Applying the branch-in-bag method, naturally seeded Scots pine (Pinus sylvestris L.) trees, 25–30 yr old, were subjected to two CO2 concentrations (350 and 700 μmol mol−1) and two soil-nitrogen-supply regimes for three growing seasons (1994–96). Gas exchange and chlorophyll a fluorescence in detached shoots were measured simultaneously in a diffuse radiation field. Elevated CO2 did not lead to a significant ‘downward regulation’ in the light-saturated rate of net photosynthesis (Pn.max), the maximum apparent quantum yield (αA.max) or the maximum photochemical efficiency (Fv/Fm) of photosystem II (PS II). However, the elevated CO2 significantly decreased the light-saturated stomatal conductance and increased the sensitivity of stomatal conductance to change in low photon-flux densities. The high soil–nitrogen supply significantly increased photosynthetic capacity, as manifested by increases in Pn.max, αA.max, Fv/Fm, and the effective photochemical efficiency (ΔF/F′m) at low photon-flux densities, did not, on the other hand, enhance the magnitude of photosynthetic response to elevated CO2 concentration. In addition, the treatment-induced modifications in fluorescence parameters are discussed in detail.

Carbodioxide uptake, oxygen evolution and chlorophyll fluorescence of leaves of Lobelia rhynchopetalum Hemsl., a giant rosette plant of the tropical alpine regions of Ethiopia, were studied under field conditions at 4000 m above sea level. Our objective was to investigate the photosynthetic adaptation to the combination of wide fluctuation in diurnal temperature, high photon flux densities (PFD) and low CO2 partial pressure encountered in these regions. At an ambient CO2 partial pressure of c. 17 Pa, maximal rates of CO2 uptake were low, ranging between 4 and 6 μmol m−2 s−1. Such rates, however, required high PFDs and were observed only at levels of 1500 μmol photons m−2 s−1. Carbon dioxide uptake was significantly inhibited when PFD was >2000 μmol photons m−2 s−1. On the other hand, at saturating CO2 levels, maximal photosynthetic oxygen evolution was higher (30 μmol O2 m−2 s−1), saturating at the same PFD as CO2 uptake. Quantum efficiency of CO2 uptake (0·006 mol CO2 mol photons−1, at high altitude and a low CO2 partial pressure of 17 Pa) and even of oxygen evolution under CO2-saturating conditions in the leaf O2 electrode (0·05 mol O2 mol photons−1) indicated reduced photosynthetic efficiency. Electron transport rate (ETR) was strongly correlated with the leaf temperature. Non-photochemical quenching (NPQ) responded inversely to leaf temperature and stomatal conductance.

The results indicated that in the morning, when the sun irradiates the partly frozen leaves with closed stomata, NPQ is the principal mechanism by which Lobelia leaves protect their photosynthetic apparatus. However, during the day, the predominant upright inclination of the leaves significantly contributes to protecting the leaves from excess light absorption. A comparison of the chlorophyll fluorescence of young and old leaves revealed that the former had high ETR and quantum efficiency of photosynthetic electron transport but a lower capacity for NPQ. Extremely high NPQ values but low ETR and low quantum efficiency were recorded for the old leaves. Thus, in the course of maturation the leaves apparently lose photosynthetic efficiency but increase their capability for protective non-photochemical quenching.

A study was conducted to quantify the ability of entrapped, monoxenically produced spores of an arbuscular mycorrhizal fungus to germinate and reproduce the fungal life cycle after cryopreservation. No germination was obtained after incubation of entrapped spores in glycerol and mannitol and subsequent cryopreservation at −70 °C, regardless of the concentration of cryoprotectants and duration of incubation. Incubation for 1 d in 0.5 M sucrose, and for 1 and 2 d in 0.5 M trehalose, led to spore germination after cryopreservation at −70 °C. Lower cryopreservation temperatures were tested with entrapped spores incubated for 1 d in 0.5 M trehalose. The highest germination rate, estimated by the percentage of potentially infective beads (%PIB), was obtained at −100 °C. A %PIB of 95% (water agar medium) to 100% (Strullu–Romand medium) was obtained at this temperature. Thereafter, %PIB rapidly decreased at −140 and −180 °C. Heavy sporulation and high internal root colonization were obtained after re-association of the entrapped spores, incubated for 1 d in 0.5 M trehalose and subsequently cryopreserved at −100 °C, with transformed carrot roots. This demonstrates the ability of entrapped spores to reproduce the fungal life cycle following cold treatment.

The objectives of this study were (i) to evaluate the relative involvement of fructans from leaf sheaths and from elongating leaf bases to regrowth after defoliation of Lolium perenne L. by following fructan exohydrolase (FEH), sucrose-sucrose fructosyl transferase (SST) and invertase (INV) activities and (ii) to examine whether gibberellins could be implicated in regulation of FEH activity. In stubble, 36% of fructans are located in leaf bases and 64% in leaf sheaths. During the first phase of regrowth, the depletion of carbohydrates was mainly due to the decline of fructans, which represented 76% and 50% of the carbohydrates mobilized from leaf sheaths and elongating leaf bases respectively. Despite a decrease in protein content, FEH activity increased 2·5-fold in leaf sheaths and six-fold in elongating leaf bases, so that the fructan-hydrolysing activity was four times higher in growing leaves than in leaf sheaths, 2 d after defoliation. INV activity also increased, whereas the pattern of SST activity was inversely related to the variations of both hydrolysing activities. SST activity, which is highest in growing leaves, declined approximately threefold in leaf sheaths and elongating leaf bases at the beginning of regrowth.

The increase in activity of FEH in stubble was strongly inhibited by an inhibitor of the biosynthesis of gibberellin, uniconazole. FEH activity was decreased to c. 67%, 45% and 33% of the level in nontreated plants 24, 48 and 72 h following defoliation, respectively. The inhibition could be overcome by a subsequent treatment with gibberellic acids (GAs). For the first time, data are provided to support the view that GAs might play a role in the regulation of FEH activity, and the implication of this result is discussed.

Geographically separated populations of Plantago major L. differ in ozone resistance, and this is correlated with the exposure to ozone at the location of each population. In addition, two populations of P. major have been demonstrated to show an increase in ozone resistance after summers when ozone concentrations were high. However, if evolution of ozone resistance has occurred in the field there must be appropriate heritable genetic variation and this must be demonstrated.

In the present study, artificial selection for ozone resistance and sensitivity was imposed on a resistant (‘Lullington Heath’) and sensitive (‘Bush’) population of P. major. Artificial selection is an efficient means of demonstrating additive genetic variance for a character. Selection was based on 2 wk growth in ozone (70 nl O3 l−1 for 7 h d−1), and selected lines were tested for ozone resistance under both short and long term ozone exposures. Changes in ozone resistance were demonstrated for each population. On the basis of a 2-wk screening test, selection from the initially sensitive Bush population led to a line that was significantly more resistant to ozone, but it was not possible to select a line with greater sensitivity than the original population. Conversely, selection from the initially resistant Lullington Heath population led to a line with increased sensitivity but not to a line with increased resistance. Differences in ozone resistance between the selected lines were maintained over the long term and were reflected in growth and seed production at the final harvest. Net assimilation rate, stomatal conductance and leaf pigments were measured. In both populations the lines selected for sensitivity showed a greater effect of ozone on net assimilation rate. Ozone had a minor effect on chlorophyll content in the Lullington Heath sensitive line but tended to increase carotenoid concentrations. However, this effect on carotenoids was not related to ozone resistance.

The experiment demonstrated that ozone resistance in P. major is heritable. The response to selection reported here supports previous evidence from field collections of a rapid evolution of ozone resistance.

Changes in polypeptide contents following inoculation of roots of Pisum sativum L. (wild type cv. Frisson (myc+, nod+)) with the arbuscular mycorrhizal fungus Glomus mosseae were analysed by two-dimensional gel electrophoresis (2D-PAGE). A different polypeptide pattern was obtained in a mycorrhiza-resistant pea genotype P2 (myc-, nod-) inoculated with G. mosseae. In order further to characterize the polypeptide modifications detected and to provide evidence for some possible symbiosis-related (SR) proteins, a time course experiment, from appressoria formation to fully developed symbiosis, was carried out on two genotypes allowing fungal colonization: the wild type and its isogenic mutant P56 (myc+, nod-). The same experiment was done with the mycorrhiza-resistant pea genotype (myc-, nod-). After G. mosseae inoculation, we characterized 12 additional polypeptides in the two mycorrhiza-compatible pea genotypes which were never observed in root extracts from the mycorrhiza-resistant mutant. Five polypeptides were first detected in the early stage of the symbiosis (5 d of inoculation) while others were observed later (8 d of inoculation). The induction and accumulation of these polypeptides seem to be more correlated to the establishment of the functional symbiosis than to the recognition stages and appressorium formation. Furthermore, none of the additional polypeptides were detected in the mycorrhiza-resistant pea genotype. This mutant was more characterized by a great repression of polypeptides. In addition, up-regulated and down-regulated polypeptides from the mycorrhiza-compatible genotypes were different from those of the mycorrhiza-resistant genotype.

The enzyme sucrose: sucrose 1-fructosyltransferase was partially purified from barley leaf growth zones. Four steps (ammonium sulphate precipitation and polyethylene glycol precipitation, followed by chromatography on Concanavalin A-sepharose and hydroxylapatite) yielded a 35-fold purification. The resulting preparation of 1-SST which still contained a number of different activities related to fructan metabolism, was subjected to preparative isoelectric focusing, and sections of the gel were analysed individually for 1-SST and related activities, using sucrose and 1-kestose as substrates. This procedure yielded a 196-fold purification and revealed the presence of two isozymes of 1-SST with pI values of 4.93 and 4.99, as determined by analytical isoelectric focusing of the corresponding fractions. Both isozymes produced glucose and 1-kestose when incubated with sucrose. In addition, small amounts of 6-kestose and tetrasaccharides were formed. In particular, one of the two 1-SST isozymes yielded fructose when incubated with 1-kestose, indicating that it also acts as a fructan exohydrolase. The other isozyme exhibited less fructan exohydrolase activity. Nystose was also degraded by the fructan exohydrolase activity but less than 1-kestose, whereas 6-kestose was not a substrate for the enzyme. Incubation of both 1-SSTs with different concentrations of sucrose showed that the enzyme was not saturated even at 500 mM. As for the barley sucrose: fructan 6-fructosyltransferase, both isozymes of 1-SST yielded two polypeptide bands of molecular weight 50 and 22 kDa upon sodium dodecylsulphate polyacrylamide gel electrophoresis, suggesting their close relationship to invertase (composed of two subunits of similar size), as previously reported for other plants.

Relatively little is known about the factors controlling ectomycorrhizal fungal communities. One possible factor is forest litter chemistry. In a series of experiments we demonstrated that the growth of ectomycorrhizal fungi able to colonize red pine (Pinus resinosa Ait.) are differentially affected by red pine needles and needle chemical components. For example, water extracts of pine needles stimulated the growth of Suillus intermedius (Smith & Thiers) Smith & Thiers and inhibited the growth of Amanita rubescens Pers. Catechin and epicatechin gallate, components of the water extract, acted similarly to the extract. The volatile compounds α- and β-pinene also had differential effects on the growth of the various species of ectomycorrhizal fungi. Our results suggest that forest litter chemistry has the potential differentially to affect the growth of ectomycorrhizal fungal species and so could affect the structure of ectomycorrhizal fungal communities.

Diel (24-h) time courses of CO2 exchange, water relations, and microclimate of the foliose lichen, Sticta tomentosa (Swartz) Ach., and responses to experimentally manipulated conditions were measured at a forest edge in a lower montane rainforest in Panama.

Similar to earlier observations on two other rain forest lichens, daily desiccation suppressed net photosynthesis (NP) during the period when irradiation was highest. Not surprisingly, the light response curves of NP showed saturation at rather low light levels. Rehydration was associated with an initial resaturation burst of short duration, which could be demonstrated both under natural conditions and experimentally. This additional loss of CO2 seems too low to be ecologically relevant. Moreover, high thallus hydration was also detrimental to NP: at maximum water content net CO2 uptake was depressed by >50%. Although NP was well adapted to the prevailing high temperatures, the latter also stimulated dark respiration. On average, almost 60% of the diurnal carbon gain was lost during the night.

In spite of these limitations, the integrated 24-h C gain was quite high, on average 0·5% of the thallus C content. Whilst these figures were determined for horizontally exposed samples, we also assessed the role of different exposures on photosynthetic performance. Diel C gain was highest under conditions of semi-shade (westerly exposure), which allowed long periods of activity, whilst much higher irradiance at other exposures could not be utilized for photosynthetic production: easterly exposed thalli dried out even faster than horizontally exposed samples.

Cuticular water permeance was manipulated in Corylus avellana L., Hypericum androsaemum L. and Populus tremula L. by (1) long-term application of low doses of various systemic herbicides inhibiting biosynthesis of cuticular waxes, (2) very short-term application of organic solvents to the leaf surface, and (3) exposure to natural strong winds. Treatment effects were very variable, but increased the natural range of permeances by a factor of 10 or so in undamaged leaves. All species had hypostomatous leaves. Relative change of leaf conductance (g) in response to stepwise increases of leaf-to-air water vapour pressure difference (VPD) was measured for individual leaves (Corylus) or groups of leaves at the shoot or branch tip. Adaxial cuticular water permeance (P) was determined for the same leaves after measurement of the VPD-response. A proportional measure of relative change of g with VPD, d(logeg)/dVPD, was then plotted against P. No increase in the strength of the closing response to increasing VPD was found with increasing P, as would have been expected if water loss through the cuticle was involved in stomatal response to changes in VPD via a direct effect on guard cell turgor. By contrast, high P coincided, most clearly in Corylus, with a reduced strength of the stomatal closing response to increasing VPD, i.e. less negative d(logeg)/dVPD. As the responses were non-linear, the value of d(logeg)/dVPD changed with VPD. With rising VPD, all three species and a fourth one previously studied showed a decline in the value of d[d(logeg)/dVPD]/d(log P), reaching negative values in one species. This is interpreted in terms of two independent and antagonistic effects of increased cuticular water permeance on guard cell response to VPD, one acting by reducing the backpressure exerted on guard cells by the epidermis, and the other one possibly causing greater depression of guard cell turgor through delivery of more chemical messengers (such as abscisic acid) to the guard cells with the cuticular transpiration stream.

Mycelial cord systems of Phanerochaete velutina (DC.: Pers.) Parmasto grown from 4 cm3 inocula on a nutrient-depleted non-sterile soil in compartmentalized laboratory microcosms were baited after 13 d of growth with either fresh, non-sterile 4 cm3 wood blocks or control Perspex® blocks of the same contact area. After 112 d, mature mycelial systems, which were in senescent phase, were subjected to disturbance by supplying a new fresh wood bait diametrically opposite the existing bait and, after 126 d, to nutrient regime amendment by application of NPK solution. At harvest (186 d) there was a significant (P[les ]0·001) linear relationship between extra-resource mycelial biomass and total wood decay over the preceding 74 d. Nutrient amendment alone did not significantly (P>0·05) affect extra-resource mycelial biomass production or wood decay rates in either disturbed or undisturbed cord systems. However, both mycelial biomass production and resource decay were significantly (P[les ]0·05) enhanced when nutrient amendment and disturbance treatments were applied concurrently. Bi-directional phosphorus translocation to inocula and wood baits (determined non-destructively) was assessed by labelling the NPK solution applied distal or proximal to the initially supplied bait with 32P. In both disturbed and undisturbed cord systems the rate of 32P uptake from a local supply was two orders of magnitude higher than from a distal supply. In disturbed cord systems uptake of 32P by inocula, which were midway between the two radiotracer supply points, was significantly (P[les ]0·05) higher when the supply point contained a newly supplied wood bait. Net translocation of 32P to newly supplied wood baits increased with time at the expense of translocation to inocula and existing wood baits. The switch in direction of net phosphorus translocation, the importance of localized nutrient scavenging and the partitioning of wood decay are discussed in relation to the ecological significance of mycelial cords.

In four field and glasshouse experiments designed to alter the supply of resources through manipulation of nutrients, root tissue, leaf area and fruit number in Alliaria petiolata (Brassicaceae), more than 99% of ovules per plant showed signs of fertilization, suggesting that seed production in this plant was not pollen limited. However, in all treatments a significant proportion of fruits and seeds did not develop to maturity. Total fruit and seed production did not differ significantly from controls when plants were given nutrient supplements at flowering. Removal of 50–75% of the root tissue in 1-yr-old plants significantly reduced fruit set, but had no effect on individual seed development. Removal of cauline leaves significantly reduced most measures of fruit and seed production, suggesting that current photosynthate is critical for fruit and seed filling. Seed maturation was significantly affected by both fruit position within an infructescence and ovule position within a fruit. Basally located fruits and ovules (within fruits) developed more mature seeds than distally positioned fruits and ovules. Plants responded to removal of basal fruits by re-allocating resources to distal fruits that would normally have aborted. Our results suggest that fruits and seeds act as reproductive sinks competing for parental photosynthate. Patterns of resource allocation within infructescences and fruits were also modified by our experiments.

The composition and morphology of leaves exposed to elevated [CO2] usually change so that the leaf nitrogen (N) per unit dry mass decreases and the leaf dry mass per unit area increases. However, at ambient [CO2], leaves with a high leaf dry mass per unit area usually have low leaf N per unit dry mass. Whether the changes in leaf properties induced by elevated [CO2] follow the same overall pattern as that at ambient [CO2] has not previously been addressed. Here we address this issue by using leaf measurements made at ambient [CO2] to develop an empirical model of the composition and morphology of leaves. Predictions from that model are then compared with a global database of leaf measurements made at ambient [CO2]. Those predictions are also compared with measurements showing the impact of elevated [CO2]. In the empirical model both the leaf dry mass and liquid mass per unit area are positively correlated with leaf thickness, whereas the mass of C per unit dry mass and the mass of N per unit liquid mass are constant. Consequently, both the N[ratio ]C ratio and the surface area[ratio ]volume ratio of leaves are positively correlated with the liquid content. Predictions from that model were consistent with measurements of leaf properties made at ambient [CO2] from around the world. The changes induced by elevated [CO2] follow the same overall trajectory. It is concluded that elevated [CO2] enhances the rate at which dry matter is accumulated but the overall trajectory of leaf development is conserved.

Previous work in the Colorado alpine ecosystem has shown that amino acids are a potentially important N source for the sedge, Kobresia myosuroides. This plant is the only known sedge to harbour associations with ectomycorrhizal fungi. The aim of the present work was to test the hypothesis that these ectomycorrhizas transfer N from amino acids in the soil solution to the host plant, and thereby have an important role in the N nutrition of this species. We used a two-chamber system (rhizoboxes) in which K. myosuroides plants were separated from a soil chamber by nylon mesh that allowed fungal hyphae, but not plant roots, to cross it. Injections of [15N, 2-13C]glycine were made into the soil chamber. The hyphal crossings on half of the rhizoboxes were regularly disrupted to control for leakage of label across the barrier. Plants in the intact rhizoboxes showed significantly higher 15N enrichment than those in controls, and mycorrhizal root tips were significantly more enriched than bulk roots. The mycorrhizas transferred an average of 1.3% of the added 15N label to plants, a figure comparable to those obtained in previous studies in which plant roots were directly exposed to label. We conclude that fungal associations have an important role in the N nutrition of K. myosuroides by transferring N from amino acids to their hosts.

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.

Mycelial cord systems, up to 50-cm diameter, of the basidiomycete Phanerochaete velutina (DC.: Pers.) Parmasto, a common woodland saprotroph, grown on non-sterile soil in model laboratory microcosms were baited, after 27 d, with pairs of fresh beech wood blocks (baits), placed at 10 d intervals behind the foraging colony margin. System development was quantified by image analysis. Mean radial extent and hyphal cover increased linearly with time until day 21, but declined before the mycelial systems reached the edges of the laboratory microcosms. The mass (DBM) and border (DBS) fractal dimensions of the mycelial systems changed with time but the ratio DBM[ratio ]DBS became constant after 14 d. A separate central compartment containing the inoculum was supplied with 32P orthophosphate and its translocation to wood baits monitored non-destructively for 73 d. Whilst total 32P acquisition by wood baits increased linearly with time, the proportion of total allocated to baits varied significantly both temporally and according to the length of time that baits had been in contact with the mycelium. Most recently supplied wood baits were not the main sink for supplied phosphorus; rather, the rate of 32P acquisition was initially greatest in baits from which egress of the fungus had already occurred. The rate of 32P acquisition by the most recently added baits increased with time, supported by efflux from other wood baits, which had initially been the main sinks for translocated phosphorus. The results raise important questions about the ecological and functional significance of nutrient partitioning in cord systems and imply that ‘observed’ translocation, rather than being an absolute measure, indicates the degree to which phosphorus is loaded from a translocation stream in regions where it is being actively utilized and/or stored.

In the monocarpic perennial Cynoglossum officinale L. the probability of flowering is related to the size of the plant. In previous work it was observed that this relation varies between years. We hypothesized that variable conditions during the winter, the period of vernalization, explain this variation.

We collected plants from the field in autumn and placed these under different simulated winter conditions in a climate room. In contrast to our hypothesis, the probability of initiating flowering at a given size was not affected by: (a) the temperature during the cold period, (b) the duration of the cold period, or (c) the application of a plant hormone (GA3) or an inhibitor of gibberellin synthesis (paclobutazol) during the cold period. Winter cold is not necessary for floral initiation, and is only required for elongation of the inflorescence. It is unlikely that winter temperature affects the fraction of plants flowering.

Subsequent morphological investigation of flower development in material collected in the field showed that large plants had primordial inflorescences well before vernalization, sometimes as early as August. In plants grown from seeds under constant conditions in a climate room, the probability of initiating the inflorescence differed for plants grown at various temperatures (34·1% at 15°C, 100% at 20°C, and 95% at 25°C). Our results suggest that environmental conditions in August and September, up to 10 months before actual flowering, could affect the fraction of flowering plants.

Mean δ15N of whole-plants of lettuce and barley varied by [les ]3‰ when given a chemically and isotopically uniform N source. This variation was related to the presence, absence and species of arbuscular mycorrhizal (AM) fungi and to external N concentration. A highly AM-susceptible plant (lettuce) responded to treatments differently than a less susceptible one (barley). The largest change in whole plant δ15N was related to the experimental combination most likely to be found in field conditions: species of fungus interacting with varying external concentrations of N.

The mechanisms underlying observed variations of plant δ15N are not understood, nor can they be approached directly using δ15N. However, descriptive data, such as presented here, are important to the development of a mechanistic model, because they suggest relationships for future research, using inter alia15N-enriched tracers. They also confirm that plant N sources, cannot be identified using plant δ15N, even when the type of mycorrhizal association (endo vs. ecto) is known.

Roots of clonal birches (Betula pendula) were inoculated with the ectomycorrhizal fungi Paxillus involutus (isolates P0 and Mi) and Hebeloma cylindrosporum (strains D1 and D105). These fungi showed different rates of mycorrhiza formation in vitro. Mature mycorrhizas were obtained after only 2–4 d with H. cylindrosporum, whereas 6–8 d were necessary with P. involutus isolate P0, and P. involutus isolate Mi was not able to form mature mycorrhiza during the 10 d of the experiment. Temporal changes in PAL activity and the expression of genes encoding intracellular pathogenesis-related proteins were followed after inoculating birch roots with these fungi. Transient increase of PAL activity, and transient induction of expression of the wound-inducible Bet v1-SC1 gene, were observed in roots challenged with both H. cylindrosporum strains and the P. involutus isolate P0. These changes were found to coincide with hyphal penetration between root cells during Hartig net formation, and were never observed in roots inoculated with the poorly aggressive P. involutus isolate Mi. Examination of mycorrhizal root sections under u.v. light indicated the presence of phenolic compounds in the host cell walls at the vicinity of the Hartig net. These results strongly suggest that hyphal penetration between the root cells triggers a transient defence response which, in turn, could limit Hartig net formation to the outer layer of the root cortex.

In the present work we compare responses to hypoxia of the carbohydrate content of related plant species, which grow naturally on sites prone to flooding (Senecio aquaticus Hill., Myosotis palustris (L.) Lehm. Rchb.) with plants from habitats with only a low risk of oxygen shortage (Senecio jacobaea L., Myosotis arvensis (L.) Hill.). Whilst the amounts of glucose, fructose and sucrose changed only slightly following hypoxia and peaked at a maximum approximately double that of the aerated control specimens, the fructan content increased fivefold to tenfold. In nearly all the plants studied, fructan became the main polysaccharide reserve. For example, the flooding-tolerant Senecio aquaticus accumulated the highest amount, in particular fructans with a degree of polymerization up to 35 compared with 10 under control conditions. Nearly 70% of the soluble carbohydrates were fructans, compared with 30% under aerated conditions.

In all species tested the starch fraction marginally increased or remained constant.

Fructans were found to accumulate as a response to oxygen deficiency in both flooding-tolerant and intolerant species but with higher absolute values and ratios between fructan and starch in the flooding-tolerant species. At 24 h after the onset of hypoxic treatment, the sugar content rose, in spite of the diminished photosynthetic rates. The ability to accumulate fructans seems to vary in plants coping with oxygen shortage but could be interpreted as advantageous in comparison to starch synthesis in terms of (i) the location of the fructan metabolism in the vacuoles, with no negative feedback in the photosynthetic apparatus and (ii) the possibility of storage of sucrose in the form of fructose polymers without the intermediate stages and energy-using processes which are indispensable for starch synthesis. These results might indicate that fructan metabolism could play a role in the tolerance of oxygen deficiency.