The ultrastructure of the stroma of the brown rot fungi

Summary Most of the hyphae forming the medulla of the stroma of the brown rot fungi are 4–7 in diameter and contain food reserves in large vacuoles and lipid bodies. Some stromatal hyphae have very thick walls and perform a protective function. Smaller hyphae (1–2 in diameter) form a network through the medulla and their structure suggests that they initiate the growth of vegetative hyphae and spores after the stroma has passed through a period of rest.     

VAM association in the shrub Myrica cerifera on a Virginia,USA barrier island

A combined laboratory and field study examined the potential for a symbiotic association between the actinorhizal shrub Myrica cerifera and vesicular arbuscular mycorrhizal (VAM) fungi on a Virginia barrier island. M. cerifera seedlings and two test species, Zea mays and Strophostyles umbellata, were grown in an environmental chamber on soils collected from four sites differing in soil age (< 5 to over 130 years), salinity (1–35 g/g total soil chloride), and edaphic characteristics. Seedling root infection was significantly lower for all three species in the youngest soils from the beach where salinity was highest. Stained M. cerifera roots revealed all the components for a functional VAM association; however, there were significantly fewer arbuscules and vesicles relative to the test species. Among field-collected M. cerifera, infection was not detected in mature shrubs from the bay side of the island, where M. cerifera thickets were in a state of degeneration. Infection was highest in soils from the young, developing thickets, and in the most stable thickets of the island interior. Despite the dynamic nature of the barrier island environment, VAM associations with M. cerifera appear to be present, especially in seedlings and developing shrub thickets.     

Growth,photosynthesis and storage of carbohydrates and nitrogen in Phaseolus lunatus in relation to resource availability

External hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi were quantified over a growing season in a reconstructed tallgrass prairie and an ungrazed cool-season pasture. In both sites, hyphal lengths increased throughout the growing season. Peak external hyphal lengths were 111 m cm^–3 of soil in the prairie and 81 m cm^–3 of soil in the pasture. These hyphal lengths calculate to external hyphal dry weights of 457 g cm^–3 and 339 g cm^–3 of soil for prairie and pasture communities, respectively. The relationships among external hyphal length, root characteristics, soil P and soil moisture were also determined. Measures of gross root morphology [e.g., specific root length (SRL) and root mass] have a strong association with external hyphal length. Over the course of the study, both grassland communities experienced a major drought event in late spring. During this period a reduction in SRL occurred in both the pasture and prairie without a measured reduction in external hyphal length. Recovery for both the pasture and prairie occurred not by increasing SRL, but rather by increasing external hyphal length. This study suggests that growth is coordinated between VAM hyphae and root morphology, which in turn, are constrained by plant community composition and soil nutrient and moisture conditions.     

Quantification of soil carbon inputs under elevated CO2: C3 plants in a C4 soil

The objective of this investigation was to quantify the differences in soil carbon stores after exposure of birch seedlings (Betula pendula Roth.) over one growing season to ambient and elevated carbon dioxide concentrations. One-year-old seedling of birch were transplanted to pots containing C₄ soil derived from beneath a maize crop, and placed in ambient (350 L L^–1) and elevated (600 L L^–1) plots in a free-air carbon dioxide enrichment (FACE) experiment. After 186 days the plants and soils were destructively sampled, and analysed for differences in root and stem biomass, total plant tissue and soil C contents and ¹³C values. The trees showed a significant increase (+50%) in root biomass, but stem and leaf biomasses were not significantly affected by treatment. C isotope analyses of leaves and fine roots showed that the isotopic signal from the ambient and elevated CO₂ supply was sufficiently distinct from that of the C₄ soil to enable quantification of net root C input to the soil under both ambient and elevated CO₂. After 186 days, the pots under ambient conditions contained 3.5 g of C as intact root material, and had gained an additional 0.6 g C added to the soil through root exudation/turnover; comparable figures for the pots under elevated CO₂ were 5.9 g C and 1.5 g C, respectively. These data confirm the importance of soils as an enhanced sink for C under elevated atmospheric CO₂ concentrations. We propose the use of C₄ soils in elevated CO₂ experiments as an important technique for the quantification of root net C inputs under both ambient and elevated CO₂ treatments.     

Changes in root growth patterns of (Picea abies) spruce roots by inoculation with an ectomycorrhizal fungusPisolithus tinctorius and jasmonic acid treatment

Symbiosis between fungi and plant roots forming a mycorrhiza involves extensive interactions at the molecular level between both partners. The role of plant hormones in the regulation of mycorrhizal infection is not known to involve jasmonates. Their endogenous levels increase during pathogen attack; however, little has been done on their involvement in mycorrhizae. In our recent work, root growth patterns of 2-month-old spruce seedlings after inoculation withPisolithus tinctorius and/or jasmonic acid (JA) treatment were studied using a paper-sandwich technique. Changes in root length, the degree of branching, presence and length of root hairs, and infection parameters were followed using a stereomicroscope. The first mycorrhizal contact of hyphae with roots was significantly accelerated upon treatment with 0.5 M JA. Interactions between root hairs and fungal hyphae were seen by scanning electron microscopy. The multiplication of root hairs of non-mycorrhized seedlings treated with 5.0 M JA and changes of the root surface were observed by the same technique.     

Extraction of extraradical arbuscular mycorrhizal mycelium from compartments filled with soil and glass beads

This study presents a novel method for the extraction and quantification of extraradical mycelium (ERM) of arbuscular mycorrhizal fungi (AMF) from a substrate that simulates soil better than previously used artificial growth media. Fungal compartments were constructed from small net pots with a latticed wall and filled with a mixture of glass beads and 40 m wet sieved soil. The net pots were surrounded by a 30-m mesh membrane through which hyphae but not roots could grow. They were inserted into soil where a Glomus intraradices (BEG 110) colonized potato plant was growing. The ERM that had grown out from roots through the membrane was successfully collected and quantified after harvest by washing out the soil/glass bead mixture through a sieve with a mesh width of 40 m. Concentrations of P, Zn, Cu and Mn in the AMF ERM were analysed.     

Vesicular-arbuscular mycorrhizal fungi,particularly Glomus tenue,in Venezuelan bromeliad epiphytes

The mycorrhizal status of water-impounding tank bromeliad epiphytes from three locales differing in altitude and moisture regime within Venezuelan cloud forest was examined. Species of vesicular-arbuscular mycorrhizal (VAM) fungi found in arboreal soils were compared to VAM fungi found in terrestrial soils. Sixteen of the 19 epiphytes examined for the presence of VAM fungi had roots with infection stages; 14 of these specimens showed growth of the fine endophyte Glomus tenue. Fine endophyte was the only VAM fungus found associated with epiphytes in the driest locale studied, while coarse VAM fungi (Gigaspora and Scutellospora spp.) were found at sampling locales receiving more moisture. Root infection was usually composed of intercellular hyphae and peletons; few arbuscules were observed. However, abundant extracellular hyphae were often observed tangled about roots in arboreal soil. It is concluded that epiphytic bromeliads probably benefit, at least periodically, from VAM fungi scavenging for sporadically available nutrients in arboreal soils. Glomus tenue may be particularly important as a colonizing VAM fungus in drier sites of Venezuelan cloud forest. The species composition of VAM fungi in arboreal soils was different to that of terrestrial soils sampled directly under epiphytic bromeliad perches, suggesting that VAM fungi species associated with bromeliads are dispersed to their hosts by vagile animal vectors.     

Influence of vesicular-arbuscular mycorrhizae on biomass production by the cactus Pachycereus pecten-aboriginum

This study reports the effect of vesicular-arbuscular mycorrhizal (VAM) fungi on dry matter production by Pachycereus pecten-aboriginum (Engelm.) Britt & Rose, an arborescent cactus of arid and tropical dry forest in Mexico. Seedlings in the presence or absence of VAM fungi were grown in soil between two plates of glass (20 × 30 cm) for 8 months inside growth chambers (30/25° C, 13/11 h day/night and a light intensity of 400 mol m^-2 s^-1). VAM seedlings had significantly (P<0.01) higher dry matter production (0.418 versus 0.169 g), root/shoot ratios (0.26 versus 0.14) and specific root length (0.65 versus 1.41 mm mg^-1) than non-VAM seedlings, suggesting a more efficient exploitation of soil resources by the VAM cacti. The data point to a role for VAM fungi in the establishment, growth, water relations and nutrition of cacti in the arid tropics.     

Arbuscular mycorrhizal fungal propagules in a salt marsh

The tolerance of indigenous arbuscular mycorrhizal fungi (AMF) to stressful soil conditions and the relative contribution of spores of these fungi to plant colonization were examined in a Portuguese salt marsh. Glomus geosporum is dominant in this salt marsh. Using tetrazolium as a vital stain, a high proportion of field-collected spores were found to be metabolically active at all sampling dates. Spore germination tests showed that salt marsh spores were not affected by increasing levels of salinity, in contrast to two non-marsh spore isolates, and had a significantly higher ability to germinate under increased levels of salinity (20) than in the absence of or at low salinity (10). Germination of salt marsh spores was not affected by soil water levels above field capacity, in contrast to one of the two non-marsh spore isolates. For the evaluation of infectivity, a bioassay was established with undisturbed soil cores (containing all types of AM fungal propagules) and soil cores containing only spores as AM fungal propagules. Different types of propagules were able to initiate and to expand the root colonization of a native plant species, but spores were slower than mycelium and/or root fragments in colonizing host roots. The AM fungal adaptation shown by this study may explain the maintenance of AMF in salt marshes.     

Bioelectric and biosynthetic aspects of cell polarity inAllomyces macrogynus

Summary In contrast to all filamentous fungi examined to date, vegetative hyphae ofAllomyces macrogynus, whether extending or not, produced an outward flow of positive electrical current, at a maximum of 0.16 A cm^–2 around 40 m behind the apex, as measured with a vibrating probe. Inward currents of up to 0.55 A cm^–2 were recorded around the rhizoids. Increases in outward current were observed in hyphae pre-grown under oxygen deficiency and then allowed to widen backwards to the hyphal base in sufficient oxygen. When spores were germinated in an applied electrical field they produced rhizoids predominantly towards the anode. Hyphae were produced initially towards the cathode but later bent around towards the anode. Experiments with a range of chemicals provided no evidence for the involvement of calcium in vegetative growth and development inA. macrogynus. Polyoxin and nikkomycin, inhibitors of chitin synthesis, had no effect on swimming zoospores, but inhibited wall formation of cysts, rhizoids and forward and backward growing hyphae.     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Soil disturbance and infection of Trifolium repens roots by vesicular-arbuscular mycorrhizal fungi

Removal and storage of the surface layers of soil is known to decrease the infectivity of vesicular-arbuscular mycorrhizal (VAM) fungi. Previous studies have mostly examined the effects of profound soil disturbance on the infectivity of VAM fungi. This study examined the effects of increasing degrees of topsoil disturbance on the infectivity of VAM fungi in two sites on sandstone soils in southeastern Australia. Intact soil blocks (20×20×15 cm) were taken from each of the two sites. Increasing degrees of topsoil disturbance were achieved by cutting the blocks longitudinally into four (dist. 1), nine (dist. 2), and 25 (dist. 3) equal portions. Seeds of Trifolium repens L. were sown into the blocks and harvested 14, 21, 28, 35 and 42 days after sowing. At each sampling date, total root length, root length colonised by VAM fungi and shoot dry mass were measured. VAM colonisation had commenced by 14 days in the roots of seedlings grown in intact, dist. 1, and dist. 2 soil blocks. The initiation of VAM colonisation was delayed by up to 6 weeks for seedlings grown in the dist. 3 soil blocks. The low (i.e. dist. 1) and intermediate (i.e. dist. 2) degrees of soil disturbance did not cause a delay in the initiation of VAM, bud did significantly reduce the proportion of root length colonised by VAM fungi after 21 days. After 21 days, shoot dry mass was significantly less in the seedlings grown in the dist. 3 soil blocks though not in the low and intermediate disturbance treatments. It is concluded that the most severe experimental disturbance probably disturbed the external hyphal network and root fragments (containing hyphae and vesicles), which in turn temporarily reduced the infective potential of the fungus to zero. The observed delay in the initiation of VAM in the most disturbed blocks can, therefore, be explained by the time required for hyphae to grow from other propagules in the soil which survived the disturbance event.     

Aluminum-induced secretion of both citrate and malate in rye

Aluminum (Al)-resistant mechanisms responsible for Al-induced secretion of organic acids are poorly understood. In this study, we characterized the Al-induced secretion of both citrate and malate from rye (Secale cereale L. cv. King). Secretion of organic acids increased with increasing concentration (10, 30 and 50 M) and duration of Al treatments. Neither phosphorous (P) deficiency up to 15 days nor addition of 50M lanthanum, 50 M lead, 10 M cadmium, or 200 M manganese caused secretion of organic acids, suggesting that this secretion was a specific response to Al stress. Aluminum activated citrate synthase, the main enzyme for the synthesis of citrate, but its activation occurred only in the root tip. The elongation of roots of an Al-sensitive cultivar of wheat (Tritium aestivum L. cv. Scout 66) was not inhibited by 50 M Al in the presence of externally applied 50 M citrate or 400 M malate. The secretion of citrate and malate from intact rye roots exposed to 50 M Al corresponded to 31.3 ± 1.7 M and 11.5 ± 2.5 M, respectively, in the rhizosphere based on an assumption of a 2 mm thick unstirred layer around root tips. This result indicated that Al-resistance in rye was achieved by the Al-induced synthesis of citrate in root apices followed by Al-induced specific secretion of citrate from root tips.     

Status of nuclear division in arbuscular mycorrhizal fungi during in vitro development

Summary The number of nuclei in spores and along hyphae of an arbuscular mycorrhizal fungiGigaspora margarita was measured in digital images of fluorescence arising from mithramycin stained cultures. Typical dormant spores (250 m diameter) contained 2000 nuclei. Eight hundred nuclei were mobilized during the first 3 days of germination. The number of nuclei in the spores nearly returned to the initial number after 22 days of hyphal growth. The average relative DNA content in the nuclei of dormant spores and in the nuclei of spores incubated for 22 days was comparable, as judged from fluorescence intensity. Hyphal elongation occurred with 460 nuclei per cm under a special set of in vitro conditions that promote extensive hyphal growth of arbuscular mycorrhizal fungi. We found an average total of 26000 hyphal nuclei per germinating spore after 22 days. The specific DNA polymerase inhibitor aphidicolin did not inhibit spore germination but it rapidly reduced the rate of hyphal growth and arrested growth after 4 days. No nuclei were produced de novo during this time. These results demonstrate thatG. margarita replicates nuclear DNA and undergoes nuclear division when grown in vitro even in the absence of a plant host.     

Effects of nitrogen and phosphorus on blue grama growth and mycorrhizal infection

Summary The purpose of this study was to examine the growth response of Bouteloua gracilis, with and without the vescular-arbuscular mycorrhiza (VAM), Glomus fasciculatus, to varying levels of phosphorus and nitrogen (as NH ₊ ⁴ ) and to determine whether nitrogen and phosphorus levels influence VAM establishment. Bouteloua gracilis was grown in 225 g of soil in a factorial experiment combining four levels of ammonium nitrogen (4, 30, 60, and 126 g/g), four levels of phosphorus (3, 7, 12, and 22 g/g), and with VAM spores or no spores. Bouteloua gracilis showed enhanced growth with increased nutrients over the entire range of experimental amendments. Shoot nitrogen concentration for all plants ranged from an average of 0.73% at the low amendment level to 1.61% at the high level, whereas shoot total averages ranged from 2.43 mg at the low amendment to 16.4 mg at the high amendment. Mean shoot phosphorus concentrations ranged from 0.109% at the low amendment level to 0.150% at the high amendment, while totals averaged 5.29 mg at the low amendment and 11.8 mg at the high amendment level. Infected plants were consistently smaller than uninfected plants. This reduction was significant at high nitrogen-low phosphorus, where percent infection was highest (71%). At low nitrogen levels, moderate infection (17%) was established at all phosphorus levels. No infection occurred when both nitrogen and phosphorus levels were high. The lack of a positive nutrient or biomass response to VAM establishment is contrary to most published reports, but is similar to a lack of response shown with certain grasses and other plants. It is possible that the parasitic nature of the response to infection represents the early phase of infection.     

Inhibition of fungal sporulation by aflatoxin

Summary The fungi Penicillium expansum, Aspergillus niger, Cladosporium herbarum, Thamnidium elegans, Mucor hiemalis and Rhizopus nigricans were cultivated on filter disks containing various concentrations of crude aflatoxin or coumarin. After 3 days the rate of sporulation was noted. 100 g/disk aflatoxin inhibits the sporulation of all fungi with the exception of P. expansum which was only slightly influenced. 10 g/disk aflatoxin also exerts a restricting influence on most of the fungi, whereas at 1 g/disk aflatoxin the quantity of newly formed spores was nearly equal to the toxin-free controls. In comparison with the action of coumarin it is evident that the influence of aflatoxin is specific for that toxin and not or only in a small degree dependent on an action of the coumarin nucleus itself.     

Clonal propagation of Callistemon by tissue culture techniques

Callus development in Callistemon viminalis was readily achieved when axillary buds derived from nodal tissue were placed in a medium containing macro- and micro-nutrients, sucrose (0.06 M), inositol (300 M), nicotinic acid (20 M), pyridoxine hydrochloride (3 M), thiamine hydrochloride (2 M), riboflavin (10 M), cytokinins (5 M) and auxins (0.1 M). The presence of benzylaminopurine (5 M) and p-chlorophenoxyacetic acid (0.1 M) promoted the most vigorous callus development and sprout formation. Rooting of nodal material was rare but occurred readily following the transference of sprouts developed on callus to a basal medium containing sucrose and salts. Root initiation was stimulated, however, by the presence of auxins. Chlorophenoxyacetic acid while stimulating root initiation repressed root growth. Indole butyric acid stimulated both root initiation and shoot growth at concentrations of 0.005 to 0.1 M. The treatment of choice for rooting and shoot growth was the addition of indole butyric acid at a concentration of 0.01 M.     

Microbial abundance in the rhizosphere: A computer model

Summary A mathematical model is described which can predict the abundance of microorganisms in the rhizosphere (as g microbial dry weight/cm³ soil) in relation to distance from the root surface and time since the root started exuding substrate. The growth rate of the microorganisms at each point in the soil is assumed to be controlled by the concentration of soluble organic substrate. The concentration of substrate changes due to (1) its production by the root and diffusion through the soil, (2) its production in the soil by breakdown of insoluble organic matter, and (3) its use by the microorganisms. Values for all of the required input parameters have been obtained from the literature.The model predicts that a high population density will develop near the root surface, but the density will fall off steeply with increasing distance from the root. At the root surface microbial growth continues for many days, provided exudation by the root continues at a steady rate, but further away the population reaches a peak and then declines. This is because the amount of substrate reaching the outer soil is no longer adequate to support the maintenance requirement of the population. Starting with a microbial concentration of 2 g/cm³, and using what are considered to be average values for other input parameters, the microbial concentrations predicted after 10 days are 1509 g/cm³ at the root surface, and 2.2 g/cm³ at 1.8 mm from the root. The model also predicts the substrate concentrations in the soil: these reach a maximum within the first day and then decline, reaching by 10 days values not very different from those in root-free soil.The model is used to predict the effect on microbial and substrate concentrations of changes in soil water content, root density, root exudation rate, initial microbial concentration and microbial response to substrate concentration. Where the predictions of the model can be tested against observed data there is good agreement. re]19760308     

Isolation and identification of plant growth inhibitors as candidate(s) for allelopathic substance(s), from aqueous leachate from mesquite (Prosopis juliflora (Sw.) DC.) leaves

As candidate(s) for allelopathic substance(s), two plant growthinhibitors were isolated from aqueous leachate of leaves of mesquite, whichshowa strong allelopathy, and which were identified as syringin and(–)-lariciresinol by their spectral analyses. Syringin inhibited root andshoot growth of lettuce seedlings at concentrations greater than 0.8M, and root and shoot growth of barnyard grass seedlings atconcentrations greater than 2.7 and 26.9 M, respectively. Onthe other hand, (–)-lariciresinol inhibited root and shoot growth oflettuce seedlings at concentrations greater than 2.8 and 0.8M,and root and shoot growth of barnyard grass seedling at concentrations greaterthan 0.8 and 2.8 M, respectively. The contents of syringin and(–)-lariciresinol in the rhizosphere soil of mesquite were 0.34 and 0.38g/g soil, respectively. These results indicate that syringinand (–)-lariciresinol are allelopathic substance(s), and may play rolesinthe allelopathy of mesquite.     

Effects of trichloroethylene,tetrachloroethylene and dichloromethane on enzymatic activities in soil

Summary Enzyme assays for -glucosidase, -acetylglucosaminidase, phosphatase, phosphodiesterase, and proteinase were made in soil samples incubated for two months after contamination with trichloroethylene, tetrachloroethylene, and dichloromethane. These volatile chlorinated hydrocarbons were added at doses of 10, 100, and 1000 g per 100 g dry soil, respectively. Almost no effect was observed in soil sample contaminated with 10 g of the chemicals when compared with control soil. When 100 g of the volatile chlorinated hydrocarbons was added, the activity of -glucosidase, -acetylglucosaminidase and, in part, also of proteinase, was reduced during the first 28 days of incubation but returned to the same or slightly higher level than in the control soil after 2 months. Trichloroethylene, tetrachloroethylene, and dichloromethane at a concentration of 1000 g per 100 g soil primarily inhibited activity of all enzymes under test. However, after two months, the enzymatic activities especially in soil samples contaminated with tetrachloroethylene and dichloromethane were found to be at the same or higher level than in the control soil.