Forests exchange large amounts of CO2 with the atmosphere and can influence and be influenced by atmospheric CO2. There has been a recent proliferation of literature on the effects of atmospheric CO2 on forest trees. More than 300 studies of trees on five different continents have been published in the last five years. These include an increasing number of field studies with a long-term focus and involving CO2×stress or environment interactions. The recent data on long-term effects of elevated atmospheric CO2 on trees indicate a potential for a persistent enhancement of tree growth for several years, although the only relevant long-term datasets currently available are for juvenile trees.

The current literature indicates a significantly larger average long-term biomass increment under elevated CO2 for conifers (130%) than for deciduous trees (49%) in studies not involving stress components. However, stimulation of photosynthesis by elevated CO2 in long-term studies was similar for conifers (62%) and deciduous trees (53%). Recent studies indicate that elevated CO2 causes a more persistent stimulation of biomass increment and photosynthesis than previously expected. Results of seedling studies, however, might not be applicable to other stages of tree development because of complications of age-dependent and size-dependent shifts in physiology and carbon allocation, which are accelerated by elevated CO2. In addition, there are many possible avenues to down-regulation, making the predicted canopy CO2 exchange and growth of mature trees and forests in a CO2-rich atmosphere uncertain. Although, physiological down-regulation of photosynthetic rates has been documented in field situations, it is rarely large enough to offset entirely photosynthetic gains in elevated CO2. A persistent growth stimulation of individual mature trees has been demonstrated although this effect is more uncertain in trees in natural stands.

Resource interactions can both constrain tree responses to elevated CO2 and be altered by them. Although drought can reduce gas-exchange rates and offset the benefits of elevated CO2, even in well watered trees, stomatal conductance is remarkably less responsive to elevated CO2 than in herbaceous species. Stomata of a number of tree species have been demonstrated to be unresponsive to elevated CO2. We conclude that positive effects of CO2 on leaf area can be at least as important in determining canopy transpiration as negative, direct effects of CO2 on stomatal aperture. With respect to nutrition, elevated CO2 has the potential to alter tree–soil interactions that might influence future changes in ecosystem productivity. There is continued evidence that in most cases nutrient limitations diminish growth and photosynthetic responses to elevated CO2 at least to some degree, and that elevated CO2 can accelerate the appearance of nutrient limitations with increasing time of treatment. In many studies, tree biomass responses to CO2 are artefacts in the sense that they are merely responses to CO2-induced changes in internal nutritional status of the tree.

There are numerous interactions between CO2 and factors of the biotic and abiotic environment. The importance of increasing atmospheric CO2 concentrations for productivity is likely to be overestimated if these are not taken into account. Many interactions, however, are simply additive rather than synergistic or antagonistic. This appears to hold true for many parameters under elevated CO2 in combination with temperature, elevated O3, and other atmospheric pollutants. However, there is currently little evidence that elevated CO2 will counteract O3 damage. When the foliage content of C, mineral nutrients and secondary metabolites is altered by elevated CO2, tree×insect interactions are modified. In most trees, mycorrhizal interactions might be less important for direct effects of CO2 than for alleviating general nutrient deficiencies.

Since many responses to elevated CO2 and their interactions with stress show considerable variability among species/genotypes, one principal research need is for comparative studies of a large variety of woody species and ecosystems under realistic conditions. We still need more long-term experiments on mature trees and stands to address critical scaling issues likely to advance our understanding of responses to elevated CO2 at different stages of forest development and their interactions with climate and environment. The only tools available at present for coping with the consequences of rising CO2 are management of resources and selection of genotypes suitable for the future climate and environment.

A two-day Discussion Meeting of the Royal Society, ‘The Nitrogen Cycle’, held in London in June 1991 (Stewart & Rosswall, 1992) reviewed the considerable progress made in understanding the N cycle in agricultural, forest and aquatic systems. The meeting included some discussion of the concerns which were already being expressed at that time over nitrate in water supplies, and the impacts of nitrogenous gases on tropospheric chemistry, the greenhouse effect and the ozone layer. Since then, disquiet over the impacts of nitrogenous compounds on the environment has increased, and numerous papers have been published on many aspects of the problem. We now have much better understanding of the size and scale of the perturbation of the N cycle, and several review papers have highlighted the complexity of the formidable issues that are challenging environmental scientists (Vitousek, 1994; Galloway et al., 1995; Vitousek et al., 1997).

Extracellular matrices associated with conidia and germ tubes of Botrytis fabae (Sard.) sporelings grown on Vicia faba L. leaves were clearly visualized by epi-fluorescence microscopy following immunolabelling with the monoclonal antibodies, BC-KH4 and BC-FD7-G9. These antibodies were raised against surface washings of B. cinerea, are directed against B. cinerea and B. fabae, and are known to recognize carbohydrate epitopes on a glycoprotein. Both BC-KH4 and BC-FD7-G9 also labelled matrix material located at the surface of penetration and infection hyphae inside the leaf tissue by epi-fluorescence microscopy. Such matrix material was not visible by DIC microscopy.

Immunoelectron microscopy of B. fabae-infected leaf tissue, prepared by progressive low-temperature dehydration and embedding in acrylic resin, allowed further investigation of the spatial distribution of the antibody-binding sites. An abundance of BC-KH4 and BC-FD7-G9 antigenic sites were observed throughout the fibrillar-like matrix material surrounding the germ tubes on the leaf surface and the infection hyphae inside the host cells. However, close examination of the V. faba–B. fabae interface inside the host tissue showed that this fibrillar material extended some distance from the surface of the infection hyphae and through the swollen epidermal and mesophyll cell walls. Such fibrillar matrix material is thought to be of fungal origin. The possible role(s) of this matrix material in the infection process are discussed.

Double-immunolabelling studies using the BC-KH4 MAb and a polyclonal antiserum directed against oligosaccharides containing β-(1→3)-glucose were carried out in order to localize and distinguish between the fungal extracellular matrix material and translucent cell wall respectively. This technique allowed a closer examination of the interactions of the fungal matrix components with the host walls and degenerate host cytoplasm. Finally, inward curling of the leaf cuticle suggested that mechanical pressure is involved in the penetration process.

Membership of our Board of Advisors to the Editors has remained almost static since the Board was founded in 1995, but significant turnover begins this summer. Accordingly, I offer a warm welcome to new members of the Board and thank those who have agreed to extend their membership. I thank especially those leaving the Board for their invaluable support during the first three years of its life. The workplaces and interests of Board members reflect both the international base of the New Phytologist and the wide range of subjects that it regularly publishes. SCI ratings support the belief that the New Phytologist is the world's leading broad-spectrum botanical journal. Given that breadth of subject matter, it is all the more remarkable that the journal has a high Impact Factor, 2·2.

The good health of the New Phytologist, and the charitable Trust that owns it, is further indicated by recent expansion of our activities. For each of the last three years, symposia, to which numerous distinguished international speakers contributed, have been organized in the UK. Proceedings have been published in the journal but they have also been made available for purchase as attractively bound separates, the most recent being Major Biological Issues Resulting from Anthropogenic Disturbance of the Nitrogen Cycle (eds. T. A. Mansfield, L. Sheppard and K. W. T. Goulding). This year, the symposium will be held in Montpellier, France, on the subject The Gap between Isolated Plants and Growing Canopies, while in 1999 it will move to the USA. In response to numerous requests, particularly from university teachers, we are now collecting recent Tansley Reviews around themes and publishing these too as separates. The first collection, Leaf Development and Function, is now available.

Like many forward looking journals, the New Phytologist is fully online (at http://www.journals.cup.org): unlike that of many other journals, our online version was launched free of charge to subscribers. This is just one of the many ways in which this independent journal can help botany and botanists. Indeed, monies from the Trust support a range of activities that promote botany (see volume 134, facing p. 197 for details). The journal will be 100 years old in 2002. The Trustees would be especially pleased to receive requests for support for activities that would mark the centenary.

As I noted in my previous Editorial, three years ago, Independent and International remain our key words.

Presentations and discussions about the impacts of nitrogenous compounds dispersed via the atmosphere, and of ozone as a secondary pollutant

Immunoelectron microscopy using the anti-pectin monoclonal antibody JIM 7 confirmed earlier observations that pectin degradation is a primary event in the process of host cell wall breakdown during the development of chocolate spot disease (causal agent: Botrytis fabae (Sard.)) of broad bean. Close examination of infected and non-infected Vicia faba L. leaves indicated a loss of JIM 7-labelling, and therefore, methyl-esterified pectin, from swollen walls of infected and contiguous epidermal cells. Modified mesophyll walls also possessed less methyl-esterified pectin than healthy walls. Enzymes which attack methyl-esterified pectin appeared to be most active in regions of host tissue close to sites of fungal infection.

Ultrastructural studies using the enzyme, cellobiohydrolase conjugated to gold (CBH1-Au) revealed that the cellulose microfibrils of outer epidermal walls of non-infected V. faba leaf tissue were heavily masked by other components of the plant cell wall. Such material was most probably pectin because the cellulose microfibrils of swollen epidermal and modified mesophyll walls of infected host tissue were heavily labelled with CBH1-Au. These results were confirmed by double-labelling studies using JIM 7 and CBH1-Au. At early stages of the infection process, limited cellulose degradation was observed in infected leaf tissue.

Double-labelling experiments using the monoclonal antibody BC-KH4 directed against Botrytis matrices and a marker for the plant cell wall (JIM 7 or CBH1-Au) confirmed previous observations that the fungal matrices extended through modified host walls and degenerate cytoplasm. It is suggested that the wall-modifying action of the pectin-degrading enzymes produced during the infection process might facilitate pervasion of matrix material associated with the infection hyphae through host cell walls. Possible role(s) of such matrix material during the post-penetration processes of the V. faba–B. fabae relationship are discussed.

The seasonal patterns of xylem embolism and xylem transport properties in Quercus pubescens Willd. and Quercus ilex L. trees growing in a natural mixed coppice stand in conditions of severe water stress were investigated. Xylem embolism was evaluated in both dehydrating branches and in apical twigs during a whole year. Measurements of xylem water potential were conducted from predawn to sunset on selected sunny days. On the same days, diurnal courses of leaf conductance were monitored. Measurements of half-hourly sap flow were made by the heat-pulse technique throughout the summer. At the onset of summer, a sharp decrease in water potential was observed in both species. Full recovery of water potentials was observed for both species after the first major rainfall event in September. Both experienced serious embolism throughout the year, ranging between minima of c. 60% (expressed as percentage loss of hydraulic conductivity) after the rains in autumn and after bud burst in spring, and maxima of c. 80% during summer and after freezing-thawing events during the winter season. A significant negative linear relationship was found between water potential and xylem embolism in branches dehydrating in air for Q. pubescens and Q. ilex. Q. pubescens had greater efficiency in hydraulic transport (higher specific conductivity and leaf specific conductivity) by the xylem than Q. ilex. In June, leaf conductance was high early in the morning and decreased gradually during the day. Midday depression of leaf conductance, as a result of high evaporative demand combined with water deficit, was observed in both species. In August, leaf conductance of both species was greatly reduced, as water potential dropped to extremely low values, and the stomata were almost completely closed during the afternoon. No hysteresis resulting from plant capacitance was observed in the relationship between shoot water potential and sap flow. Q. pubescens exhibited very high values of whole-tree hydraulic resistance between July and September, whereas Q. ilex generally showed lower values. The effect of soil moisture depletion on the relationship between sap flow and shoot water potential appears as a lowering of water potential at zero flow. A significant decrease of whole-tree hydraulic resistance in both species was observed with the onset of the autumn, preceding the partial recovery of twig hydraulic conductivity. The results demonstrate that both Q. pubescens and Q. ilex, although highly tolerant of severe water stress and tissue dehydration, operate at the limits of safety which are surpassed under severe droughts, and prolonged climatic stress might predispose these Quercus species to decline.

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.

Air quality thresholds for O3 for the protection of human health and vegetation set by the European Union (EU) have been exceeded in Europe regularly in the 1990s. Because target reductions for oxides of nitrogen (NOx) set for the year 2000 are unlikely to be achieved, these O3 exceedances are likely to continue into the next millenium. Improvements of plant tolerance towards O3 are being investigated but very little work has been done to explore NOx tolerance and plant acclimation to NO2 and NO. However, it is clear that within the populations of some plant species there is wide variation, and some individuals can fix NOx and use the nitrogen directly from the atmosphere, rather than rely upon, for example, root uptake of nitrate. It is possible that individuals capable of fixing NOx could be selected for a range of species, and genotypes with high rates of uptake could be of value as crops or for forestation in polluted areas (e.g. landscaping in the vicinity of motorways) to reduce tropospheric concentrations of NOx significantly and also to decrease the potential for O3 production.

The abundance and cellular location of Fe-containing superoxide dismutase (Fe-SOD) in trichomes of Nodularia, Aphanizomenon and Anabaena collected from various depths in the Baltic Sea, and in trichomes of a cultured Nodularia strain, BC Nod-9427, isolated from the Baltic Sea, was examined by immunogold labelling. For trichomes collected from natural populations the areal concentration of Fe-SOD labelling decreased with depth: trichomes collected from surface accumulations had between 8 and 11 gold particles μm−2 whereas trichomes collected from a depth of 18 m were unlabelled. When trichomes collected from a depth of 10 m (mean areal labelling density 0·5 gold particles μm−2) were exposed to the higher irradiances present at 1 m, the areal concentration of Fe-SOD increased to 3·5–4 gold particles μm−2 within 4 h. When cultures of Nodularia strain BC Nod-9427, adapted to low light (10 μmol m−2 s−1), were transferred to an incident irradiance of 1350 μmol m−2 s−1, a doubling of the areal concentration of Fe-SOD gold label was observed within 1 h. Addition of 3-(3,4-dichlorophenyl)-1,1′-dimethylurea (DCMU) to cultures immediately before their transfer to increased illumination resulted in a decrease in areal Fe-SOD concentrations whereas addition of CdCl2 caused an increase over and above that induced by the elevated irradiance. These results suggest that Baltic Sea cyanobacteria are able to modulate their Fe-SOD content but that this might be in response to oxidative stress rather than to light per se.

Ca2+ was localized in chemically injured internodal cells of the characean alga Nitella flexilis (L.) Ag. using alizarin red and antimonate precipitation. The presence of Ca2+ in the antimonate precipitates was verified by X-ray analysis and EGTA chelation. Callose-containing amorphous wound walls were induced by 0·1 mm chlortetracycline (CTC) and cellulosic fibrillar wound walls were induced by 50 mm CaCl2. Numerous precipitates were found in the amorphous wound walls and in the adjacent cytoplasm. Precipitates were mainly localized in single membrane-bound cisternae, probably of the endoplasmic reticulum, which accumulate at the wound and become a component of the amorphous wound wall via membrane fusion. In fibrillar wound walls, which do not contain membranous residues, precipitate density was significantly lower and similar to that in the secondary cell wall.

The data suggest that the high Ca2+ content of amorphous wound walls is due to incorporation of cytoplasmic Ca2+ stores. The possible function of amorphous wound walls in maintaining cellular Ca2+ homeostasis is discussed.

Aspects of the structure and ultrastructure of the fusiform cambial cells of the taproot of Aesculus hippocastanum L. (horse chestnut) are described in relation to the seasonal cycle of cambial activity and dormancy. Particular attention is directed at cell walls and the microtubule and microfilament components of the cytoskeleton, using a range of cytochemical and immunolocalization techniques at the optical and electron-microscopical levels. During the dormant phase, cambial cell walls are thick and multi-layered, the cells possess a helical array of cortical microtubules, and microfilament bundles are oriented axially. In the early stages of reactivation, vesicle-like profiles are associated with the cell walls, whereas arrangement of the cytoskeletal elements remains unchanged. In the succeeding active phase, the cell walls are thin, and cortical microtubules form a random array, although microfilament bundles maintain a near-axial orientation. The observations are discussed in relation to the seasonal cycle of wall structure and cortical microtubule rearrangement within the vascular cambium of hardwood trees. It is suggested that the cell-wall thickening at the onset of cambial dormancy, which is associated with the presence of a helical cortical microtubule array, should be considered to be secondary wall thickening, and that selective lysis of this secondary wall layer during cambial reactivation restores the thinner, primary wall found around active cambial cells.

We studied interspecific and ontogenetic relationships between the size of a leaf and the primary diameter of the internode bearing it. Although these two variables are known to be strongly correlated across species, the form of this relationship has not been studied. In a re-analysis of published data on interspecific comparisons of 69 temperate tree species, we showed the existence of a strong relationship between twig cross-sectional area (before secondary growth) and surface area of leaves borne by it, within each of three morphological groups, deciduous angiosperms, evergreen angiosperms, and gymnosperms. Within each of these groups, this relationship is isometric: across species, primary cross-sectional area of the stem increases proportionally with leaf surface area. When we consider the relationship between the cross-sectional area of a twig and the surface area of one leaf borne by it, the y-intercepts for this relation are different for the three groups. However, when total leaf surface area per first-year shoot is considered, no differences remained between gymnosperms and evergreen angiosperms, but deciduous angiosperms continued to be distinct. This difference between deciduous and evergreen groups could be due to differences in leaf volume (evergreen species have thicker leaves than deciduous) or in traits related to a trade-off between life span of leaves and their physiological behaviour.

We present results of the first quantitative study of the relationship between leaf size and primary diameter of the stem during ontogeny. Both these parameters increase during development of the plant from seedling to adult. For the four tree species examined, the relationship between primary cross-sectional area of the stem and leaf surface area is also isometric.

These results bear on a functional interpretation of the relationship between leaf and stem dimensions, suggesting that vascular supply is directly proportional to the requirements of leaves supported by the stem.

Possible interactions of two synthetic plant-growth retardants during the short-term response of Brassica rapa L. ssp. oleifera (DC.) Metzger plants to low root-zone temperature were investigated by pretreating with mefluidide or paclobutrazol. Water and solute transfers were studied by measuring xylem sap volume flow (under root pressure exudation) and ion flow from the roots. Relations with nitrate uptake rate were also considered. Root pretreatment with paclobutrazol strongly restricted the cold-inducible processes which normally restore water and solute flow from the root xylem. Paclobutrazol decreased the rates of nitrate uptake and exudation flow from the root xylem (principally by reducing root hydraulic conductivity) with dramatic consequences for ion flow, especially that of nitrate.

The effects of root ABA pretreatment on plant response to root cooling were then studied separately or in association with a pretreatment with paclobutrazol. Despite a slight decrease in nitrate uptake rate, ABA pretreatment of the roots enabled the plant to develop rapid mechanisms for adaptation to cold constraint at the root level. Moreover, this action of exogenous ABA greatly reduced the effect of a simultaneous paclobutrazol pretreatment and partly restored water and solute flows.

Thus, the improvement of plant resistance to cold conditions brought about by treatments with mefluidide and paclobutrazol (previously shown in long-term experiments) cannot simply be explained by their short-term effects.

Isolates of the ericoid mycorrhizal fungus Hymenoscyphus ericae (Read) Korf et Kernan, and the ectomycorrhizal fungi Suillus variegatus (Swartz ex Fr.) and Pisolithus tinctorius (Pers.) Coker & Couch, along with a Cortinarius sp. and the white rot Phanerochaete chrysosporium Burdsall were examined for the ability to oxidize carbohydrates to their corresponding lactones and to excrete the H2O2 produced thereby. All except Phanerochaete chrysosporium were found to express cellobiose oxidase (cellobiose dehydrogenase, EC 1.1.19.88) and glucose oxidase (β-d-glucose[ratio ]oxygen 1-oxidoreductase, EC 1.1.3.4) when grown on cellobiose and glucose respectively. Production of extracellular H2O2 was visualized during growth on both substrates using ABTS as the chromogen. According to the Fenton reaction, H2O2 will react with hydrated or chelated Fe(II) in the environment to produce hydroxyl (Fenton) radicals, HO·. Mycelial extracts from each of the mycorrhizal fungi produced HO· in the presence of cellobiose and Fe(II), presumably mediated by H2O2 produced by cellobiose oxidase activity in the extracts. Conditions favourable to HO· production were shown to exist in Modified Melin–Norkrans medium, and the data discussed in relation to previously observed lignin degradation by mycorrhizal fungi.

Emissions of reactive oxidized nitrogen (NO and NO2), collectively known as NOx, from human activities are c. 21 Tg N annually, or 70% of global total emissions. They occur predominantly in industrialized regions, largely from fossil fuel combustion, but also from increased use of N fertilizers. Soil emissions of NO not only make an important contribution to global totals, but also play a part in regulating the dry deposition of NO and NO2 (NOx) to plant canopies. Soil microbial production of NO leads to a soil ‘compensation point’ for NO deposition or emission, which depends on soil temperature, N and water status. In warm conditions, the net emission of NOx from plant canopies contributes to the photochemical formation of ozone. Moreover, the effect of NOx emissions from soil is to reduce net rates of NO2 deposition to terrestrial surfaces over large areas.

Increasing anthropogenic emissions of NOx have led to an approximate doubling in surface O3 concentrations since the last century. NOx acts as a catalyst for the production of O3 from volatile organic compounds (VOCs). Paradoxically, emission controls on motor vehicles might lead to increases in O3 concentrations in urban areas.

Removal of NO and NO2 by dry deposition is regulated to some extent by soil production of NO; the major sink for NO2 is stomatal uptake. Long-term flux measurements over moorland in Scotland show very small deposition rates for NO2 at night and before mid-day of 1–4 ng NO2-N m−2 s−1, and similar emission rates during afternoon. The bi-directional flux gives 24-h average deposition velocities of only 1–2 mm s−1, and implies a long life-time for NOx due to removal by dry deposition.

Rates of removal of O3 at the ground are also influenced by stomatal uptake, but significant non-stomatal uptake occurs at night and in winter. Measurements above moorland showed 40% of total annual flux was stomatal, with 60% non-stomatal, giving nocturnal and winter deposition velocities of 2–3 mm s−1 and daytime summer values of 10 mm s−1. The stomatal uptake is responsible for adverse effects on vegetation. The critical level for O3 exposure (AOT40) is used to derive a threshold O3 stomatal flux for wheat of 0·5 μg m−2 s−1. Use of modelled stomatal fluxes rather than exposure might give more reliable estimates of yield loss; preliminary calculations suggest that the relative grain yield reduction (%) can be estimated as 38 times the stomatal ozone flux (g m−2) above the threshold, summed over the growing season.

To test the ability of plants to integrate small-scale imbalances in soil nitrate and phosphate patches, plant growth and acquisition of nitrate and phosphate were measured for the perennial grass Agropyron desertorum (Fisch. ex Link) Schult. and the shrub Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle in soil where the principal supply of nitrate and phosphate came from two enriched patches. The soil was calcareous loamy-skeletal Typic Haploxerolls. These patches were applied in two treatments: either nitrate and phosphate were applied in both patches (balanced treatment) or one patch contained only nitrate and the other only phosphate (unbalanced treatment). The same total quantity of nutrients was applied in both treatments and these included 15N and 32P tracers. The plants were in large pots in open field conditions. There were no significant differences in total biomass production and nitrogen concentration between the two treatments, indicating that both species had the physiological ability to integrate soil nutrient resources. Artemisia was able to acquire more phosphate in the unbalanced treatment, probably due to the high local solution phosphate concentration. Generally Artemisia acquired more N and P than did Agropyron.

The authoritative talk by Professor Fowler (Fowler et al., 1998), emphasized the huge increase in the rate of NOx (NO and NO2) emissions into the atmosphere due to fossil fuel combustion, from 1 Tg N y−1 to over 20 Tg N y−1 during the 100 yr between 1880 and 1980. He went on to predict that this rate of emission from anthropogenic sources would increase to 46 Tg N y−1 by the year 2025. In addition, NO can also be released from the soil following microbial action, a process that is very dependent upon soil temperature, nitrogen availability and water content. Later in the meeting, Professor Raven (Raven & Yin, 1998) pointed out that terrestrial plants, though not necessarily each individual species, have over the past 450 million yr coped with large changes in nitrogenous compounds in the environment. Nevertheless, this is no basis for complacency about the current situation because the rates of change caused by man's activities are probably unprecedented. Furthermore, the fact that terrestrial plant life in some form can continue, despite massive changes in environmental chemistry, does not necessarily indicate that the systems on which we ourselves are dependent will be conserved.

Low phosphorus availability is often a primary constraint to plant productivity in native soils. Here we test the hypothesis that root carbon costs are a primary limitation to plant growth in low P soils by assessing the effect of P availability and mycorrhizal infection on whole plant C budgets in common bean (Phaseolus vulgaris L.). Plants were grown in solid-phase-buffered silica sand providing a constant supply of low (1 μm) or moderate (10 μm) P. Carbon budgets were determined weekly during the vegetative growth phase. Mycorrhizal infection in low-P plants increased the root specific P absorption rate, but a concurrent increase in root respiration consumed the increased net C gain resulting from greater P uptake. The energy content of mycorrhizal and non-mycorrhizal roots was similar. We propose that the increase in root respiration in mycorrhizal roots was mainly due to increased maintenance and growth respiration of the fungal tissue. Plants grown with low P availability expended a significantly larger fraction of their total daily C budget on below-ground respiration at days 21, 28 and 35 after planting (29–40%) compared with plants grown with moderate P supply (18–25%). Relatively greater below-ground respiration in low P plants was mainly a result of their increased root[ratio ]shoot ratio, although specific assimilation rate was reduced significantly at days 21 and 28 after planting. Specific root respiration was reduced over time by low P availability, by up to 40%. This reduction in specific root respiration was due to a reduction in ion uptake respiration and growth respiration, whereas maintenance respiration was increased in low-P plants. Our results support the hypothesis that root C costs are a primary limitation to plant growth in low-P soils.

Wheat plants were grown at a day/night temperature of 18/13°C under glasshouse conditions. Twenty-two d after anthesis, one set of plants was shaded to 50% of the normal photon fluence rate, another was ‘degrained’ by selective spikelet removal which left only the grains in the five central spikelets; a further set was left as control. Individual plants were harvested at days 22, 30 or 42 after anthesis. Extracts from the peduncle and the penultimate internode were prepared to determine the activities of sucrose phosphate synthase, sucrose synthase, fructan exohydrolase and acid invertase, and to assess the concentration of hexose sugars, sucrose and fructans. Measurements were also made of ear and individual grain weights, and stem f. wt and d. wt. There was a decline in the amount of fructans with time, more pronounced in ‘shaded’ (source-limited) than in control plants. By contrast, in ‘degrained’ (sink-limited) plants, the amount of fructans in the stem initially rose, then decreased, with a concomitant increase in the amount of fructose. The shifts in sugar content of the wheat culm reflected both the sink demand of the ear and source activity. The activity of fructan exohydrolase correlated with the carbohydrate changes. Under limited photosynthate assimilation, the mobilization of fructans from the internodes towards the ear was related to an increase in this enzyme, whereas the other enzymes played a less direct role in the mobilization of fructan reserves from the wheat stem.