The rheological properties of the seed coat mucilage of Capsella bursa-pastoris L. Medik. (shepherd's purse)

Abstract

The outer surface of myxospermous seed coats contains mucilage which absorbs large amounts of water relative to its dry weight. Ecologically, the seed mucilage can affect seed germination and dormancy. Upon hydration, a large proportion of the seed mucilage is lost to the soil and the physics of soil-seed mucilage interactions has not been assessed. Towards that end, the dynamic rheological properties of mucilage extracted from Capsella bursa-pastoris L. Medik. (shepherd's purse) seeds were 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 seed mucilage was shear thinning and was classified as a highly viscous “weak gel”. The relationship between the viscoelastic parameters (viscosity, η^*, storage and loss modulus, G′ and G″, yield and flow stresses, τ_y and τ_f) and mucilage concentration were well fitted by power law models. The values of η^*, G′ and G″ increased as temperature increased above 40°C and were also slightly frequency dependent. The shepherd's purse seed mucilage is more viscous than that from other plant parts, such as fruits and roots. These properties highlight the possibility that seed mucilage may affect soil conditions and therefore present an additional facilitative ecological role (beyond that already reported, which directly affect seed biology); and this is discussed.