bluuebunny/crossref_metadata_2025 huggingface dataset:
Virus movement proteins and other molecular probes of plasmodesmal function
Authors: WOLF S. , LUCAS W. J.
Published: April 2006

Plasmodesmata exist as supramolecular complexes that interconnect plant cells to form a symplasmic continuum. Microinjection (dye‐coupling) studies confirmed the validity of the symplasmic concept by establishing that molecules of up to 1 kDa can pass from cell to cell, via plasmodesmata. Thus, cells can exchange small molecules, such as metabolites and hormones, to coordinate biochemical and physiological processes occurring within tissues. Plasmodesmata also mediate in cell‐to‐cell transport of macromolecules, including proteins and nucleic acids. This new function was elucidated by studies on virus‐plasmodesmata interaction. Plant viruses encode for movement proteins (MPs) that are essential for cell‐to‐cell spread of infectious material. When expressed in transgenic plants, these MPs are targeted to plasmodesmata and induce a considerable increase in plasmodesmal size exclusion limit (SEL). Studies on mutant MPs confirmed that this increase in SEL is essential for viral infection. Microinjection of MP alone, or together with its conjugate infectious viral transcript, indicated that both the protein and the viral RNA (or DNA) move rapidly from cell to cell. These findings are consistent with the operation of an endogenous plasmodesmal macromolecular transport pathway. Control over this transport pathway has been probed using viral MPs, and the results support the hypothesis that plasmodesmata are involved in the creation of physiological and developmental domains. Elucidation of the molecular constituents involved in the formation and functioning of plasmodesmata will provide valuable insights into the integrated functioning of higher plants, as well as potentiating the development of new strategies for viral resistance.
Approaches to influence starch quantity and starch quality in transgenic plants
Authors: MÜLLER‐RÖBER B. , KOßMANN J.
Published: April 2006

Starch plays a major role as a transitory and long‐term storage compound in higher plants, and therefore is of prime importance for plant growth and development. Additionally, starch serves as a widely used material for a variety of industrial uses. The formation of starch can arbitrarily be divided into three types of event: (I) those leading to the supply of glucose‐1‐phosphate in the plastids; (II) the conversion of glucose‐1‐phosphate to ADP‐glucose catalysed by the enzyme ADP‐glucose pyrophosphorylase; and (III) the enzymatic reactions converting ADP‐glucose to long‐chain glucans (amylopectin, amylose). In recent years, numerous cDNA and genomic sequences encoding enzymes involved in starch metabolism have been identified. Some of these have been used to down‐regulate enzyme activities via the antisense RNA technique. Additionally, bacterial genes have been ectopically expressed in transgenic plants in order to increase corresponding enzyme activities. By modulating the activity of ADP‐glucose pyrophosphorylase in plastids, it was possible to decrease and increase, respectively, the starch content in source and sink organs of transgenic plants. In addition, down‐regulation of granule‐bound starch synthase (isoform I) resulted in the production of starch that was almost completely free of amylose. Further experiments aimed to modulate starch structure are currently underway and will briefly be discussed.
Approaches to improve stress tolerance using molecular genetics
Authors: BARTELS D. , NELSON D.
Published: April 2006

Plant productivity is greatly affected by environmental stress factors. In this review, we give an overview of molecular approaches that have been taken to study stress tolerance; in particular, we consider drought, salt and cold stress. Strategies and perspectives in using molecular biology to improve stress tolerance are outlined describing specific examples. Osmotic stress is associated with the synthesis of novel polypeptides and/or osmolytes. The spectrum and action of these different metabolites are summarized. A number of stress‐related genes have been characterized and several representatives are discussed. After transforming plants with these genes, the effect of the encoded proteins on altered stress behaviour is examined. If genes with enzymatic functions were chosen for these experiments, complete pathways may be altered and this can implement the production of novel metabolites conferring stress tolerance.
Studying plant development in mosses: the transgenic route
Authors: KNIGHT C. D.
Published: April 2006

The current status of transgenic studies in mosses is reviewed with particular attention being given to the mosses Physcomitrella patens and Ceratodon purpureus. This paper reviews the advantages of using mosses as models for higher plants in the study of plant development, and includes developmental processes, already partially characterized at the genetic level by mutant analysis, for which transgenic studies may be applicable. The P. patens transformation process is being studied in this laboratory and details are given for a class of transformants which contain extrachromosomal plasmid DNA. Publications which present the nucleic acid and/or protein sequence for nuclear, chloroplast and mitochondrial genes are reviewed. Areas of research in which transgenic studies promise to complement existing cell biological and physiological approaches are discussed. These include the measurement of calcium levels in mutant and wild‐type transformants expressing the apoaequorin gene and a role for phytochrome gene expression in the establishment of polarity.
Suitability of Arabidopsis for studies on intracellular pH regulation: correlation between H+ pump activity, cytosolic pH and malate level in wild‐type and in a mutant partially insensitive to fusicoccin
Authors: BEFFAGNA N. , ROMANI G.
Published: April 2006

Data are presented on the suitability of Arabidopsis thaliana seedlings for studies on intracellular pH regulation. In this material, grown in the dark in liquid medium, the determination of weak acid distribution at equilibrium provides an adequate method for calculating cytosolic pH values, in spite of the failure of benzylamine as a vacuolar pH probe. The stimulation of the H+ pump by K+ or K+ and fusicoccin (FC) is associated with a marked alkalinization of both cytosol and cell sap, and with a strong increase in malate level, whereas its inhibition by erythrosin B (EB) leads to the opposite effects. A good quantitative correlation is evident between the changes in net H+ extrusion and those in intracellular pH and malate content, in particular, with FC+K+. Cell sap buffer capacity is strongly influenced by the different treatments, its changes being substantially accounted for by changes in malate level. A comparison between the values of intracellular pH and malate level in wt and in the 5‐2 mutant shows that in the mutant the cytosolic pH is always more acidic, and the intracellular alkalinization induced by FC+K+ and also by K+ alone is significatively lower. These results support the view that the partial insensitivity of 5‐2 to FC is due to a reduced functionality of the H+‐extruding system on which FC acts, and that the depression of the H+ pump activity in the mutant does not depend on a possible regulation by constitutively higher cytosolic pH values.
Non‐uniform stomatal closure and the apparent convexity of the photosynthetic photon flux density response curve
Authors: OLSSON T. , LEVERENZ J. W.
Published: April 2006

The photosynthetic response of leaves to photosynthetic photon flux density (PPFD) may be described by parameters for the rate of dark respiration (R), the initial slope (Ф), the PPFD‐saturated rate of net photosynthesis (AmQ) and the apparent convexity (θa). We tested the hypothesis that non‐uniformity in stomatal aperture across a leaf results in a clearly lower θa because PPFD saturation will occur at different irradiances in different regions of the leaf. A computer model was constructed to simulate the effects of bell‐shaped and other distributions of stomatal conductance for CO2 (gs) across a model leaf. In the model, Ф and AmQ decreased by up to 50% whereas θa decreased by at most 8%, essentially negating the hypothesis. The relationship between photosynthetic rate and gs determined the size of the responses of θa and Ф to non‐uniform gs. In support of the model, experiments on sunflower leaves showed that Ф and AmQ decreased by 32 and 52%, respectively, while no measurable change in θa occurred, when abscisic acid was used to induce patchiness in gs. Although not all possible patterns of non‐uniform stomatal conductance have been tested, it appears that if large variations in θa occur in nature they do not result from non‐uniform gs.
Low‐temperature pretreatment of the root system of Brassica rapa L. plants: effects on the xylem sap exudation and on the nitrate absorption rate
Authors: BIGOT J. , BOUCAUD J.
Published: April 2006

Brassica rapa plants were exposed for a 52 h period (as pretreatment) to a differential temperature (DT) between roots (5°C) and shoots (20°C), while control plants were maintained with both shoot and roots at 20°C (warm grown = WG). Measured at 20°C, volume flow of xylem exudate from roots of DT plants was enhanced compared with that from WG plants, while transpiration flows were similar in pretreated and control plants. Both transpiration and exudation flows were dependent upon shoot/root ratio. Differences in the volume flow of exudate were principally related to increases in root hydraulic conductance. Anion fluxes (notably nitrate) into xylem exudate of DT plants were significantly greater than those into exudate of WG plants. This enhancement of nitrate flow from the pretreated roots was associated with a two‐fold increase in nitrate uptake rate. The relationship of the cold‐induced change in nitrate uptake capacity with shoot/root ratio is discussed in terms of control of nitrate absorption by shoot sink strength.
The sensitivity of C3photosynthesis to increasing CO2concentration: a theoretical analysis of its dependence on temperature and background CO2concentration
Authors: KIRSCHBAUM M. U. F.
Published: April 2006

The atmospheric CO2concentration has increased from the pre‐industrial concentration of about 280 μmol mol−1to its present concentration of over 350 μmol mol−1, and continues to increase. As the rate of photosynthesis in C3plants is strongly dependent on CO2concentration, this should have a marked effect on photosynthesis, and hence on plant growth and productivity. The magnitude of photo‐synthetic responses can be calculated based on the well‐developed theory of photosynthetic response to intercellular CO2concentration. A simple biochemically based model of photosynthesis was coupled to a model of stomatal conductance to calculate photosynthetic responses to ambient CO2concentration. In the combined model, photosynthesis was much more responsive to CO2at high than at low temperatures. At 350 μmol mol−1, photosynthesis at 35°C reached 51% of the rate that would have been possible with non‐limiting CO2, whereas at 5°C, 77% of the CO2non‐limited rate was attained. Relative CO2sensitivity also became smaller at elevated CO2, as CO2concentration increased towards saturation. As photosynthesis was far from being saturated at the current ambient CO2concentration, considerable further gains in photosynthesis were predicted through continuing increases in CO2concentration. The strong interaction with temperature also leads to photosynthesis in different global regions experiencing very different sensitivities to increasing CO2concentrations.
Photosynthetic characteristics of the shade‐adapted C4grassMuhlenbergia sobolifera(Muhl.) Trin.: control of development of photorespiration by growth temperature
Authors: SMITH M. , WU Y.
Published: April 2006

Muhlenbergia sobolifera(Muhl.) Trin., a C4grass, occurs in understory habitats in the northeastern United States. Plants ofM. soboliferawere grown at 23 and 30°C at 150 and 700 μmol photons m−2s−1. The photosynthetic CO2compensation point, maximum CO2assimilation, dark respiration and the absorbed quantum use efficiency (QUE) were measured at 23 and 30°C at 2 and 20% O2. Photosynthetic CO2compensation points ranged from 4 to 14mm3dm−3CO2and showed limited O2sensitivity. The mean photosynthetic CO2compensation point of plants grown at 30°C (4·5 mm3dm−3) was 57% lower and 80% less inhibited by O2than that of plants grown at 23°C. Photosynthesis was similarly affected by growth temperature, with 70% more O2inhibition in plants grown at 23°C; suppression over all treatments ranging from 2 to 11%. Unlike typical C4species, plants ofM. soboliferafrom both temperature regimes exhibited higher CO2assimilation rates when grown at low light. Growth temperature and light also affected QUE; plants grown at low light and 23°C had the highest value (0·068 mol CO2/mol quanta). Measurement temperature and growth light regime significantly affected dark respiration; however, O2did not affect QUE or dark respiration under any growth or measurement conditions. The results indicate thatM. soboliferais adapted to low PPFD, and that complete suppression of photorespiration is dependent upon high growth temperature.
Developmental changes in the diurnal water budget of the grape berry exposed to water deficits
Authors: GREENSPAN M. D. , SHACKEL K. A. , MATTHEWS M. A.
Published: April 2006

The diurnal water budget of developing grape (Vitis vinifera L.) berries was evaluated before and after the onset of fruit ripening (veraison). The diameter of individual berries of potted ‘Zinfandel’ and ‘Cabernet Sauvignon’ grapevines was measured continuously with electronic displacement transducers over 24 h periods under controlled environmental conditions, and leaf water status was determined by the pressure chamber technique. For well‐watered vines, daytime contraction was much less during ripening (after veraison) than before ripening. Daytime contraction was reduced by restricting berry or shoot transpiration, with the larger effect being shoot transpiration pre‐veraison and berry transpiration post‐veraison. The contributions of the pedicel xylem and phloem as well as berry transpiration to the net diurnal water budget of the fruit were estimated by eliminating phloem or phloem and xylem pathways. Berry transpiration was significant and comprised the bulk of water outflow for the berry both before and after veraison. A nearly exclusive role for the xylem in water transport into the berry was evident during pre‐veraison development, but the phloem was clearly dominant in the post‐veraison water budget. Daytime contraction was very sensitive to plant water status before veraison but was remarkably insensitive to changes in plant water status after veraison. This transition is attributed to an increased phloem inflow and a partial discontinuity in berry xylem during ripening.
The effect of growth at elevated CO2 concentrations on photosynthesis in wheat
Authors: McKEE I. F. , WOODWARD F. I.
Published: April 2006

Rising levels of atmospheric CO2 will have profound, direct effects on plant carbon metabolism. In this study we used gas exchange measurements, models describing the instantaneous response of leaf net CO2 assimilation rate (A) to intercellular CO2 partial pressure (Ci), in vitro enzyme activity assay, and carbohydrate assay in order to investigate the photosynthetic responses of wheat (Triticum aestivum L., cv. Wembley) to growth under elevated partial pressures of atmospheric CO2 (Ca). At flag leaf ligule emergence, the modelled, in vivo, maximum carboxylation velocity for RuBisCO was significantly lower in plants grown at elevated Ca than in plants grown at ambient Ca (70 Pa compared with 40 Pa). By 12 d after ligule emergence, no significant difference in this parameter was detectable. At ligule emergence, plants grown at elevated Ca exhibited reduced in vitro initial activities and activation states of RuBisCO. At their respective growth Ci values, the photosynthesis of 40‐Pa‐grown plants was sensitive to p(O2) and to p(CO2) whereas that of 70‐Pa‐grown plants was insensitive. Both sucrose and starch accumulated more rapidly in the leaves of plants grown at 70 Pa. At flag leaf ligule emergence, modelled non‐photorespiratory respiration in the light (Rd) was significantly higher in 70‐Pa‐grown plants than in 40‐Pa‐grown plants. By 12 d after ligule emergence no significant differences in Rd were detectable.
A critical comparison of the pressure‐probe and pressure‐chamber techniques for estimating leaf‐ceil turgor pressure in Kalanchoë daigremontiana
Authors: MURPHY R. , SMITH J. A. C.
Published: April 2006

The aim of the present study was to test the accuracy of the pressure‐chamber technique as a method for estimating leaf‐cell turgor pressures. To this end, pressure‐probe measurements of cell turgor pressure (Pcell) were made on mesophyll cells of intact, attached leaves of Kalanchoë daigremontiana. Immediately following these measurements, leaves were excised and placed in a pressure chamber for the determination of balance pressure (Pbal). Cell‐sap osmotic pressure (?cell) and xylem‐sap osmotic pressure (?xyl) were also measured, and an average cell turgor pressure calculated as Pcell=?cell–?xyl–Pbal. The apparent value of Pbal was positively correlated with the rate of increase of chamber pressure, and there was also a time‐dependent increase associated with water loss. On expressing sap from the xylem, ?xyl fell to a plateau value that was positively correlated with ?cell. Correcting for these effects yielded estimates of Pbal and ?xyl at the time of leaf excision. On average, the values of Pcell obtained with the two techniques agreed to within ±002 MPa (errors are approximate 95% confidence limits). If ?xyl were ignored, however, the calculated turgor pressures would exceed the measured values by an average of 0.074 ± 0.012MPa, or 48% at the mean measured pressure of 0.155 MPa. We conclude that the pressure‐chamber technique allows a good estimate to be made of turgor pressure in mesophyll cells of K. daigremontiana, provided that ?xyl is included in the determination. The 1:1 relationship between the measured and calculated turgor pressures also implies that the weighted‐average reflection coefficient for the mesophyll cell membranes is close to unity.
Isotopic heterogeneity of water in transpiring leaves: identification of the component that controls the δ18O of atmospheric O2 and CO2
Authors: YAKIR D. , BERRY J. A. , GILES L. , OSMOND C. B.
Published: April 2006

Two direct but independent approaches were developed to identify the average δ18O value of the water fraction in the chloroplasts of transpiring leaves. In the first approach, we used the δ18O value of CO2 in isotopic equilibrium with leaf water to reconstruct the δ18O value of water in the chloroplasts. This method was based on the idea that the enzyme carbonic anhydrase facilitates isotopic equilibrium between CO2 and H2O predominantly in the chloroplasts, at a rate that is several orders of magnitude faster than the non‐catalysed exchange in other leaf water fractions. In the second approach, we measured the δ18O value of O2 from photosynthetic water oxidation in the chloroplasts of intact leaves. Since O2 is produced from chloroplast water irreversibly and without discrimination, the δ18O value of the O2 should be identical to that of chloroplast water. In intact, transpiring leaves of sunflower (Helianthus annuus cv. giant mammoth) under the experimental conditions used, the average δ18O value of chloroplasts water was displaced by 3—10 % (depending on relative humidity and atmospheric composition) below the value predicted by the conventional Craig & Gordon model. Furthermore, this δ18O value was always lower than the δ18O value that was measured for bulk leaf water. Our results have implications for a variety of environmental studies since it is the δ18O value of water in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the δ18O values of atmospheric CO2 and O2, as well as in influencing the δ18O of plant organic matter.
Accumulation of glycinebetaine during cold acclimation and freezing tolerance in leaves of winter and spring barley plants
Authors: KISHITANI S. , WATANABE K. , YASUDA S. , ARAKAWA K. , TAKABE T.
Published: April 2006

A study was performed to examine whether or not betaine (glycinebetaine), a compatible solute, is accumulated in response to cold stress and is involved in mechanisms that protect plants from freezing injury. For this purpose, we used near‐isogenic lines of barley, with each line differing only in a single gene for the spring type of growth habit; the various lines were produced by back‐crosses to a recurrent cultivar of the winter type. The winter type of growth habit requires a low temperature for triggering of flower development (vernalization), whereas the spring type does not. Betaine was accumulated to five times the basal level over the course of 3 weeks at low temperature (5 °C) in the winter‐type cultivar and in a spring‐sh line having the sh gene for the spring‐type growth habit, but the level was only doubled in the spring‐Sh3 line, which carried the Sh3 gene for the spring‐type growth habit. Among near‐isogenic lines of the same cultivar, the levels of betaine accumulated in leaves at low temperature were well correlated with the percentages (on a dry weight basis) of green leaves that survived freezing injury (‐5 °C). This observation indicates the possibility, separate from the recognized role of betaine in the response to salinity and/or drought, that betaine accumulates in response to cold stress and that the accumulation of betaine during cold acclimation is associated to some extent with freezing tolerance in leaves of barley plants.
A system for imaging transverse distribution of scattered light and chlorophyll fluorescence in intact rice leaves
Authors: TAKAHASHI K. , MINEUCHI K. , NAKAMURA T. , KOIZUMI M. , KANO H.
Published: April 2006

In order to examine the transverse distribution of scattered light and chlorophyll fluorescence in intact rice leaves, a micro‐fluorescence imaging system was devised using a microscope, a CCD camera with an image intensifier, an Ar and a He‐Ne laser light source, an image processor, and a microcomputer. A laser light was projected vertically on to the surface of a rice leaf segment at a cut‐edge, and scattered light and induced fluorescence were observed at the cut‐section from a 90° angle to the axis of the laser beam. The intensity of scattered light showed a maximum at several micrometres depth from the leaf surface and a steep gradient afterwards. Fluorescence reached a maximum crossing with the decline curve of the scattered light. The maximum of fluorescence measured at 741 nm was observed at a greater depth from the leaf surface than that at 687 nm, suggesting that part of the fluorescence of the longer wavelength was emitted due to absorption of fluorescence of the shorter wavelength. Profiles of the scattered light and the chlorophyll fluorescence depended on leaf anatomy.
A feedback control system for the precise and continuous regulation of photosynthetic photon flux density
Authors: LIVINGSTON N. J.
Published: April 2006

A simple and inexpensive feedback control system that provides continuous and precise control of photosynthetic photon flux density (PPFD) in a whole plant cuvette is described. A ‘Plexiglass’ tank is interposed between a light source and cuvette and PPFD changed by varying the level of dyed liquid in the tank. The amount of liquid pumped into or drained from the tank is a function of the difference (error) between a defined set point value of PPFD and that measured in the cuvette. The set point can be varied as a function of time, can follow the output of a quantum sensor measuring ambient PPFD or can be driven by values of PPFD read from a data file. Within the 0.4 to 0.64 μm waveband, the dye acts as a neutral density filter so that there is no change in spectral distribution with PPFD. Photosynthetic photon flux density in the cuvette was controlled to better than 20 μmol m−2s−1 when the set point was varied from 200 to 1100 μmol m−2s−1 over 3 min. When the set point was held constant or changed less rapidly, errors did not exceed 5 μmol m−2s−1. Net photosynthesis of Western redcedar (Thuja plicata Donn.) seedlings held at 18 °C closely followed rapid changes in PPFD.
Symbiotic nitrogen‐fixing root nodules of Lathyrus maritimus (L.) Bigel (beach pea) from Newfoundland shore lines with special reference to oleosomes (lipid bodies)
Authors: BARIMAH‐ASARE J. , BAL A. K.
Published: April 2006

Lathyrus maritimus (L.) Bigel, commonly known as beach pea, grows along the shorelines of Newfoundland, Canada. Rhizobia have been isolated from the subterranean root nodules and the cultures were induced to nodulate seedlings raised in the laboratory. The nodules collected from the field were elongated and sometimes branched with proximal ends thickened. Histological characteristics revealed their indeterminate perennial form. Oleosomes (lipid bodies) were present in the nodule tissues. Morphometric analysis showed their presence in the nodule cortex cells throughout all developmental stages, but they were found in the infected cells only during the early stages of infection and symbiosis. It is suggested that oleosomes are utilized for membrane and rhizobial proliferation during establishment of symbiosis in the infected cells. The role of oleosomes in the cortical cells remains unclear.
Amino acid metabolism associated with N‐mobilization from the flag leaf of wheat (Triticum aestivum L.) during grain development
Authors: PEETERS K. M. U. , VAN LAERE A. J.
Published: April 2006

Field‐grown winter wheat (Triticum aestivum L. cv. Castell) was used to study changes in the free amino acid pools of different plant parts and related enzyme activities in the flag leaf throughout the grain‐filling period in three consecutive growing seasons. Amino acid analysis data indicated that, during senescence, the nitrogen flow in the flag leaf was directed towards the synthesis of glutamine as a specific nitrogen transport form. Of the enzymes involved, total glutamine synthetase (GS; EC 6.3.1.2) and especially ferredoxin‐dependent glutamate synthase (Fd‐GOGAT; EC 1.4.7.1) activities declined continuously as senescence progressed. Unlike (chloroplastic) GS2, (cytosolic) GS1 was shown to be very persistent suggesting a special role for this isoenzyme in the N‐reallocation process. Glutamate‐oxaloacetate transaminase (GOT; EC 2.6.1.1), glutamate‐pyruvate transaminase (GPT; EC 2.6.1.2) and isocitrate dehydrogenase (IDH; EC 1.1.1.42) showed a characteristic biphasic activity profile after anthesis. It is proposed that these enzymes, for each of which at least two isoenzymes were demonstrated, are involved in glutamate synthesis at the later stages of leaf senescence. Ammonium levels were fairly constant throughout the flag leafs life span, an ultimate rise often following peak values of glutamate dehydrogenase (GDH; EC 1.4.1.4) activity. The enzymology of flag leaf amino acid metabolism during grain development is further discussed in relation to observations of NH3‐volatilization from naturally senescing wheat plants.
Compartmentation and transport of zinc in barley primary leaves as basic mechanisms involved in zinc tolerance
Authors: BRUNE A. , URBACH W. , DIETZ K.‐J.
Published: April 2006

The heavy metal zinc was administered to barley seedlings by increasing its concentration in the hydroponic medium. The most dramatic effect was a severe inhibition of root elongation with little effect on root biomass production. The growth of primary leaves was little affected although the zinc content of the primary leaves increased several‐fold. A detailed compartment analysis was performed for 10‐d‐old barley primary leaves. Under low zinc nutrition (2mmol m −3), highest zinc contents were observed in the cytoplasm of mesophyll protoplasts. At inhibitory zinc concentrations in the hydroponic medium (400 μmol m −3), zinc levels dramatically and preferentially increased in the apoplastic space. Elevated zinc levels were also observed in the epidermal cells, and to a lesser extent, in mesophyll vacuoles. The cytoplasmic content of mesophyll protoplasts was unchanged, indicating perfect zinc homeostasis within the leaf. In order to understand the transport mechanisms underlying the steady‐state distribution profile, we used 65Zn to conduct uptake experiments with leaves whose lower epidermis had been stripped. The leaves were placed on zinc solutions of varying concentrations containing 65Zn for 5 min to 6 h. After the incubation, the leaves were fractionated into mesophyll and epidermis protoplasts and residue, the latter mainly representing cell wall. Adsorption of Zn to the extracellular matrix was 100 times faster than Zn uptake into the cells. By far the largest portion taken up into the mesophyll protoplasts rapidly appeared in the vacuolar compartment. These results demonstrate the importance of compartmentation and transport as homeostatic mechanisms within the leaves to handle high, possibly toxic, zinc levels in the shoot.
Behaviour of plasma membrane, cortical ER and plasmodesmata during plasmolysis of onion epidermal cells
Authors: OPARKA K. J. , PRIOR D. A. M. , CRAWFORD J. W.
Published: April 2006

During plasmolysis of onion epidermal cells, the contracting protoplast remains connected to the cell wall by an intricate, branched system of plasma membrane (PM) ‘Hechtian strands’ which stain strongly with the fluorescent probe DiOC6. In addition, extensive regions of the cortical endoplasmic reticulum (ER) network remain anchored to the cell wall during plasmolysis and do not become incorporated into the contracting protoplast with the other cell organelles. These ER profiles become tightly encased by the PM as the latter contracts towards the centre of the cell. Thus, although the cortical ER is left outside the main protoplast body, it is nonetheless still bound by the PM of the cell. As well as being anchored to the wall, the cortical ER remains intimately linked with plasmodesmata and retains continuity between cells via the central desmotubules which become distended during plasmolysis. The PM also remains in close contact with the plasmodesmatal pore following plasmolysis. It is suggested that plasmodesmata, although sealed, may not be broken during plasmolysis, their substructure being preserved by continuity of both ER and PM through the plasmodesmatal pore. A structural model is presented which links the behaviour of PM, ER and plasmodesmata during plasmolysis.
Carbon isotope discrimination and water‐use efficiency of six crops grown under wet and dryland conditions
Authors: KNIGHT J. D. , LIVINGSTON N. J. , VAN KESSEL C.
Published: April 2006

Mustard (Sinapis alba L.), Argentine canola (Brassica napus L. cv. Westar), Polish canola (Brassica campestris L. cv. Tobin), pea (Pisum sativum L.), durum wheat (Triticum durum L. cv. Kyle) and soft wheat (Triticum aestivum L. cv. Fielder) were grown at Outlook, Saskatchewan, Canada, under irrigated and dryland conditions. Carbon isotope discrimination (Δ) and water‐use efficiency (W), defined as grams of above ground dry matter produced per kilogram water used, were negatively correlated in the six field‐grown crops. In irrigated plants Δ remained relatively constant (20–21‰) throughout the growing season. However, in dryland plants, Δ declined in response to the progressive depletion of stored soil water (Polish canola, 20‐2‐18‐8‰; mustard, 19.9–18 5‰; pea, 19.9–17 2‰ durum wheat, 19.7–16.4‰; Argentine canola, 19.4–17.6‰; soft wheat, 19.0–17.4‰). Although there were genetic differences in Δ among the species, water availability was the major factor controlling Δ.
Expansion rate of young tomato fruit growing on plants at positive water potential
Authors: GRANGE R. I. , ANDREWS J.
Published: April 2006

Changes in tomato fruit expansion rate and carbohydrate content have been assessed during treatments designed to alter the carbon import rate. Because fruit expansion is sensitive to plant water status, the relationship with carbon import is difficult to assess, and thus, the diameter growth rate of young fruit was measured on plants maintained at positive water potentials. The detached top metre of a tomato plant was supplied with water, through the cut stem base, at a pressure of 0.08 MPa. Developing fruit on the stem continued to grow at high rates for up to 2 d. Fruit diameter growth rate after plant detachment was directly proportional to temperature. Plants acclimated to different continuous irradiances for 5 d before detachment gave fruit growth rates after plant detachment which were directly proportional to the irradiance up to 7 MJ m−2d−1 photosynthetically active radiation (PAR). In continuous darkness, fruit growth rate remained unchanged for 20 h and then declined to less than 40% of the original rate over the following 30 h. On re‐exposure to light, about 5 h elapsed before fruit growth rate increased but the growth rate stabilized at approximately 50% of the rate in continuously illuminated plants. During darkness, both fruit starch and hexose content decreased in comparison to illuminated controls, but on re‐illumination, carbohydrate content increased before carbon was allocated to structural growth. Heat‐killing the phloem of the fruit pedicel caused an immediate, but temporary, cessation of growth. After a partial recovery, expansion growth continued, but more slowly than in untreated fruit and at steadily declining rates. Starch and hexose sugars were not used to provide substrates for growth and starch synthesis was maintained. Continuing cell expansion was assumed to have been supported by water import via the xylem. Thus, fruit expansion may be related to carbon accumulation in most circumstances, but the changing allocation of imported carbon to storage and cell expansion may modify this relationship.
Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass
Authors: KNAPP A. K. , FAHNESTOCK J. T. , OWENSBY C. E.
Published: April 2006

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO2 levels in open‐top chambers. Within chambers or in adjacent unchambered plots, the dominant C4 grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m−2 s−1 versus 350 μmol m−2 s−1 shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO2 when compared to plants at ambient CO2. This was due primarily to the 50% reduction in stomatal conductance at elevated CO2, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO2. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO2 enhanced plant water status. CO2‐induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO2 on plant water relations in all ecosystems.
Respiration rates predict differences in growth of coast redwood
Authors: ANEKONDA T. S. , CRIDDLE R. S. , LIBBY W. J. , BREIDENBACH R. W. , HANSEN L. D.
Published: April 2006

The relation between growth rate traits (height, basal diameter, stem volume and branch diameter) and two measures of respiration rate [metabolic heat rate (q) and CO2 production rate (Rco2)] and their ratio (q/Rco2) was examined on a collection of 192 different genotypes of coast redwoods [Sequoia sempervirens (D.Don) Endl.]. Branch diameter was not correlated with any of the respiratory measures, but the other three growth traits gave highly significant (P < 0.001) correlations with positive slopes. Combining the four growth traits and the three respiratory variables (q, RCo2 and q/Rco2) to give two canonical variates, one representing growth and one representing respiration, gives an even stronger linear correlation (r= 0–85). These data suggest that simultaneous assay of multiple respiratory measures on juvenile trees can be used to predict their longer‐term growth rates.
The cellular pathway of photosynthate transfer in the developing wheat grain. I. Delineation of a potential transfer pathway using fluorescent dyes
Authors: WANG H. L. , OFFLER C. E. , PATRICK J. W. , UGALDE T. D.
Published: April 2006

A potential cellular pathway for photosynthate transfer between the crease phloem and the starchy endosperm of the developing wheat grain has been delineated using fluorescent dyes. Membrane permeable and impermeable dyes have been introduced into the grain through the crease phloem, the endosperm cavity or the dorsal surface of the starchy endosperm. The movement of the symplastic tracer 5‐(6)‐6‐carboxyfluorescein (CF) derived from 5‐(6)‐6‐carboxyfluorescein diacetate (CFDA), from either direction between the crease phloem and the endosperm cavity, indicated that the symplastic pathway was operative from the crease phloem to the nucellar projection. Furthermore, the inward movement of apoplastic tracer trisodium, 3‐hydroxy‐5,8,10‐pyrentrisulphonate (PTS) from the endosperm cavity and that of CF following plasmolysis showed that there was a high resistance to solute transfer within the apoplast of the pigment strand. All dyes entered the modified aleurone and adjacent sub‐aleurone bordering the endosperm cavity. Subsequent movement of the symplastic tracers CF and sulphorhodamine G (SRG) into and through the endosperm was rapid. However, the movement of apoplastic tracers PTS and Calcofluor White (CFW) was relatively slow and with tissue plasmolysis, CF was confined to the cytoplasm of the modified aleurone and subaleurone cells. Together, these results demonstrate that there is a high resistance to solute movement within the apoplast of the cells bordering the endosperm cavity. We propose that photosynthate transfer is via the symplast to the nucellar projection where membrane exchange to the endosperm cavity occurs. Uptake from the cavity is by the modified aleurone and small endosperm cells prior to transfer through the symplast to and through the starchy endosperm.
The effects of UV‐B radiation on loblolly pine. 3. Interaction with CO2 enhancement
Authors: SULLIVAN J. H. , TERAMURA A. H.
Published: April 2006

Projected depletions in the stratospheric ozone layer will result in increases in solar ultraviolet‐B radiation (290–320 nm) reaching the earth's surface, These increases will likely occur in concert with other environmental changes such as increases in atmospheric carbon dioxide concentrations. Currently very little information is available on the effectiveness of UV‐B radiation within a CO2‐enriched atmosphere, and this is especially true for trees. Loblolly pine (Pinus taeda L.) seedlings were grown in a factorial experiment at the Duke University Phytotron with either 0, 8.8 or 13.8 kJ m−2 of biologically effective UV‐B radiation (UV‐BBE). The CO2 concentrations used were 350 and 650 μmol mol−1. Measurements of chlorophyll fluorescence were made at 5‐week intervals and photosynthetic oxygen evolution and leaf pigments were measured after 22 weeks, prior to harvest. The results of this study demonstrated a clear growth response to CO2 enrichment but neither photosynthetic capacity nor quantum efficiency were altered by CO2. The higher UV‐B irradiance reduced total biomass by about 12% at both CO2 levels but biomass partitioning was altered by the interaction of CO2 and UV‐B radiation. Dry matter was preferentially allocated to shoot components by UV‐B radiation at 350 μmol mol−1 CO2 and towards root components at 650 μmol mol−1 CO2. These subtle effects on biomass allocation could be important in the future to seedling establishment and competitive interactions in natural as well as agricultural communities.
Stomatal responses to sodium ions inAster tripolium: a new hypothesis to explain salinity regulation in above‐ground tissues
Authors: PERERA L. K. R. R. , MANSFIELD T. A. , MALLOCH A. J. C.
Published: April 2006

A study has been made of the ionic relations of stomata ofAster tripoliumL., a maritime halophyte which colonizes coastal saltmarshes. The results obtained allow us to add this species to the growing list for which an involvement of K+transport in stomatal movements has been demonstrated. However, an additional and ecologically important characteristic was found: there was a suppression of stomatal opening by increasing NaCl concentrations. A new hypothesis is offered of the mechanism for controlling salt and water relations inA. tripolium, a species which does not possess glands or other means of excreting salt. It is suggested that when the capacity of the tissues to accumulate salt in cell vacuoles is exceeded, the concentration of Na+ions in the apoplast around the guard cells begins to rise. This causes partial stomatal closure, reduces transpiration and increases water‐use‐efficiency. Therefore, the flow of salt into the leaves is reduced but growth (and the manufacture of the new photosynthates required to support it) can continue.Aster tripoliumcan be added to the small list of known species which readily yield isolated epidermis suitable for detailed stomatal studies. Throughout this study, we have compared its stomatal physiology withC. communis, which has been thoroughly investigated in the past.
Influence of stem lacunar structure on gas transport: relation to the oxygen transport potential of submersed vascular plants
Authors: SCHUETTE J. L. , KLUG M. J. , KLOMPARENS K. L.
Published: April 2006

The influence of stem lacunar structure on the potential of diffusion and mass flow to meet estimated root O2 demands was evaluated and compared in four submersed aquatic plant species. Internodal lacunae formed large continuous gas canals which were constricted at the nodes by thin, perforated diaphragms. Gas transport studies showed that nodes had little effect on diffusion, but significantly reduced mass flow. Measured diffusive resistances approximated those predicted by Fick's first law, ranged from 203 to 5107 × 108 s m−4 and increased as lacunar area decreased in Potamogeton praelongus, two Myriophyllum species and Elodea canadensis. Our analysis suggested that diffusion could satisfy estimated root O2 demands given the development of relatively steep O2 gradients (0.15–0.35 mol O2 mor−1 per 0.5 m stem) between shoots and roots. Plants with high resistances (e.g. > 750 × 108 s m−4) and long lacunar pathlengths may be unable, even during active photosynthesis, to support the O2 demands of a large root system by diffusion alone. Measured nodal resistances to mass flow approximated those predicted by Hagen‐Poiseuille law and ranged from 46 to 2029 × 108 Pa s m−3. Our analysis suggested that these resistances were quite low and that relatively small pressure differentials (< 150 Pa per 0.5 m stem) could drive mass flow at rates which would support root O2 demands. Possible mechanisms whereby plant architecture may serve to maintain these pressure differentials are proposed.
Carbon and nitrogen content of congeneric annual and perennial grass species: relationships with growth
Authors: GARNIER E. , VANCAEYZEELE S.
Published: April 2006

Seven annual‐perennial pairs of grass species (six congeneric and one pair taken at random) were grown under productive conditions in the laboratory in order to investigate which plant characters were responsible for the higher relative growth rate (RGR) of annuals as compared to perennials under these conditions. The nitrogen and carbon concentrations of shoot organs and of the whole plant were higher in annuals than in perennials. This was also the case for the specific absorption rate for nitrate and nitrogen productivity (on whole plant and leaf basis). The range of RGR displayed by the 14 species was large enough (0.15–0.33d−1) to examine the general relationships between RGR and the various parameters measured in the present study. RGR was positively related to plant, leaf blade and sheath nitrogen concentrations, but there was no relationship between RGR and any of the carbon concentrations. RGR also strongly correlated with specific absorption rate for nitrate and with nitrogen productivity. A new factorization of this latter parameter led to the definition of the ‘leaf nitrogen productivity’ (NLP), which is likely to depend on photosynthetic nitrogen use efficiency. RGR was shown to be strongly correlated with NLP, but not with the second term of the factorization, namely the proportion of plant nitrogen allocated to the leaves.
Overexpression of oat phytochrome A gene differentially affects stem growth responses to red/far‐red ratio signals characteristic of sparse or dense canopies
Authors: CASAL J. J. , SÁNCHEZ R. A.
Published: April 2006

The effects of overexpression of oat phytochrome A on neighbour detection and on stem‐growth responses to changes in red light (R), far‐red light (FR) and blue light (B) simulating neighbours were investigated in transgenic tobacco seedlings grown under natural radiation. In wild‐type (WT) seedlings, stem extension growth was promoted: (1) by lowering the R:FR by means of daytime supplementary FR, end‐of‐day FR, neighbours reflecting FR, or selective light filters placed around the base of the shoot to reduce R without affecting FR; and (2) by lowering phytochrome‐absorbable radiation (R+FR) reaching the stem. Transgenic seedlings only responded to reductions in R:FR involving no significant changes in FR irra‐diance, i.e. end‐of‐day FR and filters placed around the stem to reduce R. Neither daytime supplementary R nor selective filters placed around the stem to reduce B affected stem growth in any genotype. In growing canopies, WT seedlings responded to the reduction of R:FR caused by FR reflected in neighbour plants. Transgenic seedlings responded to plant density about a week later, when mutual plant shading reduced R and (to a lesser extent) FR below sunlight levels. Overexpression of phytochrome A impaired early neighbour detection.
Growth, sucrose synthase, and invertase activities of developing Phaseolus vulgaris L. fruits
Authors: SUNG S. S. , SHEIH W. J. , GEIGER D. R. , BLACK C. C.
Published: April 2006

Activities of the sucrose‐cleaving enzymes, acid and neutral invertase and sucrose synthase, were measured in pods and seeds of developing snap bean (Phaseolus vulgaris L.) fruits, and compared with 14C‐import, elongation and dry weight accumulation. During the first 10 d post‐anthesis, pods elongated rapidly with pod dry weight increase lagging behind by several days. The temporal patterns of acid invertase activity and import coincided closely during the first part of pod development, consonant with a central role for this enzyme in converting imported sucrose during pod elongation and early dry weight accumulation. Later, sucrose synthase became the predominant enzyme of dry weight accumulation and was possibly associated with the development of phloem in pod walls. Sucrose synthase activity in seeds showed two peaks, corresponding to two phases of rapid import and dry weight accumulation; hence, sucrose synthase was associated with seed sink growth. Acid invertase activities in seeds were low and did not show a noticeable relationship with import or growth. All neutral invertase activities, during pod and seed development, were too low for it to have a dominant role in sucrose cleavage. Changes in activities of certain sucrose‐cleaving enzymes appear to be correlated with certain sink functions, including import, storage of reserves, and biosynthetic activities. The data supports the association of specific sucrose‐cleaving enzymes with the specific processes that occur in the developing pods and seeds of snap bean fruits; for example, acid invertase with pod elongation and sucrose synthase with fruit dry matter accumulation.