observed in the pear orchard under OF but was severe under CF. Fruit yield was 62% higher for the pear orchard farmed organically. Photosynthetic capacity (P C) was 33% higher under OF. Leaf symplastic water fraction (z sym) and osmotic concentration (C FT) were higher for pear trees under OF than for those under CF. Both stomatal (a) and cuticular (b) transpiration were faster for excised pear leaves under CF and, as a consequence, the leaf water retention ability (WRA) of leaves under CF was lower than for those under OF. Results suggest that the higher yield of pear trees with organic farming management, compared with chemical farming, is attributed to high P C and that the absence of leaf-scorch is attributed to a high water retention ability that is associated with a hi...

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.

Water movement between epidermal cells of barley leaves – a symplastic connection?

ABSTRACTUsing the cell‐pressure probe the possibility of symplastic water flow between cells of the upper epidermis of barley leaves was investigated. Cells analysed had either an intact or a more or less damaged cellular environment. Cell damage caused large pressure differentials (0·9 MPa) between damaged and adjacent intact cells. Turgor in cells adjacent to damaged cells decreased significantly. Turgor decreases were the larger the more the adjacent, damaged cell was leaking (decreases by 2·5–4·4%). In cells surrounded by a patch of leaking cells, turgor decreased the most, by 18·1–20·4%. In contrast, half‐times of water exchange (T1/2) of cells were not affected by a damaged cellular environment. Assuming that in the barley leaf epidermis, plasmodesmata close at pressure‐differentials at or exceeding 0·2 MPa as shown for other plant cells (The Plant Journal 2, 741–750; Canadian Journal of Botany 65, 509–511), it is concluded that symplastic water flow contributes insignificantly to water exchange between cells. Mechanical damage to one individual cell is enough to induce significant turgor changes in neighbouring cells.

Osmotic Adjustment and Partitioning of Turgor Responses to Drought in Grapevines Leaves

The aim of this study was to examine the ability of grapevines (<i>V. vinifera</i> L.) for osmotic adjustment using the pressure-volume technique as well as to evaluate the relative contribution of changes in both osmotic and elastic properties of leaf tissue in turgor maintenance under drought conditions. Pressure volume analysis indicated a significant increase in symplastic water fraction of the total water content in stressed plants. The calculated decrease in osmotic potential due to net solute accumulation in stressed grapevines was 0.55 MPa, suggesting that osmotic adjustment constitutes an important drought tolerance mechanism. Partitioning of turgor responses to drought into its components, revealed a negative contribution of elastic adjustment in turgor maintenance of stressed grapevines. Thus, the physiological role of tissue elasticity as a beneficial drought mechanism in grapevines might be reconsidered.

Water-content components in bryophytes: analysis of pressure-volume relationships

ing full-turgor water content for physiological work on water-stress responses in bryophytes is emphasized. The water associated with a bryophyte can be divided into (a) apoplast water held in cell-wall capillary spaces Key words: Thermocouple psychrometry, apoplast fraction, and by matric forces, (b) osmotic (symplast) water, and relative water content, osmotic potential, poikilohydry. (c) external capillary water. In many bryophytes (c) is a large and variable component, preventing easy determination of full-turgor water content and of relative Introduction water content (RWC) values physiologically comparable with those for vascular-plant leaves. Pressure‐ In contrast to vascular plants, bryophytes at full turgor volume (P‐V) curves are presented and water-relations typically carry substantial and variable amounts of extraparameters estimated for bryophytes, including cellular water. Many mosses have their main pathways species with large thin-walled cells (Hookeria lucens of water movement outside rather than inside the plant and three marchantialian thalloid liverworts), species (Buch, 1945, 1947; Proctor, 1979, 1982). This makes with notably thick cell walls (Neckera crispa), and some standard methods for studying the water relations species with wettable surfaces and well-developed of vascular plants diYcult or impossible to apply to external capillary water conduction (Tortula ruralis, bryophytes, and uncritical transfer of concepts and techAnomodon viticulosus), and for the lichen Cladonia niques from vascular-plant physiology can lead to serious convoluta. Full-turgor water content ranged from c. misunderstanding and error. In particular, ‘relative water 110% DW. in T. ruralis and Andreaea alpina to 1400% contents’ based on notional ‘saturated’ water contents of DW. or more in Dumortiera hirsuta and Conocephalum bryophytes are not physiologically comparable to RWC conicum. Osmotic potential (Y p ) at full turgor was as generally understood in vascular plant physiology. between ’1.0 and ’2.0 MPa in most species, but Dilks and Proctor (1979) considered the water content substantially less negative values were found in the of a bryophyte as divisible into three parts—external thalloid liverworts (’0.35 to ’0.64 MPa). The x-inter- capillary water, symplast water within the cells, and cept of the P‐V curve is not a reliable estimate of apoplast water in the cell walls—and emphasized that apoplast volume and may give negative values; better the external capillary water is an essential functional estimates of apoplast volume may be obtained by component in the physiology of many bryophytes. The vapour equilibration at known low water potentials. external water is held at relatively high (near-zero) water Blotting external water from shoots usually gave full- potentials related to the size of the capillary spaces turgor water content estimates in reasonable agree- provided by the morphology of the plant; much of the ment with those obtained by analysis of P‐V curves, physiologically-important part of it is likely to be held but for different reasons they could be either higher between ’0.01 and ’0.5 MPa. The symplast water declines over a range of water potentials related to Y p , or lower than the true value. The importance of know

Water relations in Azospirillum-inoculated wheat seedlings under osmotic stress

Azospirillum has been shown to improve coleoptile growth in seedlings growing in darkness under osmotic stress. However, the changes in water relations that may occur in this experimental system have not yet been studied. Two-centimetre long Triticum aestivum cv. Buck Pucará and Triticum durum cv. Balcarceño-INTA seedlings were inoculated with viable or autoclaved (control) Azospirillum brasilense Sp. 245 bacteria, at approximately 108 cells per seedling. Three days after inoculation, seedlings were exposed to osmotic stress by immersing their roots in 20% polyethylene glycol 6000 for up to 72 h. Germination and seedling growth were at 20°C in darkness. Shoots were excised after 72 h of stress, and water-status parameters were determined through pressure-volume analyses. While osmotic potential at full turgor remained constant, Azospirillum-stimulated growth in Buck Pucará seedlings was accompanied by significant decreases in osmotic potential and relative water content at zero turgor, in volumetric cell wall modulus of elasticity, and in absolute symplastic water volume and by a significant rise in apoplastic water fraction parameters. Except for a constant volumetric cell wall modulus of elasticity, similar results were obtained with Balcarceño-INTA seedlings. However, bacterial growth promotion was evident only in the less tolerant cv. Buck Pucará. Turgor at low water potential was higher in inoculated seedlings in both wheat cultivars under osmotic stress. These results are consistent with a better water status in Azospirillum-inoculated wheat seedlings under water stress, where both effects on cell wall elasticity and (or) apoplastic water are evident.Key words: Azospirillum, drought, seedlings, water status, wheat.

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.

Effects of water stress cycles on turgor maintenance processes in pear leaves (Pyrus communis).

Effects of water deficits on leaf turgor maintenance processes were analyzed for pear trees (Pyrus communis L. cv. "Barlett") grown in 120-liter containers. Four irrigation treatments were applied: a well-watered control treatment, a spring water stress cycle (Sp), a summer water stress cycle (Su), and a spring plus summer water stress cycle (Sp + Su). For the Sp treatment, water application was progressively reduced from 100 to 20% of the control dose over a period of 27 days in spring. For the Su treatment, water application was progressively reduced over 23 days in summer, from 100 to 20% of the control dose. The Sp + Su treatment comprised both the spring and summer drought stress cycles. Pressure-volume (P-V) curves were constructed and stomatal conductances were determined for pear leaves from each treatment during the spring and summer stress cycles. Leaf water potential (Psi(pi) (0)) and relative water content (R(0)) at the turgor loss point of control leaves tended to decrease from spring to summer. Changes in leaf osmotic water potential at full turgor (Psi(pi) (100)) and in symplast water fraction (R(s)) did not explain the seasonal decrease in Psi(pi) (0). The water stress treatments had no effect on Psi(pi) (100), but R(s) was reduced by the water stress treatments, particularly during the summer stress cycle of the Su and Sp + Su treatments. The decrease in R(s) was correlated with an increase in the slope of the linear region of the P-V curve. Such a coupled adjustment would lead to increased water uptake capacity of water-stressed trees only under non-turgor conditions. Furthermore, pear leaves did not actively accumulate solutes. We conclude, therefore, that changes in leaf tissue water relations as a result of leaf acclimation to water stress are unlikely to facilitate maintenance of fruit productivity under drought.

Cell Wall Elasticity as a Mechanism to Maintain Favorable Water Relations During Leaf Ontogeny in Grapevines

The aim of this study was to elucidate the influence of leaf age on water relations parameters and cell wall elasticity which is considered to be one of the most important physiological mechanism of adaptation to water stress. The relationships between the water, turgor, osmotic potentials and leaf water content were studied in both mature and immature leaves of field grown grapevines (<i>V. vinifera</i> L.) using the pressure volume technique. The symplastic water fraction of the total water content averaged 0.78 and 0.69 for immature and mature leaves respectively. The calculated volumetric modulus of elasticity was significantly lower in immature leaves contributing in turgor maintenance and plant growth under mild water stress conditions. The reduction in cell wall elasticity observed in mature leaves, which resulted in rapid changes of ψ in response to small water losses, may confer an efficient physiological mechanism to face short-term water stress conditions as well as to favor production under drought conditions.

Drought-induced structural alterations at the mycobiont-photobiont interface in a range of foliose macrolichens

Cryotechniques, such as low temperature scanning electron microscopy (LTSEM) and freeze-substitution for transmission electron microscopy (TEM), were applied to two cyanobacterial and three green algal macrolichens in order to locate free water and to visualize drought-induced structural alterations at the mycobiont—photobiont interface. The following species were examined:Peltigera canina/Nostoc punctiforme, Sticta sylvatica/Nostoc sp. (both Peltigerales),Parmelia sulcata/Trebouxia impressa, Hypogymnia physodes/Trebouxia sp. (both Lecanorales), andXanthoria parietina/Trebouxia arboricola (Teloschistales). In all species free water was confined to the symplast and the apoplast. No intercellular water reservoirs were found in the gas-filled thallus interior. Thalline fluctuations in water content reflect fluctuations in apoplastic and symplastic water. All the taxonomically diverse lichen photobionts have access to water and dissolved nutrients via the fungal apoplast only. Drought stress (i.e., water content 20%/dw and below) caused dramatic shrinkage and deformation in all cell types. At any level of hydration the fungal and algal protoplast maintained close contact with the cell wall. This applied to the cyanobacterial photobionts and their murein sacculus and gelatinous sheath too. Although the cytoplasm of both partners was strongly condensed in desiccated lichens the cellular membrane systems, usually negatively contrasted, were very well preserved. The significance of these data is discussed with regard to the functioning of the symbiotic relationship.

The measurement of water relations of plant cell culture. I. Water release in response to centrifuge‐induced water potentials

Water loss by cell suspensions during centrifugation is well defined by simple physical principles. The major factors affecting water release during centrifugation are: duration of centrifogation, depth of the cell mass, density of cells, relative centripetal acceleration and centripetal force. Water release during centrifugation was best described by an exponential decay process with a decay constant that increases with acceleration from 0.31 ± 0.01 to 0.66 ± 0.12 min−1 (mean ± SE) between 4 825 and 19 300 m s−2, respectively. The cell mass relative water content (RWC) at equilibrium was not a function of rate of water loss and was constant for each acceleration. A centripetal force was generated by the mass of the cells being accelerated away from the axis of rotation. This force generated a pressure that removed some of the cell wall and symplast water, by compression at contact points between the cells and by compression of the cytoplasm. Pressure induced by centripetal forces ranging from −0.02 to −0.23 MPa gave a linear relationship (r2 &gt; 0.99) between force and RWC. The slope (0.900 MPa) was proportional to the cell wall modulus of elasticity (±). and the intercept was interpreted to give the mass of the cells at full turgor without interstitial water (RWC=1). This interpretation is supported by the findings, of two independent experiments. Centrifuged cells suspended at 100% relative humidity for over 48 h reached the same water content as predicted by the intercept. Interstitial water was labelled with solutions of polyethylene glycol (PEG. Mr 8 000), the diameter of which was too large to enter the pores of plant cell walls. Centripetal accelerations greater than 10 900 m s−2 removed PEG‐labelled water to levels below 0.9% of cell water content. Removal of interstitial water and other loosely bound water provided a convenient method for determination of growth, RWC and ±. The centrifugal methods provide the foundation for new quantitative methods for cell culture water relations analyses.

A simple method for locating the start of symplastic water flow (flumes) in leaves.

The identification of sites in leaves where transpiration water crosses cell membranes and enters the symplast has previously been made using freeze-substitution to locate concentrations of dye [e.g. sulphorhodamine (SR)] moving with the transpiration stream, and left outside the membranes where the water passes through. These concentrations were called sumps, and the sites of entry to the symplast were called flumes. A simple method of locating sumps, and therefore flumes, is described. Fresh leaves, fed SR solution through their cut petioles for pulse periods of 0.5 h or more, followed or not by a chase of water, were sectioned by hand under paraffin oil, and the sections mounted in the same fluid. Observation of the sections by simple bright-field microscopy revealed sumps of SR at the same sites, and of the same crystalline nature as found in the freeze-substituted preparations. The saving in preparation time is of the order of > 100-fold, at the sacrifice of resolution (5-10 μm compared with 0.2 μm). A limited survey of grass, sedge and dicotyledon leaves by this method confirmed in all essentials the results found by freeze-substitution, and in addition, revealed flumes at the fusoid cells on the flanks of the veins of bamboo leaves, and at the same position next to the water tissue of Cyperus leaves. The rate of accumulation of crystalline SR in the sumps inside tracheary elements suggests that the concentration of this non-permeating solute in the xylem sap increased by about 1000-fold in the finest veins during 1-2 h of transpiration in the dye solution.

Drought and Flood Stress Effects on Plant Development and Leaf Water Relations of Five Taxa of Trees Native to Bottomland Habitats

Responses of five bottomland tree taxa to drought and flooding were studied to identify those adapted to urban environments. During one experiment, containerized `Franksred' red maple [Acer rubrum L. `Franksred' (trademark = Red Sunset)], sweetbay magnolia (Magnolia virginiana L.), black tupelo (Nyssa sylvatica Marsh.), bald cypress [Taxodium distichum (L.) Rich.], and pawpaw [Asimina triloba (L.) Dunal.] were treated with various irrigation regimes for up to 118 days. Net assimilation rate (NAR) and relative growth rate (RGR) were reduced more by flooding than by drought for plants of all taxa, except pawpaw, which showed similar NAR and RGR during flooding and drought. Only sweetbay magnolia and bald cypress maintained positive NAR and RGR during flooding, and sweetbay magnolia was the only taxon that did not produce significantly less leaf surface area, shoot dry mass, and root dry mass during flooding and drought. Apparent morphological mechanisms of stress resistance included an increase in specific mass of leaves (mg·cm-2) during drought for red maple and bald cypress and a 385% increase in the root: shoot mass ratio for droughted plants of pawpaw. Leaf water relations of drought- and flood-stressed `Franksred' red maple and sweetbay magnolia were determined in a second experiment. Predawn and mid-day leaf water potential (ψ) decreased with decreasing root-zone matric potential for both taxa, and transpiration rate was reduced by drought and flooding. Pressure-volume analysis showed that leaves of `Franksred' red maple responded to drought by shifting symplastic water to the apoplast. Leaves of drought-stressed sweetbay magnolia adjusted osmotically by reducing osmotic potential (ψπ) at full turgor by 0.26 MPa. Our results suggest that sweetbay magnolia and bald cypress will perform well at urban planting sites where episodes of drought and flooding regularly occur.

Control of pressure-chamber and rehydration-time effects on pressure–volume determination of water-relation parameters

In a study of the seasonal variation of water-relation parameters in four black spruce (Picea mariana (Mill) BSP) populations, the weekly derivation of 32 pressure–volume curves with the sap-expression method between May and October required the simultaneous use of four pressure chambers and variable sample rehydration periods. Pressure chamber had a significant effect on three of the five characteristics studied: symplastic water fraction, maximum modulus of elasticity, and osmotic potential at full turgor. These effects were constant over the growing season. On average, rehydration-time effects were significant for all water-relation parameters except osmotic potential at turgor loss point, but they were not constant over sampling dates. Variations in water-relation parameters due to chambers and rehydration times were controlled and estimated through an experimental design based on the Latin square, which allowed precise comparisons among populations despite the increase in variability due to these two factors. The efficiency of the design varied between 110 and 142% relative to a completely randomized design in which neither chambers nor rehydration times would have been controlled for. Key words: black spruce, populations, experimental design.