In memory of C. S. Pittendrigh: Does exposure in forest canopies relate to photoprotective strategies in epiphytic bromeliads?

1. Photoprotective strategies were investigated in 10 epiphytic bromeliad species in Trinidad with contrasting preferences for the light environment, according to exposure groups categorized by C.S. Pittendrigh in 1948 (viz. Exposure, Sun and Shade-Tolerant). 2, Chlorophyll alb ratios were relatively consistent under both high and low light, despite large differences in total chlorophyll content. 3. Diurnal patterns of photosynthesis were examined in three C-3 species. Carboxylation limitation was ubiquitous and despite some evidence for the operation of alternative electron sinks, utilization of photon energy was minimal. 4. Irrespective of exposure group and photosynthetic pathway (C-3 or CAM), all species exhibited a very high capacity for thermal dissipation of excess excitation. 5. Non-photochemical quenching of chlorophyll fluorescence in leaves of Exposure and Sun Group species was largely rapidly relaxing, indicative of a photoprotective down-regulation of photosynthesis. In contrast more slowly relaxing quenching (qI) was the major component in the Shade-Tolerant Group, suggesting a greater probability of photodamage for species within this group. 6, C.S. Pittendrigh supported the view that many bromeliads low in the canopy were shade-tolerant, suggesting that the epiphytic ancestor originated in exposed, open regions. Our fluorescence quenching data support this assertion.