Arbuscular mycorrhizas are beneficial under both deficient and toxic soil zinc conditions

Background and aims

Arbuscular mycorrhizas (AM) play different roles in plant Zn nutrition depending on whether the soil is Zn-deficient (AM enhancement of plant Zn uptake) or Zn-toxic (AM protection of plant from excessive Zn uptake). In addition, soil P concentration modifies the response of AM to soil Zn conditions. We undertook a glasshouse experiment to study the interactive effects of P and Zn on AM colonisation, plant growth and nutrition, focusing on the two extremes of soil Zn concentration—deficient and toxic.

Methods

We used a mycorrhiza-defective tomato (Solanum lycopersicum) genotype (rmc) and compared it to its wild-type counterpart (76R). Plants were grown in pots amended with five soil P addition treatments, and two soil Zn addition treatments.

Results

The mycorrhizal genotype generally thrived better than the non-mycorrhizal genotype, in terms of biomass and tissue P and Zn concentrations. This was especially true under low soil Zn and P conditions, however there was evidence of the ‘protective effect’ of mycorrhizas when soil was Zn-contaminated. Above- and below-ground allocation of biomass, P and Zn were significantly affected by AM colonisation, and toxic soil Zn conditions.

Conclusions

The relationship between soil Zn and soil P was highly interactive, and heavily influenced AM colonisation, plant growth, and plant nutrition.     

The symbiotic recapture of nitrogen from dead mycorrhizal and non-mycorrhizal roots of tomato plants

Aims

The aim was to quantify the nitrogen (N) transferred via the extra-radical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices from both a dead host and a dead non-host donor root to a receiver tomato plant. The effect of a physical disruption of the soil containing donor plant roots and fungal mycelium on the effectiveness of N transfer was also examined.

Methods

The root systems of the donor (wild type tomato plants or the mycorrhiza-defective rmc mutant tomato) and the receiver plants were separated by a 30 μm mesh, penetrable by hyphae but not by the roots. Both donor genotypes produced a similar quantity of biomass and had a similar nutrient status. Two weeks after the supply of ¹⁵?N to a split-root part of donor plants, the shoots were removed to kill the plants. The quantity of N transferred from the dead roots into the receiver plants was measured after a further 2 weeks.

Results

Up to 10.6 % of donor-root ¹⁵N was recovered in the receiver plants when inoculated with the arbuscular mycorrhizal fungus (AMF). The quantity of ¹⁵N derived from the mycorrhizal wild type roots clearly exceeded that from the only weakly surface-colonised rmc roots. Hyphal length in the donor rmc root compartments was only about half that in the wild type compartments. The disruption of the soil led to a significantly increased AMF-mediated transfer of N to the receiver plants.

Conclusions

The transfer of N from dead roots can be enhanced by AMF, especially when the donor roots have been formerly colonised by AMF. The transfer can be further increased with higher hyphae length densities, and the present data also suggest that a direct link between receiver mycelium and internal fungal structures in dead roots may in addition facilitate N transfer. The mechanical disruption of soil containing dead roots may increase the subsequent availability of nutrients, thus promoting mycorrhizal N uptake. When associated with a living plant, the external mycelium of G. intraradices is readily able to re-establish itself in the soil following disruption and functions as a transfer vessel.     

Uptake of zinc and phosphorus by plants is affected by zinc fertiliser material and arbuscular mycorrhizas

Background and Aims

Water solubility of zinc (Zn) fertilisers affects their plant availability. Further, simultaneous application of Zn and phosphorus (P) fertiliser can have antagonistic effects on plant Zn uptake. Arbuscular mycorrhizas (AM) can improve plant Zn and P uptake. We conducted a glasshouse experiment to test the effect of different Zn fertiliser materials, in conjunction with P fertiliser application, and colonisation by AM, on plant nutrition and biomass.

Methods

We grew a mycorrhiza-defective tomato genotype (rmc) and its mycorrhizal wild-type progenitor (76R) in soil with six different Zn fertilisers ranging in water solubility (Zn sulphate, Zn oxide, Zn oxide (nano), Zn phosphate, Zn carbonate, Zn phosphate carbonate), and supplemental P. We measured plant biomass, Zn and P contents, mycorrhizal colonisation and water use efficiency.

Results

Whereas water solubility of the Zn fertilisers was not correlated with plant biomass or Zn uptake, plant Zn and P contents differed among Zn fertiliser treatments. Plant Zn and P uptake was enhanced when supplied as Zn phosphate carbonate. Mycorrhizal plants took up more P than non-mycorrhizal plants; the reverse was true for Zn.

Conclusions

Zinc fertiliser composition and AM have a profound effect on plant Zn and P uptake.     

Net root growth and nutrient acquisition in response to predicted climate change in two contrasting heathland species

Background and aims

Accurate predictions of nutrient acquisition by plant roots and mycorrhizas are critical in modelling plant responses to climate change.

Methods

We conducted a field experiment with the aim to investigate root nutrient uptake in a future climate and studied root production by ingrowth cores, mycorrhizal colonization, and fine root N and P uptake by root assay of Deschampsia flexuosa and Calluna vulgaris.

Results

Net root growth increased under elevated CO₂, warming and drought, with additive effects among the factors. Arbuscular mycorrhizal colonization increased in response to elevated CO₂, while ericoid mycorrhizal colonization was unchanged. The uptake of N and P was not increased proportionally with root growth after 5 years of treatment.

Conclusions

While aboveground biomass was unchanged, the root growth was increased under elevated CO₂. The results suggest that plant production may be limited by N (but not P) when exposed to elevated CO₂. The species-specific response to the treatments suggests different sensitivity to global change factors, which could result in changed plant competitive interactions and belowground nutrient pool sizes in response to future climate change.     

Anisohydric behaviour in grapevines results in better performance under moderate water stress and recovery than isohydric behaviour

Aims

Arbuscular mycorrhizal fungi (AMF) can control root-knot nematode infection, but the mode of action is still unknown. We investigated the effects of AMF and mycorrhizal root exudates on the initial steps of Meloidogyne incognita infection, namely movement towards and penetration of tomato roots.

Methods

M. incognita soil migration and root penetration were evaluated in a twin-chamber set-up consisting of a control and mycorrhizal (Glomus mosseae) plant compartment (Solanum lycopersicum cv. Marmande) connected by a bridge. Penetration into control and mycorrhizal roots was also assessed when non-mycorrhizal or mycorrhizal root exudates were applied and nematode motility in the presence of the root exudates was tested in vitro.

Results

M. incognita penetration was significantly reduced in mycorrhizal roots compared to control roots. In the twin-chamber set-up, equal numbers of nematodes moved to both compartments, but the majority accumulated in the soil of the mycorrhizal plant compartment, while for the control plants the majority penetrated the roots. Application of mycorrhizal root exudates further reduced nematode penetration in mycorrhizal plants and temporarily paralyzed nematodes, compared with application of water or non-mycorrhizal root exudates.

Conclusions

Nematode penetration was reduced in mycorrhizal tomato roots and mycorrhizal root exudates probably contributed at least partially by affecting nematode motility.     

Influence of integrated weed management system on N-cycling microbial communities and N2O emissions

Aims

Integrated weed management, which allows reducing the reliance of cropping systems on herbicides, is based on the use of specific combinations of innovative agricultural practices. However the impact of the introduction of these practices in cropping systems may influence soil functioning such as biogeochemical cycling. Here, we investigated N₂O emissions and the abundances of N-cycling microorganisms in 11-year old cropping systems (i.e. conventional reference and integrated weed management) in order to estimate the environmental side-effects of long-term integrated weed management.

Methods

N₂O emissions were continuously measured using automated chambers coupled with infrared analysers. Abundances of ammonia oxidizers and denitrifiers together with total bacteria and archaea were determined monthly from 0 to 10 and 10–30 cm soil layer samples by quantitative Polymerase Chain Reaction (qPCR). The relationship between N₂O emissions and microbial abundances during the study were investigated as were their relationships with soil physicochemical parameters and climatic conditions.

Results

Over 7 months, the system with integrated weed management emitted significantly more N₂O with cumulated measured emissions of 240 and 544 g N-N₂O ha^?1 for conventional and integrated systems, respectively. Abundances of microbial guilds varied slightly between systems, although ammonia-oxidizing bacteria were more abundant in the reference system (1.7 10⁶ gene copies g^?1 dry weight soil) compared to the integrated system (1.0 10⁶ gene copies g^?1 dry weight soil). These differences revealed both the long-term modification of soil biogeochemical background and the functioning of microbial processes due to 11 years of alternative field management, and the short-term impacts of the agricultural practices introduced as part of weed management during the cropping year.

Conclusions

The abundances of the different microbial communities involved in N cycling and the intensity of N₂O emissions were not related, punctual high N₂O emissions being more dependent on favourable soil conditions for nitrifying and denitrifying activities. Future studies will be performed to check these findings for other pedoclimatic conditions and to examine the impact of such cropping systems.     

Interactions between the arbuscular mycorrhizal (AM) fungus Glomus intraradices and nontransformed tomato roots of either wild-type or AM-defective phenotypes in monoxenic cultures

Monoxenic symbioses between the arbuscular mycorrhizal (AM) fungus Glomus intraradices and two nontransformed tomato root organ cultures (ROCs) were established. Wild-type tomato ROC from cultivar “RioGrande 76R” was employed as a control for mycorrhizal colonization and compared with its mutant line (rmc), which exhibits a highly reduced mycorrhizal colonization (rmc) phenotype. Structural features of the two root lines were similar when grown either in soil or under in vitro conditions, indicating that neither monoxenic culturing nor the rmc mutation affected root development or behavior. Colonization by G. intraradices in monoxenic culture of the wild-type line was low (<10%) but supported extensive development of extraradical mycelium, branched absorbing structures, and spores. The reduced colonization of rmc under monoxenic conditions (0.6%) was similar to that observed previously in soil. Extraradical development of runner hyphae was low and proportional to internal colonization. Few spores were produced. These results might suggest that carbon transfer may be modified in the rmc mutant. Our results support the usefulness of monoxenically obtained mycorrhizas for investigation of AM colonization and intraradical symbiotic functioning.     

Fertilization and forb:graminoid ratio affect arbuscular mycorrhiza in seedlings but not adult plants of Plantago lanceolata

Aims

Nutrients play a key role in arbuscular mycorrhizal (AM) symbiosis. We quantified the response of AM symbiosis of seedlings and adult plants of Plantago lanceolata to fertilization under field conditions in managed grasslands differing in nutrient availability and soil moisture.

Methods

The AM symbiosis was measured as the total extent of AM fungal colonization and frequency of arbuscules or vesicles, and as the relative proportions of morphotypes. We further examined the effects of the surrounding vegetation upon AM symbiosis.

Results

Fertilization decreased total AM colonization and relative arbuscular frequency of the whole mycorrhizal community and of Acaulospora and “fine endophyte” morphotypes in seedling roots, but it had no effect upon the mycorrhiza in adult plants. The decline in arbuscular frequency in seedling roots due to fertilization was greater at the sites with higher nutrient availability and lower N:P ratio. Seedlings surrounded by more forbs had a greater total AM colonization and higher vesicular frequency.

Conclusions

Increased nutrient availability in the initial stages of seedling development has a prominent effect upon AM symbiosis development, but these effects seem to diminish over the long term, as evidenced by the results obtained for adult plants and from the limited effects of parameters characterizing long-term nutrient availability.     

Processes leading to N2O and NO emissions from two different Chinese soils under different soil moisture contents

Background and Aims

Great attention has been paid to N₂O emissions from paddy soils under summer rice-winter wheat double-crop rotation, while less focus was given to the NO emissions. Besides, neither mechanism is completely understood. Therefore, this study aimed at evaluating the relative importance of nitrification and denitrification to N₂O and NO emissions from the two soils at different soil moisture contents

Methods

N₂O and NO emissions during one winter wheat season were simultaneously measured in situ in two rice-wheat based field plots at two different locations in Jiangsu Province, China. One soil was neutral in pH with silt loam texture (NSL), the other soil alkaline in pH with a clay texture (AC). A ¹⁵?N tracer incubation experiment was conducted in the laboratory to evaluate the relative importance of nitrification and denitrification for N₂O and NO emissions at soil moisture contents of 40 % water holding capacity (WHC), 65 % WHC and 90 % WHC.

Results

Higher N₂O emission rates in the AC soil than in the NSL soil were found both in the field and in the laboratory experiments; however, the differences in N₂O emissions between AC soil and NSL soil were smaller in the field than in the laboratory. In the latter experiment, nitrification was observed to be the more important source of N₂O emissions (>70 %) than denitrification, regardless of the soils and moisture treatments, with the only exception of the AC soil at 90 % WHC, at which the contributions of nitrification and denitrification to N₂O emissions were comparable. The ratios of NO/N₂O also supported the evidence that the nitrification process was the dominant source of N₂O and NO both in situ and in the laboratory. The proportion of nitrified N emitted as N₂O (P _N2O ) in NSL soil were around 0.02 % in all three moisture treatments, however, P _N2O in the AC soil (0.04 % to 0.10 %) tended to decrease with increasing soil moisture content.

Conclusions

Our results suggest that N₂O emission rates obtained from laboratory incubation experiments are not suitable for the estimation of the true amount of N₂O fluxes on a field scale. Besides, the variations of P _N2O with soil property and soil moisture content should be taken into account in model simulations of N₂O emission from soils.     

Seasonal carbon allocation to arbuscular mycorrhizal fungi assessed by microscopic examination, stable isotope probing and fatty acid analysis

Background and Aim

Climate change models are limited by lack of baseline data, in particular carbon (C) allocation to – and dynamics within – soil microbial communities. We quantified seasonal C-assimilation and allocation by plants, and assessed how well this corresponds with intraradical arbuscular mycorrhizal fungal (AMF) storage and structural lipids (16:1ω5 NLFA and PLFA, respectively), as well as microscopic assessments of AMF root colonization.

Methods

Coastal Hypochoeris radicata plants were labeled with ¹³CO₂ in February, July and October, and ¹³C-allocation to fine roots and NLFA 16:1ω5, as well as overall lipid contents and AM colonization were quantified.

Results

C-allocation to fine roots and AMF storage lipids differed seasonally and mirrored plant C-assimilation, whereas AMF structural lipids and AM colonization showed no seasonal variation, and root colonization exceeded 80 % throughout the year. Molecular analyzes of the large subunit rDNA gene indicated no seasonal AMF community shifts.

Conclusions

Plants allocated C to AMF even at temperatures close to freezing, and fungal structures persisted in roots during times of low C-allocation. The lack of seasonal differences in PLFA and AM colonization indicates that NLFA analyses should be used to estimate fungal C-status. The implication of our findings for AM function is discussed.     

Liebig’s law of the minimum applied to a greenhouse gas: alleviation of P-limitation reduces soil N2O emission

Background and aims

Emission of the greenhouse gas (GHG) nitrous oxide (N₂O) are strongly affected by nitrogen (N) fertilizer application rates. However, the role of other nutrients through stoichiometric relations with N has hardly been studied. We tested whether phosphorus (P) availability affects N₂O emission. We hypothesized that alleviation of plant P-limitation reduces N₂O emission through lowering soil mineral N concentrations.

Methods

We tested our hypothesis in a pot experiment with maize (Zea mays L.) growing on a P-limiting soil/sand mixture. Treatment factors included P and N fertilization and inoculation with Arbuscular Mycorrhizal Fungi (AMF; which can increase P uptake).

Results

Both N and P fertilization, as well as their interaction significantly (P?<?0.01) affected N₂O emission. Highest N₂O emissions (2.38 kg N₂O-N ha^?1) were measured at highest N application rates without P fertilization or AMF. At the highest N application rate, N₂O fluxes were lowest (0.71 kg N₂O-N ha^?1) with both P fertilization and AMF. The N₂O emission factors decreased with 50 % when P fertilization was applied.

Conclusions

Our results illustrate the importance of the judicious use of all nutrients to minimize N₂O emission, and thereby further underline the intimate link between sound agronomic practice and prudent soil GHG management.     

Direct and indirect influences of arbuscular mycorrhizal fungi on phosphorus uptake by two root hemiparasitic Pedicularis species: do the fungal partners matter at low colonization levels?

Background and Aims

Because most parasitic plants do not form mycorrhizal associations, the nutritional roles of arbuscular mycorrhizal (AM) fungi in them have hardly been tested. Some facultative root hemiparasitic Pedicularis species form AM associations and hence are ideal for testing both direct and indirect effects of AM fungi on their nutrient acquisition. The aim of this study was to test the influence of AM inoculation on phosphorus (P) uptake by Pedicularis rex and P. tricolor.

Methods

³²P labelling was used in compartmented pots to assess the contribution of the AM pathway and the influence of AM inoculation on P uptake from a host plant into the root hemiparasites. Laboratory isolates of fungal species (Glomus mosseae and G. intraradices) and the host species (Hordeum vulgare ‘Fleet’) to which the two Pedicularis species showed obvious responses in haustorium formation and growth in previous studies were used.

Key Results

The AM colonization of both Pedicularis spp. was low (<15 % root length) and only a very small proportion of total plant P (<1 %) was delivered from the soil via the AM fungus. In a separate experiment, inoculation with AM fungi strongly interfered with P acquisition by both Pedicularis species from their host barley, almost certainly because the numbers of haustoria formed by the parasite were significantly reduced in AM plants.

Conclusions

Roles of AM fungi in nutrient acquisition by root parasitic plants were quantitatively demonstrated for the first time. Evidence was obtained for a novel mechanism of preventing root parasitic plants from overexploiting host resources through AM fungal-induced suppression of the absorptive structures in the parasites.     

Emissions of nitrous oxide from the leaves of grasses

Aims

Nitrous oxide (N₂O) emissions from pastoral agriculture are considered to originate from the soil as a consequence of microbial activity during soil nitrification and denitrification. However, recent studies have identified the plant canopy as a potentially significant source of N₂O emissions to the atmosphere. Understanding the extent and mechanisms of plant emissions may provide new mitigation opportunities as current options only target soil microbial processes.

Methods

We developed an experimental apparatus and protocol to partition N₂O emissions between the leaves of grasses and the soil and measured emissions from ten common grass species found in New Zealand pastures.

Results

The chamber design enabled us to identify measurable changes in N₂O concentration over a period of 1 h and to distinguish a range of emissions from 0.001 to 0.25 mg N₂O-N/m² leaf area/h. There was a 10-fold variation among species; Holcus lanataus, Lolium perenne and Paspalum dilatatum had the highest leaf N₂O emissions and Poa annua the lowest.

Conclusions

Grasses do emit N₂O from their leaves and the rate that this occurs varies among grass species. The emission does not appear to arise from formation of N₂O in plant leaves but more likely reflects transport of N₂O from the soil. Differences in emission rates appear to arise from a plant influence on the rate of formation of N₂O in the soil rather than the rate of transportation through the plant.     

N- and P-mediated seminal root elongation response in rice seedlings

Aims

In rice, seminal root elongation plays an important role in acquisition of nutrients such as N and P, but the extent to which different N forms and P concentrations affect root growth is poorly understood. This study aimed to examine N- and P-mediated seminal root elongation response and to identify putative QTLs associated with seminal root elongation.

Methods

Seminal root elongation was evaluated in 15 diverse wild and cultivated accessions of rice, along with 48 chromosome segment substitution lines (CSSLs) derived from a cross between the rice variety ‘Curinga’ and Oryza rufipogon (IRGC 105491). Root elongation in response to different forms of N (NH₄ ⁺, NO₃ ^? and NH₄NO₃) and concentrations of P was evaluated under hydroponic conditions, and associated putative QTL regions were identified.

Results

The CSSL parents had contrasting root responses to N and P. Root elongation in O. rufipogon was insensitive to N source and concentration, whereas Curinga was responsive. In contrast to N, seminal root elongation and P concentration was positively correlated. Three putative QTLs for seminal root elongation in response to N were detected on chromosome 1, and one QTL on chromosome 3 was associated with low P concentration.

Conclusions

Genetic variation in seminal root elongation and plasticity of nutrient response may be appropriate targets for marker-assisted selection to improve rice nutrient acquisition efficiency.     

Interactions between root hair length and arbuscular mycorrhizal colonisation in phosphorus deficient barley (Hordeum vulgare)

Aims

Phosphorus (P) limits crop yield and P-fertilisers are frequently applied to agricultural soils. However, supplies of quality rock phosphate are diminishing. Plants have evolved mechanisms to improve P-acquisition and understanding these could improve the long-term sustainability of agriculture. Here we examined interactions between root hairs and arbuscular mycorrhizal (AM) colonisation in barley (Hordeum vulgare L.).

Methods

Barley mutants exhibiting different root hair phenotypes, wild type barley and narrowleaf plantain (Plantago lanceolata L.) were grown in the glasshouse in P-sufficient and P-deficient treatments and allowed to develop AM colonization from the natural soil community. Plants were harvested after 6 weeks growth and root hair length, AM-fungal colonisation, shoot biomass and P-accumulation measured.

Results

Under P-deficient conditions, root hair length and AM colonisation were negatively related suggesting that resources are allocated to root hairs rather than to AM fungi in response to P-deficiency. There was evidence that barley and narrowleaf plantain employed different strategies to increase P-acquisition under identical conditions, but root hairs were more effective.

Conclusions

This research suggests future barley breeding programmes should focus on maintaining or improving root hair phenotypes and that pursuing enhancements to AM associations under the prevalent agricultural conditions tested here would be ineffectual.     

When do arbuscular mycorrhizal fungi protect plant roots from pathogens?

Arbuscular mycorrhizal (AM) fungi are mainly thought to facilitate phosphorus uptake in plants, but they can also perform several other functions that are equally beneficial. Our recent study sheds light on the factors determining one such function, enhanced plant protection from root pathogens. Root infection by the fungal pathogen Fusarium oxysporum was determined by both plant susceptibility and the ability of an AM fungal partner to suppress the pathogen. The non-susceptible plant species (Allium cepa) had limited F. oxysporum infection even without AM fungi. In contrast, the susceptible plant species (Setaria glauca) was heavily infected and only AM fungi in the family Glomeraceae limited pathogen abundance. Plant susceptibility to pathogens was likely determined by contrasting root architectures between plants, with the simple rooted plant (A. cepa) presenting fewer sites for infection. AM fungal colonization, however, was not limited in the same way in part because plants with fewer, simple roots are more mycorrhizal dependent. Protection only by Glomus species also indicates that whatever the mechanism(s) of this function, it responds to AM fungal families differently. While poor at pathogen protection, AM fungal species in the family Gigasporaceae most benefited the growth of the simple rooted plant species. Our research indicates that plant trait differences, such as root architecture can determine how important each mycorrhizal function is to plant growth but the ability to provide these functions differs among AM fungi.Key words: arbuscular mycorrhizal fungi, Fusarium oxysporum, root architecture, pathogen protection, multi-functionalityArbuscular mycorrhizas (AM) represent the oldest and most widespread symbiosis with land plants.¹ Most mycorrhizal research has focused on the ability of AM fungi to facilitate nutrient uptake, particularly phosphorus.² Although researchers recognize that AM fungi are multi-functional,³ it is not clear what factors determine which function an AM fungus performs or its relative importance to the plant.⁴ Newsham et al. (1995)³ hypothesized that AM function is based on root architecture: plants with simple rooting systems are dependent on mycorrhizas for nutrient uptake, while those with complex root systems are less dependent on mycorrhizas for nutrient uptake, but are more susceptible to root pathogens because of increased numbers of infections sites.³ These two functions, phosphorus uptake and enhanced pathogen protection from mycorrhizas also depend on the identity of the fungus. Arbuscular mycorrhizal fungi in the family Gigasporaceae are more effective at enhancing plant phosphorus, while AM fungi in the Glomeraceae better protect plants from root pathogens.⁵Our results support both plant and fungal control of a common pathogen, Fusarium oxysporum, and the interaction between these two factors ultimately determined the level of pathogen infection and plant mycorrhizal benefit. We inoculated two plant species that have contrasting root architectures with one of six AM fungal species from two families (or no AM fungi). After five months of growth, plants were inoculated with F. oxysporum, grown for another month and then harvested. All plant seeds and fungi were collected in a local old field community.⁶ Allium cepa (garden onion) was not susceptible to F. oxysporum likely because it has only a few adventitious roots below the main bulb that do not present many sites for infection. In contrast, Setaria glauca (yellow foxtail) was heavily infected by F. oxysporum and has fine roots with increased numbers of branching points and lateral meristems where fungi can colonize.⁷ For the susceptible plant (S. glauca), AM fungal species from the family Glomeraceae were effective at reducing pathogen abundance while species from the Gigasporaceae were not. Forming a symbiosis with a Glomus species resulted in S. glauca plants that were as large as control plants. AM fungal species from the family Gigaspoaceae were more beneficial to growth of the simple rooted A. cepa, which had fewer roots to take up soil nutrients.Reduced rooting structures may limit pathogen infection sites, but AM fungal colonization was not limited in the same way and may actually alter plant root architecture. While the simple rooted A. cepa had limited pathogen susceptibility, it had twice the AM fungal colonization of the complex rooted S. glauca. Because the simple rooted plant has a greater dependence on mycorrhizas,⁸ it likely transmits chemical signals to rapidly initiate mycorrhizal formation,⁹ but then may have less control on the spread of AM fungi within the root. In contrast, S. glauca is more susceptible to fungal pathogens and may be less mycorrhizal dependent in nature.¹⁰ As a result, S. glauca may treat all colonizing root fungi as potential parasites. Colonization by AM fungi from the Glomeraceae was also much greater than those in the Gigasporaceae due to differences in fungal life history strategy between these families.^11,12 AM fungal colonization can reduce root branching in plants and alter plant allocation to roots, thereby increasing mycorrhizal dependence for nutrients^10,13 and potentially reducing pathogen infection sites. Mycorrhizal induced changes to plant root architecture may therefore reinforce current mycorrhizal associations and alter future fungal colonization attempts.¹⁴ An important next step is to test if AM fungal families (or species) alter plant root architecture in different ways and the degree to which these effects depend on colonization timing and the plant host.Our study did not isolate the particular mechanism by which AM fungi control pathogens, but this mechanism clearly differentiates between AM fungal families. AM fungi can control pathogens through several mechanisms including direct competition for colonization sites, indirect initiation of plant defensive responses or altering other rhizosphere biota.¹⁵ Although these AM fungal families differ in the intensity of root colonization,¹¹ percentage of root length colonized by an AM fungus is a poor predictor of pathogen limitation compared to family identity,^12,16 suggesting that direct competition for space is unlikely. AM fungi share many cell surface molecules with pathogenic fungi like Fusarium.¹⁷ These molecules can act as signals that initiate plant production of defensive compounds such as phytoalexins, phenolics and other compounds.¹⁸ While AM fungi appear to evade these defenses, only AM fungal species in the family Glomeraceae would have elicited plant responses which altered future infection by F. oxysporum. AM fungi in the Gigasporaceae may differ more from F. oxysporum in their chemical signals or not colonize roots sufficiently to induce a sustained, system-wide plant response. In addition, many rhizosphere related microbes are antagonistic to pathogenic fungi¹⁵ and may differ in their response to the different AM fungal families.¹⁹ Because rhizosphere microbes also differ among plant species, plant pathogen protection may be influenced by multiple ecological interactions that determine the specific cases when mycorrhizal pathogen protection occurs. To distinguish between these mechanisms, future experiments could test whether biochemical similarity or ecological similarity (especially with other soil biota) between an AM fungus and fungal pathogen can predict mycorrhizal induced pathogen protection.Plant and fungal identity clearly affect AM fungal function and benefit, but to accurately use AM fungi in agriculture and restoration^20,21 we must clearly understand how functional mechanisms differ. Different mycorrhizal functions may be based on common plant traits like root architecture, but ecology, colonization timing and environment may alter the specific function AM fungi provide and its importance to plants. While it may be useful to establish greenhouse rules about which fungal species perform specific mycorrhizal functions, predicting their role in more complex systems relies on understanding if other factors will enhance or negate these effects. Most AM fungal species vary in their ability to perform each function and these can be locally adapted to limiting soil nutrients.²² In plants, there is also a range to which specific mycorrhizal functions may benefit plant fitness, and these responses are based on both plant traits (which change throughout a plant''s life cycle) and the local environment.^23,24 Given this variation, it is critical to understand if AM fungi can respond to cues from the plant or the environment to identify what factors limit plant growth and whether a the most effective AM fungus shows a greater response.     

The dual symbiosis between arbuscular mycorrhiza and nitrogen fixing bacteria benefits the growth and nutrition of the woody invasive legume Acacia cyclops under nutrient limiting conditions

Background and aims

Acacia cyclops is an invasive species within Mediterranean ecosystems, characteristically low in soil nutrients. Thus associations with nitrogen-fixing bacteria (NFB) and arbuscular mycorrhiza (AM) may provide an advantage to these legumes. This study investigated the role of AM and NFB in the growth and nutritional physiology of A. cyclops.

Methods

Seedlings were inoculated with?naturally occurring?NFB, Glomus mosseae or both, and grown under glasshouse conditions for 5 months. Plants were cultivated in sand and supplied with a 20 % strength nutrient solution.?Xylem sap nutrients, photosynthetic rates, biomass and chemical compositions, were recorded.

Results

The dual inoculation decreased the colonization of both symbionts, compared to a single symbiosis with either symbiont. Despite low colonization levels, the dual symbiosis increased host biomass and relative growth rates. This was associated with increased photosynthetic rates and enhanced nutrition. Additionally, dual symbiotic plants had enhanced N and P acquisition and utilization rates. Xylem sap analysis showed higher levels of NH ₄ ⁺ being exported from the roots to the shoots in the dual symbiotic plants compared with other treatments.

Conclusions

These findings suggest the dual symbiosis is an important factor in the growth and development of A. cyclops under nutrient limiting conditions.     

Effects of nitrate concentration on the denitrification potential of a calcic cambisol and its fractions of N2, N2O and NO

Background and aims

The direct measurement of denitrification dynamics and its product fractions is important for parameterizing process-oriented model(s) for nitrogen cycling in various soils. The aims of this study are to a) directly measure the denitrification potential and the fractions of nitrogenous gases as products of the process in laboratory, b) investigate the effects of the nitrate (NO ₃ ^? ) concentration on emissions of denitrification gases, and c) test the hypothesis that denitrification can be a major pathway of nitrous oxide (N₂O) and nitric oxide (NO) production in calcic cambisols under conditions of simultaneously sufficient supplies of carbon and nitrogen substrates and anaerobiosis as to be found to occur commonly in agricultural lands.

Methods

Using the helium atmosphere (with or without oxygen) gas-flow-soil-core technique in laboratory, we directly measured the denitrification potential of a silt clay calcic cambisol and the production of nitrogen gas (N₂), N₂O and NO during denitrification under the conditions of seven levels of NO ₃ ^? concentrations (ranging from 10 to 250 mg N kg^?1 dry soil) and an almost constant initial dissolved organic carbon concentration (300 mg C kg^?1 dry soil).

Results

Almost all the soil NO ₃ ^? was consumed during anaerobic incubation, with 80–88 % of the consumed NO ₃ ^? recovered by measuring nitrogenous gases. The results showed that the increases in initial NO ₃ ^? concentrations significantly enhanced the denitrification potential and the emissions of N₂ and N₂O as products of this process. Despite the wide range of initial NO ₃ ^? concentrations, the ratios of N₂, N₂O and NO products to denitrification potential showed much narrower ranges of 51–78 % for N₂, 14–36 % for N₂O and 5–22 % for NO.

Conclusions

These results well support the above hypothesis and provide some parameters for simulating effects of variable soil NO ₃ ^? concentrations on denitrification process as needed for biogeochemical models.     

Mycorrhizal responsiveness trends in annual crop plants and their wild relatives—a meta-analysis on studies from 1981 to 2010

Background and aims

Year of release of a cultivar reflects the agricultural and breeding practices of its time; we hypothesize that there are differences in mycorrhizal responsiveness of new high yielding and old crop plants and landraces. We evaluated the importance of the year of release on mycorrhizal responsiveness, arbuscular mycorrhizal (AM) fungal root colonization and P efficiency. We also analyzed the effect of experimental treatments, P efficiency (P acquisition and P utilization efficiency) and AM fungal root colonization on a potential mycorrhizal responsiveness trend for year of release.

Methods

We conducted a meta-analysis on 39 publications working on 320 different crop plant genotypes.

Results

New cultivars were less intensely colonized but were more mycorrhiza-responsive (and possibly dependent) compared to ancestral genotypes. This trend was potentially influenced by the moderator variables density, pre-germination, plant, plant type and AMF species. AM root colonization was also important for the mycorrhizal responsiveness trend for year of release, but P efficiency was not.

Conclusions

With the data available we could find no evidence that new crop plant genotypes lost their ability to respond to mycorrhiza due to agricultural and breeding practices.