Two-yr-old Norway (Picea abies (L.) Karst.), red (P. rubens Sarg.), black (P. mariana (Mill.) B.S.P.) and white (P. glauca (Moench.) Voss) spruce seedlings were exposed from bud break for 35 or 63 d (4·5 h irradiance d−1) to a gradient of biologically effective u.v.-B radiation (λ=280–315 nm) ranging from 0·61 kJ m−2 d−1 to 5·99 kJ m−2 d−1. No visible symptoms of u.v.-B injury were observed. Epicuticular wax production was not affected by needle exposure to increasing u.v.-B irradiance. Seven constituent classes were resolved by GC and confirmed by GC–MS in wax recovered from needles of the four species. Wax composition of Norway, black and red spruce was altered following needle exposure to increasing u.v.-B dose. White spruce wax composition was unaffected. Direction and magnitude of wax composition response was species-dependent. The proportion of nonacosane diols on Norway spruce needles increased (P<0·05) whereas that of alkyl esters decreased with increasing u.v.-B dose. The proportion of fatty acids in black spruce needle wax increased (P<0·05), and that of estolides (GC-identified) in red spruce needle wax increased (P<0·05) with increasing u.v.-B dose. Changes in wax chemical composition reported were induced following to daily, 4-h duration exposures of needles to u.v.-B centred on 1200 hours. Affected variables exhibited a continuum of response. The highest dose applied was within the range of measured or predicted increases in mid-northern latitudes. Such changes in conifer needle epicuticular wax chemical composition might result in increased seedling sensitivity to the changing atmospheric environment, especially from co-exposure to tropospheric ozone in mid-northern latitudes where much of Canada's productive forest is located.

Root growth and respiration in elevated CO2 (700 μmol mol−1) was studied in three tree species, Fraxinus excelsior L., Quercus petraea. L. and Pinus sylvestris L. grown in open-top chambers (OTCs) during a long-term exposure (20 months), during which root systems were allowed to develop without restriction imposed by pots. Root growth, measured as root length using root in-growth bags was increased significantly in trees exposed to elevated CO2, although the magnitude of the response differed considerably between species and with time of sampling, the greatest effect observed after 6 months in ash (ratio of elevated: ambient, e[ratio ]a; 3·40) and the smallest effect observed in oak (e[ratio ]a; 1·95). This was accompanied by changes in specific root length, with a significant decrease in all species after 6 months, suggesting that root diameter or root density were increased in elevated CO2. Increases in root length might have resulted from an acceleration in root cell expansion, since epidermal cell size was significantly increased in the zone of elongation in ash root tips (P<0·05).

Contrasting effects of elevated CO2 were observed for root carbohydrates, with significant increases in soluble sugars for all species (P<0·05), but both increases and decreases in starch content were observed, depending on species, and producing a significant interaction between species and CO2 (P<0·001). Exposure to elevated CO2 increased the total root d. wt for whole trees of all three species after 8 months of exposure, although the magnitude of this effect, in contrast to the root in-growth study, was greatest in Scots pine and smallest in ash. No significant effect of elevated CO2 was observed on the root[ratio ]shoot ratio. Further detailed analysis of whole root systems after 20 months confirmed that species differences in root responses to elevated CO2 were apparent, with increased coarse and fine root production in elevated CO2 for Scots pine and ash respectively. Lateral root number was increased in elevated CO2 for all species, as was mean root diameter. Root respiration rates were significantly reduced in elevated CO2 for all three species. These results provide firm evidence that exposure of trees to future CO2 concentrations will have large effects on root system development, growth, carbohydrate status and respiration. The magnitude and direction of such effects will differ, depending on species. The consequences of such responses for the three species studied are discussed.

The host range of ectomycorrhizal (ECM) fungi in Britain was examined by compilation of a data matrix from published literature sources, based primarily on accounts of sporocarp associations with particular host genera. Information was gathered for 577 species of ECM fungi belonging to 51 genera, and 25 genera of host trees, representing the majority of ECM fungal species and host genera recorded in Britain.

Pronounced variation was recorded in the number of ECM fungal species associated with different host genera, with over 200 species recorded with Betula, Fagus, Pinus and Quercus. There was a positive linear relationship (r2=0·47, P=0·007) between the number of species of ECM fungi associated with different host genera and the total area occupied by each tree genus in Britain (both values log-transformed). There was also variation in the number of species of ECM fungi which were apparently specific to particular host genera, values ranging from zero (in 15 genera) to >40 in the case of Betula and Fagus. In total, 233 fungal species appeared to be specific to a single host genus (i.e. 40% of those surveyed). Comparison of the ECM mycota associated with different host genera by PCA accounted for 17% of the total variation, with genera belonging to the Fagaceae (Quercus, Fagus and Castanea) tending to cluster together, indicating a degree of overlap in their ECM associates. Exotic conifer species, which displayed a lower ECM diversity than would be expected from their distributional areas, were characterized by a high degree of overlap with the ECM associates of Pinus and Betula.

These results indicate that the abundance of different genera of host trees and variation in host specificity could provide a basis for understanding patterns of diversity in ECM fungi within Britain.

TIP1 is a gene defined by an X-ray induced allele tip1–2 and a previously described EMS-induced allele tip1−1. TIP1 is involved in plant cell growth. tip1–2 plants display growth defects throughout the plant and exhibit defects in both root-hair and pollen-tube growth. tip1–2 plants are partly male sterile resulting from a combination of pollen germination and pollen-tube defects; their root-hairs are short, exhibit a tendency to branch and 2–4 hairs can initiate from each hair cell. They are also slightly dwarf in stature as a result of a general decrease in cell growth indicating that TIP1 activity is required for general cell growth. We propose a role for TIP in both the initiation and maintenance of growth in tip-growing cells. In addition TIP1 activity is required for normal cell expansion (non-tip cell growth) indicating that TIP1 is not exclusively involved in tip-growth.

A previous study had indicated that scavengers of reactive oxygen species (ROS) delayed cell death (the hypersensitive response (HR)) triggered in epidermal cells of intact, resistant, cowpea (Vigna unguiculata (L.) Walp) leaves by the monokaryotic stage of the cowpea rust fungus (Uromyces vignae Barclay race 1). This HR had been monitored by cell autofluorescence, which occurs after protoplast collapse. In the present study, when cytoplasmic disorganization was used to monitor cell death more directly, ROS-scavengers, superoxide dismutase, catalase, horseradish peroxidase, and desferal-Mn(IV) had no effect on HR development. Cytological staining for superoxide or hydrogen peroxide generation also did not reveal the presence of ROS before or during the early stages of the HR, but did, as in the previous study, suggest a role in the autofluorescence and browning of invaded cells that occur following protoplast collapse. Staining of plant mitochondria with nitroblue tetrazolium, possibly attributable to increased dehydrogenase activity but not superoxide generation, occurred transiently around invasion hyphae (monokaryotic stage of the fungus) or haustoria (dikaryotic stage) of the fungus as they entered a cell in the susceptible or resistant cultivar. Around invasion hyphae in epidermal cells in resistant plants, this staining diminished as cytoplasmic streaming stopped, and gradually disappeared as cell death progressed. These data are consistent with other evidence that rust fungi initially negate non-specific defensive responses in both resistant and susceptible cells as part of the establishment of biotrophy. They also suggest that the HR in the cowpea–cowpea rust fungus pathosystem is not triggered by an oxidative burst.

The transmission of infection by many soil-borne fungal parasites of plants depends on the ability of the fungus to grow on or through soil. Progress in analysing the effects of soil physical factors on the temporal and spatial dynamics of fungal growth has been hindered by technical difficulties of quantifying fungal biomass in soil and heterogeneity in soil properties. In this paper we use a combination of a monoclonal antibody-based immunosorbent assay and microscopy to analyse the effects of soil physical properties on the spatial and temporal dynamics of colonies of the economically important fungus Rhizoctonia solani Kühn growing in two dimensions and three dimensions in a sand. Combinations of different particle-size distributions and matric potential are used to manipulate the air-filled pore volume and pore-size distribution independently of each other. Temporal dynamics are measured by the change in fungal biomass over time whereas spatial dynamics relate to fungal spread and are measured by the colony size, the rate of colony expansion and the biomass distribution within colonies. We show that the fungus spreads more than three times further over surfaces than through sand, even though the same amount of biomass is produced in each case. Pore-size distribution and air-filled pore space both affected the extent and rate of fungal spread in three dimensions within sand, with more rapid and extensive spread in a coarse sand compared with a fine sand at identical air-filled pore volume. The spread of fungal hyphae along surfaces was affected neither by differences in surface texture nor by air-filled volume, and was substantially more homogeneous than for three-dimensional spread. We argue that the relative impermeability of sand surfaces to penetration by hyphae might be influenced by the ability of the fungus to branch within a confined space rather than simply to penetrate the pores. The broader epidemiological and ecological consequences of preferential spread by parasitic and saprophytic fungi along surfaces rather than through the dense soil volume are discussed.

To study the response of non-mycorrhizal and mycorrhizal maize plants to drought, the changes in the pools of non-structural carbohydrates and amino acids were analysed in leaves and roots of two maize cvs. Plants well colonized by the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) (60% of root length infected) and comparable non-mycorrhizal plants were subjected to moderate drought stress by reducing the water supply. This stress induced a conspicuous increase in the trehalose pool in the mycorrhizal roots, probably because it was accumulated by the fungal symbiont. Furthermore, glucose and fructose were accumulated in leaves and roots of non-mycorrhizal plants but not in the mycorrhizal ones. Starch disappeared completely from the leaves of both mycorrhizal and non-mycorrhizal plants in response to drought. Activities of soluble acid invertase and trehalase were also measured. Acid invertase activity increased during drought in the leaves of both non-mycorrhizal and mycorrhizal plants whilst in the roots it was unaffected in non-mycorrhizal plants and decreased in the mycorrhizal ones. Without drought stress, trehalase activity was considerably higher in the leaves and roots of mycorrhizal plants than in those of non-mycorrhizal plants. It increased conspicuously during drought, primarily in the leaves of non-mycorrhizal plants. A drought-induced accumulation of amino acids as well as imino acids was found in roots and leaves of both mycorrhizal and non-mycorrhizal plants; leaves of mycorrhizal plants accumulated more imino acids than those of non-mycorrhizal ones. Our results show that drought stress and the presence of a mycorrhizal fungus have a considerable effect on carbon partitioning, imino acid and amino acid accumulation in maize plants.

Effects of ozone on spring wheat (Triticum aestivum L. cv. Satu) were studied in an open-top chamber experiment during two growing seasons (1992–1993) at Jokioinen in south-west Finland. The wheat was exposed to filtered air (CF), non-filtered air (NF), non-filtered air+35 nl l−1 ozone for 8 h d−1 (NF+) and ambient air (AA). Each treatment was replicated five times. Two wk after anthesis, after 4 wk of ozone treatment (NF+, 45 nl l−1 1000–1800 hours, seasonal mean) the net CO2 uptake of wheat flag leaves was decreased by c. 40% relative to CF and NF treatments, both initial and total activity of Rubisco and the quantity of protein-bound SH groups were decreased significantly. Added ozone also significantly accelerated flag leaf senescence recorded as a decrease in chloroplast size. The effect was significant 2 wk after anthesis, and senescence was complete after 4 wk. In the CF and NF treatments senescence was complete 5 wk after anthesis. The significant effect of ozone on the chloroplasts and net CO2 uptake 2 wk after anthesis did not affect the grain filling rate. However, since the grain filling period was shorter for ozone fumigated plants, kernels were smaller. The decrease in 1000-grain weight explained most of the yield reduction in the plants under NF+ treatment. The results indicate that wheat plants are well buffered against substantial decrease in source activity, and that shortened flag leaf duration is the major factor causing ozone-induced yield loss.

A new experimental system was developed for studying the hormonal mechanisms which control tracheid differentiation. In this system the tracheids redifferentiated from parenchyma cells in the hypocotyl of young Pinus pinea L. seedlings. The experimentally induced tracheids have unique shapes and patterns, and are therefore easily distinguished from the primary and secondary tracheids formed before the experiments. Auxin (0·1–1% NAA) alone sufficed to cause the redifferentiation of short tracheids, usually in discontinuous patterns across the hypocotyl. Gibberellin by itself did not induce redifferentiated tracheids. Combinations of auxin with gibberellin (0·1% NAA+0·1–1% GA3) promoted the differentiation of long tracheids (up to threefold greater than those induced by auxin) in continuous patterns along the stem axis. Gibberellin in the presence of auxin promoted tracheid elongation by stimulating intrusive growth of both the upper and lower ends of the differentiating tracheids. The role of auxin and gibberellin in controlling the evolution of tracheary elements, from tracheids to vessels and fibres, is discussed.

A pot experiment was designed to evaluate the interactive effects of multiple microbial inoculation treatments and rock phosphate (RP) application on N and P acquisition by alfalfa plants using 15N and 32P isotopes. The microbial inocula consisted of a wild type (WT) Rhizobium meliloti strain, its genetically modified (GM) derivative, which had an enhanced competitiveness, the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe, and a phosphate-solubilizing rhizobacterium (Enterobacter sp.). Inoculated micro-organisms became established in the root tissues and/or in the rhizosphere soil of alfalfa plants (Medicago sativa L.). The GM Rhizobium strain did not interfere with AM formation. Inoculated phosphate-solubilizing rhizobacteria established in the alfalfa rhizosphere, but the level of establishment was lower where the natural population of phosphate-solubilizing bacteria was stimulated by AM inoculation and RP application. The stimulation of these indigenous bacteria was also greater in the rhizosphere of alfalfa nodulated by the GM Rhizobium. Improvements in N and P accumulation in alfalfa corroborate beneficial effects of the improved GM Rhizobium on AM performance, in RP-amended plants. Inoculation with Enterobacter did not improve the AM effect on N or P accumulation in the RP-added soil, but it did in the non RP-amended controls. Measurements of the 15N[ratio ]14N ratio in plant shoots indicated enhanced N2 fixation rates in Rhizobium-inoculated AM-plants, over that achieved by the same Rhizobium strain in non-mycorrhizal plants. Regardless of the Rhizobium strain and of whether or not RP was added, AM-inoculated plants showed a lower specific activity (32P[ratio ]31P) than did their comparable non-mycorrhizal controls, suggesting that the plant was using otherwise unavailable P sources. The phosphate-solubilizing, AM-associated, microbiota could in fact release phosphate ions, either from the added RP or from the indigenous ‘less-available’ phosphate. Deficiency in Ca concentration in soil solution in the neutral test soil might benefit P solubilization. The proportion of plant P derived either from the labelled soil P (labile P pool) or from RP was similar for AM inoculated and non-mycorrhizal controls (without Enterobacter inoculation) for each Rhizobium strain, but the total P uptake, regardless of the P source, was far higher in AM-plants. Enterobacter inoculation seems to improve the use of RP in the rhizosphere of non-mycorrhizal plants inoculated with the WT Rhizobium.

Maize (Zea mays L.) plants parasitized by the root hemi-parasitic angiosperm, Striga hermonthica (Del.) Benth., consistently display a range of symptoms similar to those found in droughted plants. The mechanisms by which these changes occur are largely unknown. However, S. hermonthica has unusually high rates of transpiration, and stomata which are relatively insensitive to water deficit. Consequently, it has often been suggested that the parasite might cause a severe depletion of the available water in the host's rooting zone. To determine whether the lower stomatal conductance and retarded growth of infected plants could be a result of parasite-induced water deficit, we have monitored the matric potential of the growth medium, water use, growth and stomatal conductance of infected vs. uninfected maize plants.

Host plant height and stomatal conductance of parasitized plants were significantly lower than those of control plants from 31 or 37 d after planting (d.a.p.) respectively. However, there was no indication of an increase in the rate of water depletion in the rooting zone of infected plants until approx. 63 d into the parasitic association. In fact, from 39 until 59 d.a.p. infected plants used less water than uninfected control plants, probably the result of the plants having fewer expanded leaves during part of this period, combined with the lower stomatal conductance exhibited by the infected plants from day 37 onwards. Leaf RWC of infected plants was unchanged in comparison with that of uninfected plants, therefore the change in stomatal conductance was not a response to dehydration of the leaf tissue. Our results indicate that parasitism by S. hermonthica does not cause an increase in water uptake/use in the host until well after most of the symptoms of infection have become fully established. It is highly unlikely, therefore, that the observed effects on the host are primarily due to soil water deficit.

The relative importance of some aspects of iron nutrition for the distribution of six Plantago species (P. maritima L., P. coronopus L., P. major ssp. major L., P. serpentina All., P. media L. and P. lanceolata L.) with different habitat requirements for soil pH and moisture was evaluated. Iron efficiency and Fe tolerance of hydroponically-grown plants were assessed by determining the decrease in relative growth rates caused by suboptimal and supra-optimal external Fe concentrations. Marked interspecific differences were observed in visual symptoms of Fe deficiency and Fe toxicity and in the external Fe concentrations leading to 50% inhibition of maximal relative growth rates (rgr50%). Whereas P. serpentina displayed a clear preference for low external Fe concentrations, growth rates of P. maritima and P. media were found to be increased at higher concentrations. Severe growth restriction at both low and high Fe concentrations was evident in P. lanceolata and P. major. A broad optimum was observed in P. coronopus exhibiting a low external Fe requirement and a high tolerance to supra-optimal Fe concentrations. Iron efficiency and Fe tolerance differed in a way which was only partly correlated with the expected Fe availability at the natural habitat of the species, suggesting that Fe concentrations are of minor importance for their distribution, or that some of the mechanisms that render Fe oxides available for uptake in situ are masked in solution experiments.

To gain insight into the impact of the physiological and morphological characteristics of the species on Fe efficiency, the effect of Fe status on shoot[ratio ]root ratio, relative root surface area, root Fe(III) reductase and proton extrusion capacity were investigated. Roots of all species showed increased Fe chelate reduction activity upon growth at suboptimal Fe concentrations; marked differences were observed with respect to the kinetic parameters of the reductase. Maximal velocity of the reduction was positively correlated with relative growth rate and was not related to the Fe efficiency of the species. By contrast, Km corresponded to Fe efficiency ranking and can therefore be regarded as an important parameter for the uptake of Fe at low availability. Enhanced ability to acidify the rhizosphere was only observed in P. major. From the morphological characteristics investigated, root surface area appears to be the most important parameter in the uptake of Fe at suboptimal external concentrations.

Developmental changes in the root apex and accompanying changes in lateral root growth and root hydraulic conductivity were examined for Opuntia ficus-indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper soil layers. During 7 d of rapid drying (in containers with a 3-cm depth of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not due to meristem exhaustion but rather to meristem mortality after 3 d of drying. The length of the meristem, the duration of the cell division cycle, and the length of the elongation zone were unchanged during rapid drying. During 14 d of gradual drying (in containers with a 6-cm depth of vermiculite), root mortality was relatively low; the length of the elongation zone decreased by 70%, the number of meristematic cells decreased 30%, and the duration of the cell cycle increased by 36%. Root hydraulic conductivity (LP) decreased to one half during both drying treatments; LP was restored by 2 d of rewetting owing to the emergence of lateral roots following rapid drying and to renewed apical elongation following gradual drying. Thus, in response to drought, the apical meristems of roots of O. ficus-indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in response to rainfall in the field can be enhanced by lateral root proliferation near the soil surface and additionally by resumption of apical growth for deeper roots.

An early successional plant community was exposed to various ozone concentrations for two growing seasons (1994–1995) in open-top chambers in Auburn, Alabama, USA. The ozone treatments were: AA, ambient air (open plots); CF, carbon-filtered air (c. 0·5×ambient air), 1×, non-filtered air, and 2×, twice ambient air. Vegetative canopy cover exhibited a pattern of accumulation in the spring, with maximum canopy cover attained in summer, then senescence of foliage in the autumn 1994. This pattern was not observed in 1995 as a result of a drought during the spring and summer. Varying ozone exposures caused shifts in the competitive interactions between plants, thereby altering community structure. Higher canopy cover, vertical canopy density (layers of foliage), species richness, diversity, and evenness existed in the CF treatments than in the other treatments. In addition, winged sumac (Rhus copallina L.) became a major component of the CF treatments only during 1995. Surprisingly, blackberry (Rubus cuneifolius Pursh.), a species considered ozone-sensitive, based on visible injury, dominated canopy cover within the 2× treatments, 41 and 33% of total canopy cover in 1994 and 1995, respectively. From these results it is concluded that plant communities existing in areas where lower ozone concentrations are prevalent might be more complex and diverse than those existing in areas with higher ozone concentrations.

During the vegetation periods 1994 and 1995, net uptake of nitrate and ammonium by roots of adult spruce (Picea abies (L.) Karst) and beech (Fagus sylvatica L.) trees was studied at a field site exposed to high loads of N (‘Höglwald’, Germany). In addition, uptake experiments were carried out under controlled conditions with young spruce and beech trees grown at normal N supply.

In the field, nitrate was not taken up by the roots of spruce trees in appreciable amounts. This was also true for beech except during September 1995. Apparently, beech trees was capable of taking up nitrate, but the environmental condition prevailing at the field site usually prevented net uptake. Net uptake of ammonium in both tree species showed a seasonal course, with maximum rates in mid summer. Rates of ammonium uptake by both species correlated with soil temperature at the field site.

Laboratory experiments on the influence of root temperature on uptake of nitrate indicated that uptake rates at temperatures found in the field were low compared with the uptake capacity at optimum temperature. At temperatures of 10 and 15°C, frequently found in the soil at the field site, net uptake of nitrate by spruce and beech amounted to c. 16% and 11%, respectively, of maximum uptake at 25°C. By contrast, net uptake of ammonium at 10°C reached 73% and 31% of the maximum uptake for spruce and beech trees, respectively. Independent of temperature, rates of nitrate uptake were considerably lower than those of ammonium. In young spruce and beech trees, net uptake of nitrate was significantly inhibited by ammonium at nitrate[ratio ]ammonium ratios found in the soil solution at the forest site. Preincubation of roots of both species, with amino acids present in the phloem of adult trees at the field site, led to an increase in the amino acid pool in the roots. For spruce trees a correlation between inhibition of uptake of nitrate and enrichment of the roots with the amino compounds Glu, γ-amino butyric acid (Gaba), Gln, and Asn was observed. In beech trees, enrichment of Asp and Gln in the roots correlated with a decrease in net uptake of nitrate. The results of laboratory experiments on the effects of temperature, the nitrate to ammonium ratio in the nutrient solution, and amino acid enrichment in the roots are discussed with special emphasis on the patterns of net uptake of ammonium and nitrate observed in the field.

Sclerophylly and synthesis of phenolic compounds are active responses of plants subjected to environmental stress (drought, low nutrient supply, u.v.-B radiation, ozone). Here we describe the morphological and histochemical alterations occurring in field-grown leaves of Fagus sylvatica L. from three sites located along an ecological gradient: from a site in cool and protected conditions to one located on a mountain ridge, where the trees grow on a thin layer of soil and are exposed to the wind and to intense solar radiation in summer. The morphological data show that, as the ecological conditions of the stand worsen, individual leaf surface decreases, while the thickness of the leaves and their specific d. wt (i.e. d. wt per unit leaf area) increases. Histochemical and ultrastructural tests show a marked increase of phenolics during the course of the year. These substances, present primarily in the leaves of trees growing in stress conditions, have been identified mainly as tannins. They accumulate in the vacuoles, especially those of the upper epidermal layer and the palisade mesophyll; at a later stage they appear to be solubilized in the cytoplasm and retranslocated, eventually impregnating the outer wall of the epidermal cells amidst the cellulose fibrils, where they cluster together and form an electron-opaque layer between the wall and the cuticle. Observation of the epidermal cells also reveals that the outer cell wall is thicker. The paper discusses the roles of secondary metabolites in protection and detoxification processes; the possible ecological significance of these alterations in the ecophysiology of beech trees.

The rate of nitrogen uptake by seven Sphagnum species, which from a gradient from hummock to hollow and from ombrotrophic to minerotrophic conditions, was measured as the decrease in the concentrations of NH4+ and NO3− from solutions in which capitula were grown under laboratory conditions.

The highest uptake rate was by individuals of each species with large capitula and a high number of ion exchange sites, i.e. lawn species (S. pulchrum, S. fallax, S. papillosum and S. magellanicum). On a dry-mass basis, the most effective species were the hummock species (S. fuscum and S. rubellum), even though these species have a low dry mass. Hummock species, which occur in high densities and have high potential N-uptake rates on a dry-mass basis, were the most effective species in retaining available nitrogen.

Direct pollen interactions, as well as interactions mediated by a recipient, can have a remarkable influence on pollen fertilization ability. Under conditions of pollen competition it could be advantageous if pollen grains interfered with the germination of other pollen. The aim of this study was to find out if there are direct negative or positive pollen–pollen interactions between pollen grains from genetically slightly different donors. The in vitro germinability of the pollen from several Betula pendula Roth clones was investigated. The pollen interactions between the clones were examined pairwise by using equal pollen mixtures. In three of the eight cases the germination percentage of the pollen mixture was significantly higher than the average germination percentage of the separate clones that formed the mixture, which indicates some type of interaction between the pollen populations. We found only positive interactions between the pollen of clones. This study also documented density-dependent germination of pollen grains in vitro (=pollen population effect). Adding an aqueous pollen extract to the incubation medium increased the germination percentages of poorly germinating pollen and small pollen populations. Germination-stimulating effects were found to exist both with fresh and dead pollen. Such direct pollen–pollen interactions could be explained by specific water-soluble substances diffusing from pollen grains.

The effects of low levels of nitrogen addition (7·7 and 15·4 kg N ha−1 yr−1) on plant sensitivity to biotic and abiotic stress were studied at a lowland heath in the south of England that has received N treatments since 1989. Larval growth rates and adult weights of heather beetles were found to be significantly higher when insects were reared on plants that had received additional N, with implications for insect survival and reproductive success. Electrolyte leakage measurements failed to reveal any significant impact of N addition on plant sensitivity to frost episodes in early winter. In April, however, there was some evidence of slightly decreased frost hardiness in plants receiving additional N. Accelerated spring bud burst also suggested earlier physiological activity in N-treated plots. The rate of water loss from excised shoots of Calluna vulgaris (L.) Hull was significantly faster in plants receiving additional N, although no difference in plant water potential was measured in the field after a prolonged dry spell. Whilst experimental N addition had only a small effect on plant sensitivity to abiotic stress, the relationship between enhanced deposition and increased insect performance was clear, with the potential for substantially increased insect damage at deposition rates around the critical load contributing to the formation of gaps in the Calluna canopy.

The impact of O3 at different stages of plant development was investigated in growth-chamber-cultivated Plantago major L. Six-d-old plants of an O3-sensitive population (‘Valsain’) were exposed to one of the following six treatments; 56 d charcoal/Purafil®-filtered air (CFA): 56 d CFA plus 70 nmol mol−1 O3 for 7 h d−1; CFA with a 14-d episode of O3 administered at days 1, 14, 28 or 42. Harvests were made every 14 d, and at the final harvest (56 d) the influence of O3 on reproductive structures was assessed.

Analysis of the effects of O3 on growth and reproductive performance confirmed the sensitivity of the population to the pollutant. In the absence of the development of typical visible symptoms of foliar damage, the total d. wt of plants maintained in O3 over a 56-d period was 35% lower than that of control plants. However, the impact of the pollutant was found to decrease as plants aged. Plant relative growth rate (R) was only affected in seedlings, suggesting that effects of O3 on seedling growth were largely responsible for the decrease in accumulated biomass; the growth rate of older plants was not affected by O3. The observed shift in O3 resistance with plant age was mediated by both ‘acclimation’ and ontogenetic changes. ‘Acclimation’ was not associated with changes in O3 uptake, and there was some evidence to support the existence of compensatory growth responses. In addition to effects on vegetative growth, plants exhibited an O3-induced decline in reproductive performance; O3 reducing the number of flower spikes and seed capsules produced per plant. Ozone episodes administered at different stages of development indicated that reproductive development was particularly sensitive to O3 during the early stages of flowering.

The findings of this study are discussed in relation to evolutionary adaptation to O3 in natural plant communities.The importance of plant age, prior exposure to the pollutant and the timing of O3 episodes in relation to plant developmental stage are highlighted.