Influence of seed size on Posidonia oceanica seedling vigor and resilience to warming: implications for seagrass restoration

Abstract Introduction Seed size (i.e. dimensions and mass) is an important trait for plant evolution and improvement and could have important implications for seedling establishment and the selection of transplant material in restoration programs. Objective This study aims to assess the role of seeds size in shaping early seedlings viability and performance, in the seagrass Posidonia oceanica , to potentially optimize strategies for future seedling restoration activities. Methods Beach‐cast seeds were classified into two size classes (small and large) to evaluate: (1) seed viability and resource content (starch and soluble sugars), (2) differences in morphological and fitness‐related traits between large‐seeded and small‐seeded seedlings grown in mesocosms under natural summer temperature conditions over 6 months, and (3) seedling performance following a 6‐week experimental heat stress and a subsequent 2‐week recovery period. Results Seed viability did not differ between the two size classes, despite higher reserves in larger seeds. Under natural temperature conditions, large seeds produced significantly larger seedlings within a shorter time frame compared to small seeds. However, exposure to heat stress did not highlight any immediate seed‐size‐related advantage in seedling performance. Conclusions Under favorable conditions, seed size does not affect the germination and survival during the first 4 weeks, however, larger seeds conferred clear advantages in terms growth, resulting in larger and more vigorous seedlings. Given the long lifespan of P. oceanica , the absence of immediate stress‐related benefits does not preclude longer‐term advantages.