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Crassulacean Acid Metabolism in the Epiphyte Tillandsia usneoides L. (Spanish Moss) ¹

RESPONSES OF CO₂ EXCHANGE TO CONTROLLED ENVIRONMENTAL CONDITIONS

Department of Botany, Duke University, Durham, North Carolina 27706

Patterns of CO₂ exchange in Spanish moss under various experimental conditions were measured using an infrared gas analysis system. Plants were collected from a study site in North Carolina and placed in a gas exchange chamber for several days of continuous measurements. No substantial seasonal effects on CO₂ exchange were observed. High rates of nocturnal CO₂ uptake were observed under day/night temperature regimes of 25/10, 25/15, 25/20, 30/20, and 35/20 C; however, daytime temperatures of 40 C eliminated nighttime CO₂ uptake and a nighttime temperature of 5 C eliminated nocturnal CO₂ uptake, regardless of day temperature. Constant chamber conditions also inhibited nocturnal CO₂ uptake. Constant high relative humidity (RH) slightly stimulated CO₂ uptake while low nighttime RH reduced nocturnal CO₂ uptake.

Reductions in daytime irradiance to approximately 25% full sunlight had no effect on CO₂ exchange. Continuous darkness resulted in continuous CO₂ loss by the plants, but a CO₂ exchange pattern similar to normal day/night conditions was observed under constant illumination. High tissue water content inhibited CO₂ uptake. Wetting of the tissue at any time of day or night resulted in net CO₂ loss. Abrupt increases in temperature or decreases in RH resulted in sharp decreases in net CO₂ uptake.

The results indicate that Spanish moss is tolerant of a wide range of temperatures, irradiances, and water contents. They also indicate that high nighttime RH is a prerequisite for high rates of CO₂ uptake.

¹ This work was partially supported by funds made available to N. L. Christensen and B. R. Strain through a Biomedical Research Support Grant from the National Institutes of Health.