bluuebunny/crossref_metadata_2025_split huggingface dataset:
The effect of source or sink temperature on photosynthesis and 14C‐partitioning in and export from a source leaf of Alstroemeria
Authors: Leonardos Evangelos D. , Tsujita M. James , Grodzinski Bernard 
Published: May 2006

The influence of source and sink temperature on leaf net C exchange rate (NCER), export, and partitioning in the C3 monocotyledon Alstroemeria sp. cv. Jacqueline were examined. Leaf (i.e. source) temperature was varied between 12 and 35°C while source leaves were exposed to photorespiratory and nonphotorespiratory conditions during a 2‐h steady‐state 14CO2 labelling period. Between 12 and 20°C, at ambient CO2 and O2, leaf NCER and export were similar with maximum rates of 9.71 ± 0.51 and 3.06 ± 0.36 μmol C m‐2 s‐1, respectively. Both NCER and export decreased above 20°C. At 35°C NCER was 30% of the rate at 20°C, but export was totally inhibited. Between 12 and 35°C, at the end of the 2‐h feeding period, 14C was partitioned in the leaf as ethanol insolubles (3–10%), H2O solubles (88–92%), and chloroform solubles (2–8%). However, above 25°C, less 14C was recovered in the starch fraction and more in the sugar fractions. At all temperatures, 86 to 94% of the labelled sugars was 14C‐sucrose. In nonphotorespiratory conditions (i.e. 1 800 μI I‐1 CO2 and 2% O2). NCER and export were higher than the rates obtained at ambient CO2 and O2 at each temperature. Carbon dioxide enrichment sustained high NCER and export rates even at 35°C, Although CO2 enrichment increased partitioning of 14C into starch, starch synthesis at 35°C was markedly reduced.Cooling the root‐zone mass (i.e. a dominant sink) to 10°C, which simulated the commercial practice used to induce flowering, had no significant effect on source leaf NCER and export rates either during a 2‐h steady‐state labelling period or subsequently during a 21‐h light‐dark chase period. Furthermore, partitioning of 14C among leaf products at the end of the feed‐chase period was not affected. Additional pulse and chase experiments using 11CO2 fed to source leaves of control and root‐cooled plants showed that there was no difference in the direction of movement of 11C‐assimilates towards the flower or the root zone as a consequence of root cooling. Together, the data indicate that changing source strength, by manipulating photosynthesis and photorespiration, by varying the leaf temperature had a more profound effect on leaf export than manipulating sink activity.
The ubiquitin system in plants
Authors: von Kampen Jan , Wettern Michael , Schulz Margot 
Published: May 2006

The small polypeptide ubiquitin participates in a variety of fundamental cellular events, such as cell differentiation, stress response, determination of steady state levels of regulatory proteins, cell cycle control, regulation of transcription, and programmed cell death. Although the complex mechanisms of these processes are not fully understood, ubiquitinylation of regulatory proteins involved in those events is obviously essential. Target proteins can be covalently coupled with one or a few ubiquitin molecules, which is supposed to present a (reversible) post‐translational modification. Polyubiquitinylation, however, marks proteins selectively for degradation by the 26S proteasome. The ubiquitin system has been studied mostly with animal systems or yeast, but all basic reactions of ubiquitin appear in plants as well.The scope of this review is to summarize several implications of recent studies directed towards the plant ubiquitin system.
Gravitropic responses of the Avena coleoptile in space and on clinostats. IV. The clinostat as a substitute for space experiments
Authors: Brown Allan H. , Johnsson Anders , Chapman David K. , Heathcote David 
Published: May 2006

Gravitropic responses of dark grown oat coleoptiles were measured in weightlessness and under clinorotation on earth. The tests in microgravity were conducted in Spacelab during the IML‐1 mission and those on clinostats were conducted in laboratories on earth. The same apparatus was used for both kinds of tests. In both cases autotropism and gravitropic responsiveness were determined. This allowed a quantitative comparison between the plants' responses after receiving the same tropistic stimulations either in weightlessness or on clinostats.Autotropism was observed with oat coleoptiles responding in weightlessness but it did not occur on clinostats. Gravitropic responsiveness was measured as the ratio between the incremental bending response (degrees curvature) and the corresponding incremental g‐dose (stimulus intensity times duration for which it was applied). Plants were tested at either of two stages of coleoptile development (i.e. different coleoptile lengths). From a total of six different kinds of critical comparisons that could be made from our tests that provided data for clinorotated vs weightless plants, three showed no significant difference between responses in simulated vs authentic weightlessness. Three other comparisons showed highly significant differences. Therefore, the validity of clinorotation as a general substitute for space flight was not supported by these results.
Evidence that cytokinin controls bud size and branch form in Norway spruce
Authors: Chen Hao‐Jie , Bollmark Marie , Eliasson Lennart 
Published: May 2006

Shoot elongation in many coniferous species is predetermined during bud formation the year before the shoot extends. This implies that formation of the primordial shoot within the bud is the primary event in annual shoot growth. Hormonal factors regulating bud formation are consequently of utmost importance. We followed the levels of the endogenous cytokinins zeatin riboside (ZR) and isopentenyladenosine (iPA) in terminal buds, whorl buds and lower lateral buds of the uppermost current‐year whorl shoots of 15‐ to 20‐year‐old trees of Norway spruce [Picea abies (L.) Karst.] from June to September. Cytokinins were isolated with affinity chromatography columns, purified by high performance liquid chromatography, and quantified by ELISA. The level of ZR was low in June but increased gradually in all buds until September. Throughout the measurement period, the ZR level was highest in terminal buds and lowest in the scattered lateral, buds, with the whorl buds intermediate. The level of iPA peaked in July and decreased later without any consistent differences among the three classes of buds. The development of different kinds of buds was followed by scanning electron microscopy. We found that bud growth was greatest during August and September. The final size of primordial shoots within the buds varied considerably and the weight of the terminal bud was three times that of the whorl buds and more than five times that of the other lateral buds.We conclude that the increase in ZR level during the period of active bud development is indicative of the importance of cytokinin for this process. Furthermore, the positive correlation between the level of ZR and bud growth during the period of predetermination of next year's branch growth suggests that this hormone indirectly controls the form of single branches in the spruce tree.
Inside or outside? Localization of the receptor relevant to auxin‐induced growth
Authors: Claussen Maike , Lüthen Hartwig , Böttger Michael 
Published: May 2006

The localization of the auxin receptor relevant to the control of elongation growth is still a matter of controversy. Auxin‐induced elongation of maize coleoptile segments was measured by means of a high resolution auxanometer. When indole‐3‐acetic acid (IAA) was removed from the bathing solution, a rapid cessation of auxin‐induced elongation was detected. This decline was delayed when the auxin efflux carrier was blocked by the phytotropins naphthylphthalamic acid (NPA) and pyrenoylbenzoic acid (PBA) or by triiodobenzoic acid (TIBA). The IAA concentration in NPA‐pretreated segments was 2–3 times higher than in NPA‐free controls 35 min after the removal of IAA in the bathing medium.A similar rapid drop of growth after removal of auxin was observed for the rapidly‐transported synthetic auxin, naphthaleneacetic acid (NAA). When the auxin efflux was blocked, growth induced by NAA was sustained much longer than IAA‐stimulated elongation.In comparison with NAA, the synthetic auxin 2,4‐dichlorophenoxyacetic acid (2,4‐D) is known to be excreted very slowly by the efflux carrier. 2,4‐D‐induced growth remained at a stimulated level when the auxin was washed off, even in the absence of any auxin efflux inhibitor. We conclude from these results that the presence of intracellular auxin is a necessary and sufficient condition for sustained auxin‐induced elongation growth, at least for the phases during the 2 h after its application. Consequently, we postulate the existence of an intracellular auxin receptor relevant to the control of growth.
Hydroxynitrile lyases: Functions and properties
Authors: Hickel Andrea , Hasslacher Meinhard , Griengl Herfried 
Published: May 2006

Plant hydroxynitrile lyases (Hnl) have attracted the attention of bioorganic scientists for more than 90 years. However, the most important increase in knowledge of this class of enzymes has only arisen in the recent decade. The industrial application of these enzymes as biocatalysts for the synthesis of enantiomerically pure α‐cyanohydrins may be responsible for the growing interest in this area.The Hnls are involved in the catabolic degradation of cyanogenic glycosides, releasing HCN which serves as defense agent against herbivores and microbial attack, or as a nitrogen source. Hydroxynitrile lyases from various plant families appear to represent a new example of enzymes that originated from the convergent evolution of different precursor proteins. The enzymes have been classified into non‐FAD‐ and FAD‐containing proteins. FAD‐containing enzymes have been isolated exclusively from the Rosaceae, whereas the FAD‐independent Hnls, which are more heterogenous in structure, have been characterized from various plant families (Poaceae, Euphorbiaceae, Linaceae, Olacaceae. Filitaceae). The aim of this review is to present a general survey of the natural function and localization of this class of enzymes and a comprehensive summary of the biochemical and genetic data of the isolated proteins.
Ascorbate synthesis and ascorbate peroxidase activity during the early stage of wheat germination
Authors: De Gara L. , de Pinto M. C. , Arrigoni O. 
Published: May 2006

Embryos from dry caryopses of wheat (Triticum durum L. cv. Norba) are completely devoid of ascorbate (ASC) but contain a low amount of dehydroascorbate (DHA). The de novo biosynthesis of ASC starts in the wheat embryos after 8–10 h of germination but before the ASC biosynthetic pathway is completely restored the embryos can provide themselves with ASC by the reduction of the stored DHA. Three different proteins having DHA‐reducing capability are present in the embryos during the early stages of germination. However, when the de novo ASC biosynthesis from sugar is completely restored, the DHA reduction capability largely drops and only one DHA‐reducing protein remains active. The presence of three proteins having DHA‐reducing capability and their behaviour during germination is discussed.Dry embryos are also devoid of ASC peroxidase (EC 1.11.1.11); this hydrogen peroxide scavenger enzyme appears after the same lag as ASC and increases during germination in parallel with the rise in ASC. When ASC biosynthesis is experimentally induced, the ASC peroxidase also appears earlier; moreover the affinities for ASC of the three ASC peroxidase isoenzymes that progressively appear during germination depend on the ASC available in the embryos: highest in the first isoenzyme, that appears when the ASC content is very low, lowest in the isoenzyme that is expressed last, when the ASC content is 10–11 times higher.
Plasmodesmatal frequency in relation to short‐distance transport and phloem loading in leaves of barley (Hordeum vulgare). Phloem is not loaded directly from the symplast
Authors: Botha C. E. J. , Cross R. H. M. 
Published: May 2006

We investigated the phloem loading pathway in barley, by determining plasmodesmatal frequencies at the electron microscope level for both intermediate and small blade bundles of mature barley leaves. Lucifer yellow was injected intercellularly into bundle sheath, vascular parenchyma, and thin‐walled sieve tubes. Passage of this symplastically transported dye was monitored with an epifluorescence microscope under blue light. Low plasmodesmatal frequencies endarch to the bundle sheath cells are relatively low for most interfaces terminating at the thin‐ and thick‐walled sieve tubes within this C3 species. Lack of connections between vascular parenchyma and sieve tubes, and low frequencies (0.5% plasmodesmata per μm cell wall interface) of connections between vascular parenchyma and companion cells, as well as the very low frequency of pore‐plasmodesmatal connections between companion cells and sieve tubes in small bundles (0.2% plasmodesmata per μm cell wall interface), suggest that the companion cell‐sieve tube complex is symplastically isolated from other vascular parenchyma cells in small bundles.The degree of cellular connectivity and the potential isolation of the companion cell‐sieve tube complex was determined electrophysiologically, using an electrometer coupled to microcapillary electrodes. The less negative cell potential (average –52 mV) from mesophyll to the vascular parenchyma cells contrasted sharply with the more negative potential (–122.5 mV) recorded for the companion cell‐thin‐walled sieve tube complex. Although intercellular injection of lucifer yellow clearly demonstrated rapid (0.75 μm s‐1) longitudinal and radial transport in the bundle sheath‐vascular parenchyma complex, as well as from the bundle sheath through transverse veins to adjacent longitudinal veins, we were neither able to detect nor present unequivocal evidence in support of the symplastic connectivity of the sieve tubes to the vascular parenchyma. Injection of the companion cell‐sieve tube complex, did not demonstrate backward connectivity to the bundle sheath.We conclude that the low plasmodesmatal frequencies, coupled with a two‐domain electropotential zonation configuration, and the negative transport experiments using lucifer yellow, precludes symplastic phloem loading in barley leaves.
A study of photorespiratory ammonia production in the C4 plant Amaranthus edulis, using mutants with altered photosynthetic capacities
Authors: Lacuesta M. , Dever L. V. , Muñoz‐Rueda A. , Lea P. J. 
Published: May 2006

Previous studies have indicated that the rate of photorespiration in C4 plants is low or negligible. In this study, wild‐type and mutant leaves of the C4 plant Amaranthus edulis were treated with the glutamine synthetase inhibitor, phosphinothricin and the glycine decarboxylase inhibitor, aminoacetonitrile, at different concentrations of CO2. The time course of ammonia accumulation in leaves of the wild type was compared with a mutant lacking phosphoenolpyruvate carboxylase activity (EC 4.1.1.31), and with three different mutants that accumulated glycine. The increase in the concentration of ammonia in the leaves, stimulated by the treatments was used as a measurement of the rate of photorespiration in C4 plants.The application of glutamine and glycine maintained the rate of photorespiratory ammonia production for a longer period in the wild type, and increased the rate in a mutant lacking phosphoenolpyruvate carboxylase suggesting that there was a lack of amino donors in these plants.The calculated rate of photorespiration in Amaranthus edulis wild‐type leaves when the supply of amino donors was enough to maintain the photorespiratory nitrogen flow, accounted for approximately 6% of the total net photosynthetic CO2 assimilation rate. In a mutant lacking phosphoenolpyruvate carboxylase, however, this rate increased to 48%, when glutamine was fed to the leaf, a value higher than that found in some C3 plants. In mutants of Amaranthus edulis that accumulated glycine, the rate of photorespiration was reduced to 3% of the total net CO2 assimilation rate. The rate of ammonia produced during photorespiration was 60% of the total produced by all metabolic reactions in the leaves. The data suggests that photorespiration is an active process in C4 plants, which can play an important role in photosynthetic metabolism in these plants.
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.
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.
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.
Sugar uptake by protoplasts isolated from tomato fruit tissues during various stages of fruit growth
Authors: Brown M. M. , Hall J. L. , Ho L. C. 
Published: May 2006

The uptake of radioactive glucose and sucrose by protoplasts isolated from pericarp and placenta tissues of tomato (Lycopersicon esculentum cv. Counter) fruit was investigated in relation to the dry matter accumulation rates of these tissues. Uptake of glucose by protoplasts isolated from pericarp tissue was highest in fruit of around 20 g fresh weight or 25 days after anthesis. Sucrose uptake by pericarp protoplasts was lower than that of glucose and did not show a peak of uptake. The maximum rate of glucose uptake by protoplasts from the pericarp was at the time when the tomato fruit was accumulating dry matter at the highest rate. Glucose uptake by placenta protoplasts was lower and at a similar level as sucrose.Protoplast uptake of glucose, but not of sucrose, was partially inhibited by (1) p‐chloromercuribenzene sulphonic acid, a sulphydryl group modifier; (2) erythrosin B, an H+‐ATPase inhibitor; and (3) valinomycin, a K+‐ionophore, suggesting that membrane transport of glucose by tomato fruit sink cells may be a carrier‐mediated, energy‐dependent process.The main route of carbohydrate accumulation by tomato fruit during the period of rapid fruit growth may be by cleavage of sucrose by apoplastic acid invertase prior to hexose transport across the plasma membrane.