Understanding seed-soil adhesion by myxospermous seeds and their biophysical function in agroecosystems

  • Ashley Gorman

    Student thesis: Doctoral ThesisDoctor of Philosophy

    Abstract

    Seedbanks provide many ecosystem services that support wildlife and regulate soil function. The fate of arable seeds is vital in shaping the aboveground composition of plant communities in agroecosystems. Understanding seed fate informs seedbank dynamics with multi-species interactions, as well as safeguarding arable biodiversity and food security. While many seed fate pathways are well known, the role of myxospermy in temperate agricultural soils is rarely addressed. Mucilage released by myxospermous seeds can modify the soil physical, hydraulic and microbial environment surrounding the seed. The principle biophysical mechanisms underlying the redistribution of weed seeds in eroded soils are unknown. It is possible that myxospermy delivers crucial services at the field scale, such as soil stability and water retention. However, modifications of the soil physical environment have been investigated using only a single-species approach and are often tested with extracted mucilage rather than seeds in-situ. As multiple taxa produce myxospermous seeds, the effects of a single species cannot be generalised. As seeds co-exist in the soil as multi-species communities, a multi-species approach is needed to further understand the functionality of seed mucilage and their implications for soil, relative to the wider seedbank. This thesis focussed on examining the mechanisms responsible for the adhesive interactions between soil and myxospermous seeds and discusses the ecological function and behaviour at the soil particle scale.

    For the first time, the structure and composition of seeds from seven myxospermous species was characterised: Arabidopsis thaliana, Capsella bursa-pastoris, Plantago lanceolata, Senecio vulgaris, Urtica urens, Veronica arvensis and Viola arvensis. Soluble and insoluble mucilage layers were visually distinct between species, with structural diversity owing to the presence of epidermal mucilage secreting cells. Previously undescribed features of mucilage secreting cells were found (e.g. elongated columellae and oval-shaped cells), raising questions over the functionality of columellae structures for mucilage release and soil adhesion mechanisms.

    Species-specific mechanisms by which mucilage adheres soil particles to seeds, and to other particles, was investigated using glass beads as a proxy for soil. Larger seeded-species with more mucilage, such as Plantago lanceolata and Urtica Urens, adhered more beads than smaller-seeded species of Arabidopsis thaliana and Capsella bursa-pastoris. The combined effect of soluble and insoluble mucilage caused a greater adherence of beads than insoluble mucilage alone. Structural characteristics of seed mucilage and seed morphology caused different adherence mechanisms between species.

    No prior study has provided evidence of seed mucilage binding soil particles to form water-stable aggregates with seeds in-situ. When testing seed amended soil aggregates, seed mucilage increased stability in water by 29-44% when compared with control aggregates without seeds. Scanning electron micrographs revealed networks of interparticle mucilage bonds, facilitating strengthening functions for aggregate stability. These results demonstrate seed mucilage can provide a biological binding agent for aggregate formation with species-specific mechanisms of seed-soil adhesion in naturally occurring soil seedbank communities. The ability to form and stabilise aggregates suggests seed mucilage may help to decrease soil detachment at the soil surface during intense runoff erosion.

    The seedbank of a long-term tillage trial was surveyed over two years. The characterisation of seed mucilage and mucilage secreting cell structures was combined with plant traits to form 22 functional group classifications. Tillage (No-Till, conventional and deep plough) and crop type (winter and spring barley) treatments shaped the presence, composition and abundance of functional groups. These results are biologically meaningful as the presence of functional groups (e.g. containing myxospermous species) within fields can alter local soil biogeochemistry and regional-landscape scale biodiversity. Where myxospermous species are abundant aboveground, localised seed rain onto the soil surface would provide a positive stabilising effect, decreasing soil erosion in tramlines. However, ploughing buries seeds in the soil profile and the effect of mucilage would be diluted. Thus, long-term functional effects of mucilage on soil properties is underpinned by the spatial distribution of specific functional groups within the seedbank community.
    Date of Award2019
    Original languageEnglish
    SupervisorJohn Rowan (Supervisor), Blair McKenzie (Supervisor) & Cathy Hawes (Supervisor)

    Keywords

    • Soil
    • Myxospermy
    • Seeds
    • Mucilage
    • Soil Erosion

    Cite this

    '