The papers in this volume form the core of a meeting called Putting Plant Physiology on the Map, which took place at the University of Wales Bangor, in April 1997. The idea for the meeting came from Professor Sid Thomas, and the scientific organization was done by him and his colleagues at the Institute for Grassland and Environmental Research, Aberystwyth. The aim was to convene a small group of physiologists and geneticists who are concerned with the genetic mapping of plant physiological traits, to produce a meeting and published volume which would summarize some of the methods and results of existing mapping programmes, and more importantly to proselytize by laying out for unconverted plant physiologists the merits of taking a genetical approach. Writing as one of the unconverted, the meeting succeeded admirably: the power and utility of a genetical approach became very clear. Not only does mapping provide unequivocal answers to a range of physiological questions, it also enables one to ask questions that simply cannot be addressed by conventional physiological approaches. The progress which has been made in understanding salt tolerance, for example, adds a completely new dimension to the experiments in the classical literature.

Those with experience of putting plant physiological characteristics onto genetical maps inevitably have a language of their own, and for the newcomer the acquisition of this language is a daunting task. Since conversion was one of our aims, we have provided a chapter (Jones, Ougham & Thomas) which explains to the beginner just how maps are made, and in so doing hopefully eases the problem of acquiring the language. Virgin mappers should read this chapter first; the experienced can move straight to the rest and sample the variety of approaches which have been used.

Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable N isotopes, 15N and 14N, in various N pools. In ecosystems such variations (usually expressed in per mil [δ15N] deviations from the standard atmospheric N2) depend on isotopic signatures of inputs and outputs, the input–output balance, N transformations and their specific isotope effects, and compartmentation of N within the system. Products along a sequence of reactions, e.g. the N mineralization–N uptake pathway, should, if fractionation factors were equal for the different reactions, become progressively depleted. However, fractionation factors vary. For example, because nitrification discriminates against 15N in the substrate more than does N mineralization, NH4+ can become isotopically heavier than the organic N from which it is derived.

Levels of isotopic enrichment depend dynamically on the stoichiometry of reactions, as well as on specific abiotic and biotic conditions. Thus, the δ15N of a specific N pool is not a constant, and δ15N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring δ15N in small and dynamic pools of N in the soil–plant system, and the complexity of the N cycle itself (for instance the abundance of reversible reactions), limit the possibilities of making inferences based on observations of 15N abundance in one or a few pools of N in a system. Nevertheless, measurements of δ15N might offer the advantage of giving insights into the N cycle without disturbing the system by adding 15N tracer.

Such attempts require, however, that the complex factors affecting δ15N in plants be taken into account, viz. (i) the source(s) of N (soil, precipitation, NOx, NH3, N2-fixation), (ii) the depth(s) in soil from which N is taken up, (iii) the form(s) of soil-N used (organic N, NH4+, NO3−), (iv) influences of mycorrhizal symbioses and fractionations during and after N uptake by plants, and (v) interactions between these factors and plant phenology. Because of this complexity, data on δ15N can only be used alone when certain requirements are met, e.g. when a clearly discrete N source in terms of amount and isotopic signature is studied. For example, it is recommended that N in non-N2-fixing species should differ more than 5‰ from N derived by N2-fixation, and that several non-N2-fixing references are used, when data on δ15N are used to estimate N2-fixation in poorly described ecosystems.

As well as giving information on N source effects, δ15N can give insights into N cycle rates. For example, high levels of N deposition onto previously N-limited systems leads to increased nitrification, which produces 15N-enriched NH4+ and 15N-depleted NO3−. As many forest plants prefer NH4+ they become enriched in 15N in such circumstances. This change in plant δ15N will subsequently also occur in the soil surface horizon after litter-fall, and might be a useful indicator of N saturation, especially since there is usually an increase in δ15N with depth in soils of N-limited forests.

Generally, interpretation of 15N measurements requires additional independent data and modelling, and benefits from a controlled experimental setting. Modelling will be greatly assisted by the development of methods to measure the δ15N of small dynamic pools of N in soils. Direct comparisons with parallel low tracer level 15N studies will be necessary to further develop the interpretation of variations in δ15N in soil–plant systems. Another promising approach is to study ratios of 15N[ratio ]14N together with other pairs of stable isotopes, e.g. 13C[ratio ]12C or 18O[ratio ]16O, in the same ion or molecules. This approach can help to tackle the challenge of distinguishing isotopic source effects from fractionations within the system studied.

Summary 373

I. Introduction: Arum-types and Paris-types 374

II. Possible functional implications 375

III. Extent of the two classes in the plant kingdom 377

1. Bryophytes and Pteridophytes 377

2. Gymnosperms 379

3. Angiosperms 379

IV. Is the distinction between classes useful? 383

V. The structural basis 383

VI. The role of the fungal genome 384

VII. Physiology revisited 384

VIII. Conclusions 385

Acknowledgements 386

References 386

This review describes diversity in the structure of (vesicular)–arbuscular (VA) mycorrhizas, i.e. endomycorrhizas formed by Glomalean fungi. In particular, we consider the extent in the plant kingdom of the two classes first described by Gallaud (1905). These are: (1) the Arum-type, defined on the basis of an extensive intercellular phase of hyphal growth in the root cortex and development of terminal arbuscules on intracellular hyphal branches; (2) the Paris-type, defined by the absence of the intercellular phase and presence of extensive intracellular hyphal coils. Arbuscules are intercalary structures on the coils. However, there have been many reports that in Paris-types arbuscules are relatively few in numbers, small, or absent altogether.

A survey of the literature has revealed that Paris-types occur more frequently in the plant kingdom than Arum-types and predominate in ferns, gymnosperms and many wild angiosperms. The cultivated herbs that are the subject of much experimental work are mostly Arum-types. Although evidence is still limited, there are differences at the family level. In 41 angiosperm families there are records of only Paris-type VA mycorrhizas and in 30 families records of only Arum-types. Another 21 families have examples of both classes, or intermediates between them. Accordingly, we consider whether the original division into two classes is still useful. We conclude that it is when considering the physiology of the symbiosis and especially the issue of whether different fungus/host interfaces have specialized roles in transfer of inorganic nutrients and organic carbon between the partners. If there is no such specialization between hyphal coils and arbuscules, then the latter might not be necessary for the function of Paris-types. This would account for reports of the infrequency or absence of arbuscules in this class. The control exerted on structures by the genomes of host and fungus, and possible reasons (anatomical and physiological) for the existence of the VA mycorrhizal structures, are discussed. The presence or absence of extensive intercellular spaces and differences in the wall structure of cortical cells might be particularly important in determining which type of VA mycorrhiza is formed.

Polyamines (PAs) and the cell wall have important roles in plant morphogenesis. An interaction between cell wall components (pectic substances) and polyamines is known and inhibition of polyamine biosynthesis induces inhibition of some morphogenic processes. This is the case of rhizogenesis in tobacco (Nicotiana tabacum L.) thin layers. This paper investigates whether inhibition of polyamine biosynthesis induces modifications in the structure, shape and size of the primary cell walls in tobacco thin layers cultured on a rhizogenic medium, and whether polyamines (putrescine and spermidine) administered in combination with their corresponding inhibitors DL-α-difluoromethylornithine +DL-α-difluoromethylarginine (DFMO+DFMA), cyclohexylamine (CHA), methylglyoxal-bis(guanylhydrazone) (MGBG) restore cell wall characteristics. In treatments with polyamine inhibitors, cell hypertrophy occurred in the lowermost layer of the explant in contact with the medium. Many unusual features were observed in the walls of the hypertrophic cells: an amorphous structure, loosening of the fibrillar component; detachment between contiguous cells, lysis of wall components and an uneven thickness often giving a wavy appearance (in transverse walls). The inhibitors reduced cell wall thickness, and caused a weak positive PATAg reaction for polysaccharides. Polyamines restored cell wall thickness and, in general, the other wall features. Our results show that polyamines are necessary for maintaining cell wall characteristics by strengthening the links between cell-wall components.

We have used a pulsed u.v. laser to ablate the cell wall of the filamentous fungus Aspergillus niger van Tiegh., and expose the plasma membrane at defined points along the hypha. Using the patch clamp technique, giga ohm seals (up to 20 GΩ) and recordings of plasma membrane ion channels were obtained. We describe, for the first time in fungi, a 43 pS plasma membrane anion efflux channel. Its potential role in cell signalling and pH homeostasis is discussed.

The use of genetics in plant biology aims at the physiological and molecular genetical characterization of the phenotypic variation for the trait under study. Efficient mutant and gene isolation procedures have been developed for a number of plant models such as Arabidopsis thaliana. For this, the map position of the genes and insertion mutagenesis are used. The latter also allows the characterization of genes that are not easily recognized in mutant approaches, by using enhancer or gene-trapping procedures and reverse genetics. In addition to mutants, natural variation present among wild and cultivated varieties of a species provides an important source of genetic variation. The use of molecular markers, advanced mapping populations and specific cytogenetic stocks in case of polyploids, enables a detailed characterization of such natural variation even when it is of a quantitative and polygenic nature. Examples of the various genetic approaches are given.

This work explored whether the natural abundances of carbon and nitrogen isotopes could be used to describe the movement of C and N within wheat plants; we also considered whether isotopic analyses of aphids or their honeydew would substitute for direct analysis of phloem exudate.

The δ13C of ears and roots (sinks) most closely matched those of the sugars+organic acids fraction (sources) in both growth stages; phloem δ13C matched that of leaf blade sugars. Xylem exudate δ13C matched no other putative (and measured) source in the ear-forming stage and matched that of whole roots and ears in the grain-filling stage. The δ15N of grain and roots (sinks) resembled that of leaf amino acids (sources) in the ear-forming stage. In the grain-filling stage, ear δ15N continued to resemble that of leaf amino acids, and δ15N of roots most closely resembled that of whole leaves. In the grain-filling stage, phloem δ15N fell between that of leaf blade amino acids and that of whole leaves and was 15N-depleted relative to internal and external NO3−-N. In both growth stages, xylem exudate δ15N was less than that of soil NO3−-N and and more than that of residual soil N after mineral N extraction. The isotopic values are generally in agreement with data from other approaches, such as isotope labelling; they show NO3−-N reduction in both shoots and roots of wheat and significant N recycling (root-shoot-phloem-root) and C movement.

Aphids might serve as a substitute for isotopic analysis of phloem δ15N, having the same value as their food source. Their excreta was 15N-enriched relative to phloem.

Cloned cuttings of Betula pendula Roth were grown in field fumigation chambers at Birmensdorf throughout one growing season in filtered air with either <3 (control) or 90/40 nl l−1 O3 (day/night; ozone generated from pure oxygen). Each ozone regime was split into high and low soil nutrient regimes by watering plants with either a 0·05% or a 0·005% solution of a fertilizer which contained macronutrients and micronutrients.

Fertilization had a strong effect on plant growth, enzyme activities and the expression of ozone-induced effects at the biochemical level. The activities of PEPC and Rubisco were enhanced about threefold in the plants with high fertilization (HF). Significant effects of ozone were in most cases found only in the older leaves of the plants with low fertilization (LF). There, sucrose, glucose and fructose levels were enhanced. In both fertilization treatments, the number of starch granules along the minor veins was increased. These ozone effects point to a decreased or inhibited phloem loading. The increased PEPC activity and the enhanced malate levels in the ozone-exposed plants might be the result of a redirection of carbon flow from sucrose synthesis and translocation towards anapleurotic processes, which can feed detoxification and repair of ozone injury as indicated by enhanced respiration. These findings agree well with the observed effects of ozone in lowering the root[ratio ]shoot biomass ratio. Although there was a marked reduction in the O3/LF plants, O3/HF plants showed no significant response. Inositol was decreased under ozone exposure in both fertilizer treatments, contrasting with the pattern for carbohydrates.

These results demonstrate the role of fertilization as an important modifier of ozone-induced effects at the plant biochemical level. Well fertilized plants appear to cope better with the impact of ozone on metabolism.

Two parameters were applied to estimate the degree of stress induced by exposure of the lichen Ramalina duriaei (De Not.) Bagl. to air pollution in urban-industrial sites in Israel. Changes in the spectral reflectance response of the thallus and in the production of stress-ethylene, were compared with the concentration of sulphate-S, V, Ni, K, Pb, P, Cu, Cr, Fe, Mn and Mg in either in situ or in transplanted thalli of the lichen.

The concentrations of Cu, Mg, Fe, Ni, Pb, V, Cr, Mn and sulphate-S in in situ thalli collected in a nature reserve in an industrial town (Ashdod, SW Israel) were higher than those detected in in situ thalli collected in a forest (HaZorea Forest) in the north-east of Israel. The high concentrations of these mineral elements in R. duriaei from the nature reserve coincided with a high rate of production of stress-ethylene and with a low normalized difference vegetation index (NDVI) indicating that changes in the spectral reflectance response relate to damage to the chlorophyll.

Lichens collected in the unpolluted forest in February 1994, either resuspended in the same site or transplanted in nine sites in the Ashdod region, and retrieved in November of the same year, contained a higher concentration of sulphate-S, Pb, Cu, Ni, V and Cr in the urban-industrial sites than in most of the rural localities. Statistical analysis revealed a positive correlation between the amount of ethylene produced by R. duriaei and the concentration of V and Ni in the same thalli.

To test the integrity of the ethylene-producing system, lichens were soaked in either 5 mM or 10 mM FeCl2 at pH 3·7. Thalli exposed to a very polluted environment produced less ethylene after this treatment. The NDVI correlated positively with the concentration of K in the thallus but negatively with the concentration of Mn, Ni, Pb and sulphate-S in the same material.

It may be concluded that the in situ thalli of R. duriaei in the nature reserve are severely stressed and possibly endangered by the pollution produced by the combustion of heavy fuel oil in the local power plant and oil refineries and by acid rain.

Developing caryopses of barley (Hordeum vulgare L. cv. Golf) were monitored for starch-branching enzyme activity for 1 month after anthesis. Homogenized caryopses from grains with the highest specific activity were used as starting material for purification of branching enzyme by FPLC chromatography. Several branching enzyme activity fractions were resolved. We have previously described the isolation of a 51/50 kDa protein with starch branching activity from one of these fractions. From three other fractions, we have now partially purified starch branching enzymes corresponding to forms I, IIa and IIb.

Microseris douglasii (DC.) Sch.-Bip. and M. bigelovii (Gray) Sch.-Bip. are two small annual autogamous species of Compositae with nearly non-overlapping distribution ranges in Western North America. Specifically, M. bigelovii occurs directly along the Pacific coast, whilst M. douglasii has an inland distribution including patches of serpentine soil. Both species are variable, and artificial hybrids between them vary widely in fertility depending on the individual parents. Segregating offspring of one hybrid (strain H27) is being used to analyse the genetic basis of characters differentiating the two species by QTL mapping with RAPDS and ALFPs as molecular markers. Technical problems with mapping dominant markers in a wide cross will be briefly listed and QTL analysis will be discussed. For the genetic analysis of physiological characters, the precise definition of the characters is crucial and the methods of scoring or measuring phenotypes in different environments eventually require more time and effort than the molecular characterization. We are establishing recombinant inbred lines to provide material for more complex physiological analyses requiring several plants per genotype. An increasing number of characters is being studied in this cross, and the possibility of shared pleiotropic QTLs is high. The potential number of QTL genotypes by far exceeds the number of actual genotypes in these lines. We are characterizing the gene interactions as closely as possible and making quantitative genetic models to predict the genotypes corresponding to all possible genotypes. These predictions are converted via computer modelling into an increasingly realistic three-dimensional representation of the growing plant useful for a simulation of plant evolution.

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

The occurrence of crassulacean acid metabolism (CAM) among the epiphyte flora of the lowland forest on Barro Colorado Island (BCI), Panama, was investigated. A total of 116 species was included, i.e. about 2/3 of the known epiphyte taxa. As judged from the carbon isotope ratios and the absence of Kranz anatomy, indications of CAM were found in 29 species of three families, Orchidaceae (20), Bromeliaceae (7), and Cactaceae (2). We estimate that about 25% of the epiphyte flora of BCI are CAM plants. CAM was most prevalent in exposed sites, but even in the understorey two epiphyte species engage in CAM.

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.

The present understanding of the genetic control of flowering in wheat is reviewed. Although this information will continue to be of value in breeding and in recognizing the role such genes have had in adapting the wheat crop to global agriculture, its use in defining the processes responsible for flowering will be limited. Progress here will depend on developments in model plant systems such as Arabidopsis, where methods for isolating genes are much more advanced than in wheat. These developments, which have already started, will probably be used to establish homologies between the genes for flowering within the cereals, as well as more widely. An understanding of the flowering process in wheat is likely to emerge from this approach. In the meantime, there are some unknowns in the genetics of flowering in wheat which need to be resolved. These include the identification of a gene(s) on the group 4 chromosomes of wheat which is homoeoallelic with the gene Sh in barley. Also, the proposed gene(s) delaying flowering and located on the group 6 chromosomes needs to be recognized and mapped. Similar needs occur for the group 1 chromosomes as well as the resolution of whether or not Vrn5 is correctly positioned on chromosome 7BS.

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

A mutation of a gene (discovered in Festuca pratensis Huds. and designated sid ) confers indefinite greenness on senescing leaves. Via intergeneric hybrids with Lolium multiflorum L. and Lolium perenne L., the mutant gene (sidy) has been introgressed into a range of Lolium backgrounds. Using genomic in situ hybridization we have identified segments carrying sidy in recombinant chromosomes of Lolium–Festuca introgression lines. We also used L. perenne lines segregating 1[ratio ]1 for the staygreen character to tag the gene with molecular markers. In two mapping populations a total of 84 genotypes were screened with isoenzymes, RAPD primers, RFLP probes and AFLP primer pairs. Over 180 polymorphic loci were identified, representing 10 linkage groups spanning 600 cM. Two AFLP markers are linked to sid at 4·6 and 14·9 cM, close enough to be usable for marker-assisted selection. Introgression of sidy into Lolium temulentum L. resulted in the production of near-isogenic inbreeding lines suitable for comparative studies of gene expression. Using a variation of the method of representational difference analysis a very small number of cDNAs have been identified as promising candidates for sid, or genes directly regulated by sid.

The aim of this study was to quantify the long-term effects of Al in solution on mineral ion uptake and associated H+ release by the roots of maize cultivated in conditions compatible with field data. Two maize (Zea mays L.) cvs of differing sensitivity to Al were compared. The culture was conducted in hydroponic conditions for culture periods of 2–5 d in the presence of 0–30 μM Al. Proton fluxes were localized along the roots using a videodensitometry method with bromocresol green on agarose gel.

Aluminium reduced the uptake of anions, particularly NO3−, whereas the uptake of K+ and NH4+ was unaffected. These effects were accompanied by an increase in net H+ release which was quantitatively comparable with the reduction in NO3− uptake. The reduction in NO3− uptake and the concomitant increase in H+ release increased with the quantity of Al accumulated in the roots, which was itself dependent on the concentration of Al in solution. The Al-sensitive cultivar bound more Al to its roots than the Al-tolerant one; the reduction in NO3− uptake and the increase in H+ release were also greater in the Al-sensitive cv. Dye indicator videodensitometry showed that the increased H+ release was general all along the roots.

These results led us to specify the relation between the Al deposition in roots, the reduction in NO3− uptake and the stimulation in rhizosphere acidification by the plants. They show that, with maize, Al induces an inhibition in NO3− uptake and an increase in net H+ release in a 1[ratio ]1 ratio. These concomitant effects suggest that inhibition in NO3−/OH− exchange unmasks active H+ excretion which is otherwise insensitive to Al. Both these effects are closely related to the Al content in roots expressed per unit root length.

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.

Three perennial grass species, Lolium perenne L., Agrostis capillaris L. and Festuca ovina L., were homogeneously labelled in phytotrons with 14CO2 at two CO2 concentrations (350 and 700 μl l−1). Plants were grown under two nitrogen regimes: one with a minor addition of 8 kg N ha−1, the other with an addition of 278 kg N ha−1. Carbon allocation over the different compartments of the plant/soil systems was measured: shoots, roots, rhizosphere soil (soil solution, microbial biomass and soil residue), and bulk soil. Elevated CO2 increased total net 14C recovery in all species by 14%, and significantly enhanced the below-ground 14C allocation by 26%, this enhancement was 24%, 39% and 21% for root, rhizosphere soil and bulk soil, respectively. Within the rhizosphere soil, the 14C amounts in the soil solution (+69%) and soil residue (+49%) increased significantly. Total microbial biomass-C in the rhizosphere soil was also increased (15%) by the elevated CO2 treatment, but only in proportion to the increased root mass. No interactions were observed between the elevated CO2 and N treatments. The N treatment increased total net 14C recovery by more than 300% and 14C was preferentially allocated to the shoots, leading to a significant increase in shoot-to-root ratio. However, N fertilization also increased (+111%) the absolute amount of 14C in soil. The three species behaved differently, but no interactions were observed between CO2 treatment and plant species. These results show that elevated CO2 induces an increased C input into soil for all three grass species at both N levels. However, the highest absolute amounts were found in the soils of the fastest growing species and at the highest N level.