Leaf pigment composition and fluorescence signatures of top canopy leaves in species of the upper Rio Negro forests

Abstract

Vegetation of the upper Rio Negro (Amazon region, Venezuela) includes a patchy arrangement of forests of distinct structure and species related to topography, soil fertility and water table fluctuation. The region receives high and non-seasonal rainfall. In all habitats, top canopy leaves are exposed to high light levels year round which saturate photosynthesis and consequently may affect the efficiency of energy conversion. In this study, we assessed top canopy leaves of dominant species in two closed forests to evaluate: 1) if species thriving in contrasting habitats have differing anatomical leaf features and pigment composition and 2) whether species experience fluorescence signature changes at hours of maximal irradiance. Two species from a mixed forest growing in oxisol soil and two species from the Caatinga forest growing in podzol soil were analyzed. Both leaf anatomy and pigment composition of all species were typical of sun-exposed leaves. Additionally, species had comparable chlorophyll a to chlorophyll b ratios (Chl (a/b) = ~3) which were independent of habitat soil fertility, suggesting equivalent photochemical capacity across species. Fluorescence signatures declined substantially (12-34%) at midday with small changes in potential quantum efficiency of photosystem II. Thus, the photosynthetic apparatus of these species seems to have adapted to cope with high midday irradiance levels in the respective Amazonian habitats. Although, this effect may result from effective defense mechanisms, it is most likely attributable to xanthophyll cycle activation. © 2011 Academic Journals Inc.