Anchorage by seed mucilage prevents seed dislodgement in high surface flow: a mechanistic investigation

Structure, mechanical and adhesive properties of the cellulosic mucilage in Ocimum basilicum seeds

Plant seeds and fruits, like those of Ocimum basilicum, develop a mucilaginous envelope rich in pectins and cellulosic fibers upon hydration. This envelope promotes adhesion for attachment to soils and other substrates for dispersal and protection of the seed for a safe germination. Initially at hydration, the mucilage envelope demonstrates low adhesion and friction, but shows increasing adhesive and frictional properties during dehydration. However, the mechanisms underlying the cellulose fiber arrangement and the mechanical properties, especially the elasticity modulus of the mucilage envelope at different hydration conditions are not fully known. In this study, which is based on scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and light microscopy, the structure of the seed coat and arrangement of the cellulose fibers of basil seeds were characterized. Moreover, we performed pull-off force measurements to estimate adhesive properties and JKR-tests to estimate E-modulus of the mucilage at different hydration levels. Microscopy results demonstrate that cellulose fibers are split at their free ends into smaller fibrils, which might enhance the adhesive properties of the mucilage. Adhesive forces in contact increased during dehydration and reached maximum of 33 mN shortly before complete dehydration. The E-modulus of the mucilage changed from 1.4 KPa in water to up to 2.1 MPa in the mucilage at the maximum of its adhesion performance. Obtained results showed hydrogel-like mechanical properties during dehydration and cellulose fiber structures similar to the nanofibrous systems in other organisms with strong adhesive properties. STATEMENT OF SIGNIFICANCE: This paper reveals the hierarchical cellulose fiber structure in Ocimum basilicum's mucilaginous seed coat, suggesting increased fiber splitting towards the end, potentially enhancing adhesion contact areas. Mechanical tests explore elasticity modulus and adhesion force during various hydration stages, crucial as these properties evolve with mucilage desiccation. A rare focus on mucilaginous seed coat mechanical properties, particularly cellulose-reinforced fibers, provides insight into the hydrogel-like mucilage of plant seeds. Adhesion forces peak just before complete desiccation and then decline rapidly. As mucilage water content decreases, the E-modulus rises, displaying hydrogel-like properties during early dehydration stages with higher water content. This study might bring the focus to plant seeds as inspiration for biodegradable glues and applications for hydrogel research.

Seed mucilage as a defense against granivory is influenced by substrate characters

Many seeds are consumed by granivores despite numerous adaptations to prevent detection or exploitation. The environment can influence the efficacy of these defensive traits. Understanding the mechanisms by which environmental factors modify defensive efficacy is important for understanding spatial patterns of granivory and seed recruitment. Seed mucilage is a sticky coating that binds imbibed seeds to substrates; this attachment has been demonstrated to lessen exploitation by granivores. Seed mucilage as a defense has been recognized for decades, though rarely studied. Here, we investigated whether the environment alters this seed defense by addressing two questions: (1) Does substrate particle size affect attachment strength? (2) Does a change in particle size lead to changes in granivore-related mortality? In the field experiment, ants removed more seeds from finer substrates than their coarser counterparts. Across that same grit range, seeds took less force to dislodge when mucilage-bound to fine sandpaper; however, an investigation across a wider range of grits demonstrated nonlinearities occurred for many species, probably due to structural and chemical mucilage properties. Small differences in substrate grit lead to differential mortality in mucilaginous seeds due to alterations in attachment strength, suggesting that the defensive efficacy of this trait differs across the landscape. This work paves the way for a more integrative look at mucilaginous seeds. Seed mucilage is a widespread trait that is easily studied and has important demographic implications. It represents an ideal system to examine dispersal, germination, and granivory to gain a more holistic view of seed ecology.

Seed Mucilage Promotes Dispersal of Plantago asiatica Seeds by Facilitating Attachment to Shoes

Understanding the mechanisms underlying seed dispersal is a fundamental issue in plant ecology and vegetation management. Several species demonstrate myxospermy, a phenomenon where the seeds form mucilage after absorbing water. Mucilage is thought to act as a glue, enabling seeds to attach to the external surfaces of dispersing agents. However, there have been no quantitative investigations of the efficacy of this function of seed mucilage. We performed a trampling and walking experiment to investigate the seed dispersal of a perennial herb, Asian plantain (Plantago asiatica L.), which forms polysaccharide mucilage upon hydration. Our experiment showed that: (1) after trampling, more seeds of P. asiatica attached to shoes in wet conditions (after rainfall), in which seed mucilage was created, than in dry conditions (no rainfall); and (2) after walking for 1000 m, more seeds remained attached to shoes in wet conditions than in dry conditions. Our results indicate that mucilage promotes the adherence of seeds to the surface of vectors. We therefore provide the first empirical evidence that seed mucilage facilitates epizoochory and human-mediated dispersal.