Modeling Imbibition of Hydrogel Coated Seeds

Plant species that grow naturally in semi-arid environments have developed various survival strategies to capture and retain water. One of them is to produce mucilage: a hydrogel seed coat that absorbs soil moisture and helps to maintain hydration in case of droughts. Inspired by these mucilage producing seeds, hydrogel coatings have been developed to improve drought resistance in seeds of other plant species. Some field tests showed encouraging results, for example, increased growth by 16% in wheatgrass, 50% in cucumber, 77% in corn, and 100% in pea shrub. However, other tests showed conflicting results as the seed coatings lowered germination rates, especially with thin coatings of less than 50% seed mass. Unfortunately, the reason behind the dichotomy of these germination rates is unknown. In order to investigate the effect of a hydrogel seed coat on water transport, we designed artificial seeds made of clay and coated them with an alginate hydrogel. We then measured the associated imbibition rate, and examined how the variation of permeability in flow direction, in this spherical geometry, affects the dynamics of water moving inwards to the clay core. By combining Darcy’s law and polymer physics, we developed an analytical model capturing imbibition dynamics in natural and artificial hydrogel coated seeds.