This paper describes the effects of hypomethylation with 5-azacytidine (5azaC) and dihydroxypropyladenine (DHPA) on protonemata of the moss Funaria hygrometrica Hedw. Following treatment with 5azaC or DHPA, hypomethylation of the EcoR II (CCA(T)GG) and Pst I (CTGCAG) sites in ribosomal DNA (rDNA) was confirmed using restriction enzyme analysis and Southern hybridization. Hypomethylation of the genome had profound effects on protonemal differentiation. Whilst apical cell organization and cell dimensions and shape remained unchanged, there was a marked retardation in both cytoplasmic and nuclear differentiation. Developmental abnormalities included: late and erratic side branch formation, some loss of the distinction between chloronema and caulonema, formation of aberrant buds, and loss of the potential to form brood and tmema cells after 5azaC treatment which in general had more profound effects than DHPA. Cytologically, caulonema cells were less highly polarized or unpolarized, and tended to retain small round chloroplasts, whilst nuclei in the hypomethylated protonema endoreduplicated to lower levels and tended to remain more spherical. Increased nucleolar volumes and loss of intranucleolar rDNA heterochromatin following hypomethylation might be the result of increased transcriptional activity of ribosomal RNA (rRNA) genes and drug-induced DNA decondensation, respectively. Growing hypomethylated protonema at 25°C induced extremely atypical cells and development. This temperature sensitivity and aberrations in development overall can be attributed to changes in the normal patterns of gene expression brought about by hypomethylation of gene promoter or regulator regions.

The effect of simultaneous N2 fixation and light limitation on the growth of two strains of Anabaena sp. Bory de St. Vincent and Aphanizomenon flos-aquae (L.) Ralfs was investigated using continuous cultures. Under severely light-limited conditions, Aphanizomenon showed a broader absorption spectrum (due to the presence of phycoerythrin), a higher maximum efficiency of photosynthesis, a higher steady-state N2 fixation activity and a higher growth affinity for light than did Anabaena. On the other hand, under light saturation, Anabaena showed a higher maximum rate of O2 production and a higher maximum specific growth rate than Aphanizomenon. These monoculture results characterize Anabaena and Aphanizomenon, in relative terms, as a ‘sun’ and a ‘shade’ species respectively, and are in accordance with field observations. The difference between the two species in their acclimatory response is discussed in terms of a species-specific alteration of the PSI[ratio ]PSII stoichiometry. Besides the species-specific modulation of the accessory pigments, such an acclimation would provide a biochemical basis for the observed physiological differences. The monoculture results were used to differentiate the niches of the two species and suggested that Aphanizomenon would competitively displace Anabaena under N2-fixing, light-limited conditions. However, when both species were grown together, Anabaena became dominant and seemed to be the superior competitor for light. In order to explain this finding, the possible effects of release of allelopathic compounds, or dynamic aspects of light supply, are discussed.

Plant roots enter symbiotic as well as pathogenic interactions with fungi in the rhizosphere. We studied the response of bean (Phaseolus vulgaris L. cv. Saxa) roots to infection by the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and the pathogen Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burkholder) Snyder & Hansen. In a time-course study of the symbiotic interaction between bean roots and G. mosseae, covering all stages of mycorrhiza development, we detected little change in the expression of the defence-related genes chitinase, β-1,3-glucanase and phenylalanine ammonia-lyase compared with non-mycorrhizal control roots. The only difference observed was a transient increase in chalcone synthase transcripts at later stages of mycorrhizal root colonization. In interactions with the pathogen, a marked induction of chitinase and phenylalanine ammonia-lyase expression was observed at the level of both the transcripts and enzyme activities. Class I β-1,3-glucanase levels strongly increased at the transcript level, but there was little change in the overall β-1,3-glucanase enzyme activity. In the non-host interaction between common bean and Fusarium solani (Mart.) Sacc. f. sp. pisi (Linford) Snyder & Hansen defence responses increased only slightly and transiently, if at all.

Arbuscular mycorrhizal (AM) fungi form mutualistic symbioses with the root systems of most plant species. These mutualisms regulate nutrient exchange in the plant–soil interface and might influence the way in which plants respond to increasing atmospheric CO2. In other experiments, mycorrhizal responses to elevated CO2 have been variable, so in this study we test the hypothesis that different genera of AM fungi differ in their response, and in turn alter the plant's response, to elevated CO2. Four species from three genera of AM fungi were tested. Artemisia tridentata Nutt. seedlings were inoculated with either Glomus intraradices Schenck & Smith, Glomus etunicatum Becker & Gerdemann, Acaulospora sp. or Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders and grown at either ambient CO2 (350 ppm) or elevated CO2 (700 ppm). Several significant inter-specific responses were detected. Elevated CO2 caused percent arbuscular and hyphal colonization to increase for the two Glomus species, but not for Acaulospora sp. or S. calospora. Vesicular colonization was not affected by elevated CO2 for any fungal species. In the extra-radical phase, the two Glomus species produced a significantly higher number of spores in response to elevated CO2, whereas Acaulospora sp. and S. calospora developed significantly higher hyphal lengths. These data show that AM fungal taxa differ in their growth allocation strategies and in their responses to elevated CO2, and that mycorrhizal diversity should not be overlooked in global change research.

Mycelial cord systems of Phanerochaete velutina (DC.: Pers.) Parmasto grown from 4 cm3 inocula on a nutrient-depleted non-sterile soil in compartmentalized laboratory microcosms were baited after 13 d of growth with either fresh, non-sterile 4 cm3 wood blocks or control Perspex® blocks of the same contact area. After 112 d, mature mycelial systems, which were in senescent phase, were subjected to disturbance by supplying a new fresh wood bait diametrically opposite the existing bait and, after 126 d, to nutrient regime amendment by application of NPK solution. At harvest (186 d) there was a significant (P[les ]0·001) linear relationship between extra-resource mycelial biomass and total wood decay over the preceding 74 d. Nutrient amendment alone did not significantly (P>0·05) affect extra-resource mycelial biomass production or wood decay rates in either disturbed or undisturbed cord systems. However, both mycelial biomass production and resource decay were significantly (P[les ]0·05) enhanced when nutrient amendment and disturbance treatments were applied concurrently. Bi-directional phosphorus translocation to inocula and wood baits (determined non-destructively) was assessed by labelling the NPK solution applied distal or proximal to the initially supplied bait with 32P. In both disturbed and undisturbed cord systems the rate of 32P uptake from a local supply was two orders of magnitude higher than from a distal supply. In disturbed cord systems uptake of 32P by inocula, which were midway between the two radiotracer supply points, was significantly (P[les ]0·05) higher when the supply point contained a newly supplied wood bait. Net translocation of 32P to newly supplied wood baits increased with time at the expense of translocation to inocula and existing wood baits. The switch in direction of net phosphorus translocation, the importance of localized nutrient scavenging and the partitioning of wood decay are discussed in relation to the ecological significance of mycelial cords.

The host range of ectomycorrhizal (ECM) fungi in Britain was examined by compilation of a data matrix from published literature sources, based primarily on accounts of sporocarp associations with particular host genera. Information was gathered for 577 species of ECM fungi belonging to 51 genera, and 25 genera of host trees, representing the majority of ECM fungal species and host genera recorded in Britain.

Pronounced variation was recorded in the number of ECM fungal species associated with different host genera, with over 200 species recorded with Betula, Fagus, Pinus and Quercus. There was a positive linear relationship (r2=0·47, P=0·007) between the number of species of ECM fungi associated with different host genera and the total area occupied by each tree genus in Britain (both values log-transformed). There was also variation in the number of species of ECM fungi which were apparently specific to particular host genera, values ranging from zero (in 15 genera) to >40 in the case of Betula and Fagus. In total, 233 fungal species appeared to be specific to a single host genus (i.e. 40% of those surveyed). Comparison of the ECM mycota associated with different host genera by PCA accounted for 17% of the total variation, with genera belonging to the Fagaceae (Quercus, Fagus and Castanea) tending to cluster together, indicating a degree of overlap in their ECM associates. Exotic conifer species, which displayed a lower ECM diversity than would be expected from their distributional areas, were characterized by a high degree of overlap with the ECM associates of Pinus and Betula.

These results indicate that the abundance of different genera of host trees and variation in host specificity could provide a basis for understanding patterns of diversity in ECM fungi within Britain.

The transmission of infection by many soil-borne fungal parasites of plants depends on the ability of the fungus to grow on or through soil. Progress in analysing the effects of soil physical factors on the temporal and spatial dynamics of fungal growth has been hindered by technical difficulties of quantifying fungal biomass in soil and heterogeneity in soil properties. In this paper we use a combination of a monoclonal antibody-based immunosorbent assay and microscopy to analyse the effects of soil physical properties on the spatial and temporal dynamics of colonies of the economically important fungus Rhizoctonia solani Kühn growing in two dimensions and three dimensions in a sand. Combinations of different particle-size distributions and matric potential are used to manipulate the air-filled pore volume and pore-size distribution independently of each other. Temporal dynamics are measured by the change in fungal biomass over time whereas spatial dynamics relate to fungal spread and are measured by the colony size, the rate of colony expansion and the biomass distribution within colonies. We show that the fungus spreads more than three times further over surfaces than through sand, even though the same amount of biomass is produced in each case. Pore-size distribution and air-filled pore space both affected the extent and rate of fungal spread in three dimensions within sand, with more rapid and extensive spread in a coarse sand compared with a fine sand at identical air-filled pore volume. The spread of fungal hyphae along surfaces was affected neither by differences in surface texture nor by air-filled volume, and was substantially more homogeneous than for three-dimensional spread. We argue that the relative impermeability of sand surfaces to penetration by hyphae might be influenced by the ability of the fungus to branch within a confined space rather than simply to penetrate the pores. The broader epidemiological and ecological consequences of preferential spread by parasitic and saprophytic fungi along surfaces rather than through the dense soil volume are discussed.

The relative importance of some aspects of iron nutrition for the distribution of six Plantago species (P. maritima L., P. coronopus L., P. major ssp. major L., P. serpentina All., P. media L. and P. lanceolata L.) with different habitat requirements for soil pH and moisture was evaluated. Iron efficiency and Fe tolerance of hydroponically-grown plants were assessed by determining the decrease in relative growth rates caused by suboptimal and supra-optimal external Fe concentrations. Marked interspecific differences were observed in visual symptoms of Fe deficiency and Fe toxicity and in the external Fe concentrations leading to 50% inhibition of maximal relative growth rates (rgr50%). Whereas P. serpentina displayed a clear preference for low external Fe concentrations, growth rates of P. maritima and P. media were found to be increased at higher concentrations. Severe growth restriction at both low and high Fe concentrations was evident in P. lanceolata and P. major. A broad optimum was observed in P. coronopus exhibiting a low external Fe requirement and a high tolerance to supra-optimal Fe concentrations. Iron efficiency and Fe tolerance differed in a way which was only partly correlated with the expected Fe availability at the natural habitat of the species, suggesting that Fe concentrations are of minor importance for their distribution, or that some of the mechanisms that render Fe oxides available for uptake in situ are masked in solution experiments.

To gain insight into the impact of the physiological and morphological characteristics of the species on Fe efficiency, the effect of Fe status on shoot[ratio ]root ratio, relative root surface area, root Fe(III) reductase and proton extrusion capacity were investigated. Roots of all species showed increased Fe chelate reduction activity upon growth at suboptimal Fe concentrations; marked differences were observed with respect to the kinetic parameters of the reductase. Maximal velocity of the reduction was positively correlated with relative growth rate and was not related to the Fe efficiency of the species. By contrast, Km corresponded to Fe efficiency ranking and can therefore be regarded as an important parameter for the uptake of Fe at low availability. Enhanced ability to acidify the rhizosphere was only observed in P. major. From the morphological characteristics investigated, root surface area appears to be the most important parameter in the uptake of Fe at suboptimal external concentrations.

An early successional plant community was exposed to various ozone concentrations for two growing seasons (1994–1995) in open-top chambers in Auburn, Alabama, USA. The ozone treatments were: AA, ambient air (open plots); CF, carbon-filtered air (c. 0·5×ambient air), 1×, non-filtered air, and 2×, twice ambient air. Vegetative canopy cover exhibited a pattern of accumulation in the spring, with maximum canopy cover attained in summer, then senescence of foliage in the autumn 1994. This pattern was not observed in 1995 as a result of a drought during the spring and summer. Varying ozone exposures caused shifts in the competitive interactions between plants, thereby altering community structure. Higher canopy cover, vertical canopy density (layers of foliage), species richness, diversity, and evenness existed in the CF treatments than in the other treatments. In addition, winged sumac (Rhus copallina L.) became a major component of the CF treatments only during 1995. Surprisingly, blackberry (Rubus cuneifolius Pursh.), a species considered ozone-sensitive, based on visible injury, dominated canopy cover within the 2× treatments, 41 and 33% of total canopy cover in 1994 and 1995, respectively. From these results it is concluded that plant communities existing in areas where lower ozone concentrations are prevalent might be more complex and diverse than those existing in areas with higher ozone concentrations.

The effects of low levels of nitrogen addition (7·7 and 15·4 kg N ha−1 yr−1) on plant sensitivity to biotic and abiotic stress were studied at a lowland heath in the south of England that has received N treatments since 1989. Larval growth rates and adult weights of heather beetles were found to be significantly higher when insects were reared on plants that had received additional N, with implications for insect survival and reproductive success. Electrolyte leakage measurements failed to reveal any significant impact of N addition on plant sensitivity to frost episodes in early winter. In April, however, there was some evidence of slightly decreased frost hardiness in plants receiving additional N. Accelerated spring bud burst also suggested earlier physiological activity in N-treated plots. The rate of water loss from excised shoots of Calluna vulgaris (L.) Hull was significantly faster in plants receiving additional N, although no difference in plant water potential was measured in the field after a prolonged dry spell. Whilst experimental N addition had only a small effect on plant sensitivity to abiotic stress, the relationship between enhanced deposition and increased insect performance was clear, with the potential for substantially increased insect damage at deposition rates around the critical load contributing to the formation of gaps in the Calluna canopy.

Following a precultivation with pedospheric nitrogen nutrition, nitrate or ammonium solutions were supplied to the shoots of Ricinus plants by spraying (during the experimental period) resulting in an increase of biotic/organic and abiotic/inorganic particles on the surface, which significantly increased wetting of the leaf surfaces. The distribution of particles on the surface of sprayed leaves, in particular crystals around and in stomata, indicated the possible entry of nutrients via thin water films through the stomatal pores in addition to diffusion through the cuticle. Ammonium was taken up more readily than nitrate by the foliage, but both at relatively low rates which caused N limitation. Interestingly, the inorganic N, both in the form of nitrate and even ammonium, was entirely assimilated in the shoots; phloem transport of inorganic N to the root was negligible. The flows of malate, and the acidification of the apoplastic washing solution of leaves in ammonium-sprayed plants pointed to the role of metabolism of malate and excretion of protons in maintaining pH during ammonium assimilation in the shoot. Ammonium-sprayed plants incorporated the N in the same amounts in shoots and roots, only 38% of the shoot-borne N being recycled in the xylem. In nitrate-sprayed plants the root was not only favoured in N partitioning, but even a net export of previously incorporated N from the shoots occurred which reflected the N limitation. The N limitation also affected carbon metabolism, in particular the flows of C, incorporation in the shoot and photosynthesis, which were decreased when compared with data from recent experiments with pedospheric well fed Ricinus. However, there was little difference in C flows between nitrate and ammonium-sprayed plants with respect to respiration, C partitioning and, most interestingly, in relative stimulation of root growth. The loss of C from dark respiration of the shoots was high on a f. wt basis as well as in relative terms, owing to exclusive N assimilation in the shoot. In general the plants invested untargeted increases in root growth as a result of N limitation irrespective of the imposed artificial treatment which made the shoot the site of mineral N uptake.

The effect of a very high CO2 mole fraction (27000–35000 μmol mol−1) on photosynthesis and water relations was studied during the dry and the rainy season in plants of Spatiphylum cannifolium (Dryand.) Schott and Bauhinia multinervia (H.B.K.) DC. growing near natural cold CO2 springs. Xylem water potential in plants of both species was lowered by drought, high CO2 growth-concentration decreasing it further in S. cannifolium. In plants of both species growing under high CO2 concentration photosynthetic rates measured at a CO2 mole fraction of 1000 μmol mol−1 were higher than in plants growing at ambient CO2 mole fraction and measured at 350 μmol mol−1. The response was the result of a direct effect of CO2 on the photosynthetic machinery. Changes in carboxylation efficiency in response to high CO2 were found during the rainy season, with an increase in S. cannifolium and a decrease in B. multinervia; a significant interaction between growth CO2 concentration and season in B. multinervia resulted from significant effects of both factors. An increase in intrinsic water-use efficiency due to high CO2 was determined in both species by an increase in photosynthetic rate as well as a decrease in leaf conductance. In high-CO2 plants of S. cannifolium a 71% decrease in stomatal density and 73% in stomatal index suggested that CO2 affected stomatal initiation, whereas in B. multinervia an 85% decrease in stomatal index and a 72% decrease in stomatal density indicated that CO2 influenced stomatal initiation as well as epidermal cell expansion. Our results indicate that very high CO2 concentrations did not inhibit photosynthesis in these species, and that growth under high CO2 allowed plants to attain carbon balances higher than those of plants growing under low CO2. This was particularly so during the dry season, since the photosynthetic rates at the corresponding ambient concentration were higher in plants nearer the springs, and carboxylation efficiency and some stomatal characteristics of both species apparently acclimated to high CO2, but patterns were not consistent and bore no obvious relationship to photosynthetic capacity.

An homologous series of hydrocarbons ranging from C21 to C37 (was detected in foliar cuticular waxes of Austrocedrus chilensis (D. Don) Pic.-Ser. & Bizz., by gas chromatography. Qualitatively, the chromatograms were similar for all individuals sampled from 29 natural populations in Chile and Argentina, showing C33 and C35 alkanes to be dominant. Quantitatively, the chromatograms varied significantly among populations. Multivariate analyses showed that arid zone populations from mediterranean Chile and the Patagonian steppe were different from one another and were distinct from mesic populations close to the Valdivian rainforest vegetation zone. Mediterranean Chilean populations were characterized by higher concentrations of the longer carbon-chain alkanes, but retained equal amounts of shorter-chain homologues as the mesic populations. A greater variation in chain lengths and higher concentrations of longer carbon chains in the mediterranean populations is consistent with a model for the adaptation to reduced cuticular permeability. Partial Mantel matrix tests revealed significant climatic and inter-population distance effects with taxonomic distances based on multivariate and univariate hydrocarbon data. Annual rainfall was overall the most significant factor, particularly in regressions with the shorter-chain hydrocarbons. Annual mean temperature was most significant for the longer-chain hydrocarbons. This suggests, on the one hand, ecogenic adaptation to both temperature and precipitation of cuticular hydrocarbon composition, and, on the other hand, a weaker, but important effect of gene flow in determining hydrocarbon composition in this species. The northernmost populations at San Felipe and San Gabriel were the most distinctive. This could result from the effects of random changes in allele frequencies and/or to founder effects in isolated and small populations. These populations show some East-Andean affinities presumably due to historic migration patterns.

Four pot experiments are reported in which Norway spruce (Picea abies (L.) Karst) seedlings, of different nutrient status, were treated with acid mist for one growing season in open-top chambers (OTCs). Combinations of H+, SO42−, NH4+ and NO3− were applied at different frequencies of application and supplying different doses of S and N kg ha−1. Plant growth, visible injury, frost hardiness and nutrient status were observed. These experiments were undertaken to improve our understanding of the interaction of environmental factors such as nutrition and mist-exposure frequency on seedling response to N and S deposition.

Both acidity ([les ]pH 2·7) and SO42− ions were necessary to induce visible injury. Mist containing SO42−, H+ and to a lesser extent NH4+ significantly reduced winter frost hardiness. Increasing the misting frequency, and to a lesser extent the overall dose, increased the likelihood of acid mist causing visible injury and reducing frost hardiness. Post-planting stress, low N status and needle juvenility increased the likelihood of acid mist causing visible injury. Increased plant vitality, adequate N status and growth rate reduced the likelihood of acid-mist-induced reductions in frost hardiness.

Principles underlying the responses of spruce seedlings treated in controlled conditions to acid mist are discussed.

The cluster roots of Grevillea robusta A. Cunn. ex R. Br. are composed of determinate rootlets that stop growing, but remain physiologically active for several months. Their apical organization, both before and after maturation, was studied by light and transmission electron microscopy. Each cell layer forms a dome, with an initial cell at its end. Xylem elements form a complicated triarch array at the base of the rootlet, passing along the rootlet as two files, and then joining at the tip to form a single file, surrounded by six pericycle cells. At the base of the rootlet, shorter xylem cells and thick-walled support cells are visible. A root cap, present in rootlets grown in vermiculite, was eventually displaced by root hair growth. Rootlets grown in Hoagland's solution lacked root caps and were significantly shorter than those grown in vermiculite. Cell fate was analysed in terms of cell position and is discussed in terms of pattern and development.

Light, fluorescence and electron microscopy were employed to follow the development of the endodermis in cluster roots and lateral roots of Grevillea robusta A. Cunn. ex R. Br. Endodermal cells had three different origins: rootlet endodermis arose from the rootlet meristem; endodermis covering the primordium shortly after initiation came from division of parental endodermis; cells at the junction between parent and rootlet endodermis developed from re-differentiated rootlet cortical cells. In the cluster root, the Casparian band formed in three ways, and was not initially present opposite the two sets of single xylem elements in the rootlet stele. A new clearing technique was developed that allowed visualization of xylem, suberized endodermis, Casparian band formation and phenolic compounds. In lateral roots, endodermal differentiation was asynchronous, but was related to position relative to protoxylem poles. However, the observed delay began before these poles had differentiated. At the tip of mature rootlets, which are determinate, the endodermis terminates in a ‘dome’ of cells, with the initial cell differentiating as an endodermal cell. Results are discussed in terms of determinate development in roots and the spatial and temporal contexts within which this development takes place.