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 l ight 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 Pleopelti s mexicana (Fee) Mickel & Beitel) with different adaptations against drough t were investigated. The plants were sampled dark adapted (treatment I), an d 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 obser ved in the field. While none of the treatments induced significant chloroph yll (chl) degradation, all plants showed strong de-epoxidation of the xanth ophyll cycle on light exposure (up to a de-epoxidation state of 70%), but w ithout an additional effect of desiccation. Most species showed a rapid inc rease (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-dec iduous 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 tocophero ls and carotenoids has not been reported for other plants under light stres s and is probably related to the epiphytic life form.