bluuebunny/crossref_metadata_2025 huggingface dataset:
Components of pathozone behaviour
Authors: GILLIGAN C. A. , BAILEY D. J.
Published: March 2003

summary

Pathozone behaviour of soil‐borne plant pathogenic fungi is characterized by curves for the change in probability of infection with distance of inoculum from a host. We identify three major components that give rise to the pathozone profile for infection efficiency: the germinability of inoculum, given that it occurs in the pathozone (P1), the growth of the resulting fungal colony outwards to make contact with the host (P2), and the infectivity of mycelium once contact is made (P3). The probability of infection (P) is then given by the product, P1× P2× P3. Using Raphanus sativus and two contrasting types of inoculum of Rhizoctonia solani as a model experimental system, we measured each of the three components by quantifying germinability, colony architecture (to determine the chance of contact by one or more hyphae) and the change in susceptibility over time as a measure of infectivity as mycelium arrives from different distances away. The germinability of inoculum is uniform across the pathozone whereas the probability of contact declines with distance, and the net effect of host susceptibility and infectivity increases. The characteristics of these components result in pathozone profiles that vary in shape. Infection efficiency can decline in an exponential or sigmoidal fashion with increasing distance from the host or follow a curve that rises close to the host and then falls asymptotically to zero with increasing distance. Colony architecture was summarized by the distribution of the furthest extent of hyphal growth amongst colonies growing out from single inoculum units (described, because of its asymmetrical pattern, by a gamma distribution) and by the radial density of hyphae on concentric circles at different distances from the centre of the colony (described by a negative binomial distribution with parameters changing with distance).

The mechanistic model for the components of pathozone behaviour is tested by comparing the magnitude and shape of pathozone profiles predicted by the model with independent estimates for P obtained by placing inoculum at fixed distances from the host, and measuring the proportion of successful infections. The shape depends on the relative magnitudes and change with distance of the three components, P1 P2 and P3, which differed between the two types of inoculum. The profiles could be reproduced accurately when the probability of contact was based on the distribution and density of hyphae reaching the host rather than on the furthest extent of hyphal growth amongst colonies. The model is used to analyse the effects of replication and stochastic variability in the components of pathozone behaviour on the comparison of treatments for disease control.
Deterrence of herbivory by zinc hyperaccumulation in Thlaspi caerulescens (Brassicaceae)
Authors: POLLARD A. JOSEPH , BAKER ALAN J. M.
Published: March 2003

Plants known as hyperaccumulators take up and sequester high concentrations of potentially toxic elements from metalliferous soils. We tested the hypothesis that zinc hyperaccumulation benefits plants by deterring herbivory. In laboratory feeding trials, three species of herbivores were allowed to choose between Thlaspi caerulescens (Brassicaceae) plants grown in low‐Zn and Zn‐amended culture solution. Locusts (Schistocerca gregaria), slugs (Deroceras caruanae), and caterpillars (Pieris brassicae) all showed significant preferences for plants with lower foliar Zn concentrations. Such differential feeding could result in selection pressures favouring the evolution of hyperaccumulation. The findings are also relevant to current proposals to exploit hyperaccumulation as a means of remediating metal‐contaminated soils.
Plant nuclear migrations as indicators of critical interactions between resistant or susceptible cowpea epidermal cells and invasion hyphae of the cowpea rust fungus
Authors: HEATH MICHELE C. , NIMICHUK ZACHARY L. , XU HAIXIN
Published: March 2003

Living epidermal cells of cowpea (Vigna unguiculata (L.) Walp.) were examined by contrast‐enhanced video microscopy during penetration by invasion hyphae of the monokaryotic stage of the cowpea rust fungus (Uromyces vignae Barclay race 1). In resistant or susceptible host cvs, the plant nucleus migrated to the penetration site before the fungus had fully penetrated the plant wall and, at sites of unsuccessful infection, remained during the formation of a callose‐containing papilla. Nuclear migration was also induced by applying hemicellulase, but not H2O2, to localized sites of wall damage. Hemicellulase‐induced migration was inhibited by calcium chelators and a protein kinase inhibitor, but not catalase. In both resistant and susceptible cvs, the plant nucleus migrated away from successful infection sites at about the time that the fungal penetration peg made contact with the plant plasma membrane, and the epidermal cell showed no further cytological responses to the growth of the fungal intra‐epidermal vesicle. In the susceptible cv., the nucleus migrated back to the fungus when the latter initiated tip growth. Inhibitors of transcription or translation did not affect this migration and only slightly reduced fungal growth. In the resistant cv. in which the invaded cell exhibited a hypersensitive response (HR), the plant nucleus changed its appearance before the cessation of cytoplasmic streaming and usually did not migrate to the fungus, even if the latter initiated tip growth. Nuclear DNA cleavage usually followed the subsequent cessation of cytoplasmic streaming. Treatments that delayed cell death and increased fungal growth also increased the frequency of nuclear migration to the fungus. It is argued that these and other data suggest that U. vignae negates nonspecific, penetration‐induced, defence responses upon entering cells of both susceptible and resistant cultivars. The results also suggest that effects on the plant nucleus are one of the earliest signs of the HR in this system, often preceding the cessation of cytoplasmic streaming and detectable changes in plasma membrane permeability.
Functioning of mycorrhizal associations along the mutualism–parasitism continuum*
Authors: JOHNSON N. C. , GRAHAM J‐H. , SMITH F. A.
Published: March 2003

A great diversity of plants and fungi engage in mycorrhizal associations. In natural habitats, and in an ecologically meaningful time span, these associations have evolved to improve the fitness of both plant and fungal symbionts. In systems managed by humans, mycorrhizal associations often improve plant productivity, but this is not always the case. Mycorrhizal fungi might be considered to be parasitic on plants when net cost of the symbiosis exceeds net benefits. Parasitism can be developmentally induced, environmentally induced, or possibly genotypically induced. Morphological, phenological, and physiological characteristics of the symbionts influence the functioning of mycorrhizas at an individual scale. Biotic and abiotic factors at the rhizosphere, community, and ecosystem scales further mediate mycorrhizal functioning. Despite the complexity of mycorrhizal associations, it might be possible to construct predictive models of mycorrhizal functioning. These models will need to incorporate variables and parameters that account for differences in plant responses to, and control of, mycorrhizal fungi, and differences in fungal effects on, and responses to the plant. Developing and testing quantitative models of mycorrhizal functioning in the real world requires creative experimental manipulations and measurements. This work will be facilitated by recent advances in molecular and biochemical techniques. A greater understanding of how mycorrhizas function in complex natural systems is a prerequisite to managing them in agriculture, forestry, and restoration.
Acclimation of Salix to metal stress
Authors: PUNSHON TRACY , DICKINSON NICHOLAS M.
Published: March 2003

summary

Nine different clones of six species of Salix (Salix cordata Muhlenb. non Michaux, 5. fragilis L., S. caprta L. S. cinerea L., S. burjatica Nazarov. and S. viminalis L.) and one hybrid (S.×calodendron Wimm.) were exposed to heavy metals in solution culture in an attempt to increase innate metal resistance. Resistance was estimated using comparative root measurements, and metal uptake was also studied. The first experiment entailed pre‐treatments with background nutrient solution, or 0.25 and 0.50 mg Cu I‐1amendments, and re‐exposure to each of the same concentrations. In a second experiment clones were exposed to sub‐toxic concentrations of single metals (0.15 mg Cu I‐1, 01 3 mg Cd I‐1or 2–5mgZnI‐1and dual‐combination treatments (0.075 mgCu I1+ 0.075 mg Cd l1, 0.075 mg Cd 1‐1+ 125 mg Zn T1or 0.075 mg Cd I‐1+ 1.25 mg Zn I1) with concentrations gradually raised 10–fold over 128 d. Plants tested in the first experiment, following pre‐exposure to Cu, were no more resistant to subsequent exposure to this metal. In the second experiment, gradual cumulative doses resulted in reduced phytotoxicity and increased resistance, most notably to Cd. There appeared to be an inverse relationship between metal uptake and resistance. Copper uptake was restricted to the roots, whereas Cd and Zn were more evenly distributed throughout the plant. Exposure to dual combinations of metals resulted in several interaction effects on uptake: increased root‐bound Cu in all combinations, and the increase in uptake of both Cd and Zn into the root tissues when supplied with Cu. The implications of these results for the use of willows in phytoremediation programmes are discussed.
Characterization and identification of black alder ectomycorrhizas by PCR/RFLP analyses of the rDNA internal transcribed spacer (ITS)
Authors: PRITSCH K. , BOYLE H. , MUNCH J. C. , BUSCOT F.
Published: March 2003

summary

The identity of black alder (Alnus glutinosa (L.) Gaertn.) ectomycorrhizas was investigated using PCR/RFLP analysis of the ITS region from 16 morphotypes sampled at a 60‐yr‐old black alder stand. A comparison was made with restriction patterns from sporocarps of 28 mycobionts, of which 16 originated from the same stand, the remaining 12 came from two geographically distant alder stands. Eight of the mycorrhizal types could thus be identified, whereas eight mycorrhizal types remained unidentified. The identified mycorrhizas belonged to the genera Russula, Lactarius, Naucoria and Cortinarius. Four of the identified ectomycorrhizal types had identical PCR/RFLP profiles to corresponding fruit bodies from all investigated stands with no detectable intraspecific variation, despite the geographical distance of c.300 km between the sampling locations. By contrast, intraspecific variation between sporocarps from the different locations was detected in Paxillus rubicundulus, mycorrhizas of which were not found. The diversity of fruiting alder mycobionts at the main experimental plot only partly matched the diversity observed from mycorrhizas when comparing their PCR/RFLP profiles. The results are discussed regarding sampling techniques, PCR/RFLP analyses and ecological aspects.
Elevated CO2 enhances below‐ground C allocation in three perennial grass species at different levels of N availability
Authors: COTRUFO M. F. , GORISSEN A.
Published: March 2003

summary

Three perennial grass species, Lolium perenne L., Agrottis capillaris L. and Festuca uvina L., were homogeneously labelled in phytotrons with 14CO2 at two CO2 concentrations (350 and 700μl l−1). Plants were grown under two nitrogen regimes: one with a minor addition of 8 kg N ha−1, the other with an addition of 278 kg N ha−1. Carbon allocation over the different compartments of the plant/soil systems was measured: shoots, roots, rhizosphere soil (soil solution, microbial biomass and soil residue), and bulk soil. Elevated CO., increased total net 14C recovery in all species by 14%, and significantly enhanced the below‐ground 14C allocation by 26%, this enhancement was 24%, 39% and 21 % for root, rhizosphere soil and bulk soil, respectively. Within the rhizosphere soil, the 14C amounts in the soil solution (+ 69%) and soil residue (+ 49%,) increased significantly. Total microbial biomass‐C in the rhizosphere soil was also increased (15 %) by the elevated CO2 treatment, but only in proportion to the increased root mass. No interactions were observed between the elevated CO2, and N treatments. The N treatment increased total net 14C recovery by more than 300% and 14C was preferentially allocated to the shoots, leading to a significant increase in shoot‐to‐root ratio. However, N fertilization also increased (+111%) the absolute amount of 14C in soil. The three species behaved differently, but no interactions were observed between CO2 treatment and plant species. These results show that elevated CO2 induces an increased C input into soil for all three grass species at both N levels. However, the highest absolute amounts were found in the soils of the fastest growing species and at the highest N level.
Regulation of photosynthetic pigments in micro‐algae by multiple environmental factors: a dynamic balance hypothesis
Authors: KANA TODD M. , GEIDER RICHARD J. , CRITCHLEY CHRISTA
Published: March 2003

summary

Environmental effects on the concentration of photosynthetic pigments in micro‐algae can be explained by dynamics of photosystem synthesis and deactivation. A model that couples photosystem losses to the relative cellular rates of energy harvesting (light absorption) and assimilation predicts optimal concentrations of light‐harvesting pigments and balanced energy flow under environmental conditions that affect light availability and metabolic rates. Effects of light intensity, nutrient supply and temperature on growth rate and pigment levels were similar to general patterns observed across diverse micro‐algal taxa. Results imply that dynamic behaviour associated with photophysical stress, and independent of gene regulation, might constitute one mechanism for photo‐acclimation of photosynthesis.
The atmospheric budget of oxidized nitrogen and its role in ozone formation and deposition
Authors: FOWLER DAVID , FLECHARD CHRIS , SKIBA UTE , COYLE MHAIRI , CAPE J. NEIL
Published: March 2003

Emissions of reactive oxidized nitrogen (NO and NO2), collectively known as NOx, from human activities are c. 21 Tg N annually, or 70% of global total emissions. They occur predominantly in industrialized regions, largely from fossil fuel combustion, but also from increased use of N fertilizers. Soil emissions of NO not only make an important contribution to global totals, but also play a part in regulating the dry deposition of NO and NO2 (NOx) to plant canopies. Soil microbial production of NO leads to a soil ‘compensation point’ for NO deposition or emission, which depends on soil temperature, N and water status. In warm conditions, the net emission of NOx from plant canopies contributes to the photochemical formation of ozone. Moreover, the effect of NOx emissions from soil is to reduce net rates of NO2 deposition to terrestrial surfaces over large areas.

Increasing anthropogenic emissions of NOx have led to an approximate doubling in surface O3 concentrations since the last century. NOx acts as a catalyst for the production of O3 from volatile organic compounds (VOCs). Paradoxically, emission controls on motor vehicles might lead to increases in O3 concentrations in urban areas.

Removal of NO and NO2 by dry deposition is regulated to some extent by soil production of NO; the major sink for NO2 is stomatal uptake. Long‐term flux measurements over moorland in Scotland show very small deposition rates for NO2 at night and before mid‐day of 1–4 ng NO2‐N m−2 s−1, and similar emission rates during afternoon. The bi‐directional flux gives 24‐h average deposition velocities of only 1–2 mm s−1, and implies a long life‐time for NOx due to removal by dry deposition.

Rates of removal of O3 at the ground are also influenced by stomatal uptake, but significant non‐stomatal uptake occurs at night and in winter. Measurements above moorland showed 40% of total annual flux was stomatal, with 60% non‐stomatal, giving nocturnal and winter deposition velocities of 2–3 mm s−1 and daytime summer values of 10 mm s−1. The stomatal uptake is responsible for adverse effects on vegetation. The critical level for O3 exposure (AOT40) is used to derive a threshold O3 stomatal flux for wheat of 0·5 μg m−2 s−1. Use of modelled stomatal fluxes rather than exposure might give more reliable estimates of yield loss; preliminary calculations suggest that the relative grain yield reduction (%) can be estimated as 38 times the stomatal ozone flux (g m−2) above the threshold, summed over the growing season.
Impact of gaseous nitrogen deposition on plant functioning
Authors: STULEN I. , PEREZ‐SOBA M. , DE KOK L. J. , VAN DER EERDEN L.
Published: March 2003

Dry deposition of NH3 and NOx (NO and NO2) can affect plant metabolism at the cellular and whole‐plant level. Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH3 dissolves to form NH4+, whereas NO2 dissolves to form NO3− and NO2−. The latter compound can also be formed after exposure to NO. There is evidence that NH3‐N and NOx‐N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re‐emission. Once formed, NO3− and NO2− can be reduced, and NH4+ can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. Fumigation with low concentrations of atmospheric NH3 increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation with low concentrations of NH3 and in vitro GDH activity. The contribution of atmospheric NH3 and NO2 deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates (RGRs). It is concluded that at current ambient atmospheric N concentrations the direct impact of gaseous N uptake by foliage on plant growth is generally small.
Vulnerability of xylem to embolism in relation to plant hydraulic resistance in Quercus pubescens and Quercus ilex co‐occurring in a Mediterranean coppice stand in central Italy
Authors: TOGNETTI R. , LONGOBUCCO A. , RASCHI A.
Published: March 2003

The seasonal patterns of xylem embolism and xylem transport properties in Quercus pubescens Willd. and Quercus ilex L. trees growing in a natural mixed coppice stand in conditions of severe water stress were investigated. Xylem embolism was evaluated in both dehydrating branches and in apical twigs during a whole year. Measurements of xylem water potential were conducted from predawn to sunset on selected sunny days. On the same days, diurnal courses of leaf conductance were monitored. Measurements of half‐hourly sap flow were made by the heat‐pulse technique throughout the summer. At the onset of summer, a sharp decrease in water potential was observed in both species. Full recovery of water potentials was observed for both species after the first major rainfall event in September. Both experienced serious embolism throughout the year, ranging between minima of c. 60% (expressed as percentage loss of hydraulic conductivity) after the rains in autumn and after bud burst in spring, and maxima of c. 80% during summer and after freezing‐thawing events during the winter season. A significant negative linear relationship was found between water potential and xylem embolism in branches dehydrating in air for Q. pubescens and Q. ilex. Q. pubescens had greater efficiency in hydraulic transport (higher specific conductivity and leaf specific conductivity) by the xylem than Q. ilex. In June, leaf conductance was high early in the morning and decreased gradually during the day. Midday depression of leaf conductance, as a result of high evaporative demand combined with water deficit, was observed in both species. In August, leaf conductance of both species was greatly reduced, as water potential dropped to extremely low values, and the stomata were almost completely closed during the afternoon. No hysteresis resulting from plant capacitance was observed in the relationship between shoot water potential and sap flow. Q. pubescens exhibited very high values of whole‐tree hydraulic resistance between July and September, whereas Q. ilex generally showed lower values. The effect of soil moisture depletion on the relationship between sap flow and shoot water potential appears as a lowering of water potential at zero flow. A significant decrease of whole‐tree hydraulic resistance in both species was observed with the onset of the autumn, preceding the partial recovery of twig hydraulic conductivity. The results demonstrate that both Q. pubescens and Q. ilex, although highly tolerant of severe water stress and tissue dehydration, operate at the limits of safety which are surpassed under severe droughts, and prolonged climatic stress might predispose these Quercus species to decline.
Selectivity of photobiont choice in a defined lichen community: inferences from cultural and molecular studies
Authors: BECK A. , FRIEDL T. , RAMBOLD G.
Published: March 2003

Axenic cultures of lichen photobionts isolated from bark‐inhabiting lichen thalli of the Physcietum adscendentis Ochsner were identified by light microscopy and sequence comparisons of internal transcribed spacer rDNAs to investigate principles of lichenization within a defined lichen sociological unit. The photobiont identity of eight lichen species is reported for the first time (photobiont species in square brackets): Lecania cyrtella (Ach.) Th. Fr. [Trebouxia arboricola Puym.], Lecania naegelii (Hepp) Diederich & v. d. Boom [Dictyochloropsis symbiontica Tscherm.‐Woess], Candelaria concolor (Dicks) B. Stein [Trebouxia jamesii (Hildreth & Ahmadjian) Gärtner], Candelariella cf. reflexa (Nyl.) Lettau [T. jamesii], Lecanora spec. [T. arboricola], Phaeophyscia orbicularis (Neck.) Moberg [T. impressa Ahmadjian], Physcia adscendens (Fr.) H. Olivier [T. impressa] and Lecidella elaeochroma (Ach.) M. Choisy [T. arboricola] and could be confirmed for another two species, Physcia stellaris (L.) Nyl. [Trebouxia impressa] and Xanthoria parietina (L.) Th. Fr. [Trebouxia arboricola]. The observation that pioneer lichens without vegetative propagules, growing on smooth bark, had Trebouxia arboricola as photobiont can be explained by the assumption of a free‐living population of Trebouxia arboricola. Species of photobionts from Xanthoria parietina were morphologically and genetically different from those of Physcia adscendens and Phaeophyscia orbicularis, respectively; a finding that does not support the previous assumption that Xanthoria parietina takes over its algal partner from a Physcia species, at least at the sites investigated.
Temporal variation in temperature and rainfall differentially affects ectomycorrhizal colonization at two contrasting sites
Authors: SWATY RANDY L. , GEHRING CATHERINE A. , VAN ERT MATT , THEIMER TAD C. , KEIM PAUL , WHITHAM THOMAS G.
Published: March 2003

We examined the roles that seasonal shifts in precipitation and temperature played in the ectomycorrhizal (ECM) colonization of pinyon pine (Pinus edulis Engelm.) at two contrasting sites in northern Arizona. Pinyons growing in ash and cinder soils experienced much greater water and nutrient stress than pinyons growing nearby in sandy‐loam soils. Over a one year period, we obtained monthly measurements of ECM colonization, root zone soil moisture and temperature, and air temperature and precipitation. Four major patterns emerged. Firstly, although climate as measured by ambient temperature and precipitation did not vary between the two sites, soil temperature was significantly higher and soil moisture significantly lower at the cinder site than at the sandy‐loam site. Secondly, ECM colonization was significantly higher at the cinder site for 5 of 12 months. Thirdly, although nearly 70% of the variation in ECM colonization of pinyons growing in cinder soil was predicted by a combination of soil moisture and soil temperature, these same variables had little predictive power for pinyons growing in sandy‐loam soils. Air temperature and precipitation were also significantly correlated with ECM colonization at the cinder site but not the sandy‐loam site. Fourthly, a watering experiment showed that ECM colonization significantly increased with supplemental water at the cinder site, but not at the sandy‐loam site. Thus, in two sites that did not differ in plant community or climate, ectomycorrhizas in cinder soils were far more sensitive to changes in moisture and temperature than ectomycorrhizas in sandy‐loam soils.
Root growth, developmental changes in the apex, and hydraulic conductivity for Opuntia ficus‐indica during drought
Authors: DUBROVSKY JOSEPH G. , NORTH GRETCHEN B. , NOBEL PARK S.
Published: March 2003

Developmental changes in the root apex and accompanying changes in lateral root growth and root hydraulic conductivity were examined for Opuntia ficus‐indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper soil layers. During 7 d of rapid drying (in containers with a 3‐cm depth of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not due to meristem exhaustion but rather to meristem mortality after 3 d of drying. The length of the meristem, the duration of the cell division cycle, and the length of the elongation zone were unchanged during rapid drying. During 14 d of gradual drying (in containers with a 6‐cm depth of vermiculite), root mortality was relatively low; the length of the elongation zone decreased by 70%, the number of meristematic cells decreased 30%, and the duration of the cell cycle increased by 36%. Root hydraulic conductivity (LP) decreased to one half during both drying treatments; LP was restored by 2 d of rewetting owing to the emergence of lateral roots following rapid drying and to renewed apical elongation following gradual drying. Thus, in response to drought, the apical meristems of roots of O. ficus‐indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in response to rainfall in the field can be enhanced by lateral root proliferation near the soil surface and additionally by resumption of apical growth for deeper roots.
Generalities in the growth, allocation and leaf quality responses to elevated CO2 in eight woody species
Authors: CORNELISSEN J. H. C. , CARNELLI A. L. , CALLAGHAN T. V.
Published: March 2003

This paper reports general patterns of relative growth rate and related traits in response to elevated atmospheric CO2 in eight woody species ranging widely in life form, leaf habit, taxonomy and ecology. Young plants of these species, all of comparable ontogenetic phases, were grown simultaneously in large containers with favourable nutrient and water availability in transparent outdoor chambers at 350 and 700 μl l−1 CO2 for one growing season. We found the following consistent responses. (1) All species grew faster at elevated CO2, whereas the following leaf and allocation traits were consistently lower in CO2‐enriched environments: specific leaf area (quotient of leaf area and leaf weight), leaf area ratio (quotient of total leaf area and plant weight), weight‐based foliar N concentration and, to a smaller extent, leaf weight fraction (quotient of leaf weight and plant weight). (2) There was important interspecific variation in the magnitude of the response of relative growth rate to CO2. Specific leaf area at ambient CO2 explained 88% of the variation in relative growth rate response to CO2 among the eight species. At ambient CO2, relative growth rate itself, was significantly correlated with the relative growth rate response to CO2 only if the leafless species Ulex gallii was excluded from analysis. (3) The four deciduous species had a significantly stronger relative growth rate response to CO2 than the four evergreens. This corresponded with their generally higher specific leaf area. (4) Specific leaf area and leaf habit might be useful for scaling up exercises, as easy‐to‐measure substitutes for growth responses of (woody) vegetation to elevated CO2. However, the usefulness of such traits in this context needs to be tested in realistic, longer‐term manipulative experiments in real ecosystems.
Evidence that differences in phosphate metabolism in mycorrhizas formed by species of Glomus and Gigaspora might be related to their life‐cycle strategies
Authors: BODDINGTON C. L. , DODD J. C.
Published: March 2003

A glasshouse experiment was done to assess the development and phosphate metabolism of mycorrhizas formed by species of arbuscular mycorrhizal fungi (AMF) from two different genera, Gigaspora and Glomus on Desmodium ovalifolium plants at three concentrations of a phosphate source. The addition of phosphate (0–100 mg P kg−1) had no effect on the alkaline phosphatase activity, stained histochemically, in the intra‐radical mycelium of Gigaspora rosea (BEG111), but decreased that of Glomus manihotis (BEG112) over a 10‐wk period. The alkaline phosphatase activity of the extra‐radical mycelium was unaffected by increasing phosphate addition (0–100 mg P kg−1) in both species of AMF over a 10‐wk period. The extra‐radical mycelium of Gi. rosea (BEG111) accumulated polyphosphate, determined by staining with 4′,6‐diamidino‐2‐phenylindole, whereas polyphosphate was not detected in the extra‐radical mycelium of G. manihotis (BEG112). This work indicates differences in the mechanisms of phosphate metabolism in the mycelium of AMF from different genera on a tropical host. This might be determined by the life‐cycle strategies of these fungi, in particular the formation of auxiliary cells in Gigaspora. The possibility of a negative‐feedback mechanism between alkaline phosphatase and polyphosphate in the extra‐radical mycelium of Gi. rosea (BEG111) and the role of polyphosphate in the symbiosis are discussed.
Biotic and abiotic consequences of differences in leaf structure
Authors: GUTSCHICK VINCENT P.
Published: March 2003

Both within and between species, leaves of plants display wide ranges in structural features. These features include: gross investments of carbon and nitrogen substrates (e.g. leaf mass per unit area); stomatal density, distribution between adaxial and abaxial surfaces, and aperture; internal and external optical scattering structures; defensive structures, such as trichomes and spines; and defensive compounds, including UV screens, antifeedants, toxins, and silica abrasives. I offer a synthesis of selected publications, including some of my own. A unifying theme is the adaptive value of expressing certain structural features, posed as metabolic costs and benefits, for (1) competitive acquisition and use of abiotic resources (such as water, light and nitrogen) and (2) regulation of biotic interactions, particularly fungal attack and herbivory. Both acclimatory responses in one plant and adaptations over evolutionary time scales are covered where possible. The ubiquity of trade‐offs in function is a recurrent theme; this helps to explain diversity in solutions to the same environmental challenges but poses problems for investigators to uncover numerous important trade‐offs. I offer some suggestions for research, such as on the need for models that integrate biotic and abiotic effects (these must be highly focused), and some speculations, such as on the intensity of selection pressures for these structures.
A comparison of specific leaf area, chemical composition and leaf construction costs of field plants from 15 habitats differing in productivity
Authors: POORTER HENDRIK , DE JONG ROB
Published: March 2003

Laboratory experiments have shown a large difference in specific leaf area (SLA, leaf area: leaf mass) between species from nutrient‐poor and nutrient‐rich habitats, but no systematic difference in the construction costs (the amount of glucose required to construct 1 g biomass). We examined how far these patterns are congruent with those from field‐grown plants. An analysis was made of the vegetation in a range of grasslands and heathlands differing in productivity. The SLA of the dominant species in 15 different habitats was determined, as well as chemical composition and construction costs of bulk samples of leaves. SLA in the field was generally lower than in the laboratory, but showed consistency in that the ranking across species remained the same. Species from highly productive habitats had higher SLA than those from sites of low productivity, although individual species sometimes deviated substantially from the general trend. Construction costs were similar for plants from different habitats. This was mainly due to the positive correlation between an expensive class of compounds (proteins) and a cheap one (minerals).
The functional significance of leaf structure: a search for generalizations
Authors: PRESS MALCOLM C.
Published: March 2003

The coupling between leaf structure and function is illustrated with reference to two examples, the C4 photosynthetic pathway and leaf pubescence. A distinction is made between function and functional significance. The latter is defined as the role, significance or consequence of a structure, whereas the former is more simply the action that a structure is capable of performing. Using the two examples, four generalizations are made concerning the relationships between structure, function and functional significance: the functional significance of leaf structure is environment‐dependent; the relationship between functional significance and structure is sometimes non‐intuitive; functional equivalency means that there is often more than one ‘solution’ to the same ‘constraint’; and the consequences of leaf structure can exert profound effects at levels of organization beyond those of the individual organism and may play a critical role in determining community structure and function, through interactions with other species and trophic levels. The importance of understanding the consequences in variation in leaf structure at the global scale is illustrated with reference to the issue of global climate change.
Heavy‐metal‐induced proline accumulation and its role in ameliorating metal toxicity in Chlorella vulgaris
Authors: MEHTA S. K. , GAUR J. P.
Published: March 2003

Exposure of Chlorella vulgaris to elevated concentrations of copper, chromium, nickel and zinc led to intracellular accumulation of high concentrations of these metals. Concomitantly, accumulation of free proline occurred, depending on the concentration of metals in the external medium or in the cell. The greater the toxicity or accumulation of a metal, the greater the amount of intracellular proline in algal cells. However, higher concentrations of copper and chromium were inhibitory to proline accumulation by the test organism. The accumulation of proline was triggered within a few hours of metal treatment. Test metals also induced lipid peroxidation; copper was the most efficient inducer whereas zinc was the least. Pretreatment of C. vulgaris with proline counteracted metal‐induced lipid peroxidation and potassium ion efflux. Thus the present work shows a protective effect of proline on metal toxicity through inhibition of lipid peroxidation.
Research review. Components of leaf dry mass per area – thickness and density – alter leaf photosynthetic capacity in reverse directions in woody plants
Authors: NIINEMETS ÜLO
Published: March 2003

The relationships of foliage assimilation capacity per unit area (PPmax) with leaf dry mass per unit area (LMA) and nitrogen content per unit area (NP) differ between species and within species grown in different habitats. To gain a more mechanistic insight into the dependencies of PPmax on LMA and NP, this literature study based on 597 species from a wide range of earth biomes with woody vegetation examines the relations between leaf photosynthetic capacity and the components of LMA (leaf density (D, dry mass per volume) and thickness (T)), and also the correlations of D and T with leaf nitrogen content and fractional leaf volumes in different tissues. Across all species, PPmax varied 12‐fold and photosynthetic capacity per unit dry mass (Pmmax) 16‐fold, NP 12‐fold, and nitrogen per unit dry mass (Nm) 13‐fold, LMA 46‐fold, D 13‐fold, and T 35‐fold, indicating that foliar morphology was more plastic than foliar chemistry and assimilation rates. Although there were strong positive correlations between PPmax and NP, and between Pmmax and Nm, leaf structure was a more important determinant of leaf assimilation capacities. PPmax increased with increasing LMA and T, but was independent of D. By contrast, Pmmax scaled negatively with LMA because of a negative correlation between Pmmax and D, and was poorly related to T. Analysis of leaf nitrogen and tissue composition data indicated that the negative relationship between D and Pmmax resulted from negative correlations between D and Nm, D and volumetric fraction of leaf internal air space, and D and symplasmic leaf fraction. Thus, increases in leaf density bring about (1) decreases in assimilative leaf compounds, and (2) extensive modifications in leaf anatomy that may result in increases in intercellular transfer resistance to CO2. Collectively, (1) and (2) lead to decreased Pmmax, and also modify PPmax versus LMA relationships.
Tansley Review No. 107. Heterocyst and akinete differentiation in cyanobacteria
Authors: ADAMS DAVID G. , DUGGAN PAULA S.
Published: March 2003

Summary 3

I. introduction 4

II. the cyanobacteria 7

III. the heterocyst 9

1. Function and metabolism 9

2. Heterocyst structure 12

(a) Overview 12

(b) The polysaccharide (homogeneous) layer 12

(c) The glycolipid (laminated) layer 12

(d) The septum and microplasmodesmata 12

3. Nitrogen regulation and heterocyst development 12

4. Heterocyst development 13

(a) The proheterocyst 13

(b) Proteolysis associated with heterocyst development 14

(c) RNA polymerase sigma factors 14

(d) Developmental regulation of heterocyst cell wall and nitrogenase gene expression 14

(e) Genome rearrangements associated with heterocyst development 15

5. Genes essential for heterocyst development 15

(a) hetR 15

(b) Protein phosphorylation and the regulation of hetR activity 16

(c) hetR in nonheterocystous cyanobacteria 16

(d) Other heterocyst‐specific genes 16

6. Heterocyst spacing 18

(a) Patterns of heterocyst differentiation 18

(b) Genes involved in heterocyst spacing 18

(c) Disruption of heterocyst pattern 18

7. Filament fragmentation and the regression of developing heterocysts 20

8. The nature of the heterocyst inhibitor 20

9. Cell selection during differentiation and pattern formation 20

(a) Cell division 20

(b) DNA replication and the cell cycle 21

(c) Competition 21

10. Models for heterocyst differentiation and pattern control 21

IV. the akinete 23

1. Properties of akinetes 23

2. Structure, composition and metabolism 24

3. Relationship to heterocysts 24

4. Factors that influence akinete differentiation 24

5. Extracellular signals 25

6. Akinete germination 25

7. Genes involved in akinete differentiation 26

V. conclusion 26

Acknowledgements 27

References 28

Cyanobacteria are an ancient and morphologically diverse group of photosynthetic prokaryotes. They were the first organisms to evolve oxygenic photosynthesis, and so changed the Earth's atmosphere from anoxic to oxic. As a consequence, many nitrogen‐fixing bacteria became confined to suitable anoxic environmental niches, because the enzyme nitrogenase is highly sensitive to oxygen. However, in the cyanobacteria a number of strategies evolved that protected nitrogenase from oxygen, including a temporal separation of oxygenic photosynthesis and nitrogen fixation and, in some filamentous strains, the differentiation of a specialized cell, the heterocyst, which provided a suitable microaerobic environment for the functioning of nitrogenase. The evolution of a spore‐like cell, the akinete, almost certainly preceded that of the heterocyst and, indeed, the akinete may have been the ancestor of the heterocyst. Cyanobacteria have the capacity to differentiate several additional cell and filament types, but this review will concentrate on the heterocyst and the akinete, emphasizing the differentiation and spacing of these specialized cells.
Enhanced UV‐B radiation under field conditions increases anthocyanin and reduces the risk of photoinhibition but does not affect growth in the carnivorous plant Pinguicula vulgaris
Authors: MENDEZ M. , JONES D. GWYNN , MANETAS Y.
Published: March 2003

The effects of enhanced UV‐B radiation were investigated in the carnivorous plant Pinguicula vulgaris in a field experiment performed in Abisko, North Sweden (68° 21′ N, 18° 49′ E, 380 m above sea level). Potted plants were exposed to either ambient or ambient plus supplemental UV‐B radiation, simulating a 15% ozone depletion. No effect was observed on either the epicuticular (external) or cellular (internal) UV absorbing capacity of the leaves. However, the anthocyanin content was more than doubled by supplemental UV‐B radiation. In laboratory experiments, the anthocyanin rich, UV‐B treated leaves were less susceptible to a low temperature/high light photoinhibitory treatment, as judged by in vivo chlorophyll fluorescence measurements. Yet, this potential benefit did not considerably affect the growth of the plant in the field (leaf area and dry mass, reproductive dry mass, flowering frequency, senescence rates, dry mass of winter buds). However, there was a marginally significant increase in root dry mass and in the root to shoot ratio, which may underlie the significant increase in the nitrogen content of the leaves. We suggest that P. vulgaris is resistant against UV‐B radiation damage and that the possible negative effects of additional UV‐B radiation on the growth of these plants may have been effectively counterbalanced by the lower risk of photoinhibition, due to the concomitant increase in anthocyanins.
Tansley Review No. 108
Authors: BLATT MICHAEL R. , LEYMAN BARBARA , GEELEN DANNY
Published: March 2003

Eukaryotic cells share a set of secretory pathways for the flux of membrane and protein material. In 1993, ideas about the functioning of three major proteins of the neurosecretory complex were consolidated in the SNARE hypothesis, which proposed that the interaction of these proteins provides both the specificity for vesicle targeting and the molecular machinery for fusion between vesicle and target membranes. Subsequetly, the organization, molecular mechanics and control of vesicle trafficking have become topics of intense research, and the hypothesis has evolved to accommodate new discoveries from the analysis of secretion in yeast and mammals. It is likely to be challenged again as more information comes to light about secretory processes in plants. New tools for measuring and manipulating vesicle traffic and secretion are now being used, drawing on in vivo fluorescence and capacitance recording as well as genetic engineering. These new technologies have already begun to yield details wholly unexpected from past studies. Here we focus on recent findings relating to the mechanisms of vesicle trafficking and the background to these developments, highlighting both current understanding of the molecular events of secretion and the gaps therein, as well as discussing emerging themes from work with plants.

contents

Summary
389

I.
introduction
389

II.
1. The SNARE hypothesis
393

III.
vesicle trafficking in plants
402

IV.
regulation of vesicle trafficking in plant cells
406

V.
conclusion
410

Acknowledgements
411

References
411
Relationships among testate amoebae (Protozoa), vegetation and water chemistry in five Sphagnum‐dominated peatlands in Europe
Authors: MITCHELL E. A. D. , BUTTLER A. , GROSVERNIER Ph. , RYDIN H. , ALBINSSON C. , GREENUP A. L. , HEIJMANS M. M. P. D. , HOOSBEEK M. R. , SAARINEN T.
Published: March 2003

To study the relationships between groups of organisms and the degree to which these relationships are consistent across major climatic gradients, we analysed the testate amoeba (Protozoa) communities, vegetation and water chemistry of one peatland in five countries: Switzerland, The Netherlands, Great Britain, Sweden and Finland, as part of the BERI (Bog Ecosystem Research Initiative) project. The relationships between the different data sets and subsets were investigated by means of detrended correspondence analysis, canonical correspondence analysis and Mantel permutation tests. The comparison of data on vegetation and testate amoebae showed that inter‐site differences are more pronounced for the vegetation than for the testate amoebae species assemblage. Testate amoebae are a useful tool in multi‐site studies and in environmental monitoring of peatlands because: (1) the number of species in Sphagnum‐dominated peatlands is much higher than for mosses or vascular plants; (2) most peatland species are cosmopolitan in their distributions and therefore less affected than plants by biogeographical distribution patterns, thus differences in testate amoeba assemblages can be interpreted primarily in terms of ecology; (3) they are closely related to the ecological characteristics of the exact spot where they live, therefore they can be used to analyse small‐scale gradients that play a major role in the functioning of peatland ecosystems. This study revealed the existence of small‐scale vertical gradients within the vegetation and life‐form niche separation in response to water chemistry. The deep‐rooted plants such as Carex spp. and Eriophorum spp. are related to the chemistry of water sampled at or near the ground water table, whereas the mosses are not. Testate amoebae were shown to be ecologically more closely related to the chemistry of water sampled at or near the water table level and to the mosses than to the deep‐rooted plants.
Cadmium accumulation in populations of Thlaspi caerulescens and Thlaspi goesingense
Authors: LOMBI E. , ZHAO F. J. , DUNHAM S. J. , McGRATH S. P.
Published: March 2003

The capacity to accumulate cadmium (Cd) and zinc (Zn) was compared in Thlaspi goesingense and four populations of Thlaspi caerulescens. Two populations of T. caerulescens were grown in hydroponics at five concentrations of Cd. In addition, plants were grown in pots containing compost in which three different concentrations of Cd and two concentrations of Zn were added. A field trial was conducted to compare Zn and Cd uptake by three populations of T. caerulescens on nine selected plots of the Woburn Market Garden Experiment (UK) which had been contaminated to different degrees with heavy metals owing to past applications of sewage sludge. Results show that the four populations of T. caerulescens had the same ability to hyperaccumulate Zn but were significantly different in terms of Cd accumulation. Two populations of T. caerulescens from Southern France accumulated much more Cd than the populations from Prayon (Belgium) and Whitesike (UK). Generally, uptake of Cd was not decreased by increased concentrations of Zn in the substrate. These results indicate that the mechanisms of Cd and Zn hyperaccumulation are not identical in this species. This is the first report of hyperaccumulation of Cd by T. goesingense, but the growth of this species was markedly reduced by the large concentrations of Zn in the substrate. Future work should focus on the differences between Cd and Zn uptake in hyperaccumulator plants at the species and population level.