bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nickel toxicity and peroxidase activity in seedlings of Triticum aestivum L.
Authors: PANDOLFINI T. , GABBRIELLI R. , COMPARINI C. 
Published: April 2006

ABSTRACTNi2+ toxicity was evaluated in Triticum aestivum L. by its effects on root and shoot length, dry matter production and water content. Over a threshold value of 20 mmol m−3 Ni2+ the degree of toxicity increases as a function of the Ni2+ concentration in the medium. Ni2+‐treated roots show enhanced lipid peroxidation; the higher Ni2+ treatment (40mmol m−3) also increases leakage of K+. In roots and shoots, Ni2+ enhances both guaiacol and syringaldazine extracellular peroxidase activity. The increase in extracellular peroxidase activity is also associated with an increase in the phenolic contents of roots and shoots. The observed growth inhibition might be partly the result of the effect of Ni2+ on cell turgor and cell‐wall extensibility. Intracellular soluble peroxidases are also stimulated by Ni2+; such effects, independently of the substrate, were detected in extracts of Ni2+‐treated shoots at a lower Ni2+ concentration than in the roots. Intracellular peroxidases might act as scavengers of peroxide radicals produced as a result of nickel toxicity.
Calcium‐salinity interactions affect ion transport in Chara corallina
Authors: WHITTINGTON J. , SMITH F. A. 
Published: April 2006

ABSTRACTDetached internodes of Chara corallina survived in solutions containing 100 mol m−3 NaCl when the external concentration of Ca2+ was greater than 1 mol m−3. Na+ influx was roughly proportional to external Na+ up to 100 mol m−3 NaCl. Na+ influx involved two components: a Ca2+‐insensitive influx which allowed the passage of Na+ independently of external Ca2+; and a Ca2+‐inhibitable mechanism where Na+ influx was inversely proportional to external Ca2+. The Ca2+‐inhibitable Na+ influx was similar to the Ca2+‐inhibitable K+ influx. Mg2+ and Ba2+ were able to substitute for Ca2+ in partially inhibiting Na+ influx in the absence of external Ca2+. The effect of Ca2+ appears specific to Na+ and K+ influx since the effects of a Ca2+‐free solution on the influx of some other cations, anions and neutral compounds is small. It is suggested that Na+ influx via the Ca2+‐inhibitable mechanism represents Na+ leakage through K+ channels and that cell death at high salinity occurs due to a cytotoxic Na+ influx via this mechanism.
Low conductances for CO2 diffusion from stomata to the sites of carboxylation in leaves of woody species
Authors: LLOYD J. , SYVERTSEN J. P. , KRIEDEMANN P. E. , FARQUHAR G. D. 
Published: April 2006

ABSTRACTConcurrent measurements of leaf gas exchange and on‐line 13C discrimination were used to evaluate the CO2 conductance to diffusion from the stomatal cavity to the sites of carboxylation within the chloroplast (internal conductance; gi). When photon irradiance was varied it appeared that gi and/or the discrimination accompanying carboxylation also varied. Despite this problem, gi, was estimated for leaves of peach (Prunus persica), grapefruit (Citrus paradisi), lemon (C. limon) and macadamia (Macadamia integrifolia) at saturating photon irradiance. Estimates for leaves of C. paradisi, C. limon and M. integrifolia were considerably lower than those previously reported for well‐nourished herbaceous plants and ranged from 1.1 to2.2μmol CO2 m−2 s−1 Pa−1, whilst P. persica had a mean value of 3.5 μmol CO2 m−2 s−1 Pa−1. At an ambient CO2 partial pressure of 33Pa, estimates of chloroplastic partial pressure of CO2 (Cc) using measurements of CO2 assimilation rate (A) and calculated values of gi, and of partial pressure of CO2 in the stomatal cavity (Cst) were as low as 11.2 Pa for C. limon and as high as 17.8Pa for peach. In vivo maximum rubisco activities (Vmax) were also determined from estimates of Cc. This calculation showed that for a given leaf nitrogen concentration (area basis) C. paradisi and C. limon leaves had a lower Vmax than P. persica, with C. paradisi and C. limon estimated to have only 10% of leaf nitrogen present as rubisco. Therefore, low CO2 assimilation rates despite high leaf nitrogen concentrations in leaves of the evergreen species examined were explained not only by a low Cc but also by a relatively low proportion of leaf nitrogen being used for photosynthesis. We also show that simple one‐dimensional equations describing the relationship between leaf internal conductance from stomatal cavities to the sites of carboxylation and carbon isotope discrimination (Δ) can lead to errors in the estimate of gi. Potential effects of heterogeneity in stomatal aperture on carbon isotope discrimination may be particularly important and may lead to a dependence of gi upon CO2 assimilation rate. It is shown that for any concurrent measurement of A and Δ, the estimate of Cc is an overestimate of the correct photosynthetic capacity‐weighted value, but this error is probably less than 1.0 Pa.
The consequences of delayed greening during leaf development for light absorption and light use efficiency
Authors: KURSAR T. A. , COLEY P. D. 
Published: April 2006

ABSTRACTMany species of rainforest plants have an unusual form of leaf development such that leaves delay greening until after full leaf expansion. Chlorophyll accumulation was measured during leaf development in five woody rainforest species, three with white young leaves, and two with ‘normal’ greening. In the three species with white leaves, the chlorophyll content of the expanding leaves was about 0.4mg dm−2, whereas in the two species with green young leaves, chlorophyll content was about 2.1 mg dm−2. Chlorophyll accumulation in greenhouse and field experiments was independent of light level. During leaf expansion, species with delayed chloroplast development only absorb 18–25% of the maximum possible light, compared with 80% for species with normal greening. Furthermore, species with delayed greening have low chlorophyll contents and reduced absorption for at least 30 d after full expansion. At a PPFD typical of the forest under story, the photosynthetic light use efficiency based upon incident radiation was 0.030–0.036 for species with delayed chloroplast development and 0.068–0.085 for the two species with normal greening. The lower light use efficiency of white species was primarily due to decreased light absorption. However, they also had a slightly lower light use efficiency based upon absorbed radiation, suggesting that development of other components of the photo‐synthetic apparatus also may be delayed. Despite the fact that delayed greening decreases light absorption and light use efficiency during leaf development, it is extremely common in shade‐tolerant species. We suggest that an advantage of delayed greening is that resources are not invested in the leaf until it is fully expanded and better defended from herbivores.
Disturbance of cell Ca2+ homeostasis as a primary trigger of Al toxicity syndrome
Authors: RENGEL Z. 
Published: April 2006

ABSTRACTThe primary toxic effects of Al are fast (taking only seconds to several minutes to develop). These effects may be due to the presence of Al ions on the apoplasmic side of the plasma membrane; Al ions may not need to enter the cytosol for a range of primary deleterious Al effects to occur. Aluminium may disturb the symplasmic Ca2+ homeostasis by altering the pattern of Ca2+ fluxes across the plasma membrane. Disturbance of transmembrane Ca2+ fluxes may prevent an increase in cytosolic Ca2+ activity necessary to trigger spindle formation during prophase as well as to initiate the metaphase/anaphase transition. The mechanism behind Al‐related inhibition of mitosis and polar cell growth is suggested to be disturbance of stimulus/response coupling effected through Ca2+ as a second messenger. Involvement of calmodulin in the Al‐related phenomena could be indirect, at least in the initial stages of the Al treatment when Al is probably confined to the apoplasm only. Al‐related disturbance of the Ca2+ homeostasis in the cytosol would alter the patterns of Ca2+ ‐calmodulin binding, resulting in the absence of activation or only partial activation of calmodulin.
Growth of Sesbania rostrata (Brem) with stem nodules under controlled conditions
Authors: KWON D.‐K. , BEEVERS H. 
Published: April 2006

ABSTRACTA series of experiments was carried out in an attempt to produce nodulated plants of Sesbania rostrata with qualities more closely resembling those in the wild than has been achieved to date. When groups of five plants were grown in a controlled climate chamber in pipes containing ∼12dm3 modified Jensen's medium with 6mol m−3 nitrate, the daily growth in height reached 5 cm and at 30 d the plants were ∼40cm high. At this time, the stems were inoculated with Azorhizobium caulinodans ORS 571 and the medium replaced with Jensen's medium without nitrate. In the subsequent 19‐d period ∼300 nodules (representing >50% of the potential infection sites) developed on each stem. The nodules increased linearly in size over this time to ∼15mg. Specific acetylene reduction activity, ARA ((μmol C2H4 mg−1 h−1) rose to 45 between days 5 and 10 after inoculation and plateaued; total ARA rose to ∼200 μmol C2H4 plant−1 h−1. Under the conditions described the plants grew vigorously, and reproducibly uniform yields of nodules with high ARA activities were obtained. As outlined, the procedure offers a standard system in which, within a 2‐week period after inoculation, individual strains of bacteria can be quantitatively compared in their ability to induce nodulation and N2‐fixation. Physiological and biochemical aspects of the nodulated system can be much more readily approached than with plants producing only root nodules. The inhibitory effects of stem nodules induced by wild type and two mutant strains of Azorhizobium on the development and activity of root nodules are described.
Magnesium deficiency enhances resistance to paraquat toxicity in bean leaves
Authors: CAKMAK I. , MARSCHNER H. 
Published: April 2006

ABSTRACTEffects of deficient (20mmol m−3) and sufficient (1000 mmol m−3) magnesium (Mg) supply and of varied light intensity (100 μmol m−2 s−1 to 580 μmol m−2 s−1) on paraquat‐dependent chlorophyll destruction in bean (Phaseolus vulgaris) plants grown in nutrient solution were studied over a 12‐d period using leaf discs or intact primary leaves. Treatment of leaf discs with 10mmol m 3 paraquat for 15h caused severe chlorophyll loss, especially with increasing light intensity. This chlorophyll destruction by paraquat was very much higher in Mg‐sufficient than Mg‐deficient leaves. The occurrence of paraquat resistance in Mg deficient leaves was already apparent after 6d growth in nutrient solution, i.e. before any decrease in chlorophyll or growth by Mg deficiency was evident. Also, following foliar application of paraquat (10–140 mmol m−3) to intact plants, Mg‐deficient plants were much more resistant to paraquat, even following longer exposure duration (72 h) and four to 14 times higher paraquat concentrations than those received by Mg sufficient plants. From experiments where exogenous scavengers of superoxide radical (O2.‐), hydroxyl radical (OH·) and singlet oxygen (1O2) were applied to leaf discs, it appears that O2.‐, and partly, OH· are the main O2 species which contribute to chlorophyll destruction by paraquat. The results demonstrate that Mg‐deficient bean plants become highly resistant to O2.‐‐mediated and light‐induced paraquat injury. The mode of this paraquat resistance is attributed to well‐known stimulative effects of Mg deficiency on O2.‐ and H2O2 scavenging enzymes and antioxidants.
Self‐assembly of plant cell walls
Authors: JARVIS M. C. 
Published: April 2006

ABSTRACTThe object of this paper is to define criteria for distinguishing between self‐assembly and template‐based assembly in plant cell walls. The example of cellulose shows that cell wall polymers biosynthesized at a membrane may retain parallel chain packing arrangements that are thermodynamically unstable and cannot be reproduced in vitro, making the experimental testing of the self‐assembly hypothesis difficult. Also, natural cellulose is ordered on a number of scales of pattern, each of which may be constructed by either self‐ or template‐based assembly independently of the rest. These conceptual problems apply equally to the self‐assembly of complete cell walls and other cell wall polymers. It is suggested that the self‐assembly concept should be applied only to one stage or level in the synthesis of a cell wall, and that an additional concept of parallel assembly may be useful for understanding the synthesis of some polysaccharides.
Zinc and salinity effects on membrane transport in Chara connivens
Authors: RYGOL J. , ARNOLD W. M. , ZIMMERMANN U. 
Published: April 2006

ABSTRACTPressure‐probe measurements showed that the pressure relaxation of internodal cells of the freshwater alga Chara connivens slowed considerably when 1–5 mol m−3 Zn2+, or more especially Zn2+ and 75 mol m−3 NaCl, were present in the medium for periods of 1 h or longer. These results indicate that the water permeability of the Chara membrane is decreased by Zn2+, and that this effect is enhanced by 75 mol m−3 NaCl. Specific values taken after 375 min exposure were: 5 mol m−3 Zn2+ and 75 mol m−3 NaCl caused the half‐time for bulk water movement to increase from 7·8±2·3 to 79·5±5·4s, corresponding to a decrease in the hydraulic conductivity (Lp) from (13·0±3·3) × 10−7 m s−1 mPa−1 to (1·25±0·23) × 10−7 m s−1 MPa−1 (mean±S.D., n= 10). These changes are not seen in the presence of NaCl alone, and to a reduced extent in the presence of 5 mol m−3Zn2+ alone (after 375 min, Lp was (2·4±0·1) × 10−7 m s−1 MPa−1, mean±S.D., n = 6). Ca2+ cannot substitute for Zn2+, but seems to competitively inhibit Zn2+. There was another, kinetically distinct effect of Zn2+: the ingress of Na+ within 15 min of exposure to 75 mol m−3 NaCl is halved by the presence of 1–5 mol m−3 Zn2+, although internal osmolality is little changed by Zn2+. In spite of this, Zn2+ does not exert the long‐term protection against NaCl that has been reported for Ca2+. Depending on the concentration of Zn2+ and the duration of the exposure, the effects on water permeability were fully or partly reversible within 24–48 h. The mechanism of these changes is difficult to identify. One possibility is a zinc‐induced restriction of trans‐membrane channels to give single‐file channels which can be blocked by salt.
Photoinhibitory responses of the epiphytic bromeliad Guzmania monostachia during the dry season in Trinidad maintain photochemical integrity under adverse conditions
Authors: MAXWELL C. , GRIFFITHS H. , BORLAND A. M. , BROADMEADOW M. S. J. , McDAVID C. R. 
Published: April 2006

ABSTRACTThe C3–CAM epiphytic bromeliad Guzmania monostachia var. monostachia may be exposed to high incident photosynthetically active radiation (PAR) during the dry season in Trinidad, and resultant variations in photochemical efficiency have been investigated for ‘exposed’ (receiving ∼100% incident PAR), ‘semi‐exposed’ (∼60% PAR) and shaded populations under natural conditions. The more succulent leaves of the plants growing fully exposed within the canopy had higher overall CAM activity (measured as ΔH+, the dawn‐dusk titratable acidity), a smaller proportion of chlorenchyma and lower total chlorophyll content. There was a gradation of morphological and physiological characteristics between these and shaded leaves. Diurnal time‐courses of photosynthetic light responses (as O2 evolution) showed marked variations in apparent quantum yield (AQY) and light‐saturated rates for both exposed and semi‐exposed populations, dependent on incident PAR during the day. Similar measurements of photosystem II fluorescence characteristics showed that Fv/Fm declined from 0·70 to 0·42 at midday for exposed plants (on a day when total incident PAR was 44 mol photon m−2), indicating non‐photochemical quenching (qNP) of photosynthesis. However, in contrast to AQY determinations, Fv/Fm recovered during the afternoon. The decrease in Fv/Fm was reduced from 0·72 to 0·64 under 24 mol photon m−2 d−1. The long–term recovery of photo‐synthetic efficiency was assessed for exposed plants placed under three shading regimes (60, 30 and 3% of incident PAR) over a 17‐d period. During this time, total chlorophyll content increased from 228 to 515 and 585 μg g−1 fresh weight (for 3 and 30%, respectively) and chlorophyll a:b declined. While AQY recovery was much longer under the lowest PAR (17d), under 30% PAR both AQY and Fv/Fm had recovered after 2d shading. The differences between timing of recovery for Fv/Fm during diurnal time courses and in the long term suggest that, while quenching associated with PSII recovers rapidly, enzyme activation and/or protein synthesis of other photosynthetic components may be limiting under low PAR. However, it is suggested that the occurrence of qNP on a daily basis may preclude long‐term photoinhibitory damage under natural conditions during the dry season.
Kinetics of chloride transport in the cyanobacterium Synechococcus R‐2 (Anacystis nidulans, S. leopoliensis) PCC 7942
Authors: RITCHIE R. J. 
Published: April 2006

ABSTRACTThe kinetics of the light‐driven Cl− uptake pump of Synechococcus R‐2 (PCC 7942) were investigated. The kinetics of Cl− uptake were measured in BG‐11 medium (pHo, 7·5; [K+]o, 0·35 mol m−3; [Na+]o, 18 mol m−3; [Cl−]o, 0·508 mol m−3) or modified media based on the above. Net36Cl− fluxes (ϕCl−o,i) followed Michaelis‐Menten kinetics and were stimulated by Na+ [18 mol m−3 Na+ BG‐11 ϕCl−max= 3·29±0·60 (49) nmol m−2 s−1 versus Na+‐free BG‐11 ϕCl−max= 1·02±0·13 (54) nmol m−2 s−1] but the Km was not significantly different in the presence or absence of Na+ at pHo 10; the Km was lower, but not affected by the presence or absence of Na+ [Km = 22·3±3·54 (20) mmol m−3]. Na+ is a non‐competitive activator of net ϕCl−o,i. High [K+]o (18 mol m−3) did not stimulate net ϕCl−o,i or change the Km in Na+‐free medium. High [K+]o (18 mol m−3) added to Na+ BG‐11 medium decreased net ϕCl−o,i [18 mol m−3K+ BG‐11; ϕCl−max= 2·50±0·32 (20) nmol m−2 s−1 versus BG‐11 medium; ϕCl−max= 3·35±0·56 (20) nmol m−2 s−1] but did not affect the Km 55·8±8·100 (40) mmol m−3]. Na+‐stimulation of net ϕCl−o,i followed Michaelis‐Menten kinetics up to 2–5 mol m−3 [Na+]o but higher concentrations were inhibitory. The Km for Na+‐stimulation of net ϕCl−o,i [K1/2(Na+)] was different at 47 mmol m−3 [Cl−]o (K1/2[Na+] = 123±27 (37) mmol m−3]. Li+ was only about one‐third as effective as Na+ in stimulating Cl− uptake but the activation constant was similar [K1/2(Li+) = 88±46 (16) mmol m−3]. Br− was a competitive inhibitor of Cl− uptake. The inhibition constant (Ki) was not significantly different in the presence and absence of Na+. The overall Ki was 297±23 (45) mmol m−3. The discrimination ratio of Cl− over Br− (δCl−/δBr−) was 6·38±0·92 (df = 147). Synechococcus has a single Na+‐stimulated Cl− pump because the Km of the Cl− transporter and its discrimination between Cl− and Br− are not significantly different in the presence and absence of Na+. The Cl− pump is probably driven by ATP.
What information is conveyed by an ABA signal from maize roots in drying field soil?
Authors: TARDIEU F. , ZHANG J. , DAVIES W. J. 
Published: April 2006

ABSTRACTDuring two seasons, ABA concentrations were monitored in roots, leaves and xylem sap of field‐grown maize. The water status of soil and plant was also measured. Plants were grown on plots with compacted or non‐compacted soil, which were irrigated or remained unwatered. ABA concentration in the xylem sap before dawn and in the roots increases 25‐fold and five‐fold, respectively, as the soil dried, with a close correlation with the soil water status, but with no clear effect of the soil structure. In contrast to the results of several laboratory experiments, no appreciable increase in xylem [ABA] and reduction in stomatal conductance were observed with dehydration of the part of the root system located in soil upper layers. These responses only occurred when the water reserve of the whole soil profile was close to depletion and the transpiration declined. Xylem [ABA] measured during the day was appreciably higher in the compacted treatment than in non‐compacted treatment, unlike that measured before dawn. Since a mechanical message is unlikely to undergo such day‐night alterations, we suggest that this was due to a faster decrease in root water potential and water flux in the compacted treatment, linked to the root spatial arrangement. These results raise the possibility that ABA concentration in the xylem sap could be controlled by two coexisting mechanisms: (1) the rate of ABA synthesis in the roots linked to the soil or root water status, as shown in laboratory experiments; (2) the dilution of ABA in the water flow from roots, which could be an overriding mechanism in field conditions. This second mechanism would allow the plant to sense the water flux through the root system.
137Cs penetration by contact exchange through isolated plant cuticles: cuticles as asymmetric transport membranes
Authors: ERTEL J. , PARETZKE H. G. , ZIEGLER H. 
Published: April 2006

ABSTRACTThe penetration of 137Cs by contact exchange through cuticular membranes from the adaxial surface of leaves of Pyrus communis and Prunus cerasus has been investigated. The resistance of the cuticles to the caesium penetration was dependent on the counter‐ions associated with the fixed negative ion exchange sites in the membrane. The mobility of hydrated potassium ions and their tenuous connection to ‐COOH−‐groups in the membrane encouraged caesium permeation in contrast to cuticular membranes with predominantly protonized ion exchange sites. Divalent calcium ions caused a strong reduction (4–20 times) of the caesium permeability which is decisive for the calculation of the caesium uptake by the intact leaf. Under these conditions, a penetration rate of the deposited caesium of 0·11±0·05% h−1 for pear cuticles and of 0·036±0·025% h−1 for cherry cuticles was measured after the adjustment to steady state conditions. Approximately 12–24% and 4·5–7·5% of initially retained caesium could be absorbed by the leaves of pear and cherry, respectively, in a rain‐free period of 7 d in the area of Munich after wet deposition of fallout from the Chernobyl reactor accident. Furthermore, the caesium penetration from the physiological inside to the outside of the membrane was found to be smaller by a factor of 100–150 compared with that of the opposite direction.
Cytokinins and tRNAs: arguments against a hypothesis of cytokinin action
Authors: JANSSON S. 
Published: April 2006

ABSTRACTArguments against the hypothesis of cytokinin action via competitive binding of cytokinin and cytokinin‐containing tRNA to specific receptor proteins [Romanov (1990) Plant, Cell and Environment 13, 751–754] are presented. The hypothesis states that cereal genes containing a large number (4–8%) of the Leu codons TTG and TTA should be regulated by cytokinin at the translational level. In this paper, it is shown that the only genes that fulfil these hypothetical requirements are the ones encoding the heavy chain of zein. It is further shown that this is a consequence of the high Leu content of the encoded proteins and the lack of GC bias of the genes.