Annual carbon balance and nitrogen-use efficiency in tropical C and CAM epiphytes.

Over the course of 12 months, diel (24 h) measurements of gas exchange were performed on leaves of three epiphytic species growing in the crown of a kapok tree on Barro Colorado Island, Panama: a drought-deciduous orchid with the C , pathway (Catasetum viridiflavum Hook.), an evergreen C fern (Polypodium crassifolium L.), and an evergreen epiphyte with an intermediate C -C -CAM pathway of photosynthesis (Clusia uvitana Pitt.), The gas exchange characteristics of all three species; were strongly affected during the four-month dry season. Compared with the rainy season, mean daily carbon gain of Clusia uvitana was reduced by almost 40° paralleled bY a strong decrease in daytime CO uptake and an increase in CO uptake at night. The orchid, growing new leaves in the second half of the dry season, showed markedly decreased stomatal conductances and greatly reduced carbon gain. In the fern, daily carbon balance became negative during the dry season and chronic photoinhibition was indicated by reduced F /F ratios and a decreased photon-use efficiency of photosynthetic O evolution. Annual carbon gain was similar far the three species (about 1000 CO m yr ) as was long-term nitrogen-use efficiency (annual carbon gain/mean leaf nitrogen content, about 1·1 g CO mg N yr ). In the C CAM epiphyte, the long-term water use efficiency of net CO uptake was more than twice as high as in the two C epiphytes.