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 andlight intensity. The role of carotenoids and tocopherols in photoprotection ofseven 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., andPleopeltis mexicana (Fée) Mickel & Beitel)with different adaptations against drought were investigated. The plants weresampled dark adapted (treatment I), and after light exposure with thesubstrate 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 resultin permanent damage and was also observed in the field. While none of thetreatments induced significant chlorophyll (chl) degradation, all plantsshowed strong de-epoxidation of the xanthophyll cycle on light exposure (up toa de-epoxidation state of 70%), but without an additional effect ofdesiccation. Most species showed a rapid increase (within hours) ofcarotenoids (mainly β-carotene and lutein) and α-tocopherol onexposure. In A. cuspidatum, a species with no apparentadaptations to drought, drought stress in combination with light resulted inan increase of tocopherols from 35 nmol mol–1 chl(treatment I) to 400 nmol (treatment IV). This effect was not significant inthe drought-deciduous species with succulent rhizomes,Po. puberulum (about 10 nmolmol–1 chl) and Ph. areolatum(5 to maximum 40 nmol), which experience little desiccation under fieldconditions. This short-term induction of tocopherols and carotenoids has notbeen reported for other plants under light stress and is probably related tothe epiphytic life form.