We tested the hypothesis that functional attributes of living leaves provide a basis for predicting the decomposition rate of leaf litter. The data were obtained from standardized screening tests on 38 British herbaceous species. Graminoid monocots had physically tougher leaves with higher silicon contents than did herbaceous dicots, and this corresponded with the lesser decomposibility of the former. Total base content of living leaves was a good predictor of litter decomposition rate, but the evolutionary and ecological basis for this relationship appeared to differ between graminoid monocots and herbaceous dicots. In the monocots, litter decomposition rate was strongly predicted by leaf potassium content, which appeared to reflect other growth-related plant attributes such as seedling maximum relative growth rate, foliar nitrogen and phosphorus content, specific leaf area and short leaf lifespan. In the dicots the relationship between total leaf base content and litter decomposition rate was not unambiguously explained by growth-related leaf attributes, possibly because of the considerable calcium uptake by dicots, which varies according to calcium availability in the soil.

Human activity has greatly perturbed the nitrogen cycle through increased fixation by legumes, by energy and fertilizer production, and by the mobilization of N from long-term storage pools. This extra reactive N is readily transported through the environment, and there is increasing evidence that it is changing ecosystems through eutrophication and acidification. Rothamsted Experimental Station, UK has been involved in research on N cycling in ecosystems since its inception in 1843. Measurements of precipitation composition at Rothamsted, made since 1853, show an increase of nitrate and ammonium N in precipitation from 1 and 3 kg N ha−1 yr−1, respectively, in 1855 to a maximum of 8 and 10 kg N ha−1 yr−1 in 1980, decreasing to 4 and 5 kg N ha−1 yr−1 today. Nitrogen inputs via dry deposition do, however, remain high. Recent measurements with diffusion tubes and filter packs show large concentrations of nitrogen dioxide of c. 20 μg m−3 in winter and c. 10 μg m−3 in summer; the difference is linked to the use of central heating, and with variations in wind direction and pollutant source. Concentrations of nitric acid and particulate N exhibit maxima of 1·5 and 2 μg m−3 in summer and winter, respectively. Concentrations of ammonia are small, barely rising above 1 μg m−3.

Taking deposition velocities from the literature gives a total deposition of all measured N species to winter cereals of 43·3 kg N ha−1 yr−1, 84% as oxidized species, 79% dry deposited. The fate of this N deposited to the very long-term Broadbalk Continuous Wheat Experiment at Rothamsted has been simulated using the SUNDIAL N-cycling model: at equilibrium, after 154 yr of the experiment and with N deposition increasing from c. 10 kg ha−1 yr−1 in 1843 to 45 kg ha−1 yr−1 today, c. 5% is leached, 12% is denitrified, 30% immobilized in the soil organic matter and 53% taken off in the crop. The ‘efficiency of use’ of the deposited N decreases, and losses and immobilization increase as the amount of fertilizer N increases. The deposited N itself, and the acidification that is associated with it (from the nitric acid, ammonia and ammonium), has reduced the number of plant species on the 140-yr-old Park Grass hay meadow. It has also reduced methane oxidation rates in soil by c. 15% under arable land and 30% under woodland, and has caused N saturation of local woodland ecosystems: nitrous oxide emission rates of up to 1·4 kg ha−1 yr−1 are equivalent to those from arable land receiving >200 kg N ha−1 yr−1, and in proportion to the excess N deposited; measurements of N cycling processes and pools using 15N pool dilution techniques show a large nitrate pool and enhanced rates of nitrification relative to immobilization. Ratios of gross nitrification[ratio ]gross immobilization might prove to be good indices of N saturation.

More than 200 angiosperms, distributed in 25 genera, develop root nodule symbioses (actinorhizas) with soil bacteria of the actinomycetous genus Frankia. Although most soils studied contain infective Frankia, cultured strains are available only after isolation from root nodules. Frankia infects roots via root hairs in some hosts or via intercellular penetration in others. The nodule originates in the pericycle. The number of nodules in Alnus is determined by the plant in an autoregulated process that, in turn, is modulated by nutrients such as nitrogen and phosphate. Except in the genera Allocausarina and Casuarina, Frankia in nodules develops so-called vesicles where nitrogenase is localized. Sporulation of Frankia occurs in some symbioses. As a group, actinorhizal plants show a large range of anatomical and biochemical adaptations in order to balance the oxygen tension near nitrogenase. In symbioses with well aerated nodule tissue like Alnus, the vesicles have a multilayered envelope composed mainly of lipids, bacterio-hopanetetrol and their derivatives. This envelope is assumed to retard the diffusion of oxygen into the nitrogenase-containing vesicle. In symbioses like Casuarina, the infected plant cells themselves, rather than Frankia, appear to retard oxygen diffusion, and high concentrations of haemoglobin indicate an infected region with a low oxygen tension. At least in Alnus spp., ammonia resulting from N2 fixation is assimilated by glutamine synthetase in the plant. The carbon compound(s) used by Frankia in nodules is not yet known. Nitrogenase activity decreases in response to a number of environmental factors but recovers upon return to normal conditions. This dynamism in nitrogenase activity is often explained by loss and recovery of active nitrogenase and has been traced to loss and recovery of the nitrogenase proteins themselves. Recovery is partly due to growth of Frankia and to development of new vesicles in the Alnus nodules. In the field, varying conditions continuously affect the plants and the measured rate of N2 fixation is a result not only of the conditions prevailing at the moment but also of the conditions experienced over preceding days. N2 fixed by actinorhizal plants is substantial and actinorhizal plants have great potential in soil reclamation and in various types of forestry. Several species are also useful in horticulture.

Subterranean clover (Trifolium subterraneum L.) in symbiosis with Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe was grown in compartmented pots. Side compartments were filled with soil where recently added (1 wk) radio-labelled cadmium (Cd) at three levels of non-labelled Cd could be accessed by either roots or arbuscular mycorrhizal hyphae. All treatments were replicated with non-mycorrhizal plants. After a growth period of 52 d roots and shoots were analysed for 109Cd, and lengths of roots and hyphae in respective labelled compartments determined. Cadmium uptake by roots was not significantly influenced by the mycorrhizal status of the plant. Uptake of Cd from hyphal compartments was higher in mycorrhizal than in non-mycorrhizal plants, corresponding to 96, 127 and 131% of that in non-mycorrhizal plants when 1, 10 and 100 mg Cd kg−1 was added, respectively. A large proportion of the increased Cd content of mycorrhizal plants was sequestered in the roots. It is concluded that extraradical hyphae of AM fungi can transport Cd from soil to plants, but that transfer from fungus to plant is restricted due to fungal immobilization. No reduction of hyphal growth into soil with up to 20 mg extractable Cd kg−1 was observed.

Arbuscular mycorrhizal (AM) fungi can reduce the incidence and importance of plant root diseases caused by pathogens. The mechanisms involved are not well characterized. We used an in vitro experimental system to test the hypothesis that the extraradical mycelium of AM fungi can interfere directly with microorganisms in the mycosphere and directly or indirectly reduce the population of plant pathogens. This system permitted the isolation of soluble substances released by the extraradical mycelium of Glomus intraradices. The AM fungus was grown on Daucus carota transformed roots in one compartment, while only the extraradical mycelium was allowed to grow in a second compartment. A freezing and centrifugation technique was developed for the extraction and concentration of substances present in the compartment containing only the AM fungal mycelium. Four soil-inhabiting microorganisms were selected, and conidial germination (fungi) or growth (bacteria) of these was studied in the presence and absence (control) of the extract. In comparison with the control, the results indicated that both the growth of Pseudomonas chlororaphis and the conidial germination of Trichoderma harzianum were stimulated in the presence of the AM fungal extract. In contrast, conidial germination of Fusarium oxysporum f. sp. chrysanthemi was reduced while the growth of Clavibacter michiganensis subsp. michiganensis was not affected. The measured effects in general were directly correlated with extract concentration. Differences in pH were noted between the extract containing substances released by the AM fungus and the non-AM control, but no significant influence of the pH on growth or conidial germination was noted, confirming that substances released by the AM fungus in the growth medium is the main factor explaining differential growth of the microorganisms tested. The results suggest that direct interactions exist between AM fungi and soil microorganisms, which might lead to changes in microbial equilibrium detrimental to pathogens.

During the vegetation periods 1994 and 1995, net uptake of nitrate and ammonium by roots of adult spruce (Picea abies (L.) Karst) and beech (Fagus sylvatica L.) trees was studied at a field site exposed to high loads of N (‘Höglwald’, Germany). In addition, uptake experiments were carried out under controlled conditions with young spruce and beech trees grown at normal N supply.

In the field, nitrate was not taken up by the roots of spruce trees in appreciable amounts. This was also true for beech except during September 1995. Apparently, beech trees was capable of taking up nitrate, but the environmental condition prevailing at the field site usually prevented net uptake. Net uptake of ammonium in both tree species showed a seasonal course, with maximum rates in mid summer. Rates of ammonium uptake by both species correlated with soil temperature at the field site.

Laboratory experiments on the influence of root temperature on uptake of nitrate indicated that uptake rates at temperatures found in the field were low compared with the uptake capacity at optimum temperature. At temperatures of 10 and 15°C, frequently found in the soil at the field site, net uptake of nitrate by spruce and beech amounted to c. 16% and 11%, respectively, of maximum uptake at 25°C. By contrast, net uptake of ammonium at 10°C reached 73% and 31% of the maximum uptake for spruce and beech trees, respectively. Independent of temperature, rates of nitrate uptake were considerably lower than those of ammonium. In young spruce and beech trees, net uptake of nitrate was significantly inhibited by ammonium at nitrate[ratio ]ammonium ratios found in the soil solution at the forest site. Preincubation of roots of both species, with amino acids present in the phloem of adult trees at the field site, led to an increase in the amino acid pool in the roots. For spruce trees a correlation between inhibition of uptake of nitrate and enrichment of the roots with the amino compounds Glu, γ-amino butyric acid (Gaba), Gln, and Asn was observed. In beech trees, enrichment of Asp and Gln in the roots correlated with a decrease in net uptake of nitrate. The results of laboratory experiments on the effects of temperature, the nitrate to ammonium ratio in the nutrient solution, and amino acid enrichment in the roots are discussed with special emphasis on the patterns of net uptake of ammonium and nitrate observed in the field.

Mineral nutrient concentrations were determined in leaves of 83 mostly herbaceous species collected from central England. Most samples were analysed for N, P, K, Ca, Mg, Na, Fe, Al, Mn, Cu and Zn. Concentrations of K, N and P showed similar levels of interspecific variability, with the highest concentrations being 6–9 times the lowest. Mg and (especially) Ca were much more variable, with the highest concentrations being 24 and 49 times the lowest respectively. Only in the case of P concentration was the majority of the variance in the data found at or below the species level. Most of the variance in Ca and Mg concentrations was between monocots and dicots. Concentrations of N and P were strongly positively correlated with each other. Only Ca and Mn were consistently associated with soil pH, positively and negatively respectively. Dicots tended to accumulate more Ca and Mn from high soil concentrations than did monocots. Concentration of P was significantly positively correlated with maximum potential relative growth rate. Plants of woodland and arable habitats contained high concentrations of P, and those of pasture and skeletal habitats contained low concentrations of P. The P[ratio ]N ratio was higher in plants of arable habitats. Species with P-rich leaves tended to be currently increasing in abundance. The results suggest that plants with nutrient-rich foliage grow quickly, dominate nutrient-rich ecosystems and are generally increasing as a result of the eutrophication and disturbance arising from human exploitation.

Responses of Medicago truncatula to colonization by two arbuscular mycorrhizal fungi, Scutellospora calospora isolate WUM 12(2) and Glomus caledonium isolate RIS 42, were compared in the light of previous findings that the former fungus can be ineffective as a beneficial microsymbiont with some host plants. The plants were grown individually in two-compartment systems in which a lateral side arm containing soil labelled with 33P was separated from the main soil compartment by a nylon mesh that prevented penetration by roots but not fungal hyphae. Fungal inoculum was applied as a root–soil mixture in a band opposite the side arm. Nonmycorrhizal controls were set up similarly, without inoculum. There were harvests at 28, 35, 42 and 49 d. Both sets of mycorrhizal plants grew better than nonmycorrhizal plants and initially had higher concentrations of P in shoots and roots. Plants grown with S. calospora grew better than plants grown with G. caledonium, and this was associated with somewhat greater fungal colonization in terms of intraradical hyphae and numbers of arbuscules. Scutellospora calospora formed denser hyphae at root surfaces than G. caledonium. By 28 d there were extensive hyphae of both fungi in the side arms, and after 35 d S. calospora produced denser hyphae there than G. caledonium. Nevertheless, there was very little transfer of 33P via S. calospora to the plant at 28 d, and thereafter its transfer increased at a rate only c. 33% of that via G. caledonium. The results showed that plants colonized by S. calospora preferentially obtained P from sites in the main soil chamber relatively close to the roots, compared with plants colonized by G. caledonium. Hence formation of a highly beneficial arbuscular mycorrhizal symbiosis does not necessarily depend on development of hyphae at a distance from the roots or on large-scale translocation of P from distant sites. The results are discussed in relation to previous studies with compartmented systems that have involved the same fungi. Possible causes of the variable effects of S. calospora in symbiosis with different host plants are briefly assessed. Differences in spatial abilities of individual arbuscular mycorrhizal fungi to acquire P might have strong ecological implications for plant growth in soils low in P.

Dehydrin proteins (late embryogenesis abundant (LEA) D11 family) are produced in a wide variety of plant species in response to environmental stimuli with a dehydrative component, including drought, low temperature, salinity, and developmental stages such as seed and pollen maturation. Despite their widespread occurrence and abundance in cells under dehydrative conditions, the biochemical role of dehydrins remains elusive. The subcellular location of dehydrins is consistent with a biochemical role as an intracellular stabilizer, possibly with surfactant characteristics, acting upon targets in both the nucleus and cytoplasm. In some species, dehydrin loci are located within quantitative trait loci (QTL) intervals for important phenotypic traits including winter hardiness in barley (Hordeum vulgare L.) and anthesis-silking interval in maize (Zea mays L.). Dehydrin loci tend to be multigenic and occur in clusters on more than one chromosome. Investigations are currently under way in our laboratory and others' to move beyond protein accumulation studies and correlations with QTL to uncover direct cause-and-effect relationships between dehydrin (dhn) genes and phenotypes associated with physiological responses to stress.

The ectomycorrhizal fungal associations of Douglas fir (Pseudotsuga menziesii D. Don) and bishop pine (Pinus muricata D. Don) were investigated in a mixed forest stand. We identified fungi directly from field-collected ectomycorrhizal (ECM) root tips using PCR-based methods. Sixteen species of fungi were found, of which twelve associated with both hosts. Rhizopogon parksii Smith was specific to Douglas fir. Three other species colonized only one of the hosts, but were too infrequent to draw conclusions about specificity. Seventy-four percent of the biomass of ECM root tips sampled in the stand were colonized by members of the Thelephoraceae and Russulaceae. All 12 species of fungi that associated with both tree species did so within a 10×40 cm soil volume, suggesting that individual fungal genotypes linked the putatively competing tree hosts.

Summary 11

I. INTRODUCTION 12

II. LICHEN BIONT CHARACTERISTICS 12

1. The mycobiont 12

2. The photobiont 13

III. LICHEN GROWTH 13

1. Allocation of resources 13

2. Growth rates and environmental limitations 14

3. Maintaining an optimal energy use efficiency 15

IV. CARBON ACQUISITION 16

1. Water relations 16

(a) Desiccation tolerance 16

(b) Activation upon re-hydration 17

(c) Diffusion of water and CO2 18

2. Photobiont CO2 fixation 20

(a) CO2 acquisition modes 20

(b) Significance of the CCM 22

3. Light and nitrogen relations 22

(a) The light-response curve 23

(b) Photosynthetic capacity 25

(c) Coping with high light 27

V. CARBON SINKS AND EXPENDITURES 28

1. Carbon translocation 28

2. Carbon sinks 29

3. Respiration 30

VI. CONCLUDING REMARKS 30

Acknowledgements 31

References 31

Lichens are nutritionally specialized fungi (the mycobiont component) that derive carbon and in some cases nitrogen from algal or cyanobacterial photobionts. The mycobiont and photobiont live together in an integrated thallus, but they lack specific tissue for the transport of metabolites and resources between them. Carbon is acquired through photosynthesis in the photobiont, which is active when the lichen is wet and exposed to light. Lichen photosynthesis is limited primarily by water, light and nitrogen, but can also be constrained by slow diffusion of CO2 within the wet thallus. The assimilated carbon is exported from photobiont to mycobiont, which also predominates in terms of biomass, and apparently regulates the size of the photobiont population. It has therefore generally been assumed that most of the carbon is used for growth and maintenance of the fungal hyphae. However, the extent of photobiont respiration in relation to mycobiont respiration has seldom been quantified; neither do we know the pool sizes of various carbon sinks within lichens. Owing to this lack of fundamental data we do not know whether, or how, carbohydrate resources are regulated to maintain an optimal balance between energy input and expenditures in these symbiotic organisms. This review summarizes data on growth, carbon gain and carbon expenditures in lichens, with particular emphasis on factors determining the photosynthetic capacity of their photobionts. An attempt is made to introduce an economic analysis of lichen growth processes, a view that has often been adopted in studies of higher plants. Areas in which more data are needed for the construction of a model on ‘lichen resource allocation patterns’ are discussed.

Three groups of species (21 herbaceous monocotyledons, 22 herbaceous dicotyledons and 16 woody dicotyledons), including representatives of a wide range of natural habitats and life forms in inland Britain, were grown in the seedling phase in a resource-rich controlled environment and assessed over a 14-day period (21 d in the case of woody species). Mean values of relative growth rate (RGR), unit leaf rate (ULR), leaf area ratio (LAR), leaf weight fraction (LWF), specific leaf area (SLA), and the root–shoot allometric coefficient were derived.

In herbaceous species, the grand mean RGR was 0·20 d−1, comparable to values previously recorded. For woody species, the mean was 0·09 d−1. An existing assumption linking high RGR to high allocation to photosynthetic biomass was upheld by comparisons made between groups. Within groups, however, no pattern of this kind could be demonstrated.

When photosynthetically active radiation was increased from 125 to 250 µmol m−2 s−1, ULR was increased almost pro rata. The parallel response in RGR was only slight, being offset by considerable reductions in LAR. The apparent mean quantum yield for photosynthesis in herbaceous species (whole-plant d. wt basis) was 0·60 g mol−1.

There was no significant dependence of RGR on ULR in any of the three groups of species, although the absolute magnitude of ULR declined in the order: herbaceous monocotyledons > herbaceous dicotyledons > woody dicotyledons. In all three groups, RGR was strongly dependent upon LAR but no differences emerged in absolute scale of LAR. The absolute scale of mean LWF decreased from herbaceous to woody species, but the dependence of LAR on LWF strengthened. Groups showed no systematic differences in magnitude of SLA, but the correlation of LAR with SLA was strong throughout.

Multiple regression showed that the leading determinants of RGR were ULR and SLA in herbaceous species and LWF in woody species. Principal components analyses (PCA) on each of the three groups explained at least 77% of variation and agreed closely with an optimal (non-hierarchical) classification. Only six cluster ‘types’ were recognized out of the 16 theoretically possible combinations of ‘high’ or ‘low’ values of the four growth parameters. Strong evidence of evolutionary trade-offs emerged, most strikingly in that high RGR was never seen in combination with low SLA. The morphological/physiological types identified by an all-groups PCA separated woody from the herbaceous species, but dicotyledons were almost congruent with the monocotyledons.

The non-growth-analytical attributes most strongly correlated with mean RGR were percentage yield at a low level of mineral nutrients, leaf nitrogen concentration, and seed weight. It was concluded that mean RGR plays a central role in the identification of pathways of evolutionary specialization in herbaceous species.

A pea (Pisum sativum cv. Puget) cultivar was grown on a medium containing different NaCl concentrations (0–160 mol m−3) in order to study the effects of salt stress on leaf water relations and on the activity of antioxidant enzymes. NaCl stress caused a rapid decline in chlorophyll content. Both leaf water (ψl) and osmotic potentials (ψs) decreased progressively with the severity of the stress (from 90–160 mol m−3 NaCl) whereas leaf turgor pressure (ψp) increased in treated plants. Pea leaves contained an iron-containing superoxide dismutase (Fe-SOD) isozyme in chloroplasts alongside a copper-zinc-containing (CuZn-SOD) form (CuZn-SOD II). The lowest NaCl concentration (70 mol m−3) had no effect on the activity of these antioxidant enzymes while higher concentrations (110–130 mol m−3) enhanced the activity of cytosolic CuZn-SOD I and chloroplastic CuZn-SOD II as well as that of mitochondrial and/or peroxisomal manganese-containing superoxide dismutase (Mn-SOD). These inductions were matched by increases in the activity of ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR). The increased activities coincided with decreased stomatal conductance and were unaffected by the severity of stress except in the case of CuZn-SOD II which fell to control values under the highest stress conditions (140–160 mol m−3 NaCl), when a concomitant increase in chloroplastic Fe-SOD activity was observed. Glutathione reductase (GR) and dehydroascorbate reductase (DHAR) activities were only induced under severe NaCl stress (130–160 mol m−3) and were accompanied by losses in the ascorbate and glutathione pools, lower ASC/DHA and GSH/GSSG ratios and increases in GSSG. Electron microscopy showed that the thylakoidal structure of the chloroplasts became disorganized and their starch content decreased in plants treated with 160 mol m−3 NaCl. Overall, the results suggest that salt stress is accompanied by oxidative stress, perhaps at the cell compartment level. The capacity of Puget cultivar to ensure cell turgor and to enhance the activity of enzymes involved in the defence against oxidative stress seems to be important in determining adaptation to moderate NaCl stress conditions. In plants exposed to severe NaCl stress (130–160 mol m−3) it seems that such resistance to oxidative stress is overcome, which might contribute to the deleterious effects of salt and significant growth reduction in these conditions.

Arbuscular mycorrhizal (AM) fungi, Rhizobium bacteria and plant-growth-promoting rhizobacteria (PGPR) were isolated from a representative area of a desertified semi-arid ecosystem in the south-east of Spain. Microbial isolates were characterized and screened for effectiveness by a single-inoculation trial in soil microcosms. Anthyllis cytisoides L., a mycotrophic pioneer legume, dominant in the target mediterranean ecosystem, was the test plant. Several microbial cultures from existing collections were also included in the screening process. Two AM fungi (Glomus coronatum, native, and Glomus intraradices, exotic), two Rhizobium bacteria (NR4 and NR9, both native) and two PGPR (A2, native, and E, exotic) were selected. A further screening for the appropriate double and triple combinations of microbial inoculants was then performed. The parameters evaluated were biomass accumulation and allocation, N and P uptake, N2-fixation (15N) and specific root length. Overall, G. coronatum, native in the field site was more effective than the exotic G. intraradices in co-inoculation treatments. In general, our results support the importance of physiological and genetic adaptation of microbes to the whole environment, thus local isolates must be involved. Many microbial combinations were effective in improving either plant development, nutrient uptake, N2-fixation or root system quality. Selective and specific functional compatibility relationships in plant response between the microbial inoculants, were observed. Despite the difficulty of selecting a multifunctional microbial inoculum, appropriate microbial combinations can be recommended for a given biotechnological input related to improvement of plant performance. This could be exploited in nursery production of target plant species endowed with optimized rhizosphere/mycorrhizosphere systems that can be tailored to help plants to establish and survive in nutrient-deficient, degraded habitats. The relevance of this microbial-based approach in the context of a reclamation strategy addressed to environmental sustainability purposes is discussed.

Summary 3

I. introduction 4

II. the cyanobacteria 7

III. the heterocyst 9

1. Function and metabolism 9

2. Heterocyst structure 12

(a) Overview 12

(b) The polysaccharide (homogeneous) layer 12

(c) The glycolipid (laminated) layer 12

(d) The septum and microplasmodesmata 12

3. Nitrogen regulation and heterocyst development 12

4. Heterocyst development 13

(a) The proheterocyst 13

(b) Proteolysis associated with heterocyst development 14

(c) RNA polymerase sigma factors 14

(d) Developmental regulation of heterocyst cell wall and nitrogenase gene expression 14

(e) Genome rearrangements associated with heterocyst development 15

5. Genes essential for heterocyst development 15

(a) hetR 15

(b) Protein phosphorylation and the regulation of hetR activity 16

(c) hetR in nonheterocystous cyanobacteria 16

(d) Other heterocyst-specific genes 16

6. Heterocyst spacing 18

(a) Patterns of heterocyst differentiation 18

(b) Genes involved in heterocyst spacing 18

(c) Disruption of heterocyst pattern 18

7. Filament fragmentation and the regression of developing heterocysts 20

8. The nature of the heterocyst inhibitor 20

9. Cell selection during differentiation and pattern formation 20

(a) Cell division 20

(b) DNA replication and the cell cycle 21

(c) Competition 21

10. Models for heterocyst differentiation and pattern control 21

IV. the akinete 23

1. Properties of akinetes 23

2. Structure, composition and metabolism 24

3. Relationship to heterocysts 24

4. Factors that influence akinete differentiation 24

5. Extracellular signals 25

6. Akinete germination 25

7. Genes involved in akinete differentiation 26

V. conclusion 26

Acknowledgements 27

References 28

Cyanobacteria are an ancient and morphologically diverse group of photosynthetic prokaryotes. They were the first organisms to evolve oxygenic photosynthesis, and so changed the Earth's atmosphere from anoxic to oxic. As a consequence, many nitrogen-fixing bacteria became confined to suitable anoxic environmental niches, because the enzyme nitrogenase is highly sensitive to oxygen. However, in the cyanobacteria a number of strategies evolved that protected nitrogenase from oxygen, including a temporal separation of oxygenic photosynthesis and nitrogen fixation and, in some filamentous strains, the differentiation of a specialized cell, the heterocyst, which provided a suitable microaerobic environment for the functioning of nitrogenase. The evolution of a spore-like cell, the akinete, almost certainly preceded that of the heterocyst and, indeed, the akinete may have been the ancestor of the heterocyst. Cyanobacteria have the capacity to differentiate several additional cell and filament types, but this review will concentrate on the heterocyst and the akinete, emphasizing the differentiation and spacing of these specialized cells.

Interspecific and intraspecific variation in the internal transcribed spacer (ITS) region of rDNA of ectomycorrhizal fungi of 44 species in 17 genera were examined using the restriction fragment length polymorphism (RFLP) method. For each species, two to five herbarium vouchers (mainly basidiocarps), collected throughout Fennoscandia, were examined. The ITS region was amplified using the polymerase chain reaction (PCR) and the universal primers ITS1 and ITS4, and subjected to RFLP analysis with three endonucleases. Intraspecific polymorphisms in the ITS region were found in seven species (in nine of the 132 herbarium vouchers). Polymorphisms were due to length mutations, ranging from 5 to 15 bp in four of the seven polymorphic species and mutations in endonuclease restriction sites in six species, mostly affecting only one endonuclease, but in two species two endonucleases. Using a single endonuclease, a unique RFLP pattern could be obtained for more than half the investigated species. By combining different endonucleases, 34 (77%) of the species could be distinguished from another. The remaining RFLP types occurred in one genus. On the basis of the low intra- but high interspecific variation in the ITS region, it is concluded that most ectomycorrhizas formed by the 44 investigated species should be recognized by comparison with this dataset, if the mycorrhizas are sampled from a site located in Fennoscandia. However, in datasets from even larger geographical areas encompassing a higher degree of intraspecific variation in the ITS region, or when mycorrhizas from several sites distant from each other are compared, it might be necessary to include local reference species.

Exposure of Chlorella vulgaris to elevated concentrations of copper, chromium, nickel and zinc led to intracellular accumulation of high concentrations of these metals. Concomitantly, accumulation of free proline occurred, depending on the concentration of metals in the external medium or in the cell. The greater the toxicity or accumulation of a metal, the greater the amount of intracellular proline in algal cells. However, higher concentrations of copper and chromium were inhibitory to proline accumulation by the test organism. The accumulation of proline was triggered within a few hours of metal treatment. Test metals also induced lipid peroxidation; copper was the most efficient inducer whereas zinc was the least. Pretreatment of C. vulgaris with proline counteracted metal-induced lipid peroxidation and potassium ion efflux. Thus the present work shows a protective effect of proline on metal toxicity through inhibition of lipid peroxidation.

Growth, tolerance and zinc and cadmium hyperaccumulation of Thlaspi caerulescens populations from three metal contaminated soils and three normal soils were compared under controlled conditions. Individuals of six populations were cultivated on five soils with increasing concentrations of zinc (50–25000 μg g−1) and cadmium (1–170 μg g−1). There was no mortality of normal soil populations in the four metal-contaminated soils, but plant growth was reduced to half that of populations from metal-contaminated soils. However, in noncontaminated soil, the growth of individuals from normal soils was greater than that of individuals from metal-contaminated soils. Individuals from normal soils concentrated three times more zinc in the aboveground biomass than those from metal-contaminated soils, but the latter accumulated twice as much cadmium. We conclude that populations of T. caerulescens from both normal and metal-contaminated soils are interesting material for phytoextraction of zinc and cadmium, but to optimize the process of phytoextraction it is necessary to combine the extraction potentials of both type of populations.

Although there is a great deal of concern about the effects of human activities on biodiversity, until recently there has been very little interest in the effects of ozone on the species that constitute the major part of the flora, the diverse herbaceous and shrubby species of natural and semi-natural communities. However, many wild species have been shown to be at least as sensitive to ozone as crops that show significant yield losses, so there is a pressing need for an evaluation of the risk to wild species posed by ozone. This review attempts to assess progress and highlight problems. It begins with a comment on semantics, discusses the difficulties involved in measuring relative ozone resistance and then proceeds to consider the effects of ozone on growth and resource allocation. The evidence for evolution of resistance is appraised and then the potential effects of several interactions (cutting/grazing, competition, soil water deficit and nutrition) are considered. The review ends with some remarks on observation of oxidant-induced changes in ecosystems.

Tropospheric ozone can affect crop yield and has been reported to cause reductions in growth and biomass of forest tree species in laboratory and glasshouse studies. However, linkages between growth and ambient ozone concentrations in the field are not well established for forest trees. Ambient ozone concentrations have been shown to cause foliar injury on a number of tree species throughout much of the eastern USA. Symptom expression is influenced by endogenous and exogenous factors and, therefore, ozone-exposure/tree-response relationships have been difficult to confirm. Clearly defined, cause-effect relationships between visible injury and growth losses due to ozone have not been validated. Generalizations of sensitivity of forest trees to ozone are complicated by tree development stage, microclimate, leaf phenology, compensatory processes, within-species variation and other interacting stresses. In general, decreases in above-ground growth at ambient ozone levels in the eastern USA appear to be in the range of 0–10% per year. However, these conclusions are based on a small number of tree species, with the vast majority of studies involving individual tree seedlings in a non-competitive environment. Comparative studies of small and large trees indicate that seedlings are not suitable surrogates for predicting responses of mature trees to ozone. Process-level modelling is a promising methodology that has been recently utilized to assess ozone effects on a stand to regional scale, indicating that ozone is affecting forest growth in the eastern USA. The extent and magnitude of the response is variable and depends on many edaphic and climatic factors. It is imperative when conducting assessment exercises, however, that forest biologists constantly keep in mind the tremendous variability that exists within natural systems. Scaling of single site/physiological response phenomena from an individual tree to an ecosystem and/or region necessitates further research.