Physiological consequences of changes in life form of the Mexican epiphyte Tillandsia deppeana (Bromeliaceae).

The heterophyllous epiphyte Tillandsia deppeana exhibits an atmospheric habit as a juvenile and a tank form as an adult. Both juveniles and adults utilize C3 photosynthesis. This is the first report of an atmospheric form of Tillandsia which does not exhibit CAM. Photosynthetic saturation occurred at approximately 10% of full sunlight in both forms, but the adults exhibited greater rates of photosynthesis at all levels of irradiance. The adults also had a higher and broader photosynthetic temperature optimum than did the juveniles. The adults transpired at greater rates than the juveniles; however, the water use efficiencies of both forms were similar and were high for C3 plants. In both forms the photosynthetic rate decreased in response to a decrease in humidity. After 8 days without water the juveniles were able to fix CO2 throughout the day. The adults, however, exhibited a net loss of CO2 on the second day without water and thereafter. These results indicate that the water-conservative atmospheric juvenile of T. deppeana is well adapted to establishment in the epiphytic habitat.