Physics of Water Imbibition in 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 maintain hydration during drought. Inspired by these mucilage-producing seeds, hydrogel coatings have been developed to improve drought resistance in other seed species. However, farm tests showed conflicting results, as the seed coatings sometimes reduced germination rates. Unfortunately, the reason behind the dichotomy of these germination rates is unknown. 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 found that the hydrogel coating did not delay the imbibition of the clay core, and the Lucas Washburn theory could capture the flow dynamics, suggesting the porous core is the driving mechanism for imbibition. We then confronted our model with experiments conducted on natural soy seeds and observed that the coated seeds had a lower yield with many decaying, indicating that the environment surrounding the seed was overly saturated with moisture. Together, our results clarify the reasons behind the conflicting yields reported when coating seeds with hydrogel.