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This study examined whether two genotypes of hybrid poplar ( Populus deltoides × Populus trichocarpa), previously classified as ozone tolerant and ozone sensitive, had differing physiological and biochemical responses when fumigated with 120 nL L−1 ozone for 6 h per day for eight consecutive days. Isoprene emission rate, ozone uptake and a number of physiological and biochemical parameters were investigated before, during and after fumigation with ozone. Previous studies have shown that isoprene protects plants against oxidative stress. Therefore, it was hypothesized that these two genotypes would differ in either their basal isoprene emission rates or in the response of isoprene to fumigation by ozone. Our results showed that the basal emission rates of isoprene, physiological responses and ozone uptake rates were all similar. However, significant differences were found in visible damage, carotenoids, hydrogen peroxide (H2O2), thiobarbituric acid reactions (TBARS) and post-fumigation isoprene emission rates. It is shown that, although the classification of ozone tolerance or sensitivity had been previously clearly and carefully defined using one particular set of parameters, assessment of other key variables does not necessarily lead to the same conclusions. Thus, it may be necessary to reconsider the way in which plants are classified as ozone tolerant or sensitive. [ABSTRACT FROM AUTHOR]

Abstract There is now strong evidence that the plant hormone abscisic acid (ABA) plays an important role in the regulation of stomatal behaviour and gas exchange of droughted plants. This regulation involves both long-distance transport and modulation of ABA concentration at the guard cells, as well as differential responses of the guard cells to a given dose of the hormone. We will describe how a plant can use the ABA signalling mechanism and other chemical signals to adjust the amount of water that it loses through its stomata in response to changes in both the rhizospheric and the aerial environment. The following components of the signalling process can play an important part in regulation: (a) ABA sequestration in the root; (b) ABA synthesis versus catabolism in the root; (c) the efficiency of ABA transfer across the root and into the xylem; (d) the exchange of ABA between the xylem lumen and the xylem parenchyma in the shoot; (e) the amount of ABA in the leaf symplastic reservoir and the efficiency of ABA sequestration and release from this compartment as regulated by factors such as root and leaf-sourced changes in pH; (f) cleavage of ABA from ABA conjugates in the leaf apoplast; (g) transfer of ABA from the leaf into the phloem; (h) the sensitivity of the guard cells to the [ABA] that finally reaches them; and lastly (i) the possible interaction between nitrate stress and the ABA signal. [ABSTRACT FROM AUTHOR]

ABSTRACTSingle leaves of 3-month-old Cedrella odorata seedlings were exposed to a step reduction in the ambient dew point. The resultant time series of dynamic variations in leaf surface water vapour concentration, leaf surface water vapour concentration gradient, transpiration rate and stomatal conductance to water vapour, are analysed using the data-based mechanistic (DBM) modelling methodology of Young (e.g. Young & Lees 1992; Minchin et al. 1996). It is shown that the identified second-order, dynamic model between transpiration rate (as the input) and stomatal conductance (as the output) provides an appropriate, physiologically meaningful, description of the system. In particular, the dynamic relationship between these two variables is remarkably linear and can be resolved in terms of two parallel, first-order, subsystems; a model which complements the results of Cowan (1977) for cotton. The model is also compared with the recently published simulation model of Haefner, Buckley & Mott (1997). [ABSTRACT FROM AUTHOR]

Transplants of beech, Fagus sylvatica, L. were planted in 1 m plastic tubes containing a mixture of peat and grit and sunk into the ground inside open-top chambers at a site in southern Britain. Chambers were ventilated with either charcoal-filtered or unfiltered air and water was withheld from half of the trees within each chamber. The influence of air quality on the growth and development of roots and interactions between air quality and drought were investigated. During May and June 1988 the growth of fine (diameter < 1.5 mm) and large roots (diameter > 15 m) was not significantly altered by air quality. A destructive harvest conducted in July revealed that trees exposed to ambient pollution had significantly longer roots (P < 0.05) compared with trees grown in filtered air, despite slightly less root dry weight (not statistically significant). Specific root length (SRL, root length per unit root dry weight) was also significantly larger (P < 0.05) for trees grown in the unfiltered chambers and exposed to pollution, suggesting a root system which was not only longer but also consisted of thinner roots, compared with the roots of trees grown in clean air. The effects of pollution were more pronounced for roots in the lower half of the 1 m profile. During August 1988, water was withheld from half of the trees within each chamber. Well-watered trees exposed to unfiltered air had significantly less root dry weight (P < 0.05) compared with well-watered trees grown in filtered air, although root length and SRL were both greater for trees grown in unfiltered air compared with those grown in filtered air. Again the effects were more pronounced lower in the root profile. Withholding water resulted in less root length being produced irrespective of air quality treatment, suggesting that trees exposed to pollution would not necessarily be more susceptible to drought-stress. [ABSTRACT FROM AUTHOR]

Sunflower plants (Helianthus annuus cv. Tall Single Yellow) were grown in the greenhouse in drain pipes (100 mm inside diameter and 1 m long) filled with John Innes No. 2 compost. When the fifth leaf had emerged, half of the plants were left unwatered for 6 days, rewatered for 2 days and then not watered for another 12 days. Measurements of water relations and abaxial stomatal conductance were made at each leaf position at regular intervals during the experimental period. Estimates were also made of soil water potentials along the soil profile and of ABA concentrations in xylem sap and leaves. Soil drying led to some reduction in stomatal conductance after only 3 days but leaf turgors were not reduced until day 13 (6 days after rewatering). When the water relations of leaves did change, older leaves became substantially dehydrated while high turgors were recorded in younger leaves. Leaf ABA content measured on the third youngest leaf hardly changed over the first 13 days of the experiment, despite substantial soil drying, while xylem ABA concentrations changed very significantly and dynamically as soil water status varied, even when there was no effect of soil drying on leaf water relations. We argue that the highest ABA concentrations in the xylem found as a result of substantial soil drying, arise from synthesis in both the roots and the older leaves, arid act to delay the development of water deficit in younger leaves. In other experiments ABA solutions were watered on to the root systems of sunflower plants to increase ABA concentrations in xylem sap. The stomatal response to applied ABA was quantitatively very similar to that to ABA generated as a result of soil drying. There was a log-linear relationship between the reduction of leaf conductance and the increase of ABA concentration in xylem sap. [ABSTRACT FROM AUTHOR]

The primary aim of the study was to quantify the effects of photosynthetically-active radiation (PAR) on extension of leaves of silver birch (Betula pendula Roth.) and sycamore (Acer pseudoplatanus L.). Plants grown at 250 μmol m-2 s1 were exposed to a range of fluence rates (0 680 μmol m-2 s-1) whilst enclosed in Perspex chambers in which temperature, vapour pressure deficit and photoenvironment were all controlled. Measurements of leaf extension, water relations, and cell wall extensibility (WEX) were made at the end of a 3 h exposure period. For leaves of birch, reducing the fluence rate resulted in reduced leaf extension and a lowering of WEX. Exposure of sycamore seedlings for 3 h to different values of PAR had little effect on the growth of leaves and WEX remained constant. The characteristics of net photosynthesis also differed for the two species and photosynthesis and wall loosening may be linked. Since natural woodland shadelight, in addition to reduced PAR, also has reduced R/FR ratio, a second group of seedlings were placed in a photoenvironment which simulated shadelight (low PAR, R/FR) for 28 d. This treatment reduced extension rate and final leaf size for both species. Measurements of extension, water relations and WEX made when seedlings were exposed to shadelight whilst held in the Perspex chambers revealed differences in the control of leaf growth for the two species. It is confirmed that leaves of birch are highly responsive to changes in PAR with cell wall loosening the most likely process controlling this response. [ABSTRACT FROM AUTHOR]

Leaf growth of first year seedlings of silver birch (Betula pendula Roth,) and sycamore (Acer pseudoplatanus L.) was measured continuously using linear variable transducers (LVTDs) when shoots were enclosed in small Perspex chambers, in which temperature, vapour pressure deficit (VPD) and photon fluence rate were all controlled. A new method was used to measure the yield turgor (Y) of growing leaves. The VPD of air in the chamber was altered so that transpiration rate was changed. Turgor pressures, measured using thermocouple psychrometers, and growth rates varied as a result of this treatment, and the relationship between leaf growth and turgor pressure (P) and the turgor pressure at which growth ceased (F) could he measured within a very short time period using well-watered plants. The value of Y was constant for illuminated and darkened leaves of both species, but Y was higher for sycamore leaves (Y = 0.250 MPa) compared with the value for birch leaves (Y = 0.071 MPa). Cell wall extensibility (m, MPa[sup-1]h[sup-1]) was calculated from the slope of the line relating turgor pressure to growth. For leaves of birch, there was a significant increase in pu when leaves were illuminated (m, dark = 1.3 x 10[sup-2] MPa[sup-1]h[sup-1]; m, light = 4.6 x 10[sup-2] MPa[sup-1]), similar in magnitude to the increase in wall extensibility measured previously by mechanical analysis of killed leaf tissue (WEX, the plasticity of tissue subjected to a uni-axial stress). No change in was detected when darkened sycamore leaves were illuminated (mn, dark = 4.2 x l0[sup-2] MPa[sup-1] h[sup-1]; m, light = 4.4 x 10[sup-2] MPa[sup-1] h[sup-1]). The results confirm earlier work on the factors controlling the diurnal patterns of leaf growth of these two species and suggest that leaf growth of birch is primarily limited by cell wall loosening hut that the growth of sycamore leaves may be limited by leaf turgor. [ABSTRACT FROM AUTHOR]

Leaf extension of one-year-old seedlings of silver birch (Betula pendula Roth.) and sycamore (Acer pseudoplatanus L.), was measured using linear variable transducers (LVDTs) interfaced to a microcomputer. Birch and sycamore seedlings exhibited contrasting patterns of leaf extension during a diurnal cycle with a 16 h photoperiod. Birch leaves grew more rapidly when illuminated; growth during the photoperiod was approximately doubled when compared with growth in the dark. Mean relative growth rates±SE at 'lights-on + 3 h' and 'lights-off+5 h' were 0.0136±0.0016 and 0.0066±0.0005 h-1 respectively. In direct contrast, growth of sycamore leaves was increased when leaves were darkened; mean relative growth rates ± SE at 'lightson± 3 h' and 'lights-off+ 5 h' were 0.0056±0.0005 and 0.0094±0.0008 h-1 respectively. When leaves of birch and sycamore were darkened, increased leaf turgor was measured in both species, but only in sycamore was this higher night-time turgor associated with a higher rate of leaf growth. Cell wall extensibility (WEX), ah indication of the ability of cell walls to loosen and extend irreversibly, and cell surface pH were assessed in darkened and illuminated leaves of both species. An increase in WEX was measured when birch leaves were illuminated (P<0.001) and this was accompanied by a decline in cell surface pH (P<0.001). However, when leaves of sycamore were illuminated, WEX declined (P < 0.05) and cell surface pH increased (P< 0.001). The ability of these species to survive beneath a woodland canopy is discussed in relation to the cellular factors controlling their leaf growth. [ABSTRACT FROM AUTHOR]

Three-month-old Cedrella odorata were exposed to a soil-drying treatment. During this period, xylem sap was periodically collected from the plant by applying pneumatic pressure to the roots. This also allowed whole-plant water status to be measured by recording the balancing pressure applied. The concentration of ABA in xylem sap (C) was related to the whole-plant transpiration rate (V) which was measured witha sap flow gauge. The analysis of these paired measurements centred on how the reciprocal of C (R) varied with respect to V. This revealed that (1) the observed increases in C could not be explained by the reductions in V alone, (2) initially, decreases in V were associated with proportional increases in the whole-plant ABA flux (M), and (3) this relationship broke down at low values of V since zero flow was associated with a finite value for C estimated to be 41 pmol ABA mmol-1 H2O. A simple static model is developed from the observations that is able to explain the data well, and the results are discussed in terms of the effects of ABA on stomatal conductance (gsw). [ABSTRACT FROM AUTHOR]

We describe here an integration of hydraulic and chemical signals which control stomatal conductance of plants in drying soil, and suggest that such a system is more likely than control based on chemical signals or water relations alone. The determination of xylem [ABA] and the stomatal response to xylem [ABAJ are likely to involve the water flux through the plant (1) If, as seems likely, the production of a chemical message depends on the root water status (Ψr), it will not depend solely on the soil water potential (Ψs) but also on the flux of water through the soil-plant-atmosphere continuum, to which are linked the difference between Ψr and Ψs. (2) The water flux will also dilute the concentration of the message in the xylem sap. (3) The stomatal sensitivity to the message is increased as leaf water potential falls. Stomatal conductance, which controls the water flux, therefore would be controlled by a water-flux-dependent message, with a water-flux-dependent sensitivity. In such a system, we have to consider a common regulation for stomatal conductance, leaf and root water potentials, water flux and concentration of ABA in the xylem. In order to test this possibility, we have combined equations which describe the generation and effects of chemical signals and classical equations of water flux. When the simulation was run for a variety of conditions, the solution suggested that such common regulation can operate. Simulations suggest that, as well as providing control of stomatal conductance, integration of chemical and hydraulic signalling may also provide a control of leaf water potential and of xylem [ABA], features which are apparent from our experimental data. We conclude that the root message would provide the plant with a means to sense the conditions of water extraction (soil water status and resisance to water flux) on a daily timescale, while the short-term plant response to this message would depend on the evaporative demand. [ABSTRACT FROM AUTHOR]

It is now clear that drying of the soil does not always result in an early change in shoot water status. This may be because stomata close and leaf growth slows to reduce water loss. When this is the case, it is necessary to ask how the change in soil water status has been 'sensed' by the shoot. The current view is that soil drying results in some type of chemical signalling between roots and shoots. The sensitivity of the response and experiments involving the manipulation of small parts of root systems suggest that the signalling involves more than a simple change in root activity in response to soil drying. In this paper, we consider the evidence for chemical signalling between roots and shoots and discuss the possible candidates for such signals. In some plants, root-sourced ABA can apparently influence shoot physiology and growth in the absence of any perturbation of shoot water relations. The ABA produced is quantitatively sufficient to account for the responses observed. Applied ABA can mimic many of the effects of soil drying on plants, including effects at the plasma membrane and on gene expression. Perhaps uniquely, ABA seems to be involved in signalling between different plant organs, and in signalling at the transmembrane and genome levels. We review the effects of ABA on leaf cells with a view to gaining some understanding of how soil drying may influence plant development. [ABSTRACT FROM AUTHOR]

Maize seedlings (Zea mays L. John Innes F1 hybrid) were grown in a greenhouse in 1-m-long tubes of soil. When the plants were well established, water was withheld from half of the tubes. Control plants were watered every day during the 20-d experimental period. The soil drying treatment resulted in a substantial restriction of stomatal conductance and a limitation in shoot growth, even though there was no detectable difference in the water relations of watered and unwatered plants. From day 7 of the soil drying treatment, xylem ABA concentrations (measured using the sap exuded from detopped plants) were substantially increased in unwatered plants compared to values recorded with sap from plants watered every day. Measurements of water potential through the profile of unwatered soil suggest that xylem ABA concentrations reflects the extent of soil drying. Leaf ABA content was a much less sensitive indicator of the effect of soil drying and during the whole of experimental period there was no significant difference between ABA concentration in leaves of well watered and unwatered plants. In a second set of experiments, ABA was fed to part of the roots of potted maize plants to manipulate xylem ABA concentration. These manipulations suggested that the increases in ABA concentration in xylem sap, which resulted from soil drying, were adequate to explain the observed variation in stomatal conductance and might also explain the restriction in leaf growth rate. These results are discussed in the light of recent work which suggests that stomatal responses to soil drying are partly attributable to an as-yet unidentified inhibitor of stomatal opening. [ABSTRACT FROM AUTHOR]

Maize plants were grown in l-m-long tubes of John Innes No. 2 potting compost. From the start of the experimental period, half of the plants were unwatered. Stomatal conductance of these plants was restricted 6d after last watering and continued to decline thereafter. This was despite the fact that as a result of solute accumulation, unwatered plants showed consistently higher leaf turgors than wellwatered plants. Leaf water potentials of unwatered plants were not significantly lower than those of plants that were watered well. Main seminal and nodal roots showed solute regulation in drying soil and continued to grow even in the driest soil, and plants growing in drying soil showed consistently higher root dry weights than did well-watered plants. Water potentials and turgors of the tips of fine roots in the upper part of the column decreased as the soil dried. Soil drying below a water content of around 0.25 g cm[SUP-3] (a bulk soil water potential of between -0.2 and -0.3MPa) resulted in a substantial increase in the ABA content of roots. As soil columns dried progressively from the top, ABA content increased in roots deeper and deeper in the soil. These responses suggest that ABA produced by dehydrating roots and which was subsequently transported to the shoots provided a sensitive indication of the degree of soil drying. [ABSTRACT FROM AUTHOR]

Experiments with isolated roots of wheat plants suggested that when water uptake rates are low, low concentrations of abscisic acid (ABA) may increase the flux of water into roots. This increase was recorded despite an ABA-stimulated reduction in the hydraulic conductance of the whole root system. Hydraulic conductances were measured under steady state conditions. A system is described where the stomatal behaviour and water movement through roots of a single intact plant may be concurrently monitored. Experiments with intact plants confirmed that application of ABA could increase the rate of water movement into roots when uptake rates were low. No such increase was observed at high flux rates. Application of ABA to roots caused partial stomatal closure and caused conductance to oscillate around a reduced mean value. An ABA-stimulated increase in the turgor sensitivity of stomata is postulated and the significance of this effect is discussed. [ABSTRACT FROM AUTHOR]

Potted seedlings of four lines of maize and Sorghum of differing drought tolerance were subjected to a single soil drying cycle and were only rewatered when the plants showed the first signs of wilting. Other plants remained well-watered throughout the experimental period. As plant water potentials decreased in the unwatered plants of three of the lines investigated (Sorghum Piper and M35-1, V-4146 and maize Farz 27), endogenous levels of farnesol-like antitranspirants increased. Closure of stomata correlated well with the increase in endogenous antitranspirant. In the fourth line (Sorghum M35-1, V-4184), stomata did not close as the level of plant water stress increased, although leaf diffusion resistance of even the well-watered plants of this line was quite high. In this line, there was no consistent relationship between plant water stress and antitranspirant level or between stomatal behavior and antitranspirant level. The involvement of farnesol-like antitranspirants in the control of stomatal behavior in water-stressed plants is discussed. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) fed in pulses to the petioles of detached cherry leaves in enclosed leaf chambers, caused a reduction in leaf conductance. The degree of inhibition was analysed with respect to the amount of ABA fed and to concentration of ABA in the feeding solution. Regression analysis of the data showed both variables to have a significant effect on leaf conductance. A hypothetical maximum ABA concentration occurring in the leaf apoplast was calculated for each pulse from a simple model. This variable explained more of the variance within the data than either the amount or the applied concentration variable. A value for the rate at which ABA is removed from the apoplast is derived from the experimental data using the model. A second experiment attempted to evaluate this rate directly, by measuring the rate of catabolism of labelled ABA within the leaf. The results suggested a half-life of 36 min for the initial rate of decay. This figure is similar to that derived from the model, the importance of ABA-metabolism for the control of leaf conductance is discussed in the context of root-to-shoot communication by ABA in the xylem stream. [ABSTRACT FROM AUTHOR]

Stomatal conductance of individual leaves, was measured In a maize field, together with leaf water potential, leaf turgor, xylem ABA concentration and leaf ABA concentration In the same leaves. Stomatal conductance showed a tight relationship With xylem ABA, but not with the current leaf water status or with concentration of ABA in the bulk leaf. The relationship between stomatal conductance and xylem [ABA] was common for variations in xylem [ABA] linked to the decline with time of the soil water reserve, to simultaneous differences between plants grown on compacted, non-compacted and irrigated soil, and to plant-to-plant variability. Therefore, this relationship is unlikely to be fortuitous or due to synchronous variations. These results suggest that increased concentration of ABA in the xylem sap in response to stress can control the gas exchange of plants under field conditions. [ABSTRACT FROM AUTHOR]

During two seasons, ABA concentrations were monitored in roots, leaves and xylem sap of field-grown maim The water status of soil and plant was also measured Plants were grown on plots with compacted or non-compacted soil, which were irrigated or remained unwatered, ABA concentration in the xylem sap before dawn and in the roots increases 25-fold and five-fold, respectively, as the soil dried, with a close correlation with the soil water status, but with no clear effect of the soil structures. In contrast to the results of several laboratory experiments, no appreciable increase in xylem [ABA] and reduction in stomatal conductance were observed with dehydration of the part of the root system located in soil upper layers. These responses only occurred when the water reserve of the whole soil profile was close to depletion and the transpiration declined. Xylem (ABA] measured during the day was appreciably higher in the compacted treatment than in non-compacted treatment, unlike that measured before dawn. Since a mechanical message is unlikely to undergo such day-night alterations, we suggest that this was due to a faster decrease in root water potential and water flux in the compacted treatment, linked to the root spatial arrangement These results raise the possibility that ABA concentration in the xylem sap could be controlled by two coexisting mechanisms: (1) the rate of ADA synthesis in the roots linked to the soil or root water status, as shown in laboratory experiments; (2) the dilution of ABA in the water flow from roots, which could be an overriding mechanism in field conditions. This second mechanism would allow the plant to sense the water flux through the root system. [ABSTRACT FROM AUTHOR]

Stomatal conductance, leaf water potential, soil water potential and concentration of abscisic acid (ABA) in the xylem sap were measured on maize plants growing in the field, in two treatments with contrasting soil structures. Soil compaction affected the stomatal conductance, but this effect was no longer observed if the soil water potential was increased by irrigation. Differences in leaf water potential did not account for the differences in conductance between treatments. Conversely, the relationship between stomatal conductance and concentration of ABA in the xylem sap was consistent during the experiment. The proposed interpretation is that stomatal conductance was controlled by the root water potential via an ABA message. Control of the stomatal conductance by the leaf water potential or by an effect of mechanical stress on the roots in unlikely. [ABSTRACT FROM AUTHOR]

Biophysical parameters controlling leaf expansion in hybrid poplar (Populus nigra L. × P. deltoides Bartr. cx Marsh = P. x euramericana) were measured for first-year cuttings growing under controlled conditions, with or without ozone fumigation. As individual leaves expanded, leaf water potential remained constant whilst solute potential declined and rurgor (P) increased from approximately 0.5 MPa to over 10 MPa as leaf size increased. Yield turgor (Y) measured by stress-relaxation of leaf discs within psychrormeters, increased from approximately 0-2 MPa to over 1.0 MPa during the same period. As the value of Y approached that of P. the effective turgor for growth (P- Y) declined and became zero as leaf growth ceased. Cell wall extensibility (measured as percentage plasticity by the `Instron' technique) was related to leaf growth rate such that highest values of plasticity corresponded to maximum rates of growth. As growth ceased, wall extensibility declined to very low values. Fumigation with 50 nl l-1 ozone for 16 h d-1 had no effect on any of the measured parameters with the result that growth and mature leaf size were unaffected by comparison with control plants receiving charcoal-filtered air. Additional episodes of 120 nl l-1 ozone on three consecutive days during the period of maximum leaf growth similarly had no effect on expansion or final leaf size. [ABSTRACT FROM AUTHOR]

Plants of Commelina communis L. were grown in media containing different concentrations of calcium from 1 to 8 mol m-3. After 4 weeks' growth stomatal behaviour was studied both on the intact plants and on isolated abaxial epidermis. The effect of Ca nutrition on stomatal opening was apparently different according to these two methods of observation. The apertures measured in light and in absence of CO2 were smaller on abaxial epidermal strips from plants grown at larger Ca concentrations than those grown at smaller concentrations. The inhibition of opening was thought to be due to the presence of free Ca2+ ions because inhibition was not found when EGTA was included in the medium used for incubating the isolated epidermis, and significant quantities of Ca were lost to the medium by epidermal tissue from plants grown on high concentrations of Ca. Studies on the leaves of intact plants were made with a diffusion porometer and by gas exchange techniques using a leaf cuvette in which the environment was closely controlled. A/c1 curves were used to partition non-stomatal and stomatal limitations to gas exchange. Porometer measurements of stomatal conductance, determined within the growth room, indicated reduced values for plants grown at low Ca compared with those grown at high Ca, i.e. the observations contradicted those obtained with isolated epidermis. Biomass production increased with Ca up to 4 mol m-3. Analysis of A/c1 curves indicated that plants grown at the larger Ca concentration showed significantly greater values for carboxylation efficiency (dA/dc1). Partitioning of stomatal and non-stomatal limitations to carbon fixation indicated that the gas-phase limitations of CO2 were similar at different Ca concentrations. Differences in carboxylation efficiency could account for the greater biomass accumulation found at the larger Ca concentration. [ABSTRACT FROM AUTHOR]