The objectives of this paper were to review the literature on the responses of root systems to elevated CO2 in intact, native grassland ecosystems, and to present the results from a 2-yr study of root production and mortality in an intact calcareous grassland in Switzerland. Previous work in intact native grassland systems has revealed that significant stimulation of the size of root systems (biomass, length density or root number) is not a universal response to elevated CO2. Of the 12 studies reviewed, seven showed little or no change in root-system size under elevated CO2, while five showed marked increases (average increase 38%). Insufficient data are available on the effects of elevated CO2 on root production, mortality and life span to allow generalization about effects. The diversity of experimental techniques employed in these native grassland studies also makes generalization difficult. In the present study, root production and mortality were monitored in situ in a species-rich calcareous grassland community using minirhizotrons in order to test the hypothesis that an increase in these two measures would help explain the increase in net ecosystem CO2 uptake (net ecosystem exchange) previously observed under elevated CO2 at this site (600 vs 350 μl CO2 l−1; eight 1.2-m2 experimental plots per CO2 level using the screen-aided CO2 control method). However, results from the first 2 yr showed no difference in overall root production or mortality in the top 18 cm of soil, where 80–90% of the roots occur. Elevated CO2 was associated with an upward shift in root length density: under elevated CO2 a greater proportion of roots were found in the upper 0–6-cm soil layer, and a lower proportion of roots in the lower 12–18 cm, than under ambient CO2. Elevated CO2 was also associated with an increase in root survival probability (RSP; e.g. for roots still alive 280 d after they were produced under ambient CO2, RSP = 0.30; elevated CO2, RSP = 0.56) and an increase (48%) in median root life span in the deepest (12–18 cm) soil layer. The factors driving changes in root distribution and longevity with depth under elevated CO2 were not clear, but might have been related to increases in soil moisture under elevated CO2 interacting with vertical patterns in soil temperatures. Thus extra CO2 taken up in this grassland ecosystem during the growing season under elevated CO2 could not be explained by changes in root production and mortality. However, C and nutrient cycling might be shifted closer to the soil surface, which could potentially have a substantial effect on the activities of soil heterotrophic organisms as CO2 levels rise.

Uptake, translocation and metabolism of 14C-labelled formaldehyde in the leaves of Epipremnum aureum (Golden Potho) and Ficus benjamina (Weeping Fig) were investigated. Plants were exposed in light and dark to 14C-formaldehyde (500 μg m−3) in gas exposure chambers. The amount of 14C-incorporation into the soluble (water-extractable) and insoluble fractions of leaves, stem sections and roots was determined. The soluble 14C-activity was fractionated by ion exchange chromatography followed by thin-layer chromatography/autoradiography. Approximately 60–70% of the applied 14C-formaldehyde was recovered from the plants. In the light about five times more 14C-formaldehyde was assimilated than in the dark. The amount of 14C-label derived from 14C-formaldehyde, which was incorporated into acid-stable metabolites, was enhanced to an even larger extent in the light. The 14C-activity pattern closely resembled the general labelling spectrum of photosynthates, obtained after a 14CO2 exposure. A substantial amount of labelled material, mostly sucrose, was translocated into the stems and roots. Our results suggest that in the light 14C enters the Calvin cycle after an enzymatic two-step oxidation process of 14C-formaldehyde to 14CO2. The activities of the respective enzymes, formaldehyde dehydrogenase and formate dehydrogenase, were determined. Among 27 ‘leafy’ indoor decorative plants, a screening experiment revealed no outstanding species with regard to its capacity for metabolism of formaldehyde, and rate of uptake through stomata was too low to justify claims that plants contribute usefully to indoor air purification.

In a study on the mechanism of stimulated petiole elongation in submerged plants, oxygen concentrations in petioles of the flood-tolerant plant Rumex palustris were measured with micro-electrodes. Short-term submergence lowered petiole partial oxygen pressure to c. 19 kPa whereas prolonged submergence under continuous illumination depressed oxygen levels to c. 8–12 kPa after 24 h. Oxygen levels in petioles depended on the presence of the lamina, even in submerged conditions, and on available light. In darkness, petiole oxygen levels in submerged plants dropped quickly to values as low as 0.5–4 kPa. It is hypothesized that prolonged submergence in the light is accompanied by a decrease in carbon dioxide in the petiole. Submergence-enhanced petiolar elongation rate was compared with emergent plants. Peak daily elongation rates occurred at the end of the dark period in emergent plants, but in the middle of the light period in submerged plants. We suggest that this shift in daily elongation pattern is induced by dependence of growth on photosynthetically derived oxygen in submerged plants. Implications of reduced oxygen for ethylene production are raised. Levels of 1- aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase and ethylene sensitivity are cited as potential factors in hypoxia-induced ethylene release.

Elevated CO2 increases root growth and fine (diam. [les ]2 mm) root growth across a range of species and experimental conditions. However, there is no clear evidence that elevated CO2 changes the proportion of C allocated to root biomass, measured as either the root[ratio ]shoot ratio or the fine root[ratio ]needle ratio. Elevated CO2 tends to increase mycorrhizal infection, colonization and the amount of extramatrical hyphae, supporting their key role in aiding the plant to more intensively exploit soil resources, providing a route for increased C sequestration. Only two studies have determined the effects of elevated CO2 on conifer fine-root life span, and there is no clear trend. Elevated CO2 increases the absolute fine-root turnover rates; however, the standing crop root biomass is also greater, and the effect of elevated CO2 on relative turnover rates (turnover[ratio ]biomass) ranges from an increase to a decrease. At the ecosystem level these changes could lead to increased C storage in roots. Increased fine-root production coupled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Although CO2 can stimulate fine-root growth, it is not known if this stimulation persists over time. Modeling studies suggest that a doubling of the atmospheric CO2 concentration initially increases biomass, but this stimulation declines with the response to elevated CO2 because increases in assimilation are not matched by increases in nutrient supply.

Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NOx emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope 15N/14N content (δ15N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ15N. Plants collected near motorways or busy urban roads had δ15N values ranging between +6 and −1‰, while in rural areas with hardly any traffic these ranged from −2 to −12‰. In a separate survey of mosses, the average δ15N of shoots from busy roadsides in London was +3.66‰, whereas from samples collected from farm buildings near poultry or cattle pens it was −7.8‰. This indicates that the two main atmospheric N sources, NOx and NHx, have different δ15N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ15N as an aid to distinguish between urban (NOx) and rural (NHx) forms of N pollution.

Three MADS-box cDNA clones and two corresponding genomic sequences (gDNAs) have been isolated from the bryophyte Physcomitrella patens and sequenced. Our findings indicate that the genes may be expressed in a tissue- or age-specific manner, and that expression of one of them is regulated by an alternative splicing mechanism. Conceptual translation of the clones reveals that the encoded MADS-domain proteins have the typical plant-domain pattern (MIKC). Additionally, there is a high degree of conservation of intron number and positions between angiosperm MADS-box genes and the moss loci. These observations confirm the homology of moss and higher plant MADS-box genes. We conclude that the MIKC pattern evolved in MADS-box genes after the separation of the plant lineage from that of fungi and animals, and that it must have been present in the common ancestor of mosses, ferns and seed plants. Therefore it evolved at least 400 million yr ago. Phylogenetic analysis of a large subset of the sequenced plant MADS-box genes, incorporating those from P. patens, indicates that the bryophyte genes are not orthologues of spermatophyte genes belonging to any of the well recognized higher plant gene subfamilies. This conclusion accords well with reports that the known fern MADS-box genes also comprise subfamilies distinct from those of higher plants. Therefore we tentatively propose that the gene duplication and diversification events that created the MADS-box gene subfamilies, discernible in extant angiosperm and other spermatophyte groups, occurred after separation of the moss and fern lineages from the lineage which produced the higher plants.

Soil temperature can influence the functioning of roots in many ways. If soil moisture and nutrient availability are adequate, rates of root length extension and root mortality increase with increasing soil temperature, at least up to an optimal temperature for root growth, which seems to vary among taxa. Root growth and root mortality are highly seasonal in perennial plants, with a flush of growth in spring and significant mortality in the fall. At present we do not understand whether root growth phenology responds to the same temperature cues that are known to control shoot growth. We also do not understand whether the flush of root growth in the spring depends on the utilization of stored nonstructural carbohydrates, or if it is fueled by current photosynthate. Root respiration increases exponentially with temperature, but Q10 values range widely from c. 1.5 to > 3.0. Significant questions yet to be resolved are: whether rates of root respiration acclimate to soil temperature, and what mechanisms control acclimation if it occurs. Limited data suggest that fine roots depend heavily on the import of new carbon (C) from the canopy during the growing season. We hypothesize that root growth and root respiration are tightly linked to whole-canopy assimilation through complex source–sink relationships within the plant. Our understanding of how the whole plant responds to dynamic changes in soil temperature, moisture and nutrient availability is poor, even though it is well known that multiple growth-limiting resources change simultaneously through time during a typical growing season. We review the interactions between soil temperature and other growth-limiting factors to illustrate how simple generalizations about temperature and root functioning can be misleading.

Relationships between nitrogen (N) content and growth are routinely measured in plants. This study determined the effects of N on the separate morphological and physiological components of plant growth, to assess how N-limited growth is effected through these components. Lettuce (Lactuca sativa) plants were grown hydroponically under contrasting N-supply regimes, with the external N supply either maintained continuously throughout the period of study, or withdrawn for up to 14 d. Richards' growth functions, selected using an objective curve-fitting technique, accounted for 99.0 and 99.1% of the variation in plant dry weight for control and N-limited plants respectively. Sublinear relationships occurred between N and relative growth rates under restricted N-supply conditions, consistent with previous observations. There were effects of treatment on morphological and physiological components of growth. Leaf weight ratio increased over time in control plants and decreased in N- limited plants. Shoot:root ratio followed a similar pattern. On a whole-plant basis, assimilation of carbon decreased in N-limited plants, a response paralleled by differences in stomatal conductance between treatments. Changes in C assimilation, expressed as a function of stomatal conductance to water vapour, suggest that the effects of N limitation on growth did not result directly from a lack of photosynthetic enzymes. Relationships between plant N content and components of growth will depend on the availability of different N pools for remobilization and use within the plant.

How ecosystems adapt to climate changes depends in part on how individual trees allocate resources to their components. A review of research using tree seedlings provides some support for the hypothesis that some tree species respond to exposure to drought with increases in root[ratio ]shoot ratios but little change in total root biomass. Limited research on mature trees over moderately long time periods (2–10 yr), has given mixed results with some studies also providing evidence for increases in root: shoot ratios. The Throughfall Displacement Experiment (TDE) was designed to simulate both an increase and a decrease of 33% in water inputs to a mature deciduous forest over a number of years. Belowground research on TDE was designed to examine four hypothesized responses to long-term decreases in water availability; (1) increases in fine-root biomass, (2) increases in fine root[ratio ]foliage ratio, (3) altered rates of fine-root turnover (FRT), and (4) depth of rooting. Minirhizotron root elongation data from 1994 to 1998 were examined to evaluate the first three hypotheses. Differences across treatments in net fine-root production (using minirhizotron root elongation observations as indices of biomass production) were small and not significant. Periods of lower root production in the dry treatment were compensated for by higher growth during favorable periods. Although not statistically significant, both the highest production (20 to 60% higher) and mortality (18 to 34% higher) rates were found in the wet treatment, resulting in the highest index of FRT. After 5 yr, a clear picture of stand fine-root-system response to drought exposure has yet to emerge in this forest ecosystem. Our results provide little support for either an increase in net fine-root production or a shift towards an increasing root[ratio ]shoot ratio with long-term drought exposure. One possible explanation for higher FRT rates in the wet treatment could be a positive relationship between FRT and nitrogen and other nutrient availability, as treatments have apparently resulted in increased immobilization of nutrients in the forest floor litter under drier conditions. Such hypotheses point to the continued need to study the interactions of water stress, nutrient availability and carbon-fixation efficiency in future long-term studies.

Ozone effects on plant species mixtures could depend on the characteristics of the species involved, their mixing ratio, or on environmental conditions. Predicting long-term effects on the dynamics of plant communities requires an understanding of the interactions involved. The present experiment was designed to determine the effects of ozone on grassland species in relation to mixing ratio and soil water content (irrigation) using binary mixtures. The grass Trisetum flavescens was grown in potted replacement-series mixtures with Centaurea jacea (Experiment A) or Trifolium pratense (Experiment B). The plants were exposed to three concentrations of ozone in open-top chambers in two irrigation treatments. Total above-ground dry weight over three growth periods was measured. The competitive ability of T. flavescens was expressed as the competitive ratio (CRT). In Experiment B, total above-ground dry weight was reduced by elevated ozone and by reduced soil moisture, and significant interactions were found for ozone × irrigation and ozone × ratio. In Experiment A these effects were not significant. Under well watered conditions, CRT tended to be reduced by elevated ozone in Experiment A, but increased significantly in Experiment B, indicating the importance of the competing species in modifying the ozone effect on T. flavescens. In both experiments reduced irrigation decreased the magnitude of ozone effects on biomass production, which could be related to observed reductions in specific leaf conductance. The results suggest that under well watered conditions the effect of elevated ozone on the competitive balance between species depends on the species mixture, but that the mixing ratio is less important.

Temperate forests are recipients of anthropogenic nitrogen (N) deposition. Because growth in these ecosystems is often limited by N availability, elevated N inputs from the atmosphere can influence above- and belowground production in forests. Although fine-root production is the largest component of belowground production in forests, it is unclear whether or how increases in Navailability to forest trees accompanying increased N deposition might influence fine-root growth. Uncertainties as to how fine-root dynamics (i.e. production and turnover) vary in relation to soil N availability contribute to this problem. Although fine-root biomass typically decreases along soil N availability gradients in forests, it is unclear whether fine-root production and turnover also decrease along these gradients. Here, four possible relationships between fine-root turnover, fine-root production, and forest soil N availability are evaluated to develop a general hypothesis about changes in rooting dynamics that might accompany increases in N deposition. The four possible relationships are as follows. (1) Fine-root turnover rates do not systematically change with N availability in forest soils. If this is true, then fine-root production rates decrease with fine-root biomass in relation to soil N availability, and increased N deposition could lead to decreased fine-root production in forests. (2) Decreases in photosynthate allocation belowground along N availability gradients will function to slow fine-root turnover (or increase life span) as N availability increases with N deposition, thereby dramatically decreasing fine-root production. (3) Fine-root production might increase with N availability even though fine-root biomass typically decreases with N availability. This could occur if fine-root metabolism and turnover increase (life span decreases) with soil N supply. Increases in fine-root production accompanying increases in N availability, if large enough, could result in constant proportions of forest production being allocated to fine roots as soil N availability increases with N deposition. (4) Although fine-root turnover and production might both increase as N becomes more available to tree roots, the proportional allocation of total primary production to fine roots could decrease. Identifying the most likely of these four possibilities requires intersite comparisons of forest root dynamics along gradients of soil N availability and N deposition. Collective results of studies that use sequential sampling of fine-root biomass to estimate production suggest that fine-root turnover and production either; do not vary systematically, or that they decrease as N availability increases. By contrast, studies using ecosystem C or N budgets suggest that fine-root turnover and production both increase with N availability and that similar increases might be expected with elevated N deposition. It is argued here that assumptions underlying most biomass-based estimates of fine-root production are more suspect than are assumptions underlying element budget-based estimates. If so, it is likely that N deposition will function to decrease forest fine-root biomass but to stimulate fine-root turnover and production. However, increases in fine- root turnover and production could eventually decrease if chronically elevated N deposition leads to forest stand mortality.

Availability of nitrogen (N) and phosphorus (P) might significantly affect N2 fixation in legumes. The interaction of N and P was studied in common bean (Phaseolus vulgaris), considering their effects on nodulation and N2 fixation, nitrate reductase activity, and the composition of N compounds in xylem sap. The effect of N on the uptake of P by plants was estimated by analysing rhizospheric pH and P concentration in xylem sap and in plant shoots. Inoculated bean plants were grown in pots containing perlite/vermiculite in two experiments with different amounts of P and N. In a third experiment, bean plants were grown on two soil types or on river sand supplied with different concentrations of N. At harvest, shoot growth, number of nodules and mass, and nitrogenase activity were determined. Xylem sap was collected for the determination of ureides, amino acids, nitrate and phosphate concentration. At low nitrate concentration (1 mM), increasing amounts of P promoted both nodule formation and N2 fixation, measured as ureide content in the xylem sap. However, at high nitrate concentration (10 mM), nodulation and N2 fixation did not improve with increased P supply. Glutamine and aspartate were the main organic N compounds transported in the xylem sap of plants grown in low nitrate, whereas asparagine was the dominant N compound in xylem sap from plants grown in high nitrate. Nitrate reductase activity in roots was higher than in shoots of plants grown with low P and high N. In both soils and in the sand experiment, increased application of N decreased nodule mass and number, nitrogenase activity and xylem ureides but increased the concentration of asparagine in xylem sap. Increasing P nutrition improved symbiotic N2 fixation in bean only at low N concentrations. It did not alleviate the inhibitory effect of high nitrate concentration on N2 fixation. A decrease in plant P uptake was observed, as indicated by a lower concentration of P in the xylem sap and shoots, correlating with the amount of N supplied. Simultaneously with the specific inhibition of N2 fixation, high nitrate concentrations might decrease P availability, thus inhibiting even further the symbiotic association because of the high P requirement for nodulation and N2 fixation.

The effects of increased nitrogen influx on Sphagnum growth and on interspecific competition between Sphagnum species were studied in a 3-yr experiment in mires situated in two areas with different rates of airborne N deposition. Sphagnum growth was recorded after various supplementary N influxes (0, 1, 3, 5 and 10 g m −2 yr−1) in hummocks and lawn communities. Sphagnum biomass production decreased with increasing N influx in both areas. After the first season at the low-deposition site, Sphagnum showed an increased growth in length with the intermediate N treatment, but in the second and third seasons the control treatment had the highest growth in length. Capitulum dry mass increased with increasing N influx. Sphagnum N concentration and N/P quotient were higher at the high- than at the low-deposition site. The low quotient at the low-deposition site, together with the initial growth increase with intermediate N supplements, indicates that growth was N-limited at this site, but our lowest N supplement was sufficient to reduce growth. The N treatments had no effect on interspecific competition between the Sphagnum species. This indicates that the species have similar responses to N. The species studied all occur naturally on ombrotrophic, N-poor sites and show low tolerances to increased N influx. Reduced Sphagnum production may affect the carbon balance, changing the mires from C sinks to sources.

Global warming over the next century is likely to be associated with a change in the extent to which atmospheric and soil temperatures fluctuate, on both a daily and a seasonal basis. The average annual temperature of the Earth's surface is expected to increase, as is the frequency of hot days. In this review, we explore what effects short-term and long-term changes in temperature are likely to have on root respiratory metabolism, and what impacts such changes will have on daily, seasonal and annual CO2 release by roots under field conditions. We demonstrate that Q10 values, and the degree of acclimation, differ between and within plant species. Changes in the temperature sensitivity of respiration with measuring temperature are highlighted. Temperature-dependent changes in adenylate control and substrate supply are likely to control the Q10 and degree of acclimation of root respiration. Limitations in respiration capacity are unlikely to control respiratory flux at most temperatures. The potential role of nonphosphorylating pathways such as the alternative oxidase in controlling Q10 values is highlighted. The possibility that potentially rapid changes in adenylate control might underlie the acclimation response (rather than slow changes in enzyme capacity) has implications for the total amount of CO2 respired by roots daily and annually. Our modelling suggests that rapid acclimation will result in near-perfect homeostasis of respiration rates and minimize annual CO2 release. However, annual CO2 release increases substantially if the speed of full acclimation is lower. Our modelling exercise also shows that high Q10 values have the potential to increase daily and annual CO2 release substantially, particularly if the frequency of hot days increases after global warming.

The dynamic-chamber technique was used to investigate the correlation between NH3 and NO2 fluxes and different climatic and physiological parameters: air temperature; relative air humidity; photosynthetic photon fluence rate; NH3 and NO2 concentrations; transpiration rate; leaf conductance for water vapour; and photosynthetic activity. The experiments were performed with twigs from the sun crown of mature beech trees (Fagus sylvatica) at a field site (Höglwald, Germany), and with 12-wk-old beech seedlings under controlled conditions. Both sets of experiments showed that NO2 and NH3 fluxes depended linearly on NO2 and NH3 concentration, respectively, in the concentration ranges representative for the field site studied, and on water-vapour conductance as a measure for stomatal aperture. The NO2 compensation point determined in the field studies (the atmospheric NO2 concentration with no net NO2 flux) was 1.8–1.9 nmol mol−1. The NH3 compensation point varied between 3.3 and 3.5 nmol mol−1 in the field experiments, and was 3.0 nmol mol−1 in the experiments under controlled conditions. The climatic factors T and PPFR were found to influence both NO2 and NH3 fluxes indirectly, by changing stomatal conductance. Whilst NO2 flux showed a response to changing relative humidity that could be explained by altered stomatal conductance, increased NH3 flux with increasing relative humidity (>50%) depended on other factors. The exchange of NO2 between above-ground parts of beech trees and the atmosphere could be explained exclusively by uptake or emission of NO2 through the stomata, as indicated by the quotient between measured and predicted NO2 conductance of approx. 1 under all environmental conditions examined. Neither internal mesophyll resistances nor additional sinks could be observed for adult trees or for beech seedlings. By contrast, the patterns of NH3 flux could not be explained by an exclusive exchange of NH3 through the stomata. Deposition into additional sinks on the leaf surface, as indicated by an increase in the quotient between measured and predicted NH3 conductance, gained importance in high air humidity, when the stomata were closed or nearly closed and/or when atmospheric NH3 concentrations were high. Although patterns of NH3 gas exchange did not differ between different months or years at high NH3 concentrations (c. 140 nmol mol−1), it must be assumed that emission or deposition fluxes at low ambient NH3 concentration (0.8 and 4.5 nmol mol−1) might vary significantly with time because of variation in the NH3 compensation point.

The gelatinous layer (G-layer) of tension-wood fibres in reaction wood of beech showed alterations as a result of the physiological processes involved in the conversion of sapwood into false heartwood or reaction-zone tissue. Using transmitted-light, fluorescence and UV microscopy, polyphenolic compounds were found to infiltrate and encrust the cellulose microfibrils within the G-layer. Experiments with naturally infected and artificially inoculated wood showed that these processes affect the rate and mode of degradation by wood-decaying fungi. Thus, although the ascomycete Ustulina deusta was able to degrade the G-layer from within the lumina of tension-wood fibres in unaltered sapwood, it failed to do so for a prolonged period within false heartwood and reaction zones. In both situations, however, there was some degradation of the underlying secondary wall in the form of erosion troughs which can be attributed to soft rot ‘type II’, and internal cavity formation typical for ‘type I’ attack. The present study indicates that not only cell type, but also alterations in the cell wall structure, affect the activity and degradation mode of decay fungi in beech.

There is a growing recognition that accurate predictions of plant and ecosystem responses to global change require a better understanding of the mechanisms that control acquisition of growth-limiting resources. One such key mechanism is root physiological capacity to acquire nutrients. Changes in kinetics of root nitrogen (N) uptake might influence the extent to which terrestrial ecosystems will be able to sequester excesses in carbon (C) and N loads. Despite its significant role in determining plant and ecosystem cycling of C and N, there is little information on whether, or how, root nutrient uptake responds to global change. In this review various components of global change, namely increased CO2 concentration, increased soil temperature and increased atmospheric N deposition and their effects on kinetics of root nutrient uptake are examined. The response of root nutrient uptake kinetics to high CO2 is highly variable. Most of this variability might be attributable to differences in experimental protocols, but more recent evidence suggests that kinetic responses to high CO2 are also species-specific. This raises the possibility that elevated CO2 might alter community composition by shifting the competitive interaction of co-occurring species. Uptake of NH4+ and NO3− seem to be differentially sensitive to high CO2, which could influence ecosystem trajectory toward N saturation. Increased soil temperature might increase N and P uptake capacity to a greater extent in species from warm and fluctuating soil habitats than in species from cold and stable soil environments. The few available data also indicate that increased soil temperature elicits a differential effect on uptake of NH4+ versus NO3−. Root uptake kinetics are generally down-regulated in response to long-term exposure to atmospheric N deposition. The extent of this down-regulation might, however, vary among species, stages of succession, land-use history and plant demand. Nonetheless, it is suggested that root N uptake kinetics might be an accurate biological indicator of the ecosystem capacity to retain N. The results reviewed here clearly highlight the scanty nature of the literature in the area of root nutrient absorption responses to global change. It is also clear that effects of one component of global change on root nutrient absorption capacity might be counterbalanced by another. Therefore, the generalizations offered here must be viewed with caution and more effort should be directed to rigorously test these initial observations in future research.

Fruiting and deblossomed plants of strawberry (Fragaria × ananassa) were exposed to 92 ppb ozone or filtered air in open-top chambers for 69 d. Flower and fruit production, relative growth rate of leaf area, leaf gas exchange and plant biomass were investigated. Ozone caused an initial acceleration in inflorescence production, which was followed by a reduction in inflorescence production, fruit set, and, later, individual fruit weight, although total fruit yield was not affected before the end of the fumigation period. Ozone accelerated leaf senescence and had a greater negative effect on the rate of photosynthesis in older than in younger leaves in fruiting and deblossomed plants, but the response of net photosynthesis to ozone did not differ between the two groups of plants. Relative growth rate of leaf area was the first parameter to be reduced by ozone fumigation, with the effect being significant in fruiting, but not in deblossomed, plants. Final above-ground biomass was also significantly decreased by ozone in fruiting plants, but not in deblossomed plants. Root and crown biomass were not significantly affected by ozone fumigation in either fruiting or deblossomed plants.

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

Defining root death in studies of root dynamics is problematic because cell death occurs gradually and the resulting effects on root function are not well understood. In this study, metabolic activity of grape roots of different ages was assessed by excised root respiration and tetrazolium chloride reduction. We investigated changes in metabolic activity and patterns of cell death occurring with root age and changes in root pigmentation. Tetrazolium chloride reduction of roots of different ages was strongly correlated to respiration (R2 = 0.786). As roots aged, respiration and tetrazolium chloride reduction declined similarly, with minimum metabolic activity reached at six weeks. Tetrazolium chloride reduction indicated that the onset of root browning corresponded to a 77% reduction in metabolic activity (P < 0.001). Anatomical examination of roots at each pigmentation stage showed that even though some cells in brown roots were still alive, these roots were functionally dead. The effect of using different definitions of root death in relation to root survivorship was determined in a study of ‘Concord’ grapes with two pruning treatments, using three criteria for root death: browning, blackening or shriveling, and disappearance. There was no effect of vine pruning on root life span when life span was defined as the time from first appearance to the onset of browning. However, if death was judged as the point when roots either became black or shriveled or disappeared, vine pruning decreased root life span by 34% and 40%, respectively (P < 0.001), and also increased the decay constant for root decomposition by about 45% (P < 0.001). We conclude that the discrepancy among determinations of root life span assessed with different definitions of death might be partly caused by the latter evaluations of root life span incorporating a portion of root decomposition in definitions of root death.