亚热带典型树种根毛特征及其与共生真菌的关系

根毛和共生真菌增加了吸收面积,提高了植物获取磷等土壤资源的能力。由于野外原位观测根表微观结构较为困难,吸收细根、根毛、共生真菌如何相互作用并适应土壤资源供应,缺乏相应的数据和理论。该研究以受磷限制的亚热带森林为对象,选取了21种典型树种,定量了根毛存在情况、属性变异,分析了根毛形态特征与共生真菌侵染率、吸收细根功能属性之间的关系,探讨了根表结构对低磷土壤的响应和适应格局。结果表明:1)在亚热带森林根毛不是普遍存在的, 21个树种中仅发现7个树种存有根毛, 4个为丛枝菌根(AM)树种, 3个为外生菌根(ECM)树种。其中,马尾松(Pinus massoniana)根毛出现率最高,为86%;2)菌根类型是理解根-根毛-共生真菌关系的关键,AM树种根毛密度与共生真菌侵染率正相关,但ECM树种根毛直径与共生真菌侵染率负相关; 3) AM树种根毛长度和根毛直径、ECM树种根毛出现率与土壤有效磷含量呈负相关关系。该研究揭示了不同菌根类型树种根毛-共生真菌-根属性的格局及相互作用,为精细理解养分获取策略奠定了基础。     

Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie

Symbiotic associations between plants and arbuscular mycorrhizal fungi are ubiquitous and ecologically important in many grasslands. Differences in species responses to mycorrhizal colonization can have a significant influence on plant community structure. The growth responses of 36 species of warm- and cool-season tallgrass prairie grasses and 59 tallgrass prairie forbs to arbuscular mycorrhizal (AM) fungal colonization were assessed in greenhouse studies to examine the extent of interspecific variation in host-plant benefit from the symbiosis and patterns of mycorrhizal dependence among host plant life history (e.g., annual, perennial) and taxonomic (e.g., grass, forb, legume, nonlegume) groups and phenological guilds. There was a strong and significant relationship between phenology of prairie grasses and mycorrhizal responsiveness, however this relationship was less apparent in forbs. Perennial warm-season C(4) grasses and forbs generally benefited significantly from the mycorrhizal symbiosis, whereas biomass production of the cool-season C(3) grasses was not affected. The root systems of the cool-season grasses were also less highly colonized by the AM fungi, as compared to the warm-season grasses or forbs. Unlike the native perennials, annuals were generally not responsive to mycorrhizal colonization and were lower in percentage root colonization than the perennial species. Plant growth responsiveness and AM root colonization were positively correlated for the nonleguminous species, with this relationship being strongest for the cool-season grasses. In contrast, root colonization of prairie legumes showed a significant, but negative, relationship to mycorrhizal growth responsiveness.     

Phosphorus,arbuscular mycorrhizal fungi and performance of the wetland plant Lythrum salicaria L. under inundated conditions

The role of arbuscular mycorrhizal (AM) fungi in aquatic and semi-aquatic environments is poorly understood, although they may play a significant role in the establishment and maintenance of wetland plant communities. We tested the hypothesis that AM fungi have little effect on plant response to phosphorus (P) supply in inundated soils as evidenced by an absence of increased plant performance in inoculated (AM+) versus non-inoculated (AM-) Lythrum salicaria plants grown under a range of P availabilities (0-40 mg/l P). We also assessed the relationship between P supply and levels of AM colonization under inundated conditions. The presence of AM fungi had no detectable benefit for any measures of plant performance (total shoot height, shoot dry weight, shoot fresh weight, root fresh weight, total root length or total root surface area). AM+ plants displayed reduced shoot height at 10 mg/l P. Overall, shoot fresh to dry weight ratios were higher in AM+ plants although the biological significance of this was not determined. AM colonization levels were significantly reduced at P concentrations of 5 mg/l and higher. The results support the hypothesis that AM fungi have little effect on plant response to P supply in inundated conditions and suggest that the AM association can become uncoupled at relatively high levels of P supply.     

The effect of soil phosphorus on the external mycelium growth of arbuscular mycorrhizal fungi during the early stages of mycorrhiza formation

The effects of soil P amendments and time of application on the formation of external mycelium by different arbuscular mycorrhizal (AM) fungi were studied. In the first experiment the external mycelium produced in the soil by the AM fungus Glomus etunicatum Beck. and Gerd., during the early stages of root colonization (7 and 14 days after inoculation), was quantified by the soil-agar film technique. A Brazilian Oxisol was used with three different phosphate levels, varying from deficient to supra-optimal for the plant. Significant differences were observed in the phosphate and inoculation treatments for plant dry weight, P content in the tissue, root length and root colonization, at fourteen days after planting. At 7 days, mycelium growth, root colonization and their relationship were reduced at supra-optimal P concentrations. Applications of P one week after planting reduced mycelium growth and root colonization more than when applied to the soil before planting. In a second experiment the arbuscular mycorrhizal (AM) fungi, Scutellospora heterogama (Nicol. and Gerd.) Walker and Sanders and E3 were tested and compared with Glomus etunicatum. For the species studied, the length of external hyphae per unit of colonized root length was affected by small P additions but no further significant differences were observed at high P levels. The three AM endophytes showed marked differences in their response to P in the soil: Scutellospora heterogama, although producing external mycelium more profusely than the Glomus spp., showed a higher sensitivity to soil P supply.     

Growth response of Atriplex nummularia to inoculation with arbuscular mycorrhizal fungi at different salinity levels

Chenopods are generally regarded as non-host plants for mycorrhizal fungi and are believed not to benefit from colonization by mycorrhizal fungi. Perennial Atriplex nummularia Lindl., growing under field conditions, showed a relatively high level of colonization by mycorrhizal fungi (10–30% of root length colonized) in spring and summer. Accordingly, two glasshouse experiments were designed to assess the effects of inoculation with mycorrhizal fungi (with a single species or a mixture of different species) on growth, nutrient uptake, and rhizosphere bacterial community composition of A. nummularia at high and low salinity levels (2.2 and 12 dSm^–1). Only low and patchy colonization by mycorrhizal fungi (1–2 of root length colonized) was detected in inoculated plants under glasshouse conditions which was unaffected by salinity. Despite the low colonization, inoculation increased plant growth and affected nutrient uptake at both salinity levels. The effects were higher at an early stage of plant development (6weeks) than at a later stage (9–10 weeks). Salinity affected the bacterial community composition in the rhizosphere as examined by ribosomal intergenic spacer amplification (RISA) of 16S rDNA, digitization of the band patterns and multivariate analysis. The effects of inoculation with mycorrhizal fungi on growth of A. nummularia may be attributed to (i) direct effects of mycorrhizal fungi on plant nutrient uptake and/or (ii) indirect effects via mycorrhizal-induced changes in the bacterial community composition.     

Can hypodermal passage cell distribution limit root penetration by mycorrhizal fungi?

* The basis for significant interspecific variability in colonization by arbuscular mycorrhizal fungi is poorly understood. Limited evidence suggests that, for species with a dimorphic hypodermis, colonization of the root cortex occurs only through hypodermal passage cells. Therefore, the hypothesis that interspecific variability in mycorrhizal colonization is accounted for by interspecific variation in passage cell distribution was tested. * The arbuscular mycorrhizal colonization and distribution of fungal penetration points and hypodermal passage cells in the root systems of eight species (seven plant families) possessing a dimorphic hypodermis were characterized. * Mycorrhizal fungal penetration of the hypodermis occurred exclusively through passage cells. Moreover, the proportion of root length with passage cells explained nearly 99% of the variability among the eight plant species in the proportion of root length with penetration points. * In dimorphic hypodermal species, passage cells appear to be key determinants of mycorrhizal colonization because they are the cells through which fungal penetration of the hypodermis occurs. Variation among such species in mycorrhizal colonization may be at least partly determined by variation in the proportion of root length with passage cells.     

土著菌根真菌和混生植物对羊草生长和磷营养的影响

植物种间相互作用直接影响植物生长、根系可塑性及养分吸收,而与植物共生的丛枝菌根真菌可以改变植物个体和种间养分资源的分配,具有协调种间竞争的潜力.以我国北方草甸草原建群种羊草(Leymus chinensis)和混生植物紫花苜蓿(Medicago sativa)及独行菜(Lepidium spetalum)为供试植物,通过模拟盆栽试验,研究了土著菌根真菌和混生植物对羊草生长、根系形态及磷营养的影响.试验结果表明,土著菌根真菌能够与羊草及紫花苜蓿形成良好共生,而独行菜根内基本未形成菌根共生结构.土著菌根真菌显著降低了羊草及独行菜的生物量,但促进了紫花苜蓿的生长;混种紫花苜蓿显著促进了羊草的生长,而混种独行菜则显著抑制了羊草的生长.土著菌根真菌对羊草根系形态的影响表现出与植株生物量类似的趋势,但不同混生植物对羊草根系生长均无显著影响.土著菌根真菌和混生植物对羊草植株磷含量均无显著影响.与混生植物相比,羊草具有较高的比根长和磷吸收能力,这也解释了其负向菌根依赖性.研究证实了菌根真菌和植物种间相互作用均是影响草原优势植物生长和根系发育的重要因素,深入研究其交互作用对于科学管理草地生态系统,维持植物群落的稳定性和生态系统生产力具有重要意义.     

Plant Kin Recognition Enhances Abundance of Symbiotic Microbial Partner

Background

The stability of cooperative interactions among different species can be compromised by cheating. In the plant-mycorrhizal fungi symbiosis, a single mycorrhizal network may interact with many plants, providing the opportunity for individual plants to cheat by obtaining nutrients from the fungi without donating carbon. Here we determine whether kin selection may favour plant investment in the mycorrhizal network, reducing the incentive to cheat when relatives interact with a single network.

Methodology/Principal Findings

We show that mycorrhizal network size and root colonization were greater when Ambrosia artemisiifolia L. was grown with siblings compared to strangers. Soil fungal abundance was positively correlated with group leaf nitrogen, and increased root colonization was associated with a reduced number of pathogen-induced root lesions, indicating greater benefit to plants grown with siblings.

Conclusions/Significance

Plants can benefit their relatives through investment in mycorrhizal fungi, and kin selection in plants could promote the persistence of the mycorrhizal symbiosis.     

Phosphorus and intraspecific density alter plant responses to arbuscular mycorrhizas

We investigated how phosphorus availability, intraspecific density, and their interaction affect plant responses to arbuscular mycorrhizas. Four facultatively mycotrophic species: chile, cilantro, tomato, and corn were examined separately in pot experiments that employed a tropical phosphorus-immobilizing soil. Each experiment comprised nine soluble phosphorus additions, two levels of intraspecific plant density, and inoculation with arbuscular mycorrhizal fungi or not. High phosphorus signi- ficantly diminished mycorrhizal colonization of corn, cilantro, and tomato, but not chile, which was highly variably colonized. Corn roots were colonized by other root-inhabiting fungi, and mycorrhizas significantly reduced colonization by these potential root parasites. High phosphorus significantly increased relative growth rates (RGR) of all species, and high density significantly reduced RGR of cilantro, tomato, and corn. Chile showed little growth at any but the highest phosphorus additions, and consequently had no RGR response to density or mycorrhizas. Mycorrhiza inoculation caused transient depression of corn growth during the first month, but mycorrhizas increased corn RGR during the second month of growth. Both RGR and dry weights at harvest, cilantro, tomato, and corn benefited from mycorrhizas at low phosphorus availability, but this benefit diminished or changed to disadvantage as phosphorus availability increased. At low phosphorus availability, high density increased the dry weight of mycorrhizal cilantro and thereby amplified the benefit of mycorrhizas. At high phosphorus availability, increased density diminished the effects of mycorrhizas on dry weight, reducing mycorrhiza benefit to tomato and chile and reducing mycorrhiza detriment to cilantro. This study demonstrates that for three of the four plant species examined, phosphorus availability, intraspecific density, and their interaction significantly modify plant responses to arbuscular mycorrhizas.     

Relating foliar dehydration tolerance of mycorrhizal Phaseolus vulgaris to soil and root colonization by hyphae

Mycorrhizal symbiosis can modify plant response to drying soil, but little is known about the relative contribution of soil vs. root hyphal colonization to drought resistance of mycorrhizal plants. Foliar dehydration tolerance, characterized as leaf and soil water potential at the end of a lethal drying episode, was measured in bean plants (Phaseolus vulgaris) colonized by Glomus intraradices or by a mix of arbuscular mycorrhizal fungi collected from a semi-arid grassland. Path analysis modeling was used to evaluate how colonization rates and other variables affected these lethal values. Of several plant and soil characteristics tested, variation in dehydration tolerance was best explained by soil hyphal density. Soil hyphal colonization had larger direct and total effects on both lethal leaf water potential and soil water potential than did root hyphal colonization, root density, soil aggregation, soil glomalin concentration, leaf phosphorus concentration or leaf osmotic potential. Plants colonized by the semi-arid mix of mycorrhizal fungi had lower lethal leaf water potential and soil water potential than plants colonized by G. intraradices. Our findings support the assertion that external, soil hyphae may play an important role in mycorrhizal influence on the water relations of host plants.     

Arbuscular Mycorrhizal Fungi Promote the Growth of Ceratocarpus arenarius (Chenopodiaceae) with No Enhancement of Phosphorus Nutrition

The mycorrhizal status of plants in the Chenopodiaceae is not well studied with a few controversial reports. This study examined arbuscular mycorrhizal (AM) colonization and growth response of Ceratocarpus arenarius in the field and a greenhouse inoculation trial. The colonization rate of AM fungi in C. arenarius in in-growth field cores was low (around 15%). Vesicles and intraradical hyphae were present during all growth stages, but no arbuscules were observed. Sequencing analysis of the large ribosomal rDNA subunit detected four culturable Glomus species, G. intraradices, G. mosseae, G. etunicatum and G. microaggregatum together with eight unculturable species belong to the Glomeromycota in the root system of C. arenarius collected from the field. These results establish the mycotrophic status of C. arenarius. Both in the field and in the greenhouse inoculation trial, the growth of C. arenarius was stimulated by the indigenous AM fungal community and the inoculated AM fungal isolates, respectively, but the P uptake and concentration of the mycorrhizal plants did not increase significantly over the controls in both experiments. Furthermore, the AM fungi significantly increased seed production. Our results suggest that an alternative reciprocal benefit to carbon-phosphorus trade-off between AM fungi and the chenopod plant might exist in the extremely arid environment.     

Responses of arbuscular mycorrhizal symbionts to contrasting environments: field evidence along a Tibetan elevation gradient

Plant adaptation to alpine ecosystems is not fully explained by plant physiological and morphological traits. Arbuscular mycorrhizal (AM) associations may be involved in mediating plant performance in response to environmental differences. Little is known, however, as to whether or not a close relationship exists between plant performance and arbuscular mycorrhizal fungus status across environmental gradients. We conducted a field investigation of the performance of six plant species and their associated AM fungi along higher and lower elevation gradients on Mount Segrila in Tibet. In most of our species, we observed higher shoot and inflorescence biomass production and a lower root-to-shoot ratio in the populations at those sites where the species was dominant (intermediate elevation sites) than in populations sampled at the limits of the distribution. The elevation pattern of root colonization differed with plant species on both gradients, and the extraradical hypha development of most species showed a unimodal pattern as did plant growth. The relationship between plant and fungus traits shows that AM fungus development generally matched host plant performance on the lower elevation gradient but not on the higher elevation gradient. This study provides evidence that plant distribution and productivity were significantly related to root and soil colonization by AM fungi, especially under less physically stressful conditions.     

Nitrogen limitation impairs plant control over the arbuscular mycorrhizal symbiosis in response to phosphorus and shading in two European sand dune species

The symbiosis of plants with arbuscular mycorrhizal fungi (AMF) may become parasitic if the cost:benefit ratio (carbon:phosphorus ratio) increases. In case of mycorrhizal parasitism, a plant may prevent growth depression through the reduction of root colonization as a form of control over the symbiosis. In this greenhouse study, we attempted to manipulate the cost:benefit ratio of the arbuscular mycorrhizal symbiosis by shading and/or phosphorus (P) fertilization in the differentially mycotrophic plant species Hieracium pilosella and Corynephorus canescens. By repeated sampling of soil cores, we assessed the temporal progress of plant investment towards mycorrhizal structures as a measure of plant control over the AMF. Unexpectedly, we found no obvious treatment effects on mycorrhizal growth dependency (MGD), most likely caused by constant N-limitation in AM plants being enhanced by P-fertilization and shade probably not exacerbating plant C-budget for AMF. This highlights the importance of N:P:C stoichiometry for the outcome of the symbiosis. Nevertheless, we found possible control mechanisms in shaded H. pilosella, with considerably higher resource investments into root than into hyphal growth, while root colonization was only marginally suppressed. This control only manifested after 4 weeks of growth under potentially detrimental conditions, emphasizing the importance of time in plant control over the arbuscular mycorrhizal symbiosis. In contrast, the less mycotrophic C. canescens did not exhibit obvious changes in mycorrhizal investments in reaction to shading and P-fertilization, possibly because the low mycotrophy and AMF colonization already imposes a functioning control mechanism in this species. Our study suggests that highly mycotrophic plants may have a stronger need to keep AMF in check than less mycotrophic plants, which may have implications for the role of mycotrophy in the outcome of symbiotic interactions in natural situations.     

干旱条件下AM真菌对植物生长和土壤水稳定性团聚体的影响

采用分室培养系统,模拟正常水分和干旱胁迫两种环境条件,探讨不同丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)对紫花苜蓿(Medicago sativa L.)生长和土壤水稳性团聚体的影响.试验条件下,Glomus intraradices对苜蓿根系的侵染率均显著高于Acaulospora scrobiculata和Diversispora spurcum接种处理.正常水分条件下,供试AM真菌均能显著提高植株生物量及磷浓度.干旱胁迫显著抑制了植株生长和菌根共生体发育,总体上菌根共生体对植株生长没有明显影响,接种D.spurcum甚至趋于降低植株生物量;同时,仅有G.intraradices显著提高了植株磷浓度.AM真菌主要影响到＞2mm的水稳性团聚体数量,以G.intraradices作用效果最为显著.在菌丝室中,G.intraradices显著提高了总球囊霉素含量.研究表明AM真菌对土壤大团聚体形成具有积极作用,而菌根效应因土壤水分条件和不同菌种而异,干旱胁迫下仅有G.intraradices对土壤结构和植物生长表现出显著积极作用.在应用菌根技术治理退化土壤时,需要选用抗逆性强共生效率高的菌株,对于不同AM真菌抗逆性差异的生物学与遗传学基础尚需进一步研究.     

Contrasting phosphate acquisition of mycorrhizal fungi with that of root hairs using the root hairless barley mutant

Comparisons between plant species or cultivars differing in root hair length have indicated a major impact of root hairs on the mycorrhizal dependency of plants with respect to phosphate (P) uptake. The current study aimed to investigate this relationship by comparing directly the mycorrhizal dependency of a spontaneous root hairless mutant, brb , in Hordeum vulgare cv Pallas and its wild type. Both brb and wild type were grown at different soil P levels in association with different mycorrhizal fungi. P uptake of brb and wild type was similar at high P levels, but P uptake by non-mycorrhizal brb plants at low P levels was substantially lower than that of the non-mycorrhizal wild-type plants. However, P uptake of the mutant was much increased by mycorrhizas and with one fungus, the additional P uptake was effectively translated into increased plant growth. Roots of the mutant contained typical colonization structures and a radioactive tracer confirmed P transport by the extraradical mycelium. This is the first direct evaluation of the relative effectiveness of root hairs and mycorrhizas. Mycorrhizas effectively substituted root hairs in P uptake, whereas the additional P was most often used less effectively in promoting plant growth than P provided by root hairs.     

Endo- and ectomycorrhizas in Quercus agrifolia Nee. (Fagaceae): patterns of root colonization and effects on seedling growth

We documented the patterns of root occupancy by Glomalean and ectomycorrhizal (EM) fungi in Quercus agrifolia, and host plant responses to inoculation with each mycorrhizal type alone or in combination. Glomalean hyphae, coils and vesicles, and EM root tips were recorded. Colonization patterns conformed to a succession from Glomalean and EM fungi in 1-year-old seedlings to predominantly EM in saplings (>11 years old); both mycorrhizal types were rarely detected within the same root segment. Inoculation of Q. agrifolia seedlings with EM or Glomalean fungi (AM) alone or in combination (EM+AM) altered the cost:benefit relationship of mycorrhizas to the host plant. Seedling survival, plant biomass, foliar nitrogen (N), and phosphorus (P) status were greatest in EM- or AM-only inoculated seedlings. Seedlings inoculated with both mycorrhizal types (AM+EM) exhibited the lowest survival rates, biomass, foliar N, and P levels. Roots of these plants were highly colonized by both EM (38% root length colonized) and Glomalean fungi (34%). Because these levels of colonization were similar to those detected in 1-year-old field seedlings, the presence of both mycorrhizal types may be a carbon cost and, in turn, less beneficial to oaks during establishment in the field. However, the shift to EM colonization in older plants suggests that mycorrhizal effects may become positive with time.     

Effects of mycorrhizal symbiosis on tallgrass prairie plant–herbivore interactions

Complex relationships occur among plants, mycorrhizal fungi, and herbivores. By altering plant nutrient status, mycorrhizas may alter herbivory or plant tolerance to herbivory via compensatory regrowth. We examined these interactions by assessing grasshopper preference and plant growth and fungal colonization responses to herbivory under mycorrhizal and non‐mycorrhizal conditions within tallgrass prairie microcosms. Mycorrhizal symbiosis increased plant regrowth following defoliation, and some strongly mycotrophic plant species showed overcompensation in response to herbivory when they were mycorrhizal. Although grasshoppers spent more time on mycorrhizal plants, herbivory intensity did not differ between mycorrhizal and non‐mycorrhizal plants. Aboveground herbivory by grasshoppers significantly increased mycorrhizal fungal colonization of plant roots. Thus mycorrhizas may greatly benefit plants subjected to herbivory by stimulating compensatory growth, and herbivores, in turn, may increase the development of the symbiosis. Our results also indicate strong interspecific differences among tallgrass prairie plant species in their responses to the interaction of aboveground herbivores and mycorrhizal symbionts.     

Association of highly and weakly mycorrhizal seedlings can promote the extra- and intraradical development of a common mycorrhizal network

Arbuscular mycorrhizal fungi (AMF) are key determinants of plant interactions in ecosystems. Through their effects on competition, they are regulators of the structure of communities. Conversely, the composition of plant assemblages may also influence the AMF colonization dynamics of plant species. Here, we tested under in vitro culture conditions the effects of Medicago truncatula, a highly mycorrhizal plant species, and Silene vulgaris, a weakly mycorrhizal plant species, grown single (monospecies treatments) or in combination (bispecies treatment) on the colonization dynamics of the AMF Rhizophagus irregularis MUCL 43194. The seedlings were placed in a pre-established hyphal network developing from a mature M. truncatula mycorrhizal donor plant. Extraradical mycelium (ERM) and root colonization parameters as well as root morphology were measured over a period of 12 days. An increased ERM length, total root colonization and proportion of arbuscules were noted in the bispecies treatment. Conversely, the bispecies treatment seemed to have no effect on root growth. This study also demonstrated the suitability of the in vitro culture system for studying the interactions between AMF and host plants grown as mono- and bispecies combinations.     

Functional diversity of arbuscular mycorrhizas extends to the expression of plant genes involved in P nutrition

This study of functional diversity considers symbiotic associations between two plant species, Medicago truncatula and Lycopersicon esculentum, and seven species of arbuscular mycorrhizal fungi (AMF). The objective was to integrate physiological analyses with molecular techniques to test whether functional diversity between AMF species is not only apparent at the level of mycorrhiza formation, plant nutrient uptake and plant growth, but also at the molecular level as observed by variation in the root expression of plant genes involved in the plant's P-starvation response. The seven species of AMF varied widely in their influence on the root expression of MtPT2 and Mt4 from M. truncatula and LePT1 and TPSI1 from L. esculentum. At one extreme was Glomus mosseae, whereby its colonization of M. truncatula resulted in the greatest reduction in MtPT2 and Mt4 gene expression and the highest level of P uptake and growth, while at the other extreme was Gigaspora rosea, whereby colonization resulted in the highest levels of MtPT2 and Mt4 gene expression and the lowest P uptake and growth. The expression of LePT1 and TPSI1 within the roots of L. esculentum was low and relatively uniform across the seven mycorrhizas, reflecting the ability of this cultivar to maintain low and constant shoot P levels despite root colonization by a broad selection of AMF. This study extends current understanding of functional diversity and shows that plants can respond differently to AMF, not only at the level of colonization, nutrient uptake and growth, but also at the level of gene expression.     

Plant species-specific changes in root-inhabiting fungi in a California annual grassland: responses to elevated CO2 and nutrients

Five co-occurring plant species from an annual mediterranean grassland were grown in monoculture for 4 months in pots inside open-top chambers at the Jasper Ridge Biological Preserve (San Mateo County, California). The plants were exposed to elevated atmospheric CO₂ and soil nutrient enrichment in a complete factorial experiment. The response of root-inhabiting non-mycorrhizal and arbuscular mycorrhizal fungi to the altered resource base depended strongly on the plant species. Elevated CO₂ and fertilization altered the ratio of non-mycorrhizal to mycorrhizal fungal colonization for some plant species, but not for others. Percent root infection by non-mycorrhizal fungi increased by over 500% for Linanthus parviflorus in elevated CO₂, but decreased by over 80% for Bromus hordeaceus. By contrast, the mean percent infection by mycorrhizal fungi increased in response to elevated CO₂ for all species, but significantly only for Avena barbata and B. hordeaceus. Percent infection by mycorrhizal fungi increased, decreased, or remained unchanged for different plant hosts in response to fertilization. There was evidence of a strong interaction between the two treatments for some plant species and non-mycorrhizal and mycorrhizal fungi. This study demonstrated plant species- and soil fertility-dependent shifts in below-ground plant resource allocation to different morpho-groups of fungal symbionts. This may have consequences for plant community responses to elevated CO₂ in this California grassland ecosystem. Received: 2 June 1997 / Accepted: 22 August 1997