bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactions between nitrate and ammonium during uptake by carob seedlings and the effect of the form of earlier nitrogen nutrition
Authors: Cruz Cristina , Lips S. Herman , Martins‐Loução Maria Amélia
Published: May 2006

Seedlings of carob (Ceratonia siliqua L. cv. Mulata) were used in two sets of experiments in order to evaluate; (1) the reciprocal effects of each nitrogen form on net uptake of nitrate and ammonium, and (2) the effect of earlier nitrogen nutrition on ammonium versus nitrate uptake. In the former group of experiments we studied the kinetics of nitrate and ammonium uptake as well as the interference of each of the two forms with net uptake of ammonium and nitrate by both nitrogen depleted and nitrogen fed carob seedlings. On the whole, nitrogen depletion led to increase in both affinity and Vmax of the system for both forms of nitrogen, at the same time as the effects of nitrate on uptake of ammonium and vice versa were concentration dependent. In the second group of experiments the effects of earlier nitrogen nutrition on nitrate and ammonium uptake were characterized, and in this case we observed that: (a) if only one form of N was supplied, ammonium was taken up in greater amounts than nitrate; (b) the presence of ammonium enhanced nitrate uptake; (c) ammonium uptake was inhibited by nitrate; (d) there was a significant effect of the earlier nitrogen nutrition on the response of the plants to a different nitrogen source. The latter was evident mainly as regards ammonium uptake by plants grown in ammonium nitrate. The interactions between nitrate and ammonium uptake systems are discussed on the basis of the adaptation to the nitrogen source during early growth.
The effect of nitrogen source on photosynthesis of carob at high CO2 concentrations
Authors: Cruz C. , Lips S. H. , Martins‐Loução M. A.
Published: May 2006

Carob seedlings (Ceratonia siliqua L. cv. Mulata), fed with nitrate or ammonium, were grown in growth chambers containing two levels of CO2 (360 or 800 μl l−1), three root temperatures (15, 20 or 25°C), and the same shoot temperature (20/24°C, night/day temperature). The response of the plants to CO2 enrichment was affected by environmental factors such as the type of inorganic nitrogen in the medium and root temperature. Increasing root temperature enhanced photosynthesis rate more in the presence of nitrate than in the presence of ammonium. Differences in photosynthetic products were also observed between nitrate‐ and ammonium‐fed carob seedlings. Nitrate‐grown plants showed an enhanced content of sucrose, while ammonium led to enhanced storage of starch. Increase in root temperature caused an increase in dry mass of the plants of similar proportions in both nitrogen sources. The enhancement of the rates of photosynthesis by CO2 enrichment was proportionally much larger than the resulting increases in dry mass production when nitrate was the nitrogen source. Ammonium was the preferred nitrogen source for carob at both ambient and high CO2 concentrations. The level of photosynthesis of a plant is limited not only by atmospheric CO2 concentration but also by the nutritional and environmental conditions of the root.
Rapid modulation of nitrate reductase in leaves and roots: Indirect evidence for the involvement of protein phosphorylation/dephosphorylation
Authors: Kaiser W. M. , Spill D. , Glaab J.
Published: May 2006

Nitrate reductase activity (NRA; NADH‐nitrate reductase, E. C. 1.6.6.1) has been measured in extracts from leaves of spinach (Spinacia oleracea L.) in response to rapid changes in illumination, or supply of CO2 or oxygen. Measured in buffers containing magnesium, NRA from leaves decreased in the dark and increased again upon illumination. It decreased also, when CO2 was removed in continuous light, and was reactivated when CO2 was added. Nitrate reductase (NR) from roots of pea (Pisum sativum L.) was also rapidly modulated in vivo. It increased under anaerobiosis and decreased in air or pure oxygen. The half time for inactivation or reactivation in roots and leaves was 5 to 30 min.

When spinach leaves were harvested during a normal day/night cycle, extractable NRA was low during the night, and high during daytime. However, at any point of the diurnal cycle, NR could be brought to a similar maximum activity by preincubation of the desalted leaf extract with AMP and/or EDTA. Thus, the observed diurnal changes appeared to be mainly a consequence of enzyme modulation, not of protein turnover. In vivo, the reactivation of the inactivated enzyme from both leaves and roots was prevented by okadaic acid, and inhibitor of certain protein phosphatases. Artificial lowering of the ATP‐levels in leaf or root tissues by anaerobiosis (dark), mannose or the uncoupler carbonyl cyanide m‐chlorophenyl hydrazon (CCCP), always brought about full activation of NR.

By preincubating crude leaf or root extracts with MgATP, NR was inactivated in vitro. Partial purification from spinach leaves of two enzymes with molecular masses in the 67 kD and 100 kD range, respectively, is reported. Both participate in the ATP‐dependent inactivation of NR.

Alltogether these data indicate that NR can be rapidly modulated by reversible protein phosphorylation/dephosphorylation, both in shoots and in roots.
Nitrogen nutrition of C3 plants at elevated atmospheric CO2 concentrations
Authors: Conroy Jann , Hocking Peter
Published: May 2006

The atmospheric CO2 concentration has risen from the preindustrial level of approximately 290 μl l−1 to more than 350 μl l−1 in 1993. The current rate of rise is such that concentrations of 420 μl l−1 are expected in the next 20 years. For C3 plants, higher CO2 levels favour the photosynthetic carbon reduction cycle over the photorespiratory cycle, resulting in higher rates of carbohydrate production and plant productivity. The change in balance between the two photosynthetic cycles appears to alter nitrogen and carbon metabolism in the leaf, possibly causing decreases in nitrogen concentrations in the leaf. This may result from increases in the concentration of storage carbohydrates of high molecular weight (soluble or insoluble) and/or changes in distribution of protein or other nitrogen containing compounds. Uptake of nitrogen may also be reduced at high CO2 due to lower transpiration rates. Decreases in foliar nitrogen levels have important implications for production of crops such as wheat, because fertilizer management is often based on leaf chemical analysis, using standards estimated when the CO2 levels were considerably lower. These standards will need to be re‐evaluated as the CO2 concentration continues to rise. Lower levels of leaf nitrogen will also have implications for the quality of wheat grain produced, because it is likely that less nitrogen would be retranslocated during grain filling.
Involvement of CO2 fixation products in the light‐dark modulation of nitrate reductase activity in barley leaves
Authors: de Cires Alfonso , de la Torre Angel , Lara Catalina
Published: May 2006

Nitrate reductase (NR, NADH:nitrate oxidoreductase, EC 1.6.6.1) activity from leaves of barley (Hordeum vulgare L. cv. Hassan) is rapidly and reversibly inactivated during a light‐dark transition. A hyperbolic correlation exists between in vivo rates of CO2 fixation and extractable NR activity from the leaves, and feeding hexose and hexosephosphate protects against the dark‐inactivation; indicating that carbon‐assimilation products are regulatory factors of NR activity mediating both the light‐dark modulation and its dependence upon CO2 fixation. To corroborate this point, the effect of inhibiting CO2 fixation on NR activity in barley leaves has been analyzed. Glycolaldehyde (50 mM), an inhibitor of the regeneration phase of the Calvin cycle, was fed through the transpiration stream and inhibited CO2 fixation by more than 80% at the same time as it produced a parallel inhibition of NR light‐activation. Feeding mannose (10 mM), inhibited CO2 fixation by 35% but did not affect NR activity in illuminated leaves and completely protected against dark‐inactivation. Interestingly, feeding inorganic phosphate, Pi, (10 mM) alone or together with mannose also protected NR activity against dark‐inactivation. The mannose effect could be interpreted in terms of accumulation of mannose 6‐phosphate, an analog of glucose 6‐phosphate. After feeding either 10 mM glucose or dihydroxyacetone phosphate, NR activity from darkened leaves was significantly higher than that of darkened control leaves fed with water (P< 0.03). These treatments, as well as Pi feeding, also produce some increase in extractable NR activity from illuminated leaves. The results indicate that factors increasing the levels of hexose‐ and triose‐phosphate have positive effects on NR activation, supporting the contention that the NR activation system is sensitive to carbon‐assimilation products.
Nitrate transport in the cyanobacterium Anacystis nidulans
Authors: Lara Catalina , Rodríguez Rocío , Guerrero Miguel G.
Published: May 2006

A significant progress in the knowledge of different aspects of nitrate transport in the unicellular cyanobacterium Anacystis (Synechococcus) has been achieved in the last few years. The main contributions of our group are summarized in this article and discussed in relation to other information available. Endergonic accumulation of nitrate into the cells, indicative of the operation of an active nitrate transport system, has been experimentally substantiated and methods established to evaluate and analyze the activity of the system. Nitrate transport activity is sensitive to regulation exerted by products of both ammonium and CO2 assimilation, thus providing evidence that photosynthetic nitrate assimilation in cyanobacteria is primarily controlled at the level of substrate supply to the cell. The expression of nitrate transport was also shown to be under nitrogen control, being repressed when ammonium is used as the nitrogen source. A 47‐kDa polypeptide, which is a major plasma membrane component in nitrate‐grown cells but is virtually absent in ammonium‐grown cells, was identified as an essential component of the nitrate transporter. More recently, evidence of a strict Na'‐dependence of active nitrate transport has been obtained, Δμ(Na+) appearing as the driving force of a sodium‐nitrate symport system. Kinetic studies indicate also that the nitrate transporter may transport nitrite into the cell.
Evidence for an indirect coupling mechanism for the nitrate‐sensitive proton pump from corn root tonoplast membranes
Authors: Brauer David , Tu Shu‐I , Hsu An‐Fei , Patterson Deidre
Published: May 2006

The nitrate‐sensitive proton‐translocating adenosine triphosphatase (H+‐ATPase) of tonoplast membranes plays an important role in regulating the flow of nutrients and metabolic waste between the cytoplasm and vacuole in the cells of plant roots. Relatively little information is available regarding the coupling between ATP hydrolysis and proton pumping by the nitrate‐sensitive, tonoplast H+‐ATPase. The coupling may be achieved either directly, i. e. the two reaction pathways share at least one common molecular step, or indirectly, i. e. the two reaction pathways do not share an intermediate step. These coupling mechanisms may be differentiated by the responses of the two events to external perturbation. The effects of the presence of nitrate in the assay medium on the rates of ATP hydrolysis and proton transport catalyzed by the tonoplast H+‐ATPase from maize (Zea mays L. cv. FRB 73) were investigated. The presence of nitrate inhibited proton transport activity of the tonoplast H+‐ATPase to a much greater degree than ATP hydrolysis. This differential response of the two activities to nitrate is the basis for a proposed reaction model for the tonoplast H+‐ATPase that features an indirect coupling mechanism between ATP hydrolysis and proton transport.
On the inducibility of nitrate transport by tobacco cells
Authors: Guy Micha , Heimer Yair M.
Published: May 2006

The question as to whether the nitrate transport system is induced by nitrate was addressed using a cell suspension of the XD line ofNicotiana tabacumL. cv. Xanthi as an experimental system. The cells were grown on area as the sole nitrogen source, and tungstate was used to render nitrate reductase non‐functional. To avoid shock due to vacuum filtration, the cells, were harvested by gravity filtration. Nitrate uptake by cells, which were harvested, transferred to fresh medium, and immediately exposed to nitrate (freshly harvested cells), displayed a lag period of about 3 h.

In cells which were given incubation periods in fresh medium before exposure to nitrate (preincubated cells), the lag period was considerably shortened. After 3 h of preincubation in the absence of nitrate (recovered cells), the lag period was almost completely eliminated. Cycloheximide inhibited nitrate uptake by recovered cells within minutes, and prevented the development of nitrate uptake in freshly harvested cells. Cycloheximide did not affect uptake of α‐aminoisobutyric acid (AIB) within the first 2 h after its addition. Recovery of the membrane potential from a low value just after the harvest of the cells to a maximal value 3 h later, was observed using the lipophilic cation methyltriphenylphosphonium (MTPP+), supplied at low concentrations, as a probe. Depolarization of the membrane potential by MTPP+, at the millimolar range, caused a rapid inhibition of nitrate uptake by recovered cells. The results indicate that nitrate transport by the XD cells depends on the membrane potential and on protein components with short half life. In addition, it requires a continuous protein synthesis. The effects of physical manipulation on nitrate uptake are discussed.
Molecular biology, genetics and regulation of nitrite reduction in higher plants
Authors: Wray John L.
Published: May 2006

Nitrite reductase (ferredoxin:nitrite oxidoreductase, EC 1.6.6.1) carries out the six‐electron reduction of nitrite to ammonium ions in the chloroplasts/plastids of higher plants. The complete or partial nucleotide sequences of a number of nitrite reductase apoprotein genes or cDNAs have been determined. Deduced amino acid sequence comparisons have identified conserved regions, one of which probably is involved in binding the sirohaem/4Fe4S centre and another in binding the electron donor, reduced ferredoxin. The nitrite reductase apoprotein is encoded by the nuclear DNA and is synthesised as a precursor carrying an N‐terminal extension, the transit peptide, which acts to target the protein to, and within, the chloroplast/plastid. In those plants examined the number of nitrite reductase apoprotein genes per haploid genome ranges from one (barley, spinach) to four (Nicotiana tabacum). Mutants defective in the nitrite reductase apoprotein gene have been isolated in barley. During plastidogenesis in etiolated plants, synthesis of nitrite reductase is regulated by nitrate, light (phytochrome), and an uncharacterised ‘plastidic factor’ produced by functional chloroplasts. In leaves of green, white‐light‐grown plants up‐regulation of nitrite reductase synthesis is achieved via nitrate and light and down‐regulation by a nitrogenous end‐product of nitrate assimilation, perhaps glutamine. A role for phytochrome has not been demonstrated in green, light‐grown plants. Light regulation of nitrite reductase genes is related more closely to that of photosynthetic genes than to the nitrate reductase gene. In roots of green, white‐light‐grown plants nitrate alone is able to bring about synthesis of nitrite reductase, suggesting that the root may possess a mechanism that compensates for the light requirement seen in the leaf.
Metabolic and developmental control of cytosolic glutamine synthetase genes in soybean
Authors: Marsolier Marie‐Claude , Hirel Bertrand
Published: May 2006

At least two genes encoding cytosolic glutamine synthetase (GS) are actively transcribed in soybean (Glycine max. L. cv. Prize) plants. The expression of the first gene, GS15, is regulated by the availability of ammonia provided externally to non‐inoculated roots or formed as the result of nitrogen fixation in nodulated plants. Moreover, in addition to transcription in roots and root nodules, GS15 is also actively transcribed in other organs such as anthers and pulvini. Separate promoter elements are able to control either the organ‐ or ammonia‐regulated expression of GS15 independently. Another cytosolic GS gene, GS21, has also been identified and is specifically expressed in nodules. The differential expression of GS15 and GS21 suggests that two independent mechanisms, one metabolic, the other developmental, may control ammonia assimilation in nodulated plants. This recent finding is discussed and compared with other results concerning the regulation of glutamine synthetase activity in legume and non‐legume plant species.
Effects of the source of inorganic nitrogen on C and N interaction in maize callus tissue: phosphoenolpyruvate carboxylase activity, cytosolic pH and 15N amino acids
Authors: Amâncio Sara , Diogo Eugénio , Santos Helena
Published: May 2006

The effect of N‐source on the interaction between carbon and nitrogen metabolism was evaluated by measuring phosphoenolpyruvate carboxylase (PEPcase; EC 4.1.1.31) activity in callus tissue of maize (Zea mays L. cv. Prisma) sub‐cultured under different N‐nutrition conditions: nitrate, ammonium or combinations of both. By comparison with the condition where both salts were supplied (control), nitrate as the sole N‐source led to an increase in PEPcase activity. Ammonium alone gave a drastic decrease of tissue growth. Extracts from calli grown on equivalent media supplied with 15N‐nitrate or 15N‐ammonium were analysed by 15N‐NMR. The labelling of amino acids in the NMR spectra showed that when 15NO−3 was the unique N‐source, 15N mainly accumulated in NδGln, Glu and Ala. With 15NH+4 only the NδGln and γ‐aminobutyric acid were labelled. The addition of both gave rise to labelled Gln, Asn, Glu, Asp, Ala, Val and γ‐aminobutyric acid independently of the origin of the label. In vivo 31P‐NMR allowed the cytoplasmic and vacuolar pH to be measured. The cytoplasmic pH showed an increase of approximately 0.3 units when nitrate was the sole source of nitrogen and a corresponding decrease when ammonium was added alone. Vacuolar pH decreased in both treatments. These results are discussed on the basis of the effect of the N‐source on carbon metabolism. A hypothesis of PEPcase activation as due to the increase of cytoplasmic pH upon nitrate uptake is proposed.
Inorganic carbon fixation and metabolism in maize roots as affected by nitrate and ammonium nutrition
Authors: Cramer M. D. , Lewis O. A. M. , Lips S. H.
Published: May 2006

The effects of NO−3 and NH+4 nutrition on the rates of dark incorporation of inorganic carbon by roots of hydroponically grown Zea mays L. cv. 712 and on the metabolic products of this incorporation, were determined in plants supplied with NaH14CO3 in the nutrient solution. The shoots and roots of the plants supplied with NaH14CO3 in the root medium for 30 min were extracted with 80%; (v/v) ethanol and fractionated into soluble and insoluble fractions. The soluble fraction was further separated into the neutral, organic acid, amino acid and non‐polar fractions. The amino acid fraction was then analyzed to determine quantities and the 14C content of its individual components.

The rates of dark incorporation of inorganic carbon calculated from H14CO−3 fixation and attributable to the activity of phosphoenolpyuvate carboxylase (EC 4.1.1.31), were 5‐fold higher in ammonium‐fed plants than in nitrate‐fed plants after a 30‐min pulse of 14C. This activity forms a small, but significant component of the carbon budget of the root. The proportion of 14C located in the shoots was also significantly higher in ammonium‐fed plants than in nitrate‐fed plants, indicating more rapid translocation of the products of dark fixation to the shoots in plants receiving NH+/sp4 nutrition.

Ammonium‐fed plants favoured incorporation of 14C into amino acids, while nitrate‐fed plants allocated relatively more 14C into organic acids. The amino acid composition was also dependent on the type of nitrogen supplied, and asparagine was found to accumulate in ammonium‐fed plants. The 14C labelling of the amino acids was consistent with the diversion of 14C‐oxaloacetate derived from carboxlyation of phosphoenolpyruvate into the formation of both asparatate and glutamate. The results support the conclusion that inorganic carbon fixation in the roots of maize plants provides an important anaplerotic source of carbon for NH+4 assimilation.
The rice coleoptile: an example of anaerobic nitrate assimilation
Authors: Reggiani Remo , Mattana Monica , Aurisano Nicoletta , Bertani Alcide
Published: May 2006

Nitrate present in rice caryopses can be reduced to ammonium and the ammonium subsequently assimilated by the coleoptile during anaerobic germination. All the enzymes of nitrate reduction and ammonia assimilation are present in the coleoptile. The supply of 15NO3 confirms that the nitrate nitrogen is anaerobically incorporated into amino acids. Under anoxia, nitrate and nitrite reductase activities are increased in the coleoptile by exogenous nitrate. The importance of nitrate utilization during the anaerobic germination of rice caryopses is discussed.
Role of ammonium accumulation in bacteria‐induced hypersensitive and compatible reactions of tobacco and cotton plants
Authors: Ullrich W. R. , Kunz G. , Stiller A. , Novacky A. J.
Published: May 2006

Leaves of 12‐week‐old tobacco plants (Nicotiana tabacumL. cv. Samsun NN) were infiltrated with suspensions ofPseudomonas syringaepv,pisi(DSM 50291) to induce hypersensitive reaction (HR). Cotyledons of 2‐week‐old cotton plants (Gossypium hirsutumL. cv. Acala 442 and Coker BR) were infiltrated withXanthomonas campestrispv.malvacearum(race 10) to induce the disease. In tobacco, HR‐related increases in NH+4levels started within 2 h after infection and continued up to the time of tissue decay. Increase of NH+4and especially K+efflux were detected in intercellular washing fluids (IWF). Antibiotics stopped and later reverted NH+4production and K+efflux, but only if applied within 2 h after infection. When 10 mMNH+4was injected into leaves, it was rapidly consumed from the IWF, and also, although more slowly, within the leaf cells. The concomitant K+efflux was strong but delayed, and most of the K+was reabsorbed after 2 h. Bacterial cell multiplication in HR stopped before the appearance of HR symptoms and cell necrosis. In the compatible reaction in cotton cotyledons, both NH+4accumulation and K+efflux proceeded much more slowly than in the HR with tobacco, and bacteria continued to multiply until general cell necrosis occurred. The compatible reaction developed faster in constant darkness than in a light/dark rhythm. Bacterial enzymes produced NH+4, mainly from proteins of host cells, in both light and darkness. Continuous light delayed the main peak of both NH+4production and K+efflux. High CO2concentration inhibited both processes, thus indicating that photorespiration plays a role in enhancing the release of free ammonium during bacterial pathogenesis. This is supported by shifts in the pattern of amino acids. The results demonstrate the accelerating and aggravating effect of ammonium in pathogenesis and HR, though ammonium is not the primary agent.
Interactions of polyamines and nitrogen nutrition in plants
Authors: Altman Arie , Levin Nitsa
Published: May 2006

Biogenic amines occupy an important position among the many nitrogenous plant compounds. Polyamines are part of the overall metabolism of nitrogenous compounds, yet they do not seem to function in the ‘normal’ nitrogen nutrition. Rather, these widespread polycations (e. g. putrescine, spermidine and spermine) are involved in the regulation of growth and stress, probably by binding to negatively charged macromolecules. In addition, some diamines and polyamines are metabolized to yield 'secondary ’metabolites such as nicotine and other alkaloids. Previous studies have indicated that the ratio of nitrate to ammonium nutrition affects polyamine biosynthesis and content in intact plants. Thus, an increase in putrescine accumulation was found under conditions of excess ammonium ions, relative to nitrate. Modifications of nitrogen sources in the culture medium of tobacco cell suspensions (depletion of ammonium nitrate, or potassium nitrate, or both) resulted in marked changes in the content of cellular free polyamines. Considerable changes in the content of specific polyamines were also found with exposure to specific inhibitors of polyamine biosynthesis (difluoromethyl ornithine, difluoromethyl arginine, cyclohexylamine, methylglyoxal‐bis‐guanylhydrazone). However, a combination of nitrogen depletion of the medium and some inhibitors resulted in a very marked over‐production of spermidine and spermine. The significance of these findings is discussed in relation to the assumption that polyamines act as a metabolic buffer, and maintain cellular pH under conditions where ammonium assimilation produces an excess of protons.
Ammonium rhythm in cultures of the cyanobacterium Microcystis firma
Authors: Abarzua S. , Altenburger R. , Callies R. , Grimme L.‐H. , Mayer A. , Leibfritz D. , Schiewer U.
Published: May 2006

Over a period of several days, rhythmic changes in extracellular NH+4 concentration take place in cultures of the cyanobacterium Microcystis firma (Bré et Lenorm.) Schmidle, strain Gromov/St. Petersb. 398, under conditions of restricted CO2 supply and light/dark alternation. The changes are enhanced by nitrate supply. Among the various processes generating intracellular NH+4 (NH44 uptake, NO−3 reduction, protein and amino acid degradation, photorespiration), NO−3 reduction appears as the one most important. This can be concluded from experiments with and without nitrate and/or ammonium in the medium. In the presence of saturating CO2, continuous light, or continuous darkness, rhythmic NH+44 oscillations are not induced. Studies of the incorporation of NH+4 nitrogen by in vivo 15N‐NMR show that if CO2 is supplied, 15N is accumulated in several components with the following time course: in the first hour in Gln (δ), in the second hour in the α‐amino groups of most nonbranched amino acids, in the third hour in γ‐aminobutyric acid (GABA), Orn (δ) and Lys (ε), and in the sixth hour in Ala. Carbon limitation, however, results in accumulation of label in the amide nitrogen of glutamine only.
Simulation of nitrogen assimilation by heterotrophic soil microbial biomass
Authors: Hadas Aviva , Molina J. A. E.
Published: May 2006

Assimilation of N by heterotrophic soil microbial biomass is associated with decomposition of organic matter in the soil. The form of N assimilated can be either low molecular weight organic N released from the breakdown of organic matter (direct assimilation), or NH+4 and NO−3 from the soil inorganic N pool, into which mineralized organic N is released (mineralization immobilization turnover). The kinetics of C and N turnover in soil is quantifiable by means of computer simulation models. NCSOIL was constructed to represent the two assimilation schemes. The rate of N assimilation depends on the rate of C assimilation and microbial C/N ratio, thereby rendering it independent of the assimilation scheme. However, if any of the N forms is labeled, a different amount of labeled N assimilation will be simulated by the different schemes. Experimental data on inorganic N and 15N and on organic 15N dynamics in soils incubated with 15N added as NH+4 or organic N were compared with data simulated by different model schemes. Direct assimilation could not account for the amount of 15N assimilated in any of the experimental treatments. The best fit of the model to experimental data was obtained for the mineralization immobilization turnover scheme when both NH+4 and NO−3 were assimilated, in proportion to their concentration in the soil.
Inhibition by 2,4‐D of somatic embryogenesis in carrot as explored by its reversal by difluoromethylornithine
Authors: Nissen Per , Minocha Subhash C.
Published: May 2006

The development of somatic embryos is, in many plants, inhibited by 2,4‐dichlorophenoxyacetic acid (2,4‐D) and other auxins. The finding that difluoromethylornithine (DFMO) can counteract this inhibition has been used to test some of the hypotheses for the mechanism of inhibition.

Inhibition of somatic embryogenesis in carrot (Daucus carota L.) by exogenous ethylene (from ethephon), antioxidants (ascorbic acid and glutathione), ethanol/acetaldehyde and abscisic acid was not counteracted by DFMO, indicating that the inhibitory effect of 2,4‐D is not manifest through the formation of these compounds. Embryogenesis was abolished by micromolar concentrations of the polar auxin transport inhibitors 2, 3, 5‐triiodobenzoic acid (TIBA), N‐1‐naphthylphthalamic acid (NPA) and 9‐hydroxyfluorene‐9‐carboxylic acid (HFCA). This inhibition was counteracted to a considerable extent by DFMO. Inhibition by relatively high concentrations of the antiauxin 2‐(p‐chlorophenoxy)‐isobutyric acid (CPIB), which does not affect polar auxin transport, was in contrast not counteracted by DFMO. These findings indicate that exogenous auxins may inhibit embryogenesis by interfering with the ability of postglobular embryos to set up internal auxin gradients necessary for polarized growth.
Changes in physical properties and cell wall polysaccharides of tomato (Lycoperskon esculentum) pericarp tissues
Authors: Sakurai Naoki , Nevins Donald J.
Published: May 2006

Green and red tomato pericarp tissues were subjected to stress‐relaxation analyses to evaluate their physical properties. Significant decreases in the initial stress, minimum stress‐relaxation and maximum stress‐relaxation times in the red tissues predict the losses of both viscosity and elasticity in the tissue. Cell walls of red fruit yielded more water‐soluble polysaccharides and less pectin, hemicelluloses and cellulose. Average molecular mass of pectin determined by gel filtration chromatography was similar in the green and red, but molecular mass of hemicellulose of red fruit walls was reduced to 50% of that of the green fruit. The decreases in the amount of hemicellulose B and in the average molecular mass were associated primarily with the degradation of xylo‐glucans. These data demonstrate that pectin solubilization, depolymerization of xyloglucans and over‐all changes in the quantity of cell wall polysaccharide fractions contribute to tomato fruit softening.
Acid‐induced changes in the in vivo wall‐yielding properties of hypocotyl sections of Vigna unguiculata
Authors: Mizuno Akiko , Nakahori Kiyoshi , Katou Kiyoshi
Published: May 2006

Elongation growth of hypocotyl sections of Vigna unguiculata under xylem perfusion was significantly enhanced when acid was applied by acid‐aerosol to an abraded hypocotyl surface in the air. The in vivo wall extensibility (φ) and the effective turgor (Pi– Y), both of which were determined by the pressure‐jump method, increased during acid‐induced growth as observed in IAA‐induced growth. The intracellular pressure (Pi), however, decreased significantly at the beginning of acid‐induced growth whereas Pi scarcely changed in IAA‐induced growth. This result indicates that protons increase the effective turgor by decreasing the yield threshold as IAA does. There seems to be no essential difference between proton and auxin in the effects on the in vivo mechanical properties of the surface cell wall.
Polyamine metabolism and ethylene biosynthesis in normal and habituated sugar beet callus
Authors: Biondi Stefania , Hagège Daniel , Rossini Paolo , Bagni Nello
Published: May 2006

The optimal assay conditions and the trend with time in culture (28 days) of arginine decarboxylase (ADE; EC 4.1.1.19), omithine decarboxylase (ODC; EC 4.1.1.17) and diamine oxidase (DAO; EC 1.4.3.6) activities in habituated (H) and normal (N) auxin‐ and cytokinin‐requiring sugar beet callus were compared. Although the response to variations in buffer pH and EDTA and pyridoxal phosphate (PLP) concentrations varied for ADC and ODC activities between the two callus types, pH 8.3, 50 μM PLP and 5 mM EDTA were generally optimal or near‐optimal for both H and N callus. In most cases the addition of ornithine or arginine in the ADC and ODC assays, respectively, given to block the interconversion between the two substrates, resulted in lower 14CO2 recovery. DAO activity was very differently affected in H and N callus by the presence of polyvinylpyrrolidone in the extration buffer. However, in both cases, this activity increased with time in culure. ADC activity was always predominant in both cell lines and always higher in N callus. In the latter, ADC activity rose sharply between days 14 and 21 and then leveled off while in H callus it incresed steadily from day 14 onwards. ODC activity was also higher in N callus and peaked sharply on day 21 while in H callus it was not detectable in the second half of the culture period. In both cell lines this activity was low or nil on day 28. 3,4‐[14C]‐methionine incorporation into ethylene and polyamines was also compared in N and H callus. In the latter, ethylene synthesis was lower and [14C]‐spermidine formation higher than in N callus. This is in accord with the significantly higher spermidine titres found in H callus.
Changes in dormancy of Sisymbrium officinaleseeds do not depend on changes in respiratory activity
Authors: Derkx M. P. M. , Smidt W. J. , Van der Plas L. H. W. , Karssen C. M.
Published: May 2006

Effects of dark incubation at different temperatures were studied on dormancy and respiratory activity of seeds of Sisymbrium officinale (L.) Scop. Because germination of this species absolutely depends on the simultaneous action of light and nitrate, changes in dormancy could be studied in darkness without the interference of early germination events. Upon the start of incubation rates of O2 uptake and CO2 release rose. This was followed by a gradual decrease until stable levels of O2 uptake and CO2 release were achieved. Seeds kept for prolonged periods at 24°C, showed neither a change in germination capacity nor in rates of O2 uptake and CO2 release. Respiratory quotients were 0.55–0.7. The initial rise in O2 uptake correlated with the rate of water uptake and with breaking of primary dormancy. However, the subsequent decline in O2 uptake was not generally linked to induction of secondary dormancy. An increased O2 uptake was not required during breaking of secondary dormancy. It is concluded that changes in dormancy are not generally related to changes in respiratory activity. However, germination strongly depends on respiration. The increase in O2 uptake started well before radicle protrusion. A far red irradiation only reversed this increase when it was given before germination escaped from its red light antagonising action. The contribution of different respiratory pathways was followed during prolonged incubation at 24°C in darkness. KCN at 1.5 mM was needed to inhibit the cytochrome pathway (CP) and benzohydroxamic acid (BHAM) at 30 mM to inhibit the alternative pathway (AP). These concentrations did not exert any side effects. Electron flow was predominantly via the CP, maximally 10% was via the AP. Flow through the CP declined during the first 6 days and residual respiration remained constant. Therefore, the contribution of residual respiration became relatively more important with prolonged incubation. KCN at concentrations that almost completely inhibited flow through the CP, did not dramatically reduce germination. BHAM already inhibited germination at concentrations that do not inhibit oxygen uptake.
Storage protein gene expression in zygotic and somatic embryos of interior spruce
Authors: Flinn Barry S. , Roberts Dane R. , Newton Craig H. , Cyr David R. , Webster Fiona B. , Taylor Iain E. P.
Published: May 2006

Storage protein gene expression was compared between zygotic and somatic embryos of interior spruce (Picea glauca/engelmanii complex). Somatic embryos, grown on medium containing 40 μM or 10 μM abscisic acid (ABA), and zygotic embryos accumulated IIS legumin, 7S vicilin and 2S albumin storage proteins. Zygotic embryos displayed a rapid, transient period of storage protein accumulation, while somatic embryos differentiated on 40 μM ABA displayed a more prolonged, gradual accumulation, with some accumulation still evident after 9 weeks of maturation. Somatic embryos on 10 μM ABA accumulated storage proteins initially, but these were rapidly degraded as the embryos germinated precociously. Legumin, albumin and vicilin transcripts were detectable in torpedo stage zygotic and somatic embryos, and increased during embryo development. Transcript levels in zygotic embryos increased during cotyledon development, but following maximum dry weight accumulation and moisture loss, transcripts declined rapidly to low levels. In contrast, somatic embryos on 40 μM ABA had high transcript levels for a prolonged period. These levels were still present after 9 weeks of maturation. A decline in storage protein transcripts similar to zygotic embryos was apparent following a mild drying treatment. These results suggest that a decline in storage protein transcripts is stimulated by embryo drying during the later stages of conifer embryogenesis. Low levels of storage protein transcripts also appeared in somatic embryos on 10 μM ABA, but declined during precocious germination. Osmotic stress induced storage protein and storage protein transcript accumulation. This could be partially inhibited by inclusion of the ABA biosynthetic inhibitor, fluridone. However, endogenous ABA levels did not differ significantly between embryos cultured in the presence or absence of fluridone.
Calcium transport across plasma membrane vesicles isolated from shoots of Stellaria media and Arena sativa
Authors: Ramón Ana M. , Cooke David T. , Clarkson David T.
Published: May 2006

Plasma membrane vesicles (ca 40% inside‐out, after one freeze‐thaw cycle) were extracted and purified from the shoots of oat (Avena sativa L.) and chickweed (Stellaria media L.) using the two‐phase aqueous polymer technique. In the presence of ATP or GTP, a rapid uptake of 45Ca2+ occurred (0.77 and 0.62 nmol Ca2+ mg‐1 protein, for ATP and GTP, respectively, in oat, and 0.53 and 0.51 nmol Ca2+ mg‐1 protein, for ATP and GTP, respectively, in chickweed). Nucleotide‐dependent Ca2+‐transport was sensitive to 1 μM Erythrosin B (with ATP. inhibited by 52% in oat and in chickweed by 72%; with GTP, inhibition was similar in both species at ca 67%); ATP‐dependent uptake was greater in oat than in chickweed, but not stimulated by calmodulin. Addition of the calcium ionophore A‐23187 resulted in the release of label from the vesicles (41% and 63% release with ATP, and 24% and 52% release with GTP, in oat and chickweed, respectively). The results obtained suggest that Ca2+‐transport is independent of the proton pump. In oat, kinetic data indicate a discontinuity in the absorption isotherm at 10 μM free calcium.
Coppicing alters ecophysiology of Quercus rubrasaplings in Wisconsin forest openings
Authors: Kruger Eric L. , Reich Peter B.
Published: May 2006

In the spring of 1987, entire shoots were removed from Quercus rubra L. saplings in two southwestern Wisconsin forest openings. Shoots possessed newly expanding leaves at the time of coppicing. All coppiced individuals sprouted from dormant stem buds near the root collar. Leaf gas exchange and water potential were monitored on these sprouts and on untreated (control) Q. rubra saplings throughout several clear warm days during the 1987 growing season. Daily maxima and averages for sprout leaf photosynthesis and stomatal conductance generally exceeded those of controls. On average, treatment differences in daily maximum photosynthetic rate were modest (11–14%) and were attributed primarily to a 30–38% enhancement of sprout leaf stomatal conductance. Relative differences in daily average photosynthetic rate (29–39%) were substantially larger than those in daily maximum photosynthesis, owing to the fact that sprouts and controls exhibited distinct diurnal gas exchange patterns. Photosynthetic rate and stomatal conductance of control leaves typically declined during the day following a mid‐morning maximum. Sprout leaves, on the other hand, tended to maintain gas exchange rates nearer to their morning maxima throughout the day. This difference in diurnal gas exchange pattern was associated with an apparent differential leaf sensitivity to leaf‐to‐air vapor pressure gradient (VPG). The relative decline in sprout leaf gas exchange rates with increasing VPG was less than that of controls. Treatment differences in gas exchange did not appear to be related to leaf water potential or tissue water relations, but sprouts had a higher soil‐to‐leaf hydraulic conductivity than controls.
Coppicing affects growth, root:shoot relations and ecophysiology of potted Quercus rubra seedlings
Authors: Kruger Eric L. , Reich Peter B.
Published: May 2006

Two experiments were conducted to examine the response of Quercus rubra L. seedlings to coppicing. In a greenhouse experiment, growth, biomass distribution, leaf gas exchange, and water and carbohydrate relations were measured for 1‐year‐old seedlings that were either coppiced when dormant at the time of planting or left intact as controls. Coppicing induced sprouting from the base of the stem, and, in general, the physiology of sprouts and controls was similar. However, the relative growth rate (RGR) of sprouts was 9% higher than that of controls, allowing sprouts to compensate fully for the initial mass lost to coppicing. In a second experiment, in an outdoor cold frame, growth, biomass distribution, leaf gas exchange and plant water relations were measured on 1‐year‐old seedlings that were either coppiced at the time of planting (dormant‐coppiced), coppiced soon after bud break (active‐coppiced) or left intact (controls). Dormant coppicing again had little impact on seedling physiology, and dormant‐coppiced plants again compensated for initial mass loss with a higher RGR. In contrast, active‐coppiced seedlings did not compensate for initial mass loss, as their RGR did not differ from that of controls. By the tenth week of the study, leaf gas exchange rates of active‐coppiced sprouts were higher than those of dormant‐coppiced and control seedlings. Active‐coppiced sprouts also had a greater soil‐to‐leaf hydraulic conductivity (expressed on a leaf area basis) and a lower ratio of leaf area to root surface area than did controls. Across treatments, photosynthetic rate and stomatal conductance were positively correlated with soil‐to‐leaf hydraulic conductivity, and gas exchange rates and hydraulic conductivity were negatively related to leaf:root area ratio. Thus, the removal of actively growing shoots may have altered subsequent leaf gas exchange largely through coppice‐induced changes in leaf‐root balance.
Effectiveness of light of different wavelengths to induce chlorophyll biosynthesis in rapidly and slowly greening tissues
Authors: Virgin Hemming I.
Published: May 2006

Action spectra for light‐induced chlorophyll formation in etiolated leaves, potato tubers, and dark‐grown epi‐ and hypocotyls of pea and bean seedlings show a correlation between rate of greening and wavelengths with the strongest greening effect. Action spectra for the greening of leaves and the upper parts of epi‐ and hypocotyls with rapid greening and with rather high concentrations of protochlorophyll650–652 have a maximum at 650 nm. While those for slowly greening material such as potato tuber parenchyma and the lower parts of epi‐ and hypocotyls show maxima around 660‐–70 nm. This shift is discussed and it is proposed to be due to a more dominating role of light absorbed by phytochrome for the formation of chlorophyll precursors in slowly greening materials than in rapidly greening tissues. in which the protochlorophyll formation is rapid also without the influence of phytochrome. Therefore, in slowly greening material, the peak in the absorption spectrum for phytochrome in its red absorbing form will have a more dominating influence on the shape of the action spectrum, and this will be reflected in a shift of the peak from 650 nm to higher wavelengths.
The glassy state and accelerated aging of soybeans
Authors: Sun Wendell Q. , Leopold A. Carl
Published: May 2006

Deteriorative changes in seeds may be expected to reflect changes in physical state as well as in chemical composition. In the present study, measurements of changes in glass transition, lipid phase transition and sugar content were made during accelerated aging of two cultivars of soybeans (Glycine max Merrill cv. Chippewa 64 and cv. Hodgson 78). The glass transition in axes, as measured by the thermally stimulated depolarization current method, showed gradual decreases in both magnitude and transition temperature during accelerated aging, and eventually, axes of seeds lost their ability to enter the glassy state. Sucrose, raffinose and stachyose contents in seed axes showed little or no change during the aging treatment. Membrane lipids in aged axes retained the liquid crystalline phase during aging. These data suggest that the changes of glass transition during accelerated aging occurred without associated changes in soluble sugar contents or changes in the liquid crystalline state of membrane lipids. The loss of the glass transition during accelerated aging could be a consequence of the annealing effect due to elevated temperature and moisture content. We propose that a loss of the glassy state during accelerated aging leads to an enhanced rate of subsequent deteriorative reactions in seeds and accelerates the loss of viability.
Frost resistance and water status of winter rape leaves as affected by differential shoot/root temperature
Authors: Kacperska Alina , Szaniawski Ryszard K.
Published: May 2006

Twenty day‐old winter rape (Brassica napus L. var. oleifera L. cv. Jantar) seedlings, grown in nutrient solution, were exposed to different shoot/root temperature (i. e. 20/20, 20/3, 3/20 and 3/3°C) for 2 or 4 weeks. Chilling treatments modified markedly the pattern of plant growth as indicated by changes in dry matter accumulation in individual plant parts (leaves, hypocotyls, roots) and decreased leaf specific area. Growth of roots was less sensitive to low temperature than that of shoots. This was reflected by a decrease in shoot/root biomass ratio. Chilling treatments increased freezing resistance, decreased water content and water potential and modified reducing sugar, soluble protein and phospholipid contents in the leaves. A biphasic character of tissue responses to chilling temperature was observed, the most remarkable changes being registered during the first 7 or 14 days of the treatment. Effects of root or shoot exposure to chilling temperature on ice nucleation temperature, LT50, water potential, accumulation of sugars and phospholipids in leaves were additive. All the observations point to the important role of the root system in plant acclimation to cold. Its impact on water status of leaves is emphasized and some mechanisms of root involvement in acclimation processes are proposed.