UV-B induces leaf reddening and supports photosynthesis in the seagrass Thalassia testudinum

Abstract

Numerous seagrass species growing in intertidal and shallow subtidal areas around the world produce red leaves, but the factors responsible for the induction of leaf reddening in seagrasses are poorly understood. We investigated the responses of transplanted green-leafed and in situ red-leafed Thalassia testudinum shoots growing in high light areas in the lower Florida Keys, USA, to four light treatments: 1) full solar radiation with UV excluded (PAR); 2) full solar radiation with UV–B excluded (PAR + UV–A); 3) full solar radiation reduced by 50% (50% Ambient); and 4) full solar radiation (Ambient). In our first experiment, green-leafed shoots were transplanted from a 1 m depth (MLW) to the four light treatments in 0.2 m depth (MLW). In our second experiment, in situ red-leafed shoots growing at depths between 0.2 m and 0.5 m were exposed to the four light treatments. Within one week, new leaf tissue from green-leafed shoots transplanted into shallow water accumulated anthocyanins and began to turn red in treatments receiving full spectrum solar radiation (Ambient; 50% Ambient) while transplanted green-leafed shoots in the two treatments that excluded UV–B (PAR and PAR + UV–A) had low anthocyanin content and remained green. Although we quickly induced red coloration in leaves of green-leafed shoots, reducing light levels (including UV–B) for seven weeks did not cause leaves of in situ red leafed shoots to decrease anthocyanin content or turn green. Instead, red leaves increased photosynthetic pigments in all treatments except Ambient. In addition, the PAR+UV -A treatment had lower effective quantum yields at midday compared to the PAR, 50% Ambient, and Ambient treatments, as well as lower relative electron transport rates compared to the PAR and Ambient treatments. We conclude that exposure to UV–B induces anthocyanin accumulation and red coloration in green-leafed shoots and contributes to the maintenance of high levels of photosynthesis in red-leafed shoots of T. testudinum. We also propose that T. testudinum in the clear, shallow waters of the lower Florida Keys produces a red-leafed variant, a genetically differentiated form of this species, with permanently red leaves since anthocyanin accumulation and red coloration in leaves of red-leafed shoots was not reversible in this and a longer-term study.