PARENT SESSION
Posters P3B Photosynthetic acclimation: Ecophysiology, diverse environments. Abstracts (479-522)

Do red carotenoids protect photosynthetic tissues by a green-light filtering mechanism? Koldobika Hormaetxe¹, Isabel Fleck², Marta Pinto², Jose Becerril¹, Jose Garcia-Plazaola^*,1, ¹ Dpt. Biologia Vegetal y Ecologia, Bilbao, Spain² Dpt. Biologia Vegetal., Barcelona, Spain

ABSTRACT- There is an evolutionary convergence to accumulate red pigments (mainly anthocyanins, but also retro-carotenoids in some species) in the outer layers of photosynthetic tissues under conditions of excess excitation energy. Such accumulation under photoinhibitory conditions, suggests a photoprotective role for these pigments. Two main hypothesis, not mutually excluding, have been proposed to explain the function of leaf reddening: antioxidative protection and green light filtering. Buxus sempervirens is a perennial shrub species with a wide ecological tolerance and also shows a large phenotypic variability so plants with different colour patterns occur simultaneously in the same location. This is due to the fact that chloroplasts undergo in winter a reversible transformation into chromoplasts in parallel with the accumulation of red carotenoids (eschscholtzxantin and anhydroeschscholtzxantin). B. sempervirens leaves can be easily split in two parts (adaxial and abaxial) because of the weak connection between the mesophyll layers. This trait have allowed us to substitute in shade green leaves an upper leaf disc (6 mm diameter) by the equivalent upper mesophyll disc of a different sun phenotype. Leaves constructed with this design were exposed to different photoinhibitory conditions in order to compare the protective effect exerted by five different phenotypes (green, red, brown, orange and yellow) in the lower mesophyll of a shade leaf. Our results indicate that the best protection is achieved in all experiments by an upper green layer, and the extent of Fv/Fm decrease is proportional to the amount of red light transmitted by the upper mesophyll. Contrasting with several reports showing that plants accumulating red anthocyanins are better protected against photoinhibition as compared with green leaves, the present results suggest a minor role for red carotenoids in light scattering as compared to chlorophylls. Other hypothesis as protection of chlorophyll biosynthetic metabolites from light or the antioxidant capacity of these carotenoids are also discussed.

KEY WORDS: Photoprotection, red carotenoids, Buxus, Chlorophyll