In addition, some species of submerged angiosperms can show morphological adaptations to available light, such that they produce longer, wider and thinner leaves at low light conditions, leading to an increase in light absorption per unit biomass ( Barko et al., 1982 Bintz & Nixon, 2001). In addition, adaptations of aquatic plants to available light include shoot elongation in low light ( Barko & Smart, 1981) and chloroplast movement, such that under weak light chloroplasts move toward a brighter area in a cell, but move away from strong light ( Lechowski & Bialczyk, 1992 Kagawa & Wada, 2002) In charophytes, however, chloroplasts are stationary ( Schubert & Blindow, 2003). These mechanisms are found in charophytes ( Küster et al., 2000, 2004). Pigmentation changes and modulation of photosynthetic efficiency, by various mechanisms, are common light acclimatization mechanisms of autotrophic organisms ( Young et al., 1997 Figueroa & Gomez, 2001). In addition, interspecific differences in light acclimatization exist within macrophyte species groups, including charophytes, reflecting their depth distribution ( Madsen & Sand-Jensen, 1994 Sorrell et al., 2001 Schwarz et al., 2002 Küster et al., 2004). Aquatic macrophyte species groups (charophytes, bryophytes, caulescent angiosperms, rosette-type angiosperms and Isoetes spp.) each respond differently to light availability ( Middelboe & Markager, 1997).
Light acclimatization strategies include mechanisms for effective light harvesting at low light conditions ( Grimshaw et al., 2002) and protection against excessive light ( Rmiki et al., 1996). Adaptations that result in an optimal value of photosynthetically active radiation (PAR) being obtained from available light should therefore play a fundamental role in plant survival. Underwater light climate is highly variable because of sun position, weather conditions, water colour, turbidity, depth and wave form ( Rorslett et al., 1997 Pott & Remy, 2000 Schubert et al., 2001). The occurrence of green plants, including charophytes, depends on a suitable light climate, among other environmental parameters ( Campbell, 1997). Each node bears a whorl of branches composed of a number of cells that cease to grow after they have reached a certain length ( Krause, 1997). The Charales have a complex morphology with apical growth and differentiation of the thallus into nodes and internodes ( Piazza et al., 2005). Their axis is attached to the substratum by rhizoids. Charophytes (Charales, Charophyceae) are submerged green algae with a macroscopic thallus and an equisetum-like growth form. Submerged macrophytes are important in lake, stream and pond ecosystems throughout the world ( Wood & Imahori, 1965 Krause, 1997 Jeppesen et al., 1998) as they provide refuge and food for various animals ( Diehl & Kornijow, 1998 Proctor, 1999) and influence the physical and chemical properties of the water ( Sand-Jensen & Frost-Christensen, 1998 van den Berg et al., 1998). In addition, the upward orientation of branches might lead to increased light transmission within dense charophyte beds, thus enabling an enhanced gross production. 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. Orientation of charophyte branches towards light is accompanied by a decrease in chlorophyll a (Chl a) content and a lower Chl a : carotenoid ratio, which clearly indicates that the plant is taking protective measures against potentially damaging excess light conditions. This indicates that branch orientation is determined by a light-dependent growth reaction. Only branches that increased in length during the experiments reacted to differences in light intensity. Two Chara species were exposed to five different intensities of photosynthetically active radiation and species traits and pigmentation were measured.īranches 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. 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.
Growth of plants or plant organs towards more light is commonly interpreted as an adaptation to low light conditions.
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