Leaf and canopy scale characterization of the photoprotective response to high-light stress of the seagrass Thalassia testudinum

The survival of tropical seagrasses in shallow and highly illuminated reef environments relies on the leaf photoacclimatory high-light response. In this study, we characterized this response for the tropical seagrass Thalassia testudinum and analysed the role of seagrass canopy in the regulation of leaf photoacclimation. We determined maximum capacity for non-photochemical quenching (NPQ(max)); xanthophyll cycle pool size and its de-epoxidation state; and the support of photosystem II (PSII) repair to maintain PSII photochemical efficiency, and compared their variation along the leaf against changes in leaf pigmentation, absorptance, and PSII maximum photochemical efficiency (F-v:F-m). Significant trends were documented: NPQ(max) increases toward the tip whereas pigmentation, absorptance, F-v:F-m and the support of PSII-repair decline. Such heterogeneous leaf condition allows minimizing photodamage and leaf maintenance costs in the upper part of the canopy while maintaining high productivity at the lowest-shaded canopy levels. NPQ(max) for laboratory and field determinations were as high (4-5) as values reported for sun-acclimated land leaves and supported by similar mechanisms. According to this, the high NPQ(max) capacity of T. testudinum leaves and the ability of canopy self-shading to reduce light stress can explain the success of this climax seagrass in the reef environment along the Caribbean. This large habitat-builder may have acquired key ecological and evolutionary advantages through its capacity to regulate canopy light field and leaf ability to respond to highly heterogeneous illumination. Our study provides a physiological basis to support the important role of seagrass canopy for large species in photoprotection and plant productivity optimization.