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

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

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

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

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

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

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

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

Root growth and respiration in elevated CO2 (700 μmol mol−1) was studied in three tree species, Fraxinus excelsior L., Quercus petraea. L. and Pinus sylvestris L. grown in open-top chambers (OTCs) during a long-term exposure (20 months), during which root systems were allowed to develop without restriction imposed by pots. Root growth, measured as root length using root in-growth bags was increased significantly in trees exposed to elevated CO2, although the magnitude of the response differed considerably between species and with time of sampling, the greatest effect observed after 6 months in ash (ratio of elevated: ambient, e[ratio ]a; 3·40) and the smallest effect observed in oak (e[ratio ]a; 1·95). This was accompanied by changes in specific root length, with a significant decrease in all species after 6 months, suggesting that root diameter or root density were increased in elevated CO2. Increases in root length might have resulted from an acceleration in root cell expansion, since epidermal cell size was significantly increased in the zone of elongation in ash root tips (P<0·05).

Contrasting effects of elevated CO2 were observed for root carbohydrates, with significant increases in soluble sugars for all species (P<0·05), but both increases and decreases in starch content were observed, depending on species, and producing a significant interaction between species and CO2 (P<0·001). Exposure to elevated CO2 increased the total root d. wt for whole trees of all three species after 8 months of exposure, although the magnitude of this effect, in contrast to the root in-growth study, was greatest in Scots pine and smallest in ash. No significant effect of elevated CO2 was observed on the root[ratio ]shoot ratio. Further detailed analysis of whole root systems after 20 months confirmed that species differences in root responses to elevated CO2 were apparent, with increased coarse and fine root production in elevated CO2 for Scots pine and ash respectively. Lateral root number was increased in elevated CO2 for all species, as was mean root diameter. Root respiration rates were significantly reduced in elevated CO2 for all three species. These results provide firm evidence that exposure of trees to future CO2 concentrations will have large effects on root system development, growth, carbohydrate status and respiration. The magnitude and direction of such effects will differ, depending on species. The consequences of such responses for the three species studied are discussed.

A previous study had indicated that scavengers of reactive oxygen species (ROS) delayed cell death (the hypersensitive response (HR)) triggered in epidermal cells of intact, resistant, cowpea (Vigna unguiculata (L.) Walp) leaves by the monokaryotic stage of the cowpea rust fungus (Uromyces vignae Barclay race 1). This HR had been monitored by cell autofluorescence, which occurs after protoplast collapse. In the present study, when cytoplasmic disorganization was used to monitor cell death more directly, ROS-scavengers, superoxide dismutase, catalase, horseradish peroxidase, and desferal-Mn(IV) had no effect on HR development. Cytological staining for superoxide or hydrogen peroxide generation also did not reveal the presence of ROS before or during the early stages of the HR, but did, as in the previous study, suggest a role in the autofluorescence and browning of invaded cells that occur following protoplast collapse. Staining of plant mitochondria with nitroblue tetrazolium, possibly attributable to increased dehydrogenase activity but not superoxide generation, occurred transiently around invasion hyphae (monokaryotic stage of the fungus) or haustoria (dikaryotic stage) of the fungus as they entered a cell in the susceptible or resistant cultivar. Around invasion hyphae in epidermal cells in resistant plants, this staining diminished as cytoplasmic streaming stopped, and gradually disappeared as cell death progressed. These data are consistent with other evidence that rust fungi initially negate non-specific defensive responses in both resistant and susceptible cells as part of the establishment of biotrophy. They also suggest that the HR in the cowpea–cowpea rust fungus pathosystem is not triggered by an oxidative burst.

A pot experiment was designed to evaluate the interactive effects of multiple microbial inoculation treatments and rock phosphate (RP) application on N and P acquisition by alfalfa plants using 15N and 32P isotopes. The microbial inocula consisted of a wild type (WT) Rhizobium meliloti strain, its genetically modified (GM) derivative, which had an enhanced competitiveness, the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe, and a phosphate-solubilizing rhizobacterium (Enterobacter sp.). Inoculated micro-organisms became established in the root tissues and/or in the rhizosphere soil of alfalfa plants (Medicago sativa L.). The GM Rhizobium strain did not interfere with AM formation. Inoculated phosphate-solubilizing rhizobacteria established in the alfalfa rhizosphere, but the level of establishment was lower where the natural population of phosphate-solubilizing bacteria was stimulated by AM inoculation and RP application. The stimulation of these indigenous bacteria was also greater in the rhizosphere of alfalfa nodulated by the GM Rhizobium. Improvements in N and P accumulation in alfalfa corroborate beneficial effects of the improved GM Rhizobium on AM performance, in RP-amended plants. Inoculation with Enterobacter did not improve the AM effect on N or P accumulation in the RP-added soil, but it did in the non RP-amended controls. Measurements of the 15N[ratio ]14N ratio in plant shoots indicated enhanced N2 fixation rates in Rhizobium-inoculated AM-plants, over that achieved by the same Rhizobium strain in non-mycorrhizal plants. Regardless of the Rhizobium strain and of whether or not RP was added, AM-inoculated plants showed a lower specific activity (32P[ratio ]31P) than did their comparable non-mycorrhizal controls, suggesting that the plant was using otherwise unavailable P sources. The phosphate-solubilizing, AM-associated, microbiota could in fact release phosphate ions, either from the added RP or from the indigenous ‘less-available’ phosphate. Deficiency in Ca concentration in soil solution in the neutral test soil might benefit P solubilization. The proportion of plant P derived either from the labelled soil P (labile P pool) or from RP was similar for AM inoculated and non-mycorrhizal controls (without Enterobacter inoculation) for each Rhizobium strain, but the total P uptake, regardless of the P source, was far higher in AM-plants. Enterobacter inoculation seems to improve the use of RP in the rhizosphere of non-mycorrhizal plants inoculated with the WT Rhizobium.

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.

The rate of nitrogen uptake by seven Sphagnum species, which from a gradient from hummock to hollow and from ombrotrophic to minerotrophic conditions, was measured as the decrease in the concentrations of NH4+ and NO3− from solutions in which capitula were grown under laboratory conditions.

The highest uptake rate was by individuals of each species with large capitula and a high number of ion exchange sites, i.e. lawn species (S. pulchrum, S. fallax, S. papillosum and S. magellanicum). On a dry-mass basis, the most effective species were the hummock species (S. fuscum and S. rubellum), even though these species have a low dry mass. Hummock species, which occur in high densities and have high potential N-uptake rates on a dry-mass basis, were the most effective species in retaining available nitrogen.

Well watered (WW) or drought-stressed (DS) saplings of ozone-sensitive and ozone-tolerant (less sensitive) birch (Betula pendula Roth) clones were exposed for 43 d to 0 nl l−1 or 100 nl l−1 ozone. Relative growth rates of leaves, stem, and roots, leaf discolouration, stomatal conductance and induction of genes encoding stress-related proteins PR-10, PAL and a LEA-group protein BP8 were determined. In general, both ozone and drought stress, singly and in combination, increased transcript levels of PR-10 in both clones. This was related to lower induction of PAL (except in older leaves of the tolerant clone), and increased proportions of visibly injured and yellowed leaves in ozone-exposed plants. The clones differed in their stomatal conductance and growth responses. In the less sensitive clone 2, ozone did not affect growth rates, but high stomatal conductance was observed in WW ozone-exposed plants. The more sensitive clone 5 showed, on the contrary, reduced growth rates and low stomatal conductance in WW ozone plants. Interestingly, clone 2 was sensitive to drought stress alone, whereas clone 5 was highly sensitive to ozone and drought stress experienced together. The results show that appearance of visible injuries (necrotic flecks) and enhanced yellowing of leaves coincided with the induction of genes for stress proteins PR-10 and PAL. The short-term growth responses, however, seemed to be separate processes. Additionally, stomatal conductance was related to leaf injuries and growth rates in a complicated manner, emphasizing the complex nature of ozone sensitivity/tolerance mechanisms in birch.

The number and type of isoforms of superoxide dismutase (SOD) and their activities were compared in mitochondria and peroxisomes isolated from cotyledons of three different oilseed seedlings. Mitochondrial and peroxisomal isoforms of SOD could be distinguished in nondenaturing polyacrylamide gels by their differential sensitivities to KCN and/or H2O2. The type of SOD was not the same for each organelle in each of the three oilseed species. For example, a single Mn–SOD was found in cotton and cucumber mitochondria, whereas four CuZn–SODs were present in mitochondria from sunflower. At least one CuZn–SOD isoform was found in the peroxisomes of all three species. Cucumber peroxisomes contained both a CuZn–SOD and a Mn–SOD, cotton peroxisomes contained a single CuZn–SOD, whilst four separate CuZn–SODs, but no Mn–SOD were found in sunflower peroxisomes. Using antibodies against CuZn–SOD from watermelon peroxisomes or from chloroplasts of Equisetum, a single polypeptide of c. 16·5 kDa was detected on immunoblots of peroxisomal fractions from the three species. Post-embedment, electron-microscopic double immunogold-labelling showed that CuZn–SOD, with malate synthase used as marker enzyme of peroxisomes, was localized in the matrix of these organelles of all three species. These results suggest that CuZn–SOD is a characteristic matrix enzyme of peroxisomes in oilseed cotyledons.

Due to their different physiological effects, elevated carbon dioxide and elevated ozone might have interactive impacts on plants, and differentially so on plants differing in photosynthetic pathway and growth rate. To test several hypotheses related to these issues, we examined the physiological, morphological and growth responses of six perennial species grown at various atmospheric concentrations of carbon dioxide and ozone. The species involved (two C3 trees: Populus tremuloides Michx., Quercus rubra L.; two C3 grasses: Agropyron smithii Rybd., Koeleria cristata L.; two C4 grasses: Bouteloua curtipendula Michx., Schizachyrium scoparium Michx.) differed in growth form, stomatal conductance and photosynthetic pathway. In situ photosynthesis, relative growth rate (RGR) and its determinants (leaf area ratio, specific leaf area, leaf weight ratio and root weight ratio) were determined via sequential harvests of seedlings that were grown in all combinations of 366 or 672 μmol mol−1 CO2 and 3 or 95 nmol mol−1 O3 over a 101-d period. Elevated CO2 had minimal effect on either photosynthesis or RGR. By contrast, RGR for all six species was lower in high O3 concentrations at ambient CO2, significantly so in A. smithii and P. tremuloides. Five of the six species also exhibited reductions in in situ photosynthesis at ambient CO2 in high-O3-grown compared with low-O3-grown plants. For all species, these O3-induced reductions in RGR and photosynthesis were absent in the elevated CO2 environment. Root weight ratio was significantly reduced by elevated O3 in A. smithii and P. tremuloides in ambient but not elevated CO2. Species with high stomatal conductance were the most susceptible to oxidant injury, while those with low stomatal conductance, such as the C4 species and Q. rubra, were not as detrimentally affected by O3. Elevated levels of CO2 will reduce stomatal conductance and O3 uptake, and might therefore reduce the potential for oxidant damage. However, there was a stronger relationship of the percent reduction in whole-plant mass due to O3, related to the ratio of photosynthesis to stomatal conductance. In general, results of this study of six functionally diverse plant species suggest that O3 pollution effects on carbon balance and growth are likely to be ameliorated by elevated concentrations of atmospheric CO2.

Embolisms in the vessels of maize axile roots of different types were observed directly after rapid freezing of intact, functioning roots in the field, by cryo-scanning electron microscopy. Quantification of the degree of embolization in each root was made by counting empty and full vessels of both the late and early metaxylem (LMX & EMX), and expressed as percent embolized vessels of the LMX, and %EMX poles containing embolized vessels. Contents of the connecting xylem (CX) at branch root junctions, and of xylem in branch roots were observed also, but not systematically quantified. Records of % embolized vessels were made from dawn to dusk on summer days in Ottawa under moderate irradiance, and in Canberra under high irradiance. Measurements in Canberra were supported by estimates of irradiance, of stomatal conductance, and of chamber balance pressure of bagged and unbagged leaves. Soon after sunrise embolisms appeared in all types of vessel, at balance pressures c. 300–400 kPa, and increased rapidly with increasing irradiance. During the middle of the day % embolized vessels reached a maximum (LMX ≈70% in Ottawa, and ≈80% in Canberra). At all times the EMX vessels were less embolized. The midday maximum was brief in Ottawa, and % embolized vessels fell to a low value during the afternoon. In Canberra the maximum was prolonged into late afternoon. By dusk nearly all vessels were once again filled with sap. The balance pressures measured during vessel refilling in Canberra ranged from 500 kPa to 1200 kPa. At all times of the day sap was seen entering some embolized vessels. Almost all were refilling by mid- to late-afternoon. Such refilling was especially frequent at junctions of branch roots with the axile roots. X-ray microanalysis of the sap entering the vessels, and of the liquid filling or partly filling vessels, showed the concentration of mineral solutes present in the sap was below the threshold of detection (≈12 mM). These results are discussed in relation to current opinions about embolisms and vessel refilling.

The major soluble carbohydrates in the desiccation-tolerant leafy liverwort Porella platyphylla (L.) Lindb. are sucrose and a homologous series of fructans including the trisaccharide 1-kestose. Exogenous glucose and fructose (10 mol m−3) did not affect the composition of the soluble carbohydrate pool. Sucrose caused an increase in the fructan pool. Sucrose also inhibited photosynthetic oxygen evolution and respiration. The fructan pool was maintained in preference to sucrose during dark starvation. Low temperature and low water potential increased the fructan pool whereas desiccation increased the proportion of high molecular weight fructan. Acid invertase activity was detected in a taxonomically diverse range of liverworts but was very low or undetectable in a range of mosses. The invertase activity from P. platyphylla was partially purified by ammonium sulphate precipitation. The reaction products of the partially purified enzyme were equimolar glucose and fructose. Kestose and higher DP fructans were not detected suggesting that, at least under the assay conditions used, the enzyme does not have sucrose[ratio ]sucrose fructosyl transferase activity. The pH optimum was 4·5–5 and the Km for sucrose was 1·7 mol m−3. Pyridoxal hydrochloride (5 mol m−3) caused 50% inhibition. The coexistence of sucrose and invertase suggests that either the invertase is inactive in vivo or is in a different subcellular compartment from sucrose. The pH response shows that it would have very low activity at cytosolic pH. A large acidic vacuole was detected in P. platyphylla leaf cells by neutral-red staining in which either invertase or sucrose could be sequestered. Rehydrating desiccated P. platyphylla for 10 min resulted in a 60% loss of extractable invertase activity. By contrast, extractable malate dehydrogenase activity increased during rehydration. Rehydrating desiccated leaves caused an increase in glucose and fructose suggesting that the sucrose pool was susceptible to invertase at this time. It is suggested that the partial inactivation of invertase during rehydration minimizes sucrose hydrolysis while membrane structure and subcellular compartmentation are re-established.

Isozyme expression of nutrient-mobilizing and defence-related enzymes were examined in different functional components of intact Scots pine (Pinus sylvestris L.) ectomycorrhizal root systems developed in natural unsterilized forest humus. Scots pine seedlings colonized by Suillus bovinus (L. ex Fr.) O. Kuntze or Paxillus involutus (Batsch ex Fr.) strains were grown on humus in two-dimensional Perspex[target] microcosms to allow development of complete ectomycorrhizal root systems. Soluble proteins from uncolonized short roots, whole mycorrhizal root tips or dissected mantle and core fractions, fungal strands and the outermost soil colonizing fine hyphae were subjected to native polyacrylamide gel electrophoresis and stained to detect esterase, acid phosphatase, peroxidase and different polyphenoloxidase isozyme activities. Tissue-specific esterase isozymes were detected in all components and most, including unique Hartig net isozymes, were of fungal origin. High fungal acid phosphatase activities were detected in mycorrhiza and fine hyphae of S. bovinus, supporting earlier findings of active phosphatase activity at the fungal interface in the Hartig net region and in the fine hyphal margins of extramatrical mycelium actively colonizing the humus. All compartments in the P. involutus mycorrhizosphere had weaker acid-phosphatase activities. Peroxidases formed a large part of the soluble-protein content in non-mycorrhizal short roots. The amount of peroxidases, on a tissue f. wt basis, was similar in mycorrhizal and non-mycorrhizal roots, but differential isozyme expression was detected, indicating a change in root peroxidase activities following mycorrhiza formation. The expression of DL-3,4-dihydroxyphenylalanine metabolizing polyphenol oxidase enzymes was reduced in mycorrhizas compared with non-mycorrhizal short roots. In the extramatrical mycelial components, fine hyphae expressed the highest polyphenoloxidase activity. P. involutus displayed some polyphenol-oxidizing isoenzymes not detected in S. bovinus systems. Laccase activity was not detected in the plant and fungal components examined. It is concluded that enzyme activities and isozyme expression are differentially regulated in the different functional components of Scots pine mycorrhizospheres.

Azolla filiculoides Lam. causes serious weed problems in Britain, but its long-term survival might be limited by winter death. The aim of this study was to establish the low temperature responses and limitations of A. filiculoides sporophytes.

In the laboratory, normal vegetative growth was shown to continue at 5°C. Reddening of plants was a response to low temperature and high light conditions which could be prevented by shading. Adult plants died after short (18 h) exposure to −4°C but survived sub-zero temperatures >−4°C. Evidence was found of seasonal changes in chill tolerance, but not in freeze tolerance.

In outdoor culture, plants survived encasement in ice and air temperatures to −5°C. Additional evidence suggested that natural populations can readily survive air temperatures much lower than this. Microclimatic effects are likely to be responsible for this discrepancy between laboratory and outdoor culture results.

Three phenotyes were identified; survival, colonizing and mat forms.

Sporulation in the floating fern Azolla filiculoides Lam. is both frequent and widespread in Britain and might therefore play a greater part in the population dynamics of the species than has been suggested by earlier reports. In laboratory experiments, increasing plant density and/or phosphorus supply resulted in increased sporulation. It was estimated that a thick mat of 8 kg m2 fresh biomass can produce 380000 microsporocarps and 85000 megasporocarps per m2.

Light and temperatures >10°C were necessary for sporocarp germination. Sporocarps could survive exposure to both low temperatures (5°C for at least 3 months) and sub-zero temperatures (−10°C for at least 18 d). Sporocarps were found to survive storage in water for 3 yr and to germinate from mud samples collected in the field. In laboratory culture, sporeling growth and survival were optimal at 15°C.

There is some evidence that A. filiculoides might have adapted to the British climate since its introduction.