How does the hypocotyl contribute to the early growth stage of Zostera marina seedlings ecologically?

Seagrasses such as Zostera marina are ecologically vital, providing essential habitats and ecosystem services that are increasingly threatened by global decline. A comprehensive understanding of seedling development, particularly the functional role of hypocotyls, is critical for effective seagrass restoration. This study investigates the ecological functions of hypocotyls during early Zostera marina seedling growth, with a focus on nutrient provisioning and the impact of hypocotyl removal on seedling performance. Hypocotyls play a critical role in early-stage nutrient allocation and water uptake-processes that are fundamental to successful seedling establishment. The developmental phase-defined as the interval from cotyledon emergence (1 cm in length) to emergence of the first true leaf (1 cm in length), followed by a fixed 20-day post-emergence growth period-is characterized by intense, developmentally regulated nutrient mobilization. During this phase, the daily degradation rates of total sugar and starch in the hypocotyl are 2.41% and 0.08%, respectively. A marked decrease in protein, total sugar, and starch content within the hypocotyls is observed as seedlings progress through early development, indicating active utilization of these reserves to support growth. Early removal of the hypocotyl leads to significantly reduced survival and impaired physiological condition, whereas removal after 27 or 37 days of growth has negligible effects. In these later treatments, seedling survival exceeded 50%, with no statistically significant difference from control groups, and survival trends approached stabilization. These findings underscore the essential contribution of the hypocotyl to early seedling viability and provide empirical evidence for optimizing the timing of seedling handling and transplantation in restoration initiatives, thereby enhancing the effectiveness of conservation strategies for these critical marine ecosystems.