Fungal Respiration Research Articles

Respiration and annual fungal production associated with decomposing leaf litter in two streams

Summary1. We compared fungal biomass, production and microbial respiration associated with decomposing leaves in one softwater stream (Payne Creek) and one hardwater stream (Lindsey Spring Branch).2. Both streams received similar annual leaf litter fall (478–492 g m−2), but Lindsey Spring Branch had higher average monthly standing crop of leaf litter (69 ± 24 g m−2; mean ± SE) than Payne Creek (39 ± 9 g m−2).3. Leaves sampled from Lindsey Spring Branch contained a higher mean concentration of fungal biomass (71 ± 11 mg g−1) than those from Payne Creek (54 ± 8 mg g−1). Maximum spore concentrations in the water of Lindsay Spring Branch were also higher than those in Payne Creek. These results agreed with litterbag studies of red maple (Acer rubrum) leaves, which decomposed faster (decay rate of 0.014 versus 0.004 day−1), exhibited higher maximum fungal biomass and had higher rates of fungal sporulation in Lindsey Spring Branch than in Payne Creek.4. Rates of fungal production and respiration per g leaf were similar in the two streams, although rates of fungal production and respiration per square metre were higher in Lindsey Spring Branch than in Payne Creek because of the differences in leaf litter standing crop.5. Annual fungal production was 16 ± 6 g m−2 (mean ± 95% CI) in Payne Creek and 46 ± 25 g m−2 in Lindsey Spring Branch. Measurements were taken through the autumn of 2 years to obtain an indication of inter‐year variability. Fungal production during October to January of the 2 years varied between 3 and 6 g m−2 in Payne Creek and 7–27 g m−2 in Lindsey Spring Branch.6. Partial organic matter budgets constructed for both streams indicated that 3 ± 1% of leaf litter fall went into fungal production and 7 ± 2% was lost as respiration in Payne Creek. In Lindsey Spring Branch, fungal production accounted for 10 ± 5% of leaf litter fall and microbial respiration for 13 ± 9%.

Networks of power and influence: the role of mycorrhizal mycelium in controlling plant communities and agroecosystem functioning

Extraradical mycelia of mycorrhizal fungi are normally the “hidden half” of the symbiosis, but they are powerful underground influences upon biogeochemical cycling, the composition of plant communities, and agroecosystem functioning. Mycorrhizal mycelial networks are the most dynamic and functionally diverse components of the symbiosis, and recent estimates suggest they are empowered by receiving as much as 10% or more of the net photosynthate of their host plants. They often constitute 20%–30% of total soil microbial biomass yet are undetected by standard measures of biomass used by soil scientists and agromomists. Mycorrhizal mycelia provide extensive pathways for carbon and nutrient fluxes through soil, often exceeding tens of metres per gram of soil. We consider the amounts of photosynthate “power” allocated to these mycelial networks and how this is used in fungal respiration, biomass, and growth and in influencing soil, plant, and ecosystem processes. The costs and functional “benefits” to plants linking to these networks are fungal specific and, because of variations in physiology and host specificity, are not shared equally; some plants even depend exclusively on these networks for carbon. We briefly assess the potential contribution of extraradical mycorrhizal mycelium to sustainable agriculture and maintenance of biodiversity and highlight technologies that promise new vistas and improved fine-scale resolution of the dynamic spatial and temporal functioning of these networks in soil.Key words: arbuscular mycorrhiza, ectomycorrhiza, extraradical mycelium, hyphal networks.

Root factors induce mitochondrial-related gene expression and fungal respiration during the developmental switch from asymbiosis to presymbiosis in the arbuscular mycorrhizal fungus Gigaspora rosea.

During spore germination, arbuscular mycorrhizal (AM) fungi show limited hyphal development in the absence of a host plant (asymbiotic). In the presence of root exudates, they switch to a new developmental stage (presymbiotic) characterized by extensive hyphal branching. Presymbiotic branching of the AM fungus Gigaspora rosea was induced in liquid medium by a semipurified exudate fraction from carrot (Daucus carota) root organ cultures. Changes in RNA accumulation patterns were monitored by differential display analysis. Differentially appearing cDNA fragments were cloned and further analyzed. Five cDNA fragments could be identified that show induced RNA accumulation 1 h after the addition of root exudate. Sequence similarities of two fragments to mammalian Nco4 and mitochondrial rRNA genes suggested that root exudates could influence fungal respiratory activity. To support this hypothesis, additional putative mitochondrial related-genes were shown to be induced by root exudates. These genes were identified after subtractive hybridization and putatively encode a pyruvate carboxylase and a mitochondrial ADP/ATP translocase. The gene GrosPyc1 for the pyruvate carboxylase was studied in more detail by cloning a cDNA and by quantifying its RNA accumulation. The hypothesis that respiratory activity of AM fungi is stimulated by root exudates was confirmed by physiological and cytological analyses in G. rosea and Glomus intraradices. Oxygen consumption and reducing activity of both fungi was induced after 3 and 2 h of exposition with the root factor, respectively, and the first respiration activation was detected in G. intraradices after approximately 90 min. In addition, changes in mitochondrial morphology, orientation, and overall biomass were detected in G. rosea after 4 h. In summary, the root-exuded factor rapidly induces the expression of certain fungal genes and, in turn, fungal respiratory activity before intense branching. This defines the developmental switch from asymbiosis to presymbiosis, first by gene activation (0.5-1 h), subsequently on the physiological level (1.5-3 h), and finally as a morphological response (after 5 h).

Fungal communities and biomass in mountain streams affected by mine drainage tab: 3

We examined fungi associated with decomposing leaves in streams to understand the effects of mine drainage on this community and to test a general model of ecological response to stress. The community composition and biomass of fungi and microbial activity associated with decomposing willow leaves were determined for 20 Colorado mountain stream sites, some of which were affected by mine drainage. The pH, concentration of dissolved zinc, and deposition rate of metal oxides were measured at each site. The community composition of fungi on willow leaves from litterbags was determined by analysis of conidia from aerated leaves and a particle-plating method. As with other communities in streams, diversity of fungal communities was sensitive to low pH and high concentrations of zinc from mine drainage, whereas biomass and functioning were stable under stress from pH or zinc. Diversity was low at sites with high concentrations of dissolved zinc (>1 mg/L) or low pH (<6). Fungal biomass (concentration of ergosterol) and microbial activity, in contrast, often were high despite the chemical conditions of the streams and the limited diversity of fungi. Microbial respiration was negatively related to the physical stress of metal oxide deposition. The concentration of ergosterol was significantly related to rates of respiration on leaves at pristine sites. Leaves from sites with high concentrations of dissolved zinc often had higher fungal biomass and microbial respiration than leaves from pristine sites. Several sites of low pH had high rates of respiration on leaves and visible fungal growth, but little or no measurable ergosterol. Leaves at these sites had ergosterol-like compounds that have yet to be identified.

CO2 exchange and thallus nitrogen across 75 contrasting lichen associations from different climate zones.

Aiming to investigate whether a carbon-to-nitrogen equilibrium model describes resource allocation in lichens, net photosynthesis (NP), respiration (R), concentrations of nitrogen (N), chlorophyll (Chl), chitin and ergosterol were investigated in 75 different lichen associations collected in Antarctica, Arctic Canada, boreal Sweden, and temperate/subtropical forests of Tenerife, South Africa and Japan. The lichens had various morphologies and represented seven photobiont and 41 mycobiont genera. Chl a, chitin and ergosterol were used as indirect markers of photobiont activity, fungal biomass and fungal respiration, respectively. The lichens were divided into three groups according to photobiont: (1) species with green algae, (2) species with cyanobacteria, and (3) tripartite species with green algal photobionts and cyanobacteria in cephalodia. Across species, thallus N concentration ranged from 1 to 50mg g-1 dry wt., NP varied 50-fold, and R 10-fold. In average, green algal lichens had the lowest, cyanobacterial Nostoc lichens the highest and tripartite lichens intermediate N concentrations. All three markers increased with thallus N concentration, and lichens with the highest Chl a and N concentrations had the highest rates of both P and R. Chl a alone accounted for ca. 30% of variation in NP and R across species. On average, the photosynthetic efficiency quotient [K F=(NPmax+R)/R)] ranged from 2.4 to 8.6, being higher in fruticose green algal lichens than in foliose Nostoc lichens. The former group invested more N in Chl a and this trait increased NPmax while decreasing R. In general terms, the investigated lichens invested N resources such that their maximal C input capacity matched their respiratory C demand around a similar (positive) equilibrium across species. However, it is not clear how this apparent optimisation of resource use is regulated in these symbiotic organisms.

Contribution of aerial hyphae of Aspergillus oryzae to respiration in a model solid‐state fermentation system

Oxygen transfer is for two reasons a major concern in scale-up and process control in industrial application of aerobic fungal solid-state fermentation (SSF): 1) heat production is proportional to oxygen uptake and it is well known that heat removal is one of the main problems in scaled-up fermenters, and 2) oxygen supply to the mycelium on the surface of or inside the substrate particles may be hampered by diffusion limitation. This article gives the first experimental evidence that aerial hyphae are important for fungal respiration in SSF. In cultures of A. oryzae on a wheat-flour model substrate, aerial hyphae contributed up to 75% of the oxygen uptake rate by the fungus. This is due to the fact that A. oryzae forms very abundant aerial mycelium and diffusion of oxygen in the gas-filled pores of the aerial hyphae layer is rapid. It means that diffusion limitation in the densely packed mycelium layer that is formed closer to the substrate surface and that has liquid-filled pores is much less important for A. oryzae than was previously reported for R. oligosporus and C. minitans. It also means that the overall oxygen uptake rate for A. oryzae is much higher than the oxygen uptake rate that can be predicted in the densely packed mycelium layer for R. oligosporus and C. minitans. This would imply that cooling problems become more pronounced. Therefore, it is very important to clarify the physiological role of aerial hyphae in SSF. © 2002 Wiley Periodicals, Inc. Biotechnol Bioeng 78: 539–544, 2002.

Strobilurin + acibenzolar‐S‐methyl controls white rust without inducing leaf chlorosis in spinach

SummaryWhite rust (Albugo occidentalis) is a major foliar fungal disease of spinach (Spinacea oleracea) in Texas and the Southeastern USA. It causes leaf yellowing, decreasing product quality and marketability. There is a need for evaluating broad spectrum fungicides and new plant defence activators that have the potential to enhance leaf quality in spinach. Two of those fungicides are acibenzolar‐S‐methyl (ACIB) and strobilurin derivatives (STRO). ACIB appears to mimic natural systemic acquired resistance response in plants, while STRO inhibits mitochondrial respiration in fungi. We conducted three consecutive studies to a) determine the effects of ACIB rates and combinations with STRO and mefenoxam + copper hydroxide (MEFE‐Cu) for the control of white rust, and b) evaluate potential phytotoxicity expressed as leaf chlorosis. After two weekly applications in 1998, ACIB, STRO and ACIB + STRO significantly reduced the % of leaf area infected with white rust in the spinach cv. Cascade. In 1999, ACIB + STRO and ACIB + MEFE‐Cu had the best white rust efficacy, with ACIB equally effective when applied at reduced rates. After three applications in 2000, STRO, ACIB + STRO, and ACIB + MEFE‐Cu had significantly lower white rust values than the control, ACIB or MEFE‐Cu. The combination of ACIB + STRO did not cause leaf chlorosis in any year. Use of preventive applications of ACIB + STRO at 7–10 day intervals appear to be an effective strategy to improve leaf quality in spinach.

Elevated-CO&lt;sub&gt;2&lt;/sub&gt;-Induced Changes in the Chemistry of Quaking Aspen (Populus tremuloides Michaux) Leaf Litter: Subsequent Mass Loss and Microbial Response in a Stream Ecosystem

Changes in chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter were examined under ambient (AMB = 360 ppm) and elevated (ELE = 720 ppm) levels of atmospheric CO 2 . Senesced ELE leaves were significantly higher in phenolic compounds, lignin, and C:N than AMB leaves. A 30-d in situ experiment in a northern Michigan stream analyzed changes in leaf mass, the concentration of phenolic compounds as a result of chemical leaching, and the growth responses of fungi and bacteria. ELE leaves lost less mass than AMB leaves after a 30-d incubation. Although ELE leaves were initially higher in total phenolic compounds and condensed tannins, differences between the treatments were no longer observed after 48 h of chemical leaching. Bacterial biomass and community respiration were higher on the AMB leaves for the first 12 d of incubation, whereas fungal biomass and community respiration were higher in the AMB treatment by the end of the 30-d experiment. Fungal biomass was negatively correlated ...

Topographical and seasonal effects on soil fungal and bacterial activity in subtropical, perhumid, primary and regenerated montane forests

This study was conducted in subtropical, perhumid, montane forests of north-eastern Taiwan. The study sites were located along a slope from the 2100 m summit to the 1670 m lakeshore of a primary coniferous forest ecosystem and at three nearby regenerated forests. Soil ergosterol content, fungal and bacterial contributions to CO 2 production (respiration rate) after glucose addition and hence ratios of fungal to bacterial respiration were evaluated. In the primary forest surface soils (depth 0–10 cm), ergosterol content and respiration rate increased from the summit to the lakeshore site, with the steep sloping shoulder site having the lowest level. At these sites, ergosterol content and CO 2 production, calculated on a per gram basis, declined sharply with depth, being 10–1000 fold less in the deepest mineral horizon (>50 cm) as compared to the surface horizon. The soil total respiration rate was found to be significantly higher in summer than winter in the surface soil, but the fungal respiration rate and ergosterol content showed no significant difference between these two seasons. The primary coniferous forest had both a significantly higher respiration rate and ergosterol content than the regenerated Japanese cedar and regenerated broadleaf forest ecosystems, but not the regenerated false cypress forest. Ratios of fungal to bacterial respiration showed no significant differences between sites or soil layers. However, a significant positive correlation ( r 2=0.746) was exhibited between soil fungal ergosterol content and soil fungal respiration rate for all sampling sites.

Interaction of the earthworm Diplocardia mississippiensis (Megascolecidae) with microbial and nutrient dynamics in a subtropical Spodosol

Direct and indirect effects of earthworm feeding and activity can alter soil microbial and nutrient dynamics. Little is known about influence of native North American earthworms on these dynamics. We investigated effects of native D. mississippiensis activity on total and soluble C, N and microbial biomass pools in Spodosols from the Apalachicola National Forest in North Florida, USA. Soil native to D. mississippiensis was collected and reconstructed by horizon (A, E, B h) in laboratory chambers and incubated with earthworms at zero, low or high density, respectively 0, three, or six individuals, for 28–31 days. Total C and N were measured by dry combustion methods. Soluble C and N and microbial biomass were measured in 0.5 K 2SO 4 extracts from non-fumigated and fumigated soil samples. Substrate induced respiration (SIR) was used with inhibitors to measure microbial respiration and shifts in bacterial and fungal components. Earthworm activity mixed soil horizons, largely through burrowing and casting. Total C in A and B h horizons averaged 16.7 mg C g −1 soil and was significantly greater than in the E (2.6 mg C g −1 soil). Soil N concentration was highest in the A (0.73 mg g −1 soil) and lowest in the E (0.11 mg N g −1 soil). N concentration was significantly increased over two fold in the E with high earthworm density. Microbial biomass C was greatest in the A and E, averaging 8.6 mg g −1 C, and decreased with earthworm activity. In all horizons, soluble C and N also increased with earthworm density. Net N mineralization increased significantly with earthworm density and ranged from –0.22 to 68 mg g −1 N across all treatments and layers. Changes in proportion of soluble to total soil nutrients indicated that greater turnover and mineralization occurred with earthworm activity. SIR indicated earthworm activity induced microbial respiration in A and E horizons. Earthworm activity stimulated fungal and bacterial respiration, respectively, by 13 and 30% in the A and by 123% and 450% in the E. Although bacterial respiration was stimulated more than fungal there was no significant shift in B:F ratio, which ranged from 0.80 to 0.99. SIR did not provide evidence of any selective microbial effects of earthworm activity. Direct and indirect effects on nutrient dynamics by earthworms were induced by casting activity and were associated with passage of material through the gut (direct effect) and nutrient and microbial enrichment through the mixing of soil horizons (indirect effect). This study established the importance of D. mississippiensis to nutrient availability in its native soil.

Spatial changes of soil fungal and bacterial biomass from a sub-alpine coniferous forest to grassland in a humid, sub-tropical region

Fungal and bacterial biomass were determined across a gradient from a forest to grassland in a sub-alpine region in central Taiwan. The respiration-inhibition and ergosterol methods for the evaluation of the microbial biomass were compared. Soil fungal and bacterial biomass both significantly decreased (P<0.05) with the shift of vegetation from forest to grassland. Fungal and bacterial respiration rates (evolved CO2) were, respectively, 89.1 μl CO2 g–1 soil h–1 and 55.1 μl CO2 g–1 soil h–1 in the forest and 36.7 μl CO2 g–1 soil h–1 and 35.7 μl CO2 g–1 soil h–1 in the grassland surface soils (0–10 cm). The fungal ergosterol content in the surface soil decreased from the forest zone (108 μg g–1) to the grassland zone (15.9 μg g–1). A good correlation (R 2=0.90) was exhibited between the soil fungal ergosterol content and soil fungal CO2 production (respiration) for all sampling sites. For the forest and grassland soil profiles, microbial biomass (respiration and ergosterol) declined dramatically with depth, ten- to 100-fold from the surface organic horizon to the deepest mineral horizon. With respect to fungal to bacterial ratios for the surface soil (0–10 cm), the forest zone had a significantly (P<0.05) higher ratio (1.65) than the grassland zone (1.05). However, there was no fungal to bacterial ratio trend from the surface horizon to the deeper mineral horizons of the soil profiles.

Mitochondrial electron transfer in the wheat pathogenic fungus Septoria tritici: on the role of alternative respiratory enzymes in fungicide resistance

Certain phytopathogenic fungi are able to express alternative NADH- and quinol-oxidising enzymes that are insensitive to inhibitors of the mitochondrial respiratory Complexes I and III. To assess the extent to which such enzymes confer tolerance to respiration-targeted fungicides, an understanding of mitochondrial electron transfer in these species is required. An isolation procedure has been developed which results in intact, active and coupled mitochondria from the wheat pathogen Septoria tritici, as evidenced by morphological and kinetic data. Exogenous NADH, succinate and malate/glutamate are readily oxidised, the latter activity being only partly (approx. 70%) sensitive to rotenone. Of particular importance was the finding that azoxystrobin (a strobilurin fungicide) potently inhibits fungal respiration at the level of Complex III. In some S. tritici strains investigated, a small but significant part of the respiratory activity (approx. 10%) is insensitive to antimycin A and azoxystrobin. Such resistant activity is sensitive to octyl gallate, a specific inhibitor of the plant alternative oxidase. This enzyme, however, could not be detected immunologically. On the basis of the above findings, a conceptual mitochondrial electron transfer chain is presented. Data are discussed in terms of developmental and environmental regulation of the composition of this chain.

Impact of environmental factors on fungal respiration and dry matter losses in wheat straw

An automatic electrolytic respirometer enabled replicated determinations of the respiration rates of individual fungi on sterile straw, and the mixed mycoflora of naturally contaminated wheat straw at different steady-state temperatures (10–30°C) and water activities ( a w, 0.75–0.98) over periods of 8–14 days. Generally, the respiratory activity of individual spoilage fungi ( Alternaria alternata, Cladosporium cladosporioides, Eurotium amstelodami, Fusarium culmorum and Penicillium aurantiogriseum) on sterile wheat straw increased linearly with increasing a w at 25°C. The calculated maximum dry matter loss of wheat straw due to colonisation by individual species was about 10%, regardless of a w. On naturally contaminated wheat straw fungal activity was also related to temperature and a w, with maximum respiration at 30°C and 0.98 a w. At the lowest temperature examined, 10°C, there was a slight lag prior to respiratory activity occurring. The respiratory activity was also significantly reduced (by half) when available water was reduced to 0.95–0.90 a w. In contrast to the colonisation of sterile straw by individual species, the maximum dry matter loss caused by fungal deterioration of naturally contaminated wheat straw was 3.4% at 0.98 a w and 30°C. The dominant fungal genera and species varied with a w and temperature. These results are discussed in relation to the storage of cereal straw without spoilage.

Diversity of kresoxim-methyl sensitivities in baseline populations ofVenturia inaequalis

Several strobilurin fungicides inhibiting fungal respiration by binding to cytochrome b have been introduced recently. A mechanism of strobilurin resistance identified as active in several plant pathogenic fungi is based on the activation of alternative respiration. Thus far, respective studies have been restricted to single isolates of respective pathogens. Here, we report a study on 250 Venturia inaequalis baseline isolates to the strobilurin kresoxim-methyl having a broad sensitivity distribution characterized by a 50-fold difference in sensitivity of the most- and least-sensitive isolates. For the majority (62%) of these isolates, differences in sensitivity were not caused by the interference of alternative respiration with the full inhibitory potency of kresoxim-methyl. Rather, variable dose-responses with largely different degrees of inhibition achieved at a low dose of kresoxim-methyl were found to be responsible. For 38% of the baseline isolates, alternative respiration was already active during the stage of conidia germination. Activation of this pathway was, again, dependent on the strobilurin dose. Selection of sub-populations of isolates resisting low doses of kresoxim-methyl by multiple mechanisms and the recombination among isolates expressing such mechanisms singly can be expected to be slowed by an anti-resistance strategy based on high strobilurin doses. © 1999 Society of Chemical Industry

An evaluation of the substrate-induced respiration method

Two ways of measuring substrate-induced respiration (SIR) following addition of glucose to soil, viz. solid glucose or glucose in solution, were tested on 13 soils sampled from arable, grassland and woodland sites ranging from pH 3.2 to 7.5. Generally similar patterns of CO 2 evolution were found between soils following addition of glucose as liquid or solid ( r=0.93) for unamended, ryegrass-amended and fumigated soils. Glucose added in solution to adjust the soils to 1.2-fold WHC was therefore preferred for analytical convenience. The optimum time of CO 2 measurement was between 0.5 and 2.5 h for routine use. It was found to be unnecessary to make any correction for CO 2 dissolved in the soil solution for soils below pH 6.5. Maximum inhibition of bacterial respiration was obtained at 4 to 8 mg added streptomycin and of fungal respiration at 8–12 mg added cyclohexamide g −1 soil. In a grassland soil (24% clay) the bacteria comprised 19±4.2% and the fungi 82±4.0% of the total biomass and in a grass ley (8% clay) the proportions were 25±1.2% for bacteria and 76±4.5% for fungal biomass.

Riparian soil response to surface nitrogen input: temporal changes in denitrification, labile and microbial C and N pools, and bacterial and fungal respiration

To investigate potential changes in soil microbial characteristics of riparian zones subject to chronic nitrogen influx, we conducted an N-addition experiment in a riparian forest in the Coastal Plain of Georgia, USA. During 6 months, we quantified temporal changes in denitrification and respiration rates, microbial biomass, and bacterial and fungal activity, following single and repeated inorganic N additions to the near-stream zone (‘zone 1’) and the upslope area (‘zone 2’) within the riparian site. Zone 1 soil had significantly higher soil moisture, larger labile and microbial C and N pools, higher denitrification, bacterial and fungal respiration rates, and a lower pH than zone 2 soil. Over time, all added N was effectively removed from zone 1 soil by denitrification, regardless of whether N was added in a single large pulse or multiple small pulses. In contrast, added N did not significantly stimulate denitrification in zone 2 soil, which lacked sufficient anaerobicity and had lower labile C levels. In neither zone did N addition result in net N immobilization into microbial biomass, suggesting that microbial storage is not a major N-removal mechanism in this riparian forest. In both zones N additions slightly reduced microbial C and basal respiration, and increased fungal activity. Although bacterial activity was not significantly affected, the observation of increased bacterivorous nematode populations in N-amended soils suggests that bacterial production had been temporarily stimulated by N addition. However, it may be expected that long-term effects of chronic N influx mostly negatively affect microbial biomass and activity, which could threaten the seemingly indefinite N-removal potential of denitrifiers in N-loaded riparian systems.

Riparian soil response to surface nitrogen input: the indicator potential of free-living soil nematode populations

Nitrogen saturation of riparian ecosystems may lead to leaching and loss of surface water quality. In the search for early warning signs of N-saturation, we conducted an N-addition experiment to evaluate the indicator potential of free-living nematodes. During 6 months, we measured changes in feeding and life strategy groups, following single high and repeated low inorganic N-additions to a near-stream zone (‘zone 1’) and an upslope area (‘zone 2’) within a riparian forest in the southeast Coastal Plain of Georgia. In both zones, N-addition significantly increased the number of bacterivores but not fungivores. Only bacterivores with r-selected life strategies increased. In this group, Rhabditinae multiplied after N-addition in both zones, but Neodiplogasteridae and Myolaimidae responded to N only in zone 1, and Cephalobidae only in zone 2. Microbivore correlations with microbial data, collected concomitantly (Ettema et al., 1999. Riparian soil response to surface nitrogen input: temporal changes in denitrification, labile and microbial C and N pools, and bacterial and fungal respiration. Soil Biology & Biochemistry 31, 1609–1624), were considerably stronger in N-amended treatments than in controls, suggesting that N-addition synchronized microbial-microbivorous dynamics. At the end of the experiment, bacterivore abundance returned to control levels, probably partly due to predation, as predator populations had considerably increased several months after the first N-addition. The increase in predator abundance was greater in the single high than repeated low N-addition treatments, as was the case for bacterivore populations. These results suggest that nematodes can be indicators of N-saturation. However, for practical application, it appears that these indicators could only be meaningful when monitored together with other system characteristics, such denitrification rates and litter N.

Approaches to in-vitro lead generation for fungicide invention†

There are increasing opportunities for the development of highthroughput in-vitro screens to aid the discovery of fungicides with novel modes of action. In the past, such screens were developed when biochemical targets were validated by fungicides with defined modes of action. However, genetic information is beginning to have a major impact both on the way in-vitro targets are selected and on the speed at which mode-of-action information is gained on current fungicides having an, as yet, undefined mode of action. This paper discusses issues concerning target selection and high-throughput screening, using examples taken from the current literature and from investigations at Zeneca Agrochemicals, using inhibition of fungal respiration as an example. Saccharomyces cerevisiae is discussed as model for fungicide research, both in terms of its sensitivity to known fungicides and its well defined molecular genetics, which makes it amenable to such techniques as gene dosage for mode of action determination. © 1998 Society of Chemical Industry

Do mites and Collembola affect pine litter fungal biomass and microbial respiration?

Effects of grazing by a common collembolan, Onychiurus subtenuis, and a common oribatid mite, Oppiella nova, singly and in combination, at a variety of densities, on lodgepole pine needle litter fungal biomass and microbial respiration were assessed in microcosms. At 3 weeks the numbers of O. subtenuis did not affect respiration. Increasing numbers of immature O. nova increased microbial respiration, accounting for 25% ( p<0.001) of the variation in respiration. At 6 weeks, respiration was not affected by numbers of either animal, but was significantly and positively affected by moisture content which accounted for 49% ( p<0.0001) of the variation. Increasing total numbers of fauna had a negative effect on fungal biomass, accounting for 24% ( p<0.002) of the variation in fungal biomass. Gut content data showed that both O. subtenuis and O. nova consumed far more dark hyphae than hyaline hyphae. It appears that densities of the two most abundant mesofaunal species in lodgepole pine needle litter have a statistically significant effect on fungal biomass and respiration. However, their effects account for only about 25% of the variation in fungal biomass and respiration.

Performance of Pinus contorta inoculated with two strains of root endophytic fungus, Phialocephala fortinii: effects of synthesis system and glucose concentration

Melanized, root-colonizing fungi are ubiquitous. Their ecological role, however, is poorly understood, and results of studies of associations between these fungi and their potential host plants are controversial. The culture system under which the association is studied may also affect the host-fungus interaction. Two experiments on the association between Pinus contorta Dougl. ex Loud., and a root-inhabiting endophytic fungus, Phialocephala fortinii Wang &amp; Wilcox, were conducted to study the host response to inoculation. First, Pinus contorta seedlings were inoculated with two strains of Phialocephala fortinii and grown under aseptic conditions with five levels of glucose in the medium. Multiple linear regression analysis was employed to study the effects of inoculation and glucose concentration. Second, the same two strains of Phialocephala fortinii were inoculated on Pinus contorta seedlings in open pot cultures. Inoculation resulted in substantial increase in all biomass components of the host plant in the aseptic culture system. Total biomass, for example, was increased approximately 60 and 90% when seedlings were inoculated with strain 1 and strain 2, respectively. No seedling mortality was observed following fungal inoculation after 6 months of incubation. Inoculation increased host biomass with increasing glucose concentration, while glucose concentration did not significantly affect host biomass when no inoculum was added. Inoculation lowered foliar nitrogen and phosphorus concentrations. The effect of glucose concentration on the foliar nutrient concentrations varied between the two strains. In the open pot cultures, inoculation did not affect biomass or foliar nutrient concentration. We hypothesise that the observed increases in host growth in the aseptic culture system are due to fungal respiration in a closed culture system, the carbohydrates made available to the host plant by the fungus, or, most likely, to a combination of both factors. The ecological role of Phialocephala fortinii and the validity of aseptic culture assays are discussed.Key words: dark-septate endophytes, deuteromycetes, fungi, host-fungus associations, root endophytes, symbiosis.