Habitat segregation of C₃ and CAM Nidularium (Bromeliaceae) in response to different light regimes in the understory of a swamp forest in southeastern Brazil

The facultative epiphytic bromeliads Nidularium procerum and N. innocentii grow terrestrially in distinct but neighbouring patches of a swamp forest understory in southeastern Brazil. N. innocentii (C₃) is restricted to a shaded, periodically flooded patch, and N. procerum (CAM) to a semi-exposed, permanently flooded patch. Semi-exposed and shaded leaves of both species were used to evaluate the efficiency of photochemical energy conversion in photosystem II and the extent of non-radiative energy dissipation of these two contrasting photosynthetic pathways. By late afternoon, both semi-exposed C₃ and CAM plants were still unable to fully recover from photoinhibition suffered earlier at the peak of irradiance, although this effect was more marked in the C₃-species. Light response curves of fluorescence parameters obtained on a cloudy and a sunny day also suggested that semi-exposed conditions are more favourable to the CAM than to the C₃ species. Both the C₃ and the CAM Nidularium are shade-adapted, but CAM allows colonization by N. procerum of patches eventually exposed to higher irradiance. This partially explains the habitat segregation of these species in this swamp forest. It remains to be seen how rhizome flood-tolerance relates to this pattern.