bluuebunny/crossref_metadata_2025 huggingface dataset:
A temperature‐sensitive nodulation mutant (sym 5) of Pisum sativum L.
Authors: FEARN J. C. , LaRUE T. A.
Published: April 2006

Abstract. In peas (Pisum sativum L.) homozygous for sym 5, nodulation has an unusual temperature dependence. These sym 5 mutants nodulate poorly at a root temperature of 20°C but nodulate better at 12°C. By lowering the root temperature of the sym 5 mutants from a lightroom temperature of 20/15°C to a constant 12°C, 8d after planting, the number of nodules can be further increased. A cool period (12°C) as short as 6h, early in the infection process, is sufficient to significantly increase nodulation of plants otherwise growing at 20/15°C. This temperature‐sensitivity of nodulation is not due to a temperature induced change of a sym 5‐related, 66‐kD peptide but may involve accumulation of a gas in the rhizosphere.
Role of root systems of eastern larch and white spruce in response to flooding
Authors: REECE C. F. , RIHA S. J.
Published: April 2006

Abstract.Soil flooding causes rapid reductions in transpiration, stomatal conductance and photosynthesis of many woody plants, which can decrease growth and ultimately result in plant death. This study was conducted to determine the role of the root system in the flooding response. Eastern larch (Larix laricina) seedlings were grown in Plexiglas tubes in which water uptake by flooded and unflooded roots was measured independently. Further flooding studies were conducted with eastern larch and white spruce (Picea glauca) in which stems were girdled. Root hydraulic properties were analysed using pressure‐flow relationships. Transpiration rates of partially flooded plants declined more slowly than fully‐flooded plants. Water uptake by unflooded roots of partially flooded seedlings increased momentarily with flooding. After lOd, flooding caused little change in root hydraulic conductance, a decrease in root system reflection coefficient, and an increase in osmotic permeability. Stem girdling had little effect on stomatal conductance and transpiration in comparison to flooding effects. The response of plant tops to flooding appears to be xylem‐mediated and in proportion to the amount of root system flooded. Root hydraulic conductance appears to be unaffected by flooding except for a possible temporary increase on the first day following flooding treatments.
Oxygen uptake by roots of Rumex species at different temperatures: the relative importance of diffusive resistance and enzyme kinetics
Authors: LAAN P. , TOSSERAMS M. , HUYS P. , BIENFAIT H. F.
Published: April 2006

Abstract. Oxygen uptake characteristics of the roots of three Rumex species were compared, and related to kinetics of the respiratory system and to root anatomy. The observed differences could not be explained by differences in fundamental characteristics of the oxygen uptake system: with all three species, cytochrome‐mediated respiration contributed 70% and cyanide‐insensitive (alternative) respiration 30% of the total respiration rate, and apparent Km values of cytochrome oxidase were lower than those obtained for the alternative oxidase in all cases. However, differences in critical oxygen pressure for respiration (COPR) and in apparent Km for oxygen, were strongly correlated with differences in root porosity and root diameter. Km(O2) values at high and low temperatures were determined, and from Arrhenius plots of oxygen uptake rates between 11 and 32°C, the role of diffusional impedance could be estimated. Root respiration of Rumex maritimus and R. crispus, both with high root porosity, but differing in root diameter, had a low Km for oxygen (3–7 mmol m−3). In contrast with this were the responses of R. thvrsiflorus, which has thin roots but low root porosity: a high Km (10‐20 mmol m−3) was found at all temperatures. The role of diffusional impedance as a function of temperature in oxygen uptake rate by the three species is discussed and related to the differential resistance of the species towards flooding.
How do plant growth substances work? II
Authors: TREWAVAS A.
Published: April 2006

Abstract. This article continues the discussion on sensitivity prompted by the publication of an earlier paper by the author in 1981. It is indicated that appropriate measurements of sensitivity can help uncover the function of growth substances in development. The experimental constraints which are necessary for unambiguous measurements of sensitivity are outlined and it is shown that a specific sensitivity measurement, termed control strength, would most readily clarify function and help obviate controversy. Methods for measuring control strength which deal with the problem of compartmentalization are considered and it is suggested that stochastic variation in single cell biochemistry might provide a constraint on the accuracy of control strength determinations.
Effects of over‐winter fumigation with sulphur and nitrogen dioxides on biochemical parameters and spring growth in red spruce (Picea rubens Sarg.)
Authors: WOLFENDEN J. , PEARSON M , FRANCIS B. J.
Published: April 2006

Abstract. Red spruce (Picea rubens Sarg.) seedlings were overwintered in two controlled environment chambers designed to simulate sub‐zero winter conditions. One of the chambers was fumigated throughout the 5‐month period with low concentrations of SO2+ NO2. Extracts of extracellular fluid from trees in this treatment revealed accumulations of sulphite and nitrite, but not of sulphate or nitrate. Analysis of the chloroplast membrane lipid monogalactosyl diglyceride (MGDG) indicated a large increase in fatty acid saturation in both treatments during mid‐winter, with a subsequent recovery to earlier levels. Although at the end of the experiment the MGDG of polluted trees contained significantly less linolenic acid, there was no overall treatment effect on fatty, acid content. In the following spring, there was some indication that flushing of leader buds began earlier and proceeded at a slower rate in polluted trees compared to controls, but no other growth parameters were affected by the winter treatment. The absorption of SO2 and NO2 and the accumulation of their products during dormancy is discussed as a potential mechanism for metabolic disruption, resulting in changes to seasonal responses which could be critical to the survival of the plant.
Photocontrol of stem elongation in plant neighbourhoods: effects of photon fluence rate under natural conditions of radiation
Authors: BALLARÉ C. L. , SCOPEL A. L. , SÁNCHEZ R. A.
Published: April 2006

Abstract.We investigated the effects of photon fluence rate on internode elongation in fully de‐etiolated plants growing under sunlight. Our goal was to find out whether perception by the stems of fluence rate changes related to canopy density may be involved in the modulation of internode growth in canopies formed by plants of similar stature (e.g. crop stands). UsingDatura feroxL. andSinapis albaL. seedlings growing under natural radiation, we found that internode elongation is promoted by localized shading. This effect was observed with internodes receiving light with a high (>0.9) or a low (0.3) red (R) to far‐red (FR) photon ratio. Selective removal of the different wavebands from the light impinging on the internodes showed that part of the response to fluence rate is due to photons in the R + FR range. The blue (B) component, most likely acting through a specific photoreceptor, also inhibited elongation. However, changes in the fluence rate of B light did not have detectable effects on the response of the internodes to R:FR ratio. Fibre‐optic studies and measurements with integrating‐cylinder sensors in even‐aged populations of seedlings showed that both the quality and quantity of radiation received by the stems are profoundly influenced by changes in canopy density. When density is very low (leaf area index = LAI ≥ 1) only the R:FR ratio is reduced, due to FR reflected from nearby leaves. In the LAI range of 1 to 2, though a large proportion of the leaf area is still receiving full sunlight, the photon fluence rate at the stem level drops dramatically. These results suggest that in even‐aged populations of LAI > 1 elongation growth is promoted by the low R:FR ratioandthe reduced fluence rate. Perception of these two factors at the stem level may elicit morphological adaptations in the canopy before the onset of severe competition among neighbours for the resource of light.
What limits nitrate uptake from soil?
Authors: ROBINSON D. , LINEHAN D. J. , CAUL S.
Published: April 2006

Abstract. An accepted view, that unless nitrate concentrations in the soil solution are very low (e.g. below 0.1–0.2 mol m−3) the growth of high‐yielding crops is not limited by the availability of nitrogen, is challenged. Conventional analyses of nutrient supply and demand, based on calculations of apparent inflow rates (uptake rates per unit total root length) are invalid. Apparent inflow rates are inversely proportional to root length. The convention of using total root length grossly overestimates the fraction of the root system active in nutrient uptake. Consequently, inflow rates based on total root lengths underestimate the true values, indicating unrealistically low nutrient concentration differentials between bulk soil and root surfaces required to drive uptake. An alternative method of analysis is suggested. This is based on total nutrient uptake rather than on inflow rate. Measurements of the former do not depend on estimates of active root length and can be made directly and reliably. The method was applied to data obtained from a pot experiment using spring wheat (Triticum aestivum L., cv. Wembley) grown in soil without nitrogen fertilizer (N0) or with nitrogen fertilizer equivalent to 200kg N ha−1 (N+). Soil nitrate concentrations calculated using the conventional method based on total root length, suggested that any increases in concentration above those measured in the N0 treatment should not have resulted in increased uptake and growth. However, the N+ plants were always bigger than those in the No treatment, refuting this suggestion. Theoretical uptakes of nitrogen (calculated initially on the basis of a fully active root system) were adjusted, by reducing the effective root length incrementally, until the theoretical uptake matched the measured net uptake of nitrogen. The mean fractions of the root systems likely to have been involved in nitrate uptake were 11% and 3.5% of the total lengths of root in the N0 and N+ treatments, respectively.
Higher plants express 3‐deoxy‐D‐manno‐octulosonate 8‐phosphate synthase
Authors: DOONG R. L. , AHMAD S. , JENSEN R. A.
Published: April 2006

Abstract. The enzymatic activity of 3‐deoxy‐D‐manno‐octulosonate 8‐phosphate (KDOP) synthase was detected in eight diverse plant species, thus providing enzymological data consistent with recent reports of the presence of 3‐deoxy‐D‐manno‐octulosonate in plant cell walls. KDOP synthase from spinach was partially purified and characterized. It possessed weak activity as 3‐deoxy‐D‐arabino‐heptulosonate 7‐phosphate (DAHP) synthase. In the presence of phosphoenolpyruvate, which conferred dramatic thermostability, KDOP synthase had a catalytic temperature optimum of about 53°C. The pH optimum was 6.2, and divalent cations were neither stimulatory nor required for activity. The Km values for arabinose 5‐P and phosphoenolpyruvate were 0.27 mol m−3 and about 35 mmol m−3, respectively. The kinetics of periodate oxidation of KDOP formed by spinach KDOP synthase indicate that the same stereochemical configuration exists as with bacterial KDOP. The possibility that an unregulated species of DAHP synthase found in some bacteria might in fact be a KDOP synthase exhibiting substrate ambiguity of the type seen in higher plants was examined. However, the DAHP synthase isozyme, DS‐O, from Acinetobacter calcoaceticus was found to be specific for erythrose 4‐P. The KDOP synthase of Acinetobacter calcoaceticus was also found to be specific for arabinose 5‐P.
Do stomata respond to relative humidity?
Authors: APHALO P. J. , JARVIS P. G.
Published: April 2006

Abstract. The objective of the work reported was to answer the following questions: (1) Do stomata respond to both humidity and temperature? (2) Do these responses interact in such a way that relative humidity at the leaf surface is a more appropriate variable than water vapour saturation deficit at the leaf surface and yields a simpler description of the compound response? To answer these questions, we measured the response of leaf conductance to humidity under constant leaf temperature, and the response to increasing leaf temperature under constant relative humidity and under constant water vapour saturation deficit. We found that, in Hedera helix subsp. canariensis (Willd.) Coutinho, there was a reversible response to humidity under constant temperature, and that there was also a response to temperature under constant relative humidity. The relationship between leaf conductance and relative humidity was different when measured at the same temperature rather than at different temperatures. An inversely proportional response was consistently obtained when stomatal conductance was expressed in relation to water vapour saturation deficit. The interaction between the effects of leaf temperature and water vapour saturation deficit was not compatible with a mechanism of response to humidity and temperature based solely on relative humidity. From these data, we conclude that water vapour saturation deficit is a more appropriate variable for describing stomatal responses to humidity.
Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO2 concentrations: Has its importance been underestimated?
Authors: LONG S. P.
Published: April 2006

Abstract. Climate change will include correlated increases in temperature and atmospheric CO2 concentration (Ca). Rising temperatures will increase the ratio of photorespiratory loss of carbon to photosynthetic gain, whilst rising Ca will have an opposing effect. The mechanism of these effects at the level of carboxylation in C3 photosynthesis are quantitatively well understood and provide a basis for models of the response of leaf and canopy carbon exchange to climate change. The principles of such a model are referred to here and used to quantitatively examine the implications of concurrent increase in temperature and Ca. Simulations of leaf photosynthesis show the increase, with elevation of Ca from 350 to 650 μmol mol‐1, in light saturated rates of CO2 uptake (Asat) and maximum quantum yields (φ) to rise with temperature. An increase in Ca from 350 to 650 μmol mol‐1 can increase Asat by 20% at 10°C and by 105% at 35°C, and can raise the temperature optimum of Asat by 5°C. This pattern of change agrees closely with experimental data. At the canopy level, simulations also suggest a strong interaction of increased temperature and CO2 concentration. Predictions are compared with the findings of long‐term field studies. The principles used here suggest that elevated Ca will alter both the magnitude of the response of leaf and canopy carbon gain to rising temperature, and sometimes, the direction of response. Findings question the value of models for predicting plant production in response to climate change which ignore the direct effects of rising Ca and the modifications that rising Ca imposes on the temperature response of net CO2 exchange.
Rising CO2 levels and their potential significance for carbon flow in photosynthetic cells
Authors: STITT M.
Published: April 2006

Abstract. In the first part of this review, I discuss how we can predict the direct short‐term effect of enhanced CO2 on photosynthetic rate in C3 terrestrial plants. To do this, I consider: (1) to what extent enhanced CO2 will stimulate or relieve demand on partial processes like carboxylation, light harvesting and electron transport, the Calvin cycle, and end‐product synthesis; and (2) the extent to which these various processes actually control the rate of photosynthesis. I conclude that control is usually shared between Rubisco (which responds sensitively to CO2) and other components (which respond less sensitively), and that photosynthesis will be stimulated by 25–75% when the CO2 concentration is doubled from 35 to 70 Pa. This is in good agreement with the published responses. In the next part of the review, I discuss the evidence that most plants undergo a gradual inhibition of photosynthesis during acclimation to enhanced CO2. I argue that this is related to an inadequate demand for carbohydrate in the remainder of the plant. Differences in the long‐term response to CO2 may be explained by differences in the sink‐source status of plants, depending upon the species, the developmental stage, and the developmental conditions. In the third part of the review, I consider the biochemical mechanisms which are involved in ‘sink’ regulation of photosynthesis. Accumulating carbohydrate could lead to a direct inhibition of photosynthesis, involving mechanical damage by large starch grains or Pi‐limitation due to inhibition of sucrose synthesis. I argue that Pi is important in the short‐term regulation of partitioning to sucrose and starch, but that its contribution to ‘sink’ regulation has not yet been conclusively demonstrated. Indirect or ‘adaptive’ regulation of photosynthesis is probably more important, involving decreases in amounts of key photosynthetic enzymes, including Rubisco. This decreases the rate of photosynthesis, and potentially would allow resources (e.g. amino acids) to be remobilized from the leaves and reinvested in sink growth to readjust the sink‐source balance. In the final part of the review, I argue that similar changes of Rubisco and, possibly, other proteins are probably also involved during acclimation to high CO2.
The interaction of rising CO2and temperatures with water use efficiency
Authors: EAMUS D.
Published: April 2006

Abstract.Recent data concerning the impact of elevated atmospheric CO2upon water use efficiency (WUE) and the related measure, instantaneous transpiration efficiency (ITE), are reviewed. It is concluded from both short and long‐term studies that, at the scale of the individual leaf or plant, an increase in WUE or ITE is generally observed in response to increased atmospheric CO2levels. However, the magnitude of this increase may decline with time. The opinion that elevated CO2may substantially decrease transpiration at the regional scale is discussed. The mechanisms by which elevated CO2may cause a change in these measures are discussed in terms of stomatal conductance, assimilation and respiration responses to elevated CO2. Finally, recent experimental data and model outputs concerning the impact of the interaction of increased temperature with elevated CO2on WUE, ITE and yield are reviewed. It is concluded that substantially more data is required before reliable predictions about the regional scale response of WUE and catchment hydrology can be made.
Canopy photosynthesis of crops and native plant communities exposed to long‐term elevated CO2
Authors: DRAKE B. G. , LEADLEY P. W.
Published: April 2006

Abstract. There have been seven studies of canopy photosynthesis of plants grown in elevated atmospheric CO2: three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO2 increased canopy photosynthesis in all cases. The relative effect of CO2 was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO2 were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO2 treatment to date, monospecific stands of a C3 sedge, Scirpus olneyi (Grey), and a C4 grass, Spartina patens (Ait.) Muhl., have been exposed to twice normal ambient CO2 concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO2 exchange per unit green biomass (NCEb) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO2 increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community.
Effects of source‐sink relations on photosynthetic acclimation to elevated CO2
Authors: ARP W. J.
Published: April 2006

Abstract. While photosynthesis of C3 plants is stimulated by an increase in the atmospheric CO2 concentration, photosynthetic capacity is often reduced after long‐term exposure to elevated CO2. This reduction appears to be brought about by end product inhibition, resulting from an imbalance in the supply and demand of carbohydrates. A review of the literature revealed that the reduction of photosynthetic capacity in elevated CO2 was most pronounced when the increased supply of carbohydrates was combined with small sink size. The volume of pots in which plants were grown affected the sink size by restricting root growth. While plants grown in small pots had a reduced photosynthetic capacity, plants grown in the field showed no reduction or an increase in this capacity. Pot volume also determined the effect of elevated CO2 on the root/shoot ratio: the root/shoot ratio increased when root growth was not restricted and decreased in plants grown in small pots. The data presented in this paper suggest that plants growing in the field will maintain a high photosynthetic capacity as the atmospheric CO2 level continues to rise.
Influence of long photoperiods on plant development and expression of Crassulacean acid metabolism in Mesembryanthemum crystallinum
Authors: CHENG S.‐H. , EDWARDS G. E.
Published: April 2006

Abstract. In the natural habitat plants of Mesembryanthemum crystallinum are induced to perform Crassulacean acid metabolism (CAM) after 3 months, and reproductive growth begins after 5 months (Winter, Liittge & Winter, 1978, Oecologia (Berlin), 34, 225‐237). The life cycle of M. crystallinum and the extent of growth required prior to induction of enzymes of Crassulacean acid metabolism (CAM) are dramatically shortened by growing seedlings with a long photoperiod (3=16h/8h light/dark). Reproductive growth begins as soon as five weeks after germination when plants are grown in continuous light (under 600μmol quanta m−2 s−1, 30°C). In plants grown under well‐watered conditions, the activities of PEP carboxylase and NADP‐malic enzyme begin increasing markedly 2 weeks after germination, with plants grown under longer photoperiods having higher enzyme activities. After 3 weeks of growth, leaves accumulated a large amount of malate, but the microequivalents of malate present were up to nine times greater than the total titratable acidities. Interestingly, plants from a 24h/0h or a 20h/4h photo‐period showed no diurnal fluctuation of malate, but did produce malate in the light as a major photosynthetic end product. That is, under these environmental conditions, principal enzymes of CAM can be induced without the plants performing CAM. However, plants grown in a 16h/8h photoperiod did exhibit nocturnal accumulation of malate after 3 weeks of growth. In plants of all three growth conditions, the activities of NADP‐malic enzyme and PEP carboxylase were further increased two‐ to live‐fold by irrigating 3‐week‐old‐plants with 350mol m−3 NaCl. Such early enhancement of these enzymes by salt and the shortened life cycle may be due to an accelerated development under the long photoperiods.
Mediation of tropisms by lateral translocation of endogenous indole‐3‐acetic acid in maize coleoptiles
Authors: IINO M.
Published: April 2006

Abstract. The hypothesis that tropic responses result from lateral auxin gradients was examined in coleoptiles of red‐light‐grown maize (Zea mays L.) by measuring endogenous IAA (indole‐3‐acetic acid) using a physicochemical method. Phototropic stimulation (unilateral blue light; 8s at 0.33 μmol m−2s−1) was found to induce a lateral gradient of solvent‐extractable IAA in a subapical zone (2‐7mm from the tip). The gradient occurred in advance of the bending response, with a decrease of IAA in the irradiated half and a compensatory increase in the shaded half. The maximal gradient measured was about 1:2 (irradiated: shaded). Diffusible IAA, obtained from the cut end of an excised coleoptile tip (3mm long, with its base split by 1mm), was similarly redistributed between the two sides, indicating that IAA is laterally translocated in the tip and that the resulting IAA gradient migrates to the subapical zone. A smaller gradient was induced in a basal zone (12‐17mm from the tip). This gradient was initiated about 20 min later than that at the subapical zone, in agreement with a similar delay of bending observed in this zone. Gravitropic stimulation (60° from the vertical) also resulted in a lateral gradient of extractable IAA in the subapical zone, the gradient preceding the bending response. It is concluded that the tropisms of maize coleoptiles are mediated by IAA gradients, which are most likely caused by lateral IAA transport as the Cholodny‐Went theory of tropisms describes. From IAA measurement data, the mean velocity of basipetally‐polar transport of endogenous IAA was estimated to be 12 mm h−1.
Effect of nitrogen nutrition on photosynthesis and growth in C4Panicum species
Authors: EPO W. D. BOWMAN
Published: April 2006

Abstract. The growth and photosynthetic responses to high and low N nutrition were measured in 2 NADP‐malic enzyme and 4 NAD‐malic enzyme C4 subtype Panicum species to evaluate whether differences in C4 photosynthetic biochemistry result in differences in the N requirement for growth. All species had lower biomass production, photosynthesis rates, and shoot N concentrations at low N, and no consistent differences between the C4 subtypes were apparent. The assimilation rates (biomass accumulated over the period of growth) for the NADP‐malic enzyme species were higher than the NAD‐malic enzyme species at high N but not at low N. When assimilation rates were evaluated on a shoot N basis a higher N‐use‐efficiency was found for the NADP‐malic enzyme species at high N. Thus the NADP‐malic enzyme Panicum species had a greater amount of growth for a given shoot N concentration, but only above a certain level of shoot N concentrations.
Quantitative changes in cytoplasmic and microsomal proteins associated with aluminium toxicity in two cultivars of winter wheat
Authors: OWNBY J. D. , HRUSCHKA W. R.
Published: April 2006

Abstract. Seedlings of two sister lines of hard red winter wheat, Tx78 (Al‐sensitive) andTx84 (Al‐tolerant), were given a 24‐h pulse of Al sufficient to irreversibly inhibit growth in the sensitive but not the tolerant cultivar. Proteins were then extracted from root tips of both cultivars, separated by 2D‐PAGE, and quantified using computer analysis of gels. The objective of the work was to determine if the major changes in protein expression associated with Al stress were cultivar‐specific, if the changes correlated with Al tolerance, and if they occurred in a specific fraction of cell proteins. Of the approximately 600 proteins examined, 14 cytoplasmic proteins and eight microsomal proteins were induced or enhanced by Al treatment in one or both cultivars, while nine cytoplasmic and 12 microsomal proteins were diminished or repressed. Among the 43 proteins significantly altered by Al treatment, three cytoplasmic proteins, and no microsomal proteins, were induced or enhanced solely in the‐AI‐tolerant cultivar. Al affected the programme of formation of both cytoplasmic and microsomal proteins, but appeared to cause the greater change in proteins associated with the cytoplasm. The results indicate that most of the changes in protein expression associated with Al stress probably result from effects of Al on cell metabolism, and that only a few proteins may be induced or enhanced as part of a cultivar‐specific mechanism of Al tolerance.
Nitrogen nutrition and the level of Crassulacean acid metabolism in Kalanchoë lateritia Engl.
Authors: SANTOS I. , SALEMA R.
Published: April 2006

Abstract. The influence of different nitrogen levels was studied in the CAM facultative Kalanchoë lateritia by watering some plants with Hoagland's solution (which contains, besides other ions, NO3−= 14.47mol m−3and NH4+= 1.04mol m−3; N group), and others with the same solution but with combined nitrogen concentration reduced to either one fifth (NO3−= 2.894mol m−3 and NH4+= 0.208mol m−3; N/5 group) or one tenth (NO3−= 1.447mol m−3and NH4+= 0.104mol m‐3;N/10 group). The influence of the three nitrogen levels on CAM expression was assessed through activities of PEP‐Case, PPD and RuBisCo, diurnal fluctuation of titratable acidity, mesophyll succulence, soluble protein, chlorophyll, nitrate, starch contents, pattern of nocturnal CO2 exchange and electron microscopy. CAM photosynthesis was more intense in N/5 plants which also had the highest Sm value. The activity of RuBisCo showed no significant differences in the three situations (expressed on chlorophyll basis) whereas both PEP‐Case and PPD had higher values in N/5” plants. Chlorophyll and soluble protein were more abundant in the N plants followed by N/5 and N/10 plants. Nitrate was higher in N plants and starch content in N/5 plants. IRGA determination of CO2 nocturnal uptake showed that N/5 plants began CO2 capture earlier and at a more intense rate and for a longer period than plants from other groups also having a daily variation of titratable acidity (97.71 ± 10.8 μeq. G−1 f.w.) indicative of performing strong CAM. Electron microscope morphometric analysis revealed larger chloroplasts in N plants and smaller in N/10 plants, with starch fractional volume higher in N/5 plants, correlating with more intense CAM activity of these plants.
Polyamine levels in relation to growth and NaCl concentration in normal and habituated sugarbeet callus cultures
Authors: LE DILY F. , BILLARD J.‐P. , BOUCAUD J.
Published: April 2006

Abstract. The concentrations of putrescine, spermidine and spermine, the only polyamines detectable in normal and habituated calli of Beta vulgaris L. ssp. altissima, were much higher in the habituated callus than the normal callus, irrespective of experimental conditions. These results suggest that, in normal (tolerant to NaCl) and habituated (sensitive to NaCl) calli, there exists a competition for the common precursor of ethylene and polyamine biosynthesis viz. S‐adenosylmethionine. A disequilibrium favouring the synthesis of putrescine and spermidine in the habituated callus might be linked to structural deterioration of the cell membrane following extended culture or severe osmotic stress (68 mol m−1 NaCl). The maintenance of membrane integrity by the normal callus coincides with ethylene production at the expense of polyamine synthesis. In contrast to the habituated callus, the salinity tolerance of the normal callus is accompanied by the accumulation of proline under hypersaline conditions (274mol m−3). The important osmoregulatory role played by quaternary ammonium compounds in the‐aerial parts of Chenopodiaceae, especially the sugarbeet, is not observed in the calli, these compounds being found in very low concentrations in saline conditions.
High carbon dioxide and sun/shade effects on isoprene emission from oak and aspen tree leaves
Authors: SHARKEY T. D. , LORETO F. , DELWICHE C. F.
Published: April 2006

Abstract. Isoprene (2‐methyl 1, 3‐butadiene) is emitted from many plants, especially trees. We tested the effect of growth at high CO2 partial pressure and sun versus shade conditions on the capacity of Quercus rubra L. (red oak) and Populus tremuloides Michx. (quaking aspen) leaves to make isoprene. Oak leaves grown at high CO2 partial pressure (65 Pa) had twice the rate of isoprene emission as leaves grown at 40Pa CO2. However, aspen leaves behaved oppositely, with high CO2‐grown leaves having just 60‐70% the rate of isoprene emission as leaves grown in 40 Pa CO2. Similar responses were observed from 25 to 35 °C leaf temperature during assay. The stimulation of isoprene emission by growth at high CO2 and the stimulation in high temperature resulted in isoprene emission consuming over 15% of the carbon fixed during photosynthesis in high‐CO2 grown oak leaves assayed at 35 °C. Leaves from the south (sunny) sides of trees growing in natural conditions had rates of isoprene emission double those of leaves growing in shaded locations on the same trees. This effect was similar in both aspen and oak. The leaves used for these experiments had significantly different chlorophyll a/b ratios indicating they were functionally sun (from the sunny locations) or shade leaves (from the protected locations). Because the metabolic pathway of isoprene synthesis is unknown, we are unable to speculate about how or why these effects occur. However, these effects are more consistent with metabolic control of isoprene release rather than a metabolic leak of isoprene from metabolism. The results are also important for large scale modelling of isoprene emission and for predicting the effect of future increases in atmospheric CO2 level on isoprene emission from vegetation.
Recovery of photosynthesis in winter‐stressed Scots pine
Authors: OTTANDER C. , ÖQUIST G.
Published: April 2006

Abstract. . Winter‐induced inhibition of photosynthesis in Scots pine (Pinns sylvestris L.) is caused by the combined effects of light and freezing temperatures; light causes photoinhibition of photosystem II (Strand & Oquist, 1985b, Physiologic Plantarum, 65, 117–123), whereas frost causes inhibition of enzymatic steps of photosynthesis (Strand & Öquist, 1988, Plant, Cell & Environment, 11, 231–238). To reveal limiting steps during recovery from winter stress, the potential of photosynthesis to recover and the actual recovery outdoors during spring, were studied in Scots pine. Studies of light dependent O2‐evolution under saturating CO2 and recordings of room temperature fluorescence induction kinetics were used. When branches of pine, in February and March, were brought into the laboratory and kept at 18°Cand 100μmol m−2 s−1, light saturated rates and apparent quantum yields of photo‐synthetic O2‐evolution recovered fully within approximately 48h. The photochemical efficiency of photosystem II, as measured by Fv/Fm ratios, recovered fully within 24h after an initial lag‐phase of 2‐3 h. Under natural winter conditions, the Fv/Fm ratio decreased more in exposed than in shaded pine, whereas the efficiency of photosynthesis was similarly inhibited in exposed and shadedpine. However, when recovery from winter stress occurred during spring, the Fv/Fm ratios of both shaded and exposed pine recovered well before photosynthesis. It is concluded that the light‐induced photoinhibition component of winter stress in photosynthesis of pine recovers well before the frost induced component(s) of winter stress. In this context, reversible photoinhibition of photosynthesis in evergreen conifers is considered as a dynamic down‐regulation of photosystem II to prevent more severe photodynamic damage of the thylakoid membrane when photosynthesis is inhibited by frost.
Intercellular location of glycoprotein in soybean nodules: effect of altered rhizosphere oxygen concentration
Authors: JAMES E. K. , SPRENT J. I. , MINCHIN F. R. , BREWIN N. J.
Published: April 2006

Abstract. A glycoprotein which occludes intercellular spaces in the inner cortex of legume nodules may be involved in controlling oxygen diffusion into rhizobial‐infected cells. Here we investigated this possibility by localizing the glycoprotein using monoclonal antibodies and immunogold labelling in nodulated roots of soybean cv. Clarke inoculated with Bradyrhizobium japonicum strain RCR3442 exposed to atmospheres with either 10, 21 (control) or 40% oxygen for 28d. Infected cells showed evidence of premature senescence when grown in above or below ambient pO2 particularly at 10% oxygen, although cortical cells appeared to be little altered by oxygen treatment. In the inner cortical cells, more glycoprotein was seen to be occluding intercellular spaces of those nodules subjected to 40% oxygen and less in those nodules exposed to 10% oxygen, when compared to controls. This observation, made at the light microscope level (using silver enhancement) was confirmed under the TEM using immunogold labelling. Therefore, it is suggested that intercellular space glycoprotein is one of the structural components of the diffusion resistance in the cortex of legume nodules.
Iron‐catalysed oxygen radical formation and its possible contribution to drought damage in nine native grasses and three cereals
Authors: PRICE A. H. , HENDRY G. A. F.
Published: April 2006

Abstract. The origin, sequence and location of molecular damage arising during sub‐lethal drought is described in 4‐week‐old wheat (Triticum aestivum L.). Short episodes of water deprivation increased the uptake of iron within 6h of drought. Superoxide, precursor of the highly reactive hydroxyl radical in the presence of iron, was formed in the thylakoid membranes. Further draughting promoted destruction of chlorophylls (but not carotenoids), which was detectable after 14 h and preceded significant losses in relative water content. Peroxidation of lipids and a decreased protein content occurred after 26h of cumulative water deprivation but was confined to the shoots. In contrast, there was no significant loss of protein nor peroxidation of lipids in the root tissue, despite quantitatively larger, more rapid, losses in water content. These results indicate that damage arising in the early phases of drought appears to be initiated within the chloroplast and to be largely confined to the photosynthetic tissue. The pattern of drought‐induced damage in 11 other non‐xerophytes of the north temperate region was qualitatively similar to wheat, although the response to drought of the three cereals was more extreme than in the indigenous grasses and less extreme in ecologically stress‐tolerant species. A method for the simultaneous quantification of carotenoids in the presence of chlorophylls is provided.
Mobility and deposition of silicon in cucumber plants
Authors: SAMUELS A. L. , GLASS A. D. M. , EHRET D. L. , MENZIES J. G.
Published: April 2006

Abstract. The mobility and deposition of silicon (Si) in cucumber plants grown in hydroponic culture were studied using scanning electron microscopy and energy dispersive X‐ray analysis under conditions of powdery mildew fungus infection. Following transfer to Si containing media, plants grown in the absence of Si showed rapid silicification of leafy tissue, primarily in the trichome bases. These plants also exhibited increased disease resistance to Sphaerotheca fuliginea (Schlecht.:Fr.) Poll, and concentration of Si in the leaf epidermis surrounding the invading pathogen. Plants grown in Si‐supplemented media and transferred into Si‐deficient media contained residual Si in the leaf trichome bases, but failed to display disease resistance or silicification of host tissue surrounding the invading pathogen.
Chlorophyll and light gradients in sun and shade leaves of Spinacia oleracea
Authors: CUI M. , VOGELMANN T. C. , SMITH W. K.
Published: April 2006

Abstract. Light gradients were measured and correlated with chlorophyll concentration and anatomy of leaves in spinach (Spinacia oleracea L.). Light gradients were measured at 450, 550 and 680 nm within thin (455 μm) and thick (630 μm) leaves of spinach grown under sun and shade conditions. The light gradients were relatively steep in both types of leaves and 90% of the light at 450 and 680 nm was absorbed by the initial 140 μm of the palisade. In general, blue light was depleted faster than red light which, in turn was depleted faster than green light. Light penetrated further into the thicker palisade of sun leaves in comparison to the shade leaves. The distance that blue light at 450 nm travelled before it became 90% depleted was 120 μm in sun leaves versus 76 μm in shade leaves. Red light at 680 nm and green light at 550 nm travelled further but the trends were similar to that measured at 450nm. The steeper light gradients within the palisade‐of shade leaves were caused by increased scattering of light within the intercellular air spaces and/or cells which were less compact than those in sun leaves. The decline in the amount of light within the leaf appeared to be balanced by a gradient in chlorophyll concentration measured in paradermal sections. Progressing from the adaxial epidermis, chlorophyll content increased through the palisade and then declined through the spongy mesophyll. Chlorophyll content was similar in the palisade of both sun and shade leaves. Chloroplast distribution within both sun and shade leaves was relatively uniform so that the chlorophyll gradient appeared to be caused by greater amounts of chlorophyll within chloroplasts located deeper within the leaf. These results indicate that the anatomy of the palisade may be of special importance for controlling the penetration of photo‐synthetically active radiation into the leaf. Changing the structural characteristics of individual palisade cells or their arrangement may be an adaptation that maximizes the absorption of light in leaves with varying mesophyll thickness due to different ambient light regimes.
Stomatal responses to humidity in air and helox
Authors: MOTT K. A. , PARKHURST D. F.
Published: April 2006

Abstract. Stomatal responses to humidity were studied in several species using normal air and a helium: oxygen mixture (79:21 v/v, with CO2 and water vapour added), which we termed ‘helox’. Since water vapour diffuses 2.33 times faster in helox than in air, it was possible to vary the water‐vapour concentration difference between the leaf and the air at the leaf surface independently of the transpiration rate and vice versa. The CO2 concentration at the evaporating surfaces (ci), leaf temperature and photon flux density were kept constant throughout the experiments. The results of these experiments were consistent with a mechanism for Stomatal responses to humidity that is based on the rate of water loss from the leaf. Stomata apparently did not directly sense and respond to either the water vapour concentration at the leaf surface or the difference in water vapour concentration between the leaf interior and the leaf surface. In addition, stomatal responses that caused reductions in transpiration rate at low humidities were accompanied by decreases in photosynthesis at constant ci, suggesting heterogeneous (patchy) stomatal closure.
Measurement of intracellular and extracellular free calcium in apple fruit cells using calcium‐selective microelectrodes
Authors: HARKER F. R. , VENIS M. A.
Published: April 2006

Abstract. Intracellular and extracellular free calcium was measured in cortical cells from apple fruit (Malus domestica Borkh cv. Cox's Orange Pippin) using calcium‐selective microelectrodes. It was difficult to position microelectrodes in the cytoplasm, and thus most intracellular measurements reflected vacuolar concentrations of about 0.6mol m−3 free calcium. Extracellular free calcium was measured in wells (0.2mm3) that had been cut through the epidermis of the fruit, then filled with osmoticum. Concentrations of free calcium in the sampling well varied between 0.02 and 1.3 mol m−3, and were related to the calcium content of the tissue. Functioning of the plasma membrane, and perhaps the supply of calcium for intracellular needs during fruit ripening and storage, may require mol m−3 concentrations of extracellular free calcium. Thus, it is suggested that low levels of extracellular free calcium in calcium‐deficient fruit may be partly responsible for development of deficiency disorders.
The use of tree root suckers to estimate root water potential
Authors: SIMONNEAU T. , INRA R. HABIB
Published: April 2006

Abstract. This paper describes a simple method to estimate the root water potential of trees using the root suckers as xylemic probes. On the day before the water potentials were measured in pressure chamber, all the sucker shoots were enclosed in plastic bags to prevent transpiration. Under the premise that the potential at two points in a flow system would be equal if there were no flux between these points, the sucker shoot water potential estimates the root water potential. The results are not dependent on the sucker architecture, are consistent with the soil‐plant‐atmosphere continuum concept, and are supported by psychrometric measurements.
Abscisic‐acid‐deficient mutants at the aba gene locus of Arabidopsis thaliana are impaired in the epoxidation of zeaxanthin
Authors: DUCKHAM S. C. , LINFORTH R. S. T. , TAYLOR I. B.
Published: April 2006

Abstract. The xanthophyll content of wild type and abscisic acid (ABA) ‐ deficient mutants of pea and Arabidopsis thaliana was determined. The wilty mutant of pea was indistinguishable from the non‐mutant control. In contrast, plants homozygous for mutant alleles at the aba locus of Arabidopsis were very different from wild type. In these mutants, zeaxanthin accumulated to abnormally high levels. The major carotenoids, violaxanthin and 9′‐cis‐neoxanthin were virually absent from the mutant chromatograms. It was concluded that the aba genetic lesion impairs the epoxidation of zeaxanthin to violaxanthin and that this results in an inability to accumulate ABA. This provides clear evidence that zeaxanthin is a precursor of ABA.
Stomatal control of xylem embolism
Authors: JONES H. G. , SUTHERLAND R. A.
Published: April 2006

Abstract. The potential role of stomatal closure in the control of xylem embolism is investigated by means of a simple model of hydraulic flow in plants. Maintenance of a maximally efficient conducting system requires the stomata to close in an appropriate fashion as evaporative demand increases in order to prevent shoot water potentials falling below the threshold value at which cavitations occur. The model showed that the optimal stomatal behaviour required depends on soil water availability. Further analysis of the model demonstrated that there could be certain circumstances where loss of a proportion of the conducting tissue by embolisms can, perhaps surprisingly, be beneficial in terms of maximizing stomatal aperture and hence short‐term productivity. The results are discussed in relation to the signals controlling stomatal aperture, and it is shown that (1) optimal control cannot be obtained using information on leaf water potential alone, and (2) information relating to soil water potential is a necessary requirement for optimal control.
Gravitropic response of primary maize rootlets as influenced by light and temperature
Authors: HORWITZ B. A. , ZUR B.
Published: April 2006

Abstract. The gravitropic curvature of primary maize rootlets was measured as a function of temperature, both in the presence and absence of light. In two different cultivars, light strongly increased the downward curvature of roots developing from horizontally‐oriented embryos. At 15–20°C, the bending angle was in the range of 70–80° in the light, and 25–50° in the dark, depending on the cultivar. When the temperature was increased above the 15–20°C range, marked differences were found between the two cultivars in their response to light. In one variety tested, JX180, the effect of light was relatively small at 30–35°C. Gravitropic curvature in another variety, Halamish, depended strongly on light throughout the temperature range tested. In both cultivars, gravitropic curvature was only slightly temperature dependent when germination and growth were in total darkness. In the dark, the extent of gravitropic curvature also depended on whether the kernels were oriented with their embryos facing upwards or downwards. Under continuous light, the gravitropic bending of roots of cultivar Halamish did not show a marked temperature dependence. When the seedlings were subjected to only a 15 min illumination, their gravitropic response was partial, and the dependence on temperature somewhat increased. In cultivar JX180, a combination of temperature and light modulates gravitropism. The gravitropic response of different maize cultivars thus differs considerably in its combined dependence on light and temperature.