Absence Of Mycorrhizae Research Articles

Effectiveness of Cadmium on Biochemical Shift of Pea Plant Treated with Mycorrhiza and Putrescine

Heavy metals like cadmium (Cd), mercury (Hg), bismuth (Bi), and arsenic (As) are potent and harmful poisonous sources that cause havoc on health conditions for the population of the world. However, the response of our crop species to these potent heavy metals-related toxicity is still left to be fully understood. It is a matter of great concern, as we are heavily dependent on crop species like rice, wheat, peas, etc. Our study here aims to learn about the defensive mechanism of Pisum sativum L. aided with putrescine and mycorrhiza against the stress created by Cd-related toxicity. We quantified physiological parameters such as the membrane-related injury and stability index. We further measured the total free proline content, lipid peroxidation content, and SOD activity. We executed our quantitative experiments on the stressed pea plants due to the exogenously applied Cd-toxicity in the presence and absence of mycorrhiza and putrescine. Insights of our significant results will improve the understanding of readers of the role of mycorrhiza and putrescine in improvising the tolerance level of a pea plant over Cd-related toxicity.

Genome-Wide Association Studies Reveal Genomic Regions Associated With the Response of Wheat (Triticum aestivum L.) to Mycorrhizae Under Drought Stress Conditions.

In the majority of wheat growing areas worldwide, the incidence of drought stress has increased significantly resulting in a negative impact on plant development and grain yield. Arbuscular mycorrhizal symbiosis is known to improve drought stress tolerance of wheat. However, quantitative trait loci (QTL) involved in the response to drought stress conditions in the presence of mycorrhizae are largely unknown. Therefore, a diverse set consisting of 94 bread wheat genotypes was phenotyped under drought stress and well watered conditions in the presence and absence of mycorrhizae. Grain yield and yield components, drought stress related traits as well as response to mycorrhizae were assessed. In parallel, wheat accessions were genotyped by using the 90k iSelect chip, resulting in a set of 15511 polymorphic and mapped SNP markers, which were used for genome-wide association studies (GWAS). In general, drought stress tolerance of wheat was significantly increased in the presence of mycorrhizae compared to drought stress tolerance in the absence of mycorrhizae. However, genotypes differed in their response to mycorrhizae under drought stress conditions. Several QTL regions on different chromosomes were detected associated with grain yield and yield components under drought stress conditions. Furthermore, two genome regions on chromosomes 3D and 7D were found to be significantly associated with the response to mycorrhizae under drought stress conditions. Overall, the results reveal that inoculation of wheat with mycorrhizal fungi significantly improves drought stress tolerance and that QTL regions associated with the response to mycorrhizae under drought stress conditions exist in wheat. Further research is necessary to validate detected QTL regions. However, this study may be the starting point for the identification of candidate genes associated with drought stress tolerance and response to mycorrhizae under drought stress conditions. Maybe in future, these initial results will help to contribute to use mycorrhizal fungi effectively in agriculture and combine new approaches i.e., use of genotypic variation in response to mycorrhizae under drought stress conditions with existing drought tolerance breeding programs to develop new drought stress tolerant genotypes.

Does mycorrhizal status alter herbivore-induced changes in whole-plant resource partitioning?

Both mycorrhizae and herbivore damage cause rapid changes in source-sink dynamics within a plant. Mycorrhizae create long-term sinks for carbon within the roots while damage by leaf-chewing herbivores causes temporary whole-plant shifts in carbon and nitrogen allocation. Thus, induced responses to herbivory might depend on the presence or absence of mycorrhizae. We examined the effects of mycorrhizal presence on induced resource partitioning in tomato (Solanum lycopersicon) in response to cues from a specialist herbivore Manduca sexta. Differences in plant size, growth and in the concentrations of carbon-based (soluble sugars and starch) and nitrogen-based (protein and total nitrogen) resources in three tissue types (apex, stem and roots) were quantified. Both mycorrhizae and simulated herbivory altered the concentrations of carbon- and nitrogen-based resources. Mycorrhizae promoted plant growth, altered sugar and starch levels. Simulated herbivory resulted in lower concentrations of most resources (sugar, starch and protein) in the rapidly growing apex tissue, while causing an increase in stem protein. There was only one interactive effect; the effects of simulated herbivory were much stronger on the sugar concentration in the apex of non-mycorrhizal plants. This clearly demonstrates that both mycorrhizal colonization and herbivore cues cause shifts in carbon- and nitrogen-based resources and further shows there is little interference by mycorrhizae on the direction and magnitude of plant responses to herbivory. Overall, our results suggest that herbivore cues, regardless of mycorrhizal status, reduce allocation to the growing apex while inducing protein storage in the stem, a possible mechanism that could increase the tolerance of plants to damage.

Influence of mycorrhizal fungi on seed germination and growth in terrestrial and epiphytic orchids.

Epiphytes constitute over 70% of orchid diversity, but little is known about the functioning of their mycorrhizal associations. Terrestrial orchid seeds germinate symbiotically in soil and leaf litter, whereas epiphytic orchids may be exposed to relatively high light levels from an early stage of development and often produce green seeds. This suggests that seedlings of the two groups of orchids may differ in their responses to light and requirements for mycorrhiza-supplied carbon. The interactive effects of light, exogenous carbon and mycorrhizal status on germination and growth were investigated in vitro using axenic agar microcosms for one tropical epiphyte and three geophytic orchid species. The geophytic species strongly depended on their mycorrhiza for growth and this could not be substituted by exogenous sucrose, whereas the epiphytic species achieved 95% of the mycorrhizal seedling volume when supplied with exogenous sucrose in the dark. Mycorrhiza status strongly interacted with light exposure, enabling germination. Light inhibited or severely reduced growth, especially for the terrestrial orchids in the absence of mycorrhiza. For the first time, this study showed the parallel ecological importance of mycorrhizal fungi in overcoming light inhibition of seed germination and growth in both terrestrial and epiphytic orchids.

Mycorrhizas influence functional traits of two tallgrass prairie species.

Over the past decade, functional traits that influence plant performance and thus, population, community, and ecosystem biology have garnered increasing attention. Generally lacking, however, has been consideration of how ubiquitous arbuscular mycorrhizas influence plant allometric and stoichiometric functional traits. We assessed how plant dependence on and responsiveness to mycorrhizas influence plant functional traits of a warm‐season, C4 grass, Andropogon gerardii Vitman, and the contrasting, cool‐season, C3 grass, Elymus canadensis L. We grew both host species with and without inoculation with mycorrhizal fungi, across a broad gradient of soil phosphorus availabilities. Both host species were facultatively mycotrophic, able to grow without mycorrhizas at high soil phosphorus availability. A. gerardii was most dependent upon mycorrhizas and E. canadensis was weakly dependent, but highly responsive to mycorrhizas. The high dependence of A. gerardii on mycorrhizas resulted in higher tissue P and N concentrations of inoculated than noninoculated plants. When not inoculated, E. canadensis was able to take up both P and N in similar amounts to inoculated plants because of its weak dependence on mycorrhizas for nutrient uptake and its pronounced ability to change root‐to‐shoot ratios. Unlike other highly dependent species, A. gerardii had a high root‐to‐shoot ratio and was able to suppress colonization by mycorrhizal fungi at high soil fertilities. E. canadensis, however, was unable to suppress colonization and had a lower root‐to shoot ratio than A. gerardii. The mycorrhiza‐related functional traits of both host species likely influence their performance in nature: both species attained the maximum responsiveness from mycorrhizas at soil phosphorus availabilities similar to those of tallgrass prairies. Dependence upon mycorrhizas affects performance in the absence of mycorrhizas. Responsiveness to mycorrhizal fungi is also a function of the environment and can be influenced by both mycorrhizal fungus species and soil fertility.

Plant Genotype Influences Mycorrhiza Benefits and Susceptibility to a Soil Pathogen

Abstract This study examines the role mycorrhizae may play in protecting host plants from a soil fungal pathogen. Asexual propagules of three Allium vineale genotypes were grown in the presence and absence of mycorrhizae and the pathogen, Sclerotium cepivorum (white rot), with some plants encountering the mycorrhizal fungi first and others encountering the pathogen first. A significant three-way interaction indicates plant genotypes vary in response to the combined effects of mycorrhizae and the pathogen, with the A. vineale genotype most resistant to the pathogen deriving the least growth benefit from mycorrhizae and the genotype most susceptible to the pathogen receiving the highest relative growth benefit from mycorrhizae, despite there being no difference in the level of mycorrhizal colonization among the plant genotypes. No evidence was found that the presence of mycorrhizae reduced the impact of the pathogen for the most susceptible plant genotype (B). These findings support the hypothesis the prima...

The problem of conifer species migration lag in the Pacific Northwest region since the last glaciation

Multiproxy evidence indicates that warmer-than-present summers became established in Eastern Beringia as early as 14,000–13,000 years ago, but the dispersal of spruces, pines, cedars and hemlocks across the Pacific Northwest (PNW) region of southern Alaska did not begin until at least 1500 years afterwards, and took many thousands of years to be completed. There are many potential reasons for this slow spread of PNW conifers towards their modern range limits. The absence of mycorrhizae in the soils of southern Alaska may have slowed conifer establishment. The availability of soil moisture was another limiting factor. With the exception of Pinus contorta, the other PNW conifers become established most readily from seeds that fall on moist, shaded substrates, thus they are not good pioneering species. Competition with alder and birch played an important role, especially along Prince William Sound and the Kenai Peninsula. Alder or alder and birch dominated these regions until the mid- to late Holocene. The other key element for most PNW conifer species is the precipitation regime. The hemlocks, cedars and Sitka spruce are not drought-hardy. So although the PNW temperature regime may have been warm enough in early postglacial times to support the growth of PNW conifers, it was probably too dry for them to successfully become established in new regions. The conflation of these environmental factors limits our present understanding of the problem, but the recent trend of multi-proxy analysis in Quaternary paleoecology will certainly sharpen our reconstructions. Such proxies as conifer needle stomata and insect fossil remains hold significant promise.

Plant growth and root morphology of Phaseolus vulgaris L. grown in a split-root system is affected by heterogeneity of crude oil pollution and mycorrhizal colonization

Response of plant performance and root properties to heterogeneous distribution of crude oil pollutants and mycorrhizal colonization is poorly understood even though (high) heterogeneity represents the normal case rather than exception in re-vegetation and phytoremediation of polluted soils. We investigated the effects of heterogeneous versus homogeneous distribution of hydrocarbon pollution (crude oil, type OMV® A) and mycorrhizal colonization on shoot and root properties of Phaseolus vulgaris L. using a split-root compartment approach in a factorial design with the main factors pollution and mycorrhiza realized in either one or both root compartments. Apart from plant responses to homogeneous pollution (e.g., decreased shoot and root biomass) or mycorrhization (e.g., larger P contents in leaves) we identified systemic interactions between split roots of heterogeneous treatments. In the absence of mycorrhiza, pollution in one root compartment resulted in locally increased root biomass and root length, and decreased root average diameter (RAD), possibly to overcome (water-)stress by better exploration of the soil volume whereas opposite effects on root biomass and length were observed in the unpolluted conjugated compartment. This systemic impact may be explained by preferential allocation of carbon by the plant to the root system in the polluted compartment on expense of the roots in the conjugated compartment. Stress indicated by decreased shoot/root biomass ratios and smaller foliar P content was not reflected in overall root and shoot biomass, likely due to sufficient supply of water and nutrient resources from the unpolluted compartment. The observed responses were not significant in the presence of mycorrhiza, indicating that mycorrhized plants were less susceptible to the observed impacts of pollutant heterogeneity. Mycorrhization in one of the conjugated compartments systemically induced shorter roots in the non-mycorrhized compartment. The resulting decrease in nutrient supply from the non-colonized roots along with increased carbon allocation to the mycorrhizal association in the conjugated compartment are likely to explain the observed overall decreases of shoot and root biomass. We conclude that heterogeneity of mycorrhizal colonization and pollutant distribution in soil can substantially change the plant response as compared to homogeneous situations studied in most pot experiments. Further studies are required to improve our understanding of mechanisms involved and to evaluate the relevance for heterogeneous field situations.

Characterizing variation in mycorrhiza effect among diverse plant varieties

Exploitation of arbuscular mycorrhizal fungi may be an important approach for development of reduced-input agriculture. We discuss the use of linear models to analyze variation in mycorrhiza response among diverse plant varieties in order to assess the value of mycorrhizas. Our approach allows elimination of variation linked to differences in plant performance in the absence of mycorrhizas and the selection of plant lines that might harbor genetic variation of use to improve the mycorrhizal symbiosis in agriculture. We illustrate our approach by applying it to previously published and to novel data. We suggest that in dealing with a relative trait such as mycorrhiza effect, the choice of measure used to quantify the trait greatly affects interpretation. In the plant populations under consideration, we find evidence for a greater potential to increase mycorrhiza benefit than previously suggested.

Arbuscular mycorrhiza reduces phytoextraction of uranium, thorium and other elements from phosphate rock

Uptake of metals from uranium-rich phosphate rock was studied in Medicago truncatula plants grown in symbiosis with the arbuscular mycorrhizal fungus Glomus intraradices or in the absence of mycorrhizas. Shoot concentrations of uranium and thorium were lower in mycorrhizal than in non-mycorrhizal plants and root-to-shoot ratio of most metals was increased by mycorrhizas. This protective role of mycorrhizas was observed even at very high supplies of phosphate rock. In contrast, phosphorus uptake was similar at all levels of phosphate rock, suggesting that the P was unavailable to the plant–fungus uptake systems. The results support the role of arbuscular mycorrhiza as being an important component in phytostabilization of uranium. This is the first study to report on mycorrhizal effect and the uptake and root-to-shoot transfer of thorium from phosphate rock.

Organic acids in root exudates from Picea abies seedlings influenced by mycorrhiza and aluminum

AbstractOrganic acid anions exuded from roots of Picea abies (Norway spruce) seedlings grown on glass beads in the presence and absence of mycorrhiza (Laccaria bicolor) and aluminum (Al) at pH 3.9 were measured. We wanted to test if the roots exuded more organic acid anions when exposed to Al and if mycorrhization influenced the exudation. Oxalate was exuded in far higher amounts than any other organic acid anion, with a maximum rate of 1.7 nmol (mg root DW)–1 d–1. Mycorrhizal roots exuded significantly more oxalate than nonmycorrhizal roots. The presence of Al did not enhance oxalate exudation. We conclude that the oxalate exuded constitutively by Picea abies/Laccaria bicolor may lead to rhizosphere oxalate concentrations that are relevant for Al resistance.

Biological flora of the British Isles: Cakile maritima Scop.

Summary This account reviews information on all aspects of the biology of Cakile maritima that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation. Cakile maritima (sea rocket) is a succulent, annual species that is confined to maritime strandlines on sand or shingle, and associated foredunes. British material is ssp. integrifolia (= ssp. maritima), found around the Atlantic and Mediterranean coasts of Europe. Cakile maritima shows considerable variation, within and between subspecies, especially in fruit morphology and leaf shape. A very closely related species, Cakile edentula, is native to the east coast of North America. Both species have been introduced to Pacific North America and Australia. Populations of C. maritima tend be ephemeral and shifting, depending on dispersal by tides and wind. The fruits are 2‐segmented: the distal segments are readily detached and can float considerable distances in seawater; the proximal segments tend to shed their seed while attached to the maternal plant. Seeds require cold stratification and do not germinate at high salinity, or usually while retained in intact fruit segments. There is often a flush of germination in strandlines left by early season, equinoctial spring tides. Cakile maritima shows great phenotypic plasticity of form and reproductive output. Work on C. edentula suggests that abundance is regulated by a combination of density‐dependent and density‐independent processes. Landward dispersal of seeds from strandline populations may subsidise foredune populations, which themselves experience severe mortality from predators. Cakile maritima is tolerant of salt spray and transient seawater inundation. Although beach and dune sand is a meagre source of macronutrients, C. maritima shows large growth responses to nitrogen addition and can exploit local nitrogen enrichment associated with mineralisation of organic detritus washed up on the strandline. Growth is stimulated by burial with blown sand and plants sometimes form the nuclei of early successional foredunes. The tissues are rich in glucosinolates; these may be responsible for the limited ranges of herbivores and fungal pathogens, and the absence of mycorrhiza.

Mycorrhiza and root hairs in barley enhance acquisition of phosphorus and uranium from phosphate rock but mycorrhiza decreases root to shoot uranium transfer

Some phosphate rocks (PR) contain high concentrations of uranium (U), which are potentially toxic via accumulation in soils and food chains, and plant uptake of U is likely to be influenced by characteristics of roots and associated microorganisms. The relative importance of root hairs and mycorrhiza in U uptake from PR was studied using a root hairless barley (Hordeum vulgare) mutant (Brb) and its wild type (WT). Both plant genotypes were grown in pots with Glomus intraradices BEG 87, or in the absence of mycorrhiza, and three P treatments were included: nil P, 2% (w/w) PR and 50 mg KH(2)PO(4)-P kg(-1) soil. Mycorrhiza markedly increased d. wts and P contents of Brb amended with nil P or PR, but generally depressed d. wts of WT plants, irrespective of P amendments. Mycorrhiza had contrasting effects on U contents in roots and shoots, in particular in Brb where mycorrhiza increased root U concentrations but decreased U translocation from roots to shoots. The experiment supports our understanding of arbuscular mycorrhiza as being multifunctional by not only improving the utilization of PR by the host plant but also by contributing to the phytostabilization of uranium.

Contrasting effects of microbial partners in the rhizosphere: interactions between Norway Spruce seedlings ( Picea abies Karst.), mycorrhiza ( Paxillus involutus (Batsch) Fr.) and naked amoebae (protozoa)

The importance of the soil microbial community for plant mineral nutrition and nutrient cycling has long been recognized. One of the most important interactions is the symbiosis of plants with mycorrhizas. In contrast, the effects of soil microfauna on plant performance have so far received little attention, although soil protozoa in particular, have been shown to beneficially affect plant growth. We investigated in a laboratory experiment the impact of mycorrhiza and protozoa and their interaction on plant performance. Spruce seedlings with or without the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr. were grown in microcosm chambers with defaunated forest soil with naked amoebae ( Acanthamoeba sp.) or without protozoa for 10 months. The presence of protozoa resulted in the development of a more complex root system by increasing root length (51%), length of fine roots (64%) and number of root tips (43%). The effects of protozoa were more pronounced in the absence of mycorrhiza. In contrast to protozoa, the presence of mycorrhiza resulted in a less complex root system, i.e. root length, length of fine roots and number of root tips were reduced by 47, 47 and 40%, respectively. Shoot height, and stem, shoot and needle mass were at a maximum in the combined treatment with both mycorrhiza and protozoa. The presence of mycorrhiza and protozoa also affected plant nutrient concentrations. In treatments with protozoa shoots of spruce seedlings contained less nitrogen, leading, e.g. to an increased C/N ratio in needles. Conversely, in treatments with mycorrhiza concentrations of phosphorus in needles were increased by a factor of almost two. Mycorrhiza and protozoa also affected rhizosphere microorganisms. Microbial biomass was reduced in the presence of mycorrhiza, mainly due to a reduction in bacterial numbers. Conversely, in the presence of protozoa the length of hyphae in the rhizosphere was reduced. It is concluded that the plant–mycorrhiza mutualism and the bacteria-mediated mutualism between plants and protozoa (microbial loop) complement each other; plant resources presumably are allocated to optimize simultaneous exploitation of both mutualistic relationships.

Interspecies competition and N transfer in a tropical grass-legume mixture

Competitiveness of Brachiaria decumbens cv. Basilisk and Stylosanthes guianensis cv. Minerao was investigated either without root restriction or by separating their root systems with a fine mesh or a solid barrier in the presence or absence of mycorrhiza (Glomus clarum). Nitrogen transfer between the legume and the grass was assessed with the 15N isotope dilution technique using a relatively stable 15N-enriched soil derived from a long-term labelling experiment. During establishment, legume development was severely restricted by competition from the grass in pots without a root barrier. However, as the system became N limited, the legume became dominant due to its access to atmospheric N2 which contributed over 80% of the legume N requirements. S. guianensis was highly mycotrophic and inoculation with mycorrhiza favoured rapid establishment even in the treatments with no root barrier. Only in the presence of root barriers, either a mesh or a complete compartment separation, was the proportion of N derived from N2 fixation positively affected by the presence of the fungus. No significant direct belowground N transfer from legume to grass was observed during the lifetime of the legume suggesting that the legume maintains a highly efficient recycling under N-limited conditions. However, after cutting the shoot at ground level, the grass assimilated significant amounts of N derived from decaying legume roots. We conclude that the main pathway of belowground N transfer from S. guianensis to associated B. decumbens occurred via decomposing roots rather than via root exudates or direct mycorrhizal hyphae transfer.

Micorriza arbuscular e fertilização do solo no desenvolvimento pós-transplante de mudas de sete espécies florestais

Mudas de sete espécies florestais foram formadas em substrato de viveiro sem e com inoculação da mistura de fungos micorrízicos arbusculares (Glomus etunicatum, Gigaspora margarita e Acaulospora scrobiculata) e transplantadas para vasos com um solo Latossolo Vermelho-Escuro com alta e baixa fertilização com NPK e submetidas, ou não, a nova inoculação. Verificou-se, após 90 dias, que tanto a inoculação na formação quanto a inoculação no transplantio garantiu elevada colonização micorrízica (>70%), estimulou o crescimento e aumentou os teores de alguns nutrientes nas plantas. Os efeitos no crescimento variaram entre as espécies e tratamentos, atingindo incrementos de matéria seca de até 800% em Colvillea racemosa. Plantas sem inoculação na formação e no transplantio, apresentaram crescimento reduzido, mesmo no solo com alta fertilidade, enquanto as plantas com inoculação na formação cresceram mais rapidamente, independentemente da reinoculação. A elevação da fertilidade não aumentou a matéria seca da parte aérea de Luehea grandiflora, Senna macranthera e Enterolobium contortisiliquum. Em Cecropia pachystachya aumentou apenas quando as mudas não foram submetidas a inoculação. Em Senna multijuga e em C. racemosa, a matéria seca da parte aérea aumentou quando as mudas foram submetidas a inoculação e em Sesbania virgata, aumentou em todos os tratamentos. Apenas C. racemosa não respondeu à inoculação no transplantio.

Effects of drought, transgenic expression of a fructan synthesizing enzyme and of mycorrhizal symbiosis on growth and soluble carbohydrate pools in tobacco plants

The effects of three conditions likely to affect soluble carbohydrate pools, namely drought, expression of barley sucrose: fructan 6‐fructosyl transferase (6‐SFT, EC 2.4.1.10) and the establishment of the arbuscular mycorrhizal symbiosis with Glomus mosseae were studied in a multifactorial experiment using tobacco (Nicotiana tabacum). Tobacco, a plant naturally unable to form fructan, accumulated fructan in leaves, and to a larger extent in the roots, when transformed with 6‐SFT. Under drought conditions, growth was considerably reduced, but neither expression of 6‐SFT nor mycorrhiza formation had an effect on growth rate. However, in response to drought, carbon partitioning was significantly altered towards accumulation of soluble sugars. In plants exposed to drought, pools of sucrose were greater than those of unstressed plants, particularly in their roots. In the transgenic plants expressing 6‐SFT, there were also increased contents of the products of 6‐SFT, namely fructan, most probably because of the increased availability of the substrate, sucrose. These effects were the same in the presence or absence of mycorrhiza. Hexoses (glucose and fructose) also increased in response to drought, primarily in the leaves. This effect of drought was little affected by the expression of 6‐SFT, except that it slightly enhanced drought‐induced glucose accumulation in roots. However, the presence of mycorrhiza led to a considerable reduction in drought‐induced accumulation of hexoses in the leaves. The content of the fungal disaccharide trehalose was greatly increased in the roots of all mycorrhizal plants upon exposure to drought, particularly in some of the transgenic plants expressing 6‐SFT.

Comparative nutrient responses by Pinus radiata, Trifolium repens, Dactylis glomerata, and Hieracium pilosella on a Mackenzie Basin outwash plain soil

The nutrient responses by a pine (Pinus radiata), a pasture grass (cocksfoot, Dactylis glomerata), a pasture legume (white clover, Trifolium repens), and a hawkweed (mouse‐ear hawkweed, Hieracium pilosella), were compared in a pot trial using a Mackenzie Basin outwash plain soil. Responses to nitrogen (N) and phosphorus (P) were tested alone and in combination, whereas responses to sulphur (S), potassium (K), magnesium (Mg), micronutrients, and lime were tested only in the presence of both N and P. All species responded to N and P, and to S and lime in the presence of N and P, but none responded to K, Mg, or micronutrients. N was the major deficient nutrient for the non‐legume species, with grass and hawkweed responding by 700% in the presence of added P, whereas pine showed a much smaller (56%) response. Clover showed the greatest response to P. Hawkweed showed the smallest response to P, but the greatest responses to S and lime. While pine responded to the same nutrients as the herbaceous species, it is suggested that the responses by pine occurred before mycorrhizae formation, and may be more pertinent to seedlings establishing from seed in the absence of mycorrhizae, than to plants with well‐developed mycorrhizal root systems. The responses by pine to N, P, S, and lime reported here therefore need to be confirmed with plants which possess mycorrhizal root systems at the time of fertiliser application. The comparative nutrient responses by mouse‐ear hawkweed are discussed. It is suggested that hawkweed is efficient at absorbing P, an attribute that would be of competitive advantage in soils with low P availability and which may have contributed to its widespread invasion of the tussock grasslands.

Physiological activity of a bioluminescent Pseudomonas fluorescens (strain 2–79) in the rhizosphere of mycorrhizal and non-mycorrhizal pepper ( Capsicum annuum L.)

Physiological status and root-colonization of Pseudomonas fluorescens 2–79RL were examined in presence or absence of mycorrhizae on the roots of pepper over 6 weeks. Due to incorporation of a ribosomal promoter-driven lux marker into the chromosome of this bacterium, the end of the lag phase and onset of exponential growth is marked by an exponential increase in bioluminescence. In vitro, the duration of the lag phase of bioluminescence after transfer of cells to rich medium increased when the cells had been grown under low nutrient conditions or subjected to prolonged starvation. Lag phase duration thus provided a marker for assessment of changes in the physiological status of P. fluorescens 2–79RL. Pepper plants, non-mycorrhizal or mycorrhizal with Glomus intraradices (GI) or Glomus deserticulum (GD), were grown for 42 days in microcosms containing soil inoculated with P. fluorescens 2–79RL. A blotting technique using filter paper strips was developed to sample cells from the rhizosphere, after which the cells were resuspended for enumeration and determination of lag phase duration. During the first 3 weeks, the population density of P. fluorescens 2–79RL (cfu mm −2) decreased by one order of magnitude on the roots and by two orders of magnitude in the bulk soil. The lag phase of bioluminescence remained stable during the first 20 days in the rhizosphere and the bulk soil, but then increased up to day 27 and remained at the higher level until day 38. The length of the lag phase was greater in the bulk soil than in the rhizosphere. Compared with the non-mycorrhizal plants, mycorrhizal infection by GD significantly decreased shoot dry weight and the population of P. fluorescens 2–79RL in the rhizosphere at the root tip and on the older root parts. GD also significantly increased the length of the lag phase of bioluminescence on the older root parts. The effect of GI was less pronounced. The results show this bacterium may be subject to moderate to severe starvation in the rhizosphere and that there are differential effects of specific mycorrhizal fungi on both the population and activity of this root-colonizing pseudomonad.

SOURCES OF MYCORRHIZAL INFECTION OF SHOREA ACUMINATA SEEDLINGS UNDER LABORATORY CONDITIONS*)

Uninoculated dipterocarp seedlings raised in normal field soil in nurseries were always found to have mycorrhizas after a few months. This study set out to determine whether dipterocarp seedlings could continue to grow and develop in the absence of mycorrhizas and also to determine possible sources of mycorrhizal infection of dipterocarp seedlings raised under laboratory conditions using Shorea acuminata as a typical example. Seedlings were planted in capped or uncapped perspex boxes containing sterile or non-sterile field soil and watered daily with sterile water or tap water. Seedling growth and development of mycorrhizas were monitored at monthly intervals for up to seven months. Seedlings grown in sterile soil remained uninfected after seven months while infection was found in some of the seedlings grown in normal soil regardless of whether they had been watered with tap water or sterile water. This showed that field soil (i.e. under grass) far from the forest contained suitable inoculum for forest tree seedlings. Tap water and the air were not important sources of infection. However, mycorrhizal infection was very uneven indicating that the inoculum was probably very unevenly distributed in the soil or that the inoculum density was rather low. Seedlings grown in sterile soil showed better growth than those grown in normal soil and infection of roots by parasitic fungi in the latter was also observed. Key words: Mycorrhizas/Plant pathology/lnfections/Shorea acuminata/Seedlings.