bluuebunny/crossref_metadata_2025_split huggingface dataset:
Xylem‐transported abscisic acid: the relative importance of its mass and its concentration in the control of stomatal aperture
Authors: GOWING D. J. G. , JONES H. G. , DAVIES W. J. 
Published: April 2006

ABSTRACTAbscisic 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.
Rapid photosynthetic adaptation to heat stress triggered in potato leaves by moderately elevated temperatures
Authors: HAVAUX M. 
Published: April 2006

ABSTRACTPhotosystem II (PSII) is considered to be one of the most thermolabile aspects of photosynthesis.In vivomeasurements of chlorophyll fluorescence and photosynthetic oxygen evolution in 25°C‐grown potato leaves (cv. Haig) indicated that the threshold temperature Tcabove which PSII denatures was indeed rather low–about 38°C–with temperatures higher than Tc causing a rapid and irreversible loss of PSII activity. The present study demonstrates the existence of adaptive processes which rapidly adjust the invivothermal stability of PSII in response to temperature increase. Transfer of potato leaves from 25°C to temperatures slightly lower than Tc(between 30 and 35°C) was observed to cause an upward shift of the Tc value without any appreciable loss of PSII activity. This increase in PSII thermotolerance was substantial (around +5°C in the Haig cultivar), rapid (with a half‐time of ∼20 min) and slowly reversible at 25°C (>24h). As a consequence, high temperatures (e.g. 40°C) which caused a complete and irreversible inhibition of the PSII function had very little effect in 35°C‐treated leaves, thus suggesting that the above‐described PSII changes could be of prime importance for the plant's behaviour in the field. Accordingly, the rise in Tc at 35°C was much larger (+8°C) in Sahel, a stress‐resistant potato variety, than in the heat‐sensitive Haig cultivar.
Role of abscisic acid in chilling tolerance of rice (Oryza sativa L.) seedlings. I. Endogenous abscisic acid levels
Authors: LEE T.‐M. , LUR H.‐S. , CHU C. 
Published: April 2006

ABSTRACTChanges of ABA levels in chilled rice (Oryza sativa L.) seedlings of two varieties were determined. On exposure to chilling, ABA concentration rapidly increased in the chilling‐tolerant cultivar (cv. Tainung 67, TNG.67) but not in the chilling‐sensitive cultivar (cv. Taichung Native 1, TN.1). Both detached shoots and roots of TNG.67 seedlings showed a significant ABA increase after exposure to chilling. TN.1 seedlings could not accumulate ABA under low temperature but well‐watered status. Exogenous application of the ABA biosynthetic inhibitor, fluridone, reduced ABA accumulation, as well as survival ratio of chilled TNG.67 seedlings. Electrolyte leakage and leaf conductance were also increased by the inhibitor and the effects could be reversed by exogenously applied ABA. ABA concentrations in xylem sap of TNG.67 seedlings increased within 4 h after chilling, and this was temporally coincident with the reduction of leaf conductance. The roles of endogenous ABA in the tolerance of rice seedlings to chilling on a whole plant basis are discussed and suggested.
Carbonic anhydrase in Ranunculus penicillatus spp. pseudofluitans: activity, location and implications for carbon assimilation
Authors: NEWMAN J. R. , RAVEN J. A. 
Published: April 2006

ABSTRACTLevels of carbonic anhydrase activity were determined on a total (60 EUmg−1 protein), external (7.36 EU), internal (50.14 EU) and protoplast (15.63 EU) basis for Ranunculus penicillatus (Dumort.) Bab ssp. pseudofluitans (Syme) S. Webster, a freshwater aquatic macrophyte, by conventional electrometric methods. The site of activity of ‘external’ carbonic anhydrase (CA) has been visualized using 5‐Dimethylaminonapthalene‐1‐ sulphonamide (DNSA)‐CA fluorescent complex formation, and is postulated to be closely associated with the epidermal cell wall. The photosynthetic rate of R. penicillatus ssp. pseudofluitans at pH 9.0 is in excess of the uncatalysed rate of production of CO2 from HCO−3, and this plant is therefore using HCO−3 for photosynthesis. The possible contribution of CA activity to inorganic carbon assimilation, and specifically to transport of HCO−3, in submerged aquatic plants is discussed.
Starch‐degrading enzymes during the induction of CAM in Mesembryanthemum crystallinum
Authors: PAUL M. J. , LOOS K. , STITT M. , ZIEGLER P. 
Published: April 2006

ABSTRACT Mesembryanthemum crystallinum plants were irrigated with 400 mol m−3 NaCl to induce CAM and levels of leaf starch, and activities of starch‐degrading enzymes were measured. During Crassulacean acid metabolism (CAM) induction, daily starch turnover gradually became more pronounced and was three‐ to four‐fold greater than in leaves of C3 plants after 3 weeks. Activities of α‐ and β‐amylase, D‐enzyme and starch phosphorylase all increased 10‐ to 20‐fold within 3 weeks of the start of salt treatment. Activities of α‐ and β‐amylase increased more than fourfold within the first 24 h of salt treatment, which is the fastest increase in enzyme activities so far measured during the induction of CAM with salt solution in intact plants of this species. Most enzyme activities were partially chloroplastic; however, the principal starch‐degrading activity was constituted by an extra‐chloroplastic β‐amylase. CAM starch‐phosphorylase activity, which was mainly chloroplastic, exhibited a two‐ to three‐fold diurnal change in parallel with starch content. CAM induction in M. crystallinum is clearly associated with greater starch turnover and enhanced starch‐degrading enzyme activities, which as catalysts of the initial reaction to release carbon for synthesis of phosphoenolpyruvate (PEP) appear highly significant for the functioning of the CAM pathway. The diurnal rhythm of phosphorylase activity may be of particular significance.
Effects of salinity and turgor on calcium influx in Chara
Authors: REID R. J. , TESTER M. , SMITH F. A. 
Published: April 2006

ABSTRACTMeasurements were made of the influx of 45Ca into internodal cells of Chara corallina in solutions containing high concentrations of NaCl. Increasing salinity in the range 4–100mol m−3 NaCl resulted in a doubling of Ca2+ influx at the plasmalemma. A time‐course of Ca2+ influx in 50 mol m−3 NaCl, 0.5mol m−3 CaCl2 showed that while influx at the plasmalemma increased only 1.5‐fold, influx to the vacuole increased by up to 15‐fold. This was interpreted as being due to inhibition of active Ca2+ efflux from the cell. The stimulation of Ca2+ influx by increasing salinity appeared to be principally a response to reduced turgor since similar stimulations were obtained when turgor was reduced by NaCl, Na2SO4 or mannitol. When cells were plasmolysed Ca2+ influx increased by 10–20‐fold. The increased permeability was relatively specific for Ca2+ and was inhibitable by La3+. Survival of cells in high salt conditions was increased by 30 mmol m−3 La3+, which inhibited Ca2+ influx. Paradoxically, survival can also be extended by increasing external Ca2+ which leads to a higher influx. Therefore, it seems unlikely that the ameliorative effect of Ca2+ on the sensitivity of plants to high NaCl is mediated by Ca2+ entry across the plasmalemma. It seems more likely that the principal role of Ca2+ under these conditions is exerted externally through the control of membrane voltage and permeability.
Modulation of leaf elongation, tiller appearance and tiller senescence in spring barley by far‐red light
Authors: SKINNER R. H. , SIMMONS S. R. 
Published: April 2006

ABSTRACTSupplemental far‐red (FR) illumination of light‐grown grass seedlings inhibits tiller production while enhancing leaf elongation. Although much is known about FR enhancement of internode elongation in dicots, relatively little research has been conducted to determine the effects of FR on monocot development. In growth chamber experiments, fibre optics were used to direct supplemental FR to elongating leaf blades, main stem bases and mature leaf blades of light‐grown barley (Hordeum vulgare L.) seedlings. Our objective was to identify specific sites of perception for FR enhancement of leaf elongation and inhibition of tiller production, and to assess potential FR effects on tiller senescence. Far‐red illumination of elongating leaves or of the main stem base reduced the total number of tillers per plant, primarily by reducing secondary and tertiary tiller production, and enhanced leaf elongation. However, leaf elongation was less sensitive to stem base treatments than to illumination of the elongating blade. Increased leaf length resulted from increased leaf elongation rate, while the duration of leaf elongation was unaffected. Exposure of mature leaf blades to FR had no effect on tillering or leaf elongation. None of the FR treatments led to tiller senescence. Localization of FR perception in vertically oriented tissues such as elongating blades and stem bases permits early detection of reflected light from neighbouring plants, allowing rapid response to impending competition.
The effect of flutter on the temperature of poplar leaves and its implications for carbon gain
Authors: RODEN J. S. , PEARCY R. W. 
Published: April 2006

ABSTRACTThe leaf temperatures of two poplar species (Populus tremuloides Michx. and P. fremontii Wats.) were characterized by attaching thermocouples to leaves that were either constrained to a fixed position or allowed to flutter naturally. There were no observed temperature differences between fluttering and constrained leaves in the lower canopy, but fluttering leaves at the top of the canopy were as much as 2–4°C cooler than constrained leaves. An increase in heat transfer, a decrease in light interception or both could account for these observed differences in the temperature of fluttering versus constrained leaves. Fluttering can increase the boundary‐layer conductance to convective heat exchange by as much as 50 and 20% for laminar and turbulent flow, respectively. The benefit that these leaf temperature differences may provide to the carbon economy of a poplar canopy was dependent on the ambient temperature. Populus fremontii, which is frequently exposed to daytime temperatures exceeding 35°C during summer months in the central valley of California, USA, could show an increase in carbon gain as a result of lower upper canopy leaf temperatures. For aspen, the benefit would be much smaller and often negative because of much lower air temperatures. Lower leaf temperatures may also increase the water use efficiency of poplars. However, the maintenance of lower leaf temperatures may not be the primary adaptive significance of leaf flutter.
Evidence for direct utilization of a siderophore, ferrioxamine B, in axenically grown cucumber
Authors: WANG Y. , BROWN H. N. , CROWLEY D. E. , SZANISZLO P. J. 
Published: April 2006

ABSTRACTTo eliminate the confounding effects of microorganisms and to examine the direct utilization of microbial siderophores as iron sources by higher plants, a hydroponic cultural system and methodology was developed to grow plants with axenic roots. This report presents a description of this system, and also its use to determine the efficacy of the microbial siderophore ferrioxamine B (FOB), compared to the synthetic iron chelate FeEDTA, and the phytosiderophores (PS) of barley as an iron source for alleviating iron stress in the model dicot cucumber. It was observed that FOB gave superior plant biomass and was preferentially utilized to restore chlorophyll synthesis in long‐term experiments when chelates were supplied at 5mmol m−3 concentrations and nutrient solution was buffered against pH change at 7.4 with solid phase CaCO3. In addition, autoradiograms indicated that 59Fe from FOB was rapidly translocated to shoots through vascular tissues and was specifically distributed to regions of rapid growth and to iron‐stressed, but still expanding young leaves. The siderophore itself could be detected within 2h in xylem exudates, regardless of whether or not plants were exposed to metabolic inhibitors. It was concluded that the FOB and iron were taken up by the axenic roots of cucumber in a highly efficient manner, most likely as the iron‐siderophore complex, and at rates that could be significant to dicot nutrition. The results also suggested that cucumber may transport FOB through the transpiration stream to upper parts of plants, where the iron would be reductively released from the siderophore for shoot nutrition.
Heat balance sap flow gauge for small diameter stems
Authors: SENOCK R. S. , HAM J. M. 
Published: April 2006

ABSTRACTApplying heat balance sap flow gauges to plant stems <10 mm in diameter has been difficult because a miniature design is needed that can be attached to a range of stem geometries. This report presents a modified gauge design for use on small plant stems of irregular geometry and shows results from Glycine max with stem diameters of 3–4 mm. The gauge was evaluated on container‐grown plants by comparing gauge measurements of flow to gravimetric estimates of transpiration. Experiments were conducted in the laboratory and greenhouse, using artificial and natural lighting, respectively. Laboratory comparisons of gauge versus gravimetric water loss measurements indicated that the instrument was accurate to within ±5% when soil water was not limiting. Similar results were obtained from greenhouse tests except when soil water availability was low and cumulative gauge estimates became 30–45% less than gravimetric measurements. Differences may have reflected reduced plant water uptake or errors in sap flow estimates associated with low flow rates. Gauge accuracy was not improved by including the rate change in heat storage (S) in the flow calculations because S was always less then 3% of the total heat balance. Relationships between system temperature and sap flow rate suggested a diagnostic test for determining optimum power input. A time constant of 15 s indicated potential application in many agronomic and physiological studies.
Does Rubisco control the rate of photosynthesis and plant growth? An exercise in molecular ecophysiology
Authors: STITT M. , SCHULZE D. 
Published: April 2006

ABSTRACTExperiments are described in which tobacco (Nicotiana tabacum L.) transformed with antisense rbcS to decrease expression of ribulose‐1,5‐bisphosphate carboxylase‐oxygenase (Rubisco) was used to evaluate the contribution of Rubisco to the control of photosynthetic rate, and the impact of a changed rate of photosynthesis on whole plant composition, allocation and growth. (1) The concept of flux control coefficients is introduced. It is discussed how, with adequate precautions, a set of wild‐type and transgenic plants with varying expression of an enzyme can be used to obtain experimental values for its flux control coefficient. (2) The flux control coefficient of Rubisco for photosynthesis depends on the short‐term conditions. It increases in high light, or low CO2. (3) When plants are grown under constant irradiance, the flux control coefficient in the growth conditions is low (<0.2) at irradiances of up to 1000μmol quanta m−2 s−1. In a natural irradiance regime exceeding 1500μmol quanta m−2 s−2 over several hours the flux coefficient rose to 0.8–0.9. It is concluded that plants are able to adjust the balance between Rubisco and the remainder of the photosynthetic machinery, and thereby avoid a one‐sided limitation of photosynthesis by Rubisco over a wide range of ambient growth irradiance regimes. (4) When the plants were grown on limiting inorganic nitrogen, Rubisco had a higher flux control coefficient (0.5). It is proposed that, in many growth conditions, part of the investment in Rubisco may be viewed as a nitrogen store, albeit bringing additional marginal advantages with respect to photosynthetic rate and water use efficiency. (5) A change in the rate of photosynthesis did not automatically translate into a change in growth rate. Several factors are identified which contribute to this buffering of growth against a changed photosynthetic rate. (6) There is an alteration in whole plant allocation, resulting in an increase in the leaf area ratio. The increase is mainly due to a higher leaf water content, and not to changes in shoot/root allocation. This increased investment in whole plant leaf area partly counteracts the decreased efficiency of photosynthesis at the biochemical level. (7) Plants with decreased Rubisco have a lower intrinsic water use efficiency and contain high levels of inorganic cations and anions. It is proposed that these are a consequence of the increased rate of transpiration, and that the resulting osmotic potential might be a contributory factor to the increased water content and expansion of the leaves. (8) Starch accumulation in source leaves is decreased when unit leaf photosynthesis is reduced, allowing a more efficient use of the fixed carbon. (9) Decreased availability of carbohydrates leads to a down‐regulation of nitrate assimilation, acting via a decrease in nitrate reductase activity.
The use of mutants and transgenic plants to study nitrate assimilation
Authors: HOFF T. , TRUONG H.‐N. , CABOCHE M. 
Published: April 2006

ABSTRACTThe nitrate assimilatory pathway has been the matter of intensive genetic and molecular analysis over the past decade. Mutants impaired in the expression of nitrate reductase have been characterized in a number of plant species. Molecular analysis of the Nia gene coding for nitrate reductase has been the basis for a three‐domain model of the structure of the enzyme, in agreement with biochemical and genetic data. Mutagenesis and antisense strategies have led to the description of nitrite reductase deficiencies. The molecular analysis of the corresponding Nii genes has provided invaluable information on the structure of nitrite reductase. Recently, a gene involved in nitrate uptake has also been identified. The regulation of the nitrate assimilatory pathway has been investigated. Analysis of the regulation of the pathway at the molecular level has shown evidence for the involvement of nitrate, light and/or sucrose, and reduced nitrogen in the regulation. Surprisingly, no bona fide regulatory mutant specific to this pathway has been identified so far in higher plants. This may reflect the redundancy of regulatory genes. The deregulated expression of one or the other step of the pathway obtained by ectopic expression of the corresponding genes is a new approach to study the physiological role of these regulations. Elements of the pathway have also been successfully used as transposon traps, or negatively selectable markers for other purposes. Finally, the identification at the molecular level of regulatory genes and structural elements involved in transport and storage of nitrate, or in the biosynthesis of cofactors of nitrate and nitrite reductases, will be the goal of the next decade.
Protection against oxygen radicals: an important defence mechanism studied in transgenic plants
Authors: FOYER C. H. , DESCOURVIÈRES P. , KUNERT K. J. 
Published: April 2006

ABSTRACTFree radicals and other active derivatives of oxygen are inevitable by‐products of biological redox reactions. Reduced oxygen species, such as hydrogen peroxide, the superoxide radical anion and hydroxyl radicals, inactivate enzymes and damage important cellular components. In addition, singlet oxygen, produced via formation of triplet state chlorophyll, is highly destructive. This oxygen species initiates lipid peroxidation, and produces lipid peroxy radicals and lipid hydroperoxides that are also very reactive. The increased production of toxic oxygen derivatives is considered to be a universal or common feature of stress conditions. Plants and other organisms have evolved a wide range of mechanisms to contend with this problem. The antioxidant defence system of the plant comprises a variety of antioxidant molecules and enzymes. Considerable interest has been focused on the ascorbate‐glutathione cycle because it has a central role in protecting the chloroplasts and other cellular compartments from oxidative damage. It is clear that the capacity and activity of the antioxidative defence systems are important in limiting photo‐oxidative damage and in destroying active oxygen species that are produced in excess of those normally required for signal transduction or metabolism. In our studies on this system, we became aware that the answers to many unresolved questions concerning the nature and regulation of the antioxidative defence system could not be obtained easily by either a purely physiological or purely biochemical approach. Transgenic plants offered us a means by which to achieve a more complete understanding of the roles of the enzymes involved in protection against stress of many types: environmental and man‐made. The ability to engineer plants which express introduced genes at high levels provides an opportunity to manipulate the levels of these enzymes, and hence metabolism in vivo. Studies on transformed plants expressing increased activities of single enzymes of the antioxidative defence system indicate that it is possible to confer a degree of tolerence to stress by this means. However, attempts to increase stress resistance by simply increasing the activity of one of the antioxidant enzymes have not always been successful presumably because of the need for a balanced interaction of protective enzymes. The study of these transformed plants has allowed a more complete understanding of the roles of individual enzymes in metabolism. Protection against oxidative stress has become a feasible objective through the application of molecular genetic techniques in conjunction with a biochemical and physiological approach.
The use of mutants and transgenic plants to study amino acid metabolism
Authors: LEA P. J. , FORDE B. G. 
Published: April 2006

ABSTRACTMutants of higher plants with alterations in amino acid metabolism have now been available for 20 years. Following the realization that at least four distinct classes of herbicides (phosphinothricins, glyphosates, imidazolinones and sulphonylureas) act by the inhibition of amino acid biosynthesis, mutants resistant to the herbicides have also been obtained. More recently, transgenic plants containing altered levels of enzymes of amino acid biosynthesis have been constructed. In this article, we have attempted to review several areas of amino acid biosynthesis including ammonia assimilation, the aspartate pathway, branched chain amino acids, aromatic amino acids and proline.