The aim of this work was to examine the correspondence between apoplastic/symplastic antioxidant status and previously reported plant age-related shifts in the ozone (O3) resistance of Plantago major L. Seed-grown plants were fumigated in duplicate controlled environment chambers with charcoal/Purafil®-filtered air (CFA) or CFA plus 70 nmol mol−1 O3 for 7 h d−1 over a 42 d period. Measurements of stomatal conductance and antioxidants were made after 14, 28 and 42 d fumigation, on leaves at an equivalent stage of development (youngest fully expanded leaf, measured c. 9 d after emergence). Ozone exposure resulted in a similar decline in stomatal conductance across plant ages, indicating that increases in O3 resistance with plant age were mediated through changes in the tolerance of leaf tissue rather than enhanced pollutant exclusion. Leaf apoplastic washing fluid was found to contain ‘unspecific’ peroxidase, ascorbate peroxidase, superoxide dismutase and ascorbate, but not glutathione and the enzymes required to facilitate the regeneration of ascorbate from its oxidized forms. A weak induction in the activity of certain symplastic antioxidants was found after 14 d O3 fumigation, despite a lack of visible symptoms of injury, but shifts in symplastic antioxidant enzyme activity were not consistent with previously observed increases in resistance to O3 with plant age. By contrast, changes in ‘unspecific’ peroxidase activity and in the small pool of ascorbate in the leaf apoplast were found to accompany age-related shifts in O3 resistance. It is concluded that constituents of the leaf apoplast may constitute a potentially important front line defence against O3.

We report a survey of four viruses (beet western yellows luteovirus (BWYV), cauliflower mosaic caulimovirus (CaMV), turnip mosaic potyvirus (TuMV), turnip yellow mosaic tymovirus (TYMV)) in five natural populations of Brassica oleracea in Dorset (UK). All four viruses were common; 43% of plants were infected with BWYV, 60% with CaMV, 43% with TuMV and 18% with TYMV. For each virus there were significant differences in the proportion of infected plants among populations, which were not completely explained by differences in the age of plants. Multiple virus infections were prevalent, with 54% of plants having two or more virus types. There were statistically significant associations between pairs of viruses. The CaMV was positively associated with the other three viruses, and BWYV was also positively associated with TuMV. There was no detectable association between BWYV and TYMV, whereas TuMV and TYMV were negatively associated. We suggest these associations result from BWYV, CaMV and TuMV having aphid vectors in common, as aphids are attracted to plants that already have a virus infection. Infected plants were distributed randomly or were very weakly aggregated within populations. The implications of widespread multiple virus infections in natural plant populations are discussed with respect to the release of transgenic plants expressing virus-derived genes.

Ozone (O3)-sensitive and tolerant clones of aspen (Populus tremuloides) were exposed to O3 at 1.0, 1.7 and 3.0 times the concentration in ambient air at Ithaca, NY, USA (×1.0, ×1.7 and ×3.0) for 3 months. Visible foliar injury occurred within a few days of the initiation of treatments and developed continuously in ×1.7 and ×3.0 treatments. Leaf senescence began earlier in trees exposed to greater-than-ambient treatments. Gas-exchange variables were measured five times over the growing season. Net photosynthesis was progressively decreased and finally reduced by 40 and 70% on tolerant and sensitive clones, respectively, but stomatal conductance was not affected by the O3 treatments. As stomatal conductance (gs) values did not differ in the two types of aspen, we conclude that this is not the defense mechanism to O3 in aspen. At the end of the season, dry weight was reduced by up to 55 and 35% in sensitive and tolerant clones, respectively. Ozone effects were significant and linear on foliar injury, biomass and growth variables such as height, diameter of stem and total leaf area. The levels of two antioxidants, ascorbic acid and glutathione, were investigated to determine if a clonal difference occurred. Concentrations of the free-radical scavengers did not differ in the clones, nor did they seem to be induced by O3. A cumulative O3 index, SUM06 (the sum of the total μl l−1 of exposure when the O3 concentration exceeded 0.06 μl l−1), was highly correlated with most foliar injury variables and gas-exchange variables except gs in higher O3 treatments. The reduction of carbon gain due to O3 is discussed with regard to foliar damage and biomass decrease.

The development of fruits of blackcurrant (Ribes nigrum) cv. Ben Alder from flower to maturity was studied non-invasively by nuclear magnetic resonance (NMR) microscopy, using attached and detached fruits, and the images were compared with those from low temperature scanning electron microscopy (LTSEM) and conventional resin histology. The NMR images derived from 2-D and 3-D datasets showed the previously unreported growth of arillar tissues to the extent that they almost completely occlude the locular cavity, but LTSEM and resin histology revealed that no fusion occurs between the arillar tissues and the gelatinous sheath surrounding each seed, or between the arillar tissues and the endocarp. The discontinuities between these tissues cause magnetic inhomogenities which result in these structures being clearly resolved by gradient echo imaging sequences. During seed maturation the endosperm changed from high (bright) to low (dark) signal intensity as lipid reserves formed and solidified, whereas the gelatinous sheath had high signal intensity throughout maturation. The high lipid concentration in the seed was manifested by chemical shift effects in the images and the increasing viscosity of the endosperm was reflected in the decrease in spin–lattice (T1) relaxation times. The funiculi, throughout development of seeds, appeared in NMR images with low signal intensity and 3-D surface-rendered reconstructions illustrated the complexity of the spatial array of seeds and funiculi arising from parietal placentas within the loculus. All other vascular tissues in the pericarp and placentas were resolved as a small bright core surrounded by a dark region, within a matrix of moderate signal intensity. Conventional microscopical studies then showed that the bright core discernible by NMR imaging encompassed an entire vascular bundle whereas the darker surrounding region represented small parenchyma cells with pronounced intercellular gas spaces. Other regions of the pericarp which included extremely large parenchyma cells, however, had few intercellular spaces and consequently gave rise to brighter regions of the image.

Floerkea proserpinacoides (Limnanthaceae) is a spring ephemeral annual species that grows in deciduous forests throughout eastern North America. Seeds germinate from late November to December, although the first leaf emerges only from late March to early April. Growth begins in early April at the onset of favourable temperatures, following snowmelt, and continues through mid-June. Senescence coincides with increasing air temperature and decreasing light level as a result of canopy closure. In this paper, we present the results of a growth chamber study designed to determine the effect of light level on growth, biomass allocation and reproduction of F. proserpinacoides. The study consists of two parts: in a first experiment, plants were grown at five constant photosynthetic photon fluence rates (PPFR: 90, 180, 360, 540 or 900 μmol m−2 s−1), and in a second experiment, PPFR was reduced from 900 μmol m−2 s−1 to 180 μmol m−2 s−1 after 0, 14, 21, 28 or 35 d of growth. Relative humidity, temperature, nutrient and water supply were kept constant in a hydroponic sand culture experiment. Total biomass, leaf mass and leaf area increased with increasing PPFR up to 540 μmol m−2 s−1. Plants grown at the highest (900 μmol m−2 s−1) and the lowest (90 μmol m−2 s−1) PPFR had a substantially lower biomass by the end of the 35-d growth period than plants grown at intermediate PPFRs (360 or 540 μmol m−2 s−1). Despite differences in total biomass, there were no significant differences in seed production among treatments. The mean relative growth rate (RGR) increased with increasing light levels between 90–540 μmol m−2 s−1, and it was reduced at 900 μmol m−2 s−1. However, differences in RGR were not significant among treatments. Specific leaf area did not vary consistently as a function of light level, whereas leaf area ratio and leaf mass ratio tended to increase with increasing PPFR, reaching maximum values at 360–540 μmol m−2 s−1. However, none of these growth variables differed significantly across the range of PPFR levels. The transfer of plants to lower PPFR had no significant effect on any of the growth components. Biomass production for the species appeared to be optimized at PPFR of 360–540 μmol m−2 s−1. Growth might be restricted by an insufficient supply of photosynthates at low PPFR and by photoinhibitory processes at higher PPFRs.

Native upland species, Nardus stricta, Eriophorum vaginatum, Erica cinerea and Vaccinium vitis-idaea were given 3 or 60 kg N ha−1 yr−1, over 2 yr, applied as a mist (NH4NO3). The high N treatment increased above-ground biomass in all four species, but only significantly in E. cinerea, E. vaginatum and N. stricta. Biomass increases in E. vaginatum and N. stricta resulted from enhanced tiller production rather than shoot elongation. Root growth increased in N. stricta, so that root[ratio ]shoot ratio in this species was unchanged by N. Root growth in E. vaginatum, E. cinerea and V. vitis-idaea did not respond to N and their root[ratio ]shoot ratios decreased. Tissue N concentrations increased in both shoots and roots of all species in response to N. The accumulated foliar N did not increase the proportion of N allocated to Rubisco and the photosynthetic capacities of N. stricta, E. vaginatum and V. vitis-idaea were unchanged. Thus growth responses to N were due to altered allocation rather than increased rate of photosynthesis per unit leaf area. The high N treatment increased flower production significantly in E. cinerea but not in the other species. Although in this experiment dwarf shrubs were more responsive than graminoids to N, in the field at current N inputs the enhanced tillering of the graminoids may be more competitively advantageous, especially where gaps develop in the canopy. Thus increasing N deposition may lead to increased grassiness of upland heath, and in particular, a spread of N. stricta.

Regular (monthly) additions of NH4NO3 (4–12 g N m−2 yr−1) were made over a period of 8 yr (1989–98) to areas of moorland in North Wales dominated by the ericaceous shrub Calluna vulgaris. Results from the early stages of the experiment (1990–94) have shown marked and dose-related increases in shoot extension and canopy height in response to the nitrogen treatments, with significantly higher shoot nitrogen contents. The nitrogen-related stimulation in the growth of the C. vulgaris canopy over this period has resulted in large accumulations of litter on the high-nitrogen-treated plots (6.6 kg m−2 in plots treated with 12 g N m−2 yr−1, compared with 3.8 kg m−2 for the water controls). Litter nitrogen concentrations were also significantly increased at the higher rates of nitrogen addition, leading to a doubling of the total return of nitrogen to the litter layer over the experimental period. These changes in vegetation structure were associated with large reductions in the abundance of the bryophyte and lichen species normally present under the untreated canopy. Results since 1994, however, show little increase in shoot extension in response to the nitrogen treatments, with no clear dose response to increasing levels of addition. These findings are associated with a dose-related increase in the susceptibility of the nitrogen-treated areas of the C. vulgaris canopy to late winter injury, characterized as browning of the shoot tips in early to late spring. These results indicate that deleterious effects are now accumulating as a result of the long-term addition of nitrogen to these moorland plots.

A field and laboratory based bioassay has been developed to investigate the effects of the quantity and duration of simulated pollutant nitrogen (N) deposition on root-surface phosphomonoesterase (PME) activities in calcareous and acid grasslands. Seedlings of Plantago lanceolata were transplanted to a calcareous grassland and Agrostis capillaris seedlings were grown in microcosms containing soil from an acid grassland that had received either 7 yr (long-term) N additions or 18 months (short-term) N and phosphorus (P) additions. The bioassay revealed that short-term N treatments had little effect on the enzyme activity, whereas long-term N additions significantly increased PME activity within 7 d of transplanting into the field plots. Root-surface PME activity of A. capillaris was significantly reduced in soil that received additions of P. In the plots receiving long-term additions of N, a strong relationship was observed between extractable soil ammonium and root-surface PME activity. Soil ammonium concentrations accounted for 67% of the variation in PME activity of P. lanceolata in the calcareous grassland, and 86% of the variation in PME activity of A. capillaris in the acid grassland. These results provide evidence that N deposition may have considerable effects on the demand and turnover of P in ecosystems that are approaching or have reached N saturation.

Previous work on Lolium perenne showed that sucrose[ratio ]sucrose fructosyltransferase (SST) activity did not increase concomitantly with fructan synthesis in regrowing leaves or in mature leaf blades of plants that have been subjected to carbohydrate-accumulating conditions. This was contrary to the pattern of SST activity in roots and stubble. To obtain further insight into the fructan synthesizing activities and to explain this discrepancy, total fructosyltransferase activity (FT) was assayed by increasing the sucrose and the soluble enzyme concentrations and was compared to sedimentable phlein sucrase activity (PS) throughout the regrowth period following defoliation in leaves, stubble and roots. Before analysis on 2-month-old plants, PS activity was characterized in dark-grown coleoptiles, using [U-14C]sucrose. PS activity had a pH optimum of 6.0 and produced 1-kestose in addition to high molecular weight fructans with a mean DP of 9. In 2-month-old plants, sedimentable PS and FT soluble reactions contained an initial sucrose concentration of 160 mM and 400 mM and proteins equivalent to 1.4 and 2.1 g f. wt of tissue, respectively. In stubble and roots, the FT preparation catalysed the synthesis of large fructans, and the overall pattern resembled the native fructans when separated by anion exchange HPLC. In regrowing leaves, the FT preparation produced low-DP fructans relatively more than in vivo but synthesized the high-DP fructan characteristic of the tissue. Moreover, FT activity did not remain at a low level like SST activity but increased from day 5 after defoliation when fructans began to accumulate. PS activity formed very few low- DP fructans and 1-kestose was the main product. Trisaccharides generated by PS activity represented 2–5% of the total trisaccharide synthesis. High-DP fructans were detectable only when the products of the reaction were concentrated 100 times. Results are discussed with respect to the relative contribution of FT and PS activities for the synthesis of 1-kestose and fructans in roots, stubble and leaves of Lolium perenne.

Two Holocene lake sequences from Lac de Creno, Corsica were analysed on the basis of 119 pollen spectra and with the support of 13 14C-calibrated dates. The lower part of one of these sequences, corresponding to the late- glacial period, has been published previously. The first third of the Post-glacial is characterized by very particular forest dynamics, namely the absence of a role for deciduous Quercus and Corylus, the presence of mesophilous vegetation types dominated by Taxus, and the major forest role of Erica arborea at lower and mean altitude. At about 7440 cal BP, the occurrence of a major anthropogenic action brought about significant changes in the vegetation, notably an increase of deciduous Quercus and the expansion of Quercus ilex. Later, three major human-induced events are identified: the first, at about 2290 cal BP, is the cause of a short local expansion of Abies; the second, at about 1150 cal BP, is the degradation of deciduous forests to the benefit of Fagus; the third, at about 310 cal BP, corresponds to the disappearance of Fagus and its replacement by Pinus. Pollen data indicate that Q. ilex, Abies and Fagus are not indigenous in Corsica but spread there during the Postglacial ; this probably took place at about 6980 cal BP for Q. ilex.

Among citrus rootstocks, higher specific root length (SRL, root length/d. wt) has been linked to several specific morphological and physiological traits, including smaller average root diameter, higher root hydraulic conductivity and higher rates of root proliferation. In this study, thickness of the outer tangential exodermal (hypodermal) wall and its suberin layer, number of passage cells, presence of epidermis, and stelar anatomy were examined and related to variation in root diameter of field roots of known maximum age. We also compared root morphology and anatomy of young roots in the field with those of potted rootstock seedlings in the glasshouse. Fibrous roots were measured separately from pioneer (framework) roots. Among the fibrous roots, only the first-order (root links having a root tip) and second-order (root links bearing first-order roots) laterals were examined. Among first-order field roots, larger root diameter was caused by larger rather than more numerous cells in the cortex. Root diameter of first-order roots was positively correlated with both number of passage cells in the exodermis and thickness of the secondary walls of the exodermis in both field and potted plants.

Exodermal walls were about 80% thicker in field- than pot-grown roots. In the field, in more than 50% of the first-order roots examined less than 30% of the root surface was still covered by epidermis, with few differences among rootstocks. In contrast, in roots of 19-wk-old glasshouse plants generally 70–100% of the epidermis was still intact. There was no evidence of secondary xylem development in second-order fibrous roots in the field; in seedling, pot-grown rootsystems, 75–97% of second-order roots had formed secondary xylem despite their small diameter (<0.8 mm).

It is argued that there can be suites of physiological, morphological and anatomical traits in roots that co-vary with specific root length. Investigations of how root morphology and anatomy are linked to root function, moreover, need to recognize trait variability and the potentially important differences between field- and pot- grown (seedling) roots.

Natural flooding is one of the major factors affecting vegetation dynamics in many regions of the world. The Flooding Pampa Grasslands (Argentina) are frequently exposed to flooding events of diverse intensity and duration, some of which Leontodon taraxacoides, an exotic dicot. frequent in these grasslands, seems to survive. Its responses to four different water depths (0, 1, 7 and 13 cm) were studied. The results indicate that plants in conditions of total submergence (depth of 13 cm) did not survive. In less severe flood conditions, increases in the leaf insertion angle resulted in the maintenance of a large proportion of the total leaf area above the water. Differences in leaf length and a decrease in the width and the proportion of lobes per leaf were also found under partial submergence conditions (depth of 7 cm). Root and leaf aerenchyma, present in unflooded plants, showed a significant increase in flood conditions. In spite of the anatomical and morphological responses, total biomass and leaf area were severely affected by water depth. Control plants allocated more biomass to reproductive organs, while partly submerged plants allocated more to leaves and less to reproductive organs. Mature L. taraxacoides plants presented a wide range of plastic adjustment as a survival strategy in soil anaerobiosis, and appear to be able to survive short spring floods in a vegetative state; in contrast, they might not tolerate total submergence conditions imposed by more intense and long-lasting floods.

The host range of the soilborne obligate biotroph, Plasmodiophora brassicae was investigated. Evidence is presented that infection by P. brassicae might occur in non-Brassica species, leading to the potential formation of resting spores. Structures resembling P. brassicae were found in the root cortex of Tropaeolum majus, Carica papaya, Reseda alba and Beta vulgaris as demonstrated by scanning electron microscopy. Inoculation of Brassica rapa roots with spores extracted from either T. majus or B. vulgaris roots which had been previously inoculated with P. brassicae led to development of clubroot in the roots of B. rapa. It was also shown that the development of the symptom might be correlated with glucosinolate content, although other host factors are implicated in the B. vulgaris interaction with P. brassicae. In the glucosinolate-containing non-Brassicas, T. majus and C. papaya, the concentrations of benzylglucosinolate increased markedly in roots inoculated with P. brassicae, compared with the controls. There were also increases in concentrations of benzylglucosinolate in leaves of T. majus after P. brassicae infection. However, in R. alba roots, the total glucosinolate content decreased after inoculation with P. brassicae compared with the controls. High root concentrations of 2-OH-2-phenylethylglucosinolate (glucobarbarin) compared with low root indole glucosinolates in this species might limit P. brassicae infection and development. The importance of our investigations in relation to cultivation of non-Brassica species on fields infested with P. brassicae is discussed.

Seeds of cherry (Prunus avium) were germinated and grown for two growing seasons in ambient (∼350 μmol mol−1) or elevated (ambient+∼350 μmol mol−1) CO2 mole fractions in six open-top chambers. The seedlings were fertilized once a week, following Ingestad principles in order to supply mineral nutrients at free-access rates. In the first growing season gradual drought was imposed on rapidly growing cherry seedlings by withholding water for a 6-wk drying cycle. In the second growing season, the rapid onset of drought was imposed at the height of the growing season on the seedlings which had already experienced drought in the first growing season. Elevated [CO2] significantly increased total dry-mass production in both water regimes, but did not ameliorate the growth response to drought of the cherry seedlings subjected to two sequential drying cycles. Water loss did not differ in either well watered or droughted seedlings between elevated and ambient [CO2]; consequently whole-plant water- use efficiency (the ratio of total dry mass produced to total water consumption) was significantly increased. Similar patterns of carbon allocation between shoot and root were found in elevated and ambient [CO2] when the seedlings were the same size. Thus, elevated [CO2] did not improve drought tolerance, but it accelerated ontogenetic development irrespective of water status.

Several DNA-based techniques, developed for identifying and differentiating fungi in the Gaeumannomyces–Phialophora complex associated with take-all diseases of cereals and grasses, were used to compare fungi from maize. Maize isolates obtained as G. graminis (Sacc.) Arx & H Olivier var. tritici Walker, from the UK, having been identified by ascospore morphology and in pathogenicity tests on wheat, were indistinguishable from isolates of the same variety obtained from wheat. Isolates of G. graminis (Sacc.) Arx & H Olivier var. maydis Yao et al., recently described as the maize take-all fungus from China, were identical in DNA tests to the anamorphic fungus Phialophora radicicola Cain and almost identical to Phialophora zeicola Deacon & Scott, whose description was originally based on isolates from South Africa and France. These species appear to represent the holomorph of the same fungus. The late wilt pathogen of maize, from India and Egypt, commonly known as Cephalosporium maydis Samra et al., but suggested as being the Phialophora anamorph of a Gaeumannomyces species, was closely related to other Gaeumannomyces species included in the tests.

Foliage construction cost (glucose requirement for formation of a unit foliar biomass, G, kg glu kg−1), chemical composition and morphology were examined along a light gradient across the canopies in five deciduous species, which ranked according to increasing shade-tolerance as Populus tremula <Fraxinus excelsior <Tilia cordata = Corylus avellana <Fagus sylvatica. Light conditions in the canopy were estimated by a hemispheric photographic technique, allowing ranking of sample locations according to long-term light input incident to the sampled leaves (relative irradiance). G and foliage carbon concentration increased with increasing relative irradiance in F. excelsior, T. cordata and C. avellana, but wereindependent of irradiance in F. sylvatica and P. tremula. However, if G of non-structural-carbohydrate-free dry mass was considered, it also increased with increasing relative irradiance in P. tremula. A positive correlation between the concentration of carbon-rich lignin and irradiance, probably a result of the acclimation to greater water stress at higher light, was the major reason for the light-dependence of G. Lignin concentrations were highest in more shade-tolerant species, resulting in greatest carbon concentrations in these species. Since carbon concentration and G are directly linked, the leaves of shade-tolerant species were also more expensive to construct. As the result of these effects, G increased faster with increasing leaf dry mass per area which was mainly determined by relative irradiance, in shade-tolerators. Given that shade-tolerant species had lower leaf dry mass per area at common irradiance and that this saturated at lower relative irradiance than leaf dry mass per area in the intolerant species, it was concluded that enhanced energy requirements for foliage construction might constrain species morphological plasticity and the upper limit of leaf dry mass per area attainable at high light.

Cherry seedlings (Prunus avium) were grown from seed for two growing seasons in three ambient [CO2] (∼350 μmol mol−1) and three elevated [CO2] (ambient+∼350 μmol mol−1) open-top chambers, and in three outside blocks. A drying cycle was imposed in both the growing seasons to half the seedlings: days 69–115 in the first growing season, and in the second growing season days 212–251 on the same seedlings which had already experienced drought. Stomatal conductance was significantly reduced in elevated [CO2]-grown, unstressed seedlings in both the first and second growing seasons, but was not caused by a decrease in stomatal density. Droughted seedlings showed little or no reduction in stomatal conductance in response to elevated [CO2]. However, stomatal conductance was highly correlated with soil water status. Photosynthetic rate increased significantly in response to elevated [CO2] in both water regimes, leading to improvement in instantaneous transpiration efficiency over the whole duration of the experiment, but there was no relationship between instantaneous transpiration efficiency and long-term water use efficiency. The Amax was strongly reduced in the second growing season, but unaffected by [CO2] treatment. Although photosynthetic rate was not down-regulated, Rubisco activity was decreased by elevated [CO2], possibly because of the increased leaf carbon: nitrogen ratio which had occurred by the ends of the two growing seasons. Elevated [CO2] did not improve plant water relations (for example, bulk leaf – water potential, osmotic potentials at full and zero turgor, relative water content at zero turgor, bulk modulus of elasticity of the cell) and thus did not increase water-stress tolerance of cherry seedlings.

For a number of generations, we followed the frequency, performance and fecundity of an initial mixture (50[ratio ]50) of triazine resistant (R) and susceptible (S) but otherwise near-isogenic lines of Brassica rapa (for 3 yr) and of Chenopodium album (for 2 yr) in neighbouring experimental garden plots, which differed in aspect (north-facing versus south-facing), shading, and transparent cover from precipitation. Each of eight treatments was replicated four times per species. Seed from each plot was kept separate and sown back in its plot of origin to provide the next generation. For both Brassica and Chenopodium, R frequency changed between generations. In Brassica R frequency declined consistently from an initial 0.5 and, after 2 yr, R plants had disappeared in all replicates in two treatments and averaged <0.126 in the remaining treatments. However, R frequencies in the smaller second generation samples in each year were less consistent. In Chenopodium, R frequency increased consistently from an initial 0.5 in all treatments and replicates during 1995 to average 0.772 on south-facing sites and 0.675 on north-facing sites. However, in 1996 Chenopodium R frequency did not change significantly from that in 1995 for any treatment. Brassica S plants were more fecund that R plants, except in shaded conditions, and south-facing plots produced significantly more S seeds than north-facing plots. Both Brassica plants and plots were more productive in 1996 than in 1995. By contrast, Chenopodium was much less productive in 1996, a season with more climatic extremes between plots than 1995. In 1995, R plants were more fecund than S plants in south-facing plots and in covered conditions, but this difference was not detected in 1996. South-facing Chenopodium plots were more productive than north-facing plots in 1995, but the reverse was the case in 1996. Cover resulted in an increase in seed production in Chenopodium on south-facing plots in 1995, but a decline in 1996. We conclude that selection might act against triazine-resistant plants in B. rapa under the climatic conditions in the UK, but in C. album, which is more susceptible to extreme conditions of light and water stress, triazine-resistant phenotypes might predominate in low stress cool moist conditions. We suggest that a relationship might exist between the overall climatic tolerance of a species, and climate thresholds beyond which herbicide-resistant phenotypes might spread. Thus, in the absence of herbicide, it is not inevitable that the frequency of R phenotypes will decline in field conditions.

Phialocephala fortinii is among the few identified hyphomycetes belonging to the Mycelium radicis atrovirens complex. This ‘dark-septate endophyte’ has a global distribution and colonizes a wide variety of host plants. In this study, the spatial distribution of discrete genets of P. fortinii on the forefront of a receding glacier was analysed using the randomly amplified polymorphic DNA (RAPD) technique to determine plants colonized and patterns of colonization. In two consecutive years of sampling, a total of 74 isolates of P. fortinii were obtained from nine plant species, typically ectomycorrhizal, ericoid mycorrhizal or non-mycorrhizal. The isolates showed substantial variation, sharing on average approx. half the RAPD markers. In the first year, three isolates belonging to a single genet were obtained from two plants separated by a distance of nearly 1.5 m. The continuity of this genet was assessed by a sampling the following year. No isolates similar to that, or any of the genets collected the year before were observed. Consequently, the identical isolates from the previous year were concluded to represent discontinuous ramets. Two additional large genets were observed during the second year of sampling, which inhabited roots of several plants representing three different species. These data suggest that the sharing of P. fortinii genets among plant species might play a fundamental role in adaptation and interaction within the whole plant community in a system undergoing primary succession.

The time course of initiation and development of root nodules was investigated in the South American actinorhizal shrub Discaria trinervis (Rhamnaceae). A local strain of Frankia (BCU110501) which was isolated from D. trinervis nodules, was used as inoculum. Inoculated seedlings were periodically studied under the light microscope after clearing with aqueous NaClO. In parallel, semithin and ultrathin sections were analysed by light and electron microscopy. Infection by Frankia BCU110501 involved intercellular penetration among epidermal and cortical root cells. Nodule primordia were detected from 6 d after inoculation, while bacteria were progressing through intercellular spaces of the outer layers of cortical cells. Invasion of host cells by the symbiont occurred 7–9 d after inoculation, and hypertrophy of the primordium cells was associated with Frankia penetration. Root hairs were not deformed during the early events of nodule formation. From 13 to 16 d after inoculation, the proximal cellular zone of the primordia behaved differently from the other tissues after NaClO treatment and remained darkly pigmented. At the same time, differentiation of Frankia vesicles started to occur inside already infected cells. By 16 d after inoculation, spherical vesicles of BCU110501 were homogeneously distributed in the host cells. These vesicles were septate and surrounded by void space. Frankia spores or sporangia were not observed in the nodule tissue. This study has clarified the mode of Frankia penetration in D. trinervis, one of the Rhamnaceae which also includes Ceanothus. The events involved in infection, nodule induction, host-cell infection and vesicle differentiation have been characterized and identified as time-segregated developmental processes in the ontogeny of D. trinervis root nodules.