9 Aug 2022

Mycorrhizal fungi in a wildflower meadow

Raphaella Hull

Raphaella Hull is a visiting PhD student from the University of Cambridge, where she researches arbuscular mycorrhizal symbiosis in plants. She is interested in how to grow with soil fungi in mind.

With its biodiversity-rich wildflower meadows, the National Botanic Garden of Wales provides a rare opportunity to experience a habitat that has all but disappeared from Great Britain.

If you spend some time on a summer evening in one of the meadows at the Garden, you will find yourself in the company of a diverse community of wildflowers, insects, and fungi.

The brilliant pinks of the southern marsh-orchids Dactylorhiza praetermissa and striking flower spikes of the greater butterfly-orchids Platanthera chlorantha, classified as ‘near threatened’ on the Vascular Plant Red Data List for Great Britain, stand out most but, in between, is a spread of yellow rattle Rhinanthus minor, red clover Trifolium pratense, and great burnet Sanguisorba officinalis, and hidden among the tangle of taller flowers, the sparkling, delicately marked flowers of eyebright Euphrasia sp. Over the heads of this multitude there is a sea of grass flowers and insects.

The complexity found above ground is, surprisingly, sustained by soil that is typically low in the mineral nutrients essential for plant growth. Wildflower meadows establish on soils with low fertility, which contain low concentrations of nitrogen, phosphorus, and potassium – the key nutrients that plants require in large quantities for healthy growth.

Counter-intuitively, it is the low nutrient conditions that are the reason why diversity is so high in wildflower meadows.

The low soil fertility means that coarse grasses struggle to establish, which suppresses the formation of grassland and allows wildflowers to thrive. Potentially, the low nutrient concentrations mean that wildflower species grow relatively slowly. This slow growth rate may contribute to maintaining high biodiversity in a wildflower meadow, as no single plant can grow so fast that it outcompetes its neighbours.

Wildflower meadow plants have several strategies to cope with the low nutrient concentrations in the soil.

Perennial species, like great burnet, common knapweed Centaurea nigra, selfheal Prunella vulgaris and most of the grasses, often form extensive mats of fibrous roots in the surface soil layers, where nutrient concentrations are highest. Besides being modest in their nutrient requirements, perennial plants are also efficient at recycling nutrients from their old tissues.

Two important annual wildflower meadow species – yellow rattle and eyebright – have evolved their own strategy for nutrient uptake from a low nutrient environment.

Yellow rattle and eyebright are both hemiparasitic, meaning that, although the plants can generate their own sugars by photosynthesis, they also obtain some nutrition through parasitism. They do this by attaching themselves to the roots of other plants, subsidising the limited quantity of nutrients they can obtain through their own small root systems by stealing some from their neighbours.

Specifically, yellow rattle and eyebright both parasitise and weaken grass species, which helps to suppress the development of grassland and provides space for wildflowers to grow. In fact, to turn a lawn into a wildflower meadow, you can begin with soil scarification and the sowing of yellow rattle seeds in the autumn.

An alternative method to parasitism for obtaining nutrients is used by most other wildflower meadow species. Examination of the roots of wildflowers show that they contain beneficial mycorrhizal fungi living intimately within the plant root cells. Mycorrhizae are formed when arbuscular mycorrhizal fungi in the soil enter plant roots, where they supply plants with nutrients that they have foraged from beyond the reach of the plant root system.

In return, plants provide the fungi with photosynthetically fixed carbon in the form of sugars and lipids. Physiologically, arbuscular mycorrhizal fungi form highly branched intracellular structures in roots termed ‘arbuscules’, derived from the Latin for tree. These tiny treelike structures are the main sites of nutrient exchange between plants and fungi. Arbuscular mycorrhizal symbiosis is ubiquitous, found not only in wildflower meadows but in habitats globally, with plants that associate with arbuscular mycorrhizal fungi making up an estimated 70 per cent of global plant biomass. Arbuscular mycorrhizal symbiosis is also ancient, playing a key part in plant territorialisation some 450 million years ago.

Just how much these mycorrhizal associations help a particular plant depends on the species and can be difficult to study in situ.

While both annual and perennial plants can be colonised by mycorrhizal fungi and their growth improved by the symbiosis, it is thought that perennials generally respond more favourably to colonisation than annuals.

In a natural environment, it is difficult to prove just how beneficial mycorrhizae are to different plants, since if a plant can be colonised by mycorrhizal fungi, then it is likely that all members of that species in the area will already contain fungi in their roots. However, by growing seedlings on a sterile substrate and allowing only certain plants to become colonised, you can observe the growth enhancements of mycorrhizal symbiosis. In some cases, the non-colonised plants hardly develop beyond the seedling stage. This investigation removes any of the interactions with other plants and fungi that you would find in nature but can demonstrate the significant growth enhancement that mycorrhizal fungi often impart on their plant hosts.

Despite the difficulties in studying mycorrhizal symbiosis in situ, it is thought that most plants in a wildflower meadow will be growing with support from mycorrhizal symbionts.

The more widespread arbuscular mycorrhizal associations are very different from those which have long been known to support orchids.

All orchids require their mycorrhizae for successful germination in the wild, as orchid seed reserves are very limited and carbon delivered by the fungus is essential for germination.

In contrast, arbuscular mycorrhizae in non-orchid plants form when seedling roots meet a germinating fungal spore or an existing colonised root in soil. The fact that seedlings often become colonised through contact with neighbouring plants means that mycorrhizal fungi, as well as supplying nutrients, are also connecting plants with one another belowground. While this is particularly well-studied for forests (the “Wood Wide Web”), it is also believed that herbaceous plants are connected to one another in common mycorrhizal networks. It is likely that these networks help to distribute nutrients between plants. Like the hemiparasitic members of a meadow, the mycorrhizal plants are therefore also obtaining some of their nutrients from neighbouring plants.

Interestingly, the low nutrient soil conditions required for diverse wildflower meadows are also generally required for the establishment of mycorrhizal symbiosis. High nutrient conditions following the application of synthetic fertilisers are known to block the symbiosis between plants and fungi. In these conditions, plants can take up nutrients directly and do not need to give away their carbon in return for fungi-derived nutrients.

While plants can grow vigorously, the fungi perish as they are completely dependent on their plant hosts for essential lipids. In contrast, low nutrient concentrations promote mycorrhizal symbiosis, so that plants can benefit from the foraging skills of their fungal symbionts. Therefore, in a wildflower meadow, low nutrients not only suppress grass growth and prevent out-competition but they encourage the formation of complex mycorrhizal networks for nutrient distribution, carbon sequestration, and plant diversity.

As such, low nutrient concentrations in the soil can be considered the cornerstone of a wildflower and wild fungi meadow.

When mycorrhizal relationships are inhibited by artificial fertilisation, we lose the very foundation of an ecosystem. Without mycorrhizal symbiosis, there is no mycelial mesh to provide soil structure and promote aggregation. The mycorrhizal networks that form between plants, which are important not only for nutrient distribution but also transport of water and defence signals between neighbouring plants, are absent.

And, strikingly, we lose major global carbon sinks. It is estimated that 75 per cent of terrestrial carbon is stored in the soil. When mycorrhizal relationships are inhibited, carbon from plants that is naturally locked in the soil by fungi is released to the atmosphere and soil carbon content decreases. In contrast, under the low nutrient conditions typically found in nature, plants and fungi share resources for the benefit of both partners, the soil, and the atmosphere.

Further reading

A powerful and underappreciated ally in the climate crisis? Fungi

Mycorrhizal Fungi as Mediators of Soil Organic Matter Dynamics

Finding the Mother Tree by Suzanne Simard

Braiding Sweetgrass by Robin Wall Kimmerer

Entangled Life by Merlin Sheldrake

Blue Image: Strawberry Fragaria vesca roots inoculated with the arbuscular mycorrhizal fungus Rhizophagus irregularis, stained with Trypan blue, and observed under a brightfield microscope. Labelled are the fungal structures termed the ‘arbuscules’, the major sites for resource exchange between plant and fungus.