Heidi-Jayne Hawkins

Uptake and transport of organic and inorganic nitrogen by arbuscular mycorrhizal fungi

New information on N uptake and transport of inorganic and organic N in arbuscular mycorrhizal fungi is reviewed here. Hyphae of the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (BEG 107) were shown to transport N supplied as 15N-Gly to wheat plants after a 48 h labelling period in semi-hydroponic (Perlite), non-sterile, compartmentalised pot cultures. Of the 15N supplied to hyphae in pot cultures over 48 h, 0.2 and 6% was transported to plants supplied with insufficient N or sufficient N, respectively. The increased 15N uptake at the higher N supply was related to the higher hyphal length density at the higher N supply. These findings were supported by results from in vitro and monoxenic studies. Excised hyphae from four Glomus isolates (BEG 84, 107, 108 and 110) acquired N from both inorganic (15NH415NO3, 15NO3− or 15NH4+) and organic (15N-Gly and 15N-Glu, except in BEG 84 where amino acid uptake was not tested) sources in vitro during short-term experiments. Confirming these studies under sterile conditions where no bacterial mineralisation of organic N occurred, monoxenic cultures of Glomus intraradices Schenk and Smith were shown to transport N from organic sources (15N-Gly and 15N-Glu) to Ri T-DNA transformed, AM-colonised carrot roots in a long-term experiment. The higher N uptake (also from organic N) by isolates from nutrient poor sites (BEG 108 and 110) compared to that from a conventional agricultural field implied that ecotypic differences occur. Although the arbuscular mycorrhizal isolates used contributed to the acquisition of N from both inorganic and organic sources by the host plants/roots used, this was not enough to increase the N nutritional status of the mycorrhizal compared to non-mycorrhizal hosts.

The importance of nutritional regulation of plant water flux

Transpiration is generally considered a wasteful but unavoidable consequence of photosynthesis, occurring because water is lost when stomata open for CO2 uptake. Additionally, transpiration has been ascribed the functions of cooling leaves, driving root to shoot xylem transport and mass flow of nutrients through the soil to the rhizosphere. As a consequence of the link between nutrient mass flow and transpiration, nutrient availability, particularly that of NO3−, partially regulates plant water flux. Nutrient regulation of transpiration may function through the concerted regulation of: (1) root hydraulic conductance through control of aquaporins by NO3−, (2) shoot stomatal conductance (gs) through NO production, and (3) pH and phytohormone regulation of gs. These mechanisms result in biphasic responses of water flux to NO3− availability. The consequent trade-off between water and nutrient flux has important implications for understanding plant distributions, for production of water use-efficient crops and for understanding the consequences of global-change-linked CO2 suppression of transpiration for plant nutrient acquisition.

Studies of iron transport by arbuscular mycorrhizal hyphae from soil to peanut and sorghum plants

Abstract The influence of an arbuscular mycorrhizal (AM) fungus on phosphorus (P) and iron (Fe) uptake of peanut (Arachis hypogea L.) and sorghum (Sorghum bicolor L.) plants was studied in a pot experiment under controlled environmental conditions. The plants were grown for 10 weeks in pots containing sterilised calcareous soil with two levels of Fe supply. The soil was inoculated with rhizosphere microorganisms only or with rhizosphere microorganisms together with an AM fungus (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe). An additional small soil compartment accessible to hyphae but not roots was added to each pot after 6 weeks of plant growth. Radiolabelled P and Fe were supplied to the hyphae compartment 2 weeks after addition of this compartment. After a further 2 weeks, plants were harvested and shoots were analysed for radiolabelled elements. In both plant species, P uptake from the labelled soil increased significantly more in shoots of mycorrhizal plants than non-mycorrhizal plants, thus confirming the well-known activity of the fungus in P uptake. Mycorrhizal inoculation had no significant influence on the concentration of labelled Fe in shoots of peanut plants. In contrast, 59Fe increased in shoots of mycorrhizal sorghum plants. The uptake of Fe from labelled soil by sorghum was particularly high under conditions producing a low Fe nutritional status of the plants. These results are preliminary evidence that hyphae of an arbuscular mycorrhizal fungus can mobilise and/or take up Fe from soil and translocate it to the plant.

Growth and sporulation of the arbuscular mycorrhizal fungus Glomus caledonium in dual culture with transformed carrot roots

Glomus caledonium was established in a dual culture with Ri T-DNA-transformed carrot roots. A modification of the minimal M medium buffered at pH 6.50 with 10 mM MES and solidified with 0.4% unpurified gellan gum allowed spore germination and formation of the symbiosis, together with the development of an extensive extramatrical mycelium and sporulation. Spore production increased with culture generation and most spores were viable. These spores colonized carrot roots and completed the fungal life cycle. In many cultures, sporulation was accompanied by the formation of arbuscule-like structures on short and thickened lateral branches of main hyphae. Root colonization was of the Paris-type with hyphae spreading intracellularly. Most colonized root cells contained coils of thickened hyphae, sometimes surrounded by fine hyphae, but no typical arbuscules were observed.

Hydroponic culture of the mycorrhizal fungus Glomus mosseae with Linum usitatissimum L., Sorghum bicolor L. and Triticum aestivum L.

Linum usitatissimum, Sorghum bicolor and Triticum aestivum plants were further colonised by the arbuscular mycorrhizal fungus, Glomus mosseae, during a four week period of hydroponic culture after a pre-culture period of three weeks with the fungus in perlite substrate. The viability of mycorrhizal colonisation of T. aestivum was indicated by an initial experiment where G. mosseae from mycorrhizal plants colonised non-mycorrhizal plants when the plants were grown together in the same hydroponic container using modified Long Ashton nutrient solution. Intermittant aeration of the plant roots (2 h periods, four times per day) provided a compromise between adequate aeration and minimal disturbance of the fungus. In a second experiment, two nutrient media, modified Long Ashton and modified Knop plus Hoagland medium were compared for culturing G. mosseae on T. aestivum. A significantly higher root dry weight was found for the mycorrhizal versus the non-mycorrhizal wheat plants in modified Long Ashton nutrient medium, which contained 10 µM P and an organic buffer. Modified Knop plus Hoagland nutrient medium contained a high P concentration (0.9 mM) and did not produce viable cultures of mycorrhizal colonisation. In a third experiment, modified Long Ashton medium was used for hydroponic culture of mycorrhizal L. usitatissimum, S. bicolor and T. aestivum. The root colonisation percentages for T. aestivum (73%), S. bicolor (36%) and L. usitatissimum (65%) were within the range of colonisation rates obtained with solid substrate culture in perlite. Viability of the mycorrhizal structures in hydroponic culture was assessed by monitoring activity of fungal succinate dehydrogenase and found to be similar to cultures in perlite. No difference in the P concentration of mycorrhizal and non-mycorrhizal plants was observed, possibly owing to the lack of diffusion limits for P in hydroponic solution. This report describes a system for the viable culture of G. mosseae with different plant species where a high mycorrhizal colonisation rate was produced under conditions of a short culture period using intermittent aeration, a low concentration of P supply and an organic buffer.

Root respiratory quotient and nitrate uptake in hydroponically grown non-mycorrhizal and mycorrhizal wheat

Abstract Oxygen and CO2 fluxes were measured in hydroponically grown mycorrhizal and non-mycorrhizal Triticum aestivum L. cv. Hano roots. The NO3– uptake of the plants was used to estimate the amount of root respiration attributable to ion uptake. Plants were grown at 4 mM N and 10 μM P, where a total and viable mycorrhizal root colonisation of 48% and 18%, respectively, by Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (BEG 107) was observed. The O2 consumption and NO3– uptake rates were similar and the CO2 release was higher in mycorrhizal than in non-mycorrhizal wheat. This resulted in a significantly higher respiratory quotient (RQ, mol CO2 mol–1 O2) in mycorrhizal (1.27±0.13) than in non-mycorrhizal (0.79±0.05) wheat. As the biomass and N and P concentrations in mycorrhizal and non-mycorrhizal wheat were the same, the higher RQ resulted from the mycorrhizal colonisation and not differences in nutrition per se.

A global assessment of Holistic Planned Grazing™ compared with season-long, continuous grazing: meta-analysis findings

It has been claimed that Holistic Planned Grazing™ (HPG), a type of rotational grazing, can increase productivity in rangelands and reverse climate change while doubling the stocking rate, mainly through the impact of densely bunched animals on primary production. Previous reviews have found similar or greater plant and animal production in continuous (season-long) compared with rotational grazing. Here season-long continuous grazing is compared with HPG alone to explore the evidence for animal impact. Three quantitative meta-analysis models were used to assess data sets from literature between 1972 and 2016. Weighted mean differences (effect sizes) between HPG and continuous grazing showed that there was no difference in plant basal cover, plant biomass and animal gain responses (p > 0.05). Thus, from the balance of studies, if animal impact is occurring during HPG, it has no effect on production. As interesting as the overall result is the significant between-study heterogeneity assessed using Cochran’s Q (p = 0.007 to <0.0001). Studies with positive effect sizes tended to have higher precipitation (p < 0.05), suggesting that only some rangelands have the resources to support HPG. Furthermore, there is scope for investigating the impact of HPG on socio-ecological aspects of rangelands, such as management.

Does Holistic Planned Grazing™ work on native rangelands?

The balance of papers in this Special Issue show that how Holistic Planned Grazing™ is managed and where it is used impacts the efficacy of the approach. While we will do well to develop more mechanistic models that can identify these thresholds and test them in real-life situations, it is certain that broad generalisations will not do. We can neither dismiss Holistic Planned Grazing out of hand nor claim that it will work anywhere. Both land-users and scientists should consider the evidence at hand along with their management goals (production, conservation or restoration) before deciding what livestock management approach is appropriate.

Increasing crop diversity increased soil microbial activity, nitrogen-sourcing and crop nitrogen, but not soil microbial diversity

The relationships between crop diversity, soil microbial diversity and agroecosystem functioning are not well understood. Soils under wheat monoculture, wheat–medic, and wheat–medic plus clover rotations from a 19-year-old wheat rotation trial in South Africa were measured for soil microbial functional and genetic diversity using community-level physiological profiling and automated rRNA intergenic spacer analysis. A 15N–13C dual isotope mixing model determined the nitrogen (N) sourcing when wheat was supplied with N from fertiliser and sheep dung (monoculture), or fertiliser, diazotrophy from one or two legumes, and sheep dung (wheat in rotation). Historical wheat yields and foliar [N] were 28% and 106% higher in wheat–legume rotations compared with wheat monoculture. Increasing crop diversity was related to increased soil microbial activity, but not increased microbial richness or diversity, which depended more on known abiotic drivers of microbial community structure. The δ15N mixing models revealed an increasing dependence on legume-derived N with increasing legume species in rotation. This suggests that enhanced N cycling and yield in crop–legume rotations is not a result of microbial diversity per se, but rather increased microbial activity when keystone legume species and their associated N2-fixing symbionts were present.

Beitrag der Mykorrhiza zur Aufnahme von Anorganischem und Organischem N in Weizen

Es wurde gezeigt, das der Mykorrhizapilz Glomus mosseae anorganischen und organischen N aufgenommen und zur Weizenpflanze transportiert hat. Die Hyphen des Mykorrhizapilzes transportierten 15N zu Weizenpflanzen, das in anorganischer N-Form (als 15NH4 15NO3, 15NO3 − oder 15NH4 +) oder in organischer N-Form (als 15N-Glycin) den Hyphen fur 48 h angeboten wurde. Die Form des anorganischen N hatte einen Einflus auf Hyphenlange, 15N-Aufnahme und Trockenmasse der Pflanzen (NH4NO3 ≥ NO3 − > NH4 +). Der N-Ernahrungszustand der Pflanzen hatte einen Einflus auf Kolonisationsrate, Hyphenlange, 15N-Aufnahme und Trockenmasse der Pflanzen (ausreichendes N-Angebot > nicht ausreichendes N-Angebot).