bluuebunny/crossref_metadata_2025 huggingface dataset:
Endocytosis in plants: fact or artefact?
Authors: HAWES C. , CROOKS K. , COLEMAN J. , SATIAT‐JEUNEMAITRE B.
Published: April 2006

Whilst plant cells are apparently equipped with all the necessary molecular machinery for receptor‐mediated endocytosis, the physiological role of this process in these cells remains an enigma. In this article, we consider current opinions of endocytosis in plants and define some of the problems that have impeded progress in our under‐standing of the part played by endocytosis in the vesicle trafficking pathway.
Alteration in carbon partitioning induced by the movement protein of tobacco mosaic virus originates in the mesophyll and is independent of change in the plasmodesmal size exclusion limit
Authors: BALACHANDRAN S. , HULL R. J. , VAADIA Y. , WOLF S. , LUCAS W. J.
Published: April 2006

The influence of the 30 kDa movement protein of tobacco mosaic virus (TMV‐MP) on carbon partitioning in trans‐genie tobacco plants (Nicotiana tabacum cv. Xanthi) expressing the TMV‐MP was investigated. Using reciprocal grafting of transgenic tobacco plants expressing this movement protein and vector control plants, as well as transgenic tobacco plants expressing the TMV‐MP in phloem cells only, we showed that the interactive site involved in carbon allocation to roots is localized to the mesophyll tissue. Biomass partitioning experiments conducted on transgenic plants, in which various deletion mutant forms of the TMV‐MP (two of which included deletions in the domain responsible for increasing the size exclusion limit) were expressed, revealed that the TMV‐MP exerts its influence on carbon allocation via a mechanism that is completely independent of the TMV‐MP‐induced increase in the plasmodesmal size exclusion limit. Furthermore, small N‐ and C‐terminal deletions in the MP revealed the complexity of the interactions likely to be involved between the MP and an endogenous regulatory mechanism. We propose that the TMV‐MP interferes with an endogenous signal transduction pathway that involves macromolecular trafficking through plasmodesmata to regulate biomass partitioning between the source and various sink tissues.
Effects of photoinhibitory treatment on CO2 assimilation, the quantum yield of CO2 assimilation, D1 protein, ascorbate, glutathione and xanthophyll contents and the electron transport rate in vine leaves
Authors: CHAUMONT M. , MOROT‐GAUDRY J.‐F. , FOYER C.H.
Published: April 2006

The responses of photosynthesis to high light and low temperature were studied in vines cultivated in the greenhouse in low light. Exposure to high light (1000 /umol m−2 s−1) or low temperature (5 °C) alone had no measurable effect on the photosynthetic processes, but the combination of high light and low temperature caused rapid loss of photosynthetic capacity and a decrease in the efficiency of photosynthetic energy conversion. After a 15 h exposure to 5°C at high light, the Fv/sb/Fmratio had decreased by 80% and the apparent quantum yield by 75%. Nevertheless, when the leaves were returned to low light at 22°C, these parameters recovered rapidly. The foliar pools of ascorbate and glutathione decreased in the first hours of photoinhibitory treatment while the zeaxanthin content increased from negligible levels to about 50% of the total foliar xanthophyll pool. There was a clear correlation between the zeaxanthin content of the leaves and their Fv/Fm ratio during both photoinhibition and recovery. However, there was also a good correlation between the decrease in theFv Fm ratio and the measured decrease in the total foliar levels of the antioxidants ascorbate and glutathione. The amount of D, protein diminished over the same period as the zeaxanthin levels were increasing. This approach, involving simultaneous measurements of several parameters considered to influence photosystemy II activity, clearly demonstrates that measured decreases in Fv/Fm may not simply be related to zeaxanthin levels or to amounts of D1 protein alone but result from multifactoral influences.
Calmodulin levels in radish (Raphanus sativus L.) seeds germinating at low calcium availability induced by EGTA treatments
Authors: NEGRINI N. , RIVETTA A. , COCUCCI M.
Published: April 2006

Incubation of radish (Raphanus sativus L.) seeds in the presence of 1 or Smol m−3 Ca‐EGTA, which increased Ca2+ activity in the incubation medium (c. 0.24 or 0.37 mol m−3 at 24 h with respect to c. 0.13 mol m−3 in the control), did not affect germination, the restoration of K+ net influx, the increase in DNA and RNA levels or protein synthesis. Incubation in 1 mol m−3 Na‐EGTA, which reduced Ca2+ activity in the incubation medium (20 mmol m−3 at 24 h), decreased the total Ca2+ level in embryo axes (‐21%), but only slightly inhibited the increase in fresh weight without affecting the restoration of K+ net influx, the increase in DNA and RNA levels or protein synthesis. In the presence of 5 mol m−3 Na‐EGTA (Ca2+ activity in the incubation medium was 0.6 mmol m−3), the decrease in the total Ca2+ level was greater (c. ‐27%) and the increases in fresh weight, DNA and RNA were inhibited by about 50, 39 and 40%, respectively. These results indicate that increased Ca2+ availability does not affect germination and suggest that the effect of Na‐EGTA, at least up to 5 mol m−3, is a result of an induction of Ca2+ deficiency. The amount and specific activity of calmodulin (CaM) present in the soluble fraction (100 000g) of radish embryo axes greatly increased during the first 24 h of incubation (c. 5‐fold and 7‐fold, respectively). This increase was very similar in the Ca‐EGTA‐treated seeds but was inhibited (c. ‐38%) by 1 mol m−3 Na‐EGTA, even if the increases in DNA and RNA levels and protein synthesis were not significantly reduced. The lower amount of CaM after 24 h of incubation in 1 mol m−3 Na‐EGTA (c. ‐30%) was due to a reduction in the fraction of CaM bound to a proteinaceous CaM inhibitor present in radish seeds [M. Cocucci & N. Negrini (1988) Plant Physiology 88, 910–914] and not involved in the metabolic reactivation of the seed. These results suggest that the level of CaM is controlled by Ca2+ availability and that the CaM inhibitor has a role in controlling the amount of Ca‐CaM available for the Ca‐CaM‐dependent enzymes.
Depolarization of tomato leaf cells by oligogalacturonide elicitors
Authors: THAIN J.F. , GUBB I.R. , WILDON D. C.
Published: April 2006

The electrical potential difference (Em) across the plasma membrane of tomato leaf mesophyll cells consists of a cyanide‐sensitive component, presumably produced by an H+‐ATPase, and a cyanide‐insensitive component. Variation of Em between different batches of tissue is mainly caused by variation in the cyanide‐sensitive component. Oligogalacturonide elicitors that induce the synthesis of proteinase inhibitors in tomato seedlings depolarize the Em of tomato leaf mesophyll cells. This depolarization closely resembles that caused by cyanide: they are of similar magnitude and vary in a similar manner with variation in the initial Em of different batches of tissue. Treatments with cyanide and with the elicitors have similar effects on the small depolarization caused by KCl at 10 mol m−3. The results suggest that the elicitors depolarize Em by inhibiting the plasma membrane H+‐ATPase, but that the detailed mechanism of inhibition by the elicitors is different from that caused by cyanide.
Growth of coniferous trees exposed to SO2 and O3 using an open‐air fumigation system
Authors: HOLLAND M. R. , MUELLER P. W. , RUTTER A. J. , SHAW P. J. A.
Published: April 2006

Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and Sitka spruce (Picea sitchensis Bong. Carr.) were planted as 2‐year‐old seedlings in an open‐air fumigation facility at Liphook in southern England in March 1985. The soil was a humoferric podzol of pH 4. SO2 fumigation began in May 1987 and continued until December 1990. Long‐term mean SO2 concentrations were 4,13 and 22 nmol mo−1. Three plots, one at each SO2 level, were also exposed to O3 at an average of 1–3.times the ambient level. O3 fumigation ran from March to December 1988, May to December 1989 and February to December 1990. Each species reacted differently to treatment. Scots pine showed no growth response to either pollutant, although other work on the site demonstrated a number of deleterious effects of SO2 on this species, including increased leaf loss and foliar injury. Stem basal diameter growth of Norway spruce was depressed in SO2‐treated plots. In contrast, extension growth of shoots of Sitka spruce increased in SO2‐treated plots, in apparent response to codeposition of NH3‐N. However, diameter growth of Sitka spruce main stems did not increase. No effects of O3 on growth were recorded for any species.
The Liphook Forest Fumigation Project: an overview
Authors: McLEOD A. R. , SKEFFINGTON R. A.
Published: April 2006

The aim of the Liphook Project was to assess the effects of SO2 and O3, singly and in combination, on coniferous forest ecosystems. More than 4000 trees of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and Sitka spruce (P. sitchensis) were fumigated for nearly 4 years using an open‐air fumigation technique especially developed for the purpose. The technique eliminated artifacts due to chambers and enabled a variety of effects of the pollutant gases on forest ecosystems to be studied. Most symptoms of forest decline did not occur, but each species reacted in a different way to SO2 stress, providing no evidence for universal forest decline symptoms. However, some of the mechanisms hypothesized to underlie forest declines were observed as an effect of SO2 treatment, though others were not, notably any major effect of O3. The results are assessed against proposed regulatory standards (critical loads and levels) for the protection of forest ecosystems against pollution.
Interpretation of an empirical model for stomatal conductance in terms of guard cell function
Authors: DEWAR R. C.
Published: April 2006

An empirical model for stomatal conductance (g), proposed by Leuning (1995, this issue) as a modification of Ball, Woodrow & Berry's (1987) model, is interpreted in terms of a simple, steady‐state model of guard cell function. In this model, stomatal aperture is a function of the relative turgor between guard cells and epidermal cells. The correlation between g and leaf surface vapour pressure deficit in Leuning's model is interpreted in terms of stomatal sensing of the transpiration rate, via changes in the gradient of total water potential between guard cells and epidermal cells. The correlation between g, CO2 assimilation rate and leaf surface CO2 concentration in Leuning's model is interpreted as a relationship between the corresponding osmotic gradient, irradiance, temperature, intercellular CO2 concentration and stomatal aperture itself. The explicit relationship between osmotic gradient and stomatal aperture (possibly describing the effect of changes in guard cell volume on the membrane permeability for ion transport) results in a decrease in the transpiration rate in sufficiently dry air. Possible extension of the guard cell model to include stomatal responses to soil water status is discussed.
Activated oxygen and antioxidant defences in iron‐deficient pea plants
Authors: ITURBE‐ORMAETXE I. , MORAN J. F. , ARRESE‐IGOR C. , GOGORCENA Y. , KLUCAS R. V. , BECANA M.
Published: April 2006

Iron (Fe) deficiency in pea leaves caused a large decrease (44–62&) in chlorophyll a, chlorophyll b and carotenoids, and smaller decreases in soluble protein (18&) and net photosynthesis (28&). Catalase, non‐specific peroxidase and ascorbate peroxidase activities declined by 51& in young Fe‐deficient leaves, whereas monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase activities remained unaffected. Ascorbate peroxidase activity was highly correlated (r2= 0. 99, P < 0. 001) with the Fe content of leaves, which allows its use as an indicator of the Fe nutritional status of the plant. Fe deficiency resulted in an increase of CuZn‐superoxide dismutase but not of Mn‐superoxide dismutase. The content of ascorbate decreased by only 24& and those of reduced and oxidized glutathione and vitamin E did not vary. The low‐molecular‐mass fraction of Fe‐sufficient leaves contained 30–65 μg (g dry weight)−1 Mn. This concentration was 15–60 times greater than that of Fe and Cu in the same fraction, and was further enhanced (1. 5‐ to 2. 5‐fold) by Fe deficiency without causing Mn toxicity. The concentration of catalytic Fe, that is, of Fe active for free radical generation, was virtually zero and that of catalytic Cu did not change with severe Fe deficiency. Because catalytic metals mediate lipid and protein oxidation in vivo, the above findings would explain why oxidatively damaged lipids and proteins do not accumulate in Fe‐deficient leaves.
Structural changes in the innercortex cells of soybean root nodules are induced by short‐term exposure to high salt or oxygen concentrations
Authors: SERRAJ R. , FLEURAT‐LESSARD P. , JAILLARD B. , DREVON J. J.
Published: April 2006

After a 2 h exposure of intact soybean nodules to high concentrations of NaCl (100mol m−3) or oxygen (8OkPa O2), morphometric computations carried out using an image analysis technique on semi‐thin sections showed that both treatments induced a decrease in the area of the inner‐cortex cells, which were then characterized by a tangential elongation. In contrast, no significant change in area occurred in the middle‐cortex cells although their elongation decreased. Electron microscopic observations showed that in the inner‐cortex cells changes included the presence of wall infoldings, an enlarged periplasmic space and a lobate nucleus whose chromatin distribution differed from that of the control. Structural changes also occurred in the endoplasmic reticulum, microbodies, mitochondria and plastids. From several of these changes, which are similar to those noted in osmocontractil cells in response to external stimuli, it can be hypothesized that the inner cortex may provide a potential mechanism for the control of oxygen diffusion through the nodules.
Response of the photosynthetic apparatus in maize leaves grown at low temperature on transfer to normal growth temperature
Authors: NIE G. ‐Y. , ROBERTSON E. J. , FRYER M. J. , LEECH R. M. , BAKER N. R.
Published: April 2006

Low temperatures are known to restrict chloroplast development and prevent the attainment of photosynthetic competence in maize leaves. The responses of the photosynthetic apparatus of mature maize leaves grown at 14°C on transfer of the plants to 25°C are examined. The synthesis of thylakoid proteins increased immediately on transfer of leaves from 14 to 25°C, with a dramatic accumulation of thylakoid proteins and chlorophylls occurring after 3 d at 25°C. Thylakoid structure and organization also became similar to those observed in leaves grown at 25°C over this period. However, no comparable development of photosynthetic competence in photosystems I and II or in the rate of CO2 assimilation was observed on transfer of leaves from 14 to 25°C. Immunocytological analyses demonstrated heterogeneity in the distribution of a range of thylakoid proteins (cy tochrome f, the α and β subunits of the coupling factor, Dl of the photosytem II reaction centre, the 33kDa protein of the extrinsic oxygen‐evolving complex of photosystem II, and subunit II of photosystem I between mesophyll cells in leaves grown at 14°C, and in the responses of individual proteins to transfer of the leaves to 25°C. Such heterogeneity between mcsophyll cells would account for the inability of the leaves to develop the expected degree of photosynthetic competence on transfer to 25°C. The effects of low growth temperatures on chloroplast biogenesis are complex, as are the changes induced by the transfer ofleaves grown at low temperatures to optimal growth temperature, and both these factors may limit the canopy development and photosynthetic productivity of crops in temperate regions.
A chilling‐sensitive mutant of Arabidopsis is deficient in chloroplast protein accumulation at low temperature*
Authors: SCHNEIDER J. C. , NIELSEN E. , SOMERVILLE C.
Published: April 2006

Mutations at the chsl locus in Arabidopsis thaliana (L.) confer a chilling‐sensitive phenotype in which plants become chlorotic and die after more than 3 d of exposure to temperatures in the range of 18 to 10°C. Within 8h after transfer of mutant plants from growth at 23 to 13°C, accumulation of the newly synthesized chloroplast‐localized polypeptides stearoyl‐ACP desaturase, glutamine synthetase and OEC 23 was severely impaired. By contrast, there was no apparent deleterious effect on the accumulation of two extrachloroplastic proteins examined: the elongation factor lα, and the phloem‐specific, extrachloroplastic isoform of glutamine synthetase. In one instance examined in detail, chilling did not decrease the accumulation of mRNA for stearoyl‐ACP desaturase, indicating that the effect was post‐transcriptional. Chloroplasts isolated from chilled wild‐type and chsl plants were equally able to process and protect from protease digestion in vitro synthesized pre‐plastocyanin. The results suggest that the chsl mutant may be defective in some aspect of chloroplast protein accumulation at low temperature.
Comparison of electric and growth responses to excision in cucumber and pea seedlings. II. Long‐distance effects are caused by the release of xylem pressure
Authors: STAHLBERG R. , COSGROVE D. J.
Published: April 2006

Excision of a growing stem causes local wound responses, such as membrane depolarization and growth inhibition, as well as effects at larger distances from the cut. In this study, cucumber hypocotyls were excised 100mm below the hook, so that the growing region was beyond the reach of the wound‐induced depolarization (up to 40mm). Even at such a distance, the cut still caused a considerable and rapid drop in the hypocotyl growth rate. This growth response is not a direct wound response because it does not result from the cut‐induced depolarization and because it can be simulated by root pressure manipulation (using a pressure chamber). The results indicate that the growth response resulted from the rapid release of the xylem pressure upon excision. To test this conclusion we measured the xylem pressure by connecting a pressure probe to the cut surface of the stem. Xylem pressure (Px) was found to be +10 to +40kPa in cucumber hypocotyls and ‐5 to ‐10 kPa or lower in pea epicotyls. Excision of the cucumber hypocotyl base led to a rapid drop in Px to negative values, whereas excision in pea led to a rapid rise in Px to ambient (zero) pressure. These fast and opposite px changes parallel the excision‐induced changes in growth rate (GR): a decrease in cucumber and a rise in pea.

The sign of the endogenous xylem pressure also determined whether excision induced a propagating depolarization in the form of a slow wave potential (SWP). Under normal circumstances pea seedlings generated an SWP upon excision whereas cucumber seedlings failed to do so. When the Px in cucumber hypocotyls was experimentally inverted to negative values by incubating the cumber roots in solutions of NaCN or n‐ethylmaleimide, excision caused a propagating depolarization (SWP). The experiment shows that only hydraulic signals in the form of positive Px steps are converted into propagating electric SWP signals. These propagating depolarizations might be causally linked to systemic ‘wound’ responses, which occur independently of the short‐distance or direct wound responses.
Limitation of net CO2 assimilation rate by internal resistances to CO2 transfer in the leaves of two tree species (Fagus sylvatica L. and Castanea sativa Mill.)
Authors: EPRON D. , GODARD D. , CORNIC G. , GENTY B.
Published: April 2006

Using a combination of gas‐exchange and chlorophyll fluorescence measurements, low apparent CO2/O2 specificity factors (1300 mol mol−1) were estimated for the leaves of two deciduous tree species (Fagus sylvatica and Castanea sativa). These low values contrasted with those estimated for two herbaceous species and were ascribed to a drop in the CO2 mole fraction between the intercellular airspace (Ci) and the catalytic site of Rubisco (Cc) due to internal resistances to CO2 transfer. Cc. was calculated assuming a specificity of Rubisco value of 2560 mol mol−1. The drop between Ci and Cc was used to calculate the internal conductance for CO2 (gi). A good correlation between mean values of net CO2 assimilation rate (A) and gi was observed within a set of data obtained using 13 woody plant species, including our own data. We report that the relative limitation of A, which can be ascribed to internal resistances to CO2 transfer, was 24–30%. High internal resistances to CO2 transfer may explain the low apparent maximal rates of carboxylation and electron transport of some woody plant species calculated from A/Ci curves.
The phototropic response of Triticum aestivum coleoptiles under conditions of low gravity
Authors: HEATHCOTE D. G. , BROWN A. H. , CHAPMAN D. K.
Published: April 2006

Wheat (Triticum aestivum L. cv Broom) coleoptiles were stimulated by unilateral blue light pulses, the duration of which varied between 3 s and 30 min, under microgravity conditions on the IML‐1 Spacelab mission and in ground controls. The stimuli covered first positive, indifferent and second positive response regions. When phototropic responses were observed under low‐gravity conditions, slightly fewer seedlings responded compared with 1 g conditions. The latent period was similar in flight and 1g treatments (10–20 min), except for the response to 300,μmol m−2 in the indifferent response region, where a positive response in flight plants followed a 130 min latent period, while no response was observed in 1 g plants. First positive responses at Og were slightly enhanced, both in magnitude and in duration, compared to the 1 g controls, but not to the extent predicted by clinostat studies. The response kinetics in the second positive region at 1g showed a plateau at 120 min, in contrast to the single maximum at Og. The fluence‐response relationship was similar in both flight and ground controls. Only the responses to 4 and 6μmolm‐2 showed significantly greater curvatures at 9g. This contrasts with previous clinostat studies, which reported substantial response enhancement at all fluence levels.
Effects of salinity on seed set in rice
Authors: KHATUN S. , FLOWERS T. J.
Published: April 2006

Salinity reduces fertility in rice (Oryza sativa L.), but little is known of the underlying cause(s). In order to determine the relative importance of pollen viability and stigmatic receptivity for seed setting, plants of the rice cultivar IR36 were treated with ‘artificial’ sea water (0,10, 25 or 5Omol−3 with respect to NaCl) from 1 month after germination until the main tiller flowered.

An increase in the salinity in the medium resulted in a decrease in the number of fertile florets and in the viability of pollen as determined both by pollen germination and by pollen staining with the tetrazolium salt 3‐(4,5‐dimethyl‐ithyazolyl)‐2,5‐diphenyl monotetrazolium bromide.

In order to assess the effects of salt on stigmas, seed production was measured for salt‐grown and non‐salt‐grown female plants pollinated with viable pollen (from plants grown in the absence of salt). The percentage of seed set was reduced by 38% when the female plants were grown in 1Omol m−3 Na and by 72% at 25mol m−3 Na: no seed setting was recorded for plants grown in 50mol m−3 Na. Comparisons between crosses involving male and female parents grown at different salinities indicated that effects on the female plants dominated those on pollinator plants. Mineral analysis of leaves of different ages showed that there was a gradient of K concentration from leaf to leaf which was opposite to that of Na and Cl at all levels of applied salinity: K was maximal in the flag leaf, where Na and Cl were minimal. Analysis also revealed that there was an increase in the concentrations of Na and Cl and a decrease in the concentration of K in the pollen grains and stigmas of plants subjected to saline conditions. Correlations between the concentration of Na and Cl in pollen and pollen staining and pollen germination in vitro suggest that Na and Cl perse were responsible for the poor viability. The change in ionic concentrations in pollen and stigmas was much larger than that in the younger leaves, and in particular very much larger than that in the lemmas and paleas.
Relations between vulnerability to xylem embolism and xylem conduit dimensions in young trees of Quercus corris
Authors: LO GULLO M. A. , SALLEO S. , PIACERI E. C. , ROSSO R.
Published: April 2006

Measurements of xylem conduit length and width and the distribution of xylem conduit ends were made in inter‐nodes (I), nodes (N) and twig junctions (J) of 1‐, 2‐ and 3‐year‐old twigs of plants of Quercus cerris L. Parallel measurements were also made of the loss of hydraulic conductivity of twigs subjected to pressure differentials across conduit pit membranes, equalling the leaf water potential at the turgor loss point. The loss of theoretical hydraulic conductivity was calculated as the ratio of i esivr4 (where r is the conduit radius) of the non‐conducting conduits to that of all the conduits in the outermost wood ring of I, N and J. Stem zones such as 1‐year‐old nodes and junctions were localized with narrower and shorter xylem conduits and with higher percentages of conduit ends than internodes. Such ‘constricted zonesrsquo; were less vulnerable to embolism than internodes. Latewood conduits were consistently narrower, shorter and less vulnerable to embolism than earlywood ones. A positive relation therefore existed between conduit diameter and length and vulnerability to embolism. The overall vulnerability to embolism of Q. cerris plants is discussed in terms of xylem conduit width and length and of the distribution of conduit ends.
Shoots, roots and ectomycorrhiza formation of pine seedlings at elevated atmospheric carbon dioxide
Authors: INEICHEN K. , WIEMKEN V. , WIEMKEN A.
Published: April 2006

The effect of elevated atmospheric CO2 concentration on the growth of shoots, roots, mycorrhizas and extraradical mycorrhizal mycelia of pine (Pinus silvestris L.) was examined. Two and a half‐month‐old seedlings were inoculated axenically with the mycorrhizal fungus Pisolithus tincto‐rius (Pers.) by a method allowing rapid mycorrhiza formation in Petri dishes. The plants were then cultivated for 3 months in growth chambers with daily concentrations of 350 and 600 μmol mol−1 CO2 during the day. Whereas plants harvested after 1 and 2 months did not differ appreciably between ambient and increased CO2 concentrations, after 3 months they developed a considerably higher root biomass (%57%) at elevated CO2, but did not increase significantly in root length. The mycorrhizal fungus Pisolithus tinctorius, which depended entirely on the plant assimilates in the model system, grew much faster at increased CO2: 3 times more mycorrhizal root clusters were formed and the extraradical mycelium produced had twice the biomass at elevated as at ambient CO2. No difference in shoot biomass was found between the two treatments after 91 d. However, since the total water consumption of seedlings was similar in the two treatments, the water use efficiency was appreciably higher for the seedlings at increased CO2 because of the higher below‐ground biomass.
Phytochrome A, phytochrome B and HY4 are involved in hypocotyl growth responses to natural radiation in Arabidopsis: weak de‐etiolation of the phyA mutant under dense canopies
Authors: YANOVSKY M. J. , CASAL J. J. , WHITELAM G. C.
Published: April 2006

The roles of phytochrome A (phyA), phytochrome B (phyB) and a putative blue‐light (BL) photoreceptor (HY4) in the control of hypocotyl growth by natural radiation were investigated using phyA, phyB and hy4 mutants of Arabidopsis thaliana. Full sunlight inhibited hypocotyl growth to a larger extent in wild‐type (WT) than in phyA, phyB and, particularly, hy4 seedlings. In WT seedlings, hypocotyl growth was promoted by selectively lowering BL irradiance, lowering red‐light (R) plus far‐red‐light (FR) irradiance or lowering the R/FR ratio (which was achieved either by increasing FR or by reducing R). The effects of lowering BL were reduced in hy4 and exaggerated in phyA seedlings. The effects of lowering R+FR were reduced in phyA and exaggerated in hy4 seedlings. Neither phyB nor hy4 mutants responded to low R/FR ratios. Neighbouring plants reflecting FR without shading caused subtle reductions of the R/FR ratio. This signal promoted hypocotyl growth in WT but not in phyA, phyB or hy4 seedlings. Intermediate canopy shade produced similar effects in all genotypes. Under deep shade, de‐etiolation was severely impaired in phyA seedlings, which died prematurely. Thus, the FR ‘high‐irradiance reaction’ mediated by phyA could be important for seedling survival under dense canopies.
Guard cell pressure/aperture characteristics measured with the pressure probe
Authors: FRANKS P. J. , COWAN I. R. , TYERMAN S. D. , CLEARY A. L. , LLOYD J. , FARQUHAR G. D.
Published: April 2006

Pressure within guard cells in strips of intact epidermis of Tradescantia virginiana was controlled with a pressure probe apparatus after the guard cells had been filled with silicone oil. Pressure was increased and decreased incrementally between 0.0 and 4.1 MPa to cause inflation and deflation of the guard cells. At steady‐state guard cell pressures, the width of the stomatal pore was recorded and plotted against pressure. The pressure required for near‐maximum aperture was 4.1 MPa. Aperture as a function of pressure was sigmoidal.
Two proteins regulate the cell wall extensibility and the yield threshold in glycerinated hollow cylinders of cowpea hypocotyl
Authors: OKAMOTO A. , OKAMOTO H.
Published: April 2006

Glycerinated hollow cylinders of hypocotyl segments excised from the elongation region of cowpea seedlings were heated for 15s in 50% glycerol at 70, 80 or 90°C. Their in vitro yield threshold tension (y) and extensibility (φ) were determined by stress‐strain experiments under the perfusion of solutions of pH 4·0 or 6·2. The decrement in y and the increment in φ with acidification were extinguished at 80 and 90°C, respectively. Moreover, such changes in φ and y with acidification were prevented by proteinase treatment for 6 and 10 h, respectively. These results suggest that these two cell wall mechanical properties are controlled, respectively, by two functional proteins activated by acid.
Photosynthetic inorganic carbon uptake and accumulation in two marine diatoms
Authors: COLMAN B. , ROTATORE C.
Published: April 2006

Some physiological characteristics of photosynthetic inorganic carbon uptake have been examined in the marine diatoms Phaeodactylum tricornutum and Cyclotella sp. Both species demonstrated a high affinity for inorganic carbon in photosynthesis at pH7.5, having K1/2(CO2) in the range 1.0 to 4.0mmol m−3 and O2− and temperature‐insensitive CO2 compensation concentrations in the range 10.8 to 17.6 cm3 m−3. Intracellular accumulation of inorganic carbon was found to occur in the light; at an external pH of 7.5 the concentration in P. tricornutum was twice, and that in Cyclotella 3.5 times, the concentration in the suspending medium. Carbonic anhydrase (CA) was detected in intact Cyclotella cells but not in P. tricornutum, although internal CA was detected in both species. The rates of photosynthesis at pH 8.0 of P. tricornutum cells and Cyclotella cells treated with 0.1 mol m−3 acetazolamide, a CA inhibitor, were 1.5‐ to 5‐fold the rate of CO2 supply, indicating that both species have the capacity to take up HCO3− as a source of substrate for photosynthesis. No Na+ dependence for HCO3− could be detected in either species. These results indicate that these two marine diatoms have the capacity to accumulate inorganic carbon in the light as a consequence, in part, of the active uptake of bicarbonate.
Gravity perception in plants: a multiplicity of systems derived by evolution?
Authors: BARLOW P. W.
Published: April 2006

The origin and subsequent evolution of life on Earth have taken place within an environment where a 1g gravitational field is omnipresent. Living organisms, at whatever stage in their evolution, have accommodated this variable in both their structure and their function. Systems have also evolved whereby gravitational accelerations are perceived by gravisensors and these, in turn, have led to responses that give particular spatial orientations to living processes. It is proposed that, the higher the evolutionary status of an organism, the more likely it is that it will possess multiple systems for gravisensing because evolution discards little that assists fitness and hence supplements with new gravisensing systems those which already existed within evolutionary older, less complex organisms. Moreover, in comparison with a single gravisensing system, a multiplicity of systems permits gravity to participate in a wider range of developmental programmes, such as taxes, morphisms and tropisms, through the action of different sensory mechanisms coupled to distinct signalling and response pathways. Whatever the precise mechanism of graviperception in any given set of conditions, all may transduce the g‐forcc by means of a membrane system. Transduction may involve the endoplasmic rcticulum and thence the plasma membrane.
Above‐ground nitrogen dynamics following the complete pruning of a nodulated woody legume in humid tropical field conditions
Authors: NYGREN P.
Published: April 2006

Above‐ground nitrogen fluxes following a complete pruning of the neotropical agroforestry tree Erythrina poeppi‐giana (Walpers) O.F. Cook (Leguminosae: Phaseoleae) were studied during pruning intervals of 26 and 23 weeks in the humid tropics of Costa Rica. The experiment included four clones and a half‐sib family of the species. The leaf‐level N fluxes were modelled as a function of leaf age, and were scaled to the canopy level on the basis of the leaf age distribution. The N content of young leaves increased exponentially, followed by relatively small changes at maturity. Slight net N retranslocation started at the middle of leaf life span, but a major N efflux occurred only a few days before leaf shedding. It was estimated that, depending on the tree source, 29‐44& of the total N accumulation in the above‐ground biomass occurred during 11 weeks after pruning (WAP), before initiation of nodulation. The nodule biomass peaked at the time leaf shedding started (15–17 WAP). Considerable rapid N efflux from the foliage was observed at this time, suggesting regulation of nodulation by a feedback from the foliage. The implications of the N economy of E. poep‐pigiana on its management in agroforestry systems are discussed.
Stomatal behaviour in a beech canopy: an analysis of Bowen ratio measurements compared with porometer data
Authors: HERBST M.
Published: April 2006

On the basis of measurements or stand transpiration and microclimate, the bulk stomatal or bulk leaf conductance (gL) of a beech forest in northern Germany was calculated for periods in which leaves were fully expanded and the canopy was dry. This conductance depends strongly on light and humidity conditions above the forest. During periods with photosynthetic photon flux densities Q > 1200 μmol m−2s−1, gL was reduced from 1500mmol m−2s−1 at a vapour pressure deficit D= 0.5kPa to 500 mmol m−2s−1 at D= 2kPa. Light saturation of gL was not reached until Q= 1200 μmol m−2s−1 at low D, or until even higher Q at higher D. The dependence of gL, on Q and D was described mathematically by a non‐linear equation that requires two empirical parameters. Values for gL as simulated by this equation provided a satisfactory agreement with independent porometer data collected on single leaves and scaled up to the canopy. A comparison of stomatal and aerodynamic conductances showed a strong coupling between the forest canopy and the atmosphere, indicating that transpiration of the beech forest is controlled mainly by the stomata.
Scaling processes and problems
Authors: JARVIS P. G.
Published: April 2006

A personal view is presented of current approaches to scaling processes and variables in the context of better understanding ecosystem function and predicting the consequences of global environmental change. Issues considered include spatial and temporal scales of interest, the scaling process, scaling strategy, scaling problems, heterogeneity, patchiness and non‐linearity, aggregation methodology and feedbacks.

Knowledge of processes in plants and vegetation is largely at small scales. The transfer of this knowledge up to larger spatial and longer temporal scales is an open‐ended process with potential errors arising from heterogeneity and patchiness in the distribution of processes and non‐linearities in the functional relationships between processes and environmental variables. Scaling now covers several orders of magnitude with respect to spatial and temporal scales with attendant risks of propagating errors.

At larger scales the wide diversity of vegetation classes poses a problem, and it is suggested that this can be countered by classifying classes of vegetation (not species) into a small number of ‘functional types’ of vegetation. Scaling through summation of component processes and through derivation of appropriately averaged parameters is considered. However, the increasing role of feedbacks at larger spatial and longer temporal scales is an essential feature of the scaling process. Thus, understanding the feedbacks and including them in upscaling schemes is a major priority.

A scaling strategy is outlined to minimize the propagation of errors. Because the scaling process is open‐ended it is essential that good models are used and tested at each increase in scale.
The role of nitrogen in a simple scheme to scale up photosynthesis from leaf to canopy
Authors: KULL O. , JARVIS P. G.
Published: April 2006

A simple analytical scheme, involving the distribution of nitrogen, to scale up photosynthesis from leaf to canopy is proposed. The scheme is based on the assumption that there are two pools of nitrogen in leaves: nitrogen in photosynthetic, degradable structures (Np) and nitrogen in non‐photosynthetic and non‐degradable structures (Ns). The rate of photon‐saturated photosynthesis, Fm, is assumed to be proportional to Np and is distributed inside the canopy similarly to photon flux density (PFD). Prior assumptions of an optimum distribution of nitrogen are not a prerequisite. Calculations made with the scheme lead to development of the hypothesis that the canopy can be treated as a ‘big leaf’ on the time scales involved in acclimation of photosynthesis to PFD. Simulations using parameters for tree species with different requirements for PFD show that shade‐tolerant species may have denser canopies than sun‐demanding species because of smaller amounts of non‐photosynthetic structural nitrogen and/or supporting tissue in their leaves.
Identification of amino compounds synthesized and translocated in symbiotic Parasponia
Authors: BAKER A. , DODD C. D. , PARSONS R.
Published: April 2006

We investigated the synthesis and translocation of amino compounds in Parasponia, a genus of the Ulmaceae that represents the only non‐legumes known to form a root nodule symbiosis with rhizohia. In the xylem sap of P. andersonii we identified asparagine. aspartate. glutamine, glutamated significant quantities of a non‐protein amino acid. 4‐methylglutamte(2‐amino‐4‐methylpentanedioic acid). This identification was confirmed by two methods, capillary gas chromatography (GC) electron ionization (El) mass spectrometry (MS) and reverse phase high pressure liquid chromatography (HPLC) analysis of derivatized compounds. In leaf, root and nodule samples from P. andersonii and P. parviflora we also identified the related compounds 4‐methyleneglutamate and 4‐methyleneglulamine. Using 15N2 labelling and GC‐Ms analysis of root nodule extracts we followed N2 fixation and ammonia assimilation in P. andersonii root nodules and observed Label initially in glutamine and subsequently in glutamate, suggesting operation of the glutamine synthetase/glutamine:2‐oxoglutarate aminotransferase (GS/GOGAT) pathway. Importantly, we observed the incorporation of significant quantities of 15N into 4‐methylglutamate in nodules, demonstrating the de nova synthesis of this non protein amino acid and suggesting a role in the translation of N in symbioticParasponia.
Relative growth rate correlates negatively with pathogen resistance in radish: the role of plant chemistry
Authors: HOFFLAND E. , NIEMANN G. J. , VAN PELT J. A. , PUREVEEN J. B. M. , EIJKEL G. B. , BOON J. J. , LAMBERS H.
Published: April 2006

Plant growth rate has frequently been associated with herbivore defence: a large investment in quantitative defence compounds occurs at the expense of growth. We tested whether such a relationship also holds for growth rate and pathogen resistance. For 15 radish (Raphanus sativus L.) cultivars, we determined the potential growth rate and the resistance to fungal wilt disease caused by Fusarium oxysporum. We subsequently aimed to explain a putative negative relationship between growth rate and resistance based on plant chemical composition. Both growth rate and resistance level varied greatly among cultivars. Moreover, there was a strong negative correlation between growth rate and resistance, i.e. there are costs associated with a high resistance level. Roots of slow‐growing, resistant cultivars have a higher biomass density. Using pyrolysis mass spectrometry. we part1y explained variation in both growth rate and resistance in terms of the same change in chemical composition. Leaves of slow‐growing, resistant cultivars contained more cell wall material. Surprisingly, roots of slow‐growing, highly resistant cultivars contained significantly less cell wall material, and more cytoplasmic elements (proteins). We speculate that this higher protein concentration is related to high construction and turn‐over costs and high metabolic activity. The latter in turn is thought to be responsible for a rapid and adequate resistance reaction, in which phenols may be involved.
The relationship between the relative growth rate and nitrogen economy of alpine and lowland Poa species
Authors: ATKIN O. K. , BOTMAN B. , LAMBERS H.
Published: April 2006

This study investigates the nitrogen economy of six altitudinally contrasting Poa species which differ in their relative growth rate (R). Two alpine (Poa fawcettiae and P. costiniana), one sub‐alpine (P. alpina)and three temperate lowland species (P. pratensis, P. campressa and P. trivialis) were grown hydroponically under identical conditions in a growth room. The low R exhibited by the alpine species was associated with lower plant organic nitrogen concentration (np) and lower nitrogen productivity (Πp, amount of biomass accumulation per mol organic nitrogen and time). The differences in Πp between the alpine and lowland species did not appear to be due to differences in the carbon concentration or the proportion of total plant organic nitrogen allocated to the leaves, stems or roots. Variations in ΠP were also not due to variations in photosynthetic nitrogen use efficiency (ΨN, the rate of photosynthesis per unit organic leaf nitrogen) or shoot or root respiration rates per unit organic nitrogen (ΛSH and ΛR, respectively) per se. Rather, the lower Λp in the alpine species was probably due to a combination of small variations in several of the parameters (e.g. slightly lower ΨN, slightly higher ΛSH and ΛR, and slightly higher proportions of total plant organic nitrogen allocated to the roots). The alpine species exhibited lower organic acid and mineral concentrations. However, no differences in whole‐plant construction costs (grams of glucose needed to synthesize one gram of biomass) were observed between She alpine and lowland Poa species. The lack of sub‐stantial differences in ΨN between the alpine and lowland species contrasts with the large differences in ΨN between slow‐ and fast‐growing lowland species that have been reported in the literature. The reasons for the unusually high ΨN values exhibited by the alpine Poa species are discussed.
Cell wall composition of leaves of an inherently fast‐ and an inherently slow‐growing grass species
Authors: GROENEVELD H. W. , BERGKOTTE M.
Published: April 2006

Differences in the relative growth rules of the inherently slow‐growing Deschampsia flexuosa L. and the inherently fast‐growing Holcus lanatus L. were reflected in cell wall synthesis in the elongation zone of the leaves. Leaf elongation rates depended on the size of the plant and ranged from 6 to 14 mm d−1 in Deschampsia and from 12 to 42 mm d−1 in Holcus. Anatomical data showed that the epidermis and vascular tissue are the important tissues controlling leaf extension. The cell wall polysaccharides of fully expanded leaves of the two species were identical in sugar composition. Enzymatic hydrolysis of polymeric sugars in the cell walls of the sheath and the lamina gave glucose (85%), arabinose (3.5%), fucose (0.5%), xylose (5.0%), mannose (0.5%), galaclose (0.8%) and galacturonic acid (3–4%). This composition applied throughout the blade and the sheath and did not change with ageing. Polysaccharides in the meristems of the two species showed identical sugar compositions with 51–55% glucose, 13–15% galactoronic acid and 13–14% arabinose as the main components. The extension zone was marked by a gradual increase of driselase‐digestable polymers (per mm tissue) and a concurrent shift in sugar composition. The massive increase of glucose in the cell wall polymers of the elongation zone is probably caused by cellulose synthesis. The rate of synthesis of cell wall polysaccharides in Holcus was twice as high as that in Deschampsia. The slower‐growing Deschampsia has more ferulic acid esterified with cell walls, which might contribute to the slowing of leaf growth. Lignin is not significantly deposited until growth has essentially ceased and is not responsible for the difference in growth rate.
Location of sugars in barley seeds during germination by NMR microscopy
Authors: ISHIDA N. , KOIZUMI M. , KANO H.
Published: April 2006

The distribution and fluctuation of sugars in germinating barley seeds were examined by13C nuclear magnetic resonance (NMR) spectroscopy,1H‐NMR imaging and1H‐NMR localized spectroscopy in relation to morphology. Maltose, sucrose, fructose and oils were detected in intact imbibed seeds by13C‐NMR spectra. During the first 6 d of germination, the maltose content increased and the oil content gradually decreased, whilst the levels of sucrose and fructose remained constant. Sugars were located by1H‐NMR images and1H‐NMR localized spectra in the vascular bundle of the seeds as well as in the solubilized endosperm. They were also detected in the shoots. The sugars detected in an 80% ethanol shoot extract were sucrose and glucose, which were located in the vascular bundles but not in the mesophyll cells of the coleoptile. They were also located in the basal part of the shoot, but not above 7 mm from the scutellum. The data suggest that the sugars are primarily transported through the vascular bundles and, at the same time, rapidly incorporated into mesophyll cells in the leaves.