The significance of carotenoids and tocopherols in photoprotection of seven epiphytic fern species of a Mexican cloud forest

Epiphytes experience frequent and rapid changes in water availability and light intensity. The role of carotenoids and tocopherols in photoprotection of seven fern species (Asplenium cuspidatum Lam., Phlebodium areolatum (HB ex Willd.) Smith, Polypodium puberulum Schl. & Cham., Po. plebeium Schl. & Cham., Elaphoglossum glaucum Moode, E. petiolatum (Sw.) Urb., and Pleopeltis mexicana (Fee) Mickel & Beitel) with different adaptations against drought were investigated. The plants were sampled dark adapted (treatment I), and after light exposure with the substrate present (treatment II), with the substrate removed (treatment III) and with substrate and rhizomes removed (treatment IV) to induce increasing degrees of drought stress. The degree of dehydration reached does not result in permanent damage and was also observed in the field. While none of the treatments induced significant chlorophyll (chl) degradation, all plants showed strong de-epoxidation of the xanthophyll cycle on light exposure (up to a de-epoxidation state of 70%), but without an additional effect of desiccation. Most species showed a rapid increase (within hours) of carotenoids (mainly beta -carotene and lutein) and alpha -tocopherol on exposure. In A. cuspidatum, a species with no apparent adaptations to drought, drought stress in combination with light resulted in an increase of tocopherols from 35 nmol mu mol(-1) chl (treatment I) to 400 nmol (treatment IV). This effect was not significant in the drought-deciduous species with succulent rhizomes, Po. puberulum (about 10 nmol mu mol(-1) chl) and Ph. areolatum (5 to maximum 40 nmol), which experience little desiccation under field conditions. This short-term induction of tocopherols and carotenoids has not been reported for other plants under light stress and is probably related to the epiphytic life form.