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.

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.

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.

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.

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.

Different aspects of salt excretion from leaves of the xero-halophyte Reaumuria hirtella Jaub. et Sp. (Tamaricaceae) were investigated under the extremely arid conditions of the desert. The diurnal excretion pattern, which gradually decreased toward midday, showed a negative correlation with the daily transpiration pattern. The relative excretion, which is the ratio between the absolute excretion of Na+ and the change in its internal content, was maximal at sites with low salinity, and decreased when the concentration of NaCl in the root environment increased. By contrast, the absolute excretion increased from 253 to 323 mmol Na+ g−1 f. wt per 12 h with increase in the total soil salinity from 0·24 to 1·23%, respectively. Water stress conditions, caused either by increasing soil salinity or by dryness of the atmosphere, greatly influenced the efficiency of excretion process.

Six ions: Na+, K+, Ca2+, Mg2+, Cl− and SO42− constituted 96·8% of the d. wt of the field-collected secreted salts. Na+ and Cl−, which were the predominating ions in the soil solution of the root environment, were also the predominantly secreted, constituting c. 89% of salts secreted. The proportion of Na+ which was prevented from entry into the roots increased with increasing Na+ concentration in the soil solution. More than 67% of the absorbed NaCl was secreted by leaves during the day. However, the accumulated salts contributed to the osmotic potential (ψs) and improved the plant water status. The osmotic potential of the plant sap at the end of the day or during periods of low excretion rates was shown to be decreased −7·3 to −13·7 bar from that in the morning. The fraction of osmotic potential decreased during the day (Δψs) was positively correlated with the soil salinity. These retained salts are expelled from the leaves by increased efficiency of extraction during the night and early morning, so that the plant begins the day with low internal salt content, and the leaf reservoirs become suitable for a new supply. The rejection mechanism at the plant roots and the secretion mechanism at shoots allow the plant to maintain its internal salt content at an approximately constant level despite the great variation in soil salinity.

Leaf sucrose, starch, hexose and maximum extractable soluble acid invertase activity were compared throughout the day in source leaves of 13 plant species chosen for their putative phloem-loading type (apoplastic or symplastic). Four species which represent the different phloem-loading types (tomato, barley, maize and Fuchsia) were studied in detail. Using this information we wished to determine whether a positive correlation between foliar carbohydrates and acid invertase activity exists in leaves from different species and, furthermore, whether this relationship is determined by phloem-loading type. Acid invertase activity was relatively constant throughout the day in all species. The extent of sucrose, hexose and starch accumulation and the sucrose: starch ratio measured at a given time were species-dependent. No correlations were found between foliar soluble acid invertase activity and the hexose, sucrose or starch content of the leaves in any of the species, regardless of phloem-loading type. The species examined could be divided into three distinct groups: (1) high sucrose, low invertase; (2) low sucrose, low invertase; and (3) low sucrose, high invertase. The absence of an inverse relationship between leaf sucrose, hexose or starch contents and endogenous soluble acid invertase suggests that this enzyme is not directly involved in carbon partitioning in leaves but serves an auxiliary function.

Gene-specific probes were used to assess the expression patterns of four different phenylalanine ammonia-lyase (pal) genes in infected or elicitor-treated leaves and suspension-cultured cells of barley. Genes corresponding to hpal2, hpal3, hpal4, and hpal6 were all induced by mercuric chloride and fungal infection by Bipolaris sorokiniana Sacc. (Shoem.) in barley (Hordeum vulgare L. cv. Pokko) leaves, but with considerable variation in their expression level and timing. The expression patterns of hpal2 and hpal6 were similar, both showing a rapid, strong induction after treatment with mercuric chloride and a slower induction after fungal inoculation, whereas the more divergent hpal3 was induced at a later time and at a lower level after both treatments. Hpal4 was expressed with timing like that of hpal2 and hpal6 in infected or mercuric chloride-treated leaves but its expression was much weaker. Hpal2 and hpal4 were induced in elicitor-treated, suspension-cultured barley cells, whereas the expression of hpal3 was nearly undetectable, and hpal6 was strongly and constitutively present. All pal genes except hpal4 were developmentally regulated, but differentially expressed in various barley tissues. The results suggest that the four pal genes studied here might be responsible for the activation of different branches in the phenylpropanoid biosynthesis of barley.

The study characterized the influence of cucumber mosaic virus (CMV) on the growth response of two annual weeds to nitrogen. Plants were grown individually along a N gradient from 4 to 32 mmol l−1 and data were interpreted using growth analysis. Plant biomass increased with N concentration and was significantly higher for healthy than infected plants at the two highest N concentrations. Healthy plants of Portulaca oleracea L. were characterized by lower biomass allocation to leaves and higher biomass allocation to roots than infected plants; no change in biomass allocation was recorded for Stellaria media Vill. Relative growth rate (rgr), net assimilation rate (nar) and specific leaf area (sla) of plants increased with increasing N concentration. Healthy plants of P. oleracea were characterized by a higher rgr and nar and a lower sla than infected plants, whereas healthy S. media had a higher rgr but a similar nar and sla or leaf weight ratio (lwr) compared with infected plants. The consequences of these results on the population dynamics of weeds and virus spread are discussed.

The frequent co-occurrence of ectomycorrhizas and arbuscular mycorrhizas in the same ecosystem raises the question about how two functionally different mycorrhizal associations exploit a spatially heterogeneous habitat. Horizontal (in plots along a transect of 3750 m crossing forest types of low and high abundance of ectomycorrhizal (ECM) host species) and vertical (across profiles of litter and 0–5 cm, 10–15 cm, 25–35 cm soil layers) distribution of ectomycorrhizas and arbuscular mycorrhizas in relation to edaphic parameters (pH, moisture, total carbon, total nitrogen and extractable-phosphorus concentration) was compared in Korup National Park rain forest, to test the hypothesis that the two types of mycorrhiza exploit different niches. No pattern was observed in either ECM or arbuscular mycorrhizal (AM) fractional colonization with depth in the profile even though most of the variation in soil parameters was vertical. Only local (between monoliths within plots) horizontal differences were found in soil parameters. By contrast, horizontal differences in fractional colonization were found between forest types, and between monoliths, for ectomycorrhizas but only between monoliths for arbuscular mycorrhizas. The lack of a clear relationship between both horizontal and vertical distribution of ECM and AM fractional colonization and soil parameters suggests that both fungal groups are equally able to colonize the same niche, at the scale of our experiment. By contrast, a negative relationship was found between AM and ECM fractional colonization in the litter and 0–5 cm and 10–15 cm soil layers. This, in addition to the more heterogeneous horizontal distribution of arbuscular mycorrhizas in forest areas dominated by ectomycorrhizas and a patchy vertical distribution of the former as opposed to a more coherent distribution of the latter, suggests that both types of mycorrhizal fungi compete for the same niche. The co-occurrence of ectomycorrhizas and arbuscular mycorrhizas might be explained by differences between ECM and AM hosts and fungi in their colonizing abilities.

This study compares the effects of the fungicide Aliette® (fosetyl-Al) on non-mycorrhizal and mycorrhizal onion (Allium cepa L.) with effects of the degradation products, aluminium (applied as aluminium chloride) and phosphonate (applied as dimethyl phosphonate). We sought to determine the extent to which the plants absorb and accumulate phosphonate and ascertain why, as shown previously, toxic effects of fosetyl-Al on mycorrhizal plants are less severe than effects on equivalent non-mycorrhizal plants.

Fosetyl-Al markedly reduced growth, especially of roots, and also inhibited mycorrhizal colonization. Dimethyl phosphonate caused smaller effects on growth and did not decrease colonization. Aluminium chloride did not affect growth of non-mycorrhizal or mycorrhizal plants, or mycorrhizal colonization. In all cases, mycorrhizal plants grew better than equivalent non-mycorrhizal plants (with no added soil phosphate), and non-mycorrhizal plants given supplementary soil phosphate grew best.

Concentrations and contents of total P in shoots were increased by dimethyl phosphonate and more so by fosetyl-Al. Concentrations of P were also increased in roots. 31P nuclear magnetic resonance (NMR) spectroscopy was used to determine relative concentrations of phosphonate and phosphate. Application of fosetyl-Al led to accumulation of more phosphonate than did application of dimethyl phosphonate, and non-mycorrhizal plants treated with fosetyl-Al accumulated much more phosphonate than did equivalent mycorrhizal plants (both with no added soil phosphate). Internal phosphate concentrations increased, especially in mycorrhizal plants, following application of both dimethyl phosphonate and fosetyl-Al. Non-mycorrhizal plants given supplementary soil phosphate also showed restricted growth when treated with fosetyl-Al even though they accumulated phosphate in relatively high amounts and had lower ratios of phosphonate to phosphate, particularly in shoots. In general, high internal concentrations of phosphonate were correlated with large reductions in plant growth among treatments.

Possible causes of these effects include conversion of phosphonate to phosphate in the tissues (especially in mycorrhizal plants), enhanced uptake of phosphate via mycorrhizal fungi and additional accumulation of P in plants with restricted growth. Competition for uptake between phosphate and phosphonate is also discussed.

The ectomycorrhizal fungal associations of Douglas fir (Pseudotsuga menziesii D. Don) and bishop pine (Pinus muricata D. Don) were investigated in a mixed forest stand. We identified fungi directly from field-collected ectomycorrhizal (ECM) root tips using PCR-based methods. Sixteen species of fungi were found, of which twelve associated with both hosts. Rhizopogon parksii Smith was specific to Douglas fir. Three other species colonized only one of the hosts, but were too infrequent to draw conclusions about specificity. Seventy-four percent of the biomass of ECM root tips sampled in the stand were colonized by members of the Thelephoraceae and Russulaceae. All 12 species of fungi that associated with both tree species did so within a 10×40 cm soil volume, suggesting that individual fungal genotypes linked the putatively competing tree hosts.

Basidiocarps of Pisolithus associated with indigenous (Afzelia quanzensis Welw.) and introduced (Pinus caribaea Mor. and Eucalyptus camaldulensis Dehnh.) hosts in the lowland forests of the Coast Province of Kenya are morphologically distinct. Genetic variability among 52 Pisolithus basidiocarps, collected beneath the various host plants, was examined based on sequence polymorphism within the internal transcribed spacer (ITS) and intergenic spacer (IGS1) of ribosomal DNA genes. Variability in ITS and IGS1 sequences indicated that the three host-associated morphotypes were genetically different. Consensus trees generated by bootstrap analysis of sequence data of Pisolithus isolates from Australia and Kenya are polyphyletic and strongly suggest that the three different morphotypes/genotypes present in Kenya represent separate biological species. In addition, our data indicate that little genetic exchange occurs in silva between these species.

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

Roots of Lablab purpureus (L.) Sweet were treated with tri-iodobenzoic acid (TIBA), kinetin or with nodulation factors (Nod factors) purified from Rhizobium sp. NGR234 and grown in the presence of a mycorrhizal inoculum (Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe. Colonization by the mycorrhizal fungus was increased from <30% to c. 65% of root length when roots were treated with these growth regulators. Moreover, treatment of mycorrhizal L. purpureus roots with Nod factors or TIBA strongly induced sporocarp formation of Glomus mosseae. In parallel, the pool size of the fungal disaccharide trehalose was significantly affected in roots treated with TIBA and Nod factors alone, and with TIBA combined with all effectors, and increased from 0·06 mg g−1 d. wt in control roots to up to 1·7 mg g−1 d. wt (TIBA+kinetin). Conversely, the sucrose pool decreased from 5% d. wt to less than a half in roots treated with Nod factors. Activities of trehalase were significantly enhanced in mycorrhizal roots by the treatment with Nod factors or TIBA. When Nod factors and TIBA were added in combination, these activities were strongly enhanced suggesting synergism between these growth regulators.

Plantago lanceolata L. was grown for 104 d with (M) or without (NM) arbuscular mycorrhizal colonization under conditions of ambient (CAMB=350 μl l−1) and elevated (CELEV=540 μl l−1) CO2. Sequential harvests (H) were taken at 41 (H1), 76 (H2) and 104 d (H3) to determine the time-course of mycorrhizal influence on the response of the plant to CO2 enrichment. Total yields of M plants were greater than those of NM from H2 onwards. Plants in the MELEV treatment were significantly larger than those in the MAMB at 104 d. There were significant but much smaller differences in yield between NMELEV and NMAMB. The differences in total yield arose through impact of CELEV on both shoots and roots. Total root length was greater in MELEV than in MAMB only at H3, but total length of mycorrhizal root was greater at H2 and H3. The percentages of root length colonized and that occupied by arbuscules and vesicles were greater in MELEV than in MAMB at the last two harvests, indicating increased sequestration of carbon in internal fungal structures. Though extraradical hyphal lengths were greater in MELEV than in MAMB at H2 and H3, the differences were not significant. Phosphorus inflow and P content of M plants were higher than those of NM plants at H2 and H3, and were higher in MELEV than in MAMB at H3. anova revealed no significant interactions between CO2 and mycorrhizal treatment.

The results are discussed in relation to carbon sequestration in mycorrhizal systems and likely impacts of CO2 enrichment on P. lanceolata grown under field conditions. The importance of sequential harvesting for realistic determination of responses to CO2 is stressed.

The present work describes the morphogenesis and cytological characteristics of ‘branched absorbing structures’ (BAS, formely named arbuscule-like structures, ALS), small groups of dichotomous hyphae formed by the extraradical mycelium of arbuscular mycorrhizal (AM) fungi. Monoxenic cultures of the AM fungus Glomus intraradices Smith & Schenck and tomato (Lycopersicum esculentum Mill.) roots allowed the continuous, non-destructive study of BAS development. These structures were not observed in axenic cultures of the fungus under different nutritional conditions or in unsuccessful (asymbiotic) monoxenic cultures. However, extraradical mycelium of G. intraradices formed BAS immediately after fungal penetration of the host root and establishment of the symbiosis. The average BAS development time was 7 d under our culture conditions, after which they degenerated, becoming empty septate structures. Certain BAS were closely associated with spore formation, appearing at the spore's substending hypha. Branches of these spore-associated BAS (spore-BAS) usually formed spores. Electron microscopy studies revealed that BAS and arbuscules show several ultrastructural similarities. The possible role of BAS in nutrient uptake by the mycorrhizal plant is discussed.