bluuebunny/crossref_metadata_2025 huggingface dataset:
Nitrogen metabolism of external hyphae of the arbuscular mycorrhizal fungus Glornus intraradices
Authors: JOHANSEN ANDERS , FINLAY ROGER D. , OLSSON PÅL AXEL
Published: April 2006

summary

The arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck and Smith was grown in symbiosis with Cucumis sativus L. ev. Aminex (Fl hybrid) in mesh bags surrounded by a sand‐filled hyphal compartment (HC), allowing only the fungal hyphae to protrude into the HC. The hyphae in the HC were supplied with 15N‐labelled NH4+ or NO3− after 60 d (expt 1). Following a 48 h labelling period, the sand was removed from the HC and the hyphae extracted. In another experiment (expt 2), the hyphae were extracted from the sand before being incubated in vitro in a nutrient solution containing 15N‐labelled NH4+ for 15 h. The hyphal material was incubated in a 0 or 2.5 mM solution of the GOGAT‐inhibitor albizzine prior to labelling. In both experiments the hyphal content of free amino acids and fatty acids were measured as well as the ammo acid l5N enrichment.

Asparagine was the hyphal amino acid measured in highest concentration followed by glutamine, glutamate, aspartate and alanine. Ornithine, serine and glycine were detected in lower concentrations, but γ‐aminobutyric acid and citrulline were not detectable. This pattern was the same in both experiments and was unaffected by the type of N applied to the hyphae or by preincubation with albizzine, although the amino‐acid concentration decreased considerably in expt 2 compared with expt 1. Both NH4+‐N and NO3−‐N were assimilated into amino acids but the levels of l0N enrichment following application of NO3− were much lower than those following application of NH4+ indicating that the latter was more readily assimilated. Albizzine decreased the hyphal amino acid concentration by c. 30% (without affecting the 15N enrichment of the individual amino acids) indicating that the AM‐fungal hyphae might possess a GS‐GOGAT enzyme system for assimilation of inorganic N. The fatty‐acid profiles (especially phospholipid fatty acids 16: 1ω5 and 20:5) obtained from the hyphae of G. intraradices showed that contamination of the samples by fungi other than G. intraradices and bacteria was insignificant, and confirmed the usefulness of specific fatty‐acid measurement to estimate soil AM‐fungal content.
Physiological responses of Festuca arundinacea to fungal endophyte infection
Authors: MARKS SUSAN , CLAY KEITH
Published: April 2006

summary

Enhanced growth rates of several grass species infected by fungal endophytes are known, but the underlying changes in plant physiology are not. Carbon exchange rates (CER) and leaf conductances (g) of 13 genotypes of tall fescue (Festuca arundinacea Schreb. var. KY 31) infected by the fungal endophyte Acremomum coenophialum Morgan‐Jones and Gams were measured at ambient conditions. Endophyte‐free ramets of the same genotypes were also measured. Correlations were calculated between environmental conditions at the time of measurement, and physiological responses. The only differential response of infected and uninfected ramets was to temperature. At low leaf temperatures no difference was seen between infected and uninfected plants. However, at leaf temperatures above 35°C infected tall fescue plants photosynthesized at a significantly greater rate (20–25%) than uninfected plants. This resulted from a decrease in the CER of uninfected plants, not an increase in the rate of infected plants, at high temperature. There were also significant infection × genotype interactions, indicating that the response to infection was specific to a given genotype. These results indicate that physiological responses of host plants to fungal endophyte infection depend both on the physical environment and the genotype of the plants.
Reduction of risk for growers: methods for the development of disease‐resistant crops
Authors: UKNES SCOTT , VERNOOIJ BERNARD , MORRIS SHERICCA , CHANDLER DANIELLE , STEINER HENRY‐YORK , SPECKER NICOLE , HUNT MICHELLE , NEUENSCHWANDER URS , LAWTON KAY , STARRETT MARK , FRIEDRICH LESLIE , WEYMANN KRIS , NEGROTTO DAVID , GÖRLACH JÖRN , LANAHAN MIKE , SALMERON JOHN , WARD ERIC , KESSMANN HELMUT , RYALS JOHN
Published: April 2006

Systemic acquired resistance (SAR) appears to be a ubiquitous higher plant defence response. Resistance is dependent on the high‐level expression of SAR genes. Mutations, transgenes or chemicals that cause high‐level SAR gene expression also cause resistance to pathogens. Furthermore, when SAR is eliminated, through the reduction of salicylic acid accumulation or activity, generalized severe disease‐susceptibility occurs. Therefore, SAR appears to be an essential component of the defensive repertoire that ensures plant health in nature. Chemicals that activate SAR in the field, as well as crop varieties with constitutive SAR gene expression, will provide new solutions to disease problems for growers in the near future.
Cellular and molecular defence‐related root responses to invasion by arbuscular mycorrhizal fungi
Authors: GIANINAZZI‐PEARSON VIVIENNE , DUMAS‐GAUDOT ELIANE , GOLLOTTE ARMELLE , ALAOUI A. TAHIRI‐ , GIANINAZZI S.
Published: April 2006

Arbuscular mycorrhizal (AM) fungi, which date from the Devonian era, must have dealt very early on with problems of plant defence, a tactic which has enabled them to colonize roots of most extant plant taxa. Conversely, plants forming arbuscular mycorrhiza must exert some sort of control over the fungi during symbiotic interactions since mycelial proliferation within roots is confined to the cortical parenchyma tissue. Plants possess a panoply of defence mechanisms which are triggered by microbial attack. Here we review present‐day knowledge of defence‐related root responses to colonization by AM fungi, and assess their possible implications in the symbiosis. Weak, transient, unco‐ordinated or extremely localized activation of inducible defence responses occurs during compatible interactions in AM which differs from that in plant–pathogen interactions. When symbiosis‐related plant genes are mutated, typical defence responses are associated with resulting root resistance to arbuscular mycorrhizal fungi whilst, contrastingly, plants constitutively expressing defence genes are unaffected in their symbiotic capacities. The mechanisms by which plants can modulate defence responses during symbiotic interactions and the way in which A1V1 fungi might contend with these are discussed.
Ecological implications of recognition events in the pre‐infection stages of root pathogens
Authors: DEACON J. W.
Published: April 2006

Host‐specificity and recognition by root‐infecting pathogens are reviewed, with emphasis on fungi but drawing parallels with other root parasites, symbionts and rhizosphere micro‐organisms. Two major levels of specificity are identified: family‐specific parasitism, exemplified by many parasites that characteristically infect Gramineae, and the possibly recent segregation of pathogen species into crop‐specific strains. In both of these cases there are examples where host‐specificity is paralleled by host‐recognition in the pre‐infection stages, including host‐specific triggering of propagule germination. The pre‐infection sequence of zoosporic pathogens (Pythium, Phytophthora, Aphanomyces spp.) is reviewed. These fungi seem to have both general (low‐affinity) and host‐specific (high‐affinity) receptor‐based recognition systems for chemotaxis to root diffusates and for induction of encystment by host surface components. Zoospores orientate (dock) during encystment, and the cysts germinate from a fixed point, whereas the non‐motile spores of at least some root parasites (e.g. Idriella bolleyi) can germinate from any point, which is influenced by the host.

The possession of both general and host‐specific recognition systems by a parasite could enable it to exploit rhizosphere niches or to invade compromised plants in the absence of preferred hosts. Evidence from graminicolous (family‐specific) Pythium spp. suggests that host‐specific recognition is quantitative rather than absolute but, when compounded over successive cycles of infection, is a powerful selective mechanism.
Mycorrhizal diversity in arctic and alpine tundra: an open question
Authors: GARDES M. , DAHLBERG A.
Published: April 2006

Current knowledge of mycorrhizal diversity in arctic and alpine tundra is based mainly on static surveys of mycorrhizal associations of plant taxa and fruiting patterns of selected ectomycorrhizal fungal species in specific habitats. Within these limitations, it appears that: (1) non‐mycorrhizal plants are widespread and predominate in certain plant communities; (2) typical arbuscular mycorrhizal associations are ubiquitous in low arctic and alpine areas but that the level of root colonization is highly variable; (3) root colonization by dark septate fungi is a common event but that their ecological significance is still unknown; (4) a large number of ectomycorrhizal fungal species are present as symbionts of a relatively few widely distributed shrubs and herbaceous plant taxa; (5) ericaceous plants with erieoid mycorrhizas dominate large arctic and alpine areas covered by heath communities. Physical environmental features strongly limit and shape species diversity in arctic and alpine tundra. Cold‐dominated environments provide extreme conditions for the establishment and functioning of mycorrhizal associations. Therefore, such systems are simple models to address the ecology and evolution of mycorrhizal symbioses. Molecular methods to identify mycorrhizal fungi on plant roots will resolve questions related to the structure and dynamics of communities of mycorrhizal fungi in arctic and alpine tundra.
Lux gene technology – a strategy to optimize biological control of soil‐borne diseases
Authors: WHITE DUNCAN , LEIFERT CARLO , RYDER MAARTEN H. , KILLHAM KEN
Published: April 2006

Lux gene technology is based on the transformation of environmental bacteria with genes of the lux operon from the marine bacteria Vibrio fischeri and V. hurveyi. Transformed strains are bioluminescent and thus provide a rapid and very accurate tool for the study of population dynamics, metabolic activity and spatial distribution of specific bacteria in environmental samples. This review summarizes the recent progress in using lux‐marked rhizobacteria to optimize biological control methods for soil‐borne diseases. Special emphasis has been placed on the advantages and limitations of the new technology.
A model for the partitioning of photosynthetically fixed carbon during the C3‐CAM transition in Sedum telephium
Authors: BORLAND ANNE M.
Published: April 2006

Sedum telephium is a succulent plant in which CAM‐cycling is rapidly induced or enhanced in response to drought stress. Diel measurements of leaf malate content and gas exchange demonstrate that as drought stress increased, daytime uptake of CO2 declined and less respiratory CO2 was lost at night as a result of re‐fixation by phosphoenolpyruvate carboxylase (PEPC) with subsequent accumulation of malate. The enhancement of CAM‐cycling in response to drought stress under high and low photon flux density (PFD) results in up to fourfold increases in daily water use efficiency. Moreover, potential conservation of water resulting from CAM‐cycling varied from 4 to 77 % of total daytime transpiration, depending on PFD and severity of drought. Measurements of instantaneous C‐isotope discrimination demonstrated the activation of PEPC activity in vivo with a minimal direct contribution from C4 carboxylation to net CO2 uptake during the day. The δ13C composition of carbohydrates and organic acids indicated that products of malate breakdown are preferentially directed towards starch during the day, which subsequently supplies phosphoenolpyruvate (PEP) for the dark reactions of CAM. The data are summarized using a model which quantitatively describes the source of carbon in the leaf during the day (C3 or C4) and the partitioning of this carbon between the generation of substrate for CAM, production of respiratory CO2 or export. The resulting carbon budgets illustrate that the breakdown of starch at night increases up to twofold with severe drought stress, whilst the flux of carbon into starch during the day is maintained or even increased, largely at the expense of the soluble sugar pool. Overall, under high PFD the enhancement of CAM‐cycling permits continued growth under conditions of severe drought, albeit at a reduced rate. However, under low PFD, the induction of CAM‐cycling serves mainly as a maintenance mechanism whereby water is conserved and a positive carbon balance is preserved by recycling carbon skeletons at the expense of growth during periods of severe water shortage.
Estimations of apoplastic concentrations of K+ and Ca2+ in the vicinity of stomatal guard cells
Authors: DE SILVA D. L. R. , HONOUR SARAH J. , MANSFIELD T. A.
Published: April 2006

A method which determines the null point for stomatal aperture has been used to estimate the apoplastic concentrations of potassium and calcium adjacent to the stomatal guard cells of Commelina communis. These two ions are contributors to important aspects of stomatal physiology: the determination of guard cell turgor (K+) and intracellular signalling (Ca2+) when the guard cells respond to the various stimuli that effect changes in stomatal aperture. We obtained estimates of apoplastic K+ concentrations in the range of 50–75 mol m‐3, which are in general agreement with those of Bowling (1987). Ca2+ concentrations appear to be in the region of 0·05 mol m‐3 adjacent to the guard cells even though much higher concentrations (up to 4 mol m‐3) may be delivered to the leaf in the xylem sap. Thus it is suggested that the gradient in apoplastic Ca2+ may be very large over short distances, and may be strictly controlled by cells within the epidermis.
The genotoxicity of Zn(II) and Cd(II) in Allium cepa root meristematic cells
Authors: BORBOA L. , DE LA TORRE C.
Published: April 2006

Applications of 5 × 10‐4m Zn(II) and 3 × 10‐5m Cd(II) depressed but did not block root growth in Allium cepa L. Both heavy metals were indirect clastogens at these moderate concentrations, for they induced the appearance of chromosomal bridges, but no breaks were detected in the first anatelophases. On the other hand, Zn and Cd respectively potentiated six and eleven times the direct clastogenic effect of the β‐irradiation produced by [3H]thymidine incorporation into DNA. The chromosomal breaks at mitosis were more abundant when [3H] was incorporated into early replicating DNA. On the other hand, when the incorporation took place in the last half of interphase, the subsequent G2 as well as the prophase were extended (1·7‐fold for Zn and 2·7‐fold for Cd), and the Sexcess lesions were repaired before anaphase. The whole cycle experienced 1·5 and 2·0‐fold increases in the presence of Zn and Cd in relation to that in control meristems (15 h).
Factors determining the growth curve of the foliose lichen Parmelia conspersa
Authors: ARMSTRONG R. A. , SMITH S. N.
Published: April 2006

Growth curves of the foliose lichen Parmelia conspersa (Ehrh. ex Ach.) Ach. were obtained by plotting radial growth rate (RGR, mm yr‐1) of the fastest measured lobe, the slowest measured lobe, a randomly selected lobe, and by averaging a sample of lobes from each thallus against thallus diameter. Growth curves derived from the fastest‐growing lobe and by averaging lobes were asymptotic and could be fitted by the growth model of Aplin and Hill. Mean lobe width increased with thallus size, reaching a maximum at approx. 4· cm thallus diameter. In four out of six thalli, radial growth of lobes over 4 months was positively correlated with initial lobe width or area. The RGR of isolated lobes was unaffected until the base of the lobe was removed to within 1–2 mm of the tip. The concentration (μg mg‐1 biomass) of ribitol, arabitol and mannitol was greater in the marginal lobes of large than in small thalli. These results suggested that the growth curve of P. conspera is determined by processes that occur within individual marginal lobes and can be explained by the Aplin and Hill model. Changes in lobe width and in the productive capacity of individual lobes with thallus size are likely to be more important factors than the degree of translocation within the lobe in determining the growth curve.
Aerial dispersal of lichen soredia in the maritime Antarctic
Authors: MARSHALL WILLIAM A.
Published: April 2006

An aerobiological monitoring programme was carried out for over a year on Signy Island, South Orkney Islands, Antarctica. Collections were made using arrays of rotorod samplers at three sites. Lichen soredia were found to be the most abundant air borne propagules, more so than ascospores, the sexual propagules of lichen fungi. The dominance of soredia over ascospores appeared to decrease with increasing maturity of fellfield sites. No correlations were found with temperature, relative humidity or wind speed. Collections at 1 m above ground level were shown not to be significantly different to those at 0·15 m at two of the sites. Size range distribution also differed at two of the sites. Soredial clumps in excess of 100 μm in diameter were collected at 1 m above ground level and at some distance from potential source plants, though most fell in the range 30–60 μm. Peaks in numbers of air borne soredia were found after winter snow melt, demonstrating that soredial production continues at subzero temperatures.
Effects of soil pH on the ectomycorrhizal response ofEucalyptus urophyllaseedlings
Authors: AGGANGAN NELLY S. , DELL BERNARD , MALAJCZUK NICHOLAS
Published: April 2006

To examine the effects of soil pH on ectomycorrhizal formation and function onEucalyptus urophyllaS. T. Blake, seedlings inoculated with nine ectomycorrhizal fungi (seven isolates ofPisolithusspp.,Scleroderma cepaandLaccaria laccatacollected under eucalypt stands in Australia and the Philippines) were transplanted into pots containing a non‐sterile acid (pH 4·6) sandy loam amended with four levels of CaCO3that raised the soil pH from 4·6 to 6·6 (5 mM CaCl2). Pots were placed in temperature‐controlled water baths (28 ± 2 °C) inside an evaporatively cooled glasshouse for 9 wk.

Increase in soil pH from 4·6 to 6·6 significantly decreased plant d. wt and shoot nutrient content of uninoculated and inoculated seedlings. Inoculation with fourPisolithusspp. (H445, H2144, M56 and H4003) significantly increased the growth ofE. urophyllaseedlings at pH 4·6. At pH 6·6, eight ectomycorrhizal isolates significantly improved total d. wt compared with those of the uninoculated seedlings.Pisolithusisolates stimulated seedling growth more thanL. laccatawhereasS. cepawas ineffective at all pH levels. Total d. wt of H445 inoculated plants grown in P‐deficient (8 mg P kg‐1soil) soil was 147% more than that of uninoculated plants given the same P rate and was 70% that of plants fertilized with 64 mg P kg‐1soil (P64) at pH 46. At soil pH 5·8 and 6·6, M56 was the best growth‐promoter forE. urophylla.These results indicate that soil pH can significantly alter the development and function of ectomycorrhizal fungi. Soil pH did not significantly affect mycorrhizal formation by the different ectomycorrhizal fungi. However, the percentages of mycorrhizal root tips formed by the different ectomycorrhizal fungi differed significantly.Pisolithusisolate H445 formed the highest percentage of colonized roots and highest total d. wt at pH 4·6 and 5·2, implying its potential for commercial use in acidic conditions.
Xylem recovery from cavitation‐induced embolism in young plants of Laurus nobilis: a possible mechanism
Authors: SALLEO SEBASTIANO , GULLO M. ASSUKTA LO , DE PAOLI DOROTEA , ZIPPO MANUELA
Published: April 2006

Xylem recovery from cavitation‐induced embolism was studied in l‐yr‐old twigs of Laurus mibilis L. Cavitation was induced by applying pre‐established pressure differentials (AP0,1) across the pit membranes of xylem conduits. ΔP0) were 1.13, 1.75 and 2.26 MPa, corresponding to about 50, 77 und 100% of the measured leaf water potential at the rurgor loss point ΔP0,1 were obtained either by increasing xylem tensions or by applying positive pressures from outside, or by a combination of the two. The percentage loss of hydraulic conductivity (PLC) did not change, regardless of how the ΔP0–1 were obtained. This confirmed that xylem cavitation was nucleated by microbubbles from outside the vessels. Positive pressures, however, amplified (up to 75%) and sped up the xylem refilling (20 min) in comparison with that measured in unpressurized twigs (c.50% in 15 h). Twigs girdled proximally to their pressurized segment 1 nun after the desired pressure value had been reached, did not recover from embolism. The later the twigs were girdled with respect to when they were tested for PLC, the higher was their recovery from embolism, suggesting that some messenger was transported in the phloem which stimulated xylem refilling. Indol‐3‐acetic acid (1AA] applied to the exposed cortex of both pressurized and unpressurized twigs, induced an almost complete recovery from PLC. We hypothesize that the refilling of cavitated xylem might be a result of an auxin‐induced increase in the phloem loading with solutes. This would cause radial transport of solutes to cavitated xylem conduits via the rays, thus decreasing their osmotic potential and making them refill. No positive xylem pressure potentials were measured during xylem recovery from PLC.
Growth and sugar accumulation in durum wheat plants under water stress
Authors: KAMELI A. , LÖSEL D. M.
Published: April 2006

The effect of water stress on growth of Trillium durum L. was investigated in relation to sugar accumulation and water status of wheat plants before, during and after a period of water stress. The slight decrease in water potential in the first few days after withholding water had no detectable effect on growth. Inhibition of growth was only apparent when the water content started to decline. Dry weight continued to increase during water stress, even under severe stress (after day 27) which was associated with n sharp rise in sugar content, accounting for 20% of the gain in dry matter between days 27 and 31. The increase in leaf length and leaf area of stressed plants fallowing re‐watering, from day 31, was owing to the leaves regaining turbidity after witting. Growth inhibition coincided with a considerable increase in sugar content. The role of growth inhibition and other factors in sugar accumulation under water stress is discussed, Photosynthesis rather than reserve starch might be the major source of sugar accumulated under water stress in durum wheat.
Tillering and partitioning of dry matter and nutrients in Lolium perenne growing with neighbours of different species: effects of nutrient supply and defoliation
Authors: MARRIOTT CAROL A. , ZUAZUA M. TERESA
Published: April 2006

The hypotheses that performance of Lolium perenne L. (ryegrass) differs when plants grow with conspecific or heteraspecific neighbours, and that nutrient supply and severity of defoliation modifies the effect, were tested in a target: neighbour pot experiment. Target L. perenne plants were grown with neighbours of five grass species (L. perenne, Poa pratensis L., P. trivialis L., Festuca ovina L. and F. rubra L.) and Trifolium repens L. (white clover), at three nutrient levels (high, medium and low) and three defoliation severities (no defoliation or cut every 2 wk to 4 cm or 8 cm) over a 12 wk experimental period. Performance of L. perenne plants differed depending on the species of neighbour. Tiller numbers and dry weights of clippings of L. perenne, at both individual cuts and the total accumulated over all seven cuts, were greater with P. pratensis, F. ovina or T. repens as neighbours than in a monoculture. The differences were generally confined to medium and high nutrient levels. The pattern of clippings depended on species association: clippings were similar at successive cuts except in mixtures with P pratensis, F. ovina and T. repens, where they increased until the 4–5th cuts. The total accumulated dry matter (DM) and nutrients of L. perenne and the percentage contribution of DM to the pot total were also greater in these mixtures. Individual linear regressions of tiller numbers and accumulated N for each species combination best described the relationships between L. perenne and its neighbour. Neighbouring species had no significant effect on the relative losses of dry weight of nutrients in clippings of L. perenne when expressed as a proportion of the total accumulated, but partitioning of DM to L. perenne roots (root weight ratio) was greater in associations with Poa species than in a monoculture. It was concluded that the species of neighbour affected the accumulation and partitioning of dry matter and nutrients in L. perenne and that nutrient supply, but not defoliation, modified the responses. The results are discussed in relation to species interactions in grazed swards.
Response to phosphorus supply of tropical tree seedlings: a comparison between a pioneer species Tapirira obtusa and a climax species Lecythis corrugata
Authors: RAAIMAKERS DORINNE , LAMBERS HANS
Published: April 2006

The highly‐weathered acid sandy soils in Guyana, South America, art very low in nutrients, especially in phosphorus (P) Earlier experiments demonstrated that P was growth‐limiting for some tree seedlings on these soils, but other species failed to increase their growth in response to greater P‐availability. To investigate this, we measured growth and distribution (if biomass and P of tree seedlings, of a pioneer tree species, Tapirira obtusa (Benth.) Mitchell, and of a climax tree species, Lecythis corrugata Poit., at 10 levels of P‐supply under controlled conditions in a glasshouse. At intervals of 3 wk. dry weights of plant parts and their phosphorus concentrations were measured. The pioneer and the climax species took up similar amounts of P when grown at high P‐supply. The pioneer tree T. obtusa maintained a low P concentration (0.25 mg g−1) independent of P‐supply, and used the P taken up Hi increase growth., At high P‐supply it invested little biomass in roots, and reached a relative growth rate (RGR) of 40 mg g−1 d−1. The climax, tree species, L, corrugate, maintained a low RGR of 10 mg g−1 d−1 and a constant distribution of biomass at all P‐supply rates. It stored the extra P in a structure between the stem and root derived from the former hypocotyl, which persisted for over 6 months after germination. The differences m growth and distribution of biomass and P in response to P‐supply of the two species are likely to contribute to the establishment of their seedlings in the field. If L. corrugata is capable of re‐translocating P from the hypocotyl, this storage of P has ecological advantages for long‐term survival, which might be important under low light conditions. Together with a low RGR, it enables a seedling to maintain P‐reserves until a gap occurs. In a newly created gap, P‐availability per seedling increases, and pioneers, with their higher P uptake and growth potential, can benefit from these relatively higher levels of P‐availability. This is an important advantage in high‐light gap environments where the tallest tree seedling is generally the most competitive one.
Benomyl inhibits phosphorus transport but not fungal alkaline phosphatase activity in a Glomus–cucumber symbiosis
Authors: LARSEN J. , THINGSTRUP I. , JAKOBSEN I. , ROSENDAHL S.
Published: April 2006

Short‐term effects of benomyl on the arbuscular mycorrhizal fungus Glomus caledonium (Nicol. & Gerd.) Trappe and Gerdeman associated with Cucumis sativus L. were studied by measuring effects on fungal P transport and on fungal alkaline phosphatase activity. Mycorrhizal plants were grown in three compartment systems where nylon mesh was used to separate n root‐free hyphal compartment (HC) and a root + hyphal compartment(RHC) from The main root compartment (RC). Non‐mycorrhizal control plants were grown in similar growth units. After 6 wk benomyl was applied to the plants in three ways: as soil drenches to RHC or HC, or as u spray to the leaves. Benomyl was added in three concentrations. Equal amounts of 32P and 33P were added to the HC and to the RHC respectively, immediately after the application of benomyl. Plants were harvested 4–6 d later. Hyphal transport of 32P from the HC was inhibited when benomyl was applied to the HC at 10 μg g−1 soil, whereas the uptake of 32P from RHC I roots + hyphae) was reduced only at the highest dose of application to the RHC (100 μ g g−1 soil). In contrast to the marked reduction of benomyl on fungal P transport, the activity of fungal alkaline phosphatase inside the roots was unaffected by benomyl.
The effects of ozone and nutrient supply on stomatal response in birch (Betula pendula) leaves as determined by digital image‐analysis and X‐ray microanalysis
Authors: FREY BEAT , SCHEIDEGGER CHRISTOPH , GÜNTHARDT‐GOERG MADELEINE S. , MATYSSEK RAINER
Published: April 2006

Cuttings of Betula pendula Roth were grown in field fumigation chambers throughout one growing season in filtered air with < 3 nl l−1 O3 (control; C) or day/night = 90/40 nl 1−1 O3 (ozone fumigation: O3). Plants were watered with either low (0·005 %: LF) or high‐concentrated (0·05%: HF) fertilizer solution. Discs between second‐order veins in the central portion of the leaves were excised and immediately cryofixed in liquid nitrogen for low‐temperature scanning electron‐microscopy (LTSEM) at 1000 hours and 1400 hours. Stomatal width, area and density were measured by digital image‐analysis. X‐ray counts of potassium (K) and calcium (Ca) ions were determined by means of energy‐dispersive X‐ray microanalysis in guard and subsidiary cells. Accurate and fast measurements of stomatal apertures by image analysis were possible in birch leaves, because the darkness of the stomatal pore contrasts with the brightness of the guard cells and the cuticular ledges. Regression analysis showed a close relationship between the stomatal width and the pore area (r= 0·938, P < 0·01). At all harvest times, the stomatal pores were significantly narrowed in the high fertilization control treatment (O3/HF vs. O3/LF), and in the ozone treatment at 1400 hours (O3/HF vs. C/LF). In addition to this fertilization effect, ozone had also narrowed the stomatal pores (O3/HF vs. C/HF at 1400 hours, O3/LF vs. C/LF at all harvest times). In addition to these functional effects, morphological effects (individual leaf area, stomatal density) were determined. Single‐leaf area was increased by high fertilization, with a tendency to decrease with O3 fumigation. The stomatal density in intercostal fields was increased by O3 but decreased by high fertilisation. Stomatal widening was accompanied by increased K count rates in the guard cells, in contrast to constant K values in the subsidiary cells, irrespective of the fumigation or fertilization regimes. Calcium counts in the guard cells were similar to those in the subsidiary cells, and were independent of the aperture width. In samples with established ozone injury, the K/Ca ratio in collapsed guard cells increased compared with turgid guard cells irrespective of the pore aperture. Collapsed subsidiary cells only differed from turgid subsidiary cells when the guard cells had also collapsed and thus closed the pore.
Late Quaternary vegetation history of the Western Isles of Scotland
Authors: FOSSITT J. A.
Published: April 2006

Palacoecoiocical investigations of two lake sediment sequences are used to reconstruct the vegetation history of the Western Isles. Pollen records begin in the late glacial and show a clear progression From herb‐rich grassland with abundant Salix, Rumex and Poly‐podium to widespread Empetrum nigrum heath. Woodland developed widely in the early Holocene, and regional vegetation patterns emerged. Western Lewis supported extensive areas of birch woodland with Corylus avellana, Salix, Populus and Sorbus aucuparia. Open birch‐hazel woodland dominated other lowland regions. Regional variation was accentuated when woodland underwent a sudden decline in western Lewis at about 7900 BP. Trees were replaced by blanket peat and woodland failed to recover on a regional scale after the decline‐ Woodland continued to diversify in central and eastern regions and, by 6000 BP, Quercus, Ulmus, Pinus sylvestris, Alnus glutinosa and Fraxinus excelsior had become established. Open mixed woodland persisted until the second major decline began between 5200 and 4000 UP. Woodland contracted gradually as blanket peat expanded. The Western Isles were predominantly treeless at 2500 BP. Blanket peat started to form in localized areas on the Western Isles between 9000 and 8000 BP; two main expansion phases are associated with periods of woodland decline. Sublossil wood or macrofossils are reported from a total of 40 sites. Radiocarbon ages are presented for 13 samples of these.
Fructan metabolism in expanding leaves, mature leaf sheaths and mature leaf blades of Lolium perenne. Fructan synthesis, fructosyltransferase and invertase activities
Authors: GUERRAND DAVID , PRUD'HOMME MARIE‐PASCALE , BOUCAUD JEAN
Published: April 2006

summary

Accumulation of water‐soluble carbohydrates was studied in leaf tissues of 8‐wk‐old plants of perennial ryegrass (Lolium perenne L. var. Bravo). The roots and leaf bases were cooled to low temperatures to reduce sink activity while source activity was enhanced by continuous illumination of the shoots. This resulted in accumulation of fructans, first in sheaths, subsequently in expanding leaves, and finally in blades. Fructan concentration increased from 6 to 23 mg g−1f. wt in sheaths, from 8 to 30 mg g−1f. wt in expanding leaves and from 6 to only 17 mg g−1f. wt in mature leaf blades. Increase in concentration of low‐DP fructans preceded accumulation of high‐DP fructans. Expanding leaves accumulated significantly more glucose and fructose than did mature leaf tissue. Leaf blades contained a higher concentration of sucrose than either leaf sheaths or expanding leaves. Expanding leaves exhibited the greatest activity of SST (0.77 nkat g−1f. wt), the mature leaf blades the least (0.09 nkat g−1f. wt), whilst that of mature leaf sheaths was intermediate (0.27 nkat g−1f. wt). In each leaf tissue, invertase activity was higher than SST activity. Under conditions of fructan accumulation, SST activity increased threefold in mature leaf sheaths and 1.2‐fold in expanding leaves. In mature leaf blades, however, the activity remained constant at a low level. Invertase activity never increased. Inhibitors of protein synthesis (cycloheximide) and of RNA metabolism (actinomycin D) prevented any increase of SST activity in leaf sheaths. The results are discussed in relation to fructan metabolism in leaf tissues of Lolium perenne.
Physiological and biochemical changes in shoots of coconut palms affected by lethal yellowing
Authors: LEÓN RUBÉN , SANTAMARÍA JORGE M. , ALPIZAR LUCELY , ESCAMILLA JOSÉ A. , OROPEZA CARLOS
Published: April 2006

summary

Noon stomatal conductance of lethal‐yellowing (LY) affected coconut palms (Cocos nucifera L. Atlantic Tall ecotype) decreased progressively as the disease developed. In the early stages of the disease and before leaf yellowing started, stomatal conductance decreased to a minimum in leaves at both the top, the middle and the base of the crown. Since altered stomatal behaviour might affect gas exchange and related processes in the plant, such as water movement and photosynthesis, these results support the hypothesis that LY‐induced stomatal closure is central to the development of LY symptoms in coconut palms, and therefore to the mode of pathogenicity of the LY mycoplasma‐like organism. Leaf yellowing occurred simultaneously with a decrease in photosynthetic rates and a decrease in protein, chlorophyll and carotenoid contents. Based on these biochemical changes a second hypothesis is proposed, that LY‐associated leaf yellowing is part of a leaf senescence process. The concentration of abscisic acid in the leaves and the capacity of leaf tissue to form ethylene increased in diseased palms, suggesting that a hormonal imbalance occurs in LY‐affected palms. Furthermore, treatment of symptomless palms with an ethylene‐releasing agent (ethephon) resulted in symptoms that mimicked some of the LY symptoms. A third hypothesis is therefore proposed, that a hormonal imbalance might be related to LY symptom development, at least with respect to nutfall and leaf senescence.
Interactive effects of ozone and soil volume on Plantago major
Authors: WHITFIELD C. P. , DAVISON A. W. , ASHENDEN T. W.
Published: April 2006

summary

Plantago major L. ssp. major usually grows on trampled paths, tracks and gateways, and its root system is often restricted by compaction or by a limited volume of soil. In this heterogeneous environment, individuals close to each other will experience variable degrees of root restriction. As some populations of P. major have been demonstrated to be sensitive to ozone, the aim of this study was to investigate the effect of root restriction on the response of P. major to ozone.

It was hypothesized that root restriction should reduce the effects of ozone. If this is true it would have implications for evolution of ozone resistance. A single population of P. major was grown in controlled‐environment chambers and exposed to either charcoal/Purafil‐filtered air or to ozone (70 nl O3 l−1for 7 h d−1). Plants were grown in pots of four different volumes: 0.175 1; 0.3651; 2.2801; and 5.8301. Four harvests were performed, at 21, 37, 51 and 91 d from germination. At the final harvest, the total nitrogen content of leaf tissue was analysed and compared with that of plants collected from the field. Stomatal conductance (gs) and net carbon assimilation (A) rate were also recorded during development.

Plants grown in the smallest pots exhibited severely impaired growth, lower gs and lower A than non‐restricted plants. Root growth was less impaired than shoot growth. However, only plants in the smallest pots resembled those found in the field, which implies that realistic assessment of ozone effects on this species should involve a degree of root restriction. Appearance and leaf nitrogen content of leaves suggested that root restriction may have caused nitrogen deficiency. Contrary to the initial hypothesis, exposure to ozone reduced the growth of plants in all sizes of pot. Stomatal conductance was consistently lower in plants exposed to O3, but there was no effect of ozone on A.
Effects of leaf age, basal meristem and infection with powdery mildew on photosynthesis in barley grown in 700 μmol mol−1CO2
Authors: HIBBERD J. M. , RICHARDSON P. , WHITBREAD R. , FARRAR J. F.
Published: April 2006

summary

The rate of net photosynthesis in the second leaf of barley was higher in 700 than 350 μmol mol−1CO2, when measured in the CO2 concentration in which the plants were grown, but the magnitude of this difference decreased as the leaf aged. Infection by powdery mildew accelerated the decline in net photosynthesis of leaves grown in either 350 or 700 μmol mol−1CO2. A/Ci curves allowed the reduction in net photosynthesis of plants exposed to 700 μmol mol−1CO2 or after infection by powdery mildew to be related to changes in the carboxylation efficiency or in the regeneration of ribulose 1,5‐bisphosphate. The carboxylation efficiency declined in plants exposed to 700 μmol mol−1CO2. In plants infected with powdery mildew, the reduction in net photosynthesis was associated with both reduced carboxylation efficiency and reduced ability to regenerate ribulose 1,5‐bisphosphate. Reduced carboxylation efficiency of the second leaf of plants grown in 700 μmol mol−1CO2, was not associated with a reduction in the concentration of rubisco within the leaf. In contrast to the presence of a close exogenous sink, leaf age had large effects on the acclimation of photosynthesis to 700 μmol mol−1CO2.
Superoxide dismutase activity in arbuscular mycorrhizal Lactuca sativa plants subjected to drought stress
Authors: RUIZ‐LOZANO J. M. , AZCÓN R. , PALMA J. M.
Published: April 2006

summary

Total superoxide dismutase (SOD; EC 1.15.1.1) activity, SOD specific activity and changes in the SOD isoenzymatic pattern were determined in roots and shoots of plants of Lactuca sativa L. cv. Romana colonized by the arbuscular mycorrhiza (AM) fungi Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or Glomus deserticola (Trappe, Bloss & Menge), and in non‐mycorrhizal phosphorus‐fertilized controls. Lettuce plants were grown under well watered or drought conditions and were harvested 5 wk or 8 wk after planting. Previous studies have reported that mechanisms that decrease oxidative stress can play an important role in drought tolerance of plants. However, there is little information on either the role of superoxide dismutase in the symbiosis between AM fungi and plants, or about the response at biochemical level of the mycorrhizal symbiosis to drought stress. In our experimental conditions the protein concentration in shoots decreased in all plants as a result of drought stress (except in plants colonized by Glomus mosseae after 8 wk of growth). In general, shoots of mycorrhizal plants had more protein than did shoots of P‐fertilized plants, G. deserticola being the treatment with the highest protein concentration. However, in roots the protein concentrations did not vary as clearly as in shoots. The SOD specific activity of 5‐wk‐old plants under stress conditions was considerably higher in shoots and roots of mycorrhizal than in those of P‐fertilized plants, showing increases which ranged from 99% in G. mosseae‐mycorrhizas to 150% in G. deserticola‐mycorrhizas. Under well watered conditions shoots showed no significant differences in SOD specific activity between treatments, whereas in roots mycorrhizal colonization increased it by 16–18 %. Drought stress increased SOD activity of shoots of control plants by 17%, and that of shoots of plants colonized by G. mosseae and G. deserticola by 93 % and 128 %, respectively. Similar results were also found for roots, regardless of the plant age. The increased SOD activity in mycorrhizal plants was not related to the nutritional status of the plants. Instead, it could be ascribed to a direct mechanism of AM association in response to drought exposure. However, no changes in the SOD isozyme pattern as a consequence of mycorrhizal association or water treatment were observed.
Effect of arbuscular mycorrhizal fungi on growth, mineral nutrition and nitrogen fixation of three varieties of common beans (Phaseolus vulgaris)
Authors: IBIJBIJEN J. , URQUIAGA S. , ISMAILI M. , ALVES B. J. R. , BODDEY R. M.
Published: April 2006

summary

A pot experiment was conducted under glasshouse conditions to investigate the effect of inoculation with four arbuscular mycorrhizal (AM) fungi (Glomus clarum, G. etunicatum, G. manihotis, Gigaspora margarita), either individually or in a mixture, and withRhizobium leguminosarumbiovarphaseoli, on the growth, mineral nutrition, and contribution of biologically fixed N2, to three nodulating bean (Phaseolus vulgaris) varieties (Carioca, Negro Argel and Rio Tibagi). The15N isotope dilution technique and three control crops (non‐nodulating bean,Panicum maximumand sorghum) were used to quantify the contribution of N2fixation. There were significant differences between the capacity of the different AM fungi to infect and colonize the roots of the different bean varieties. Inoculation with AM fungi significantly increased production of dry matter by 8–23 % and the concentration of P in plants by 160–335%. A strongly positive correlation of % root colonization by AM fungi and plant P concentration and accumulation was observed which suggests that the observed inoculation responses were a result of improved P acquisition by the mycorrhizal roots. N derived from the atmosphere increased significantly with AM fungi inoculation, with Carioca showing the best response, followed by Negro Argel and Rio Tibagi, thereby showing the indirect and positive effect of AM fungi inoculation on the N2‐fixing association betweenRhizobiumandPhaseolus vulgaris.
Effect of a genetically modified Rhizobium meliloti inoculant on the development of arbuscular mycorrhizas, root morphology, nutrient uptake and biomass accumulation in Medicago sativa
Authors: BAREA J. M. , TOBAR R. M. , AZCÓN‐AGUILAR C.
Published: April 2006

summary

A soil microcosm system was used to evaluate the impact of a genetically modified (GM) Rhizobium meliloti strain on development and function of arbuscular mycorrhiza in alfalfa plants (Medicago sativa L.). There was no indication that this GM Rhizobium strain, which had an enhanced nodulation competitiveness ability, interfered with mycorrhiza formation by Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe. Indeed, inoculation with the GM Rhizobium strain greatly increased the number of mycorrhizal entry points in the alfalfa root system in comparison to the wild‐type strain. Mycorrhizal development and quality of nodulation increased with time and coincided with increased biomass of and nutrient (N, P) uptake by the host plant. The establishment of the symbiotic interactions also induced changes in root morphology; in particular, the degree of branching increased and the number of lateral roots was greater in plants inoculated with the GM Rhizobium strain. These results demonstrate that the GM Rhizobium strain does affect adversely the performance of arbuscular mycorrhizal symbiosis, a biosafety model system based on a functional rhizosphere.
Atomic force microscopy study of isolated ivy leaf cuticles observed directly and after embedding in Epon®
Authors: CANET D. , ROHR R. , CHAMEL A. , GUILLAIN F.
Published: April 2006

summary

The first results obtained by atomic force microscopy (AFM) of the fine structure of isolated ivy leaf cuticles are reported. Observations of transverse sections embedded in Epon® allow easy recognition of the general shape of cuticles as viewed by light microscopy. The surface profile shows irregularities not revealed by transmission electron microscopy (TEM). The lamellate and reticulate zones, distinguishable by TEM, were also located by AFM. The outer zone appears as an irregularly thick (c. 4 μm) homogeneous region; lamellae can be visualized only after image‐processing to subtract relief defects produced by sectioning. The second region or internal zone represents the largest part of the cuticle thickness; it appears heterogeneous and disordered. The cuticle internal surface images show imprints of the epidermal cells. At high magnification, the cell imprint central regions appear to be made up of a network of fibres of c. 50 nm diameter. Some images show that these fibres have a preferential orientation. They disappear after acid‐hydrolysis is used to eliminate polysaccharides. This study shows that AFM can produce reproducible images of isolated plant cuticles at a subcellular scale leading to new high‐resolution representations of cuticle substructure.