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After the imposition of soil drying treatments, an elevation of xylem sap pH is one of the earliest observable responses in many herbaceous model plant species. It is theorized that alkalization of sap results in a concurrent elevation in abscisic acid (ABA) concentration delivered to transpiring tissues by preventing Henderson–Hasselbalch-regulated partitioning between the apoplast and symplast. However, here it is demonstrated that the sap alkalzation response to soil drying is far from universal in higher plant species. Tests were conducted to determine how universal the pH response to drying soil was in a range of perennial species from a diverse range of plant families. The response was not found in the majority of the 22 species tested. Four species exhibited significant increases in pH, but the majority showed no significant change in xylem sap pH. There was no evolutionary relationship between the species that showed alkalization under drought stress. However, the species that alkalized sap also exhibited good control over internal water status and were the most isohydric species of those tested. None of the species exhibiting anisohydric responses alkalized xylem sap under drought stress. Regardless of alkalization response, plants still retain the ability to respond to changes in xylem sap pH when manipulated by alkaline buffer foliar sprays. This finding indicates that plants have conserved the ability to respond to changes in xylem pH and redistribute ABA, even if they do not currently utilize the mechanism when exposed to drought stress. It was found in Buddleja davidii, Euonymus fortunei, and Hydrangea serrata that the xylem sap pH response to water deficits mirrored the natural pH changes that occur as sap is transported to the leaves, indicating that plants need to be able to have naturally occurring alkalization processes in place for them to be up-regulated under drought stress. [ABSTRACT FROM PUBLISHER]

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]

Several mechanisms enable leaf stomata to optimize water loss with respect to carbon gain. Stomatal responses to environmental variation constitute a plant's first and second lines of defense against damaging water deficits. Changes in the concentrations of endogenous growth regulators and their influence on stomata may well be important to both defense strategies. [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]

In two experiments, the effect has been investigated of a mild and more prolonged drought on the spatial distribution of growth, epidermal cell lengths and cell wall peroxidase activities in the leaf elongation zone of the grass species Lolium temulentum L. In both experiments drought reduced the size of the elongation zone and local rates of elongation within it. Abrupt increases in cell wall-associated peroxidase activity occurred at or close to the position where elongation ceased in the leaf elongation zones of well-watered and mildly drought-stressed plants. More prolonged drought caused a 200-300% increase in the cell wall-associated peroxidase activity in the elongation zone only. The significant increase in the elongation zone cell wall peroxidase activity and it spatial variation provides evidence of a potentially causal role for cell wall-associated peroxidase in restricting cell expansion during drought. [ABSTRACT FROM PUBLISHER]

Background and Aims Bioenergy crops are central to climate mitigation strategies that utilize biogenic carbon, such as BECCS (bioenergy with carbon capture and storage), alongside the use of biomass for heat, power, liquid fuels and, in the future, biorefining to chemicals. Several promising lignocellulosic crops are emerging that have no food role – fast-growing trees and grasses – but are well suited as bioenergy feedstocks, including Populus , Salix , Arundo , Miscanthus , Panicum and Sorghum. Scope These promising crops remain largely undomesticated and, until recently, have had limited germplasm resources. In order to avoid competition with food crops for land and nature conservation, it is likely that future bioenergy crops will be grown on marginal land that is not needed for food production and is of poor quality and subject to drought stress. Thus, here we define an ideotype for drought tolerance that will enable biomass production to be maintained in the face of moderate drought stress. This includes traits that can readily be measured in wide populations of several hundred unique genotypes for genome-wide association studies, alongside traits that are informative but can only easily be assessed in limited numbers or training populations that may be more suitable for genomic selection. Phenotyping, not genotyping, is now the major bottleneck for progress, since in all lignocellulosic crops studied extensive use has been made of next-generation sequencing such that several thousand markers are now available and populations are emerging that will enable rapid progress for drought-tolerance breeding. The emergence of novel technologies for targeted genotyping by sequencing are particularly welcome. Genome editing has already been demonstrated for Populus and offers significant potential for rapid deployment of drought-tolerant crops through manipulation of ABA receptors, as demonstrated in Arabidopsis , with other gene targets yet to be tested. Conclusions Bioenergy is predicted to be the fastest-developing renewable energy over the coming decade and significant investment over the past decade has been made in developing genomic resources and in collecting wild germplasm from within the natural ranges of several tree and grass crops. Harnessing these resources for climate-resilient crops for the future remains a challenge but one that is likely to be successful. [ABSTRACT FROM AUTHOR]

The role of bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity in the interaction between tomato (Lycopersicon esculentum=Solanum lycopersicum) and Pseudomonas brassicacearum was studied in different strains. The phytopathogenic strain 520-1 possesses ACC deaminase activity, an important trait of plant growth-promoting rhizobacteria (PGPR) that stimulates root growth. The ACC-utilizing PGPR strain Am3 increased in vitro root elongation and root biomass of soil-grown tomato cv. Ailsa Craig at low bacterial concentrations (106 cells ml−1 in vitro and 106 cells g−1 soil) but had negative effects on in vitro root elongation at higher bacterial concentrations. A mutant strain of Am3 (designated T8-1) that was engineered to be ACC deaminase deficient failed to promote tomato root growth in vitro and in soil. Although strains T8-1 and 520-1 inhibited root growth in vitro at higher bacterial concentrations (>106 cells ml−1), they did not cause disease symptoms in vitro after seed inoculation, or in soil supplemented with bacteria. All the P. brassicacearum strains studied caused pith necrosis when stems or fruits were inoculated with a bacterial suspension, as did the causal organism of this disease (P. corrugata 176), but the non-pathogenic strain Pseudomonas sp. Dp2 did not. Strains Am3 and T8-1 were marked with antibiotic resistance and fluorescence to show that bacteria introduced to the nutrient solution or on seeds in vitro, or in soil were capable of colonizing the root surface, but were not detected inside root tissues. Both strains showed similar colonization ability either on root surfaces or in wounded stems. The results suggest that bacterial ACC deaminase of P. brassicacearum Am3 can promote growth in tomato by masking the phytopathogenic properties of this bacterium. [ABSTRACT FROM PUBLISHER]

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]