bluuebunny/crossref_metadata_2025 huggingface dataset:
Isolation and characterization of plasma membranes from cyanobacteria
Authors: Norling B. , Zarka A. , Boussiba S.
Published: May 2006

The two‐phase partition system in comparison to the traditional methods used thus far (density gradients) for the isolation of the plasma membrane from cyanobacteria is described. The advantages of the two‐phase system are: A short‐time preparation of 3–4 h compared to 16–48 h required for the density gradient method; a purer fraction, resulting from separation according to membrane surface charge and hydrophobicity, not specific density; and, ease of scaling‐up for obtaining large quantities.

Also, the different biological activities attributed to this membrane to date are summarized. Findings on the typical plasma membrane ATPase (P‐type ATPase) as well as the nutrient transporters and the corresponding proteins are included. As for the electron transport chain, one may conclude that this membrane contains a complete system (similar to that of the mitochondrion), portraying apparently F‐type (F0F1) ATPase activity.
Is negative turgor fallacious?
Authors: Rhizopoulou Sophia
Published: May 2006

In early studies, negative turgor pressure was obtained by indirect methods and was mainly related to inaccurate measurements of osmotic and/or water potential. The uncertainties in the measurements made any discussion of the underlying reasons and phenomena premature and risky, perhaps even useless. Yet, the idea of relating negative turgor to the strength of small‐sized cells with thick walls, to resist collapse when subjected to water deficit, seemed attractive. Negative turgor pressures are difficult to detect in higher plants, but they are theoretically possible. Regarding their origin, a hypothesis is discussed here, that the negative turgor detected in leaves might indicate a reversal of the water flow that has occurred in roots, under conditions of severe and prolonged drought.
An evolutionary perspective on the ‘death hormone’hypothesis in plants
Authors: Wilson J. Bastow
Published: May 2006

Many plants die after first reproduction, as if committing suicide, a phenomenon known as ‘monocarpic senescence’. The process is different from the ageing seen in most animals. A ‘death hormone’has been proposed to explain the phenomenon, both for plants and also separately for animals. This paper considers for plants whether and how such a death hormone could have evolved.

Monocarpic senescence has sometimes been attributed to starvation of the vegetative parts for photosynthate or mineral nutrients, by nutrient diversion to developing fruit. In this case, no evolutionary problem arises. In other cases, the phenomenon seems to be one of programmed senescence, since details of the process conflict with an explanation in terms of nutrient diversion, since there is apparently a hormonal senescence signal, and since, e.g., the same type of senescence occurs even in male plants that do not bear fruit.

A range of evolutionary mechanisms are considered, by which a death hormone could have evolved: species selection, kin selection, direct Darwinian selection, pleiotropy, functional correlation, orthogenesis, genetic drift and founder effects. The two most likely are identified as: (1) kin selection, in which the mother plant dies, and thus facilitates establishment of her offspring, and (2) selection for efficiency of assimilate mobilisation. Neither of these explanations seems completely satisfactory.
Spectral reflectance and integrity of cell membranes and chlorophyll relative to the concentration of airborne mineral elements in a lichen
Authors: Garty Jacob , Karnieli Arnon , Wolfson Racheli , Kunin Paola , Garty‐Spitz Rachel
Published: May 2006

To determine the environmental impact of oil‐combustion pollutants and soil dust on a lichen, we examined the spectral reflectance of thalli of the epiphytic fruticose lichen, Ramalina duriaei, expressed as values of NDVI (the normalized difference vegetation index). We analyzed electrolyte leakage caused by degradation of cell membranes in terms of electric conductivity of water, apart from chlorophyll degradation, the latter expressed as changes in the A435 nm/A415 nm ratio to indicate the physiological status of the lichen. The concentrations of Al, Cr, Fe, K, Ni, P, sulfate‐S, Ti and V in the lichen thallus were measured to quantify the degree of pollution. Thalli of R. duriaei, growing in a nature reserve on the periphery of a 40‐year‐old industrial town, Ashdod, in southwest Israel were compared with thalli of R. duriaei from an unpolluted forest in the northeastern part of the country transplanted to the polluted areas in and around the town. After an exposure for 10 months, many transplants exhibited lower NDVI values, higher electric conductivity values as well as a lower A435 nm/A415 nm ratio. The three physical/physiological parameters thus reflected severe injury in the lichen transplants. The concentrations of Al, Cr, Fe, Ni, sulfate‐S, Ti and V in the lichen transplants were found to correlate inversely with the NDVI values, whereas the concentrations of Fe, Ni, Ti and V were found to correlate with electric conductivity. The decrease in the A435 nm/A415 nm ratio was found to correlate with high concentrations of Al, Fe, Ni, sulfate‐S, Ti and V in the lichen transplants, whereas the concentration of K and P correlated with both the NDVI value and the A435 nm/A415 nm ratio. It is concluded that in situ thalli of R. duriaei, the only indigenous fruticose lichen growing in the region of Ashdod, are endangered by the presence of pollutants and by acid rain due to the combustion of heavy fuel oil.
Light stress provokes plastic and elastic modifications in structure and function of photosystem II in camellia leaves
Authors: Krüger Gert H. J. , Tsimilli‐Michael Merope , Strasser Reto J.
Published: May 2006

Adaptation of plants to a continuously changing environment may be approached in terms of an optimisation strategy that is dictated by the thermodynamic demand for minimal entropy production. Any change in environmental input is a stressor in the sense that it disturbs optimality. Stress is defined by this disturbance and is therefore relative, with non‐stress serving as the reference condition. Because of the thermodynamic demand for optimality, suboptimality forces the system to undergo a state change to a new optimal state. These state changes bring about the adaptation of the system. Any physical or chemical change that occurs during the adaptation processes is denoted as strain. In the present study, the polyphasic rise in chlorophyll (Chl) a fluorescence (OJIP) is used to investigate the behaviour of photosystem II in camellia leaves under different physiological steady states established by adaptation to different light regimes at both room temperature and 10°C. Analysis of the fast fluorescence rise according to the JIP‐test allows establishment of structural and functional parameters, providing a quantification of the system's behaviour. The calculated functional parameters are the specific (per reaction center, RC) and phenomenological (per cross section of the sample, CS) energy fluxes for absorption (ABS), trapping (TR) and electron transport (ET). The flux ratios or the yields (the maximum quantum yield of primary photochemistry, φp0, the efficiency with which a trapped exciton can move an electron into the electron transport chain, ψ0, or the probability that an absorbed photon will move an electron into the electron transport chain, ψE0 are structural parameters provided by this test, as well as an expression of the concentration of the reaction centers (RC/CS). The photochemical (kp) and nonphotochemical (kN) de‐excitation rate constants are also calculated. The present study focuses mainly on the response of photosystem II to cyclic environmental changes, denoted as the residual strain. The study investigates the adaptive processes in relation to the regulation of the different functional and structural parameters, as well as reversibility (elasticity) and irreversibility (plasticity). Having screened camellia leaves for several environmental changes, we observed that the various structural parameters undergo modification that differ in both their extent and degree of elasticity, which indicates that different survival strategies are employed in response to stress.
Polygalacturonase isozyme 2 binding and catalysis in cell walls from tomato fruit: pH and β‐subunit effects
Authors: Chun Jong‐Pil , Huber Donald J.
Published: May 2006

The catalytic activity of endopolygalacturonase (PG, EC 3.2.1.15) against pectic polymers in vitro is typically not expressed in vivo. In the present study, the binding and catalytic properties of PG isozyme 2 and the influence of the β‐subunit protein were investigated in cell walls prepared from tomato fruit expressing an antisense gene to the β‐subunit protein. Cell walls prepared from mature‐green fruit were employed for binding and assay of PG2. Walls were provided with rate‐limiting quantities of purified PG2 and incubated at 100 mM KCl, pH 4.5, or 25 mM KCl, pH 6.0. Cell walls of both β‐subunit antisense and wild‐type fruit retained comparable quantities of added PG2. The release of pectin from PG2‐loaded walls was proportional to the quantity of added enzyme, consistent with a finite catalytic capacity of individual PG proteins. β‐Subunit‐antisense cell walls released 2‐ to 3‐fold higher levels of pectin in response to PG2 than did wild‐type walls. Cell walls incubated at pH 6.0 released lower quantities and showed less extensive depolymerization of pectins than did walls incubated at pH 4.5. Pectins recovered from ripe fruit were similar in size distribution to polymers released by PG2 at pH 6.0, indicating that pH can influence both quantitative and qualitative aspects of pectin metabolism and may be responsible for the restricted hydrolysis of pectins in vivo. Molecular mass differences were not evident in the polymers rendered freely soluble in response to PG2‐mediated hydrolysis of β‐subunit‐antisense compared with wild‐type cell walls. The solubilization of pectin from cell walls was not the sole indicator of the extent of PG‐mediated cell wall hydrolysis. Hydrolytic modifications were also evident in a pectic fraction extracted from postcatalytic cell walls with 50 mM CDTA (trans‐1,2‐cyclohexanediamine‐N,N,N′,N′‐tetraacetic acid), and were more extensive for the β‐subunit‐antisense cell walls compared with the wild‐type walls. Pectic polymers derived from ethanol insoluble‐powders showed molecular mass downshifts during ripening but differences between the β‐subunit‐antisense and wild‐type fruits were not observed.
Characterization of proton and sugar transport at the tonoplast of sweet lime (Citrus limmetioides) juice cells
Authors: Echeverria Ed , Gonzalez Pedro C. , Brune Andreas
Published: May 2006

Citrus fruits accumulate high levels of sucrose and hexoses, although most photoas‐similates arrive in the form of sucrose. In sweet limes, faster rates of sugar accumulation take place early in development when sucrose catabolic enzymes are most active. The present investigation was aimed at providing information on the mechanisms of sucrose (and hexose) uptake into the vacuole of cells containing high levels of sucrose hydrolytic activity. Tonoplast vesicles of high purity were isolated in a discontinuous sucrose gradient. The vesicles were capable of forming a pH gradient in the presence of ATP. Both bafilomycin and NO3− (but not vanadate) inhibited ATP hydrolysis and prevented the formation of the pH gradient, confirming the tonoplast origin. Energized vesicles (either by addition of ATP or by artificial pH gradient) did not accumulate sucrose or hexoses against a concentration gradient. In the presence of either sucrose or hexoses, the established ΔpH; was not disrupted as was the case with tonoplast vesicles from red beet hypocotyl. Therefore, a sucrose/H+ (hexose) antiport may not be the mechanism of sucrose and hexose transport into the vacuoles of sweet lime juice cells. The data indicated that sucrose uptake into vacuoles of sweet lime occurs by facilitated diffusion. Hexoses originate from the hydrolytic action of acid invertase on sucrose within the vacuole, and by the action of cytosolic sucrose synthase.
Changes in jasmonic acid concentration during early development of apple fruit
Authors: Fan Xuetong , Mattheis James P. , Fellman John K. , Patterson Max E
Published: May 2006

Apple fruits (Malus domestica Borkh.) were harvested from 24 to 136 days after full bloom (DAFB) and endogenous jasmonic acid was analyzed by GC‐MS. There were two isomers of jasmonic acid in apple fruit with a ratio of 37:63 (cis:trans). The cis:trans ratio remained relatively constant throughout this period of fruit development. The endogenous jasmonic acid concentration was 138 ng g−1 fresh weight 24 DAFB and decreased as fruit developed. Changes in jasmonic acid concentration were coincident with those of respiration, ethylene production, and anthocyanin accumulation in patterns consistent with the reported responses to exogenous jasmonates. Possible roles for jasmonic acid during early fruit development are discussed.
Increased ethylene synthesis enhances chilling tolerance in tomato
Authors: Ciardi Joseph A. , Deikman Jill , Orzolek Michael D.
Published: May 2006

Exposure to chilling temperatures (0–10°C) increases ethylene synthesis in several species, but it is not clear whether this increase in ethylene synthesis is involved in the development of chilling tolerance. To determine this relationship, chilling tolerance development of the ethylene‐insensitive Never‐ripe (Nr) tomato (Lycopersicon esculentum Mill. cv. Pearson) mutant was compared to that of a normal isogenic line. Plants were grown for 3 weeks at 25/20°C day/night, and then half the plants of each genotype were chill‐hardened at 25/5°C day/night for 10 days, while the other half remained at 25/20°C. All plants were then exposed to a severe chill (5/5°C day/night) for 7 days, and a recovery period (25/20°C day/night) of 4 days. Chill hardening increased ethylene synthesis in both genotypes, but increased the rate of leaf development and dry weight accumulation during the recovery period only for the normal plants. Hardening also caused a greater decrease in lesion development in the normal plants following the severe chill. Therefore, chill hardening was more effective in the ethylene‐sensitive normal plants than in the ethylene‐insensitive mutants, indicating that a response to ethylene is involved in chilling tolerance development. However, chill hardening was effective for both genotypes in maintaining chlorophyll a fluorescence levels (Fv/Fm), suggesting that ethylene does not control all aspects of chilling tolerance. In a separate experiment, application of a 500 ppm ethephon solution to normal ‘Pearson’ plants led to higher rates of ethylene synthesis and a higher rate of leaf development following a 6 day severe chill (5/5°C day/night). These results further indicate that ethylene is involved in the development of chilling tolerance.
Changes in free polyamine levels induced by salt stress in leaves of cultivated and wild tomato species
Authors: Santa‐Cruz Ana , Acosta Manuel , Pérez‐Alfocea Francisco , Bolarin Maria C.
Published: May 2006

The effects of NaCl on endogenous free levels of the poluamines putrescine, spermi dine and spermine, and the relationships between polyamines, K+ levels and Na+ accumulation were determined in leaves of the cultivated tomato (Lycopersicon esculentum Mill.) and its wild, salt‐tolerant relative L. pennellii (Correll) D' Arcy at different exposure times during a 32‐day period. Both stress treatments (100 and 200 mM NaCl) decreased the levels of putrescine and spermidine, although to a different degree for the cultivated and wild tomato species. The spermine levels did not decrease with salinity in L. pennellii over the salinization period, whereas they decreased in L. esculentum, except at the first application of the 100mM NaCl treatment. In both species, the changes induced by salinity in total polyamines and K+ were very similar, with the accumulation of Na+ in the leaf being concomitant with a decrease in both total polyamines and K+. This suggests that the main role of the polyamines in the leaf tissues. In this sense, a direct relationship between total polyamines and K+, and inverse relationship between polyamines and Na+ and between K+ and Na+ were found for both species. In the short term (up to 4 days) a peculiar physiological behavior was found in L. pennellii, as the total polyamine and K+ levels decreased at 100 mM but not at 200 mM NaCl, while after this time the latter plants had values lower than those of the 100 mM NaCl‐treated plants at day 11.
Quantification of the export of cytokinins from roots to shoots of Rosa hybrida and their degradation rate in the shoot
Authors: Dieleman Janneke A. , Verstappen Francel W. A. , Perik René R. J. , Kuiper Daan
Published: May 2006

Cytokinins from the roots may be involved in regulating rose (Rosa hybrida) shoot growth and development. The objective of this study was to estimate the export of cytokinins from the roots and their degradation rate in the shoot, which were expected to be correlated with plant development. Hence, the total cytokinin content of the shoot, the concentration of zeatin riboside (ZR) in bleeding sap, and the transpiration rates in three stages of development were determined. The estimations performed are based on the assumption that the cytokinin concentration in bleeding sap is representative for the cytokinin concentration in xylem sap in situ. This was verified by comparing the ZR concentration in bleeding sap and in sap obtaíned after pressurizing the root system to a level equivalent to the leaf water potential; no significant differences could be found. The import of cytokinins could not be correlated with plant development, as it increased linearly with time. The estimated relative degradation rate of cytokinins in the shoot decreased as the plants matured. The half‐life of cytokinins in the shoot was found to be approximately 1 day, indicating that cytokinins are rapidly metabolized in the shoot.
The photosynthetic potential of canola embryos
Authors: Asokanthan Ponita S. , Johnson Robert W. , Griffith Marilyn , Krol Marianna
Published: May 2006

Canola embryos are green during development, but the role of embryo chloroplasts is not known. The objective of this study was to characterize the structure and function of embryo chloroplasts. Observations obtained by SDS‐PAGE and transmission electron microscopy showed that thylakoids from embryo chloroplasts contained the same chlorophyll‐protein complexes, but exhibited a greater proportion of granal stacking, compared with thylakoids from leaf chloroplasts. When assayed using an oxygen electrode, photosynthetic electron transport and respiration were enhanced in canola embryos bathed in concentrations of sucrose below 0.47 M. Photosynthesis, measured as the rate of incorporation of 14CO2, was much lower in embryos than leaves even though significant electron transport was detected. These results indicate that the primary role of chloroplasts in embryos is not to photoassimilate CO2. Instead, canola embryos are photoheterotrophic and may use the light reactions to generate the ATP and NADPH required to fuel the conversion of maternally supplied sucrose to the fatty acids used in oil synthesis and storage. These results led us to propose that the current model of plastid differentiation be modified to include the development of photoheterotrophic chloroplasts in sink tissues. This pattern of plastid differentiation can then be used to explain patterns of embryo development.
Water and sucrose regulate canola embryo development
Authors: Johnson Robert W. , Asokanthan Ponita S. , Griffith Marilyn
Published: May 2006

The effect of water and sucrose on the growth and development of zygotic, 30‐day‐old canola (Brassica napus L. cv. Bounty) embryos was examined in vitro by manipulating the levels of sucrose and/or sorbitol present in the culture medium. In some experiments, the medium water potential was allowed to vary with sucrose concentration, while in other experiments, the medium water potential was held constant by adding sorbitol to varying amounts of sucrose. Our results showed that embryos cultured on sorbitol alone exhibited two developmental patterns: embryos germinated precociously on media containing up to 0.70 M sorbitol, whereas embryos became yellow and quiescent on media with higher concentrations of sorbitol. For embryos cultured on media containing sucrose alone, three distinct developmental patterns were noted: at low sucrose concentrations, embryos germinated precociously; at intermediate concentrations, embryos continued to grow in an embryonic mode; and, at high concentrations, embryos became yellow and quiescent. Continued embryonic growth was never observed in embryos cultured on media containing sorbitol alone. Embryos never germinated precociously when cultured on media maintained at a constant water potential of ‐1.4 MPa, rather dry weight increased in these embryos with an increase in sucrose concentration. We envision the effect of sucrose on embryo growth and development to be nested within the effect of water availability. When water availability is restricted, embryos become quiescent. When water is available, embryos have the potential to grow, but the developmental growth pattern depends on the availability of sucrose. In the absence of sucrose, embryos germinate and initiate the transition to autotrophy. If sufficient sucrose is available, embryos remain photohet‐erotrophic and continue to grow in an embryonic mode.
Cyanogenic glycosides and their metabolic enzymes in barley, in relation to nitrogen levels
Authors: Forslund Karin , Jonsson Lisbeth
Published: May 2006

The possible role for cyanogenic glycosides as nitrogen storage compounds was studied in barley, Hordeum vulgare (cv. Golf), cultivated under different nitrogen regimes. Cyanogenic glycosides were absent in seeds and roots but were synthesized in seedlings where they accumulated at a level of about 150 nmol shoot−1 in control plants and 110 nmol shoot−1 in nitrogen‐starved plants. An enzyme involved in the breakdown of cyanogenic glycosides, β‐glucosidase (EC 3.2.1.‐) exhibited high activity in seeds and was also detected in roots and shoots. The activity of β‐cyanoalanine synthase (EC 4.4.1.9), which is involved in the metabolism of HCN, was low in seeds but very high in roots and shoots. There was no correlation between the activities of the two enzymes and the content of cyanogenic glycosides or nitrogen. The relative content of nitrogen in cyanogenic glycosides never exceeded 0.3% of total nitrogen, and the amount of cyanogenic glycosides decreased at a low rate even at a stage when nitrogen limitation inhibited growth.
The transiently pubescent young leaves of plane (Platanus orientalis) are deficient in photodissipative capacity
Authors: Bisba Aspasia , Petropoulou Yiola , Manetas Yiannis
Published: May 2006

We have analyzed reflectance changes and carotenoid composition of young and mature leaves of Platanus orientalis L. in order to test the hypothesis that the transient occurrence of highly absorptive and reflective leaf hairs of young leaves (M. Ntefidou and Y. Manetas 1996, Aust. J. Plant Physiol. 23: 535–538) may be correlated to a weakly developed photodissipative capacity in the chloroplast. Compared to mature leaves, young leaves showed negligible reflectance changes at 530 nm upon sudden illumination, possibly indicating a limited production of zeaxanthin. In addition, actual pigment analysis confirmed lower pools of xanthophyll cycle components and reduced capacity for violaxanthin photoconversion in young leaves. Accordingly, the epoxidation state at saturating photon fluence rates was particularly high. A notable feature of xanthophyll cycle interconversions in young leaves was the inability to drive the system to complete de‐epoxidation, as antheraxanthin in the light was always higher than zeaxanthin. Among the rest of the carotenoids, the levels of β‐carotene were particularly low. Moreover, most of the photosynthetic pigments were considerably bleached when young leaves were exposed to high light. The above results strongly suggest that young leaves possess a limited photodissipative capacity and therefore, the presence of leaf hairs affords protection against excess light. When the leaf has matured and presumably the concentrations of photoprotective compounds are adequate, the loss of hairs is not of consequence. In fact, their presence on mature leaves may reduce the photosynthetically active radiation to non‐saturating levels for photosynthesis.
Molecular cloning of a Brassica napus cysteine protease gene inducible by drought and low temperature stress
Authors: Stroeher Virginia L. , Maclagan James L. , Good Allen G.
Published: May 2006

We have identified a drought‐inducible gene in Brassica napus, bcp‐15, which encodes a putative protein with significant amino acid similarity to wound and stress‐induced plant cysteine proteases. We detected basal levels of bcp‐15 transcript in leaves of nonstressed B. napus plants. Subjecting plants to drought resulted in a maximum 3‐fold increase in expression at 63% relative water content (RWC), while further dehydration resulted in decreased levels of expression. Low temperature caused the greatest accumulation of bcp‐15 transcript; an 8.5‐fold increase was seen after 1 day of exposure to 4°C. After 4 days of exposure, transcript levels decreased to 6.4‐fold over basal levels. These results suggest that the encoded BCP15 protease is required during the early stages of stress response. Exposure to 40°C heat shock resulted in a moderate increase in expression, although transcript levels were less than those seen for dehydrated or low‐temperature‐treated plants. Addition of the plant hormone abscisic acid had only a minor effect on bcp‐15 transcript levels. High salt caused a decrease, not an increase, in bcp‐15 expression over basal levels. These results suggest that bcp‐15 responds to specific conditions of stress and is not simply an adaptation to general osmotic stress.
Comparison of spectrophotometric and radioisotopic methods for the assay of Rubisco in ozone‐treated plants
Authors: Reid Chantal D. , Tissue David T. , Fiscus Edwin L. , Strain Boyd R.
Published: May 2006

Radioisotopic and spectrophotometric assays for ribulose‐1,5‐bisphosphate carboxy‐lase/oxygenase (Rubisco) initial and final activities and Rubisco content were compared in plants chronically exposed to ozone (O3) in a greenhouse and the field. In a greenhouse experiment, Glycine max was treated in exposure chambers with either charcoal‐filtered air (CF air) or 100 nl O3 1−1 for 6 h daily during vegetative growth. Samples were collected after 7 days of exposure. In a field experiment, G. max was treated in open‐top chambers with either CF air or nonfiltered air with O3 added at 1.5 times ambient O3 for 12 h daily. Average daily O3 concentrations were 21 and 92 nl T1 in the CF and O3 treatments, respectively. Samples were collected during vegetative and reproductive growth. Both assays generally yielded comparable Rubisco initial and final activities for greenhouse‐grown plants regardless of the O3 treatment. However for field‐grown plants, Rubisco initial and final activities averaged 15 and 23% lower when assayed by the spectrophotometric rather than the radioisotopic method. For Rubisco content estimated by the spectrophotometric method, lower r2 values for the regression of Rubisco activity vs concentratio of carboxyarabinitol‐1,5‐bisphos‐phate were observed in O3 than in CF‐treated plants. Both assays yielded comparable Rubisco contents in the greenhouse and in the field although the variation was larger with the spectrophotometric method in field‐grown plants. Growth conditions, field vs greenhouse, were more critical to the spectrophotometric assay performance than the O3 treatments for measurement of Rubisco activity and content.
Endo‐β‐mannanase activity during dormancy alleviation and germination of white spruce (Picea glauca) seeds
Authors: Downie Bruce , Hilhorst Henk W. M. , Bewley J. Derek
Published: May 2006

It is not known how embryos of seeds of the Pinaceae protrude from their enclosing tissues to complete germination. Prior to protrusion of the radicle there is an increase in endo‐β‐1,4‐mannanase (EC 3.2.1.78) activity associated with weakening of the micropylar megagametophyte/nucellus from seeds of white spruce (Picea glauca [Moench.] Voss). Mannanase activity is present as three isoforms (pI values 5.0, 4.8, 4.7) in both the embryo and surrounding structures (megagametophyte and nucellus) prior to and during imbibition. Activity of all the isoforms increases in the chalazal and micropylar megagametophyte during germination. Activity then declines after the testa splits, typically 1 day prior to radicle protrusion, due partially to its leaching from the seed into the surrounding water. Activity increases in the cotyledons and axis as the embryo commences elongation. Seeds from dormant seedlots exhibit a lower germination percentage, relative to seeds from nondormant seedlots, and the force necessary for the embryo to puncture the surrounding structures tends to be greater. Although similar mannanase activities are present in unimbibed seeds of dormant and nondormant seedlots, during germination, enzyme activity in seeds of dormant seedlots is lower. Moist chilling alleviates dormancy in the seeds of the Pinaceae and, during 3 weeks of this treatment, mannanase activity slowly increases. After 3 weeks of moist chilling and regardless of whether the seedlot was dormant or not prior to moist chilling, the force necessary to puncture the micropylar megagametophyte and nucellus is lower, and the speed of germination greater. Seeds from previously dormant seedlots also complete germination to a greater percentage, relative to unchilled seeds from dormant seedlots. Upon transfer to 25°C, mannanase activity in moist‐chilled seeds decreases during germination of all seedlots regardless of their previous dormancy status.
Biosynthesis and accumulation of D‐ononitol in Vigna umbellata in response to drought stress
Authors: Wanek Wolfgang , Richter Andreas
Published: May 2006

Metabolic responses to water deficit that lead to an accumulation of cyclitols, have been examined in rice bean (Vigna umbellata [Thunb.] Ohwi et Ohashi). Imposition of drought stress by withholding water from the soil for 9 days led to an accumulation of D‐ononitol (lD‐4‐O‐methyl‐myo‐inositol) which was most pronounced in leaves (from 33 to 88 umol g−1 dry mass). However, the activity of the enzyme myo‐inositol 6‐O‐methyltransferase (m6OMT, EC 2.1.1.X), which catalyzes the synthesis of ononitol from myo‐inositol and S‐adenosyl‐L‐methionine (AdoMet), increased in stems but not in leaves during the drought stress experiment. Detailed analysis of different plant parts revealed that the accumulation of ononitol in leaves was linearly related to stem m6OMT activity during drought stress, indicating that m6OMT may control the in vivo biosynthetic rate of this cyclitol. The availability of myo‐inositol, required for enhanced rates of ononitol synthesis by m6OMT, increased during the stress experiment, while the capacity to synthezise AdoMet by S‐adenosyl‐L‐methionine synthetase (SMS, EC 2.5.1.6) decreased. However, the high capacity for degradation of S‐adenosyl‐L‐homocysteine (AdoHcy; a potent competitive inhibitor of m6OMT) by the enzyme S‐adenosyl‐L‐homocysteine hydrolase (SHH, EC 3.3.1.1) provided favourable conditions for ononitol biosynthesis during the whole stress treatment.
Role of light in changes in free amino acid and polyamine contents at chilling temperature in maize (Zea mays)
Authors: Szalai Gabriella , Janda Tibor , Bartók Tibor , Páldi Emil
Published: May 2006

The effect of irradiance on changes in photosynthesis, free amino acids and polyamines was investigated. Two‐week‐old maize (Zea mays L.) plants were chilled at 5°C in the light (250 μmol m−2 s−1 PAR) or dark. The chlorophyll fluorescence ratio, Fv/Fm, decreased in the light by ca 50% but did not change in the dark. Similarly to the Fv/Fm, there was no change in the transpiration rate or the stomatal conductance in the dark, while these parameters decreased by ca 55% in the light. The net photosynthesis rate declined in both cases, but to a far greater extent in the light (73%) than in the dark (40%). The intercellular CO2 partial pressure increased by ca 50% in all cases. The free amino acid contents increased compared to the control during the cold treatment. In most cases this increase was more pronounced in the light than in the dark. There was a continuous increase in the putrescine level, which was more pronounced in the light than in the dark. The spermidine content increased one and a half times after one day in the light but decreased by 70% in the dark compared to the control values. From the second day a 50% decline in the spermidine content was observed in the light and an 80% decline in the dark. These results suggest that light has an influence not only on the photosynthetic processes during chilling stress but also on other stress markers such as polyamines and free amino acids.
The wheat wcs120 gene family. A useful model to understand the molecular genetics of freezing tolerance in cereals
Authors: Sarhan Fathey , Ouellet François , Vazquez‐Tello Alejandro
Published: May 2006

Winter, as compared with spring cereals, possess better acclimation mechanisms that allow them to overwinter and survive freezing temperatures. This difference is genetically programmed and involves a complex genetic system. To understand the nature of this system and its regulation by low temperature, genes associated with freezing tolerance in wheat (Triticum aestivum L.) were identified and characterized. Among these, the wcs120 gene family encodes a group of proteins ranging in size from 12 to 200 kDa. As shown by biochemical, immunohistochemical, molecular and genetic analyses, this gene family is specific to the Poaceae, highly abundant and coordinately regulated by low temperature. Furthermore, accumulation of WCS protein is directly correlated with the development of freezing tolerance. These analyses also revealed a regulatory control of the vernalization process over low temperature gene expression in winter cereals. Recent studies suggest that the molecular mechanisms controlling the expression of these genes involve negative regulatory factors that are modulated by phosphorylation.
A shift of paradigm is needed in plant science
Authors: Ingestad Torsten
Published: May 2006

The generally accepted norm for controlled experiments with plants has traditionally been restricted to a simplified maintenance of a constant state of the surrounding medium. The ‘treatments’ and a ‘control’, as a reference for comparisons, have been applied without a clear specification of causality and function. However, for pure mathematical reasons, plants are in a constant physiological state (steady‐state) only when the internal concentrations of carbon and nutrients and the ratios between them remain constant. Consequently, experimental conditions are controlled in a meaningful sense only when carbon and nutrients are taken up with a constant relative uptake rate, Ru, that is equal to the relative growth rate, RG. The successful application of these fundamental rules to experimental procedures has been verified in numerous experiments. As for acclimatized plants in steady‐state, responses to limiting factors have been graded relative to maximum performance, which constitutes the ultimate reference value. Despite the comprehensive and straightforward material of evidence, the methodology is still a matter of argument.

It is concluded that the driving variable commonly used for uptake, external concentration, should be abandoned since its form (lack of time dimension) makes it incompatible with the uptake rate. This calls for a shift of paradigm since the failure of recognition both of fundamental relationships and of experimental shortcomings has been harmful to the development of a sound scientific basis for specification of plant properties. Although fragmented knowledge of basic mechanisms may be good, understanding of causes and effects is poor in relationships between environmental, internal and inherited conditions on the one side, and plant performance on the other. A pseudo‐science has in an empirical ‘trial‐and‐error’ research been based upon statistical evaluation of data of low quality. This is a problem of major significance that needs open discussion and public attention.
The photoreceptors in the high irradiance response of plants
Authors: Tanada Takuma
Published: May 2006

Several studies show that the high irradiance response (HIR) of plants is probably due to two photoreceptors. One of the photoreceptors is phytochrome, and the other is an unidentified pigment provisionally named heliochrome. One of the functions of heliochrome is the synthesis of phytochrome, using far‐red and blue radiations of high intensities, to replace the phytochrome destroyed by light. Another possible function could be an interaction of heliochrome with a substance produced by phytochrome. The data presented show that heliochrome is a pigment with different properties from phytochrome. It shows a far‐red/green reversibility. Heliochrome has been shown to participate with phytochrome in such HIRs as leaf movement in Albizzia and flowering in a long‐day plant. The first event initiated by phytochrome and by heliochrome could be the generation of a strong positive, electrostatic charge in the cell membrane.
Oxidative damage to chloroplasts from Chlorella vulgaris exposed to ultraviolet‐B radiation
Authors: Malanga Gabriela , Calmanovici Gabriela , Puntarulo Susana
Published: May 2006

Upon UV‐B irradiation, Chlorella vulgaris cells and isolated chloroplasts increased in size and starch accumulation. Photosynthetic capacity and chlorophyll content of chloroplasts isolated from irradiated algae decreased by 72 and 66%, as compared to chloroplasts isolated from control cells. Dihydrorhodamine 123 conversion to rhodamine 123 was used as a sensitive method for detection of peroxide (presumably hydrogen peroxide) formation in isolated chloroplasts. The accumulation of rhodamine 123 is higher in irradiated than in nonirradiated chloroplasts and the increased accumulation of rhodamine 123 depended on the UV‐B dose. Quantitation of alkyl radical‐EPR signals in chloroplasts indicated that UV‐B exposure significantly increased radical content in the membranes. The content of an oxidized DNA base (8‐hydroxy‐2′‐deoxyguanosine) in chloroplasts was increased by 72 and 175% after irradiation of the algal culture with 17.3 and 42.6 kJ m−2, respectively. The chloroplastic activity of superoxide dismutase decreased by 50% as compared with control values after irradiation with 42.6 kJ m−2 and no changes in ascorbate peroxidase activity and ascorbic acid content were detected at the irradiation doses tested. The β‐carotene content in chloroplasts was not affected by the irradiation, but the α‐tocopherol content increased approximately 4‐fold after UV‐B irradiation. The results suggest that oxidative damage related to UV‐B exposure is responsible for alterations in chloroplasts function and integrity, and that an antioxidant response is triggered in chloroplasts through an increase in α‐tocopherol content.
Binding and activation of pea chloroplast fructose‐1,6‐bisphosphatase by homologous thioredoxins m and f
Authors: Reche Ascensión , Lazaro Juan José , Hermoso Rosario , Chueca Ana , Gorgé Julio López
Published: May 2006

Fructose‐1,6‐bisphosphatase (FBPase; EC 3.1.3.11) binds its putative physiological activator thioredoxin f (Trxf) at pH 7.9, the pH in the stroma of the illuminated chloroplast. Since Trx m, described as specific in NADP+‐malate dehydrogenase (NADPMDH) activation, appears in pea (Pisum sativum L.) also to be functional in FBPase modulation, we have here analyzed the effect of pH and the redox status of the chloroplast stroma in the pea FBPase binding of homologous Trx f and m. Both pea Trx were strongly bound by purified FBPase when they were preincubated at pH 7.9 with 2.5 mM dithiothreitol (DTT), but not when the reductant was omitted. As occurs with Trx f the Trx m/FBPase ratio of the complex was 4, but this was only observed with a Trx m/FBPase concentration ratio > 10 in the preincubation mixture. The FBPase‐Trx m binding disappeared in the presence of 100 mM NaCl, even with 2.5 mM DTT at pH 7.9, with a concomitant appearance of different aggregation states of the FBPase subunit. A similar FBPase‐Trx m complex was detected in the stromal solution when pea chloroplasts were lysed at pH 7.9 in the presence of DTT. No interaction was observed between NADP‐MDH and Trx f or m, either in the presence or in the absence of DTT. Pea FBPase showed sigmoidal activation kinetics with pea Trx m, and an S0.5 of 133 nM versus 6.6 nM with pea Trx f. About 10‐fold higher concentration of the former than that of the latter was required for obtaining maximum activity; however, the Vmax with Trx f was only 2‐fold higher than that with Trx m. We conclude that pea FBPase binds and is activated by the homologous Trx m, even though to a lesser extent than with Trx f. We also deduce that in the light the conditions in the chloroplast stroma are optimal for forming an FBPase‐Trx complex.
Heavy metal‐induced accumulation of free proline in a metal‐tolerant and a nontolerant ecotype of Silene vulgaris
Authors: Schat Henk , Sharma Shanti S. , Vooijs Riet
Published: May 2006

Accumulation of free proline in response to Cu, Cd and Zn was studied in nontolerant and metal‐tolerant Silene vulgaris (Moench) Garcke. In the nontolerant ecotype these metals induced a massive accumulation of proline, especially in the leaves. When compared at equimolar concentrations in the nutrient solution, Cu was the most effective inducer, followed by Cd and Zn, respectively. However, when compared at equal toxic strength, as estimated from the degree of root growth inhibition, proline accumulation decreased in the order Cd > Zn > Cu. The threshold exposure levels for proline accumulation coincided with the highest no‐effect‐concentrations for root growth. In the metal‐tolerant ecotype the constitutive proline concentration in the leaves was 5 to 6 times higher than in the nontolerant ecotype. Exposure to Cu and Zn, however, was without any effect on the leaf proline concentration, even at exposure levels that caused a 50% root growth inhibition. Only Cd, when present at concentrations above the highest no‐effect‐concentration for root growth, induced a further increase of the leaf proline content. Reducing transpiration by placing the plants under a transparent polyethylene cover almost completely inhibited proline accumulation, even at metal accumulation rates in the leaves that caused a 10‐fold increase of the proline level in leaves of uncovered plants. The results demonstrate that metal‐induced proline accumulation depends on the development of a metal‐induced water deficit in the leaves. Differential metal‐induced proline accumulation in distinctly metal‐tolerant ecotypes is a consequence, rather than a cause of differential metal tolerance.
Phloem loading in peach: Symplastic or apoplastic?
Authors: Moing Annick , Carbonne Francis , Zipperlin Bénédicte , Svanella Laurence , Gaudillère Jean‐Pierre
Published: May 2006

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in other rosaceous trees. The respective role of these two soluble carbohydrates in the leaf carbon budget, and their phloem loading pathway, remain poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose‐transporting species, far less is known about sorbitol in species transporting both sucrose and sorbitol. Sorbitol and sucrose concentrations were measured in several tissues and in sap, in 2‐month‐old peach (Prunus persica L. Batsch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap collected using a pressure bomb, and phloem sap collected by aphid stylets. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol concentration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the leaf apoplast, estimated by infiltration with 3H‐inulin, represented about 17% of the leaf blade water content. This volume was used to calculate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated concentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the symplastic loading of the phloem of this alditol. However, infiltration of 14C‐labelled source leaves with 2 mMp‐chloromercuribenzenesulfonic acid (PC‐MBS), a potent inhibitor of the sucrose carrier responsible for phloem loading in sucrose‐transporting plants, had a significant effect on the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative symplastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.
Effect of UV‐B radiation on the shoot dry matter production and stable carbon isotope composition of two Arabidopsis thaliana genotypes
Authors: Ormrod Douglas P. , Schmidt Anna‐mary , Livingston Nigel J.
Published: May 2006

An application of stable carbon isotope analysis to the mechanistic interpretation of ultraviolet‐B (UV‐B) effects on growth inhibition is described that is particularly useful for small plants such as Arabidopsis thaliana that are not well suited for gas exchange studies. Many investigators use tissue δ13C, relative abundance of 13C and 12C, as a proxy for water use efficiency and as an indicator of environmental effects on stomatal behaviour and on photosynthesis during growth. Discrimination against 13C is enhanced by both high stomatal conductance and damage to photosynthetic machinery. Because the thinning of the stratospheric ozone layer is permitting more UV‐B to enter the biosphere, the mechanisms of action of UV‐B radiation on plants are of particular current interest. Arabidopsis thaliana wild‐type Landsberg erecta (Ler) and the UV‐B‐sensitive mutant fah I, deficient in UV‐absorbing sinapate esters, were grown in a controlled environment and exposed to UV‐BBE doses of 0 or 6–7 kJ m−2 day−1. UV‐B exposure decreased dry matter production and δ13C in both genotypes, but growth inhibition was generally greater in fah I than in Ler. The fah I mutant also had less leaf greenness than Ler. Changes in leaf tissue δ13C were detected before growth inhibition and were evident in treatments of both genotypes that did not cause marked growth effects. This suggests that the effects of UV‐B contributing to increased carbon isotope discrimination in Ler may have been primarily associated with high stomatal conductance, and in fah I with both high stomatal conductance and damage to photosynthetic machinery.