Seasonal variations in the spatial distribution of root tips in teak (Tectonia grandis Linn. f.) plantations in the Varanasi Forest Division,India

Summary Seasonal variations in the spatial distribution of root tips were studied in 19 and 29 year old teak plantations, located on red and alluvial soils respectively. The pattern was essentially similar at both sites, but generally the alluvial soil site exhibited a greater number of root tips. Root tips decreased with increasing distance from the tree base. Through-out most of the year the relative distribution of root tips decreased with depth; the difference between 0–10 and 10–20 cm depths was marginal, but 20–40 cm depth contained distinctly fewer root tips. At all distances a similar seasonal trend was noticed, a mid rainy season peak being followed by a steady decline until the dry summer except for an abrupt rise to a smaller peak in February after the winter rains.The root tip density was positively correlated with the 2 mm root biomass and both showed a similar bimodal annual cycle. Of three environmental variables studied, soil moisture and rainfall were significantly positively correlated with root tip densityl the relationship between soil temperature and root tip density was negative and non-significant. The combined effect of soil moisture and temperature on root tip density, evaluated by a multiple regression model, accounted for 80–95% of the variation in root tip density.     

Spatial and temporal distribution of a bioluminescent‐marked Pseudomonas putida on soybean root

The ability of rhizobacteria to compete with other microorganisms for root colonization may be critical for its establishment on a root. Over a 6 day period, visualization of the spatial and temporal rhizosphere distribution of a bioluminescent‐marked rhizobacterium, Pseudomonas putida, strain GR7.4lux, was examined on soybean grown in non‐sterile soil conditions. Luminometry technologies showed a rapid root distribution of rhizobacteria where bioluminescence was particularly intense on the seed and upper root parts. The results provide new information on rhizobial root distribution, where, using enrichment broth, 50% of the root tips were still colonized by rhizobacteria up to 6 days after sowing. This suggests that rhizobial enrichment is required to detect low populations at the root tip. Bioluminescent technology represents a promising alternative to previous methods for studying rhizobial growth and distribution on roots. Copyright © 2003 John Wiley & Sons, Ltd.     

A novel method for sampling bacteria on plant root and soil surfaces at the microhabitat scale

This study reports the first method for sampling bacteria at a spatial scale approximating a microhabitat. At the core of this method is the use of tungsten rods with laser-cut tips of known surface area (0.013 mm(2)). Exposed plant root or soil surfaces were viewed with a dissecting microscope and micro-sampling rods were guided to sample sites using a micro-manipulator. Bacteria that adhered to the sampling tips were then recovered for microbiological analyses. The efficiency of this method for removing bacteria from root surfaces was similar to that with which bacteria are recovered from dissected root segments using the conventional technique of washing. However, as the surface area of the micro-sampling tips was known, the new method has the advantage of eliminating inaccuracy in estimates of bacterial densities due to inaccurate estimation of the root or soil surface sampled. When used to investigate spatial distributions of rhizoplane bacteria, the new technique revealed trends that were consistent with those reported with existing methods, while providing access to additional information about community structure at a much smaller spatial scale. The spatial scale of this new method is ca. 1000-times smaller than other sampling methods involving swabbing. This novel technique represents an important methodological step facilitating microbial ecological investigations at a microhabitat scale.     

Diversity of an ectomycorrhizal fungal community studied by a root tip and total soil DNA approach

Molecular methods based on soil DNA extracts are increasingly being used to study the fungal diversity of ectomycorrhizal (EM) fungal communities in soil. Contrary to EM root tip identification, the use of molecular methods enables identification of extramatrical mycelia in soil. To compare fungal diversity as determined by root tip identification and mycelial identification, six soil samples were analysed. Root tips were extracted from the six samples and after amplification, the basidiomycete diversity on the root tips was analysed by denaturing gradient gel electrophoresis (DGGE). The soil from the six samples was sieved, total soil DNA was extracted and after amplification, the basidiomycete diversity in the soil fractions was analysed by DGGE. Fourteen different bands were excised from the DGGE gel and sequenced; fungal taxon names could be assigned to eight bands. Out of a total of 14 fungal taxa detected in soil, 11 fungal taxa were found on root tips, of which seven were EM fungal taxa. To examine whether the sieving treatment would affect EM species diversity, two different sieve mesh sizes were used and in addition, the organic soil fraction was analysed separately. DGGE analysis showed no differences in banding pattern for the different soil fractions. The organic fraction gave the highest DGGE band intensities. This work demonstrates that there is a high correspondence between basidiomycete diversity detected by molecular analysis of root tips and soil samples, irrespective of the soil fraction being analysed.     

东北红松纯林菌根外生菌根真菌群落与环境因子的相关性

外生菌根是木本植物根系与真菌形成的共生结构,外生菌根真菌在红松等外生菌根树种的定植与森林生态系统的保持方面起到至关重要的作用。明确菌根系统内外生菌根真菌群落组成是揭示菌根共生机制的前提条件。本研究利用Illumina Hiseq测序平台对生长季内红松纯林内根围土壤及菌根样品ITS2区进行高通量测序,分析其外生菌根真菌群落结构随季节的变化规律,同时通过统计学的方法分析了红松根系微生态中外生菌根真菌群落结构组成变化与其他生物因素、非生物因素的相关性。结果如下：（1）从6月份到10月份,5个月的菌根样品测序共得到741个真菌OTUs,利用FUNGuild数据库分析,其中85个OTUs归类为外生菌根真菌,优势属（相对丰度>5）为蜡壳菌属Sebacina、乳牛肝菌属Suillus、Meliniomyces、红菇属Russula、棉革菌属Tomentella、须腹菌属Rhizopogon和缘腺革菌属Amphinema。6月份菌根中外生菌根真菌的多样性最大,显著高于其他月份。（2）红松林外生菌根真菌群落组成受到土壤pH、有效磷含量、有效钾含量和土壤有效氮含量的影响,它们与外生菌根真菌优势属相对丰度呈现正相关或负相关。（3）根围土壤内真菌是影响红松根系外生菌根真菌相对丰度的另一重要因素,其中,包括普可尼亚属Pochonia、产丝齿菌属Hyphodontia、镰刀菌属Fusarium、Collembolispora、枝穗霉属Clonostachys、Apodus、鹅膏属Amanita在内的土壤真菌与根内外生菌根真菌的相对丰度呈线性关系。同时,超过85%的根内外生菌根真菌与同一取样地的土壤共有,可以认为侵染和扩散是红松根内外生菌根真菌群落形成的主要方式,同时兼有植物根系的选择,因为根内并不包括所有土壤中存在的外生菌根真菌,其机制需要进一步人工模拟试验验证。     

Lifetime and temporal occurrence of ectomycorrhizae on ponderosa pine (Pinus ponderosa Laws.) seedlings grown under varied atmospheric CO2 and nitrogen levels

Climate change (elevated atmospheric CO2, and altered air temperatures, precipitation amounts and seasonal patterns) may affect ecosystem processes by altering carbon allocation in plants, and carbon flux from plants to soil. Mycorrhizal fungi, as carbon sinks, are among the first soil biota to receive carbon from plants, and thereby influence carbon release from plants to soil. One step in this carbon release is via fine root and mycorrhizal turnover. It is necessary to know the lifetime and temporal occurrence of roots and mycorrhizae to determine the capacity of the soil ecosystem to sequester carbon assimilated aboveground. In this study, ponderosa pine (Pinus ponderosa Laws) seedlings were grown under three levels of atmospheric CO2 (ambient, 525 and 700 mol CO2 mol-1) and three levels of annual nitrogen additions (0,100 and 200 kg N ha-1) in open-top chambers. At a two-month frequency during 18 months, we observed ectomycorrhizal root tips observed using minirhizotron tubes and camera. The numbers of new mycorrhizal root tips, the numbers of tips that disappeared between two consecutive recording events, and the standing crop of tips at each event were determined. There were more mycorrhizal tips of all three types seen during the summer compared with other times of the year. When only the standing crop of mycorrhizal tips was considered, effects of the CO2 and N addition treatments on carbon allocation to mycorrhizal tips was weakly evident. However, when the three types of tips were considered collectively, tips numbers flux of carbon through mycorrhizae was greatest in the: (1) high CO2 treatment compared with the other CO2 treatments, and (2) intermediate N addition treatment compared with the other N addition treatments. A survival analysis on the entire 18 month cohort of tips was done to calculate the median lifetime of the mycorrhizal root tips. Average median lifetime of the mycorrhizal tips was 139 days and was not affected by nitrogen and CO2 treatments.     

Seasonal root growth in the arctic tussock tundra

Summary The quantity of growing root tips per unit of soil volume was analyzed in a central Alaskan tussock tundra site. By June 10, the aboveground fraction of the vegetation had initiated the flush of spring growth and flowering while less than 5 active root tips cm^-3 were found. By June 25 this value had increased to 30 root tips cm^-3. Similar values in July were followed by a complete cessation of root growth after the first week of August. By then, leaf senescence had also become visible. In the spring, low root temperatures were responsible for the time lag between shoot growth initiation and the beginning of root growth. In early August, root growth stopped in spite of adequate soil temperatures and accumulated carbohydrate for root growth. It is proposed that use of reserve carbohydrate for root growth in August would compromise the flush of spring growth in the following year.     

Contrasting below-ground views of an ectomycorrhizal fungal community

Ectomycorrhizal fungal communities have been characterized in a number of ways. Here we compare colonized root-tip and mycelia views of an ectomycorrhizal fungal community. Ectomycorrhizal fungi, both as mycelia and colonized root tips, were identified in soil samples taken from a pine plantation. We determined that for some ectomycorrhizal fungal species multiple root tips from a single soil sample were not independent. Therefore in the comparison of root-tip and mycelia views, we considered species to be present or absent from each soil sample irrespective of the number of root tips colonized by the species. We observed 39 ectomycorrhizal fungal species in total, but 12 were observed exclusively as mycelia and 11 exclusively colonizing root tips. The relative frequencies of 10 species occurring as both mycelia and root tips were not independent of the method of observation. Our results suggest that ectomycorrhizal fungal species differ in their spatial distributions on root tips, and that root-tip and mycelia views of the community are different.     

Plant-induced changes in the redox potentials of rice rhizospheres

Redox potentials in microsites of the rhizosphere of flooded rice were continuously measured for several days. Close to the root tips redox potential markedly increased. The highest increase was measured in the rhizosphere of the tips of short lateral roots. Aerobic redox conditions were reached there, except in a very strongly reduced soil. Both the extension of the oxidation zone around the root tips and the maximum redox potential reached were influenced by the reducing capacity of the soil. The radius of the redox rhizosphere varied from less than 1 mm in a strongly reduced soil up to 4 mm in a weakly reduced one. The root-induced oxidation processes in the rhizosphere depended on the atmospheric oxygen supply to the roots.     

Ecophysiology of ectomycorrhizal fungi associated with Pinus spp. in low rainfall areas of Western Australia

As a potential means of monitoring functional properties of plantations of Pinus pinaster and Pinus radiata established as part of land rehabilitation in the wheatbelt of Western Australia, we examined aspects of the ecophysiology of ectomycorrhizal fungi associated with tree roots. A single species of ectomycorrhizal fungi, Rhizopogon roseolus, dominated the mycorrhizal flora. Sporocarps of Rhizopogon roseolus appeared with the onset of winter rains in May, increased in number and total biomass to peak in September, and decreased to negligible levels at the beginning of the summer drought in December. A greater number of sporocarps, and consequently a greater biomass of sporocarp tissue, was associated with roots of P. radiata than P. pinaster. A similar seasonal pattern of mycorrhizal root infection was determined by counts of individual ectomycorrhizal root tips from bimonthly collection of soil core samples. At the low rainfall (380 mm annually) site, greater numbers of live root tips were more strongly correlated with soil moisture than organic matter content of soil. In contrast, in wetter areas closer to Perth (800 mm annually), highest numbers of active root tips and greatest amounts of organic matter were both within 0–10 cm depths. Results suggest an overriding importance of soil moisture rather than nutrient status of the soil as the key determinant of spatial and temporal distribution of the fungus. Results from a range of assays determining enzyme activity of soil (protease, phosphomonoesterase, cellulase, L-asparaginase, L-glutaminase and β-glucosidase) surrounding mycorrhizal roots indicated seasonal patterns to be similar to those described for reproductive activity of mycorrhizal fungi. Factors responsible for patterns of seasonal activity and distribution of ectomycorrhizal roots are discussed in terms of managing systems in order to maximise tree growth and form while effectively restoring soil water balance.     

Ectomycorrhizal root development in wet Alder carr forests in response to desiccation and eutrophication

Effects of desiccation and eutrophication on ectomycorrhizal (ECM) root development in wet Alder carr forests in The Netherlands were studied. In northwestern Europe, wet Alder carr forests are found mostly in peatlands and along streams, forming an important component of wetland ecosystems. The dominant tree species in wet Alder carr forests is Alnus glutinosa (L.) Gaertn. (Black alder), which associates with ectomycorrhizal fungi. During recent decades, wet Alder carr forests in Europe have declined because of desiccation and eutrophication, particularly in The Netherlands. In the present study, the number of root tips of A. glutinosa trees was highest in an undisturbed wet Alder carr forest in a peatland area. Eutrophication in the peatland area significantly inhibited ectomycorrhizal (ECM) root development of A. glutinosa. In the eutrophied forest, ECM root tips were observed only close to A. glutinosa trees growing on hummocks. The concentrations of nitrate and potassium in soil water of the eutrophied forest were significantly higher than in the undisturbed forest, while magnesium and iron concentrations and the pH were significantly lower. The number of ECM root tips of A. glutinosa in a desiccated forest along a stream was generally lower than in an undisturbed wet Alder carr forest on waterlogged soil in the same area. The sulphate concentration in soil water in the desiccated forest was significantly higher than in the forest on waterlogged soil. ECM root development of A. glutinosa may have been negatively affected by the chemical composition of the soil water.     

Performance of two Picea abies (L.) Karst. stands at different stages of decline

Summary The development of root tips and apparent ectomycorrhizas was compared in the Fichtelgebirge (FRG) over one growing season in two 30-year-old Picea abies stands, both on soils derived from phyllite but showing varying symptoms of decline. Visual symptoms of tree decline reflected a lower relative and absolute mycorrhizal frequency, a lower number of ectomycorrhizas per m² leaf area and an uneven vertical distribution of root tips and ectomycorrhizas. The number of apparent ectomycorrhizas per ground area was correlated with the amount of magnesium, calcium, and ammonium, and the pH in the free-drainage soil solution, and with the molar calcium to aluminium ratio in mineral soil extracts. The foliage concentrations of magnesium and calcium were correlated with the numbers of apparent ectomycorrhizas per m² leaf or ground area. These observations were used to formulate testable hypotheses concerning the role of the root system and the soil environment in forest decline.     

A new approach for the quantification of root-cap mucilage exudation in the soil

Direct evidence on the functions of root-cap mucilage during plant root growth in soil is limited mainly due to the lack of a method for in situ measurements. In this paper, we offer a method that facilitates the measurement of mucilage exudation when roots are growing in soil. We observed the mucilage exudation directly through a transparent panel located on the side of a root box in which plant roots were growing. We used a CCD camera attached to a microscope to observe and record mucilage exudation. Using image analysis, the activity of mucilage exudation was evaluated based on the area occupied by the mucilage on the root tip. The area of mucilage observed on the root tips after 1-h growth in soil corresponded with the weight of mucilage that was originally observed on the tips before they were transplanted. This relationship suggests that the observed area on root tip relates to total exudation. The area of mucilage exudation on the root tips was high (0.48 mm²) at night and low (0.35 mm²) at midday, suggesting that the activity of mucilage exudation follows diurnal changes. Furthermore, the mucilage exudation positively correlated with the root elongation rate, implying that fast-growing roots exude more mucilage.     

Effects of ectomycorrhizal colonization and nitrogen fertilization on morphology of root tips in a <Emphasis Type="Italic">Larix gmelinii</Emphasis> plantation in northeastern China

Root morphology is important in understanding root functions in forest ecosystems. However, the effects of ectomycorrhizal colonization and soil nutrient availability on root morphology is not clear. In this study, root morphology in relation to season, soil depth, soil nitrogen (N) availability, and mycorrhizal fungal colonization were investigated in a larch (Larix gmelinii) plantation in northeastern China. The first-order roots (or root tips) of larch were sampled four times in May, July, and September of 2005, and May of 2006 from two depths of upper soil layer (0–10 and 10–20 cm) in the control and the N-fertilized plots. The results showed that ectomycorrhizal (ECM) colonization rates for the first-order roots were reduced by 17% under N fertilization. The peak of root colonization rates occurred in summer and was positively correlated with soil temperature. ECM colonization significantly altered root morphology: root diameter was increased by 19 and 29%, root length shortened by 27 and 25%, and specific root length (SRL) reduced by 16 and 19% for the control and the N-fertilized plots, respectively. N fertilization led to decreased root length, but did not affect root diameter and SRL. In addition, effects of ECM colonization on root morphology varied with season and soil depth. The observed relationships among ECM fungal colonization, soil N availability, and root-tip morphology should improve our understanding of how root tips respond to environmental changes in soil in temperate forest ecosystems.     

Salinity effects on germination,growth, and seed production of the halophyte Cakile maritima

The growth ofEucalyptus regnans seedlings in forest soil is enhanced when it has been air-dried. In undried forest soil seedlings grow poorly and develop purple coloration in the foliage, indicating P deficiency. This paper reports the results of pot experiments designed to investigate the relationship between growth and P acquisition, ectomycorrhizal infection and age of seedlings grown in air-dried and undried soil. The effect on seedling growth of their inoculation with air-dried or undried soil or with ectomycorrhizal roots from plants growing in air-dried or undried soil was also investigated. Ectomycorrhizal root tips were detected in 3-week-oldE. regnans seedlings in both air-dried and undried soil, and from then on the frequency of ectomycorrhizal root tips increased rapidly. In air-dried soil, seedlings were fully ectomycorrhizal at 9 weeks, and the occurrence of maximum ectomycorrhizal infection coincided with enhanced P acquisition and the initiation of rapid seedling growth. In undried forest soil, seedling growth remained poor, even though the seedlings had well-developed ectomycorrhizae and the incidence of ectomycorrhizal root tips was the same as in air-dried soil. The dominant ectomycorrhizae in airdried soil were associated with an ascomycete fungus, whereas in undried, undisturbed soil they were commonly associated with basidiomycete fungi. Inoculation of sterile soil/sand mix with washed ectomycorrhizal roots from air-dried soil increased the P acquisition and growth of the seedlings significantly compared with controls, whereas ectomycorrhizal inocula from undried soil had no effect on seedling growth, although both inocula resulted in a similar incidence of ectomycorrhizal root tips. Similarly, addition of a small amount of air-dried soil into sterile soil/sand mix resulted in a significantly greater increase in the P content and dry weight of the seedlings, compared with the control, than addition of undried soil. In both treatments, the incidence of ectomycorrhial root tips was similar. As (i) the differentiation in seedling growth between air-dried and undried soil occurred after seedlings became ectomycorrhizal, (ii) the dominant ectomycorrhizae in air-dried soil were different from those in undried soil, and (iii) inocula from air-dried soil, but not from undried soil, stimulated seedling growth in sterile soil/sand mix, it is concluded that development of particular ectomycorrhizae may be involved in seedling growth stimulation and enhanced P acquisition associated with air drying of forest soil.     

Effects of liming on ectomycorrhizal community structure in relation to soil horizons and tree hosts

Liming is a forestry practice used to correct tree cation deficiency induced by soil acidity. Ectomycorrhizal (ECM) community structure and functioning is closely linked to soil nutrient availability, which is strongly affected by liming. The aim of this study was to assess the impact of liming on ECM community structure depending on soil horizon and tree host. Acidophilic species occurring in untreated plots, such as Russula ochroleuca, were absent from limed plots and were replaced by more generalist morphtoypes. The abundance of ECM root tips in the untreated plots was higher in topsoil layers, whereas most of the ECM root tips in the limed plots were in the organomineral layer, whatever the tree host. Liming was the major determinant of fungal community structure, then tree host.     

Enzymatic activities and stable isotope patterns of ectomycorrhizal fungi in relation to phylogeny and exploration types in an afrotropical rain forest

? Ectomycorrhizal (ECM) fungi obtain both mineral and simple organic nutrients from soil and transport these to plant roots. Natural abundance of stable isotopes ((15) N and (13) C) in fruit bodies and potential enzymatic activities of ECM root tips provide insights into mineral nutrition of these mutualistic partners. ? By combining rDNA sequence analysis with enzymatic and stable isotope assays of root tips, we hypothesized that phylogenetic affinities of ECM fungi are more important than ECM exploration type, soil horizon and host plant in explaining the differences in mineral nutrition of trees in an African lowland rainforest. ? Ectomycorrhizal fungal species belonging to extraradical mycelium-rich morphotypes generally displayed the strongest potential activities of degradation enzymes, except for laccase. The signature of (15) N was determined by the ECM fungal lineage, but not by the exploration type. ? Potential enzymatic activities of root tips were unrelated to (15) N signature of ECM root tip. The lack of correlation suggests that these methods address different aspects in plant nutrient uptake. Stable isotope analysis of root tips could provide an additional indirect assessment of fungal and plant nutrition that enables enhancement of taxonomic coverage and control for soil depth and internal nitrogen cycling in fungal tissues.     

The toxicity of peach tree roots

Summary The HCN content of peach plants and the effect of this and other substances formed from peach root residues on growth of peach trees and on soil microorganisms were investigated. Peach root bark contained appreciable amounts of HCN. HCN was released from live roots after mechanical injury. Benzaldehyde and KCN were toxic to rooted peach trees in the greenhouse and they inhibited respiration of peach root tips. A similar suppression of respiration was caused by benzoic acid, mandelonitrile, and water extracts of peach root bark incubated in peach and non-peach soils. Extracts from peach soil caused greater inhibition to respiration of peach root tips than extracts from non-peach soil.KCN, mandelonitrile, benzaldehyde, peach root bark, and amygdalin reduced the total micro-organism, actinomycete, pythium, and pathogenic nematode population of an old peach soil.     

Effects of Liming on Potential Oxalate Secretion and Iron Chelation of Beech Ectomycorrhizal Root Tips

Liming is used to counteract forest decline induced by soil acidification. It consists of Ca and Mg input to forest soil and not only restores tree mineral nutrition but also modifies the availability of nutrients in soil. Ectomycorrhizal (ECM) fungi are involved in mineral nutrient uptake by trees and can recover them through dissolution of mineral surface. Oxalate and siderophore secretion are considered as the main agents of mineral weathering by ECMs. Here, we studied the effects of liming on the potential oxalate secretion and iron complexation by individual beech ECM root tips. Results show that freshly excised Lactarius subdulcis root tips from limed plots presented a high potential oxalate exudation of 177 μM tip⁻¹ h⁻¹. As this ECM species distribution is very dense, it is likely that, in the field, oxalate concentrations in the vicinity of its clusters could be very high. This points out that not only extraradical mycelium but also ECM root tips of certain species can contribute significantly to mineral weathering. Nonmetric multidimensional scaling (NMDS) separated potential oxalate production by ECM root tips in limed and untreated plots, and this activity was mainly driven by L. subdulcis ECMs, but NMDS on potential activity of iron mobilization by ECM root tips did not show a difference between limed and untreated plots. As the mean oxalate secretion did not significantly correlated with the mean iron mobilization by ECM morphotype, we conclude that iron complexation was due to either other organic acids or to siderophores.     

Tribology of the root cap in maize (Zea mays) and peas (Pisum sativum)

Frictional resistance to a penetrating body can account for more than 80% of the total resistance to penetration of soil. We measured the frictional resistance between growing root caps of maize and pea and ground and smooth glass surfaces, which was linearly correlated to load, allowing calculation of the coefficient of kinetic friction and adhesion. Coefficients of kinetic friction between the root caps and the ground and smooth glass surfaces were approximately 0.04 and 0.02, respectively, the first measurements of the frictional properties of root tips at rates approaching those of root elongation, and an order of magnitude smaller than those previously reported. Results suggest that roots are well designed for penetrating soil, and encounter only small frictional resistance on the root cap. These data provide important parameters for modelling soil stresses and deformation around growing root tips.