Showing papers in "New Phytologist in 1995"

Assessment of photosynthetic radiation-use efficiency with spectral reflectance.

Abstract: Reflectance changes at 531 nm, associated with the zeaxanthin-antheraxanthin-violaxanthin interconversion and the related thylakoid energization, are widespread among plant species. We evaluated an index based on 531 nm, reflectance ('PRI', Photochemical Reflectance Index, calculated as (R 531 -R 570) /(R 531 +R 570 )) as an indicator of efficiency of photosynthetic radiation use in seven species representing both C 3 and CAM photosynthetic pathways. Leaves exposed to a dark-light-dark transition in a steady-state laboratory gas exchange system exhibited nearly parallel changes in PRI and PS II quantum yield (ΔF/F m '). Similar PRI and ΔF/F m ' responses were seen in leaves exposed to diurnally changing sunlight levels outdoors. PRI was linearly related to ΔF/F m ', and both ΔF/F m ' and PRI were exponentially related to instantaneous efficiency of photosynthetic radiation-use in different species over a range of different field conditions. These results extend previous studies by indicating a functional relationship between PRI, ΔF/F m ', and photosynthetic radiation-use efficiency. The narrow-band PRI index offers a simple, portable means of assessing PS II radiation-use efficiency, analogous to ΔF/F m ', and with the potential for remote applications at scales larger than the leaf.

Gametes with the somatic chromosome number: mechanisms of their formation and role in the evolution of autopolyploid plants.

Abstract: summary The production of 2n gametes in plants, i.e. gametes with a somatic chromosome number, is considered to be the dominant process involved in the origin of polyploid plants. In this review, we provide a synthesis of current knowledge concerning the production of 2n gametes. Firstly, we describe the different methods used to detect and quantify the production of 2n gametes in plants, which include morphological and flow cytometry screening of the occurrence of 2n pollen, the analysis of crosses among diploid and tetraploid parents and the instigation of micro-and mega-sporogenesis. Secondly, the high level of inter- and infra-specific variation in 2n gametes production is described. Thirdly, the various cytological anomalies responsible for the production of 2n gametes are reviewed, with particular reference to the relative genetic consequences of the first and second restitution divisions that give rise to 2n gametes. Fourthly, the significance of 2n gametes in crop plant improvement is discussed, in relation to somatic chromosome doubling to obtain new polyploid varieties. In particular, we compare the genetic and yield consequences of methods based on unilateral and bilateral sexual polyploidization. Finally, we outline how knowledge of the variety of mechanisms involved in 2n gamete production have increased our understanding of the evolutionary significance of polyploidy and the population biology of polyploid plants.

Ectomycorrhizal fungal succession in jack pine stands following wildfire

Abstract: SUMMARY A study was conducted to determine if ectomycorrhizal fungi in an age sequence of jack pine (Pinus banksiana Lamb.) stands which had regenerated following wildfire disturbance followed a successional pattern. Ectomycorrhizal development and number of symbionts were assessed in the forest floor and 0-20 cm deep mineral soil in 6, 41, 65 and 122-yr-old stands by conducting a macrofungal fruit body survey and examining pine root tips microscopically for mycorrhizal status and types of fungi forming the symbiosis. The majority of roots were located in the mineral soil with no substantial invasion of the forest floor except in the 122-yr-old stand. Over 90 % of the jack pine root tips were mycorrhizal and the majority of fruit bodies were produced by ectomycorrhizal species, regardless of stand age. There was no decrease in ectomycorrhizal colonization of roots with stand age. Both fruit body and root assessments revealed a distinct sequence of mycorrhizal fungi with stand age consisting of early-stage fungi [Coltricia perennis (L: Fr.) Murr., Thelephora spp., E-strain]; multi-stage fungi [Suillus brevipes (Pk.) Kuntze, Inocybe spp., Cenococcum geophilum Fr., Mycelium radicis atrovirens Melin] and late-stage fungi [Cortinarius spp., Lactarius spp., Russula spp., Tricholoma spp., Hygrophorus spp., Hydnellum peckii Banker, Suillus tomentosus (Kauff.) Sing., Snell & Dick, Piloderma byssinum (Karst.) Jul. and Sarcodon scabrosus (Fr.) Karst.]. Many of the basidiomycete species fruiting above ground were detected also on the roots below ground. Fruit bodies of 50 species of ectomycorrhizal fungi were recorded while 39 distinct mycorrhizal types were identified on the roots. There was a significant increase in mycorrhizal species richness between the 6 and 41-yr-old stands and this was primarily the result of partial replacement of Suillus brevipes on the 6-yr-old trees by late-stage fungi in the older stands. Very few species present in the 6-yr-old stand were completely replaced in the older stands; rather, the multi-stage species present in the young stand were joined by late-stage species in the mature stands. The species abundance distribution of fungi on the roots in the 6-yr-old stand was best described by a geometric series which is typical of an early successional community while the distributions in the three oldest stands conformed to a lognormal series which is indicative of a stable, species rich community. Both the composition and structure of the ectomycorrhizal community had stabilized 41 yr after wildfire.

Physiological ecology of cyanobacteria in microbial mats and other communities

Abstract: In this review some aspects of the physiological ecology of cyanobacteria are discussed by taking a microbial mat as an example. The majority of microbial mats are built and dominated by cyarsobacteria which are primary producers at the basis of the microbial foodweb in microbial mats. These micro-scale ecosystems are characterized by steep and fluctuating physico-chemical gradients of which those of light, oxygen and sulphide are the most conspicuous. Light is strongly attenuated in the sediment, and owing to constant sedimentation, the mat-forming cyanobacteria have to move upwards towards the light. However, at the sediment surface, light intensity, particularly in the u.v. part of the spectrum, is often deleterious. The gliding movement of the cyanobacteria, with photo- and chemotaxis, allows the organism to position itself in a thin layer at optimal conditions. The organic matter produced by cyanobacterial photosynthesis is decomposed by the ruicrobial community. Sulphate-reducing bacteria are important in the end-oxidation of the organic matter. These organisms are obligate anaerobes and produce sulphide. Gradients of sulphide and oxygen move up and down in the sediment as a response to diurnal variations of light intensity. Cyanobacteria, therefore, are sometimes exposed to large concentrations of the extremely toxic sulphide. Some species are capable of sulphide-dependent anoxygenic photosynthesis. Other cyanobacteria show increased rates of oxygenic photosynthesis in the presence of sulphide and have mechanisms to oxidize sulphide while avoiding sulphide toxicity. Iron might play an important role in this process. Under anoxic conditions in the dark, mat-forming cyanobacteria switch to fermentative metabolism. Many species are also capable of fermentative reduction of elemental sulphur to sulphide. The gradients of sulphide and oxygen are of particular importance for nitrogen fixation. Very few microbial mats are formed by heterocystous cyanobacteria, which are best adapted to diazntrophic growth. However, these organisms probably cannot tolerate greater concentrations of sulphide or anoxic conditions or both. Under such conditions non-heterocystous cyanobacteria become dominant as diazotrophs. These organisms avoid conditions of oxygen supersaturation. In the ecosystem, nitrogen fixation and photosynthesis might be separated temporally as well as spatially. In addition, non-heterocystous diazotrophic cyanobacteria have mechanisms at the subcellular level to protect the oxygen-sensitive nitrogenase from inaction. CONTENTS Summary 1 I. Introduction 2 II. Microbial mats 3 III. Cyanobacteria in light gradients 7 IV. Dark metabolism 10 V. Interactions with sulphide 13 VI. Nitrogen fixation 16 VII. References 28.

The structure and function of the vegetative mycelium of ectomycorrhizal plants

Abstract: SUMMARY Using perspex observation chambers, the uptake, translocation and distribution of 32P-labelled phosphorus was studied in ectomycorrhizal mycelial networks of Suillus bovinus (Fr.) O. Kuntze interconnecting plants of Pinus contorta Dougl. ex Loud and Pinus sylvestris L. Label was fed either directly to the cut ends of individual mycelial strands, to plant roots, or to the unsterile peat in the vicinity of advancing mycelial fans. Where 32P was fed to individual strands or mycelial fans it was taken up and translocated through the mycelium, over distances exceeding 40 cm, to all host plants connected to the mycelial network. Ectomycorrhizal roots acted as major sinks for phosphate but the label did not move exclusively towards the plant and was distributed throughout the mycelial system. Calculated translocation rates and flux rates suggest that transport is primarily by symplastic flow rather than turgor driven bulk flow. The amount of label accumulated by each plant was significantly related both to the size of the plant shoot and to the number of mycorrhizal root tips but did not appear to be influenced by the transpiration rates of individual plants. Phosphorus supplied directly to plant roots did not move to other plants via the mycelial connections suggesting that movement of phosphorus between the fungus and host is unidirectional.

Diversity of fungal symbionts in arbuscular mycorrhizas from a natural community

Abstract: summary The arbuscular mycorrhizal (AM) association between fungi in the order Glomales and the roots of a very wide range of vascular plants is of global ecological significance but has proved particularly intractable to study in the field. We have developed a reliable technique to identify the fungal symbionts in roots taken directly from natural communities. Selective Enrichment of Amplified DNA combines the use of recently-developed specific DNA primers with a novel method based on the principle of subtractive hybridization to remove interfering plant-derived DNA after amplification with the polymerase chain reaction. Using this technique we have shown that endomycorrhizas of bluebells (Hyacinthoides non-scripta) sampled directly from a woodland habitat are multispceies communities of varying composition which contain at least three genera of mycorrhizal fungi. The technique works well on a range of plant species and should have wide application to the identification of other symbionts, including pathogens. A spore survey has indicated that two particular AM types are associated with bluebells and this observation corroborates the molecular data. The presence of a Glomus species in bluebell roots was not expected from the spore data.

The influence of CO2 concentration on stomatal density

Abstract: summary A survey of 100 species and 122 observations has shown an average reduction in stomatal density of 14.3% (SE ±2.2 %) with CO2 enrichment, with 74% of the cases exhibiting a reduction in stomatal density. A sign test demonstrated that stomatal density decreases, in response to CO2, significantly more often than expected by chance. Repeated observations on the same species indicated a significant repeatability in the direction of the stomatal response. Analyses which removed the potential effect of taxonomy on this data set showed no significant patterns in the dependency of the degree of stomatal change on growth form (woodiness vs. non-woodiness; trees vs. shrubs), habitat (cool vs. warm) or stomatal distribution on the leaf (amphi- vs. hypostomatous). Forty-three of the observations had been made in controlled environments and under a typical range in CO., enrichment of 350–700 μmol mol−1. For these cases the average stomatal density declined by 9% (SE ± 3.3%) and 60% of the cases showed reductions in stomatal density. When analyses were restricted to these 43 observations, amphistomatous samples more frequently had greater changes in stomatal density than did hypostomatous samples. The degree of reduction in stomatal density with increasing CO2 increases with initial stomatal density, after the influence of taxonomy is removed using analyses of independent contrasts. When the data were examined for patterns that might be due explicitly to the effects of relatedness, the subclasses of the Hamamelidae and the Rosidae showed highly significant reductions in stomatal density with CO2 (87% of the species studied in the Hamamelidae and 80% of the species in the Rosidae showed reduction with CO2 enrichment) and correlations between initial stomatal density and degree of reduction in stomatal density. The species sampled in the Hamamelidae were dominantly trees, whereas herbs dominated the species in the Rosidae. There were insufficient species studied at lower taxonomic levels to warrant further statistical analyses. This problem results from experimental and observational data being most often restricted to one species per taxonomic level, typically up to the level of order, a feature which can severely limit the extraction of taxonomically-related and ecologically-related plant responses.

Biochemistry and molecular biology of lignification

Abstract: SUMMARY Lignins, which result from the dehydrogenative polymerization of cinnamyl alcohols, are complex heteropolymers deposited in the walls of specific cells of higher plants. Lignins have probably been associated to land colonization by plants but several aspects concerning their biosynthesis, structure and function are still only partially understood. This review focuses on the modern physicochemical methods of structural analysis of lignins, and on the new approaches of molecular biology and genetic engineering applied to lignification. The principles, advantages and limitations of three important analytical tools for studying lignin structure are presented. They include carbon 13 nuclear magnetic resonance, analytical pyrolysis and thioacidolysis. The use of these methods is illustrated by several examples concerning the characterization of grass lignins,‘lignin-like’materials in protection barriers of plants and lignins produced by cell suspension cultures. Our present limited knowledge of the spatio temporal deposition of lignins during cell wall differentiation including the nature of the wall components associated to lignin deposition and of the cross-links between the different wall polymers is briefly reviewed. Emphasis is placed on the phenylpropanoid pathway enzymes and their corresponding genes which are described in relation to their potential roles in the quantitative and qualitative control of lignification. Recent findings concerning the promoter sequence elements responsible for the vascular expression of some of these genes are presented. A section is devoted to the enzymes specifically involved in the synthesis of monolignols: cinnamoyl CoA reductase and cinnamyl alcohol dehydrogenase. The recent characterization of the corresponding cDNAs/genes offers new possibilities for a better understanding of the regulation of lignification. Finally, at the level of the synthesis, the potential involvement of peroxidases and laccases in the polymerization of monolignols is critically discussed. In addition to previously characterized naturally occurring lignin mutants, induced lignin mutants have been obtained during the last years through genetic engineering. Some examples include plants transformed by O-methyltransferase and cinnamyl alcohol dehydrogenase antisense constructs which exhibit modified lignins. Such strategies offer promising perspectives in gaining a better understanding of lignin metabolism and functions and represent a realistic way to improve plant biomass.

Atmospheric CO2, soil nitrogen and turnover of fine roots

Abstract: summary In most natural ecosystems a significant portion of carbon fixed through photosynthesis is allocated to the production and maintenance of fine roots, the ephemeral portion of the root system that absorbs growth-limiting moisture and nutrients. In turn, senescence of fine roots can be the greatest source of C input to forest soils. Consequently, important questions in ecology entail the extent to which increasing atmospheric CO2 may alter the allocation of carbon to, and demography of, fine roots. Using microvideo and image analysis technology, we demonstrate that elevated atmospheric CO2 increases the rates of both fine root production and mortality. Rates of root mortality also increased substantially as soil nitrogen availability increased, regardless of CO2 concentration. Nitrogen greatly influenced the proportional allocation of carbon to leaves vs. fine roots. The amount of available nitrogen in the soil appears to be the most important factor regulating fine root demography in Populus trees.

Identification of ribosomal DNA polymorphisms among and within spores of the Glomales: application to studies on the genetic diversity of arbuscular mycorrhizal fungal communities

Abstract: summary Little information currently exists on species diversity in communities of arbuscular mycorrhizal fungi (AMF), mainly owing to difficulties in identification of field extracted spores on the basis of morphology. The possibility was explored to identify individual AMF spores from the field on the basis of a molecular marker, namely the nuclear ribosomal DNA encoding the highly conserved 5.8S rRNA with the two flanking internal transcribed spacers (ITS region), known to vary between species. A technique involving polymerase chain reaction followed by restriction fragment length polymorphism analysis (PCR–RFLP) was developed to amplify and characterize the ITS region from single AMF spores. PCR reactions with extracts from single spores of three AMF species, raised under glasshouse conditions, yielded reproducibly a single amplification product of the ITS region in sufficient amounts to allow cleavage with several restriction enzymes. The size of the ITS region, c. 600 base pairs, varied only slightly between species. Digestion of the PCR products with the restriction enzymes Hinfl and Taq I resulted in banding patterns that were reproducible for different individual spores of a given species, but showed clear differences between the three species tested. The sum of the fragment sizes was sometimes greater than the size of the original PCR product, e.g. in Glomus mosseae. Clones of the amplification product from a single spore of this fungus were obtained and sequenced. This yielded two closely related but different sequences, indicating that two different ITS regions co-existed in the spore. The RLFP pattern of the amplification product of the spore was a result of an amalgamation of these two sequences. The technique was applied to AMF spores collected from a species-rich grassland. Spores were sorted into morphological groups on the basis of their colour, size, and shape, and then subjected to PCR–RFLP analysis. In some morphological groups, a large percentage of spores failed to yield an amplification product, probably because they had lost their contents. A group of Glomus spores yielding amplification products in the majority of cases was further investigated: PCR RFLP analysis on 10 individual spores from the field produced 10 different patterns. Similar results were obtained with other groups of spores. The results suggest that the diversity in natural AMF communities and the genetic diversity within individual spores might he much greater than previously thought.

Tansley Review No. 82. Strategies of arbuscular mycorrhizal fungi when infecting host plants

Abstract: summary Arbuscular mycorrhizal (AM) fungi are ancient Zygomycetes, thought to have colonized the first land plants; today, they are associated with the roots of about 80% of plant species. The symbiosis they form is potentially valuable not only for developmental programmes based on low-input agriculture, but also as a complex experimental model, where both fungal and host plant growth are regulated. Here we review some recent progress m the area of cell and molecular biology of arbuscular mycorrhizas. Particular attention is given to strategies followed by AM fungi when, as obligate biotrophs, they establish a successful symbiosis with their host plants. Four topics are analysed: (i) parameters controlling fungal growth in the absence and presence of the host root, Le. events of DNA replication and timing of the cell cycle; (ii) mechanical and enzymatic mechanisms which allow the fungus to colonize root tissues, maintaining host viability; (iii) morphological changes induced by AM fungus host cells and, in particular, the formation de novo of a subcellular compartment termed the interface, and (iv) modifications of plant gene expression during fungal colonization, including those involving structural, symbiotic and defence-related genes.

Effects of disease in wild plant populations and the evolution of pathogen aggressiveness

Abstract: SUMMARY Fungal pathogens can alter host fitness by affecting a plant's ability to survive, reproduce, compete, grow or defend itself against herbivores and other parasites. However, infections need not have uniformly negative effects, and the outcome of a fungus-plant interaction can range from strongly parasitic, through commensalistic, to mutualistic. The outcome of the interaction is determined by characteristics of the fungus and plant, as well as ecological conditions. Damping off and root rot diseases severely reduce plant survivorship. These pathogens appear to be highly aggressive, a trait that may be evolutionarily constrained because of the necrotrophic infection habit. Incidence is locally high with patch size being variable. Canker and wilt pathogens reduce plant survivorship and growth, and tend to be highly aggressive. Two diseases within this group, chestnut blight and Dutch elm disease, have caused sustained highly destructive pandemics on their hosts. Neither pathogen appears to be evolving towards genetically based reduced aggressiveness despite the fact that disease incidence is near 100 % over a wide area. Less aggressive strains may not be selectively favoured because plants can be multiply infected, which would allow more aggressive strains to invade trees previously infected by less aggressive strains. Obligate foliar diseases can affect plant fitness by reducing survivorship, reproduction, growth or competitive ability of the host. However, overall effects are often slight and the relationship between plant and pathogen range from strongly parasitic to nearly commensalistic. Disease incidence is highly variable across space and time, and much of the variability appears to be influenced by climatic and environmental variables. Non-systemic foliar diseases reduce plant reproduction, but some interactions may be nearly commensalistic because of low seed set in healthy individuals. Thus, disease effects are minimized in environments where reproduction by seed is not important. Systemically infecting pathogens can alter plant survivorship, reproduction, growth, competitive ability and susceptibility to herbivores. The effects can be positive as well as negative, and the net outcome of the interaction can range from strongly parasitic to unconditionally mutualistic. Trends within this group support Clay's New Function Hypothesis that pathogens can reduce their aggressiveness by acquiring new functions which increase plant fitness, and not by the amelioration of the original disease symptoms.

Endophyte infection effects on stomatal conductance, osmotic adjustment and drought recovery of tall fescue

Abstract: summary Presence of the endophytic; fungus Acremonium coenophialum Morgan-Jones and Gams in tall fescue (Festuca artuidinacea- Sehreb.) enhances host persistence in drought-prone environments. However, the physiological mechanism is not well understood. We investigated the influence of endophyte infection and water deficit on osmotic adjustment, stomatal conductance, tiller survival and leaf elongation rate of genotype GA87-122 and cv. Kentucky-31 (KY31) of tall fescue. Plants were grown in a greenhouse in pots containing fine-silty topsoil (experiment 1) or sand (experiment 2). In expt 1, endophyte-infected (EI) and endophyte-free (EF) isolines of GA87-122 were exposed to two drought preconditioning cycles, after which all pots were re-watered and osmotic adjustment determined. Osmotic adjustment was −0.46 and −0.51 MPa in El leaf blades and tiller bases and −0.29 and −0.13 MPa in EF leaf blades and tiller bases, respectively. In expt 2, EI and EF GA87-122 and KY31 Mere exposed to severe drought after two preconditioning cycles, then re-watered. During the second preconditioning drying cycle of KY31, stomatal conductance tended to be lower in EI than in EF plants, but the difference was significant (P 0–05) only at two sampling times. Shoot tissues osmotically adjusted 0.17–0.31 MPa more in EI than in EF plants. Tiller survival and leaf elongation rates were higher (P 0.05) in preconditioned EI than in EF plants. Basal-zone osmotic adjustment was correlated with tiller survival rate in GA87-122 and KY31 (r = 0.87, P 0.01 for both). Enhanced osmotic adjustment in the meristematic and growing zone might account far improved survival of tillers by facilitating protection of the apical meristem.

B chromosomes in plants

Abstract: summary This review of supernumerary B chromosome systems in flowering plants deals mainly with work published in the last 15 yr, hut also includes older material which has not hitherto been presented systematically. The term B chromosome (B) is defined, and there is an introductory overview dealing with general characteristics and the significance of Bs as a widespread chromosome polymorphism. Detailed sections are then presented covering the occurrence of Bs in different taxa, their structure and molecular organization, their irregular modes of inheritance, their phenotypic effects, population dynamics and origin. Particular attention is paid to the research growth points in molecular analysis of the structure and genome organization of Bs, and to transmission genotypes in the context of their adaptive versus their selfish properties. Where appropriate, reference is also made to likely future lines of research, and also to the usefulness of B chromosomes in genetic analysis and as model systems to study general phenomena of genome organization and evolution, nuclear physiology and architecture, chromosome polymorphism and selfish DNA.

Functional compatibility in arbuscular mycorrhizas measured as hyphal P transport to the plant

Abstract: summary The functional compatibility of symbioses between host plants and arbuscular mycorrhizal fungi was measured as hyphal P transport to plants. Cucumber (Cucumis sativus L.), wheat (Triticum aestivum L.) and flax (Linum usitatissimum L.) were inoculated with Glomus caledonium (Nicol. & Gerd.) Trappe & Gerdemann (isolate RIS42, BEG 15) and Glomus invermaium Hall (isolate WUM10) or left uninoculated and grown for 28 days in soil divided into three compartments. One side compartment was separated from the main compartment by a 20 um nylon mesh, and radioactive labelling with 32P was used to determine hyphal P uptake. The other side compartment was separated from the main compartment by a 700 um nylon mesh and 33P was used to determine the combined P‘uptake by roots and hyphae. All plant-fungus combinations were compatible with respect to mycorrhiza formation, measured both as root colonization and growth of external hyphae. In contrast, the symbioses differed markedly with respect to functional compatibility as phosphorus uptake by each fungus depended on the species of host plant. The hyphal transport of 32P was high in G. caledonium in symbioses with all three plant species, whereas G. invermaium transported significant amounts of 32P only when associated with flax. Consequently, to determine the P-transport effectiveness of a mycorrhizal fungus is meaningful only in the context of its associated host plant species.

Chlorophyll breakdown in senescent cotyledons of rape, Brassica napus L.: Enzymatic cleavage of phaeophorbide a in vitro

Abstract: Solubilization of senescent thylakoids from rape cotyledons in the presence of Triton X-100 was employed to establish an in vitro system that allowed the assessment of enzymatic conversion of phaeophorbide a into an uncoloured fluorescent chlorophyll catabolite, Bn-FCC-2. The action of the putative dioxygenase responsible for the cleavage of the porphyrin macrocycle depends on reduced ferredoxin as reductant. Apart from this thylakoidal catalyst, stromal protein is also required for the production of FCC-2 in vitro. The cleavage reaction does not occur with phaeophorbide b as substrate. Saturation kinetics with phaeophorbide a as substrate yielded an apparent Km -value of c. 200 μ. The enzyme contains iron as suggested by inhibitory effects of appropriate chelators. Enzyme activity lost upon treatment with bipyridyl was partly restored in the presence of Fe-ions; other metal ions such as Cu, Zn and Co were ineffective. The enzyme is absent in the thylakoids of mature green cotyledons. It appears upon the induction of foliar senescence and reaches the highest levels towards the end of the yellowing process.

Root-induced iron oxidation, pH changes and zinc solubilization in the rhizosphere of lowland rice

Abstract: summary Rice plants (Oryza sathiva L., cv. IR34) were grown with their roots sandwiched between cylinders of an anaerobic low-Zn Mollisol. After periods of root-soil contact of up to 12 d (total plant age c. 28 d) the profiles of different Zn fractions, reduced and oxidized Fe, and pH in the soil near the root plane' were determined. The concentration of easily plant-extractable Zn in the soil (measured by extraction in I M KCl) was negligible, and so it was necessary for the plants to induce changes in the soil to solubilize Zn. After 6 d, there was a substantial accumulation of Zn associated with organic matter and amorphous ferric hydroxide within 4–5 mm of the root plane. Over the next 6 d, the accumulation continued but there was a substantial depletion of the accumulated fractions within 2 mm of the root plane. The zones of accumulation and depletion coincided with zones of Fe(III) accumulation and soil acidification in which the pH decreased from the bulk soil value of 7.3 by over 0.2 pH units; i.e. a two-fold increase in H+ concentration, The acidification was the result of H+ released from the roots to balance excess intake of cations over anions, and H+ generated in the oxidation of Fe(H) by root-released O2. At the high pH and CO., pressure of the experimental soil (7.3 and c. 0.9 kPa. respectively), soil acidity diffusion is fast and consequently the pH drop at the root surface was small. The rate of Fe oxidation peaked before 6 d, but the acidification and Zn accumulation continued beyond 6 d unabated. It is concluded that Fe oxidation released Zn from highly insoluble fractions, and that this Zn was re-adsorbed on Fe(OH)3 and on organic matter in forms that were acid-soluble and therefore accessible to the plants.

Root hair length determines beneficial effect of a Glomus species on shoot growth of some pasture species

Abstract: SUMMARY Differences between plant species in the benefit derived from arbuscular mycorrhizal colonization have often been attributed to differences in physical properties of their roots, especially in root hair development. To test this hypothesis, the growth response to phosphate of five pasture species which differed markedly in the length of their root hairs was measured. Plants in the mycorrhizal treatments were inoculated with a Glomus sp. (isolate WUM 10(1)) while non-mycorrhizal plants received control inoculum. Benefit was described as the relative effectiveness of phosphorus (P) for the mycorrhizal plants compared with non-mycorrhizal plants. The beneficial effect of Glomus sp. was inversely related to root hair length of the host plant but it was not well related to root diameter, root length per plant or root/shoot ratio. It is suggested that root hairs and external hyphae of Glomus sp. act as alternative, similar ways of shortening the distance for the diffusion of phosphate in soils.

Field study of leaf photosynthetic performance by a Mediterranean deciduous oak tree (Quercus pubescens) during a severe summer drought

Abstract: SUMMARY Quercus pubescens Willd. is a winter deciduous oak species which co-occurs in the Mediterranean area with an evergreen oak, Q. ilex. As the knowledge of Q. pubescens assimilation patterns is poorly documented, we monitored leaf physiological parameters throughout a growing season. In summer, drought was particularly severe, and some trees experienced pre-dawn water potentials (Ψpd) as low as – 4·5 MPa. Diurnal courses of net photosynthesis, stomatal conductance and I photochemical efficiency of photosystem II (PS II) were recorded on four trees in the field. Maximum assimilation rates (10–16·5 μmol m−2 s−1, depending on sampled trees) occurred in spring. Leaf carbon gain recovered in autumn even after water potential fell to – 3·4 MPa. During progressive drought, Ψpd decreased substantially, leaf gas exchange was reduced and showed an important diurnal depression. At – 2 MPa (Ψpd), net photosynthesis and stomatal conductance were still about 5 μmol m−2 s−1 and 50 mmol m−2 s−1, respectively. Pre-dawn maximal PS II photochemical efficiency was stable to – 4 MPa. The diurnal decrease in photochemical efficiency of PS II (Fv/Fm), measured after 30 min of dark adaptation, was enhanced during drought but maximal values always recovered at sunset, i.e. leaves only experienced short-term reversible PS II inhibition induced by high light and temperature. In conclusion, Q. pubescens displays a strategy of tolerance to drought and an efficient protection against high irradiance.

Variation in moisture contents between bulk soil and the rhizosheath of wheat (Triticum aestivum L. cv. Wembley)

Abstract: SUMMARY There is conflicting evidence as to whether rhizosheath soil can be wetter or drier than bulk soil. The gravimetric moisture contents of bulk soil and the rhizosheath of wheat were determined for plants grown in a glasshouse over a range of dry bulk densities (10, 1-1 and 1-3 Mg m-3). Plants were also grown in soil at 1P1 Mg m-3 at 15 ?C in a controlled environment. No significant differences in soil moistures over the initial dry bulk density range examined were found. Overall, rhizosheath soil was significantly (P < 0 05) wetter than bulk soil. In the second experiment, under controlled environment conditions, significant differences (P < 005) were found only at the lowest depth range examined. The mechanism behind preferential wetting of the rhizosheath is examined with reference to other work on transfer of water to soil from roots, physical changes in the rhizosheath, and root exudates. The most likely explanation of these results is that the presence of mucigel within the rhizosheath increases the water-holding capacity of that soil.

Differential response of leaf conductance, carbon isotope discrimination and water‐use efficiency to nitrogen deficiency in maritime pine and pedunculate oak plants

Abstract: SUMMARY Growth, CO2 assimilation rate (A), leaf conductance (g), transpiration efficiency (W= ratio biomass production/plant water use) and carbon isotope discrimination (Δ) were assessed in maritime pine (Pinus pinaster Ait.) and pedunculate oak (Quercus robur L.) grown on a sand–peat mixture with three levels of fertilization: F100, optimal complete fertilization; F25, 25% of the optimal fertilizer supply; F0, no fertilization. Leaf phosphorus (P) and potassium (K) concentrations were affected little by the diminishing nutrient availability. Reduced fertilization decreased plant nitrogen (N) concentration in both species but leaf N concentration was less affected in oak than in pine. In pine W was markedly reduced in response to reduced leaf or whole plant N concentration, which was consistent with the sharp decrease also observed for plant intrinsic water-use efficiency (ratio A/g) both at the instantaneous (gas exchange data) and time-integrated (A/g derived from Δ measurements) levels. In this species, lowered W in the N deficient conditions was primarily associated with enhanced values of g. The existence of such a stomatal response pattern, confirmed by the increase in plant transpiration between F100 and F25, has not been reported before. In oak, both A and g were decreased in F25 and F0 as compared with F100. W was not affected – and instantaneous as well as time-integrated A/g values were only slightly decreased – in relation to decreasing plant N concentration. For F100, no difference in W was noticed between pine and oak though the Δ values were 2·6‰ lower in oak. We speculate that this discrepancy was linked with higher plant-carbon losses through processes like respiration, fine-root mortality or root exudation in oak. The isotopic approach proved useful for assessing the effects of nutritional status on W, but has to be used with caution when comparing different species.

Root‐induced solubilization of phosphate in the rhizosphere of lowland rice

Abstract: SUMMARY Lowland rice (Oryza sativa L., cv IR74) was grown in cylinders of a P-deficient reduced Ultisol separated into upper and lower cells by a fine nylon mesh so that the roots formed a planar layer above the mesh. This enabled changes in soil P fractions and other root-induced changes in the soil near the root plane to be measured. In both P-fertilized and unfertilized soil, the quantity of readily plant-available P was negligible in comparison with the quantity of P extracted by the plants, and the plants therefore necessarily induced changes in the soil so as to solubilize P. After 6 wk of growth, 90 % of the P taken up was drawn from acid-soluble pools. The remainder was from an alkali-soluble inorganic pool which was on balance depleted, although its concentration profile contained zones of accumulation corresponding to zones of Fe(III) accumulation. There was also a small accumulation of alkali-soluble organic P. There were no changes in the more recalcitrant soil P pools. The zone of P depletion was 4–6 mm wide, increasing with P addition, and coincided with a zone of acidification in which the pH fell from near 6 in the soil bulk to less than 4 near the roots. The acidification was due to H+ generated in oxidation of Fe2+ by root-released O2, and to H+ released from the roots to balance excess intake of cations over anions. With increasing P deficiency there were increases in the ratio of root: shoot d. wt; the ratio of shoot d. wt to total P in the plant; the excess intake of cations over anions per unit plant d. wt and corresponding release of H+ to the soil; and the quantity of Fe oxidized per unit plant d. wt and corresponding release of H+ to the soil. Independent, in vitro measurements confirmed that acid addition increased the P concentration in the soil solution and the quantity of P that could be desorbed per gram of soil. A mathematical model of the diffusion of acid away from the roots, acid reaction with the soil in solubilizing P, and the diffusion of P back to the absorbing roots showed that, under the conditions of the root-plane experiments, solubilization by acidification accounted for at least 80% of the P taken up in both P-fertilized and unfertilized soil, but that less than 50% of the P solubilized could be taken up by the roots.

Chlorophyll breakdown in senescent leaves: identification of the biochemical lesion in a stay-green genotype of Festuca pratensis Huds.

Abstract: Chlorophyll breakdown in senescent leaves proceeds in essentially three steps: dephytylation by the action of chlorophyllase; conversion of chlorophyllide to phaeophorbide by Mg-dechelatase; and oxygenolytic cleavage of the chlorine-macrocycle by a newly discovered dioxygenase. The metabolic lesion responsible for high retention of chlorophyll during foliar senescence in a mutant genotype of meadow fescue (Festuca pratensis Huds.) was located in the third step of the breakdown pathway. Senescent leaves of both the normally yellowing reference genotype, c.v Rossa, and the non-yellowing mutant Bf993 were shown to be competent with regard to chlorophyllase and Mg-dechelatase. On the other hand, thylakoids isolated from senescent leaves of cv. Rossa were able to carry out oxygenolysis of phaeophorbide into a colourless fluorescent catabolite in vitro, whereas Bf993 thylakoids were deficient in this activity. It is concluded that the Sid locus, a mutant allele of which is responsible for the stay-green character, encodes or regulates the gene for, phaeophorbide a dioxygenase.

Chemical changes in selected wetland plant species with increasing Fe supply, with specific reference to root precipitates and Fe tolerance

Abstract: summary A range of wetland plant species (44) was screened in solution culture experiments for sensitivity to large concentrations of supplied Fe. Two distinct root precipitates were observed (see ‘Appendix’ for a tabulated summary of symptoms). Energy dispersive spectroscopy showed that an ochreous precipitate was probably a hydrated ferric oxide whilst a pale yellow or yellow-grey one was an Fe-P solid, possibly a ferric phosphate. Ochre precipitates were found almost exclusively cm the roots of Fe-tolerant species and were particularly intense on the most tolerant ones, Monocotyledonous species (generally more Fe-tolerant than dicotyledonous species and with higher root porosities), tended to produce ochre most freely. The Fe-P precipitate was frequently found on roots of the less tolerant species and was particularly abundant on the most sensitive ones. Blackening of roots (with flaccidity I, another common response of roots of Fe-sensitive plants exposed to large Fe concentrations, was not a result of any obvious external precipitate. There was no evidence that differential pH changes induced by roots helped to determine either Fe tolerance or the nature of the root precipitate observed. Roots of all species tested caused the pH of an agar medium to rise. Elevated Ft- supply resulted in greater external acid phosphatase activity in roots of two test species. The increase corresponded with the species' mean relative growth rate (RGR) (and presumed P requirement), and with me fertility of sites in which it grows. Chemical analyses of shoots and roots of four species of differing Fe tolerance showed that, with increasing Fe supply, mean shoot Fe concentrations increased significantly whereas for most other elements (except Na and Zn) a significant reduction in concentrations in the shoot was observed. Mean root concentrations of most elements also fell significantly with increasing Fe supply, though for three of the four test species there was a significant increase in root P concentrations. It is clear that only some wetland plant species form ochreous root precipitates when grown in Fe-rich conditions, and have an effective Fe-exclusion mechanism. Growth of those species that lack this capacity is affected adversely by high Fe concentrations. This might be owing both to direct effects of Fe uptake and to indirect effects, particularly of Fe upon P metabolism. No evidence was found to suggest chat the Fe-tolerant species examined could accommodate higher tissue Fe concentrations than the Fe-sensitive species.

Arbuscular mycorrhizas and water relations in maize under drought stress at tasselling

Abstract: SUMMARY A greenhouse experiment was conducted to investigate the influence of an arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) on drought tolerance in tropical maize. Freshly regenerated seeds of selection cycles 0 (cv. CO, drought-sensitive) and 8 (cv. C8, drought-resistant) of the lowland tropical population 'TuxpefiQ sequia' were used for this study. Maize plants were subjected to drought stress for three weeks following tasselling (75-95 days after sowing). During the drought stress period, midday leaf water potential (LWP), stomatal resistance (SR) and transpiration rate (TR) were monitored daily, and green leaf area (GLA) determined at alternate days to assess the effects of mycorrhizal inoculation. Mycorrhizal plants of the cultivars CO and C8 had higher (less negative) LWP and TR, and lower SR throughout the experiment. The GLA was 27-5 % higher in mycorrhizal than non-mycorrhizal CO plants under drought conditions. Results indicate that mycorrhizas significantly improve tolerance to moderate drought stress imposed at tasselling, especially for the sensitive CO maize cultivar.

Enzyme activities of the internal hyphae and germinated spores of an arbuscular mycorrhizal fungus, Gigaspora margarita Becker & Hall

Abstract: SUMMARY A method for isolation of metabolically active internal hyphae from arbuscular mycorrhizas of onion (Allium cepa) is described. Roots of onion plants colonized by Gigaspora margarita were treated with a digestion solution containing cellulase and pectinase for 1 h at 30 °C, then homogenized with a Waring blender at low speed. The internal hyphae were collected from the homogenate by centrifugation on a discontinuous gradient of Percoll, then purified further by filtration. Enzyme histo chemical staining showed that the collected internal hyphae had active succinate dehydrogenase (SDH), alkaline and acid phosphatases (ALP, ACP). Specific activities (protein basis) of several enzymes in a crude extract of the internal hyphae were compared to those of axenically germinated spores of G. margarita. The specific activities of hexokinase, and of the enzymes involved in phosphate metabolism (ALP, ACP) and the TCA cycle (SDH, malate dehydrogenase) were much greater in the internal hyphae than in the germinated spores. The specific activity of phosphofructokinase was similar in both. By contrast, the specific activity of glucose-6-phosphate dehydrogenase was higher in the germinated spores. The metabolic change from non-symbiotic to symbiotic status of the fungus is discussed in relation to these results.

Differential toxicity of heavy metals is partly related to a loss of preferential extraplasmic compartmentation: a comparison of Cd-, Mo-, Ni- and Zn-stress

Abstract: SUMMARY Barley was grown at inhibitory concentrations of cadmium, molybdenum, nickel and zinc. Primary leaves were analyzed for cellular and subcellular compartmentation of the heavy metals. Epidermis and mesophyll protoplasts, mesophyll vacuoles and chloroplasts were isolated and apoplasmic washing fluid prepared, and the heavy metal contents of the various fractions determined. Efflux experiments showed that heavy metals were not lost from the preparations within the time span of the experiment. The different heavy metals were subjected to distinct distribution mechanisms within the leaves: (1) On a relative basis, the order of preferred epidermal accumulation was Cd = Zn > Mo > Ni (P < 0-01). (2) Within the mesophyll, Mo showed the highest degree of vacuolar compartmentation, whereas Ni was compartmentalized into the cytoplasm including chloroplasts to almost 80 %. (3) The low degree of vacuolar compartmentation was correlated with the development of damage in the leaves, as visualized by chlorosis and decreased quantum yield efficiency. (4) Damage was inversely correlated with apoplasmic compartmentation. (5) Interestingly, sulfhydryl contents of stressed leaves were neither positively nor negatively correlated with toxicity of the heavy metals: maximum induction was seen in the presence of Cd, followed by Zn, and no changes under Ni and Mo-stress. Increases in leaf SH-contents were small as compared with induction in the roots.

The role of plant-parasitic nematodes and soil-borne fungi in the decline of Ammophila arenaria (L.) Link.

Abstract: summary In coastal foredunes, Ammophila arenaria (L.) Link grows vigorously when it is buried regularly by windblown sand and degenerates at stabilized sites. Nematodes and soil-borne fungi were found to be involved in its decline. In order to establish their role in the disease complex, seedlings of A. arenaria were inoculated with several groups of potentially harmful soil organisms that were isolated from its root zone. Inoculation of single species of fungi did not reduce the growth of the seedlings, but combining all fungi that were commonly found in the Dutch coastal foredunes significantly reduced growth to about 80% of that in sterilized soil. This indicates synergistic effects between commonly found plant pathogenic fungi. The addition of large numbers of the nematode Telotylenchus ventralis, the only species that could be successfully grown on A. arenaria in the laboratory, reduced plant growth to the same level as in non-sterile soil, but numbers were 80 times greater than in the latter soil. Inoculation with relatively large numbers of T. ventralis in combination with the commonly occurring fungi reduced plant growth in sterilized soil to a level similar to that in non-sterile soil. The involvement of other species of nematodes, such as Heterodera spp. or Meloidogyne maritima, in the decline of A. arenaria in non-sterile soil could not be proven in inoculation experiments with sterile soil, but it is likely that these species may also be involved. It is concluded, therefore, that several different combinations of soil organisms can be harmful to A. arenaria, so that natural decline is not caused by one simple well defined pathosystem.

Interacting effects of soil fertility and atmospheric CO2 on leaf area growth and carbon gain physiology in Populus × euramericana (Dode) Guinier

Abstract: SUMMARY Two important processes which may limit productivity gains in forest ecosystems with rising atmospheric CO2are reduction in photosynthetic capacity following prolonged exposure to high CO2 and diminution of positive growth responses when soil nutrients, particularly N, are limiting. To examine the interacting effects of soil fertility and CO2 enrichment on photosynthesis and growth in trees we grew hybrid poplar (Populus × euramericana) for 158 d in the field at ambient and twice ambient CO2 and in soil with low or high N availability. We measured the timing and rate of canopy development, the seasonal dynamics of leaf level photosynthetic capacity, respiration, and N and carbohydrate concentration, and final above- and belowground dry weight. Single leaf net CO2 assimilation (A) increased at elevated CO2 over the majority of the growing season in both fertility treatments. At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO2, leading to a 49% increase in total dry weight. In contrast, at low fertility leaf area growth was unaffected by CO2 treatment. Total dry weight nonetheless increased 25% due to CO2 effects on A. Photosynthetic capacity (A at constant internal p(CO2), ((C1)) was reduced in high CO2 plants after 100 d growth at low fertility and 135 d growth at high fertility. Analysis of A responses to changing C1 indicated that this negative adjustment of photosynthesis was due to a reduction in the maximum rate of CO2 fixation by Rubisco. Maximum rate of electron transport and phosphate regeneration capacity were either unaffected or declined at elevated CO2. Carbon dioxide effects on leaf respiration were most pronounced at high fertility, with increased respiration mid-season and no change (area basis) or reduced (mass basis) respiration late-season in elevated compared to ambient CO2 plants. This temporal variation correlated with changes in leaf N concentration and leaf mass per area. Our results demonstrate the importance of considering both structural and physiological pathways of net C gain in predicting tree responses to rising CO2 under conditions of suboptimal soil fertility.

Mycorrhizal fungus propagules in the jarrah forest II. Spatial variability in inoculum levels

Abstract: Spatial variations in the capacity of propagules of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi to form associations in their natural habitats were investigated using bioassays with bait plants grown in intact cores of forest soil. These cores were collected from a sclerophyllous forest community dominated by Eucalyptus marginata Donn ex Smith (jarrah) and E. calophylla Lindley (marri) trees with a diverse shrub understorey in the mediterranean (winter rainfall) climatic zone of Western Australia. Small-scale (adjacent core) variations in the capacity of AM fungi to form associations were found to be as substantial as differences between locations 1 5 m apart. Comparisons of AM fungus colonization patterns within the roots of seedlings growing in the same core indicated that there was considerable spatial heterogeneity in the inoculums potential of 'individual' fungi within these 1 1 volumes of soil. A second experiment included bait plants to measure ECM formation as welt as AM formation and also considered the impact of soil disturbance. The disruption of hyphal networks reduced mycorrhizal formation somewhat, but it still remained highly variable. Some of this spatial heterogeneity could be attributed to differences in the organic matter content, length of fungal hyphae, or length of old mycorrhizal roots, measured within soil cores. In jarrah forest soil, mycelial systems of AM and ECM fungi apparently were localized in separate domains, and there were also zones where non-mycorrhizal roots (mostly cluster roots produced by members of the Proteaceae) predominated. More research is required to determine the size of domains of mycorrhizal mycelial systems in soils, how these spatial patterns change with time, and if they are associated with zones of resource utilization by different 'functional groups' of roots.