Poster presentation: Biomechanics of Posidonia oceanica: relation with leaf ageing and effect of collection procedure

Seagrasses are specialised unique marine plants formed by shoot bundles holding several leaves. By the leaf growth pattern, the older leaves on the outside of the shoot bundle protect inner younger leaves against biotic and abiotic factors, such as herbivory or mechanical loads. We hypothesised this may be reflected in both the biomechanical and nutritional properties of the leaves, with the outer ones being stronger and less nutritious than the inner ones. Hence, we studied how morpho-mechanical and nutritional traits vary within a seagrass shoot of Posidonia oceanica (collected in Bay of Calvi, Corsica, France). Moreover, we also investigated how the collection procedure (classical: uprooted shoot vs. Non-Destructive Shoot Method: cutting the shoot without a portion of rhizome) and time span between sampling and testing event (24 vs. 48 h) may influence those traits. Results showed that sampling with a Non Destructive Shoot Method (measuring both within 24 – 48 h) did not compromising the research objective of measuring mechanical leaf traits. A clear intra-shoot pattern was obtained, with younger leaves having i) higher nitrogen content, ii) lower C:N ratio, and iii) lower fiber content. Leaf-mechanical properties also showed a clear intra-shoot pattern, with the youngest and oldest leaves having the lowest breaking forces, and intermediate leaves displaying the highest values. Breaking stress (force per unit of cross-section area) and modulus of elasticity (a measure of deformation in tension) negatively correlated with age, that is, old leaves resisted less for per unit of area and were more elastic than inner younger leaves. The combined intra-shoot variability in mechanical and nutritional properties may well explain herbivory patterns in Posidonia oceanicaleaves.