Changes in various cell functions were examined during a shift of the cyanobacterium Anabaena sp. strain PCC7120 to acidic external pH (pHex) in the presence and absence of added calcium. In the presence of 0·25 mM Ca (the standard Ca concentration of the growth medium), growth and O2 evolution were inhibited at pH values lower than 6. The cyanobacterium was unable to maintain a relatively constant internal pH (pHi) at pH 6 and below, which led to acidification of the cytoplasm. In the absence of Ca, the acidification found at acid pH values was even more pronounced. The elimination of Ca did not affect pHi at pHex of 6·5 and above. Increased acidification of the internal cell contents correlated well with a general impairment of growth in Ca-deficient cells exposed to external acid pH values. On the other hand, Ca enrichment of cells grown under acidic conditions resulted in a significant improvement of several physiological processes. The protein pattern of cell extracts of Anabaena sp. strain PCC7120 was altered by acidity. The most significant modifications of the protein profiles induced at low pH were not evident in the presence of high concentrations of Ca. Increasing concentrations of Ca allowed Anabaena sp. strain PCC7120 to perform better at lower pH.

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.

In Nicotiana sylvestris Spegazzini and Comes (Solanaceae), we examined the relationships among wounding, endogenous leaf jasmonic acid (JA) pools, and whole-plant (WP) nicotine accumulation over a range of wounding intensities and spatial distributions, in order to explore optimal defence (OD) theory predictions. We quantitatively wounded one or four leaves and then quantified: (1) JA in damaged and undamaged leaves 90 min after wounding; (2) WP nicotine concentration after 5 d (the times when JA and nicotine attain the largest wound-induced concentrations). We find: (1) statistically significant, positive relationships on a leaf-by-leaf basis among the number of leaf punctures, endogenous leaf JA, and WP nicotine accumulation; (2) that young, undamaged leaves have a higher concentration of JA than do older, undamaged leaves, and produce a greater amount of JA per puncture than older leaves, but that all leaves have the same JA content (ng JA per leaf); and (3) that a damaged leaf produces less JA when other leaves in the canopy are wounded than when it is the only wounded leaf in the canopy, but that when it is the only wounded leaf, the phylotactically adjacent, undamaged leaves do not increase their JA concentrations. The observation that younger leaves produce more JA per puncture than do older leaves is consistent with OD theory predictions. The observation that a small amount of damage localized to a single leaf is as effective as a larger amount of damage dispersed across the canopy in increasing leaf JA and WP nicotine accumulation shows the plant's ability to differentiate between dispersed and localized damage. Because the quantity of JA in a wounded leaf 90 min after wounding is a reliable indicator of the WP nicotine response to wounding, this trait provides insight into how plants integrate information about environmental insults and tailor their defence responses.

Recent advances in the cytogenetics of the Lolium–Festuca complex provide new opportunities for understanding and manipulating complex physiological mechanisms such as drought and cold resistance. This paper describes breeding programmes involving hybrids between two species: L. multiflorum, which offers good early growth and high nutritive value, and F. arundinacea, which is more stress tolerant. The programmes are designed to allow a rapid recovery of the Lolium genome and to restrict numbers of recombinants derived from Festuca. Use of genomic in situ hybridization (GISH) and an isozyme marker demonstrates how gene complexes from any part of the Festuca genome can be introgressed into Lolium. This enables us to construct genotypes combining desirable traits of both Lolium and Festuca species. By introgressing different Festuca genes into Lolium, quantitative traits such as tolerance to drought and cold can be ‘dissected’ into their different components, to clarify their function. Festuca genes for stress tolerance can be located by genomic in situ hybridization (GISH) and assigned to regions of chromosome arms in Lolium. Two Lolium genotypes are described, in which genes for drought resistance transferred from the F. pratensis sub-genome of F. arundinacea onto chromosome 2 of Lolium. The two drought-resistant lines have the high water conductance of Festuca on their adaxial leaf surface and the low abaxial conductance of Lolium. The paper also describes how androgenesis of L. multiflorum × F. arundinacea hybrids has led to the selection and characterization of genotypes with coacclimation to drought and freezing stress, in some cases exceeding that in the Festuca parent.

Seedlings of 12 provenances of European beech (Fagus sylvatica) were exposed to ambient, non-filtered air (NF) or NF+50 nl l−1 ozone (NF50) for 8 h d−1 in open-top chambers (OTCs), from 1 June to 4 October 1995. In 1996 exposure was continued from 31 May to 1 October at four levels: charcoal-filtered air (CF), NF, NF50 and NF+100 nl l−1 ozone (NF100). Provenances were grown for both seasons in outside reference plots. All treatments were replicated. Ozone did not affect gas exchange in the provenances until late in the second season. NF100 reduced photosynthesis by 18% in August 1996 compared to CF. In September, photosynthesis was reduced by 22% in NF50 and by 29% in NF100. After two seasons, ozone reduced the root:shoot ratio by 24% when comparing CF and NF100; this was caused by reductions in the root biomass. Ozone did not affect height growth or stem diameter, and there were no ozone×provenance interactions for any growth parameter. There was, however, a significant ozone×provenance interaction for photosynthesis, showing northwest European provenances to be more sensitive to ozone than southeast European provenances when comparing dose–response estimates. This is interpreted in terms of genetic adaptation of the photosynthetic apparatus to regional growing conditions. Seedlings in the chambers grew 45% taller, and had 28% more shoot biomass and 29% smaller root biomass, resulting in a 44% reduction of root:shoot ratios compared to seedlings outside. Increased temperature and decreased PAR inside the chambers relative to the outside were probably the main causes for the differences. The magnitude of the chamber effects in OTCs raises doubts about conclusions drawn from ozone exposures in such chambers. This and previous ozone experiments with OTCs may have reached inaccurate conclusions concerning the size of ozone responses due to chamber effects.

Simulated swards of perennial ryegrass (Lolium perenne) growing in 1-m3 soil blocks in the glasshouse were either well watered or deprived of water for 57 d and then rewatered. The first aim was to measure effects of drought on sugar (water-soluble carbohydrate) composition of laminae and sheaths of mature laminae, and bases and laminae of young (growing) leaves. The second aim was to use pulse labelling with 14CO2 to follow the partitioning of recently-fixed assimilates, and the assembly and consumption of reserve sugars (fructans). Over the last 7 d of drought growth almost stopped, old leaves died faster than they were replaced, and total sugar (which had doubled in concentration during drought) was rapidly consumed. Leaf laminae had lower content of total sugars and of large fructan (DP>5) than did growing bases and sheaths. Drought greatly reduced the rate at which sugar was exported from the laminae to the sheaths and growing leaf bases, and the rate at which it was converted to fructan. Nevertheless, fructan accumulated over the first 50 d of drought. Rewatering did not result in depolymerization and remobilization of sugars that had been formed during the last 7 d of drought, but stimulated their further assembly into high-DP fructans. Our hypothesis, that accumulation of neo-kestose (a DP-3 fructan) in droughted laminae was a symptom of sugar remobilization just before death, was disproved. It is concluded that sugar reserves contribute to drought resistance only under extreme conditions. The specific role of fructan in dry environments might be to improve regrowth when drought is relieved, rather than to enhance growth during drought.

In the ‘F’ horizons of acid mor-humus soils of heathland ecosystems, mycorrhizal roots of the dominant ericaceous species form a large fraction of the soil biomass. Rapid turnover of these roots provides the potential for recycling of substantial amounts of nitrogen contained in their fungal and plant components. Here, we first determine the amount of N in the biomass of ericoid roots growing in heathland and show it to constitute a large proportion of total soil N. In order to assess the accessibility of this N to ericaceous plants, experiments were then conducted using aseptically produced shoot and root necromass of Vaccinium macrocarpon Ait., the roots being grown with or without mycorrhizal colonization. These materials were provided as sole nitrogenous substrates in growth experiments using the ericoid mycorrhizal fungus Hymenoscyphus ericae (Read) Korf & Kernan in pure culture and V. macrocarpon in the mycorrhizal (M) or non-mycorrhizal (NM) condition as test organisms. The experiments were designed to test the hypothesis that the N contained in these substrates can be mobilized by the mycorrhizal endophyte. The ability of the endophyte to utilize the substrates was determined by measuring fungal yields and by assessing the presence of its extra-cellular protease and chitinase enzymes. Transfer of N to the host by the endophyte was determined through measurements of plant yield and tissue N contents. H. ericae produced a significantly greater yield on shoot and mycorrhizal root necromass than on non-mycorrhizal root necromass. The extra-cellular enzymes protease and chitinase were produced by the fungus when grown on the M root necromass. The fungus also transferred N to the host plant, up to 76% of N contained in the substrate being found in M plants whereas less than 5% was present in their NM counterparts.

Feeding dynamics in the spittlebug Philaenus spumarius were analysed to determine the food source. Evidence from previous reports indicates that this species and similar insects feed from the xylem. However, it is not clear whether they extract sap from the main transpiration stream, or from some subsidiary compartment of xylem, such as immature or cavitated vessels. In feeding from such subsidiary compartments, the insects could avoid the large hydraulic tensions thought to prevail in xylem on the main transpiration stream. Philaenus, for example, is shown here to initiate feeding, and to feed rapidly, from plants in which the full xylem tension is estimated at −10 bar. From measurements of feeding rates and feeding patterns in stressed and unstressed plants, it is shown that Philaenus cannot feed from subsidiary xylem compartments. It is concluded that this insect feeds directly from the main transpiration stream, and at the full hydraulic tension which prevails there. The anatomy of the feeding pump in Philaenus appears consistent with this conclusion. Spittlebugs therefore offer an experimental tool for quasi- non-invasive sampling of the transpiration stream.

Potted specimens of 27 Swedish native herbs and grasses were exposed to three different ozone concentrations, CF (charcoal filtered air), NF (non-filtered air) and NF+ (1·5 × non-filtered air) in open-top chambers from 2 July to 5 August 1994. The species represented a wide range of different plant strategies according to the C-S-R model. The results show that the stress-tolerators, S, had a smaller mean relative growth rate, R¯, during the exposure period and also a smaller accumulation of biomass compared with other strategies. The species with the intermediate strategy, CSR, had a R¯ similar to that of the species with a large component of C, the competitor strategy, and/or R¯ the ruderal strategy, whereas the net accumulation of biomass was smaller in the CSR group. This difference between R¯ and net growth for the CSR species compared to the species of other strategies, can be explained by a slower growth for the CSR group during the establishment phase. In the present investigation the response to ozone was very small regardless of plant strategy, although there was a weak trend towards slower growth with higher ozone concentration, 18 out of 27 species having a greater growth in CF than in NF+. The growth of one typical stress-tolerant species, Festuca ovina L., was stimulated significantly by ozone. Visible injury, most likely caused by ozone, was noticed in three species, Dactylis glomerata L., Dactylis aschersoniana Graebn. and Phleum alpinum L. Although there were no major restrictions to plant growth in terms of water, nutrients or space in the present open-top chamber experiment, the variability in all climatic factors was as large as in the ambient air. This might harden the plants and make them less sensitive to ozone compared to plants grown in the laboratory under controlled and optimal conditions with low variability.

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.

Rates of carbon flux in 14 species of Antarctic bryophytes were measured under controlled conditions using an infra-red gas analysis system. The results were used to produce estimates of model parameters for respiration and photosynthesis. The relationships between respiration, photosynthesis, irradiance and temperature followed standard patterns. Temperature optima for gross and net photosynthesis were 10–20 and 0–20°C respectively, suggesting that the plants were not truly psychrophilic. Photosynthesis was saturated at 30–270 μmol m−2 s−1, consistent with the view that bryophytes are, physiologically, shade plants, although there was no evidence of photoinhibition over the range of irradiances tested (up to 700 μmol m−2 s−1). Comparison of the results with environmental data suggests that photosynthesis is usually temperature-limited during daylight in the growing season. Therefore, any change in the temperature of the habitat could affect the productivity of the bryophytes. Rates of photosynthesis varied widely between species, and these relationships were largely maintained over the range of temperatures and irradiances tested. Photosynthetic rankings were correlated with the water availability in the plant habitats, supporting the hypothesis that water is the important factor in determining the distribution of populations in Antarctic habitats.

Biological rhythms are ubiquitous in eukaryotes, and the best understood of these occur with a period of approximately a day – circadian rhythms. Such rhythms persist even when the organism is placed under constant conditions, with a period that is close, but not exactly equal, to 24 h, and are driven by an endogenous timer – one of the many ‘biological clocks’. In plants, research into circadian rhythms has been driven forward by genetic experiments using Arabidopsis. Higher plant genomes include a particularly large number of genes involved in metabolism, and circadian rhythms may well provide the necessary coordination for the control of these – for example, around the diurnal rhythm of photosynthesis – to suit changing developmental or environmental conditions. The endogenous timer must be flexible enough to support these requirements. Current research supports this notion most strongly for the input pathway, in which multiple photoreceptors have been shown to mediate light input to the clock. Both input and output components are now related to putative circadian oscillator mechanisms by sequence homology or by experimental observation. It appears that the pathways linking some domains of the basic clock model may be very short indeed, or the mechanisms of these domains may overlap. Components of the first plant circadian output pathway to be identified unequivocally will help to determine exactly how many output pathways control the various phases of overt rhythms in plants.

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

This is the first extensive investigation that quantifies natural mycoparasitic relationships in the phyllosphere. The presence of Ampelomyces spp. was quantified in naturally occurring powdery mildew fungi collected in Hungary and Romania between 1992 and 1995. A total of 570 samples was studied representing 27 species (nine genera) of the Erysiphaceae infecting 41 host plant genera. The incidence of Ampelomyces spp., determined as the proportion of samples in which intracellular pycnidia were present, varied between 4.3 and 68.8% in the host fungal genera studied. The intensity of mycoparasitism, defined as a percentage of the powdery mildew mycelia parasitized by Ampelomyces, ranged from 0.15 to 65%. Both the incidence and the intensity of mycoparasitism showed the lowest values in Blumeria graminis (DC.) Speer collected from wild and cultivated monocotyledons, while the highest values were found in Arthrocladiella mougeotii (Lév.) Vassilkov infecting Lycium halimifolium Mill. plants. The paper reports for the first time the natural occurrence of Ampelomyces in Sawadaea bicornis (Wallr.[ratio ]Fr.) Homma on maple.

The interaction between the race 1 of the cowpea rust fungus (Uromyces vignae Barclay) and the resistant cowpea (Vigna unguiculata (L.) Walp.) cv. Queen Anne is characterized by the deposition of callose around intracellular fungal structures. Ultrastructural examination of the early stages of infection by basidiospores of the fungus revealed two types of deposits induced by intracellular hyphae: a non-callose collar at the penetration site and a callose encasement around the invasion hypha. The callose encasement was developed from the site where the fungus encountered the inside of the plant cell wall and was separated from the fungus by the extrahyphal membrane and an extension of plant plasma membrane. The incidence of encasements was reduced in plants treated with inhibitors of transcription (actinomycin D), protein synthesis (cycloheximide), protein glycosylation (tunicamycin) and microfilament polymerization (cytochalasin E). Inhibitors of Golgi-associated vesicle transfer (monensin, brefeldin A) and anti-microtubule agents (colchicine, oryzalin) had no effect. When the fungus was killed by heat treatment in either the resistant or the susceptible cultivar, callose was deposited at various locations along the fungus, mostly in the extrahyphal matrix. The data suggest that unlike the extrahaustorial membrane surrounding D-haustoria of this fungus, the extrahyphal membrane is capable of generating callose. Since this process does not normally occur in the susceptible or the resistant cv. even when callose is deposited in the latter by regions of the plasma membrane not associated with the fungus, we conclude that the deposition of callose by the extrahyphal membrane is inhibited by the living fungus.

Littorella uniflora (L.) Ascherson is a small, perennial, amphibious rhizophyte of rosette life-form which is common along the margins of lakes, tarns and reservoirs where water-level fluctuations are often rapid and unpredictable. The majority of plants are continuously submersed and reproduce vegetatively, but a small proportion become completely emersed for variable lengths of time, when flowering and seed set occur. To find out how L. uniflora adjusts to sudden emersion we studied the plants at a reservoir where water level falls each spring and remains low throughout the summer; L. uniflora adjusted very quickly showing a degree of phenotypic plasticity not expected in a ‘stress tolerator’, including the production of a new set of terrestrial leaves with reduced lacunal volume and increased stomatal density, a rapid increase in leaf growth rate, and flowering within 3–4 wk. Comparison of terrestrial L. uniflora with aquatic plants growing permanently submersed in lake and tarn habitats showed that three to fourfold more carbon (C) and nitrogen (N) was incorporated into above-ground biomass by emersed plants. However, ramet production in the aquatic environment appeared to be more costly, in terms of C and N invested, than terrestrial flower and seed production. The combination of continuous, submersed vegetative spread with the capacity for a high degree of phenotypic plasticity allowing some flower and seed production to occur during brief periods of emersion seems to account for the success of this plant in the amphibious niche.

Sixty-two isolates of Pisolithus were obtained from a carpophore population collected from a 90-km2 area of New South Wales, Australia. Random amplified polymorphic DNA (RAPD) analysis was used initially to screen isolates for variation, and restriction fragment length polymorphism (RFLP) analysis of the internal transcribed spacer (ITS) region and ITS sequence analysis were subsequently performed on selected isolates. ITS–RFLP and ITS sequence analysis indicated that two isolates (LJ07 and WM01) from different sites were considerably different from all other isolates, a strict consensus tree generated by bootstrap analysis of sequence data strongly suggesting that isolates LJ07 and WM01 represent a separate Pisolithus species. Although the general morphology of carpophores and basidiospores of the majority of collected isolates was consistent with previous descriptions of P. tinctorius (Pers.) Coker and Couch, basidiospore characteristics suggest that isolates LJO7 and WMO1 might represent a Pisolithus species distinct from P. tinctorius and the previously described Australian species, P. microcarpus (Cooke & Massee) G. Cunn.

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

Following a precultivation with pedospheric nitrogen nutrition, nitrate or ammonium solutions were supplied to the shoots of Ricinus plants by spraying (during the experimental period) resulting in an increase of biotic/organic and abiotic/inorganic particles on the surface, which significantly increased wetting of the leaf surfaces. The distribution of particles on the surface of sprayed leaves, in particular crystals around and in stomata, indicated the possible entry of nutrients via thin water films through the stomatal pores in addition to diffusion through the cuticle. Ammonium was taken up more readily than nitrate by the foliage, but both at relatively low rates which caused N limitation. Interestingly, the inorganic N, both in the form of nitrate and even ammonium, was entirely assimilated in the shoots; phloem transport of inorganic N to the root was negligible. The flows of malate, and the acidification of the apoplastic washing solution of leaves in ammonium-sprayed plants pointed to the role of metabolism of malate and excretion of protons in maintaining pH during ammonium assimilation in the shoot. Ammonium-sprayed plants incorporated the N in the same amounts in shoots and roots, only 38% of the shoot-borne N being recycled in the xylem. In nitrate-sprayed plants the root was not only favoured in N partitioning, but even a net export of previously incorporated N from the shoots occurred which reflected the N limitation. The N limitation also affected carbon metabolism, in particular the flows of C, incorporation in the shoot and photosynthesis, which were decreased when compared with data from recent experiments with pedospheric well fed Ricinus. However, there was little difference in C flows between nitrate and ammonium-sprayed plants with respect to respiration, C partitioning and, most interestingly, in relative stimulation of root growth. The loss of C from dark respiration of the shoots was high on a f. wt basis as well as in relative terms, owing to exclusive N assimilation in the shoot. In general the plants invested untargeted increases in root growth as a result of N limitation irrespective of the imposed artificial treatment which made the shoot the site of mineral N uptake.

It has been suggested that the populations of planktonic cyanobacteria that occupy the metalimnion of stratified lakes during the summer months may be aestivating between the main periods of growth during entrainment in the epilimnion in spring and summer. We determined the vertical distribution of the biomass and daily integral of photosynthesis of the population of Planktothrix (Oscillatoria) rubescens in Lake Zürich for 136 d from July to November 1995. The population showed an 80-fold increase during the stratified period but it only doubled over the subsequent period of entrainment. During the first eight days, part of the increase was attributed to recruitment of filaments floating up from greater depths but all of the subsequent production could be accounted for by photoautotrophic growth. On sunny days the biomass-specific photosynthesis of this population reached some of the highest values over the whole period despite its depth (>13 m). On very cloudy days, however, primary productivity was very low and on 4 days, when the mean depth of the population exceeded 15 m, there was no net production. Over the whole period of the study, the accumulated photosynthetic production exceeded the increase in biomass of the population by a factor of 9·5. Although much of this production occurred during the period of entrainment only a small proportion was translated into growth of the population. It is concluded that the growth that takes place in the period of stratification in the metalimnion is essential to subsequent production.