Young spruce (Picea abies [L.] Karst; 2-yr) and beech trees (Fagus sylvatica L.; current-year and 1-yr-old seedlings) were grown under field conditions at a rural site near the city of Basel (Schönenbuch; 400 m above sea level) and 2-yr-old spruce trees at an elevated montainous site (Wengernalp; 1890 m above sea level). The plants were exposed in open-top-chambers (OTC) either to charcoal-filtered air or ambient air with ozone being the predominant air pollutant at both sites. The exposures lasted up to 1 yr. At the end of the growth period the plants were harvested and the pool sizes of the major non-structural carbohydrates were determined. Exposure to ambient air compared with filtered air controls caused an increase of glucose, fructose, pinitol, starch and, in some cases, sucrose in needles whereas carbohydrate contents in roots and twigs of spruce were reduced, in particular at Wengernalp. In beech saplings at Schönenbuch, a significant rise of sucrose and, in some cases, of raffinose, together with a significant reduction of glucose and fructose was observed in buds, leaves, twigs, and roots in ambient air compared with filtered air. In phloem exudates of current year beech seedlings, a 54–160% higher content of carbohydrates was found in ambient air than in filtered air. In roots of the beech saplings, the amount of soluble carbohydrates increased and starch contents mostly decreased. In general, starch storage in roots and stems was more abundant in beech trees. Spruce, especially at the mountainous site, accumulated high amounts of soluble carbohydrates in needles, in particular glucose.

Two arbuscular mycorrhizal fungi, Scutellospora calospora and Glomus sp. ‘City Beach’, were grown in soil conditions suitable for colonization of Allium porrum. Effects on plant growth and phosphate uptake were examined. Isolates of S. calospora (including the one used here) have been shown by others to vary in their stimulation of plant growth, and appear to be inefficient in the transfer of P from the fungus to the host. Our hypothesis was that such isolates of S. calospora may be more aggressive colonizers than other fungi, thus preventing growth increases or producing strong growth depressions in the host plant. In fact, inoculation with either fungus increased growth of plants in their respective soils both low in P (P0) and with added P (P1) by 42 d. The effect on growth due to mycorrhizal symbiosis (i.e. mycorrhizal growth response, % MGR) at 42 d was higher in plants grown in P0 soil. Plants colonized by Glomus sp. ‘City Beach’ had a greater % MGR than plants colonized by S. calospora. Both fungi colonized plants to high levels. The percentage of root length colonized was higher in P0 soil than in P1 soil at 21 d. The internal development of S. calospora appeared less affected by addition of P than Glomus sp. ‘City Beach’ at the early harvests. Formation of arbuscules followed the same trends as total colonization. Shoot P concentration was significantly higher in mycorrhizal plants than in non-mycorrhizal plants, by 21 d in P1 soil and by 28 d in P0 soil. Different percentage responses to added P based on total plant dry weight (% PGR) were observed at 28 and 42 d between the plants colonized by the two fungi. The increased P content due to mycorrhizal colonization (% MPR) differed with soil P. For both fungi grown in their respective soils, the response was greater in plants grown in P0 soil. Although the isolate of S. calospora used is an aggressive arbuscular mycorrhizal fungus with some hosts, it promotes a strong positive plant growth response in A. porrum after a mild initial growth depression.

Birch cuttings (Betula pendula Roth) were grown in a sand-culture system with two concentrations (0·05, HF and 0·005%, LF) of fertilizer containing macronutrients and micronutrients, and were exposed to 90/40 nl l−1 O3 (day/night) and <3 nl l−1 O3 (control) for one growing season in the field fumigation chambers at Birmensdorf (Switzerland). Leaves of different ages were analysed for gas exchange, contents of chlorophyll, protein, and for metabolites as well as enzyme activities of carbohydrate metabolism.

Ozone reduced net photosynthesis and chlorophyll contents in mature leaves of both fertilization treatments, whereas that of protein was only reduced in high-fertilized plants (HF). However, net photosynthesis, chlorophyll, and protein increased in young leaves of low-fertilized plants (LF). The effects of ozone on enzyme activities of carbohydrate metabolism were most pronounced in leaves of LF plants. Specific activities of the sucrose-cleaving enzymes, sucrose synthase and alkaline invertase, were induced, whereas acid invertase was unchanged. Extractable activity of sucrose phosphate synthase, which is a key enzyme of sucrose synthesis, was reduced. Levels of fructose 2,6-bisphosphate, an inhibitor of sucrose synthesis, were increased in leaves of O3/LF plants, but reduced in O3/HF plants. In addition, activities of enzymes involved in starch metabolism, ADP-glucose pyrophosphorylase and starch phosphorylase, were lowered in ozone-treated samples and the ratio of ATP[ratio ]ADP was increased.

It is concluded that chronic ozone exposure leads to an inhibition of sucrose synthesis and favours sucrose degradation. This effect is modulated by the nutrient status of the plants, indicating higher O3 tolerance in HF plants. Furthermore, as the metabolic responses in the ozone-treated samples resemble very closely those observed under end-product inhibition of photosynthesis, we assume that the O3 effect is mainly due to reduced photosynthate export.

The representation of root activity in models is here confined to considerations of applications assessing the impacts of changes in climate or atmospheric [CO2]. Approaches to modelling roots can be classified into four major types: models in which roots are not considered, models in which there is an interplay between only selected above-ground and below-ground processes, models in which growth allocation to all parts of the plants depends on the availability and matching of the capture of external resources, and models with explicit treatments of root growth, architecture and resource capture. All models seem effective in describing the major root activities of water and nutrient uptake, because these processes are highly correlated, particularly at large scales and with slow or equilibrium dynamics. Allocation models can be effective in providing a deeper, perhaps contrary, understanding of the dynamic underpinning to observations made only above ground. The complex and explicit treatment of roots can be achieved only in small-scale highly studied systems because of the requirements for many initialized variables to run the models.

IAA production by the ectomycorrhizal fungus Paxillus involutus grown in pure culture was investigated by testing IAA accumulation or IAA-synthesizing activity. All the Paxillus involutus strains studied released indole compounds either when incubated with 0·5 mm tryptophan or in its absence. HPLC analyses of indole compounds all yielded two main peaks identified by reference to controls as being IAA and indole-3-carboxylic acid and a small peak of indole-3-aldehyde. Mass spectrometry unequivocally confirmed IAA production in the culture filtrates and in the incubation solution used to study in vitro IAA-synthesizing activity. The IAA-synthesizing activity was found to vary widely, from 7·6 to 137·4 nmol IAA h−1 g−1 f. wt, between strains. The mycorrhizal activity of these strains with seedlings of Pinus sylvestris cultured under aseptic conditions was tested. Fungal strains that were characterized in pure culture conditions by high IAA-synthesizing activity induced more fine roots and significantly higher numbers of mycorrhizas than strains which showed a low activity of IAA synthesizing enzymes. The increased ectomycorrhiza formation was generally accompanied by an increased growth of seedlings.

Most natural and semi-natural communities are limited by major nutrients such as nitrogen (N) or phosphorus (P), but most experiments on the effects of ozone on wild plants have used nutrient-rich composts or soil. In order to investigate interactions between ozone and low nutrient supply, two artificially selected lines (ozone-resistant and ozone-sensitive) of two populations Plantago major ssp. major L. were grown on a sandy loam, with (HN) and without (LN) addition of fertilizer. The soil was from a semi-natural grassland that has never been fertilized. Plants were exposed to either charcoal/Purafil®-filtered air (CF=<5 nl O3 l−1) or 70 nl O3 l−1 7 h d−1 from the seedling stage to seed production.

Poor growth (c. 25% of that in HN) of the low-nutrient plants, and leaf concentrations of N and P showed that the LN plants were severely nutrient-limited. In addition to affecting the total dry mass of the plants, the nutrient supply altered seed production, reproductive effort (number of seeds per total mass) and root-to-shoot allocation. Exposure to ozone had significant effects on physiology, growth, and seed production that varied with population, selection line, time, and plant development. There also were significant interactions between ozone effects and nutrient regime. In the Lullington Heath population, ozone reduced plant dry weight at 4 wk only in the LN treatment, and in the sensitive line of the Bush population, seed production was reduced by ozone only in LN. Therefore, contrary to what was expected, in the present experiment, plants given the LN treatment were often more sensitive to ozone than those grown under the high-nutrient regime. This increase in sensitivity was despite the fact that the LN treatment reduced stomatal conductance and ozone flux. It is concluded that there are potentially important interactions between ozone and low nutrient supply that need further investigation, particularly under field conditions.

Endogenous polyamine content of the ectomycorrhizal fungus Paxillus involutus, as well as the activity of its biosynthetic enzymes in relation to mycelia ageing were investigated in this work. Polyamines in free, PCA-soluble and insoluble conjugated forms, are present in Paxillus involutus mycelia in relatively high amounts and the ratio of putrescine to spermidine is age-dependent. Both arginine- and ornithine-decarboxylases are present, but putrescine biosynthesis proceeds mostly via ornithine decarboxylase and decreases with the age of mycelia. There was a large release of free polyamines from mycelia which showed age-dependent features. Clear polyamine uptake was observed in 2-wk-old mycelia and no competition between putrescine and cadaverine was detected. Putrescine uptake seems to reduce ornithine decarboxylase activity, but does not affect arginine decarboxylase.

To test the hypothesis that fructan structure can be used as a taxonomic character for the Poaceae we examined the accumulation of the linear, β-2,6-linked fructan series with a terminal glucose residue within the tribes Aveneae and Poeae. Only Dactylis glomerata L. (Poeae) has been shown to contain this series, making it unique among fructan structures and a potential taxonomic marker. To this end, 13 members of the tribes Aveneae and Poeae were surveyed for the presence of this fructan series by comparing extracts of water-soluble carbohydrate by TLC with an extract of D. glomerata. The fructans of four species were similar to those of D. glomerata, as determined by TLC. Further comparison by anion-exchange chromatography and linkage analysis demonstrated that Phalaris aquatica L., Puccinellia stricta (Hook. f.) C. H. Blom and Holcus lanatus L. contained a linear, β-2,6-linked series of fructan with a terminal glucose residue but that Lagurus ovatus L., although containing predominantly β-2,6-linked fructan was composed of more than one series of fructan, one with a terminal glucose residue and one with an internal glucose residue. A linear β-2,6-linked series of fructans with a terminal glucose residue was the dominant fructan accumulated by some species of genera belonging to both the Aveneae (Phalaris and Holcus) and the Poeae (Dactylis and Puccinellia). As both of these tribes also contain genera that accumulate fructans with an internal glucose residue, e.g. Avena and Lagurus (Aveneae) and Lolium (Poeae), structure of fructan cannot be used to distinguish these tribes. Review of the literature, however, showed that it is possible to separate the supertribes Triticodae and Poodae on the basis of the structure of the fructan that they accumulate.

Ammonia was necessary for the origin of life. However, NH3 would not have been a significant component of the neutral or mildly reducing (N2, CO2, H2O) atmosphere which characterized the Hadean earth (>3·8 Gyr ago), especially in view of the u.v. lability of NH3, the greater u.v. output of the young sun than pertains today, and the absence of an atmospheric u.v. screen. Heterogeneous phase reactions, following atmospheric chemistry, have been proposed as a prebiotic NH3 source. Lightning, or bolides (meteorites and comets), generated NOx., and Fe2+ in the sea could have reduced NO2− resulting from the NOx. to NH4+; this disequilibrium NH4+ level could have supported the production of the nitrogenous organic building blocks of life. NOx. and NHy thus could both have had important roles in the origin and evolution of life. Burgeoning biota could soon have depleted abiotically generated NH4+, and biological N2 fixation could have evolved in its present Fe-demanding, O2-sensitive forms in the Archaean O2-free, Fe2+-rich environment. Organic matter could have driven biological denitrification reactions based on NO2− and NO3− generated abiologically using some redox components which evolved in earlier chemolithotrophs and photolithotrophs, regenerating atmospheric NOx. and producing N2O. Any atmospheric NH3 leaking from oceanic biology would have been subject to u.v. breakdown and rain-out. Although O2-evolving photosynthesis probably began in the Archean some 3·5 Gyr ago, any O2 accumulation was local until 2·0 Gyr ago (Proterozoic) due to consumption by oxidation of Fe2+ and S2−. However, localized O2 accumulation before 2·0 Gyr ago could account for the observed early evolution of cytochrome oxidase and the possibility of O2-consuming chemolithotrophic and chemoorganotrophic nitrification, with further possibilities of NOx. production. Oxygen accumulation globally from 2·0 Gyr onwards coincided approximately with the evolution of eukaryotes, which contributed phagotrophy to the reactions of the N cycle as well as the nutrification-like aerobic production of NO. by nitric oxide synthetase, while lightning and bolides could now generate NO. from N2 and O2.

Evidence for terrestrial ecosystems is found from 1·0 Gyr onwards; NOx. and NH3 generated by terrestrial biota stands a greater chance of escaping to the atmosphere than do these compounds generated in the sea where recycling within the water body is likely. As CO2 levels fell and O2 levels rose, NH3 cycling in the photorespiratory carbon oxidation cycle might have been evident as early as 1 Gyr ago, although this does not seem to be a major contributor to atmospheric NH3 today. Embryophyte evolution on land 450 Myr ago, together with symbionts and biophages, increased primary productivity and N cycling on land, with greater quantitative possibilities for NOx. and NH3 escape to the atmosphere. The evolution of lignin (and related phenylpropanoids) at least 400 Myr ago, with associated NH3 recycling in vascular land plant, does not seem (on present evidence) to increase NH3 loss to the atmosphere significantly. Biomass burning occurred at least 350 Myr ago with lightning as the likely ignition source; such burning yielded NO., NH3, N2O and N2 from organic N. As in earlier times, the existence of terrestrial embryophyte vegetation has been punctuated by major bolide impacts, with the Cretaceous-Tertiary boundary impact generating perhaps 104 as much NO. as a year's thunderstorms do today, although the toxicity of NOx.per se to vegetation might not have been the major effect of the NOx. on biota. Terrestrial plants suffered fewer extinctions at this time than did many other major taxa. Despite such very significant generators of atmospheric combined N as major bolide impacts, the mean levels of NHy, NOx. and N2O (and O3) in the atmosphere during the 450 Myr existence of embryophyte vegetation was lower than the current, globally averaged, anthropogenically influenced values. Current globally averaged levels of atmospheric combined N are thus not without precedent in the history of life, globally for NOx., and locally for NHy, so vegetation has had evolutionary experience of high atmospheric combined N. However, this should not make us complacent about the impact of current wide-spread and continuing anthropogenic inputs of combined N, in view of the rate and extent of the inputs, and their combination with other aspects of local anthropogenic influence (e.g. SO2 from burning of high-S coal), stratospheric O3 depletion, and global environmental change (increasing CO2, temperature and sea-level).

Production of enzymes which degrade plant cell wall macromolecules has been studied in relatively few ericoid fungal isolates, although these polymers are a major component of the organic litter and an important source of nutrients for these fungi. Our aims were to investigate whether the ability to degrade the wall pectic component, only reported for one isolate, is a general feature of ericoid fungi. Of about 35 isolates from different geographic regions, all were capable of growing on pectin as the sole carbon source. Polygalacturonase (PG) activity was detected to a different degree in the culture filtrates and independently of the fungal growth rate. Solid and liquid isoelectric focusing allowed separation and identification of several polygalacturonase isoforms. Among the fungal isolates investigated, those from the northern hemisphere produced mostly acidic isoforms, whereas isolates from South Africa secreted more abundantly basic isoforms. However, purification and biochemical characterization of several PG isoforms from the different isolates revealed an optimal activity in the acidic pH range for all the PG enzymes tested. Polygalacturonase enzymes seem to be an important component of the enzymatic arsenal secreted by ericoid fungi during their saprotrophic life. In addition, they could also play a role during root colonization, since penetration across the plant cell wall is a prerequisite for the establishment of endomycorrhizal symbiosis.

‘Tiny Tim’ tomato plants were exposed to five CO2 treatments (375 (ambient), 450, 525, 600 or 675 μmol mol−1) in combination with O3 (0 or 80 nmol mol−1). Biomass was evaluated following 3, 5, 7 and 13 wk exposure. Biomass following 13 wk exposure also included weekly harvests of mature tomato fruit beginning week 8. Carbon dioxide enrichment significantly enhanced total vegetative plant d. wt at each harvest, as well as cumulative yield of mature fruit, whereas O3 significantly suppressed total vegetative plant d. wt at each harvest and reduced total cumulative fruit yield. The magnitude of these changes varied with the development of tomato from early growth to mature fruit yield. Carbon dioxide enrichment reduced the detrimental effects of O3 on total vegetative plant d. wt of tomato following 3, 5, 7 and 13 wk exposure. Final mature fruit yield was 24% higher under enriched CO2 treatments than in ambient CO2. Ozone suppressed final yield by 31% following exposure to 80 nmol mol−1 O3 when compared with exposure to charcoal-filtered (CF) air. The impacts of both CO2 and O3 on yield were, however, dependent upon the presence or absence of the other gas. In the absence of O3, yields were very similar for the ambient and elevated CO2 treatments, but in the presence of O3, yields under ambient CO2 were greatly suppressed whereas yields under elevated CO2 were similar to those in the absence of O3. Thus, enriched CO2 ameliorated most of the suppressive effect of O3 on yield of mature fruit.

Since the beginning of the Bronze Age the acquisition of metals and the mastering of metal processing has been a driving force in human civilisation. It is difficult to imagine, however, how a plant could possibly profit from accumulating excessive amounts of a metal for which there is no metabolic need. Leaving behind an environmentally degraded wasteland, human mining and smeltering activities contributed to providing the niches for such plants, sometimes even accomplishing seed dispersal – through seeds attached to the boots of migrating miners. A stunning example of this is found in the healthy populations of Thlaspi caerulescens in the Cevennes (southern France), the leaves of which have been found to contain the toxic and non-essential metal cadmium (Cd) at concentrations between 1000 and 3000 mg kg−1 dry biomass (R. D. Reeves, unpublished). In this issue, experiments under controlled experimental and glasshouse conditions are reported, confirming an extraordinary level of Cd accumulation and tolerance in these populations (Lombi et al., pp. 11–20).

Two tetraploid subspecies of Dactylis glomerata L., subsp. reichenbachii (Hausm.) Stebbins et Zohary and subsp. glomerata, occur in the French Alps. The former is confined to dolomitic, south-facing, alpine lawns above 2000 m, whereas the latter occurs in non-dolomitic habitats in subalpine meadows mainly below 1900 m. Previous studies of allozyme variation have shown that genetic introgression between the two subspecies occurs over large areas. By contrast, morphologically intermediate individuals only occur in an extremely narrow area, suggesting that the morphological and physiological differences between the two subspecies is of adaptive significance. A reciprocal clone transplant experiment was set up to examine (1) any genetic differences between subspecies indicative of ecotypic differentiation in relation to habitat characteristics and (2) the level of phenotypic plasticity in the two subspecies. Genetic differentiation was confirmed by a statistically significant taxon × site interaction effect in anova for all traits studied. The glomerata populations produced more tillers, longer leaves and higher culms in all sites, especially in their home environment. However, reichenbachii populations produced more seeds than the glomerata populations in the original reichenbachii environment, suggesting ecotypic differentiation between the two subspecies. This result might also explain why the glomerata subspecies is unable to colonize dolomitic habitats occupied by the reichenbachii subspecies. The reichenbachii populations showed less plasticity than the glomerata populations for leaf length and floriferous tiller number, a result which is discussed in the context of the response of plants from productive and non-productive habitats to environmental variation.

In Plantago lanceolata L. the effect of Cu(II) additions to the nutrient solution on root-associated Fe(III) reductase was studied in a factorial design with different Cu(II) and Fe(III) concentrations. Iron starvation resulted in approx. an eightfold increase in root Fe reduction at the level of intact plants and twofold enhancement in the specific activity of both NADH-linked FeEDTA reductase and H+-ATPase in isolated root plasma membrane vesicles. In plants exposed to low (0·3–0·7 μM) Cu and suboptimal Fe levels, reduction activity at the root surface was further increased and associated with more severe interveinal chlorosis than plants grown in Cu-free medium. In Fe-sufficient plants, withholding Cu over a prolonged period slightly enhanced the reduction activity.

Addition of high (5 μM) Cu concentrations to Fe-free medium inhibited the induction of the physiological responses by Fe-deficiency stress. In plants without Fe supply but with adequate Cu supply, short-term application of 5 μM CuSO4 completely inhibited the reduction activity. Neither incubation of the plasma membrane vesicles before measurement nor incubation of intact plants with Cu before isolation caused a significant decrease in reductase activity. The results are interpreted as indicating different mechanisms underlying Cu-induced alterations in iron nutrition.

Adventitious rooting might be induced in recalcitrant woody genotypes by infection with Agrobacterium rhizogenes, and, in some cases, might also require exogenous auxin. The objective of the present study was to determine how agrobacteria trigger root formation in the stem of a recalcitrant woody microcutting, which cytological events result from the combined presence of infection and exogenous auxin, and which types of roots are induced by infection. Microcuttings of a recalcitrant walnut (Juglans regia), infected or not with A. rhizogenes strain 1855, were cultured with either indolebutyric acid (IBA), IAA, or without exogenous hormones, to induce rhizogenesis. They were cytohistologically and ultrastructurally investigated at various times in culture. Southern blot and PCR analyses were performed to verify the frequency of transgenic, chimeric and bacterium-containing roots. The infection was sufficient per se to stimulate rhizogenesis. Rooting on the infected cuttings was enhanced by exogenous IBA, which accelerated and increased root meristemoid formation, in comparison with without-hormone treatment. Meristemoids were organized both directly by the cambial cells and indirectly by the callus, and showed a pluricellular origin. Inter and intracellular bacteria were observed in the stem throughout the culture period (30 d). They were preferentially present in the vessels, and mainly in those showing polyphenol deposition. In the infected IAA-treated cultures, a high level of secondary xylem formation occurred instead of rhizogenesis. Nontransformed roots were preferentially produced by the infected cuttings treated with the auxins. Bacterium-containing and chimeric roots were produced by infected cuttings independently of the treatment. Thus, in a recalcitrant walnut, nontransformed root meristemoids are stimulated by combining infection and exogenous indolebutyric acid. Furthermore, the persistence of bacteria in the stem during the culture and the pluricellular origin of the meristemoids explain the presence of the bacterium-containing and chimeric roots.

The relationship between precipitation chemistry and the concentrations of nitrogen ([N]) and phosphorus ([P]) in the cushion-forming lichen Cladonia portentosa (Dufour) Coem. (=C. impexa (Harm)) was investigated. Samples of C. portentosa were collected from heathlands and upland moorlands close to 31 rural sampling stations in the UK Acid Deposition Monitoring Network, which provides data on wet deposition and NO2 concentrations in air. The [N] and [P] were measured in the top 5 mm of lichen thalli (thallus apices) and also in a horizontal stratum between 40–50 mm from the apices (thallus base). The [N] (per unit dry mass) was 0·08–1·82% and [P] was 0·04–0·17%, depending on collection site and lichen fraction analysed. Concentrations of both elements were c. 2–5 times greater in the apices than in the basal strata, and [N] and [P] values in each stratum were strongly positively correlated. Lichen [N] was positively correlated with N deposition: this relationship was stronger when using [N] values for thallus bases than for the apices. By contrast, thallus [N] was poorly correlated with [N] values in precipitation. When [NO2] in air was included together with NO3− deposition in a linear regression model explaining thallus base N, the model fit was significantly improved, whereas modelled values of NH3 deposition rate for the heathland sites did not correlate with lichen [N]. It is suggested that the proportionately greater enrichment of [N] in the thallus base might reflect a perturbation of internal recycling of thallus N at polluted sites. Thallus [P] was generally weakly linked to wet N deposition but positively correlated with [NO2] in air. It is not known whether the trend for increasing thallus [P] values indicates decreasing lichen growth rate and reduced growth dilution of P in polluted areas, or is due to regional variation in P deposition rate.

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.

The inter- and intracellular distribution of phosphate in non-mycorrhizal and ectomycorrhizal roots of Pinus sylvestris was analysed by energy-dispersive X-ray spectroscopy after cryofixation, freeze-drying, pressure infiltration with resin and dry cutting of the material. The results show (1) that in non-mycorrhizal roots the distribution of phosphate across the root cross section is relatively homogenous, (2) that the intracellular distribution of phosphate depends on the external supply, and (3) that the phosphate absorption can be increased by a supply of NH4. Under phosphate starvation, the phosphate is mainly localized in the cell cytoplasm, whereas under high external phosphate this element accumulates in the vacuoles of cortical cells. It can be inferred that at low external conditions phosphate is mainly located in the cytoplasm of cells to protect metabolism from deficiency, that phosphate is translocated to the shoot via symplastic and apoplastic pathways, and that especially the vacuolar pool size is regulated by the external phosphate supply. The nutrition of the host plant under phosphate starvation was improved by a mycorrhizal infection with Suillus bovinus. However, this effect might not be the same for all ectomycorrhizal infections and supply conditions. With high external phosphate an ectomycorrhizal infection with S. bovinus had no effect on intracellular phosphate within the roots, the shoot contents of mycorrhizal plants were less than those of non-mycorrhizal seedlings and translocation rate across the mycorrhizal interface appeared to be independent of the availability of phosphate. Intracellular phosphate in fungal cells of mycorrhizal roots was independent of the external supply and seemed to be regulated by the formation of polyphosphates. A supply of (NH4)2HPO4, which led to an increase in cytoplasmic phosphate levels in the hyphae of the Hartig net of the Paxillus involutus mycorrhiza also led to higher phosphate in the plant cell compartments. Therefore, it can be inferred that the translocation of phosphate from fungus to host plant across the mycorrhizal interface is regulated by the phosphate concentration in the cytoplasm of the Hartig net and by the efflux rate into the interfacial apoplast.

To clarify the role of seasonal change, competitive response and nutrient availability in the competitive asymmetry of grassland species a competition experiment was conducted on Holcus lanatus, Anthoxanthum odoratum and Festuca ovina, which represent a successional sequence of decreasing nutrient availability. Seven harvests were taken over two growing seasons. At each harvest the dry weight of plant parts, dead leaves, leaf area and plant height were measured. Three key traits that determine the successional status of the species were studied: specific leaf area, specific shoot height, and dead leaf fraction.

The response of these traits to competition appeared to be limited and insufficient to change the competitive relations in the experiment. However, all three traits showed marked seasonal changes which resulted in superior growth and survival in winter of the species adapted to nutrient-poor environments. The findings support the theory that competitive asymmetry increases at higher nutrient levels. It is postulated that the directionality of light makes it possible for the dominant species to monopolize this resource more easily than nutrients.

Ascospore germination, thallus initiation, and areole and prothallus development in the lichen Rhizocarpon lecanorinum Anders were examined using light, fluorescence and scanning electron microscopy. The ascospore germ hyphae remain very short and do not form a prothallus-like mycelium. Instead, a compact soredium-like granule develops directly from sporeling contact with a compatible species of Trebouxia. Diffuse initial stages involving non-trebouxioid algae are lacking. The onset of thallus differentiation is marked by the deposition of rhizocarpic acid in an incipient cortical layer within the apical part of the granule. As pigmentation and cortex-formation transform this structure into a typical areole, radiating prothallus hyphae are simultaneously initiated from its basal margin. Most areoles formed subsequently in the marginal prothallus lack subtending melanized hyphae and apparently stem from overgrowth by the prothallus of photobiont cells on, or in, the substratum. Apothecia reach maturity in thalli as small as 2 mm in diameter. It is proposed that the lack of diffuse hyphal growth in sporelings and telescoped morphogenesis of R. lecanorinum are part of a life history strategy geared to precocious, heavy investment in ascospore production. The R. lecanorinum–Trebouxia symbiosis has a number of features which make it well-suited for further studies of the life history and development of prothallus-forming crustose lichens with sexually reproducing mycobionts.