Seagrasses with red leaves have been observed growing in areas with high light intensities at numerous locations around the world. To test whether variations in light conditions affect anthocyanin accumulation and red coloration in leaves, we performed reciprocal transplants of green-
and red-leafed Thalassia testudinum banks & sol. ex K.D. Koenig shoots among patches with high and low self-shading (determined from canopy height and leaf area index measurements) located along a depth related gradient of light availability in the lower florida Keys, USA, and monitored
them for 3 yrs. All red-leafed shoots continuously produced red leaves with high concentrations of anthocyanins regardless of self-shading or depth. Transplanted green-leafed shoots transiently turned red during periods of high solar UV and visible light intensity, with the reddening process
influenced by self-shading and depth. Green-leafed shoots also increased UV absorbing compounds, but increases were not related to anthocyanin content or reddening. There was a significant relationship between UV-B absorbing compounds and anthocyanin content in red-leafed shoots. We conclude
that anthocyanin accumulation and the expression of red coloration can be permanent or temporarily photo-induced in T. testudinum leaves. We also suggest that this species produces a genetic variant with permanently red leaves in the lower florida Keys.
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