Utilization and Dissipation of Absorbed Light Energy in the Epiphytic Crassulacean Acid Metabolism Bromeliad Tillandsia Ionantha

Abstract

Past studies of the ability of epiphytic Crassulacean acid metabolism bromeliads to acclimate to different light levels yield conflicting findings; some indicate that these plants are similar to shade plants whereas others stress their similarity to sun plants. This study investigates the ability of individuals of Tillandsia ionantha to acclimate to low or high irradiance. Plants were exposed to 100 and 800 jitmol m~2 s"1 photosynthetic photon flux density under controlled conditions for 4 wk. Individuals exposed to the lower light level exhibited higher chlorophyll concentrations and higher photosynthetic rates at low light relative to plants exposed to high light. Low-light plants also exhibited a greater efficiency in the photochemical utilization of absorbed light energy and a lower ability to dissipate excess energy nonphotochemically, relative to the plants exposed to the higher light level. Photosynthetic rates at high light were similar in both sets of plants, reflecting the higher efficiency of energy conversion in the low-light plants and an apparent saturation of photosynthetic capacity in the high-light plants. The latter may have resulted from high-light-induced damage to the photosynthetic apparatus in addition to an increase in nonphotochemical dissipation of excess light energy. The higher capacity for harmless dissipation of excess light energy in the high-light plants should prove beneficial in plants growing in exposed locations and subject to drought and nutrient stresses. Thus, the results support and expand those of previous studies: T. ionantha can acclimate to both low and high light but does so in different ways. Such flexibility in adjusting the photosynthetic apparatus to varying light levels constitutes a valuable adaptation to growing throughout the canopy of a host tree.