Journal of Experimental Botany vol:65 issue:13 pages:3705-3714
Temporal compartmentation of carboxylation processes is a defining feature of crassulacean acid metabolism and involves circadian control of key metabolic and transport steps that regulate the supply and demand for carbon over a 24 h cycle. Recent insights on the molecular workings of the circadian clock and its connection with environmental inputs raise new questions on the importance of light quality and by analogy certain photoreceptors, for synchronizing the metabolic components of CAM. The present work tested the hypothesis that optimal coupling of stomatal conductance, net CO2 uptake and the reciprocal turnover of carbohydrates and organic acids over the diel CAM cycle requires both blue and red light input signals. Contrasting monochromatic wavelengths of blue, green and red light (i.e. 475, 530, 630 nm) with low fluence rates (10 μmol m-2 s-1) were administered for 16 hours each diel cycle for a total treatment time of 48 hours to the obligate CAM bromeliad, Aechmea ‘Maya’. Of the light treatments imposed, low fluence blue light was a key determinant in regulating stomatal responses, organic acid mobilization from the vacuole and daytime decarboxylation. However, the reciprocal relationship between starch and organic acid turnover that is typical for CAM was uncoupled under low fluence blue light. Under low fluence red or green light, the diel turnover of storage carbohydrates was orchestrated in line with the requirements of CAM but a consistent delay in acid consumption at dawn, compared to plants under white or low fluence blue light was noted. In line with the acknowledged influences of both red and blue light as input signals for the circadian clock, the data stress the importance of both red and blue-light signaling pathways for synchronizing the metabolic and physiological components of CAM over the day/night cycle.