Comparative community physiology: nonconvergence in water relations among three semi-arid shrub communities

Plant adaptations to the environment are limited, and therefore plants in similar environments may display similar functional and physiological traits, a pattern termed functional convergence. Evidence was examined for functional convergence among 28 evergreen woody shrubs from three plant communities of the semi-arid winter rainfall region of southern California. Both leaf and water relations traits were examined, including seasonal stomatal conductance (gs), specific leaf area (SLA), leaf specific conductivity (Kl), seasonal water potential (Psi w), stem cavitation resistance (Psi 50), and xylem density. Species display community-specific suites of xylem and leaf traits consistent with different patterns of water use among communities, with coastal sage scrub species utilizing shallow pulses of water, Mojave Desert scrub species relying on deeper water reserves, and chaparral species utilizing both shallow and deep moisture reserves. Communities displayed similar degrees of water stress, with a community-level minimum Psi w (Psi wmin) of c. -4.6 Mpa, similar to other arid communities. Pooled across sites, there was a strong correlation between Psi wmin and xylem density, suggesting that these traits are broadly related and predictive of one another. This comparative community physiology approach may be useful in testing hypotheses of functional convergence across structurally similar semi-arid communities.     

Cavitation resistance and seasonal hydraulics differ among three arid Californian plant communities

Vulnerability to water stress-induced cavitation was measured on 27 woody shrub species from three arid plant communities including chaparral, coastal sage and Mojave Desert scrub. Dry season native embolism and pre-dawn water potential, and both wet and dry season xylem specific hydraulic conductivity (Ks) were measured. Cavitation resistance, estimated as water potential at 50% loss in conductivity (Psi50), was measured on all species during the wet season and on a subset of species during the dry season. Cavitation resistance varied with sampling season, with 8 of 13 sampled species displaying significant seasonal shifts. Native embolism and water potential were useful in identification of species displaying seasonal shifts. The Ks was not different among sites or seasons. The Psi50 varied among species and communities. Within communities, interspecific variation may be partially explained by differences in rooting depth or leaf habit (evergreen, semi-deciduous, deciduous). Communities diverged in their Psi50 with chaparral species displaying the greatest cavitation resistance regardless of sampling season. The greater cavitation resistance of chaparral species is surprising, considering the greater aridity of the Mojave Desert site. Adaptation to arid environments is due to many plant traits, and aridity does not necessarily lead to convergence in cavitation resistance.     

Contrasting physiological responses of six eucalyptus species to water deficit

BACKGROUND AND AIMS: The genus Eucalyptus occupies a broad ecological range, forming the dominant canopy in many Australian ecosystems. Many Eucalyptus species are renowned for tolerance to aridity, yet inter-specific variation in physiological traits, particularly water relations parameters, contributing to this tolerance is weakly characterized only in a limited taxonomic range. The study tests the hypothesis that differences in the distribution of Eucalyptus species is related to cellular water relations. METHODS: Six eucalypt species originating from (1) contrasting environments for aridity and (2) diverse taxonomic groups were grown in pots and subjected to the effects of water deficit over a 10-week period. Water potential, relative water content and osmotic parameters were analysed by using pressure-volume curves and related to gas exchange, photosynthesis and biomass. KEY RESULTS: The six eucalypt species differed in response to water deficit. Most significantly, species from high rainfall environments (E. obliqua, E. rubida) and the phreatophyte (E. camaldulensis) had lower osmotic potential under water deficit via accumulation of cellular osmotica (osmotic adjustment). In contrast, species from low rainfall environments (E. cladocalyx, E. polyanthemos and E. tricarpa) had lower osmotic potential through a combination of both constitutive solutes and osmotic adjustment, combined with reductions in leaf water content. CONCLUSIONS: It is demonstrated that osmotic adjustment is a common response to water deficit in six eucalypt species. In addition, significant inter-specific variation in osmotic potential correlates with species distribution in environments where water is scarce. This provides a physiological explanation for aridity tolerance and emphasizes the need to identify osmolytes that accumulate under stress in the genus Eucalyptus.     

Drought acclimation among tropical forest shrubs (Psychotria,Rubiaceae)

Summary Mechanisms of dry-season drought resistance were evaluated for five evergreen shrubs (Psychotria, Rubiaceae) which occur syntopically in tropical moist forest in central Panama. Rooting depths, leaf conductance, tissue osmotic potentials and elasticity, and the timing of leaf production were evaluated. From wet to dry season, tissue osmotic potentials declined and moduli of elasticity increased in four and five species, respectively. Irrigation only affected osmotic adjustment by P. furcata. The other seasonal changes in leaf tissue properties represented ontogenetic change. Nevertheless, they made an important contribution to dry-season turgor maintenance. Small between-year differences in dry season rainfall had large effects on plant water status. In 1986, 51 mm of rain fell between 1 January and 31 March, and pre-dawn turgor potentials averaged <0.1 MPa for all five Psychotria species in March (Wright 1991). In 1989, 111 mm of rain fell in the same period, pre-dawn turgor potentials averaged from 0.75 to 1.0 MPa for three of the species in April, and only P. chagrensis lost turgor. The relation between leaf production and drought differed among species. P. limonensis was buffered against drought by the lowest dry-season conductances and the deepest roots (averaging 244% deeper than its congeners) and was the only species to produce large numbers of leaves in the dry season. P. chagrensis was most susceptible to drought, and leaf production ceased as turgor loss developed. For the other species, water stress during severe dry seasons may select against dry-season leaf production.     

Effect of water deficit on the PSII photochemical phases in two olive trees varieties

The effect of the water deficit, on two olive tree varieties 'Chetoui' and 'Chemlali' at the level of photosystem II photochemistry (PSII) was studied through the following parameters: leaf water potential (Psi(Hb)), quantum yield of PSII (PhiPSII), maximum quantum yield of PSII (Phi(max) PSII), electron transfer rate (J(T)) and photochemical quenching (qP). The results obtained show a reduction in the leaf water potential and a decrease in quantum efficiency of PSII. Besides, electron transfer rate and photochemical quenching showed an increase in response to water deficit. These modifications present some differences according to the variety. These observations are discussed in relation to the strategies developed to grow drought-resistant olive trees in arid areas.     

Seasonal changes in activity of subterranean termite species (Isoptera) in Western Australian wheatbelt habitats

Subterranean termite assemblages in woodland, mallee and heath habitats of the Western Australian wheatbelt were sampled for seasonal changes in species richness and species abundance. The study was carried out in Durokoppin and Kodj Kodjin Nature Reserves between January and November 1988, and a trench method was used to sample termites. Species richness changed over time, with monthy means of: woodland 10 species (range: 5–15), mallee 8 species (range: 4–15), heath 8 species (range: 5–15). Species richness was highest in autumn and spring when termite activity in the soil was also highest due to favourable soil moisture. Amitermes neogermanus and Tumulitermes petilus were the most abundant species in woodland and mallee in all seasons. During summer, the pooled observations of these two species accounted for 50% (woodland) and 82% (mallee) of all observations. Their predominance may be due to greater tolerance of low soil moisture and a more diverse diet than obligate wood-feeders (e.g. Rhinotermitidae). The latter were generally absent near the soil surface in summer, possibly because food sources deeper within the soil were utilized. Four species were abundant in heath: Tumulitermes dalbiensis (monthly mean 20% of all species observations), T. comatus (13%), Amitermes. sp.‘R’ (18%)and Drepanotermes rubriceps (12%). These four species showed no seasonal change in their relative abundance, possibly because their diet restricted them to surface-foraging.     

Growth-induced water potentials and the growth of maize leaves

Profiles of water potential (Psi(w)) were measured from the soil through the plant to the tip of growing leaves of fully established maize (Zea mays L.). The profiles revealed gradients in transpiration-induced Psi(w) extending upward along the transpiration path, and growth-induced Psi(w) extending radially between the veins in the elongating region of the leaf base. Water moving upward required a small gradient while that moving radially required a much larger gradient primarily because the protoxylem vessels were encased in many small, undifferentiated cells that were likely to act as a barrier to radial flow. Upon maturation, these small cells enlarged and some began to conduct water, probably decreasing the barrier. In the mature leaf, the growth-induced Psi(w) were absent but the transpiration-induced Psi(w) remained. When leaves were growing, the growth-induced Psi(w) moved water into the elongating cells during the day and night, and it shifted with changes in transpiration-induced Psi(w). The shift involved solutes accumulating in the growing region. When water was withheld, the growth-induced Psi(w) disappeared and leaf elongation ceased even though turgor pressure was at its highest. Turgor was maintained by osmotic adjustment that doubled the osmotic potential of the elongating cells. If elongation resumed at night or with rewatering, the growth-induced Psi(w) reappeared. If pressure was applied to the soil/root system to cause guttation and re-establish the growth-induced Psi(w), elongation resumed immediately. These findings support the hypothesis that the primary control of growth is the disappearance and reappearance of the growth-induced Psi(w) because the potential changed in the xylem and nearby cells, blocking or permitting radial water movement and thus blocking or permitting growth.     

Increases in water potential gradient reduce xylem conductivity in whole plants. Evidence from a low-pressure conductivity method

A new method using hydrostatic suctions (less than 0.02 MPa) was used to measure whole-root conductivity (K(r)) in saplings of two angiosperm pioneer trees (Eucalyptus regnans and Toona australis) and two rainforest conifers (Dacrycarpus dacrydioides and Nageia fleurii). The resultant K(r) was combined with measurements of stem and leaf hydraulic conductivity to calculate whole-plant conductivity and to predict leaf water potential (Psi(l)) during transpiration. At normal soil temperatures there was good agreement between measured and predicted Psi(l) during transpiration in all species. Changes in the soil-to-leaf water potential gradient were produced by root chilling, and in three of the four species, changes in Psi(l) corresponded to those expected by the effect of increased water viscosity on K(r). In one species, however, root chilling produced severe plant wilting and a decline in Psi(l) significantly below the predicted value. In this species Psi(l) decreased to a value close to, or below, the Psi(l) at 50% xylem cavitation. It is concluded that decreased whole-plant conductivity in T. australis resulted from a decrease in xylem conductivity due to stress-induced cavitation.     

西鄂尔多斯地区强旱生小灌木的水分参数

应用PV技术研究了西鄂尔多斯地区绵刺、红沙、四合木和霸王柴4种超旱生灌木的水分关系参数膨压(ψ_P)、细胞弹性模量(ε)、细胞体积比(RCV)及其相互关系.结果表明：在4种荒漠旱生灌木中,红沙保持最大膨压的能力最强(a=2.4593).不同荒漠旱生灌木保持膨压的方式不同：绵刺通过弹性调节保持膨压(ε_max=8.4005 MPa);红沙通过渗透调节来保持膨压(ψ_π100=-3.1302 MPa;ψ₀=-3.5074 MPa);四合木通过渗透调节和弹性调节的协同作用来维持膨压;霸王柴通过渗透调节来保持膨压,而弹性调节能力较弱.绵刺具有柔软而高弹性的细胞壁,是构成其根茎系统快速吸收和传导水分能力的因素之一.四合木具有较柔软而高弹性的细胞壁且ψ_P的变化随RCV减小而趋于缓慢,说明四合木具有较强的持水能力和抗脱水能力.     

Grasstree (Xanthorrhoea preissii ) leaf growth in relation to season and water availability

Abstract Water stress usually arrests growth of even the most deep‐rooted species during summer drought in Mediterranean‐type climates. However, scant evidence suggests that grasstrees may represent an unusual exception. We used weather data and plant water potential to investigate the relationship between leaf growth and season in the grasstree, Xanthorrhoea preissii Endl. (Xanthorrhoeaceae). Leaf production in two contrasting habitats revealed continuous annual growth, oscillating between maximum rates (2.5–3.2 leaves/d) in late‐spring to autumn, to a minimum rate of 0.5 leaf/d during winter but never stopping. While the rate of leaf production during the fast‐growth season was positively correlated with temperature above 17–18°C, leaf elongation commenced substantially earlier in the year (from 12°C). Leaf water potentials cycled annually, with predawn readings commonly measured as zero during winter–spring and as low as ?1.26 MPa during summer, but never indicating stress by exceeding the turgor loss point. Leaf death was synchronized with summer drought. The fast (summer) growth period was characterized by rapidly fluctuating leaf production, particularly in banksia woodland, where plant growth reliably responded quickly to >18 mm of rainfall. Within 24 h of 59 mm of simulated rainfall, grasstrees in banksia woodland showed a marked increase in water potential, and leaf production reached 7.5 times the controls, confirming their capacity to respond to temporary spasmodic summer rains. Rainfall was the best climatic variable for predicting woodland grasstree leaf production during summer, whereas leaf production of forest grasstrees was most closely correlated with daylength. This plastic response of grasstrees between seasonal weather extremes is relatively rare among other mediterranean floras, and has implications for a recently proposed technique for ageing grasstrees.     

Roles of leaf water potential and soil-to-leaf hydraulic conductance in water use by understorey woody plants

Diurnal changes of leaf water potential and stomatal conductance were measured for 12 deciduous shrubs and tree saplings in the understorey of a temperate forest. Sunflecks raised the leaf temperature by 4°C, and vapor pressure deficit to 2 kPa. Although the duration of the sunflecks was only 17% of daytime, the photon flux density (PFD) of sunflecks was 52% of total PFD on a sunny summer day. Leaf osmotic potential at full turgor decreased in summer, except in some species that have low osmotic potential in the spring. Plants that endured low leaf water potential had rigid cell walls and low osmotic potential at full turgor. These plants did not have lower relative water content and turgor potential than plants with higher leaf water potential. There were three different responses to an increase in transpiration rate: (i) plants had low leaf water potential and slightly increased soil-to-leaf hydraulic conductance; (ii) plants decreased leaf water potential and increased the hydraulic conductance; and (iii) plants had high leaf water potential and largely increased the hydraulic conductance.     

Leaf water relations and net gas exchange responses of salinized Carrizo citrange seedlings during drought stress and recovery

BACKGROUND AND AIMS: Since salinity and drought stress can occur together, an assessment was made of their interacting effects on leaf water relations, osmotic adjustment and net gas exchange in seedlings of the relatively chloride-sensitive Carrizo citrange, Citrus sinensis x Poncirus trifoliata. METHODS: Plants were fertilized with nutrient solution with or without additional 100 mm NaCl (salt and no-salt treatments). After 7 d, half of the plants were drought stressed by withholding irrigation water for 10 d. Thus, there were four treatments: salinized and non-salinized plants under drought-stress or well-watered conditions. After the drought period, plants from all stressed treatments were re-watered with nutrient solution without salt for 8 d to study recovery. Leaf water relations, gas exchange parameters, chlorophyll fluorescence, proline, quaternary ammonium compounds and leaf and root concentrations of Cl(-) and Na(+) were measured. KEY RESULTS: Salinity increased leaf Cl(-) and Na(+) concentrations and decreased osmotic potential (Psi(pi)) such that leaf relative water content (RWC) was maintained during drought stress. However, in non-salinized drought-stressed plants, osmotic adjustment did not occur and RWC decreased. The salinity-induced osmotic adjustment was not related to any accumulation of proline, quaternary ammonium compounds or soluble sugars. Net CO(2) assimilation rate (A(CO2)) was reduced in leaves from all stressed treatments but the mechanisms were different. In non-salinized drought-stressed plants, lower A(CO2) was related to low RWC, whereas in salinized plants decreased A(CO2) was related to high levels of leaf Cl(-) and Na(+). A(CO2) recovered after irrigation in all the treatments except in previously salinized drought-stressed leaves which had lower RWC and less chlorophyll but maintained high levels of Cl(-), Na(+) and quaternary ammonium compounds after recovery. High leaf levels of Cl(-) and Na(+) after recovery apparently came from the roots. CONCLUSIONS: Plants preconditioned by salinity stress maintained a better leaf water status during drought stress due to osmotic adjustment and the accumulation of Cl(-) and Na(+). However, high levels of salt ions impeded recovery of leaf water status and photosynthesis after re-irrigation with non-saline water.     

Tissue water relations and leaf growth of tropical corn cultivars under water deficits

Abstract This study reports on the effect of water deficit on the tissue water relations and leaf growth of six corn cultivars, growing in glasshouse conditions, in order to understand growth responses to drought of tropical corn. A mild water-stress treatment was imposed slowly; plants reached a minimum pre-dawn leaf water potential of about –1.5 MPa by day 12 after watering was withheld. Analysis of the water relation characteristics of growing leaves using the pressure–volume technique demonstrated that under water deficits all the cultivars changed their moisture-release curves compared with irrigated plants. Osmotic potential at full turgor was lowered in water-stressed plants of all the genotypes and the degree of such change was between 0.34 MPa and 0.58 MPa. Thus, turgor pressure was lost at a lower water potential in water-stressed plants than in irrigated plants of all the varieties. Volumetric elastic moduli were also increased under water deficits and the increase ranged between 10% and 141% among the cultivars. In all the genotypes, the stress imposed led to a reduction of leaf area and dry matter accumulation. Leaf expansion was very sensitive to low turgor pressure and it ceased when turgor reached 0.2 MPa. Thus, varieties able to maintain a higher degree of turgor pressure (i.e. by osmotic adjustment) under water deficits may be able to prolong leaf growth.     

Photosynthetic limitations of a halophyte sea aster (Aster tripolium L) under water stress and NaCl stress

To understand the mechanisms of salt tolerance in a halophyte, sea aster ( Aster tripolium L.), we studied the changes of water relation and the factors of photosynthetic limitation under water stress and 300 mM NaCl stress. The contents of Na(+) and Cl(-) were highest in NaCl-stressed leaves. Leaf osmotic potentials ( Psi(s)) were decreased by both stress treatments, whereas leaf turgor pressure ( Psi(t)) was maintained under NaCl stress. Decrease in Psi(s) without any loss of Psi(t) accounted for osmotic adjustment using Na(+) and Cl(-) accumulated under NaCl stress. Stress treatments affected photosynthesis, and stomatal limitation was higher under water stress than under NaCl stress. Additionally, maximum CO(2) fixation rate and O(2) evolution rate decreased only under water stress, indicating irreversible damage to photosynthetic systems, mainly by dehydration. Water stress severely affected the water relation and photosynthetic capacity. On the other hand, turgid leaves under NaCl stress have dehydration tolerance due to maintenance of Psi(t) and photosynthetic activity. These results show that sea aster might not suffer from tissue dehydration in highly salinized environments. We conclude that the adaptation of sea aster to salinity may be accomplished by osmotic adjustment using accumulated Na(+) and Cl(-), and that this plant has typical halophyte characteristics, but not drought tolerance.     

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around –2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.     

Components of water potential estimated from xylem pressure measurements in five tree species

Summary Pressure volume curves were measured with a pressure bomb in leaves collected in the field from Ilex opaca, Acer rubrum, Liquidambar styraciflua, Liriodendron tulipifera and Cornus florida. Water potential components were calculated from the curves. The species differed in the relationships measured. In all species the trends from summer to fall were toward lower (more negative) osmotic potentials, lower matric potentials more rapid loss of turgor with increasing leaf water deficit, and the occurrence of incipient plasmolysis at lower values of leaf water deficit. Initial osmotic potentials ranged from-14.8 to-19.8 bars, similar to values reported in the literature for other mesophytic plants. These values, however, were much higher than those reported for halophytes and xerophytes. The fraction of leaf water which contributes to the osmotic potential ranged from 0.74 to 0.98 in this study. Values reported for other mesophytes and for halophytes and xerophytes all fall well within this range. Patterns of component water potentials are discussed in relation to potential growth rates and water flow in the total plant system.     

Quercitol and osmotic adaptation of field-grown Eucalyptus under seasonal drought stress

This study investigated the role of quercitol in osmotic adjustment in field-grown Eucalyptus astringens Maiden subject to seasonal drought stress over the course of 1 year. The trees grew in a native woodland and a farm plantation in the semi-arid wheatbelt region of south Western Australia. Plantation trees allocated relatively more biomass to leaves than woodland trees, but they suffered greater drought stress over summer, as indicated by lower water potentials, CO₂ assimilation rates and stomatal conductances. In contrast, woodland trees had relatively fewer leaves and suffered less drought stress. Plantation trees under drought stress engaged in osmotic adjustment, but woodland trees did not. Quercitol made a significant contribution to osmotic adjustment in drought-stressed trees (25% of total solutes), and substantially more quercitol was measured in the leaves of plantation trees (5% dry matter) than in the leaves of woodland trees (2% dry matter). We found no evidence that quercitol was used as a carbon storage compound while starch reserves were depleted under drought stress. Differences in stomatal conductance, biomass allocation and quercitol production clearly indicate that E. astringens is both morphologically and physiologically 'plastic' in response to growth environment, and that osmotic adjustment is only one part of a complex strategy employed by this species to tolerate drought.     

Using multiple trait associations to define hydraulic functional types in plant communities of south-western Australia

Assessing the hydrological imbalance and associated land degradation issues facing much of southern Australia and other parts of the world requires a better understanding of the defining features of ecosystem water use and the design of sustainable agroecosystems. Thus, by grouping species with similar water-use strategies into 'hydraulic functional types' (HFTs), we investigated the characteristics of water use in species-rich plant communities of south-western Australia. HFTs were determined using multiple-trait associations between morphological and physiological traits relating to water transport, water-use efficiency and response to water deficit. Sixteen traits were assessed from a subset of 21 species from three plant communities located along a topographically determined soil- and water-availability gradient. Multivariate analyses showed that trait variation was least at sites with shallower soils and putatively lower water availability, suggesting a convergence of water-use strategies at sites where plants are exposed to large seasonal water deficits. Stem hydraulic parameters, including stem-specific hydraulic conductivity, conduit diameter and maximum percentage embolism, were positively correlated, indicating the generality that larger conduit diameter permits greater hydraulic efficiency and is associated with greater seasonal reductions in hydraulic conductivity in this ecosystem. Wood density was not correlated with these traits, but closely associated with species' ability to withstand more negative water potentials during summer. Long-term integrated water-use efficiency was lower in shallow-rooted species that exhibited more negative summer water potentials. Specific leaf area and minimum leaf water potential were correlated with a number of separate traits, and appear to represent key axes of trait variation that describe the water-use strategies of different HFTs along the topographic gradient. Five HFTs were classified using a resemblance analysis according to combinations of traits that pertain to different water-use strategies among species; year-round active tree, year-round active shrub, hemiparasite, drought-suppressed broad-leaved shrub and drought-suppressed narrow-leaved shrub.     

High apoplastic solute concentrations in leaves alter water relations of the halophytic shrub, Sarcobatus vermiculatus

Predawn plant water potential (Psi(w)) is used to estimate soil moisture available to plants because plants are expected to equilibrate with the root-zone Psi(w). Although this equilibrium assumption provides the basis for interpreting many physiological and ecological parameters, much work suggests predawn plant Psi(w) is often more negative than root-zone soil Psi(w). For many halophytes even when soils are well-watered and night-time shoot and root water loss eliminated, predawn disequilibrium (PDD) between leaf and soil Psi(w) can exceed 0.5 MPa. A model halophyte, Sarcobatus vermiculatus, was used to test the predictions that low predawn solute potential (Psi(s)) in the leaf apoplast is a major mechanism driving PDD and that low Psi(s) is due to high Na+ and K+ concentrations in the leaf apoplast. Measurements of leaf cell turgor (Psi(p)) and solute potential (Psi(s)) of plants grown under a range of soil salinities demonstrated that predawn symplast Psi(w) was 1.7 to 2.1 MPa more negative than predawn xylem Psi(w), indicating a significant negative apoplastic Psi(s). Measurements on isolated apoplastic fluid indicated that Na+ concentrations in the leaf apoplast ranged from 80 to 230 mM, depending on salinity, while apoplastic K+ remained around 50 mM. The water relations measurements suggest that without a low apoplastic Psi(s), predawn Psi(p) may reach pressures that could cause cell damage. It is proposed that low predawn apoplastic Psi(s) may be an efficient way to regulate Psi(p) in plants that accumulate high concentrations of osmotica or when plants are subject to fluctuating patterns of soil water availability.     

Gradients in water potential and turgor pressure along the translocation pathway during grain filling in normally watered and water-stressed wheat plants

The water relations parameters involved in assimilate flow into developing wheat (Triticum aestivum L.) grains were measured at several points from the flag leaf to the endosperm cavity in normally watered (Psi approximately -0.3 MPa) and water-stressed plants (Psi approximately -2 MPa). These included direct measurement of sieve tube turgor and several independent approaches to the measurement or calculation of water potentials in the peduncle, grain pericarp, and endosperm cavity. Sieve tube turgor measurements, osmotic concentrations, and Psi measurements using dextran microdrops showed good internal consistency (i.e. Psi = Psi(s) + Psi(p)) from 0 to -4 MPa. In normally watered plants, crease pericarp Psi and sieve tube turgor were almost 1 MPa lower than in the peduncle. This suggests a high hydraulic resistance in the sieve tubes connecting the two. However, observations concerning exudation rates indicated a low resistance. In water-stressed plants, peduncle Psi and crease pericarp Psi were similar. In both treatments, there was a variable, approximately 1-MPa drop in turgor pressure between the grain sieve tubes and vascular parenchyma cells. There was little between-treatment difference in endosperm cavity sucrose or osmotic concentrations or in the crease pericarp sucrose pool size. Our results re-emphasize the importance of the sieve tube unloading step in the control of assimilate import.