As with all higher plants, seagrasses have diurnal patterns of photosynthetic activity, which vary according to the daily cycles of irradiance, as well as the daily tidal cycle. Seagrasses have the ability to tolerate high light during solar noon by dynamically down-regulating their photosynthetic apparatus (Ralph et al., 1998). This process, whereby excess irradiance is dissipated as heat without being absorbed into the photosystems and without causing damage, is largely entrained by the xantho-phyll cycle (see below). In the absence of down-regulation, seagrasses can suffer photoinhibition by excess irradiance. Photoinhibition is the caused largely by irreparable damage to D1 protein of PSII
necessitating the replacement of the damaged D1 protein. An essential aspect of understanding the dynamic nature of diurnal rhythms is the ability of PSII to recover during the afternoon or overnight, i.e. a recovery from photoinhibition over a period of hours in the shade or overnight.
Seagrasses growing near the intertidal region will be exposed to high light stress, which can result in either down-regulation or, if irradiance is sufficiently high, can result in photoinhibition. Photoinhibition can be defined as damage to the PSII reaction centres, mainly to D1 protein, requiring several hours of low light (or overnight) for repair. Down-regulation is associated with increased NPQ. Photoinhibition is generally associated with reduced Fv/Fm and with increase in Fo.
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