High light-induced switch from C₃-photosynthesis to Crassulacean acid metabolism is mediated by UV-A/blue light

The high light-induced switch in Clusia minor from C₃-photosynthesis to Crassulacean acid metabolism (CAM) is fast (within a few days) and reversible. Although this C₃/CAM transition has been studied intensively, the nature of the photoreceptor at the beginning of the CAM-induction signal chain is still unknown. Using optical filters that only transmit selected wavelengths, the CAM light induction of single leaves was tested. As controls the opposite leaf of the same leaf pair was studied in which CAM was induced by high unfiltered radiation (c. 2100 μmol m^-2s^-1). To evaluate the C₃-photosynthesis/CAM transition, nocturnal CO₂ uptake, daytime stomatal closure and organic acid levels were monitored. Light at wavelengths longer than 530 nm was not effective for the induction of the C₃/CAM switch in C. minor. In this case CAM was present in the control leaf while the opposite leaf continued performing C₃-photosynthesis, indicating that CAM induction triggered by high light conditions is wavelength-dependent and a leaf internal process. Leaves subjected to wavelengths in the range of 345-530 nm performed nocturnal CO₂ uptake, (partial) stomatal closure during the day (CAM-phase III), and decarboxylation of citric acid within the first 2 d after the switch to high light conditions. Based on these experiments and evidence from the literature, it is suggested that a UV-A/blue light receptor mediates the light-induced C₃-photosynthesis/CAM switch in C. minor.