PHYSIOLOGICAL AND ANATOMICAL RESPONSES TO WATER DEFICITS IN THE CAM EPIPHYTE TILLANDSIA-IONANTHA (BROMELIACEAE)

Although physiological responses to drought have been examined in seve ral species of epiphytic bromeliads, few have included a comprehensive methodological approach to the study of the carbon and water relation s of a single species undergoing drought: stress. Thus, physiological and anatomical responses to an imposed drought treatment were examined in the atmospheric Crassulacean acid metabolism (CA-M) epiphyte Tilla ndsia ionantha. From 0 through 20 d without water, nocturnal malic aci d accumulation and CO2 uptake rates did not change despite a 17% reduc tion in relative water content. In addition, water potentials averaged -0.40 MPa and, unlike leaf water content, did not decline. The avoida nce of further declines In leaf water content was attributed to thr re striction of stomatal opening to the night (a characteristic feature o f CAM), to low stomatal densities and small stomatal pores, and to a t hick boundary layer resulting from a dense foliar trichome cover. The maintenance of high physiological activity during the first 20 d of th e drought treatment was most likely a result of the high water potenti als in the chlorenchyma, which were attributed, in part, to water move ment from the water-storage parenchyma (= ''hydrenchyma'') to the chlo renchyma. Nocturnal malic acid accumulation and the rate of net CO2 ex change declined in a linear fashion from 30 to 60 d without water, as did leaf water potential and osmotic potential. During this time, CO2 recycling increased from ca. 20% to nearly 75%. Though declining throu ghout this later stage of the drought treatment, metabolic activity re mained relatively high, possibly as a result of the observed osmotic a djustment as well as a potentially high cell wall elasticity.