Variation in pigment content of Thalassia testudinum seedlings in response to changes in salinity and light

Changes in pigment content allow seagrasses to photo-acclimate to dynamic light environments, but lightand salinity-induced stress may reduce this ability. In this study, Thalassia testudinum seedlings were exposed to two light treatments (50 – 70% reduction and full sun) and three salinity treatments (20, 35, and 50) in a 44-day mesocosm experiment. Variation in leaf pigment content (chlorophylls and carotenoids) was determined three times a day (0900, 1200, and 1800 h) during pretreatment, at treatment target conditions, and following a recovery period in the pretreatment conditions using reverse-phase high-performance liquid chromato graphy (HPLC). Seedlings in the salinity 50 treatment had reduced chlorophyll and carotenoid content after 6 days at their target salinity, a common stress response in plants. High salinity also induced increased nonphotochemical quenching (NPQ) during midday. The shaded seedlings had significantly lower chlorophyll a : b ratios, due largely to the increased chlorophyll b content. Epoxidation state (EPS) and NPQ significantly varied during the day among all the treatments, suggesting that the seedlings responded quickly to the diurnal fluctuations in light. No significant residual differences among the treatment seedlings were detected for chlorophylls or carotenoids from leaves sampled at the conclusion of the recovery period. The results indicate that hypersalinity caused increased sensitivity to high irradiances in T. testudinum seedlings, requiring an increased photoprotection.