Growth towards light as an adaptation to high light conditions in Chara branches

Growth of plants or plant organs towards more light is commonly interpreted as an adaptation to low light conditions. Here, we show for the first time, in a study of charophyte branches, a growth-based orientation towards light functioning as a mechanism to protect the plant from excessive light. Two Chara species were exposed to five different intensities of photosynthetically active radiation and species traits and pigmentation were measured. Branches of plants exposed to higher light intensities were convergent and pointed steeply upwards, whereas those exposed to lower light intensities grew nearly straight and were less inclined. Only branches that increased in length during the experiments reacted to differences in light intensity. This indicates that branch orientation is determined by a light-dependent growth reaction. Orientation of charophyte branches towards light is accompanied by a decrease in chlorophyll a (Chla) content and a lower Chla : carotenoid ratio, which clearly indicates that the plant is taking protective measures against potentially damaging excess light conditions. We suggest that the growth-based orientation of Chara branches towards light may protect sexual organs, which grow on adaxial branch sides, from light damage. In addition, the upward orientation of branches might lead to increased light transmission within dense charophyte beds, thus enabling an enhanced gross production.