Seasonal changes in photosynthesis for the epiphytic bromeliad Tillandsia brachycaulos in a tropical dry deciduous forest

Abstract

Background: Sunlight stress and drought affect plants by inducing various biochemical and physiological responses, which reduce growth. Seasonal changes in light and water availability that occur in forest canopies, where epiphytes occur, are extreme.
 Questions: What are the seasonal changes in photosynthesis for an abundant epiphytic bromeliad in contrasting microenvironments? Is Crassulacean acid metabolism (CAM) an important feature of photoprotection for this epiphyte?
 Studied species: Tillandsia brachycaulos Schltdl. (Bromeliaceae)
 Study site and dates: Canopy of the tropical dry deciduous forest of Dzibilchaltún National Park, Yucatan, Mexico during the rainy season 2008 and dry season 2009.
 Methods: Diurnal measurements of photosystem II efficiency, titratable acidity, leaf water potential, and photosynthetic pigment concentration were measured during the dry and rainy seasons in adult plants of T. brachycaulos in shaded and exposed microenvironments. The prevailing environmental conditions (photon flux density, precipitation, air temperature and relative humidity) were also seasonally characterized.
 Results: The highest irradiance occurred during the dry season caused photo-inactivation, a decrease of the quantum efficiency of photosystem II and a reduction in CAM activity of about 40 % in leaves of exposed plants of T. brachycaulos. During the rainy season, the leaf water potential of exposed and shaded plants of T. brachycaulos was lower at midday than at predawn, indicating water loss during the day.
 Conclusions: Individuals of T. brachycaulos reduced CAM activity during the dry season; and, during the rainy season, increased carbon gain by stomata opening during phase II and IV of CAM.