Experimental and numerical analysis of behaviour of the myxospermous seeds of Capsella bursa-pastoris (L.) Medik. (shepherd’s purse) in soil

  • Wenni Deng

    Student thesis: Doctoral ThesisDoctor of Philosophy


    Myxospermy is a term that describes the ability of a seed (or seed pod) to produce mucilage. The myxospermous trait is prevalent among those plant species adapted to surviving in arid sandy soils. The seed mucilage is composed of polysaccharides, which are capable of absorbing large volumes of water and it has been shown to be of significant ecological importance in terms of seed dormancy, germination and dispersion. However, the effects of the mucilage on the physical properties of soil have not been examined. In this thesis, the Capsella bursa (L.) Medik. (shepherd's purse) is chosen as a model plant species to be studied. The shepherd’s purse seed mucilage consists two distinct layers. The outer layer is composed of only pectin and is readily dispersed/extracted in water. The inner layer is comprised of cellulose fibres which are anchored to the seed coat and these are embedded in pectinaceous matrix. The aim of this study is to discern the physical properties of the seed mucilage especially in relation to its expansion in response to hydration, and to develop a mathematical model that simulates the seed mucilage expansion, and understand the rheology of the mucilage. Furthermore, this insight is extrapolated to understand the role of the extractable component of the seed mucilage (only) and the whole seeds to affect the soil and its potential application as soil conditioner to change soil stability and soil water related properties.Firstly, the structures underpinning the mucilage swelling process was described using light, electron and time-lapse confocal micrographs. The geometry size, weight of seed and mucilage were measured from images. The amount of mucilage which was present in the inner and out layer was discerned. The mucilage swelling was assessed at different osmotic pressure environment created by Polyethylene glycol (PEG) solution. The experimental data were then used to create a mathematical model of myxospermous seed mucilage swelling based on diffusion equations. The swelling time of mucilage at different PEG solution was simulated and the influence of mucilage height was assessed. The results showed that mucilage accounted for 25.2 % of seed weight and took 16 x water of its weight. The mucilage had a low osmotic potential property which helped it compete water from environment. The swelling degree was largely reduced as PEG solution concentration increased and the time required to reach equilibrium state was reduced too. Secondly, the dynamic rheology of extractable seed mucilage was assessed as a function of mucilage concentration (1–10 % [w/w]), temperature (0–80 °C) and shear frequency (0.1–100 rad s-1). The relationship between the viscoelastic parameters (?, G', G", ty and tf) and mucilage concentration were well fitted by power law models. The dynamic rheology results showed that mucilage was a high viscous material and was able to be classified as a ‘weak gel’ material. The seed mucilage exhibited relative higher viscosity compared with mucilage from other plant parts which indicated the special role of seed mucilage. These properties highlight the possibility that myxospermous seed mucilage may affect soil conditions and therefore present an additional facilitative ecological role Thirdly, a comparative analysis of soil water retention, hydraulic conductivity and rheology using clay and sandy-loam soil containing a range of seed mucilage, or myxospermous seed (MS) or demucilaged seeds (DS) was carried out. The applied densities for mucilage were 0.5 and 1 % [w/w] and for seeds (MS or DS) were 5 % and 10 % [w/w]. The water retention curve of soil was fitted by van Genuchten model and their fitted parameters were used in numerical modelling later. The mucilage or seeds had greater influence on sandy-loam soil compared with clay type soil, both the water retention and the rheology properties. It was also found that seeds, whether myxospermous or not, increase sandy-loam soil water retention and rheology properties while reducing the soil hydraulic conductivity. The myxospermous seeds had higher impact on soil water retention than demucilaged soil and the effect was more significant in drier soil. At last, a numerical model was developed to simulate the flow of water in seed conditioned soil. The model was developed on the basis of the experimental data and findings of the earlier chapters. The mucilage swelling properties was considered as a blocking effect and modelled by reducing the hydraulic conductivity. The numerical geometry was the same as the soil column used in the water retention tests. The water flow was modelled by Darcy’s law combined with continuum equation. The two dimensional symmetry model was established and was solved using COMSOL Multiphysics. The influence of both MS and DS were simulated at different seed density (Cs), different amended soil layer thickness (hs) and different position from the soil surface (ds). The velocity of water flow and the relative permeability were compared at different cases. Cs and h had a great influence on water flow velocity in soil while the influence of ds was not significant. The simulation showed that 2 – 4 % [w/w] at the surface layer of soil may have the most significant influence on water flow in soil.
    Date of Award2012
    Original languageEnglish
    SupervisorDong-Sheng Jeng (Supervisor) & Pietro Iannetta (Supervisor)

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