A common feature of many pollutant exposure studies is that data from these experiments often consist of either plant biomass or yield, and the response to the pollutant is usually based on a simple comparison of means determined at the end of each growing season. This type of data is now being used to quantify critical levels for different types of vegetation. Such an approach is, however, inappropriate for the relatively short-term exposure studies with trees since, due to their longevity, it is not possible to determine a final yield. Instead these studies should be regarded as a type of intervention experiment in which only a small part of the life cycle of the tree is investigated. Moreover, critical levels as they are now defined focus on the cumulative exposure to ozone concentrations over time. Hence, any analysis of the effects of ozone on the growth of trees should similarly focus on the behaviour of growth functions over time and not on a comparison of biomass at the end of a growing season or experiment.

Here we report on the statistical analysis of a longitudinal study, where the term ‘longitudinal’ refers to the analysis of repeated measurements over time, and which was used to investigate relative differences in the growth of Sitka spruce and Norway spruce seedlings during three summers' exposure to ozone over three (Norway) or four (Sitka) growing seasons. Measurements of total seedling height and stem diameter were made at frequent intervals over the period of the experiment and the above-ground growth of individual trees (as log d2h) analysed for each growing season using a statistical model of the form: m(t)=a+b(t−t0). No statistically significant differences in the growth of Norway spruce were observed after three summers of ozone exposure. The growth of Sitka spruce was, however, reduced by ozone during the third growing season and in the following year, even in the absence of the pollutant.

For the Sitka spruce, the fitted model was used to calculate the time at which a 10% reduction in growth had occurred in the ozone-exposed trees. In combination with ozone AOT40 indices for the relevant growing seasons, this was then used to determine a critical level of 21·3 ppm-h for this species. Since the growth of the Norway spruce was unaffected by exposure to ozone it was not possible to calculate a critical level other than to surmise that it is in excess of 30 ppm-h.

This review considers the reasons for, and research governing, the regulation and monitoring of genetically engineered micro-organisms and viruses (GEMs) released into the environment. The hazards associated with releasing GEMs into the environment are the creation and evolution of new pests and diseases, and damage to the ecosystem and non target species. The similarities and differences between GEMs and conventional micro-organisms are discussed in relation to risk assessment. Other issues covered include the persistence of micro-organisms in the environment, transgene dispersal to non-engineered microbes and other organisms, the effects of transgenes and transformation on fitness, and the evolution of pests and pathogens that are given or acquire transgenes. Areas requiring further research are identified and recommendations for risk assessment made.

Net CO2 assimilation rates and chlorophyll a fluorescence were measured on leaves of an ABA-unresponsive poplar (Populus trichocarpa × koreana cv. Peace) during a period of drought. Stomata of fully expanded leaves of ‘Peace’ partly closed during drought, while those of Populus euramericana cv. Robusta closed almost completely. The measured data were used to calculate total light-driven electron flows and to derive an estimate of a CO2 mole fraction in the chloroplasts. Two major results were obtained. (i) In well watered plants, photosynthesis operated at lower CO2 mole fractions in the chloroplasts of cv. Peace than in those of cv. Robusta, whereas CO2 mole fractions in the substomatal spaces were of the same order of magnitude. We concluded that a higher resistance to CO2 influx in the mesophyll contributed to the lower net assimilation rates in ‘Peace’. (ii) Drought induced an important decrease in CO2 availability in ‘Robusta’ but not in ‘Peace’. This suggested that reduced CO2 influx was a major cause of the limitation of net CO2 assimilation during drought in ‘Robusta’, but not in ‘Peace’, where drought probably reversibly reduced the apparent carboxylation efficiency of Rubisco. Measurements of O2 evolution under saturating CO2 supported this view, as photosynthesis decreased in droughted plants of ‘Peace’ but not of ‘Robusta’. Moreover, estimates of the 13C/12C isotope ratio in the leaves of both cultivars showed drought-related decreases in discrimination, which, in the case of ‘Peace’, could only be explained by changes in activity of the photosynthetic carbon reduction cycle.

We compared the plasma membrane, tonoplast and mitochondrial Mg-ATPase activities of microsomes isolated from mycorrhizal and non-mycorrhizal sunflower (Helianthus annuus L.) and onion (Allium cepa L.) roots during a time-course experiment (21–60 d). No significant difference was observed between the protein levels of root microsomes from mycorrhizal and non-mycorrhizal sunflower. However, the protein content of microsomes obtained from mycorrhizal onion roots increased when the symbiosis was well established (more than 50% of root length colonized by the arbuscular mycorrhizal fungus). Kinetic studies revealed that vanadate-sensitive (plasma membrane-associated) ATPase activity of sunflower root microsomes increased with mycorrhizal colonization. However, the vanadate-sensitive ATPase activity of microsomes from mycorrhizal onion roots decreased compared with those from non-mycorrhizal controls. Increased activity of nitrate (tonoplast-associated) and azide (mitochondrial membrane-related)-sensitive ATPase activities was detected in extracts obtained from 45- and 60-d-old mycorrhizal onion plants. A slight increase in azide-sensitive ATPase activity was detected in microsomes of mycorrhizal sunflower. The existence of different physiological and biochemical strategies in sunflower and onion in relation to the establishment of this mutualistic symbiosis is discussed.

‘The view that nutrient acquisition by most plants growing in natural ecosystems is mediated by mycorrhiza-forming symbiotic fungi is now largely accepted’ (Read, 2000). Is this bold claim really true for the whole suite of mineral nutrients that plants require? The case is strongest for nutrients that are not very mobile in soil, especially when present in growth-limiting amounts, and phosphate (P) is the classic example. Arbuscular mycorrhizas are by far the most widespread mycorrhizal symbioses, and the ability of arbuscular mycorrhizal (AM) fungi to take up soil nutrients such as P and transfer them to the host plant is an area of intense research. However, there is great variation in the extent to which AM plants benefit in measurable terms from the symbiosis under a given set of environmental conditions, and a paper in this issue, by Koide et al., addresses this problem (Koide et al., pp. 163–168). The variability is especially apparent in the field, thus obscuring the possible roles of mycorrhizas in community structure and succession (Fitter, 1985; McGonigle, 1988).

Plants of Chondrus crispus Stackhouse, collected from mid-littoral tidepools, were treated as follows. Some plants were kept for a few weeks under controlled starvation conditions in order to decrease their initial content of total tissue phosphorus, then incubated for up to 48 h in phosphorus (15 μM) and nitrogen (25 μM) enriched sea water. Other plants were directly incubated in enriched sea water. Chemical analyses showed that the total phosphorus content of fresh and starved plants remained stable, reflecting the nutritional status of the plants. The predominant acid-soluble phosphate fraction was larger in fresh than in starved plants. The content of acid-soluble polyphosphates, similar in both types of plants at the beginning of the experiment, doubled in starved plants, and increased by a factor of 2·7 in fresh plants, over 48 h. The content of acid-insoluble polyphosphates was lower than that of acid-soluble polyphosphates.

Transmission electron microscopy and energy dispersive X-ray microanalysis confirmed the presence, mostly in medullary cells, of acid-insoluble polyphosphates in the form of cytoplasmic granules and precipitates along the plasmalemma, particularly near pit plugs. This is the first report of such phosphorus storage structures in a red macroalgal species.

Measurements were made of the rates of nitrogenase activity (acetylene reduction) by cyanobacteria collected from the Baltic Sea at 2-h intervals, over a period of 24 h, and incubated under natural light. By relating the chlorophyll-specific rate of N2 fixation (PN) to the mean photon irradiance (I) at different periods, a PN/I curve was constructed. A mathematical description of this relationship was used in the calculation of rates of N2 fixation at different depths and times of day from continuous measurements of surface irradiance and light attenuation. By relating these calculations to the vertical distribution of chlorophyll due to Aphanizomenon flos-aquae, an estimate of the daily integral of N2 fixation by the population of this cyanobacterium was obtained: it varied from 0·39 to 0·71 mmol m−2 with a mean value of 0·53 mmol−2 over 9 d. Comparisons with similar calculations of the daily integral of photosynthesis over the same period indicated the atomic ratio of N/C fixed is about 0·22, not very different from the N/C ratio expected for the elemental composition of these cyanobacteria. It is demonstrated that when buoyant gas-vacuolate colonies float up during calm periods the increased irradiance that they experience supports an increased rate of N2 fixation. This increase is less than that seen for carbon fixation because photosynthesis is negated by respiration at low irradiances and is less inhibited at high irradiances.

We previously found that the ethylene inhibitor Ag+ could overcome the inhibitory effect of nitrate on nodulation of soybean (Glycine max) cv. Bragg. The same treatment increased nodulation quantitatively under non-inhibitory conditions, strongly suggesting involvement of ethylene in the control of nodulation in this species. Supernodulation mutants that lack internal autoregulation of nodulation, however, had biosynthesis capacity similar to the wild type. In the present work, the effects of ethylene on nodulation of ‘Bragg’ and two separate, but allelic, supernodulating mutants (nts382 and nts1007) were compared. The nodulation process appeared much more sensitive than plant growth and development to ethylene, which reduced the number of nodules per plant, but nearly twofold more in the wild type than in the supernodulation mutants. The cause–effect relationship is established by the counteracting effect of Ag+ and the fact that the stronger the inhibition by ethylene, the higher the recovery of nodulation ability with the ethylene antagonist. This higher tolerance of or lower sensitivity to ethylene in nts382 persists even under low inoculum dose, where nodule number and mass could be decreased to wild-type levels. Differences between the mutant and the wild type in the triple response test do not appear to support differences in ethylene perception on a whole-plant basis. The results suggest that sensitivity of nodulation to ethylene might have been affected in supernodulation mutants.

A range of marine photosynthetic picoeukaryote phytoplankton species grown in culture were screened for the presence of extracellular carbonic anhydrase (CAext), a key enzyme in inorganic carbon acquisition under carbon- limiting conditions in some larger marine phytoplankton species. Of the species tested, extracellular carbonic anhydrase was detected only in Micromonas pusilla Butcher. The rapid, light-dependent development of CAext when cells were transferred from carbon-replete to carbon-limiting conditions was regulated by the available free- CO2 concentration and not by total dissolved inorganic carbon. Kinetic studies provided support for a CO2- concentrating mechanism in that the K0.5[CO2] (i.e. the CO2 concentration required for the half-maximal rate of photosynthesis) was substantially lower than the Km [CO2] of Rubisco from related taxa, whilst the intracellular carbon pool was at least seven fold greater than the extracellular DIC concentration, for extracellular DIC values [les ]1.0 mm.

It is proposed that when the flux of CO2 into the cell is insufficient to support the photosynthetic rate at an optimum photon irradiance, the development of CAext increases the availability of CO2 at the plasma membrane. This ensures rapid acclimation to environmental change and provides an explanation for the central role of M. pusilla as a carbon sink in oligotrophic environments.

Three stands of Phragmites australis (Cav.) Trin. ex Steudel were investigated regarding the relationship between the number of efflux culms and convective ventilation efficiency affecting the hypoxic status of roots and rhizomes. The lack of old (efflux) culms after mowing the preceding winter caused a significantly higher counterpressure within the rhizome, thereby diminishing air flushing rate, i.e. oxygen supply, of rhizomes. The levels of alanine and c-aminobutyric acid in basal culm internodes increased significantly. Both amino acids indicate the hypoxic status of the root and rhizome metabolism of P. australis. Amino acid patterns of the basal culm internodes are discussed with respect to the maintenance of aerobic root metabolism and nutrient availability.

Apparent Km and Vmax for net NO3− uptake and short-term translocation patterns of recently absorbed N were compared in a stay-green mutant and wild-type selection line of Lolium perenne L. by means of a series of depletion studies using 15NO3−, performed over 12 d under conditions of progressively increasing N deprivation. In view of the greater retention of N in senescent leaves of the stay-green phenotype, it was predicted that NO3− uptake would be up-regulated relative to the normal line, and that a proportionally higher fraction of recently absorbed N would be allocated to young leaves. It was shown that the stay-green trait had significant phenotypic consequences for plant N relations, with higher ‘sink strength’ of shoots for recently absorbed N, and higher Vmax for NO3− uptake compared with those of normal plants. The stay-green mutation had no effect on the Km of the nitrate uptake system. Although the N-use efficiency might be expected to be lower in stay-green than in normal plants, there were no differences in rates of dry matter production.

The assumption that the mushroom stem has the ability to undergo autonomic straightening enables a mathematical model to be written that accurately mimics the gravitropic reaction of the stems of Coprinus cinereus. The straightening mechanism is called curvature compensation here, but is equivalent to the ‘autotropism’ that often accompanies the gravitropic reactions of axial organs in plants. In the consequently revised local curvature distribution model, local bending rate is determined by the difference between the ‘bending signal’ (generated by gravitropic signal perception systems) and the ‘straightening signal’ (proportional to the local curvature at the given point). The model describes gravitropic stem bending in the standard assay with great accuracy but has the virtue of operating well outside the experimental data set used in its derivation. It is shown, for example, that the mathematical model can be fitted to the gravitropic reactions of stems treated with metabolic inhibitors by a change of parameters that parallel the independently derived physiological interpretation of inhibitor action. The revised local curvature distribution model promises to be a predictive tool in the further analysis of gravitropism in mushrooms.

Mycelial cord systems, up to 50 cm in diameter, of the basidiomycete Phanerochaete velutina, a common woodland saprotroph, were grown on unsterile soil in model laboratory microcosms from either 4 or 16 cm3 wood inocula. After 48 d, systems were supplied at 10-d intervals with pairs of new 4 cm3 beechwood resources placed behind the foraging colony margin, where possible on opposite sides of the system. Image analysis was used to quantify radial extension, hyphal cover, the mass (DBM) and surface fractal (DBS) dimensions of the mycelial systems and wood-resource bleaching activity. Mycelial systems developing from small inocula had significantly (P[les ]0.05) lower radial extension rates, hyphal cover and DBM[ratio ]DBS ratio than those from large inocula. Initially, systems developing from small inocula also displayed significantly (P<0.05) slower wood-resource bleaching activity than those from large inocula, suggesting that carbon limitation affected both foraging behaviour and resource utilization. A separate central compartment containing the inoculum was supplied with 32P orthophosphate and its partitioning amongst wood resources was monitored nondestructively for 44 d. Total 32P acquisition by wood resources was not significantly (P>0.05) affected by inoculum resource size. However, the proportion of total acquired P allocated to resources, which varied according to the length of time that resources had been in contact with the mycelium, was dependent upon inoculum size. The results support the hypothesis that phosphorus translocation is not a demand-driven process. We suggest that the time taken before the greatest rate of phosphorus withdrawal from a uniform translocation stream is dependent upon prior availability of carbon within colonized resources.

Seedlings of Pinus sylvestris L. were grown in Plexiglas® observation chambers in limed (CaCO3, pH5·0 and 5·9) and untreated (pH 4·1) peat. The seedlings were either colonized by the mycorrhizal fungus Paxillus involutus (Batsch: Fr.) Fr. Or were non-mycorrhizal. After 18 wk in the observation chambers, 15N-labelled organic nitrogen, as lyophilized and ground mycelium of Suillus variegatus (Swartz: Fr.) O. Kuntze, or ammonium, was added to the peat. The plants were harvested after an uptake period of 14 d.

Irrespective of the nitrogen form added, liming decreased both the content and concentration of 15N in non-mycorrhizal plants, and, to a lesser extent, those in mycorrhizal plants. In mycorrhizal plants the uptake of 15N was not correlated with area colonized by the mycorrhizal mycelium. The amount of KCl-extractable 15N in peat without plants and mycorrhizal fungi decreased with liming. It is proposed that liming induced chemical or microbial immobilization of the added 15N. This is suggested to be the main reason for the decreased uptake of 15N in lime treatments.

Nitrogenous inorganic compounds impact plants as nutrients, signals and toxins. We are dissecting a regulatory network that controls nitrate assimilation at the level of nitrate reductase (NR) activity. The identification of protein kinase cascades, protein phosphatases and 14-3-3 proteins as regulators of NR are giving clues about how plants sense their nutrient availability, and use the information to signal changes in their metabolism and developmental strategies to cope with supplies. We hope that understanding these controls might lead to the design of transgenic plants with deregulated signalling networks, which would make them more efficient in using nitrogen fertilizers, and improving quality and yield of crops. There are circumstantial indications that gaseous anthropogenic nitrogenous emissions might also have complex regulatory influences on plant growth and development.

A model of seed germination is proposed which uses a variable with the units of an osmotic potential (virtual osmotic potential) to integrate the effect of a constant or a varying water potential. This differs from existing models that describe the effects of fixed water potentials on germination, or the effects of fixed priming water potentials on the subsequent germination at a fixed water potential. When a seed is sown, the virtual osmotic potential is assumed to fall at a rate that depends on the ambient water potential, and on the difference between its current and a minimum value. Radicle growth is assumed to initiate when the difference between the ambient water potential and the virtual osmotic potential exceeds a threshold. The germination of carrot and onion seeds at various fixed potentials below 0 MPa was well described by the virtual osmotic potential model. The model was also used to simulate the results of experiments in which seeds were given a single step change in water potential.

Barley (Hordeum vulgare L. cv. Agneta) was cultivated hydroponically under continuous light, constant temperature and relative humidity. The relative growth rate (RGR) was kept at 25% by an exponential supply of complete medium for 2 wk. The cultures were then transferred to phosphorus-free media and the amounts of total P, fructans, starch, sucrose, glucose and fructose in sink and source leaves were measured at 0, 12, 24, 48, 72, 120 and 156 h. The capacity of net fructan synthesis (i.e. total activity of sucrose[ratio ]sucrose 1-fructosyltransferase (1-SST) and sucrose[ratio ]fructan 6-fructosyltransferase (6-SFT)), the activity of fructan exohydrolase (FEH), as well as the activity of acid invertase were also measured in the two types of leaves. Omission of P from the growth medium caused a decrease in the P content in both sink and source leaves, whereas the RGR was not affected during the 156 h of the experiment. The fructan concentration increased in low-P plants whilst there was little or no effect on the concentrations of starch, sucrose, glucose and fructose. There was a slight increase in the capacity of net fructan synthesis in both sink and source leaves at 156 h after the onset of P-starvation. There was no change in the activity of FEH or acid invertase. The results indicate that in barley leaves the mechanism for carbon partitioning into fructans is more sensitive towards low P conditions than the mechanism for carbon partitioning into starch.

Addition of 2 mM nitrite or ammonium to aerobically incubated cultures of Gloeothece rapidly inhibited N2 fixation (measured as acetylene reduction). In contrast, 2 mM nitrate inhibited N2 fixation less rapidly and less extensively, and often temporarily stimulated nitrogenase activity. The inhibitory effects of both nitrate and ammonium could be prevented by addition of 3 mM L-methionine-DL-sulphoximine, suggesting that the true inhibitor of N2 fixation was an assimilatory product of ammonium rather than either ammonium or nitrate itself. The inhibition of N2 fixation by nitrite could not, however, be prevented by addition of L-methionine-DL- sulphoximine. On the other hand, nitrite (unlike nitrate and ammonium) did not inhibit N2 fixation in cultures incubated under a gas phase lacking oxygen. These findings suggest that the mechanism whereby nitrite inhibits N2 fixation in Gloeothece differs from that of either nitrate or ammonium. The inhibitory effect of nitrite on N2 fixation did not involve reduction of nitrite to nitric oxide, though nitric oxide was a potent inhibitor of nitrogenase activity in Gloeothece. Nitrate and nitrite inhibited the synthesis of nitrogenase in Gloeothece, while ammonium not only inhibited nitrogenase synthesis but also stimulated degradation of the enzyme. In addition, all three compounds favoured the appearance of the Fe-protein of nitrogenase in its larger, presumed inactive, form.

The growth of the marine flagellate Isochrysis galbana was followed in batch cultures at four concentrations of dissolved inorganic carbon (DIC), from C- and N-replete lag phase into C- and/or N-deplete stationary phase. Organic buffers were omitted from the growth medium, and culture pH was maintained at 8.30±0.05 by the addition of acid or alkali. The responses of the flagellate to N stress included an increase in the C[ratio ]N ratio, and decreases in the ratios of glutamine (Gln)[ratio ]glutamate (Glu) and Chl a[ratio ]C, and the cell Chl a quota. Conversely, the responses to C stress included a decrease in the C[ratio ]N ratio, and increases in the ratios of Gln[ratio ]Glu and Chl a[ratio ]C, and the cell Chl a quota. The relationship between carbon-specific growth rate (C-μ), and the concentration of extracellular DIC, [DIC]ext, exhibited Michaelis–Menten type kinetics with a half saturation constant, KG(DIC), of 81 μM. Comparative studies of the diatom Phaeodactylum tricornutum showed similar results, although the value of KG(DIC) was lower at 30 μM.

Contrary to the long-held belief that, internal to the cortical sterome, the central region of Sphagnum stems comprises unspecialized parenchyma, the present light- and electron-microscope study has revealed that these cells in fact have a highly specialized cytoplasmic organization. Their key features are: (a) the absence of large central vacuoles; (b) a spindle-shaped nucleus positioned internally; (c) a prominent axial system of endoplasmic microtubules associated with the nucleus, mitochondria, pleomorphic vacuoles, and membrane-bounded tubules and vesicles; (d) a distinct cytoplasmic polarization, with the cellular region near the capitulum being richer in organelles than the basal region; and (e) a high frequency of plasmodesmata in the cross walls with an enlarged median region containing no discernible desmotubule. Such a distinctive combination of cytological features has been hitherto only described for putative food-conducting cells in bryoid mosses. The results introduce a major new character common to Sphagnum and bryoid mosses and strongly suggest that this cytological organization underlines cellular specialization in symplasmic transport.