A great diversity of plants and fungi engage in mycorrhizal associations. In natural habitats, and in an ecologically meaningful time span, these associations have evolved to improve the fitness of both plant and fungal symbionts. In systems managed by humans, mycorrhizal associations often improve plant productivity, but this is not always the case. Mycorrhizal fungi might be considered to be parasitic on plants when net cost of the symbiosis exceeds net benefits. Parasitism can be developmentally induced, environmentally induced, or possibly genotypically induced. Morphological, phenological, and physiological characteristics of the symbionts influence the functioning of mycorrhizas at an individual scale. Biotic and abiotic factors at the rhizosphere, community, and ecosystem scales further mediate mycorrhizal functioning. Despite the complexity of mycorrhizal associations, it might be possible to construct predictive models of mycorrhizal functioning. These models will need to incorporate variables and parameters that account for differences in plant responses to, and control of, mycorrhizal fungi, and differences in fungal effects on, and responses to, the plant. Developing and testing quantitative models of mycorrhizal functioning in the real world requires creative experimental manipulations and measurements. This work will be facilitated by recent advances in molecular and biochemical techniques. A greater understanding of how mycorrhizas function in complex natural systems is a prerequisite to managing them in agriculture, forestry, and restoration.

Plants of Chondrus crispus Stackhouse, collected from mid-littoral tidepools, were treated as follows. Some plants were kept for a few weeks under controlled starvation conditions in order to decrease their initial content of total tissue phosphorus, then incubated for up to 48 h in phosphorus (15 μM) and nitrogen (25 μM) enriched sea water. Other plants were directly incubated in enriched sea water. Chemical analyses showed that the total phosphorus content of fresh and starved plants remained stable, reflecting the nutritional status of the plants. The predominant acid-soluble phosphate fraction was larger in fresh than in starved plants. The content of acid-soluble polyphosphates, similar in both types of plants at the beginning of the experiment, doubled in starved plants, and increased by a factor of 2·7 in fresh plants, over 48 h. The content of acid-insoluble polyphosphates was lower than that of acid-soluble polyphosphates.

Transmission electron microscopy and energy dispersive X-ray microanalysis confirmed the presence, mostly in medullary cells, of acid-insoluble polyphosphates in the form of cytoplasmic granules and precipitates along the plasmalemma, particularly near pit plugs. This is the first report of such phosphorus storage structures in a red macroalgal species.

In two open, flow-through systems 11 lichen species were fumigated with concentrations between 0·036 and 2·0 ppm (c. 94·3–5240 μg m−3) SO2. Sulphur dioxide uptake was monitored concurrently as a mass balance between incoming and outgoing SO2 concentrations. The rate of uptake ranged from 0·025 (±0·006) to 0·7 (±0·16) nmol SO2 g−1 d. wt s−1, for all species. After the first hour of fumigation the uptake rate was almost constant during the following 5 h for concentrations up to 1·0 ppm SO2. At higher concentrations the uptake declined over time continuously. Within the range of measured SO2 concentrations the uptake rate was linearly correlated with SO2 concentrations (r2=0·90). Thalli which were killed by heat treatment or in which respiration was inhibited by azide treatment did not show significantly different SO2 uptake from living thalli. No differences in uptake were found during fumigation in the light versus in the dark.

The potential role of sclerophylly (leaf hardness and rigidity) in the control of leaf dehydration and rehydration was investigated in two sclerophylls (Viburnum tinus and Ilex aquifolium) and two non-sclerophylls (Hedera helix ssp. helix and Sambucus nigra). After leaves were dehydrated in the pressure chamber, water transport from the apoplast (mainly consisting of xylem conduits and mechanical cells) to symplast was detected 15 min from pressure release, in terms of a spontaneous increase in leaf water potential (Ψ1). This Ψ1 increase was much larger in sclerophylls than in non-sclerophylls.

Positive pressures applied to leaves simulated the tensions developing in the leaf apoplast under water stress conditions, causing water to be expelled from leaf xylem conduits and mechanical cells and transferred to the leaf symplast, thus leading to symplast rehydration and to the consequent Ψ1 increase.

No correlation was found between the leaf modulus of elasticity at full turgor or the degree of sclerophylly (in terms of the ratio of leaf d. wt to surface area) and the characteristic rehydration time of the leaves, i.e. between the two main parameters expressing the rigidity of the leaf blade and the rate of leaf rehydration. However, when changes in Ψ1 were measured as a function of leaf water deficit (RWD), equal Ψ1s corresponded with larger RWDs during leaf rehydration than during leaf dehydration in the two non-sclerophylls. In particular, the two sclerophylls showed rehydration of their leaf apoplast and symplast completely and simultaneously. By contrast, the two non-sclerophylls showed a persisting water loss, localized, it is likely, in their xylem conduits and mechanical cells.

In other words, the two sclerophylls did not recover from water loss more rapidly than non-sclerophylls but they recovered from xylem cavitation more completely. The major elasticity of the cavitation strain shown by the two sclerophylls studied was interpreted as of advantage to plants subjected to diurnal large drops in Ψ1 followed by nocturnal recovery. This is the case in Mediterranean sclerophylls growing in areas characterized by high humidity of the air condensing on the soil at night. The same mechanism of cavitation recovery, however, would be useless in very xeric areas.

The hypothesis is advanced that sclerophylly of Mediterranean species may derive from similar anatomical structures developed in species formerly adapted to more humid environments which later migrated to more arid zones.

A dry soil is generally a hard soil. Thus, the effects of water stress and mechanical impedance on plant growth are difficult to separate. To achieve this we have developed a growth cell that allows manipulation of the strength of growth media (i.e. mechanical impedance) without altering the availability of water or nutrients. We monitored leaf elongation rates of barley and wheat seedlings before and after the mechanical impedance to root growth was increased. Results show that a large and rapid reduction (within 10 min) of leaf elongation rates occurred after impedance to the roots was increased. The average reductions for barley and wheat, with associated standard errors, were 22·6% (4·84) and 36·2% (5·48), respectively. The data are consistent with the hypothesis that mechanical impedance of roots might have a direct negative effect on leaf growth even where nutrients and water are in plentiful supply to the plant. The implications of the rate of the response are examined with respect to the underlying mechanisms controlling root–shoot signalling.

Topology is an important component of the architecture of whole root systems. Unfortunately, most commonly applied indices used for characterizing topology are poorly correlated with one another and thus reflect different aspects of topology. In order to understand better how different methods of characterizing topology vary, this paper presents an exploration of several different methods for assigning order within branched root systems on the basis of (a) developmental (centrifugal) vs. functional (centripetal) ordering sequences and (b) whether orders are assigned to individual links or groups of adjacent links (segments). For each ordering system, patterns of scaling in relation to various aspects of link and segment size are explored using regression analyses. Segment-based ordering systems resulted in better fits for simple scaling relationships with size, but these patterns varied between developmental vs. functional ordering as well as the different size metrics examined. The functional (centripetal), link-based ordering system showed complex, non-linear scaling in relation to numbers of links per order. Using a simple simulation model of root growth, it is demonstrated that this method of characterizing root topology in relation to root size might be a more powerful tool for characterizing root system architecture than in the use of simple, single-index characterizations of topology.

Traits associated with leaf energy balance are especially important for desert plants because decoupling leaf and ambient temperatures can be critical for maximizing productivity and survival in these hot, dry regions. Deserts are also one of the most climatically unpredictable and heterogeneous biomes; thus, variation of energy balance characters due either to plasticity or genetic differentiation might also be crucial to the success of desert plant species. In a common garden environment we examined variation in the traits associated with energy balance and productivity in Encelia farinosa A. Gray (brittlebush) plants. Comparing two populations from contrasting rainfall and drought climates, we found that plants from the wetter site (Superior, Arizona) always maintained higher leaf absorptances than plants from the more xeric region (Oatman, Arizona) when at similar water potentials. Superior plants also increased stomatal conductance and photosynthesis in response to mid-spring rainfall, whereas Oatman plants did not. Oatman plants, however, tended to have greater leaf areas and continuous spring growth, which made them larger than Superior plants, yet both populations produced the same number of flower heads. The differences for these traits, and the associations among them, agree with predictions based on the contrasting drought and rainfall environments of these two populations. Our results suggest that the differences might represent alternative suites of characters of selective importance for maximizing carbon gain over the climatically variable range of this widespread species. Furthermore, these differences, detected in common environment conditions, might have a genetic basis, which might contribute to a greater potential for local adaptation in this species.

The potential to improve the salt tolerance of white clover (Trifolium repens L.) was evaluated in populations developed by selecting plants within the cv. Haifa which had high and low concentrations of Cl− in the shoots. Under saline conditions (40 mm NaCl), the low-Cl populations consistently maintained lower concentrations of both Na and Cl in all plant parts and produced more dry matter than the original parent cultivar (Haifa), or the high-Cl populations. Under non-saline conditions, there were no differences between populations for these characters. In young seedlings, concentrations of Cl− in the shoots of the low-Cl and high-Cl populations started to diverge after 4 d exposure to saline conditions and there were significant differences between populations after 6 d growth at 40 mm NaCl. Results using 36Cl suggested that the salt-tolerant low Cl population was better able to retain 36Cl in the roots and to limit its translocation to the leaves and petioles than the high-Cl population. Realized heritability values for shoot Cl− concentration were found to be moderate in the first cycle of selection (0·24 and 0·37 for high and low Cl− concentrations respectively), but were substantially lower (0·10 and 0·09 respectively) in the second selection cycle. We propose that it is possible to increase levels of salt tolerance in white clover by selecting for low shoot Cl− concentrations under saline conditions and that this criterion could be used at very early stages of exposure to NaCl (i.e. day 4 or 6).

Plants known as hyperaccumulators take up and sequester high concentrations of potentially toxic elements from metalliferous soils. We tested the hypothesis that zinc hyperaccumulation benefits plants by deterring herbivory. In laboratory feeding trials, three species of herbivores were allowed to choose between Thlaspi caerulescens (Brassicaceae) plants grown in low-Zn and Zn-amended culture solution. Locusts (Schistocerca gregaria), slugs (Deroceras caruanae), and caterpillars (Pieris brassicae) all showed significant preferences for plants with lower foliar Zn concentrations. Such differential feeding could result in selection pressures favouring the evolution of hyperaccumulation. The findings are also relevant to current proposals to exploit hyperaccumulation as a means of remediating metal-contaminated soils.

In Plantago lanceolata L. the effect of Cu(II) additions to the nutrient solution on root-associated Fe(III) reductase was studied in a factorial design with different Cu(II) and Fe(III) concentrations. Iron starvation resulted in approx. an eightfold increase in root Fe reduction at the level of intact plants and twofold enhancement in the specific activity of both NADH-linked FeEDTA reductase and H+-ATPase in isolated root plasma membrane vesicles. In plants exposed to low (0·3–0·7 μM) Cu and suboptimal Fe levels, reduction activity at the root surface was further increased and associated with more severe interveinal chlorosis than plants grown in Cu-free medium. In Fe-sufficient plants, withholding Cu over a prolonged period slightly enhanced the reduction activity.

Addition of high (5 μM) Cu concentrations to Fe-free medium inhibited the induction of the physiological responses by Fe-deficiency stress. In plants without Fe supply but with adequate Cu supply, short-term application of 5 μM CuSO4 completely inhibited the reduction activity. Neither incubation of the plasma membrane vesicles before measurement nor incubation of intact plants with Cu before isolation caused a significant decrease in reductase activity. The results are interpreted as indicating different mechanisms underlying Cu-induced alterations in iron nutrition.

Chickweed plants (Stellaria media Vill.), either inoculated with cucumber mosaic virus (CMV) or not, were grown to reproductive maturity at a range of densities, both in monoculture and in mixture. CMV infection did not influence the vegetative growth of plants grown in monoculture. Similarly, the total vegetative production of a population containing infected and healthy plants was the same as that of a monoculture of healthy plants. However, infected plants were disadvantaged when in competition with healthy plants, the disadvantage increasing with density and the proportion of healthy plants. Investment in sexual reproduction was reduced in infected plants at all densities and in healthy plants at high densities. In very hard conditions, plants did not even produce seeds.

Water contents in healthy xylem, reaction zones (column boundary layers) and decayed wood were determined in sycamore (Acer pseudoplatanus L.) naturally infected by sooty bark disease (Cryptostroma corticale (Ellis & Everh.) Gregory & Waller) or wound-inoculated with Ustulina deusta (Fr.) Petrak. or Ganoderma adspersum (Schulz) Donk. Water contents determined gravimetrically were greater in reaction zones than in healthy wood by factors of 1·1–2·1 times. Maps of imageable proton density (M0), essentially equivalent to relative water contents, were also calculated from two independent nuclear magnetic resonance (NMR) imaging (MRI) data sets, acquired from T1 or T2-weighted experiments. M0 values derived from these, corrected for T2 and T1 respectively, were in good agreement, and paralleled the results obtained when total water was determined gravimetrically, although the proportional increase in M0 in reaction zones (1·3–3·7 times) was greater than the corresponding increase in total water.

Living wood saturated with water by vacuum infiltration contained approx. 1·3 times more water, determined gravimetrically, than uninfiltrated wood. The proportional increase in M0 was greater (1·8–2·4 times). This was attributable to the alleviation of magnetic susceptibility effects by elimination of the gas–water interfaces normally present in the uninfiltrated wood. The similarities between infiltrated wood and reaction zone tissues in both total gravimetric water content and NMR-visible water suggest that the biophysical environment of the water might be similar in both. This is compatible with the hypothesis that in reaction zones the gases normally present in the lumens of empty (dead) xylem fibres are replaced by an aqueous solution of phytoalexin-like compounds. This has been confirmed by microscopy. The implications of this in relation to antimicrobial defence and xylem function are discussed.

Although contrast in NMR images of lesions was predominantly attributable to changes in M0, a decrease in the spin–spin relaxation time (T2) was seen in the vicinity of some lesions. Spin–lattice relaxation times (T1) were not altered to the same extent.

Results obtained from both early and late stages of decay were much more variable than those from healthy wood or from reaction zones, and reflected both the low water content of decayed wood (and hence poor signal-to-noise ratios for NMR data) and the changing physical environment as the wood was degraded.

Living epidermal cells of cowpea (Vigna unguiculata (L.) Walp.) were examined by contrast-enhanced video microscopy during penetration by invasion hyphae of the monokaryotic stage of the cowpea rust fungus (Uromyces vignae Barclay race 1). In resistant or susceptible host cvs, the plant nucleus migrated to the penetration site before the fungus had fully penetrated the plant wall and, at sites of unsuccessful infection, remained during the formation of a callose-containing papilla. Nuclear migration was also induced by applying hemicellulase, but not H2O2, to localized sites of wall damage. Hemicellulase-induced migration was inhibited by calcium chelators and a protein kinase inhibitor, but not catalase. In both resistant and susceptible cvs, the plant nucleus migrated away from successful infection sites at about the time that the fungal penetration peg made contact with the plant plasma membrane, and the epidermal cell showed no further cytological responses to the growth of the fungal intra-epidermal vesicle. In the susceptible cv., the nucleus migrated back to the fungus when the latter initiated tip growth. Inhibitors of transcription or translation did not affect this migration and only slightly reduced fungal growth. In the resistant cv. in which the invaded cell exhibited a hypersensitive response (HR), the plant nucleus changed its appearance before the cessation of cytoplasmic streaming and usually did not migrate to the fungus, even if the latter initiated tip growth. Nuclear DNA cleavage usually followed the subsequent cessation of cytoplasmic streaming. Treatments that delayed cell death and increased fungal growth also increased the frequency of nuclear migration to the fungus. It is argued that these and other data suggest that U. vignae negates nonspecific, penetration-induced, defence responses upon entering cells of both susceptible and resistant cultivars. The results also suggest that effects on the plant nucleus are one of the earliest signs of the HR in this system, often preceding the cessation of cytoplasmic streaming and detectable changes in plasma membrane permeability.

Microtubule arrays were studied in Spiranthes sinensis (Pers.) Ames protocorms cultured in vitro either asymbiotically or symbiotically with the fungus Ceratobasidium cornigerum (Bourdot) Rogers by localizing β-tubulin in hand sections observed with laser scanning confocal microscopy. Cortical microtubules present in uncolonized cells disappeared when cells became colonized by the fungus. However, microtubules were observed between the hyphae forming hyphal coils (pelotons) in colonized cells. In these cells a close relationship between pelotons, microtubules and nuclei was often observed, and microtubules associated with hyphae formed a net-like structure through the pelotons. During senescence and condensation of the pelotons, microtubules were observed surrounding the pelotons, between collapsing hyphae and as circular profiles within the collapsed hyphal masses. In cells which contained completely collapsed hyphal masses and which were not recolonized, cortical microtubules reappeared and continued from the cell periphery through the hyphal remains to the collapsed hyphal masses. Microtubules associated with hyphae seemed to be involved in the condensation of hyphal masses in the host cells. Microtubules were also observed within hyphae, except in those which were collapsing or had collapsed.

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.

Leaves of a mottled stripe disease-susceptible cultivar (B-4362) and of a mottled stripe disease-resistant cultivar (SP 70–1143) of sugar cane (interspecific hybrids of Saccharum) were inoculated with the diazotrophic endophytes, Herbaspirillum rubrisubalbicans or Herbaspirillum seropedicae, via injection into the apex of the stem. At 7 and 20 d.a.i., H. seropedicae could be isolated only from a small necrotic area around the point of inoculation, where there was considerable degradation of host cells, and was not detected in any other part of the leaves. This suggested a hypersensitive response by the host to this bacterium, and no disease symptoms formed on either cultivar. By contrast, H. rubrisubalbicans could be re-isolated from throughout the infected leaves of both cultivars at both harvests and produced widespread disease symptoms on the leaves of cv. B-4362.

Symptoms consisted of necrotic regions near the point of inoculation, and red stripes and red patches along the vertical axis of the leaves, where the bacteria had spread in the primary and secondary veins. The xylem-conducting elements in diseased regions of leaves were filled with bacteria and, at the edges of disease symptoms, the vessels were filled with a gum which stained blue-green with toluidine blue. This material probably contained phenolic compounds, and was produced as a host defence response. Leaves of cv. SP 70–1143 only developed small red stripes near the point of inoculation. These symptoms did not spread along the leaves, and the infected xylem vessels were never seen to be completely full of bacteria. Instead, the vessels contained encapsulated bacterial colonies attached to secondary wall deposition; these colonies were surrounded by blue-green material that might have been host-defence gums. In cv. B-4362, bacteria were abundant in the intercellular spaces of mesophyll adjacent to infected xylem, and also filled sub-stomatal cavities. Immunogold labelling using polyclonal antisera raised against H. rubrisubalbicans gave a weak signal with the bacteria in cv. SP 70–1143, showing that few binding sites were available to the antibodies. By contrast, bacteria in cv. B-4362 reacted strongly with the antibody, suggesting that they had a denser coating of immunoreactive mucus. In the later stages of infection of cv. B-4362, lysed bacteria were seen within degraded plant cells surrounded by a matrix of plant gums and bacterial mucus. This matrix reacted strongly to the H. rubrisubalbicans antibodies. Immunogold labelling using antibodies against nitrogenase component II showed that nitrogenase was expressed by bacteria in the early stages of infection of cv. B-4362, but not in later stages, or by bacteria infecting cv. SP 70–1143.

The presence of host plants was found to be a key determinant of soil infectivity for endophytes of epacrids growing in south-west Australian jarrah forest carrying a sparse epacrid understorey. A comparable Banksia woodland with higher density of epacrids showed more uniform levels of infective endophytes, presumably because of closer overlap of host-supported patches of endophyte. Disturbed sites in jarrah forest, in which topsoil had been recently returned following bauxite mining, initially showed extremely low endophyte infectivity, probably due to disruption of the hyphal network, absence of host plants and/or increased competition from soil micro-organisms antagonistic to the endophytes. As the system stabilized and epacrids recruited and grew, inoculum potential of the soil re-established and by 12 yr it equalled that of adjacent undisturbed native forest. At disturbed sites with sparsely distributed adult members of the Epacridaceae, endophyte inoculum proved to be adequate only directly adjacent to the epacrids and declined steeply to negligible levels at only 40 cm radially distant from a host plant. The significance of live epacrid root systems on survival of endophyte inoculum in Banksia woodland was examined following removal of shoots of adult plants of the endemic epacrid, Leucopogon conostephioides DC. Coincident with demise of roots of the detopped plants, endophyte inoculum potential of closely adjacent soil declined by 50% during the growing season. However, the effect was short lived and infectivity rose the following year to equal that of undisturbed adjacent woodland as mycelial matrices supported by neighbouring epacrids invaded the depleted study areas.

Abuscular mycorrhizal fungi are thought to survive adverse environmental conditions primarily as spores. Extraradical mycelia of two Glomus species were produced in fine mesh pouches which excluded roots but not hyphae. The mycelia in these pouches were exposed to freezing conditions, either in the field or in a controlled-temperature chamber. Bioassay plants were grown directly in the pouches and mycorrhizal colonization was assessed after 1 month. The mycelia remained infective in frozen soil over winter. This survival was not dependent on either the presence of root pieces or on the connection of mycelia to roots. Spores were not an effective inoculum in these bioassays. Overwinter survival of mycelia would enable plants to become incorporated into functional mycorrhizal associations early in spring.

This study was conducted to determine whether xylem abscisic acid (ABA) concentration is altered by mycorrhizal symbiosis of cowpea plants grown in drying soil, and to determine whether stomatal sensitivity to xylem ABA is altered by the symbiosis. We allowed the entire root zone to dry and found that at high soil water contents (θ), mycorrhizal plants had higher stomatal conductance (gs) and lower xylem [ABA] than did non-mycorrhizal plants, but the difference disappeared at low θ, probably because of stomatal closure. The altered gs and xylem [ABA] were apparently not related to plant water status since shoot water potential, xylem sap osmotic potential and shoot water content were similar for mycorrhizal and non-mycorrhizal plants across the range of soil moisture. These differences were also not related to P nutrition or plant size. The relationship of gs to xylem [ABA] was not affected by the symbiosis, indicating that either stomatal sensitivity to xylem ABA was not affected by other xylem constituents, or that more than one xylem constituent was altered by the symbiosis but was offsetting in its effect on gs. We conclude that the symbiosis altered gs non-hydraulically, and that the factor might be xylem ABA.

Seedlings of Pinus sylvestris L. were grown in Plexiglas® observation chambers in limed (CaCO3, pH5·0 and 5·9) and untreated (pH 4·1) peat. The seedlings were either colonized by the mycorrhizal fungus Paxillus involutus (Batsch: Fr.) Fr. Or were non-mycorrhizal. After 18 wk in the observation chambers, 15N-labelled organic nitrogen, as lyophilized and ground mycelium of Suillus variegatus (Swartz: Fr.) O. Kuntze, or ammonium, was added to the peat. The plants were harvested after an uptake period of 14 d.

Irrespective of the nitrogen form added, liming decreased both the content and concentration of 15N in non-mycorrhizal plants, and, to a lesser extent, those in mycorrhizal plants. In mycorrhizal plants the uptake of 15N was not correlated with area colonized by the mycorrhizal mycelium. The amount of KCl-extractable 15N in peat without plants and mycorrhizal fungi decreased with liming. It is proposed that liming induced chemical or microbial immobilization of the added 15N. This is suggested to be the main reason for the decreased uptake of 15N in lime treatments.