This review describes the life cycle of Mesembryantheum crystallinum L. (the common ice plant, Aizoaceae, Caryophyllales), a halophyte with a developmentally programmed switch from C3 photosynthesis to Crassulacean acid metabolism (CAM) which is accelerated by salinity and drought. Since there has been controversy regarding the interplay between genes and environmental stimuli during the development of M. crystallinum, it is timely to summarize the life cycle for a defined set of conditions. We seek to establish the framework whereby five stages of development can be described in terms of morphology, physiology, and molecular biology. Stages 1 and 2, representing germination and growth of a juvenile form, show a determinate pattern of growth. Although specific genes for salt tolerance can be induced at these stages, stress early in development prevents progression to the mature form (stages 3–5) in which the plants advance to mature growth, flowering, and seed development. Growth in stage 3 is indeterminate in the absence of stress, but development and flowering are accelerated by environmental stresses, and CAM is constitutively expressed. Depending on the severity of the stress, plants start to flower (stage 4) and then die from the roots, ultimately with only seed capsules remaining viable, with salt sequestered into large epidermal bladder cells (stage 5). We highlight responses to salinity leading to compartmentation of ions and compatible solutes, turgor maintenance, and CAM. Finally, the molecular genetics of the ice plant are characterized, emphasizing selected genes and their products. We conclude with an analysis of the multiple stages of growth as an ecological adaptation to progressive stress. The initial determinate and inflexible juvenile phase provides a critical mass of plant material which supports the indeterminate, mature phase. Depending on the degree of stress, the mature form is then propelled towards flowering and seedset.

During the growing session of 1995, the total soluble non-protein nitrogen (TSNN) composition and contents of mycorrhizal fine roots, xylem sap and phloem exudates of roots from a coniferous (Picea abies L.(Karst)) and a deciduous (Fagus sylvatica L.) tree species were analysed at a field site (‘Höglwald’, Germany) exposed to high loads of N. In April, TSNN in fine roots of spruce and beech trees amounted to 16 μmol N g−1 f. wt and 23·3 μmol N g−1 f. wt, respectively. It decreased to 9·2 μmol N g−1 f. wt and 18·1 μmol N g−1 f. wt, respectively, after bud break in June. The seasonal maximum of TSNN in fine roots of spruce was observed in July (32·7 μmol N g−1 f. wt) followed by a decline of c. 30% until the end of the growing season in September. TSNN in fine roots of beech trees showed a further decline between June and July, when its seasonal minimum was determined (15·6 μmol N g−1 f. wt), and increased to c. 29 μmol N g−1 f. wt until September. In spruce roots Gln and Arg were the most abundant TSNN compounds during the entire growing season. In roots of beech Asn played an important role alongside Gln and Arg, especially in April, when it was the most abundant TSNN compound. Other proteinogenic and non-proteinogenic N compounds comprised c. 20–30% of TSNN. Nitrate made up <1%, and ammonium <7% of TSNN in the fine roots of both species.

In April, TSNN in the xylem sap of roots of spruce and beech trees amounted to 3·4 and 8·6 μmol N ml−1, respectively. In roots of spruce trees xylem sap TSNN increased after bud break up to 12·7 μmol N ml−1 in July. At the end of the growing season TSNN had declined again to 3·9 μmol N ml−1. TSNN in the root xylem sap of beech trees decreased after bud break until July (2·4 μmol N ml−1 in July) followed by a slight increase until September (2·9 μmol N ml−1). Arg, Gln and Asp were the most abundant TSNN compounds in the xylem sap of spruce trees contributing together c. 90% to TSNN. The same TSNN compounds prevailed in the root xylem sap of beech trees in April and July, whereas in June and September Asp was replaced by Asn comprising 57% of TSNN in June. In addition to the N compounds mentioned above, a number of other proteinogenic and non-proteinogenic amino compounds were found in root xylem sap of both species. In either species, nitrate and ammonium were present in small amounts, contributing <1% and <4% to TSNN, respectively. Apparently, inorganic N taken up by the mycorrhizal roots is mainly assimilated in root tissues or by the mycorrhiza and N uptake by the roots is largely adapted to the assimilatory capacity of this organ.

In phloem exudates of spruce roots, TSNN amounted to 10·7 μmol N g−1 f. wt in April, increased in June to 23·4 μmol N g−1 f. wt and decreased again until September to a seasonal minimum of 4·8 μmol N g−1 f. wt. In contrast to spruce, TSNN content in phloem exudates of beech roots showed a seasonal maximum (c. 20 μmol N g−1 f. wt) in April with a subsequent decrease in June after bud break (c. 2 μmol N g−1 f. wt). A fourfold increase in July was followed by a decrease in September, when TSNN in phloem exudates of beech roots amounted to 4·3 μmol N g−1 f. wt. Arg was the most abundant N compound in the phloem of roots from spruce trees and made up c. 60–85% of TSNN during the entire growing season. In beech trees the seasonal course of TSNN correlated with the relative abundance of Arg. Arg comprised 69 and 57% of TSNN in April and July, respectively, but contributed <20% in June and September. Besides Arg, other proteinogenic and non-proteinogenic amino compounds could be detected in the phloem of both species. In addition, nitrate and ammonium were present in considerable amounts.

From these results and a previous report on TSNN in above-ground parts of spruce and beech at the same site, a whole-plant model for the cycling of TSNN in both species is proposed. Differences in the location of storage pools are assumed to be responsible for the differences in the seasonal course of TSNN composition and contents observed between the two tree species.

The photosynthate costs of processes (amino acid and protein synthesis and turnover, and pH regulation) associated with the utilization of nitrate (NO3−), ammonium (NH4+) or glutamine (Gln) for plant growth were estimated. Based on these estimates, the effects of these forms of nitrogen (N) on the carbon balance of plants and on shoot–root biomass allocation were evaluated. The results indicated that NO3− as an N source for plant growth is not substantially more expensive to utilize than either NH4+ or Gln, particularly in the long term when costs due to protein turnover dominate the total costs of N utilization. It is also suggested that the photosynthate use in processes associated with N assimilation has little impact on the carbon balance of plants, and hence on shoot–root biomass allocation.

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 hydration of selected lichens (Cladonia mitis, Cladonia bellidiflora, Cetraria islandica, Parmelia saxatilis, and Xanthoria parietina) was investigated using gravimetry and proton magnetic free induction decays (FIDs).

The hydration from gaseous phase and dehydration to gaseous phase showed first-order kinetics. The amount of water which was non-removable in the air-dry state (relative humidity p/p0=9%) did not depend significantly on the lichen species and was found to be 5·6±1·0% of the d. wt.

The proton FID Gaussian component from the solid matrix of thallus structure, and two (or, depending on lichen species, one averaged) liquid signals coming from water tightly bound on the surface of thallus solid matrix and from loosely bound or free water, were recorded. The bound-water component was distinguished by its motional properties and by its proximity to endogenous paramagnetic centres present in solid matrix (presumably PS II reaction centres of the photobiont). Mild dehydration (from gaseous phase) could completely remove the loosely bound water fraction, leaving the system below the water percolation threshold and below the water clustering point, emphasizing the passivity of lichen response to desiccation shock. In the species in which the one average liquid component was recorded, bound water behaved similarly.

The hydration at which free water pool vanishes (ΔM/m0) and the relative (scaled to water) proton densities of solid matrix of lichen (β) were evaluated for all lichens investigated.

To develop further the methods for estimation of NOx absorption by plants supplied with 15N-labelled fertilizer, we proposed a new calculation method, total N fixed method (TNF), and compared with the 15N dilution method and the classical mass balance method (MB).

Hydroponically grown soybean plants were supplied with 15N-labelled nitrate and exposed to 200–250 nl l−1 NO2 for 7 d. The proportions of the N derived from NO2 to total N in exposed plants were estimated by the three methods.

The reported rates of NO2 absorption by several plant species, estimated by the 15N dilution method, were recalculated using the TNF method. The results of the two methods were compared and showed that: (1) The 15N dilution method overestimated the content of NO2-N in exposed plants compared with the MB method whilst the TNF method produced estimations of NO2-N closer to those by the MB method when the plants were supplied with 5 mM nitrate. (2) The differences in estimations between the MB method and either the 15N dilution method or the TNF method increased with decreasing supply of 15N-labelled nitrate to roots.

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.

Aqueous extracts of homogenized shoot and root tissue of alfalfa (Medicago sativa L.), white mustard (Sinapis alba L.), and cress (Lepidium sativum L.), with the exudates of sterile roots of these crop plants, were examined spectrophotometrically for the activities of 20 oxidoreductase enzymes by standard procedures. In tissue extracts and root exudates, the reactions of laccase (EC 1.10.3.2), ascorbate oxidase (EC 1.10.3.3), monophenol monooxygenase (EC 1.14.18.1), and phenol 2-monooxygenase (EC 1.14.13.7) were readily detected. Of the aromatic-ring cleavage dioxygenases, those of the meta-cleavage pathway (EC 1.13.11.2 and 1.13.11.8) could also be detected. Guaiacol peroxidase (EC 1.11.1.7) was dominant in all samples. In sterile root exudates of alfalfa, this enzyme was represented by at least seven acidic isoforms. The formation of the ligninolytic Mn3+/malonate and Mn3+/citrate complexes from Mn2+ occurred in all tissue extracts and in root exudates of alfalfa. In root extracts of soybean (Glycinemax L.), the rate of Mn3+ generation correlated (P=0·993) with the activities of endogenous plant guaiacol peroxidase and horseradish peroxidase (HRP) supplements and also with the total phenol content in tissue extracts (P=0·984). Plant guaiacol peroxidase and purified HRP decolorized four aromatic dyes, an activity reported to be involved in ligninolysis. Although no enzymes capable of generating H2O2 as a consequence of the oxidation of simple sugars, amino acids, organic acids, and aldehydes were found, traces of peroxide were detected in tissue extracts and in the root exudate of alfalfa. It is concluded that the oxidoreductases found in plant tissues also occur in root exudates of aseptic whole plants. The significance of interrelations between oxidoreductase enzymes and enzymically generated higher-valency metal ions is discussed in the context of the oxidative conversion of phenolic compounds in soil and plant tissue.

To investigate nitrogen assimilation in Lolium perenne L. colonized by the arbuscular mycorrhizal (AM) fungus Glomus fasciculatum (Thax. sensu Gerd.), nitrate uptake, key enzyme activities, and 15N incorporation into free amino acids were measured. After a 4-h labelling period with [15N]nitrate, 15N content was higher in roots and shoots of AM-plants than in those of control plants. Glutamine synthetase (GS) and nitrate reductase (NR) activities were increased in shoots of AM-plants, but not in roots. More label was incorporated into amino acids in shoots of AM plants. Glutamine, glutamate, alanine and γ-aminobutyric acid were the major sinks for 15N in roots and shoots of control and AM plants. Interactions between mycorrhizal colonization, phosphate and nitrate nutrition and NR activity were investigated in plants which received different amounts of phosphate or nitrate. In shoots of control plants, NR activity was not stimulated by high levels of phosphate nutrition but was stimulated by high levels of nitrate. At 4 mM nitrate in the nutrient solution, NR activity was similar in control and AM plants. We concluded that mycorrhizal effects on nitrate assimilation are not mediated via improved phosphate nutrition, but could be due to improved nitrogen uptake and translocation.

Homogeneously developed oak (Quercus robur L.) microcuttings were challenged in a Petri-dish system with the mycobionts Piloderma croceum J. Erikss. & Hjortst. and Paxillus involutus (Batsch) Fr. Non-destructive observations over 10 wk followed by d. wt measurements at the end of the assays served to precisely characterize root and shoot development, dynamics of mycorrhizal colonization and morphological ratio. In the system, plant development, and especially root morphogenesis, had more similarities to those of stump cuttings or of older seedlings than to those of 3-month-old seedlings. Whereas Paxillus involutus displayed early mycorrhizal colonization and had no significant morphological effects on the host Piloderma croceum modified markedly the entire plant development before a delayed mycorrhiza formation. The latter mycobiont stimulated elongation and production of the lateral root system and also increased the leaf surface. However, no corresponding weight increases were noted, which was reflected by significant increase of both specific root length and specific leaf area. These differential effects are discussed in relation to data concerning carbon requirement and auxin production of the mycobionts. The developed system was shown to be highly suitable for comparative studies with diverse mycobionts on recognition and physiological balance between partners before, and in the early stage of, formation of mycorrhizas.

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.

Mean δ15N of whole-plants of lettuce and barley varied by [les ]3‰ when given a chemically and isotopically uniform N source. This variation was related to the presence, absence and species of arbuscular mycorrhizal (AM) fungi and to external N concentration. A highly AM-susceptible plant (lettuce) responded to treatments differently than a less susceptible one (barley). The largest change in whole plant δ15N was related to the experimental combination most likely to be found in field conditions: species of fungus interacting with varying external concentrations of N.

The mechanisms underlying observed variations of plant δ15N are not understood, nor can they be approached directly using δ15N. However, descriptive data, such as presented here, are important to the development of a mechanistic model, because they suggest relationships for future research, using inter alia15N-enriched tracers. They also confirm that plant N sources, cannot be identified using plant δ15N, even when the type of mycorrhizal association (endo vs. ecto) is known.

The photosynthetic properties of cyanobiont lichens from contrasting habitats were measured to identify whether the increased assimilation rates which characterized Peltigera membranacea (Ach.) Nyl. from an exposed habitat were correlated with increased carbon-concentrating mechanism (CCM) activity. The results were contrasted with data obtained from two populations of Peltigera praetextata (Flörke ex Sommerf.) Zopf collected from dry and damp microhabitats within a shaded woodland and Peltigera leucophlebia (Nyl.) Gyelnik, which has been shown to lack a carbon-concentrating mechanism. The differences in assimilation rates between the cyanobiont lichens were not accounted for by differences in chlorophyll content. Peltigera membranacea from the exposed habitat which had the highest assimilation rates had the lowest Gamma; and K0·5 values and accumulated the greatest Ci-pool indicating that increased Ci accumulation contributed towards the higher assimilation rates shown by these species. The convexity of the light response curve for the cyanobiont lichens decreased with increasing assimilation rates. This might have indicated a diversion of electron transport to energize the carbon-concentrating mechanism. The apparent quantum efficiency of CO2 assimilation (ΦCO2) was correlated with the genus of lichen photobiont. All cyanobiont lichens had comparable values for ΦCO2 which were greater than that of the tripartite Peltigera leucophlebia. Light compensation points reflected the exposure of the habitats with higher compensation points characterizing the cyanobiont population from the exposed crag and the tri-partite population from the open grassland. Carbon isotope discrimination values for organic matter and measured instantaneously were the same for all cyanobiont lichens and were comparable with values recorded for species with a carbon-concentrating mechanism. Carbon isotope measurements for P. leucophlebia were typical of those recorded for species without a carbon-concentrating mechanism. Variation in source isotope signature and refixation of respiratory CO2 were considered to be significant factors in determining organic matter and instantaneous carbon-isotope discrimination. These factors might have masked any subtle variation in carbon-isotope discrimination which resulted from variable CCM activity. The functional significance of increased carbon-concentrating mechanism activity in cyanobiont lichens occupying exposed habitats is discussed.

An experiment was carried out in open-top chambers located in eastern Spain. One-yr-old Pinus halepensis Mill. seedlings were exposed during three consecutive summers to the following ozone (O3) treatments: charcoal-filtered air (CFA), non-filtered air (NFA) or non-filtered air plus 40 nl l−1 O3, 9 h d−1, 5 d wk−1 (NFA+40). Seasonal variations in Aleppo pine performance were observed since reductions in chlorophyll and cellular peroxidase levels associated with increases in superoxide dismutase activity, were recorded during the summer. Similarly, a reduction in epoxidation state was found at midday during the summer, derived from an activation of the xanthophyll cycle associated to an increment in radiation and temperature levels.

The first O3-induced effects were recorded in previous-year needles (1991) during the first summer exposure as an increase in extracellular and total peroxidase activities and in zeaxanthin levels in the NFA+40 treatment along with a trend to a higher SOD activity in this treatment. A carry-over effect was detected since a lower winter recovery of chlorophyll levels was found in the NFA+40 seedlings along with a reduction of xanthophyll levels. A reduction in chlorophyll levels was observed in the previous-year needles (1992) from the NFA+40 treatment at the end of the second fumigation period. Realistic ozone exposures induced alterations in plant antioxidative systems and plant pigments as shown in this paper. These observations together with the reductions in stomatal conductance and net photosynthesis recorded in the same experiment, indicate that Aleppo pine is a species sensitive to ozone.

Two cvs of alfalfa (Medicago sativa L.), Gilboa and Moapa 69, were inoculated in glasshouse pots with three arbuscular mycorrhizal (AM) fungi to investigate the efficacy of mycorrhizas with respect to the extent of colonization and sporulation. Paspalum notatum Flugge also was inoculated to describe fungal parameters on a routine pot culture host. Percentage root length of P. notatum colonized by Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe, Glomus intraradices Schenck & Smith, and Gigaspora margarita Becker & Hall increased from 10 to 21 wk, and all fungi sporulated during that period. In alfalfa, only colonization by G. intraradices increased over that time period, and it was the only fungus to sporulate in association with alfalfa at 10 wk. Glomus mosseae did not sporulate after 16–21 wk despite having colonized 30–35% of the root length of both alfalfa cvs. In vitro experiments in which Ri T-DNA-transformed roots of alfalfa were inoculated with AM fungi showed normal mycorrhizal formation by G. intraradices and a hypersensitivity-like response to Gi. margarita. Colonized cells became necrotic, and HPLC analysis indicated increased concentrations of phenolics and isoflavonoids in these root segments. These data strongly support the existence of a degree of specificity between AM fungi and host that might rely on specific biochemical regulatory processes initiated in the host as a result of the attempts at colonization by the fungus.

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.

Responses of gas exchange and photosynthesis to changes in CO2 concentration and PPFD were examined in well watered plants of Delosperma tradescantioides Bgr. to establish the relative importance of these environmental changes on the photosynthetic machinery in this CAM-cycling species which grows naturally in both exposed and partly shaded environments. Plants were grown at two PPFDs (220 [LL] and 550 [HL] μmol m−2 s−1). HL plants had larger leaves with higher specific weight, water content and diurnal malic acid fluctuation. Photosynthetic PPFD responses were typically those of sun and shade species for HL and LL plants, both under 21% O2 and non-photorespiratory (2% O2) conditions. The CO2 compensation point in the absence of non-photorespirational CO2 evolution in the light (Γ∗) was c. 30 μmol mol−1. Irradiation reduced mitochondrial respiration by >50%. Comparison of the PPFD responses of linear electron flow rates derived from gas exchange measurements and from fluorescence analysis ([Jscr ]F) indicated effective photosynthetic control. [Jscr ]F was always larger than electron flow rates calculated from gas exchange, indicating that processes other than carboxylation and oxygenation were consistently important in energy consumption under all sampled environmental conditions. Regardless of PPFD during growth, electron flow to carboxylation and [Jscr ]F were linearly correlated, demonstrating that the photosynthetic apparatus was well adapted to PPFD during growth. In HL plants, non-photochemical quenching increased, and photochemical quenching and the quantum yield of linear electron transport through PS II decreased more slowly with increasing PPFD than in LL plants. In plants of both treatments non-photochemical energy dissipation seemed to be exhausted when the proportion of photons not utilizable by photochemistry exceeded 0·7. Results illustrate a pronounced ability of D. tradescantioides to acclimate to a 100% change in the prevailing PPFD and lend support to the hypothesis that CAM cycling might act as a photoprotective process.

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.

A series of glasshouse experiments was used to determine mycorrhiza-specific isozymes (MSIs) produced by five species of Glomus colonizing roots of a desert shrub legume (Anthyllis cytisoides L.), Thymus vulgaris L. and Allium porrum L. over time. Extracts of colonized roots were electrophoresed on non-denaturing polyacrylamide gels (PAGE) and stained for 10 different enzymes. Staining protocols for esterase, glutamate oxaloacetate transaminase, alkaline phosphatase and malate dehydrogenase provided MSIs for the mycorrhizas formed by different arbuscular mycorrhizal (AM) fungi that had colonized roots of the three host plants. There was no apparent correlation between levels of colonization or arbuscular intensities, at or between each sampling, and the presence of MSIs. The development of colonization by the AM fungi differed little between the three plants when assessed with two methods of estimating fungal biomass. The variety of MSIs detected might reflect the diversity of metabolic activities of these Glomus species and, possibly, differing ecological functions. The high-level induction of two alkaline phosphatase MSIs in the mycorrhizas of Anthyllis cytisoides colonized by Glomus microaggregatum BEG56 was used to track the fate of this fungus when the same plant was inoculated and transplanted into a semi-arid site in south-east Spain. The probable fungal origin of the isozyme was indicated by detection of the same isozyme in the extraradical mycelium formed by Glomus microaggregatum BEG56 on Allium porrum. The use of MSIs to detect the mycorrhizas of species of Glomus in colonized roots is discussed.

Using field plots, three species (Mercurialis perennis L., Rubus fruticosus L., and Trientalis europaea L.) were tested for their potential to emit gaseous ammonia to the atmosphere. Canopies were misted with 5 mM methionine sulphoximine (MSO) to inhibit glutamine synthetase (GS), the enzyme of ammonium assimilation. Leaf tissue NH4+ concentration of control plants was 0·03–0·1 μmol g−1 f. wt. Although NH4+ accumulated in the leaf tissue of MSO-treated plants of all three species to similar concentrations (6-10 μmol g−1 f. wt after 4 d), emissions were only detected from the leaves of M. perennis, with potential rates of 2·5 nmol m−2 leaf s−1. Experiments carried out in a controlled environment confirmed this rate of emission over 9 d, during which time leaf tissue ammonium increased to 66 μmol g−1 f. wt. Comparisons with Hordeum vulgare grown under the same conditions showed that tissue NH4+ concentration reached a plateau of about 40 μmol g −1f. wt after 2 d. Emissions of NH3 during the 5 d of treatment reached a maximum rate of 10 nmol m−2 s−1 by the third day.

Apoplastic pH of the plants was determined, and it is suggested that this is an important factor explaining the differences in NH3 emission between species. The higher the apoplastic pH, the greater the likelihood of loss of NH3 from sub-stomatal spaces to the atmosphere. T. europaea (non-emitter) had an apoplastic pH of 5, R. fruticosus (non-emitter) a pH of c. 5·6, whereas that of M. perennis (emitter) was c. pH 6·3. The apoplastic pH is thought to be dictated in part by the N nutrition of a species, nitrophilous species tending to have high pH. Without NO3− fertilization, H. vulgare had an apoplastic pH of 6·8 but this increased to 7·3, 3 d after feeding with NO3−.

Short-term fumigation (2 h) of shoots of H. vulgare with 60 μg (≈32 mg NH3 m−3) of labelled gaseous 15N-NH3 (in the absence of MSO) showed that a substantial proportion (60%) of the applied label was found in the leaves, as well as in stems and roots (3%). There was also a change in amino acid pools, with an increase in shoot amino acids and a decrease in those in the root, while tissue NH4+ was very low in both shoots and roots. This provided indirect evidence that some of the applied label had been incorporated into an organic form. Following the fumigation treatment, emissions of NH3 were collected for 3 h, then c. 6·5 μg of N was recovered, of which c. 17% was 15N-labelled. Some of this label could have resulted from desorption of NH3 from leaf surfaces, but it was more likely that the remaining 14N isotope was from sub-stomatal emissions of NH3.

It is argued that non-nitrophilous plants tend to rely on mixed sources of N (NO3−, NH4+ or organic-N) and are more likely to favour root rather than shoot assimilation. Under these circumstances, their apoplastic pH is relatively low (compared with that of nitrophiles, which tend to assimilate NO3− mainly in their shoots), and at atmospheric concentrations most wild species are likely to be net assimilators, rather than emitters, of atmospheric ammonia.