Resistance is useful: diurnal patterns of photosynthesis in C3 and crassulacean acid metabolism epiphytic bromeliads

Abstract

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001

Diurnal patterns of photosynthesis in response to environmental variables were investigated in an obligate C₃ and a facultative C₃-crassulacean acid metabolism (CAM) bromeliad species. A midday depression of photosynthesis occurred in both C₃ groups, mediated as a decrease in stomatal conductance in response to increased vapour pressure difference. The response was associated with a reduction in Rubisco activation state during the period of maximum photon flux density. In contrast, the switch to CAM resulted in a strong shift in the pattern of Rubisco carbamylation, with full enzyme activation delayed until the midday period. For the first time it is demonstrated that the pattern of Rubisco activation differs between C₃ and CAM plants of the same species under identical conditions. Despite large differences in Rubisco content between C₃ and CAM plants, neither the amount of Rubisco or enzyme activity is thought to be limiting for photosynthesis, and it is suggested that Rubisco may function as a nitrogen store. Extreme CO₂ diffusion limitation resulted in low rates of atmospheric CO₂ assimilation that were associated with high rates of photosynthetic electron transport, and it is likely that photorespiration constitutes a significant electron sink over the entire diurnal course. Leaf morphological and physiological adaptations to drought stress are necessary for the epiphytic lifestyle but limit CO₂ assimilation and confound the likelihood of high productivity.