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Xanthophyll cycle-related photoprotective mechanism in the Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa under normal and stressful hypersaline conditions

Marín-Guirao, L., Ruiz, J.M., Sandoval-Gil, J.M., Bernardeau-Esteller, J., Stinco, C.M., Meléndez-Martínez, A.
Aquatic botany 2013 v.109 pp. 14-24
energy, leaves, light intensity, lutein, meadows, photochemistry, photoinhibition, photosystem II, salinity, seagrasses
In the present study, we investigated the capacity for non-radiative energy dissipation operated by the xanthophyll cycle pigments of the Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa, under normal and increased salinity conditions. After being chronically stressed by hypersalinity during several months in a mesocosm system, the efficiency of PSII photochemistry and non-photochemical quenching (NPQ) kinetics and the leaf content in pigments (including xanthophylls) were analysed in unstressed and stressed plants under two approximations: (i) along a daily cycle in the mesocosm under moderate light levels; and (ii) exposing leaf segments to direct high light and allowed latterly to recover under low light conditions. C. nodosa leaves exhibited not only a greater pool of xanthophyll cycle compounds but also a higher xanthophyll de-epoxidation state compared to P. oceanica leaves, which is indicative of a greater capacity for thermal energy dissipation. These differences in the photoprotective capacity of both seagrasses reflect the different light climates that naturally exist inside both meadows due to the stronger self-shading produced by P. oceanica canopies. Under direct high light, both species enhanced photoprotective down-regulation of photosystem II (PSII), and although they both finally exhibited signs of photodamage, this was mainly in P. oceanica, which also showed pigment degradation due to sudden exposure to harmful irradiances. Chronic hypersaline stress did not alter the functioning of the xanthophyll-cycle-photoprotection of both species; nonetheless in combination with high light, it predisposes P. oceanica leaves to photoinhibition, most likely through a reduction in the capacity for reparation damaged PSII reaction centres.