Phenolics and condensed tannins in relation to altitude in neotropical Pteridium spp.: A field study in the Venezuelan Andes

Two species of neotropical bracken, Pteridium caudatum (L.) Maxon (P.c.) and P. arachnoideum (Kaulf.) (P.a.), thrive dominantly in habitats extending from near the sea level to the Andean alpine zone around 3000 m altitude. The scope of sources of stress associated with elevation-has three main components: plant competition for resources, activity of herbivores/pathogens, and climate. The contribution of each component varies greatly from place to place and bracken must respond with considerable physiological/genetic plasticity to maintain its dominant status in all these habitats. Phenolic compounds have a number of properties for plant adaptation to evolutionary selective forces of biotic and climatic nature, in particular with reference to their protective role against excessive solar radiation. We have examined the second of these properties by studying the dynamics of low (LMP) and high (HMP) molecular weight phenolics in neotropical bracken ferns in two groups of field experiments: (1) their variation during the phenologic development of blades in sympatric populations of both bracken species to examine the synthesis and allocation of these materials under similar conditions of competition, predation and solar UV-B radiation; (2) the contents of LMP and HMP in six bracken populations of Re. along its typical altitudinal range [1180-2140 m above sea level (asl)] and six additional populations of P. a. (1880-3190 m as]), which grows at higher elevations in the western Andes of Venezuela, to investigate the phenolics' response to the ecological gradients associated with elevation in these plants. From series (1), we recorded the progressive accumulation of both types of phenolics with frond expansion, and hence increased surface of lamina exposed to sunlight, in Re. and P.a. However, phenolics in mature blades of P.a. were 1.6 (LMP) and 3.26 (HMP) greater in amount than in Re. From series (2), synthesis and accumulation of LMP was found to be independent of altitude, except for the subalpine population of P. a. at 3190 m asl, where a 167% increment of the average contents was recorded. By contrast, the levels of HMP correlated positively with elevation in both bracken species. The independent response of both types of phenolics to solar radiation may be interpreted as either more effective and metabolically cost-effective protection of HMP against UV-B light than LMP or differential light activation of key steps in the biosynthesis of HMP and LMP in bracken. (C) 2004 Elsevier Ltd. All rights reserved.