EFFECTS OF DROUGHT STRESS ON CO2 EXCHANGE AND WATER RELATIONS IN THE CAM EPIPHYTE TILLANDSIA-UTRICULATA (BROMELIACEAE)

Water was withheld from plants of Tillandsia utriculata L. for up to t wo months in a growth chamber. Nocturnal CO2 uptake declined substanti ally in the first week without water, then declined slowly between 7 a nd 31 d of the drought treatment. Net CO2 exchange after two months of desiccation was nearly zero, although substantial nocturnal increases in malic acid concentrations were still measureable. Thus, the degree of CO2 recycled internally via CAM increased dramatically throughout the drought treatment. In spite of the slow, yet substantial declines in nighttime CO2 uptake observed throughout the drought treatment, lea f water potentials remained relatively high (above -1.5 MPa) and decli ned by only 0.5 MPa after two months without water. Based on pressure- volume analyses, the leaf bulk elastic modulus was approximately 3.3 M Pa, a very low value indicative of highly elastic cell walls. Thus, ma intenance of high turgor pressures as a result of elastic cell walls m ay constitute an important adaptation of this species and presumably o ther epiphytic species of Tillandsia that reduces the negative effects of drought stress. This, as well as other adaptations to drought stre ss such as the preferential utilization of water from water-storage pa renchyma, osmotic adjustment, and low rates of transpiration, should c ontribute to survival during drought in these unusual plants.