Turgor Maintenance Research Articles

Competitive Turgor Maintenance in Tall Fescue

Economical, rapid methods are needed to identify drought‐tolerant turfgrasses. Objectives of this study were to (i) examine water relations of three tall rescue (Festuca arundinacea Schreb.) selections, (ii) determine the relationship between these characteristics and recovery from water deficit under greenhouse conditions, and (iii) assess the feasibility of using a competitive soil moisture extraction technique to select tall rescue germplasm with superior drought tolerance. Three tall rescue selections were vegetatively propagated and grown together in 38‐L weighing lysimeters to allow interplant competition for available soil moisture. Adequate irrigation and fertilizer were applied for 12 wk after planting; irrigation was then withheld for 8 wk. Tiller survival among selections ranged from 2 to 37% and plant survival ranged from 22 to 76% at 4 wk after irrigation was resumed. Survival was associated with low basal osmotic potential before stress and osmotic adjustment, prolonged positive turgor maintenance, and delayed leaf rolling during stress. Leaf rolling scores, and tiller and plant survival during competitive soil moisture extraction were easily assessed indicators of tall fescue germplasm drought tolerance.

Evaluation of contrasting cellular-level acclimation responses to leaf water deficits in three wheat genotypes

Osmotic adjustment, increased apoplastic water fraction (AW) and increased cell wall elasticity have been cited as cell physiological responses to leaf water deficits which could allow acclimation to water stress. All three cell responses to stress should allow the maintenance of greater turgor (Ψp) at low water potential (Ψw). The physiological significance of drought-induced changes in these cell water relations parameters were evaluated in plants of three wheat ( Triticum sp.) genotypes exposed to successive water stress episodes using results from pressure/volume curve analysis. Maintenance of Ψp at low leaf Ψw during subsequent water stress episodes was taken as a measure of plant acclimation to stress. Net photosynthesis was used as a measure of cell metabolic function during the stress episodes. Results indicated that stress-induced increases in AW and cell wall elasticity were not associated with acclimation to water deficits, while osmotic adjustment did result in acclimation and enhanced photosynthesis at low Ψw. It was concluded that among these wheat genotypes osmotic adjustment, and not the other cell responses to water deficits, is an important acclimation mechanism which can allow for the maintenance of relatively greater metabolic function at low leaf Ψw.

Maintenance of Turgor by Rapid Sealing of Puncture Wounds in Leaf Epidermal Cells

When leaf epidermal cells are puncture wounded with a glass microcapillary tip, a small droplet of fluid is discharged and then evaporates, leaving a solid residue on the cell surface. For puncture wounds of about 3.5 micrometers in diameter, this process is complete within 2 to 3 seconds. A second puncture wound also exhibits a similar discharge, indicating the persistence of some turgor pressure within the cell, despite damage to the cell wall. Direct measurement of turgor on the large epidermal cells of Tradescantia virginiana L. demonstrated that turgor was substantially maintained (91-96%) after puncture wounding. Anatomical and histochemical evidence suggests that the damaged portion of the cell wall was sealed with an amorphous plug of material comprised of pectinaceous polysaccharides. Rapid sealing of puncture wounds and the maintenance of turgor in epidermal cells may be an important functional component of plant adaptation to physical damage such as that caused by insect feeding.

Carbohydrate accumulation and turgor maintenance in seedling shoots and roots of two boreal conifers subjected to water stress

Water potential components and organic solutes were examined in shoots and roots of potted jack pine (Pinus banksiana Lamb.) and white spruce (Picea glauca (Moench) Voss) seedlings after exposure to 7 days of water stress. The osmotic potential at the turgor loss point (ψπTLP) decreased in shoots and roots of water-stressed seedlings of both species, resulting in the maintenance of positive turgor at lower xylem water potentials (ψX) compared with nonstressed seedlings. Following water stress, ψπTLP of shoots and roots declined by 0.28 MPa and 0.14 MPa, respectively, in jack pine, and 0.19 MPa and 0.28 MPa, respectively, in white spruce. The osmotic potential at saturation (ψπ100) was significantly lower after water stress only in jack pine roots. Active osmotic adjustment during water stress was confirmed by higher concentrations of organic solutes in white spruce shoots (1.4 × increase relative to nonstressed plants) and roots (1.7 ×) and in the roots (2.2 ×) but not the shoots of jack pine. Carbohydrates, particularly fructose and glucose, were the primary organic solutes accumulating in both species. Tissue elasticity was greater in the roots than the shoots of both jack pine and white spruce regardless of treatment. Consequently, the relative water content at the turgor loss point was 22% and 18% lower in the roots than in the shoots of jack pine and white spruce, respectively. Osmotic adjustment in the roots and shoots of these two boreal conifers suggests that preconditioning planting stock by exposure to water stress may increase carbohydrate concentrations and enhance seedling drought tolerance. Key words: carbohydrate accumulation, drought tolerance, organic solutes, osmotic adjustment, Picea glauca, Pinus banksiana, water potential components.

Acclimation of CO2 Assimilation in Cotton Leaves to Water Stress and Salinity

Cotton (Gossypium hirsutum L. cv Acala SJ2) plants were exposed to three levels of osmotic or matric potentials. The first was obtained by salt and the latter by withholding irrigation water. Plants were acclimated to the two stress types by reducing the rate of stress development by a factor of 4 to 7. CO(2) assimilation was then determined on acclimated and nonacclimated plants. The decrease of CO(2) assimilation in salinity-exposed plants was significantly less in acclimated as compared with nonacclimated plants. Such a difference was not found under water stress at ambient CO(2) partial pressure. The slopes of net CO(2) assimilation versus intercellular CO(2) partial pressure, for the initial linear portion of this relationship, were increased in plants acclimated to salinity of -0.3 and -0.6 megapascal but not in nonacclimated plants. In plants acclimated to water stress, this change in slopes was not significant. Leaf osmotic potential was reduced much more in acclimated than in nonacclimated plants, resulting in turgor maintenance even at -0.9 megapascal. In nonacclimated plants, turgor pressure reached zero at approximately -0.5 megapascal. The accumulation of Cl(-) and Na(+) in the salinity-acclimated plants fully accounted for the decrease in leaf osmotic potential. The rise in concentration of organic solutes comprised only 5% of the total increase in solutes in salinity-acclimated and 10 to 20% in water-stress-acclimated plants. This acclimation was interpreted in light of the higher protein content per unit leaf area and the enhanced ribulose bisphosphate carboxylase activity. At saturating CO(2) partial pressure, the declined inhibition in CO(2) assimilation of stress-acclimated plants was found for both salinity and water stress.

A preliminary study of the carbohydrate metabolism in Parthenium argentatum

The decline in starch observed in guayule at the end of summer cannot be attributed to a decrease in temperature, but rather that starch can, upon hydrolysis, provide carbon for the synthesis of fructans. In this plant, fructans are important winter reserves arising from a changed ratio of carbon utilization to carbon fixation, and are seen as important in the maintenance of turgor during stress. Evidence suggests that bud break, leaf initiation, and flowering are times of major seasonal metabolic shifts with reallocation of carbohydrate reserves. It is concluded that growth has priority on carbon allocation, while rubber is seen as a secondary product and the result of metabolic ‘overspill’. The amount of rubber produced is the result of an interaction between enzyme and substrate availability and the compartmentation of cells. These findings have important implications in studies aimed at increasing rubber biomass.

Leaf Water Relations and Maintenance of Gas Exchange in Coffee Cultivars Grown in Drying Soil

Plant water status, leaf tissue pressure-volume relationships, and photosynthetic gas exchange were monitored in five coffee (Coffea arabica L.) cultivars growing in drying soil in the field. There were large differences among cultivars in the rates at which leaf water potential (Psi(L)) and gas exchange activity declined when irrigation was discontinued. Pressure-volume curve analysis indicated that increased leaf water deficits in droughted plants led to reductions in bulk leaf elasticity, osmotic potential, and in the Psi(L) at which turgor loss occurred. Adjustments in Psi(L) at zero turgor were not sufficient to prevent loss or near loss of turgor in three of five cultivars at the lowest values of midday Psi(L) attained. Maintenance of protoplasmic volume was more pronounced than maintenance of turgor as soil drying progressed. Changes in assimilation and stomatal conductance were largely independent of changes in bulk leaf turgor, but were associated with changes in relative symplast volume. It is suggested that osmotic and elastic adjustment contributed to maintenance of gas exchange in droughted coffee leaves probably through their effects on symplast volume rather than turgor.

Tissue water relations in elfin cloud forest tree species of Serrania de Macuira, Guajira, Colombia

Diurnal courses of stomatal conductance, leaf water potential, and the components of tissue water potential were measured in six canopy species in an elfin cloud forest. High values of stomatal conductance were measured on cloudy days and during early morning and late afternoon of sunny days. Decreases in stomatal conductance with increases in vapour pressure deficit may have been a response to avoid further water deficits and suggested a stomatal response to changes in relative humidity. Daily transpiration varied between 470 and 1014 g m-2 day-1 during cloudy days and between 532 and 944 g m-2 day-1 during clear days. Stomatal conductance may have also responded to changes in leaf water potential, which was minimum at noon. The minimum tissue water potential measured in the field was -1.8 MPa in Myrcianthes fragrans, and the minimum turgor pressure was 0.49 MPa also in M. fragrans. There was a correlation between the osmotic potential and the minimum tissue water potential, suggesting that osmotic potential plays a major role in the maintenance of turgor in these species, in spite of the great variability in the elastic properties of leaf tissues. Turgor pressure decreased during the day following the course of water potential but never approached the turgor loss point, as it has been measured in some lowland rain forest trees. This is a strong indication that elfin cloud forest trees do not suffer severe water deficits, and that small tree stature is not directly related to water shortage.

Turgor Maintenance in Rosa rugosa Grown at Three Levels of Nitrogen and Subjected to Drought

Abstract The influence of N fertilization on turgor maintenance was determined in leaves from well-watered and droughted Rosa rugosa L. Plants were fertilized for 60 days with a complete fertilizer with N at levels of 0, 200 or 500 ppm and then subjected to several drought cycles for 22 days. Plants receivmg no N were stunted and chlorotic, but had the greatest full saturation turgor pressure and symplastic osmolality, and the lowest full saturation osmotic potential, after drought. These plants also maintained higher turgor across a range of leaf water potentials and relative water contents. Leaf water content at full turgor and relative water content at the turgor loss point were also lower in plants without N. N treatments of 200 and 500 ppm had similar water relations under both well-watered and drought conditions.

Salinity Tolerance in Sorghum. I. Whole Plant Response to Sodium Chloride in S. bicolor and S. halepense

Based on casual qualitative observations, grain sorghum [Sorghum bicolor (L.) Moench[ is less tolerant to salt than the noxious weed and potential biomass energy crop, johnsongrass [S. halepense (L.) Pers.]. The objective was to quantitatively compare whole plant growth and physiological responses to salt stress of these two sorghum species. Salt stress was induced by adding incremental levels of NaCI to a vermiculite medium until concentrations of 0.1 and 0.2 M were attained. Leaf number and leaf area reduction and dry weight reduction in the culm and leaves in response to salinity compared to controls were greater in S. bicolor than in S. halepense. Larger leaf growth reductions in response to salinity in S. bicolor were associated with higher tissue levels of Na and CI. Sorghum halepense had a lower Na/K ratio in the leaves as well as in the roots; the cuim ratio was the same in both species. Higher ψp (0.65 MPa) and lower ψs(−2.17 MPa) S. bicolor leaves compared to the ψp (0.28 MPa) and ψs (−1.71 MPa) S. halepense leaves indicated more osmotic adjustment and more turgor maintenance in S. bicolor than in S. halepense; this response was due largely to CI and sucrose accumulation. The greater growth reduction observed in S. bicolor was associated with higher levels of CI higher Na/K ratios, and a greater capacity for osmotic adjustment. A Na exclusion mechanism appeared to be operative in both species but was more apparent in S. halepense.

INFLUENCE OF GENOTYPE, WATER AND N ON LEAF WATER RELATIONS IN NO-TILL WINTER WHEAT

Field studies were established at four locations in 1985 and 1986 to study the physiological responses of two winter wheat (Triticum aestivum L.) cultivars under different water and nitrogen regimes. Measurements of leaf water potential (ψ1), osmotic potential (π) and leaf conductance (g1) were made on individual leaves at regular intervals throughout the growing season. Values of ψ1 ranged from −1.0 to −4.5 MPa, indicating that winter wheat in Saskatchewan experiences moderate to extreme levels of water stress. Leaf ψ1 levels generally decreased as evaporative demand for water increased. Osmotic potential values ranged from −1.0 to −4.0 MPa while ψp (calculated as ψp = ψ1 − π) ranged from −0.4 to 0.9 MPa. Despite strong osmotic adjustment in response to stress, positive ψp was not always maintained. Additional water through limited irrigation increased both ψ1 and π but did not significantly affect ψp. Fertilizer N additions consistently decreased both ψ1, and π, and in some cases ψp levels. The tall cultivar Norstar displayed greater turgor maintenance than the semidwarf cultivar Norwin under both stress and nonstress conditions. Leaf conductance ranged from approximately 0.04 – 1.10 cm s−1 and was higher in irrigated plots. Fertilizer N additions often decreased g1 per unit area of leaf. On a leaf area basis, g1 for Norwin at high N levels was often greater than for Norstar. However, similar g1 vs. ψ1 responses for Norwin and Norstar indicated a similar stomatal response to increased water stress for these cultivars.Key words: Drought stress, nitrogen, leaf conductance, turgor potential

Water relations of stem succulent trees in north-central Baja California.

Water relations of several stem succulent trees were measured in north-central Baja California in comparisons to other growth forms in the same habitat. Our research concentrated on three stem succulent species (Idria collumnaris, Pachycormus discolor and Bursera microphylla) each with a different succulent stem morphology. The stem succulent trees had 1 to 4 kg H2O/m3 of trunk while the other trees and shrubs in the same habitat had 0.6 to 0.8 kg H2O/m3. The diurnal and seasonal variation in leaf water potential was small for the stem succulent species in comparison to deciduous and evergreen species as a consequence of the stem-water, buffering capacity. In addition, the leaf conductance of the stem succulent species was low (60 mmol m-2 s-1) and yet, the leaf conductance decreased through the day similar to adjacent evergreen and deciduous species. The leaves of the stem succulent trees lost turgor at low saturated water deficits (0.06 to 0.14), had comparatively high osmotic potentials, and high values of elastic modulus in comparison to adjacent evergreen and deciduous species. The stem acts as an important buffering mechanism allowing for the maintenance of leaf turgor in these stem succulent trees. The low transpiration rates of the stem succulent trees may be a mechanism to minimize leaf saturated water deficit and extend leaf longevity.

Contrasts between Citrus species in response to salinisation: An analysis of photosynthesis and water relations for different rootstock‐scion combinations

Leaf gas exchange, water relations and ion content were measured on two‐year‐old Valencia orange (Citrus sinensis [L.] Osbeck), Washington Navel orange (C. sinensis) and Marsh grapefruit (C. parodisi Macfad) scions budded to either Trifoliata (Poncirus infoliata [L] Raf) or Cleopatra mandarin (C. reticuLua Blanco) rootstoeks. Trees were watered with dülute nutrient solution containing either 0 or 50 mM NaCl for 77 days. Leaf chloride concentrations (cell sap basis) were higher in all scions budded on “Trifoliata but sodium levels were lower than in equivalent foliage budded on Cleopatra mandarin rootstock. Foliar salt levels also varied according to scion. Leaves of Marsh grapefruit had higher levels of both sodium and chloride than leaves of either Valencia orange or Washington Navel orange on both rootstocks. Accumulation of sodium and chloride in salinised leaves caused a reduction in leaf osmotic potential of 0.2–1.4 MPa. and leaf water potential declined by as much as 0.5 MPa. Turgor pressure in salinised leaves was thus maintained at or above the control level. Osmotic potentials determined by psychrometry compared with pressure‐volume curves were taken to imply that some accumulation of sodium or chloride in the apoplast of salinised leaves may have occurred.Despite turgor maintenance both co2 assimilation and stomatal conductance were reduced by salinity. Following onset of leaf response to salinisation, gas exchange was impaired to a greater extent in scions budded to Cleopatra mandarin compared to those on Trifoliata. Amongst those scions. leaves of salt‐treated Marsh grapefruit showed greater reductions in gas exchange than Valencia orange or Washington Navel orange budded on either rootstock. Increased sensitivity of 1Marsh grapefruit was correlated with a higher foliar sodium and chloride content in this scion. Scion differences in sensitivity of leaf gas exchange to solute concentration were independent of rootstock and appeared unrelated to leaf prolinebetaine concentrations. This implies an inherent difference between scion species with respect to salt tolerance, rather than variation in their capacity to acquire that type of compatible solute.In terms of rootstock effects, all scions proved more sensitive to salinity when budded to Cleopatra mandarin compared with Trifoliata. That response was attributed to a disproportionately higher concentration of leaf sodium in scions on Cleopatra mandarin.

Leaf water relations characteristics of Lupinus angustifolius and L. cosentinii.

Lupins (Lupinus angustifolius and L. cosentinii) growing in 321 containers in a glasshouse were exposed to drought by withholding water. Leaf water potential (Ψ1), and leaf osmotic potential (Ψs) were measured daily as soil water became depleted. Leaf water relations were further assessed by a pressure-volume technique and by measuring Ψs and relative water content of leaves after rehydration. Analysis by pressure-volume or cryoscopic techniques showed that leaf osmotic potential at saturation (Ψs100) decreased from -0.6 MPa in well watered to -0.9 MPa in severely droughted leaves, and leaf water potential at zero turgor (Ψzt) decreased from about -0.7 to -1.1 MPa in well watered and droughted plants, respectively. Relative water content at zero turgor (RWCzt) was high (88%) and tended to be decreased by drought. The ratio of turgid leaf weight to dry weight was not influenced by drought and was high at about 8.0. The bulk elastic modulus (ɛ) was approximately halved by drought when related to leaf turgor potential (Ψp) and probably mediated turgor maintenance during drought. The latter was found to be negatively influenced by rate of drought. Supplying the plants with high levels of K salts did not promote adjustment or turgor maintenance.

Adaptive Radiation of the Hawaiian Silversword Alliance (Compositae- Madiinae): Ecological, Morphological, and Physiological Diversity

The ecological, morphological, and physiological diversity of species in the Hawaiian silversword alliance is exceptional. The 28 species, which belong to the endemic genera Argyroxiphium, Dubautia, and Wilkesia, have a wide variety of geographical distributions and elevational ranges within the archipelago. They grow in habitats as varied as dry scrub and woodland, wet scrub and forest, cinder and lava, and bog. Ecological diversity is also evident among sympatric species. At a site of sympatry on the island of Hawaii, for example, D. ciliolata and D. scabra are restricted to different lava flows, even though individuals of the two species may grow within a meter of one another. The 28 species have growth forms as varied as rosette shrubs, shrubs, trees, and lianas. They have a wide range of leaf sizes and shapes, with the Dubautia species exhibiting significant variation in leaf turgor maintenance capacities. Morphological and physiological diversity is also evident among sympatric species. At a site of sympatry on the island of Maui, for example, A. sandwicense and D. menziesii exhibit different suites of morphological and physiological traits enabling them to cope with the severe environmental conditions. The patterns of diversity and the genomic relationships among the 28 species suggest that a variety of factors may have played important roles in their adaptive radiation. The Hawaiian silversword alliance is a premier example of adaptive radiation in plants (Carlquist, 1980; Carr et al., 1989). The alliance includes 28 species in three endemic genera: Argyroxiphium, Dubautia, and Wlilkesia (Carr, 1985). The species grow in a wide range of habitats and have a wide variety of growth forms. They are also closely related, as evidenced by the high frequency of spontaneous interspecific and intergeneric hybrids in nature, coupled with the ease of production of artificial hybrids in the laboratory (Carr & Kyhos, 1981, 1986). The detailed analysis of the hybrids and parental taxa using cytogenetic, electrophoretic, and molecular approaches has provided compelling evidence that the silversword alliance is a genetically cohesive group whose origin and diversification probably trace to a single colonizing ancestor (Baldwin et al., 1988; Carr et al., 1989). Our objective in this review is to provide insight into the ecological, morphological, and physiological diversity of species in the silversword alliance. With respect to ecological diversity, we compare the geographical distributions, habitats, and elevational ranges of the 28 species, then analyze the local distributions of several sympatric Dubautia species. With respect to morphological and physiological diversity, we compare the growth forms, leaf sizes, and leaf shapes of the 28 species, then examine the turgor maintenance capacities of the Dubautia species and the water and temperature balances of two sympatric Argyroxiphium and Dubautia species. Our primary theme is that the ' This paper is dedicated to Dr. Sherwin Carlquist in honor of his pioneering research on the Hawaiian silversword alliance. The research was supported by NSF Grant DEB-82064 11 and a gift from the Atlantic Richfield Foundation to the senior author. We thank Lani Stemmermann for invaluable companionship, botanical insight, and technical assistance in the field. We also thank the staff of Haleakala National Park, particularly R. Nagata and L. Loope, for critical logistical support, and J. Canfield and N. Friedman for generous photographic assistance. 2 Department of Ecology & Evolutionary Biology, University of Arizona, Tucson, Arizona 85721, U.S.A. < Department of Botany, University of Hawaii, Honolulu, Hawaii 96822, U.S.A. 4 Department of Plant & Soil Sciences, University of Maine, Orono, Maine 04469, U.S.A. Department of Botany, University of California, Davis, California 95616, U.S.A. ANN. MISSOURI BOT. GARD. 77: 64-72. 1990. This content downloaded from 152.1.161.146 on Thu, 27 Mar 2014 19:39:59 PM All use subject to JSTOR Terms and Conditions Volume 77, Number 1 Robichaux et al. 65 1990 Adaptive Radiation of Hawaiian Silversword Alliance

Field Drought Tolerance of a Soybean Plant Introduction

Rainfall is seldom sufficient to meet the evaporative and transpirational demands of a soybean crop [Glycine max (L.) Merr.] in the southeastern USA. Development of new drought‐tolerant cultivars would thus seem an effective way in which to address the problem of moisture stress. Unfortunately, few drought‐tolerant genotypes have been identified for use as breeding stock. The objective of this study was to compare the relative droughtolerance of plant introduction PI 416937, a visually slow‐wilting accession, with that of ‘Forrest’, a popular cultivar of similar maturity. Leaf water potential (Ψw), solute potential (Ψs), and relative water content (RWC) these genotypes were measured under two levels of soil water adjustment (well‐watered and water‐stressed at early pod‐fill) during 2 yr in the field (on a Varina loamy sand, a clayey, kaolinitic, thermic Plinthic Paleudult). Although water stress reduced Ψw equally for both genotypes, PI 416937 maintained lower levels of Ψs and. higher levels of pressure potential and RWC than Forrest. A comparison of the relationship between RWC and Ψs for the two genotypes indicated that the PI may accumulate more solutes in leaf tissue under stress than Forrest. That is, at a normalized leaf pressure potential of zero, the Ψs of the PI was estimated to be 0.3 MPa lower than that of Forrest. Seed yields of Forrest and the unadapted PI were comparable under stress. However, stress reduced the yield of Forrest by more than half while reducing yield of the PI by only a third. The superior turgor maintenance and competitive yielding ability of the unadapted PI under stress indicated that the PI may be an important source of drought tolerance for breeding programs.

Intraspecific variation in the water relations of Salix arctica, an arctic-alpine dwarf willow

The seasonal and diurnal water relations were investigated within arctic and alpine populations of the dwarf willow Salix arctica. Marked differences that were habitat dependent (e.g. xeric vs. mesic) occured both within and between the populations. The environmental variables that most affected plant water balance and the bulk tissue water relations were soil water potential (Ψsoil) and the leafto-air water vapor pressure gradient (Δw), however, low soil temperature (<4.0° C) also had a marked effect in the wet to mesic habitats. The effects of declining Ψsoil and increased ‡w were most pronounced in the plants growing in xiric habitats in both populations. Stomatal response to increased ‡w was two-fold greater in alpine versus arctic plants and is hypothesized to have arisen in response to more frequent exposure to the higher evaporative conditions of alpine existance. Seasonal fluctuations in the osmotic potential closely followed changes in Ψsoil, suggesting that these were active rather that passive changes. Additionally, plants from xeric habiats had a lower bulk tissue elastic modulus (more elastic tissues) in both arctic and alpine populations. The osmotic and elastic properties enhanced turgor maintenance over a broad range of leaf water potentials and during periods when ‡w was high. Turgor maintenance also correlated to continued transpiration despite fluctuations in soil and atmospherically induced water deficits. Arctic habitats have a shorter growing season, lower soil temperatures due to the presence of permafrost, but higher soil water potentials and lower leaf-to-air vapor pressure gradients than alpine habitats. The observed variation in patterns of stomatal conductance and in values of tissue water relations characteristics between arctic and alpine populations of S. arctica is hypothesized to have arisen in response to these different environmental regimes which represent different selective regimes that occur along the arctic-alpine environmental continuum inhabited by this wide ranging species.

Root-zone Temperature Affects Water Status and Growth of Red Maple

Abstract Red maple (Acer rubrum L.) plants were evaluated for their responses to 5 weeks of constant root-zone temperatures from 18 to 36C. Shoot lengths of plants grown with 18 to 30C root zones did not differ significantly from one another at any time during the study, and shoot dry weights of these plants were similar. However, after 21 days of exposure, shoot length of plants grown with roots at 36C was significantly less than that of plants with roots grown at 30C and below. Leaf area was greatest among plants with roots at 24C, and mean shoot and root dry weights of plants in the 36C treatment were 57% and 68% less, respectively, than those for plants with roots at 30C. Leaf diffusive resistance of plants grown at 36C was five times greater than for plants with root zones at 30C or below. Shoot water potential decreased with increasing temperature, but increased solute concentration in leaves of 36C-grown plants probably contributed to turgor maintenance.

Response of orchard 'Washington Navel' orange, Citrus sinensis (L.) Osbeck, to saline irrigation water. I. Canopy characteristics and seasonal patterns in leaf osmotic potential, carbohydrates and ion concentrations

Effects of irrigation water salinity on tree canopy volume, leaf area, rates of leaf abscission and production, as well as seasonal patterns in leaf osmotic potential (=), starch, soluble sugars and sodium and chloride concentrations were determined for 24-year-old 'Washington Navel' orange trees (Citrus sinensis [L.] Osbeck) on sweet orange (C. sinensis) rootstock. Trees had been irrigated with water containing either 5 or 20 mol NaCl m-3 for 5 years prior to measurements.Trees irrigated with 20 mol NaCl m-3 had a greater number of vegetative flushes in spring. This occurred at the expense of flowering, as numbers of reproductive and mixed flushes were reduced by salinity. Despite a high number of vegetative buds on trees irrigated with 20 mol NaCl m-3, leaf area was still less than low salinity trees.Extensive abscission of spring flush leaves occurred from mid-summer onwards for trees irrigated with 20 mol NaCl m-3. This was not a consequence of leaf water deficit, as more negative leaf osmotic potentials resulting from increased foliar sodium and chloride concentrations resulted in maintenance of leaf turgor. Excessive concentrations of sodium and/or chloride may have been responsible for abscission observed. Some acclimation of foliage to salinity was evident.Irrespective of salinity treatment, leaf osmotic potential became more negative as the season progressed. This was partly due to increased concentrations of soluble sugars in foliage during autumn and winter. Levels of soluble sugars and starch were consistently lower in leaves on trees irrigated with high salinity water, indicating that production rather than utilization of carbohydrate may limit citrus productivity under saline conditions.

Effect of Water Stress on Stomatal Conductance and Leaf Water Relations of Leaves Along Current-Year Branches of Peach

A gradation, that reflects the maturity of the leaves, exists in the leaf water, osmotic and turgor potential and stomatal conductance of leaves along current and 1-year-old branches of peach. Predawn leaf water potentials of immature folded leaves were approximately 0.24 MPa lower than mature leaves under both well-watered and dry conditions. During the daytime the leaf water potential of immature leaves reflected the water potential produced by water flux for transpiration. In well- watered trees, mature and immature unfolded leaves had a solute potential at least 0.5 MPa lower than immature folded leaves, resulting in a turgor potential that was approximately 0.8 MPa higher. The turgor requirement for growth appeared to be much less than that maintained in mature leaves. As water stress developed and leaf water potentials decreased, the osmotic potential of immature folded leaves declined to the level found in mature leaves, thus maintaining turgor. In contrast, mature leaves showed little evidence of turgor maintenance. Stomatal conductance was lower in immature leaves than in fully mature leaves. With the onset of water stress, conductance of mature leaves declined to a level near that of immature leaves. Loss of turgor in mature leaves may be a major factor in early stomatal closure. It was concluded that osmotic adjustment played a role in maintenance of a leaf water status favorable for some growth in water-stressed immature peach leaves.