Soil and weed management for enhancing arbuscular mycorrhiza colonization of wheat

Tillage and weed control are critical components of cropping systems that need to be combined such that crops benefit from reduced competition. However, weeds may also contribute to the biological diversity within the agro‐environment. This greenhouse study investigated whether common weeds of arable cropping systems were suitable host plants for arbuscular mycorrhizal fungi (AMF), allowing the development of extraradical mycelium (ERM) that can contribute to the early colonization of a following wheat crop, especially in the absence of soil disturbance. Weeds were allowed to grow for up to 2 months before being controlled by soil disturbance or herbicide application (glyphosate or paraquat). Pregerminated wheat seeds were then planted. Chemical control of the weeds prior to sowing enhanced the early arbuscular mycorrhiza (AM) colonization rate of wheat roots, whereas mechanical disturbance was less acceptable as a method of weed control for rapid AM colonization. The type of herbicide (contact or systemic) had no impact on colonization of the wheat crop. Enhanced AM colonization promoted early P acquisition and growth of the crop. Appropriate management of weeds emerging between two consecutive cropping seasons coupled with no‐till soil management could ensure a quick and efficient AM colonization of the following wheat plants.     

Differential effects of soil disturbance and plant residue retention on function of arbuscular mycorrhizal (AM) symbiosis are not reflected in colonization of roots or hyphal development in soil

The effects of soil disturbance and residue retention on the functionality of the symbiosis between medic (Medicago truncatula L.) and arbuscular mycorrhizal fungi (AMF) were assessed in a two-stage experiment simulating a crop rotation of wheat (Triticum aestivum L.) followed by medic. Plants were inoculated or not with the AMF, Glomus intraradices and Gigaspora margarita, separately or together. The contribution of the arbuscular mycorrhizal (AM) pathway for P uptake was determined using ³²P-labeled soil in a small hyphal compartment accessible only to hyphae of AMF. In general AM colonization was not affected by soil disturbance or residue application and disturbance did not affect hyphal length densities (HLDs) in soil. At 4 weeks disturbance had a negative effect on growth and phosphorus (P) uptake of plants inoculated with G. margarita, but not G. intraradices. By 7 weeks disturbance reduced growth of plants inoculated with G. margarita or AMF mix and total P uptake in all inoculated plants. With the exception of plants inoculated with G. margarita in disturbed soil at 4 weeks, the AM pathway made a significant contribution to P uptake in all AM plants at both harvests. Inoculation with both AMF together eliminated the negative effects of disturbance on AM P uptake and growth, showing that a fungus insensitive to disturbance can compensate for loss of contribution of a sensitive one. Application of residue increased growth and total P uptake of plants but decreased ³²P in plants inoculated with the AMF mix in disturbed soil, compared with plants receiving no residue. The AMF responded differently to disturbance and G. intraradices, which was insensitive to disturbance, compensated for lack of contribution by the sensitive G. margarita when they were inoculated together. Colonization of roots and HLDs in soil were not good predictors of the outcomes of AM symbioses on plant growth, P uptake or P delivery via the AM pathway.     

Time-course of heavy metal uptake in maize and clover as affected by root density and different mycorrhizal inoculation regimes

A pot experiment was conducted to test the effect of three microbial regimes on the time course of heavy metal uptake in clover and maize from an industrially polluted soil. The three treatments included: (1) an intact flora of bacteria and fungi, including indigenous arbuscular mycorrhizal (AM) fungi together with soil microfauna; (2) the indigenous bacterial/fungal flora except AM fungi, reintroduced into sterilized soil; or (3) the same bacterial/fungal flora plus an AM fungus. For the final harvest, two pot sizes were included to assess the effect of root density. Plant uptake of P and heavy metals varied according to plant species, harvest time and soil treatment. For both plant species, shoot concentration of Zn, Cd and Cu decreased and Ni increased with plant age. Plants growing in sterilized soil with reintroduced AM fungi generally grew better, but contained higher concentrations of heavy metals than those colonized by indigenous AM fungi. Plants with mycorrhiza frequently contained more P, Zn, Cd, Cu and Pb in roots and shoots compared to nonmycorrhizal plants. Elevated root/shoot concentration ratios of P and metals indicate a sequestration of metal phosphates in mycorrhizal roots. Mycorrhizal performance was influenced by root density. At low root densities, metal concentrations in mycorrhizal plants were reduced, whereas it had no effect at high root densities when the entire soil volume was efficiently exploited by roots. We conclude that root density data are essential for interpretations of the influence of AM on metal uptake in plants.     

不同种植密度下丛枝菌根真菌对玉米磷吸收的影响

【目的】利用土著丛枝菌根真菌(arbuscular mycorrhizal fungi,AM真菌)与作物形成互惠互利的共生关系提高作物对土壤磷的利用效率是解决农业生产中磷供需矛盾的主要途径之一,本研究在大田玉米不同种植密度条件下,研究AM真菌对玉米根系的侵染及磷吸收作用,为揭示集约化玉米高效获取磷的机理提供理论依据。【方法】以大田作物玉米的两种种植密度(5104 plants/hm2和9104 plants/hm2)体系为研究对象,在田间原位埋设PVC管装置,通过测定菌丝生长室中的菌丝密度和有效磷耗竭来确定不同种植密度体系条件下AM真菌对玉米磷吸收的作用。【结果】相对于低密度种植群体,高密度群体显著降低了玉米拔节期土壤有效磷的耗竭量,同时增加了玉米地上部的磷含量,即磷吸收效率,增幅达20%; 在玉米拔节期,增加种植密度使根际的根外菌丝生物量(菌丝密度)降低了4%,而非根际土壤中的根外菌丝生物量(菌丝密度)增加了37%; 高密度玉米种植密度群体中AM真菌的根外菌丝对土壤有效磷耗竭的贡献增加了22%。【结论】集约化玉米生产中土著AM真菌依然帮助植株从土壤中吸收有效磷; 高密度体系下玉米对磷的吸收更加依赖于AM真菌。高密度种植增加AM真菌对玉米的侵染、 根外菌丝量和对土壤有效磷的吸收。     

Effects of indigenous and introduced arbuscular mycorrhizal fungi on the growth of Allium fistulosum under field conditions

ABSTRACT

Enhanced phosphorus (P) uptake from the soil and increased plant growth related to arbuscular mycorrhizal (AM) fungi in pot culture, using sterilized soil, are well-known phenomena. However, these enhancements are not widely observed under field conditions because field sterilization is difficult. The aim of this study was to investigate the effects of AM fungi on P uptake and the growth of Allium fistulosum in non-fumigated and fumigated fields, under different levels of P availability. Plants were inoculated with the AM fungus Glomus R-10 and grown in fumigated soil. For the uninoculated treatment, a sterilized inoculum was applied directly. The field was fumigated using dazomet. Superphosphate was applied to the field at the rates of 0 (P0) or 500 (P500) kg P₂O₅ ha^?1. The inoculated and uninoculated plants were transplanted into the fields and sampled three times to measure AM fungal colonization, shoot P concentration, and shoot dry weight of the plants. At the transplanting stage, AM fungal colonization was observed in the inoculated plants (>70%) but not in the uninoculated plants. At the third sampling, irrespective of P treatment, AM fungal colonization was observed both in the uninoculated and inoculated plants in the non-fumigated field, and there was no difference in shoot P content and shoot dry weight between the inoculated and uninoculated plants. AM fungal colonization in the fumigated field was higher in the inoculated than uninoculated plants, irrespective of P treatment; shoot P content and shoot dry weight were both higher in the inoculated plants than in the uninoculated plants with P0. These results suggest that the responses of A. fistulosum to AM fungal inoculation under the low-P and fumigated conditions are similar to those observed in sterilized pot culture conditions.     

Impact of plant species grown as monocultures on sporulation and root colonization by native arbuscular mycorrhizal fungi in potato

Rhizosphere soils from 12 different plant species grown as monocultures at a field site of biodiversity and ecological processes in terrestrial herbaceous ecosystems (BIODEPTH) in northern Sweden were used as inoculum on potato to investigate mycorrhizal traits. Potato roots showed significantly higher mycorrhizal colonization when inoculated with soil samples from Festuca ovina and Leucanthemum vulgare compared to soil samples from other plants. The soil samples of F. ovina, L. vulgare, Phalaris arundinacea and Trifolium pratense rhizospheres were chosen for arbuscular mycorrhizal fungi identification based on spore morphology and large subunit (LSU) ribosomal DNA sequences amplified from single spores and roots. Spore morphological identification showed that Glomus mosseae and Glomus intraradices were found in F. ovina and L. vulgare soils at the site as well as in our potato trap experiment. Also, Glomus geosporum spores were present in all four plants’ soils in the potato trap experiment. LSU rDNA sequences were obtained from AM fungal spores from the collection site or potato trap experiment and colonized potato roots inoculated with L. vulgare soil. Sequences showed highest similarity to G. mosseae. Our results suggest that the host F. ovina and L. vulgare could be considered in crop rotation to enhance AM fungal inocula for potatoes.     

Mycorrhizal phosphorus enhancement of plants in undisturbed soil differs from phosphorus uptake stimulation by arbuscular mycorrhizae over non-mycorrhizal controls

Previous greenhouse and field studies have shown arbuscular mycorrhizal (AM) plants usually have greater P uptake and growth when raised in undisturbed soil compared to soil disturbed between plantings, such as by tillage. We report here for the first time that AM fungi able to stimulate shoot P uptake in experimental comparisons to non-mycorrhizal plants differ in their ability to bring about similar responses in undisturbed soil compared to disturbed soil. This outcome indicates a difference in functional character between the two stimulation processes. Three isolates of AM fungi were tested for growth promotion of maize (Zea mays L.) in pots in a soil disturbance experiment that included non-mycorrhizal controls. All three fungi colonized roots well and promoted shoot P uptake compared to non-inoculated controls, but only Glomus mosseae was able to stimulate growth in undisturbed soil compared to disturbed soil. This effect was seen when Glomus mosseae was alone or in combination with Gigaspora margarita. However, the presence of Glomus aggregatum in combination with Glomus mosseae prevented any stimulation, presumably due to domination by Glomus aggregatum. The ability of AM fungi to be beneficial to plants in comparison to non-mycorrhizal situations likely relates to the spread of mycelium in the soil and the capacity for nutrient transfer to the root. The ability of an AM fungus to promote growth in undisturbed soil appears to be related to these features and, in addition, a capacity for persistence and retention of functional capacity of the extraradical mycelium from one plant generation to the next.     

Crop residue and Collembola interact to determine the growth of mycorrhizal pea plants

The effects of collembolan grazing on arbuscular mycorrhizal (AM) fungi and plant growth were studied in a controlled experiment utilizing a mix of AM fungi and the dominant collembolan species (Isotoma sp.) indigenous to the experimental soil. Collembolan (+/– Col) effects were examined in the presence and absence of crop residue (+/– Litter) incorporated into the experimental soil. Significant interactions between collembolans and crop residue occurred for mycorrhizal colonization of roots and plant growth. In the absence of crop residue, collembolans reduced root length colonized by AM fungi, total plant dry mass and seed pod yield. However, in the presence of crop residue, collembolans had no effect on root colonization by AM fungi, and increased total plant mass and pod yield. Crop residue increased root colonization by AM fungi, numbers of bacteria and saprophytic fungi (colony forming units), small- (<5 m) and large- (>5 m) diameter hyphal lengths in soil, and the final population of collembolans in soil. Collembolans reduced both small- and large-diameter hyphae in soil and the number of saprophytic fungi (colony forming units, p =0.052). Feeding preference experiments conducted in vitro showed that Isotoma sp. preferred to graze on mycorrhizal roots over nonmycorrhizal roots when given no other food choice. However, when crop residue was added as a food choice, Isotoma sp. showed a clear feeding preference for crop residue. We conclude that collembolan grazing on mycorrhizae can be detrimental to plant growth when other fungal food sources are limited, but grazing on mycorrhizal fungi does not occur when ample organic matter and associated saprophytic fungi are present in soils.     

Arbuscular Mycorrhizal Mediated Nutrition in Plants

ABSTRACT

Arbuscular mycorrhizae (AM) are the symbiotic fungi that predominate in the roots and soils of agricultural crop plants. The most recognized beneficial effect of these fungi is to enhance host plant uptake of relatively immobile nutrients, in particular phosphorus (P), and several micronutrients. The AM fungi absorb inorganic P either from the soluble P pools in the soil, or from insoluble forms such as rock phosphates as well as from insoluble organic sources. Recent studies show that mycorrhizal fungi would have access to rock phosphate through localized alterations of pH and/or by the production of organic acid anions that may act as chelating agents. The AM colonization also improves plant N nutrition. Generally mycorrhizal symbiosis more influences on nitrogen (N) uptake and translocation if ammonium (NH₄ ⁺) rather than nitrate (NO₃ ^?) is the nitrogen source. However, under drought stress the role of mycorrhizae in NO₃ ^? transport to the root surface may be significant as the NO₃ ^? mobility is severely restricted due to its low concentration and diffusion rate under such circumstances. However, as yet little is known about the mechanism of N uptake by the AM fungi. Uptake of micronutrients is also influenced by mycorrhizal colonization.     

供磷水平对玉米丛枝菌根侵染及其对异质养分吸收的影响

【目的】 探究酸性土壤玉米丛枝菌根侵染对植物磷素吸收的促进作用,以加深理解根外菌丝对局部磷养分的获取如何受丛枝真菌侵染和环境磷养分的影响。 【方法】 以玉米为宿主植物,进行盆栽试验。在低磷酸性土壤上设置供P 0、50、500 mg/kg 3个水平 (P0、P50、P500),供试磷肥为磷酸二氢钾。每个处理再设置局部养分处理,即在每个重复中埋置两个各装有120 g灭菌土 (提前加 P 50 mg/kg) 的塑料小管,分别用孔径为0.45 μm(根系、菌丝均不能进入,以“–H”表示处理) 和50 μm(根系不能进入,菌丝可以进入,以“+H”表示处理) 的尼龙膜封住管口。测定了玉米的生长与磷吸收、土著丛枝菌根真菌的侵染和根外菌丝密度以及菌丝对局部磷养分的获取。 【结果】 1) 玉米株高、叶片SPAD值、全株干重、磷浓度及吸收量都随供磷水平升高而增加,以P50处理的根系干重最高,根冠比随供磷水平上升而降低。3个供磷水平下玉米根系均有不同程度的丛枝菌根真菌侵染。以P50处理的丛枝菌根侵染率、丛枝和孢囊结构发育最好;P0处理的丛枝菌根侵染率、丛枝丰度与P50处理没有显著差异,但孢囊丰度明显下降;P500处理虽然87.2%的根系具有侵染点,但整个根系形成的真菌结构、丛枝和孢囊比例远低于P0和P50处理,丛枝菌根的发育受到严重抑制。2) 土体土 (除塑料管之外的土) 菌丝密度随供磷水平升高而降低,但P0和P50处理差异不显著。–H处理塑料管中的菌丝密度在3个供磷水平下基本不变,保持在极低水平,而+H处理塑料管中的菌丝密度随供磷水平升高而下降。在相同供磷水平下,土体土的菌丝密度最高,其次是+H处理,–H处理的菌丝密度最低。根外菌丝从+H处理塑料管中获取的磷随环境供磷水平的升高而减少。 【结论】 酸性土壤条件下,适当地供磷可以促进玉米根系生长和丛枝菌根真菌的侵染。根外菌丝对局部磷养分的获取受环境磷养分的调控,在环境磷养分较低而局部磷养分高于环境磷养分时,较多的菌丝会进入局部区域获取磷。      

Impact of soil nitrogen concentration on Glomus spp.-Sinorhizobium interactions as affecting growth, nitrate reductase activity and protein content of Medicago sativa

Our objective was to evaluate how increasing levels of N in the medium (0, 4, 8 and 16 mmol N added kg^-1 soil) affect the interaction between Sinorhizobium and arbuscular mycorrhiza (AM) fungi in the tripartite symbiosis with Medicago sativa. Growth response, nutrient acquisition, protein content, and nitrate reductase (NR) activity were measured both in plant shoots and roots. Results showed that N levels in soil did not affect mycorrhizal colonization but they strongly influenced nodulation, particularly of mycorrhizal plants. Mycorrhizal colonization was required for a proper nodulation when no N was applied to soil. In contrast, the addition of 4 mmol N kg^-1 soil reduced nodulation only in mycorrhizal plants and 8 mmol N added kg^-1 soil allowed nodule formation only in non-mycorrhizal plants. Nodulation was totally inhibited in all treatments with the addition of 16 mmol N added kg^-1 soil. N addition enhanced NR activity in all the treatments, while AM colonization increased the proportion of NR allocated to roots. This effect was more pronounced under the lowest N levels in the medium. The two AM fungal species showed different distribution pattern of enzymatic activities in plant tissues indicating specific physiological traits. Protein content as well as the relative proportion of protein in roots were greatly increased after mycorrhizal colonization. Glomus intraradices-colonized plants had the highest protein content in shoot and root. Mycorrhizal effects on growth, N acquisition and biochemical variables cannot be interpreted as an indirect P-mediated effect since P content was lower in mycorrhizal plants than in those which were P fertilized. Mycorrhizal colonization increased the N content in plants irrespective of the N level, but the effectiveness of AM fungi on plant N acquisition depended on the AM fungus involved, G. intraradices being the most effective, particularly at the highest N rate. N₂ fixation, enhanced by AM colonization, contributed to N acquisition when a moderate N quantity was available in the soil. Nevertheless, under a high N amount the nodulating process and/or fixing capacity by Sinorhizobium was reduced in AM plants. In contrast, the AM fungal mycelium from a particular mycorrhizal fungus may continue to contribute efficiently to the N uptake from the soil even at high N levels. These results demonstrate the particular sensitivity of AM fungal species in terms of their growth and/or function to increasing N amounts in the medium. A selection of AM fungi used to address specific environmental conditions, such as N fertilization regimes comparable to those used in agronomic practices, is required for a better use of N applied to soil.     

Variable response of growth and arbuscular mycorrhizal colonization of maize plants to preceding crops in various types of soils

The effects of the preceding crops, sunflower (mycorrhizal host) and mustard (nonhost), on arbuscular mycorrhizal (AM) colonization and growth of succeeding maize were examined in 17 soils in an attempt to clarify the influence of soil characteristics on the effects of preceding crops. Shoot weight and P uptake of maize planted after sunflower were much higher than those after mustard in 14 soils, although the preceding crop had little effect on soil-P availability. AM colonization of maize after sunflower was much higher than that after mustard. The effect of the preceding crop was eliminated by soil sterilization. These results suggested that the differences in maize growth were caused by differences in the AM colonization. Correlation analysis of the effect of the preceding crop and soil properties showed that the difference in the effects on maize growth could not be explained by soil chemical properties, but only by the AM colonization of the preceding sunflower crop. In one of the 17 soils, however, the effect was not evident despite the higher AM colonization of sunflower. This soil was sterilized, and the effect of inoculation by AM fungi (AMF) on maize was examined. However, it was found that the inoculation increased AM colonization but did not improve maize growth at any P level, suggesting that the effect of AMF was unusually inhibited in this soil by unknown soil physicochemical properties. In most soils, however, the preceding mycorrhizal host crop, sunflower, improved the growth and AM colonization of maize depending on the AM colonization of sunflower.     

Short-term microbial responses to soluble inorganic P input in a tropical lowland rain forest in Amazonia

In non-flooded lowland rain forests with low soil phosphorus (P) in parts of Amazonia, P cycling largely occurs via leaf litter recycling by arbuscular mycorrhizal (AM) fungal symbionts. Occasional high input of P into these ecosystems occurs during drought years with increased litterfall. As the length and frequency of drought events are projected to increase in the region, a single-dose nutrient addition experiment was carried out to test how this would impact P cycling. An application rate of 4 kg P ha^-1 was used, which corresponds to twice the amount of litter-derived P in an average year. It was hypothesized that i) the added mineral P would be immobilized by soil microorganisms, leading to measurable increase in soil microbial biomass carbon (C) and P and ii) AM colonization rate would be reduced by the pulse in mineral P available for plant uptake. The results did not support either of our hypotheses. The addition of P did not have an effect on AM root colonization, nor was P immobilized by soil microbiota during the experimental period. The lack of a difference between the control and treatment at our study site could be attributed to the relatively low one-off dose of P applied that did not change either the colonization rate of roots by AM fungi or the amount of soil available labile P. To obtain a mechanistic understanding of the availability, capture, and use of P by plant-symbiont associations in tropical rain forest ecosystems, further integrated studies of the soil-plant system combining long-term nutrient manipulations, modeling, and experimental approaches are required.     

Wheat plants invest more in mycorrhizae and receive more benefits from them under adverse than favorable soil conditions

Soil chemistry and biota heavily influence crop plant growth and mineral nutrition. The stress-severity and optimal resource allocation hypotheses predict mutualistic symbiotic benefits to increase with the degree of metabolic imbalance and environmental stress. Using two cross-factorial pot experiments with the same biologically active calcareous soil, one time highly saline and nutrient-deficient, and the other time partially desalinated and amended with mineral soil fertilizer, we explored whether these general predictions hold true for zinc (Zn) nutrition of bread wheat in mycorrhizal symbiosis. Increased arbuscular mycorrhizal (AM) fungal root colonization positively correlated with plant Zn nutrition, but only when plants were impaired in growth due to salinity and nutrient-deficiency; this was particularly so in a cultivar-responsive to application of mineral Zn fertilizer. Evidence for direct involvement of AM fungi were positive correlations between Zn uptake from soil and frequency of fungal symbiotic nutrient exchange organelles, as well as the quantitative abundance of AM fungi of the genera Funneliformis and Rhizophagus, but not Claroideoglomus. Combined partial soil desalination and fertilization swapped the dominance ranking from Claroideoglomus spp. to Funneliformis spp. Positive growth, nitrogen, and Zn uptake responses to mycorrhization were contingent on moderate soil fertilization with ZnSO₄. In agreement with the predictions of the stress-severity and optimal resource allocation hypotheses, plants limited in growth due to chemically adverse soil conditions invested relatively more into AM fungi, as evident from heavier root colonization, and took up relatively more Zn and nitrogen in response to mycorrhization, than better growing and less mycorrhized plants. It thus appears that crop plant cultivar-dependent mycorrhization and Zn fertilizer-responsiveness may reinforce each other, provided that there is bioavailable Zn in soil and plant growth is impaired by suboptimal chemical soil conditions.     

内蒙古盐碱土中AM真菌的多样性与分布

在内蒙古盐碱土13种主要植物分离到3属26种AM真菌,其中球囊霉属(Glomus)22种,无梗囊霉属(Acaulospora)3种,原囊霉属(Archaeospora)1种。地球囊霉(Glomus geosporum)和地表球囊霉(Glomus ver-siforme)是该区域盐碱土中的优势种。13种主要植物均能被AM真菌侵染,其中玉米和马蔺的侵染率最高,达100%;根际土壤中AM真菌孢子密度范围为29~182个g-1烘干土,其中稻的孢子密度最高,达182个g-1烘干土;在不同土壤类型条件下植物的菌根侵染率具有明显的差异,其规律为草甸盐土>碱化盐土>盐化草甸土>碱化草甸土;根际土壤中孢子密度以碱化草甸土最高(101个g-1烘干土),其次为碱化盐土、草甸盐土和盐化草甸土。相关分析表明,根际土壤中AM真菌孢子密度与菌根侵染率无显著相关性。     

Mycorrhizai associations with some plant species in a soil strip of different successional stages in Egypt

In this study, soil and plant samples were collected from a strip of soil comprising four successional stages from the eastern desert plateau to the Nile Valley, Egypt. On one hand, some essential elements [i.e., potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)] and water contents decreased on transition from the desert soil to the Nile Valley. Soil nitrite content was the highest in Nile Valley soil and decreased toward desert soil. pH values of the soils did not strongly differ and were moderately alkaline. Root colonization with arbuscular‐mycorrhizae (AM) fungi of 33 plant species (11 cultivated and 22 wild) collected from the study area was highest in eastern desert plateau and decreased toward the Nile Valley. Mycorrhizai spore counts reflected the root colonization data. The decrease in mycorrhizai colonization was explained on the basis of difference in soil porosity, soil water contents, and toxicity of nitrite. Contents of K, Ca or Mg in some species were increased by increasing the colonization percentage of roots by AM fungi. Increasing the infection percentage of roots decreased or maintained the ratio of Na K+Ca+Mg (in meq) in the shoots of investigated plants. The contents of chlorophyll, soluble sugars, and protein significantly increased as a result of increasing colonization value. Also, mycorrhizal root colonization improved the water status of most plant species. These results suggest that increasing infection by AM fungi in the newly reclaimed soil may enable the plants to maintains its internal water status and mineral balance through decreasing the ratio of distressing ions to the nutrient ones.     

Effects of nitrogen feeding for extraradical mycelium of Rhizophagus irregularis maize symbiosis incorporated with phosphorus availability

In terrestrial ecosystems, plants are frequently in symbiosis with arbuscular mycorrhizal fungi (AMF) with mineral nutrients and photosynthesis carbon exchanges in between. This research sought to identify the effects of phosphorus (P) levels on the nitrogen (N) uptake via extraradical mycelium (ERM) and the mycorrhizal growth response (MGR) of maize plants within the AMF symbiosis. Pots were separated into root compartments and hyphae compartments (HCs) with two layers of a 30‐μm mesh membrane and an air gap in between, where only hyphae could pass through, to avoid both N diffusion and root growth effects. Maize plants were inoculated with Rhizophagus irregularis with different N fertilization in HCs under two different P fertilization levels. Our results indicated that a strong increase in MGR with low‐P fertilization. The same tendency was not observed with high‐P fertilization, although both had a large increase in P concentration as a potential source of growth in shoot tissue of mycorrhizal plants. Substantial effects (10.5% more N) were observed in the case of high‐P availability for the host plants from ERM fed with N, whereas under low‐P conditions ERM may prioritize P uptake rather than N uptake. The AM fungi increase the uptake of N and P, which are most limiting in the soil with fewer forces from soil resources. In addition, there was still more P accumulated than N due to the high N for ERM with high‐P supply. Low N in HCs corresponded with a lower colonization rate in roots but with high hyphae density in HCs; this result suggest that N and P availability might change the ratio of extraradical to intraradical hyphae length.     

Influence of arbuscular mycorrhizal fungi,inoculum level and phosphorus placement on growth and phosphorus uptake of <Emphasis Type="Italic">Phyllanthus calycinus</Emphasis> under jarrah forest soil

Phosphorus uptake and biomass production may vary between arbuscular mycorrhizas (AM) species of contrasting life cycles and their modes of interaction with host plants. This research investigated differences among three AM fungi from different genera isolated from the jarrah forest with respect to their colonisation, biomass production and P uptake over time. The understorey plant Phyllanthus calycinus, has been shown to be extensively mycorrhizal. We examined differences in the capacity of fungi associated with this plant to access a P point source from increasing distances from the root. The methodology simulated “pockets” of P in forest soil. Phosphorus sources in root-exclusion mesh bags were inserted 2, 4 and 6 cm from plant roots restrained in separate mesh bags. Acaulospora laevis colonised plants to the greatest extent, producing higher plant biomass and P uptake compared to the other two fungi. Plant biomass and P uptake were enhanced where P sources were closest to the root, as expected, but there were differences among fungi in response to level of inoculum used for each fungus. The capacity of individual AM fungi to access a point source of P at different distances varied in a highly mycorrhizal native forest understorey plant in parallel with plant growth and P uptake.     

Effects of hexachlorocyclohexane on rhizosphere fungal propagules and root colonization by arbuscular mycorrhizal fungi in Plantago lanceolata

Lindane ( γ‐hexachlorocyclohexane or γ‐HCH) is an organochlorine insecticide previously used extensively for the control of agricultural pests. We studied the effects of soil HCH contamination on vegetation and its associated arbuscular mycorrhizas (AM). The polluted and unpolluted plots had similar plant cover, with the same species richness and abundance. Plantago lanceolata plants were selected for mycorrhizal analysis because of their presence in both plots and known mycotrophy. The presence of HCH appeared to have no significant effect on the extent of colonization of Plantago roots by AM, suggesting a similar functionality of the fungal symbionts. However, infective AM propagules, the density of AM spores and viable AM hyphae in the rhizosphere were much less in the HCH‐polluted soil than in the unpolluted plot. Pre‐inoculation of four plant species with an isolate of Glomus deserticola obtained from the HCH‐contaminated soil resulted in increased growth and fungal colonization of roots compared with plants pre‐inoculated with the introduced fungus G. macrocarpum or colonized by the consortium of indigenous AM fungal species, when those plants were transplanted to an HCH‐contaminated soil. This suggests that the fungus increases the tolerance of plants to the toxic soil environment. We conclude that herbaceous and woody plants can grow in soil with little P contaminated with <100 mg HCH kg^?1 with the help of tolerant AM, despite the detrimental effect of HCH on AM fungal propagules in soil. The effects of AM fungi on plant growth and soil microbial community structure in HCH‐polluted sites could be important for remediation of the pollutant through the microbial activity in the rhizosphere.     

Improved zinc tolerance in Eucalyptus globulus inoculated with Glomus deserticola and Trametes versicolor or Coriolopsis rigida

The potential of interactions between saprophytic and arbuscular mycorrhizal (AM) fungi to improve Eucalyptus globulus grown in soil contaminated with Zn were investigated. The presence of 100 mg kg ⁻¹ Zn decreased the shoot and root dry weight of E. globulus colonized with Glomus deserticola less than in plants not colonized with AM. Zn also decreased the extent of root length colonization by AM and the AM fungus metabolic activity, measured as succinate dehydrogenase (SDH) activity of the fungal mycelium inside the E. globulus root. The saprophytic fungi Trametes versicolor and Coriolopsis rigida increased the shoot dry weight and the tolerance of E. globulus to Zn when these plants were AM-colonized. Both saprophytic fungi increased the percentage of AM root length colonization and elevated G. deserticola SDH activity in the presence of all Zn concentrations applied to the soil. In the presence of 500 and 1000 mg kg⁻¹ Zn, there were higher metal concentrations in roots and shoots of AM than in non-AM plants; furthermore, both saprophytic fungi increased Zn uptake by E. globulus colonized by G. deserticola. The higher root to shoot metal ratio observed in mycorrhizal E. globulus plants indicates that G. deserticola enhanced Zn uptake and accumulation in the root system, playing a filtering/sequestering role in the presence of Zn. However, saprophytic fungi did not increase the root to shoot Zn ratio in mycorrhizal E. globulus plants. The effect of the saprophytic fungi on the tolerance and the accumulation of Zn in E. globulus was mediated by its effect on the colonization and metabolic activity of the AM fungi.