Light quality modulates metabolic synchronization over the diel phases of crassulacean acid metabolism

Abstract

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 24h 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, 630nm) 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 with plants under white or low-fluence blue light was noted. Consistent 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 signalling pathways for synchronizing the metabolic and physiological components of CAM over the day/night cycle.

Keywords: CAM; PEPC; PEPCK; carbohydrate; gas exchange; light quality; titratable acidity..

Fig. 1.

Net 24h CO₂ uptake (μmol m^–2 s^–1) for young fully developed leaves of Aechmea ‘Maya’ under control light-dark cycle (A, white light, 100 μmol m^–2 s^–1), continuous dark (B), and different monochromatic light-dark cycles (10 µmol m^–2s^–1; C=blue 475nm; D=green 530nm; E=red 630nm). The dark period is indicated in grey. Gas exchange curves are representative of three replicate runs with SE<15%.

Fig. 2.

Net 24h evapotranspiration (mmol m^–2s^–1) for young fully developed leaves of Aechmea ‘Maya’ under blue (A) and red (B) illumination of 10 μmol m^–2 s^–1. The dark period is indicated in grey. Curves are representative of three replicate runs with SE<15 %.

Fig. 3.

Diel patterns of titratable acidity (μmol H⁺ g^–1fw; left panel), starch (μmol Glc eq g^–1fw; middle panel) and sucrose (μmol g^–1fw; right panel) for young fully developed leaves of Aechmea ‘Maya’ under control light-dark cycle (A, F, K; white light, 100 μmol m^–2 s^–1), continuous dark (B, G, L), and different monochromatic light-dark cycles (10 μmol m^–2 s^–1), i.e. blue (C, H, M), green (D, I, N), and red (E, J, O). The dark period is indicated in grey. Data are means±SE (n=5 plants).

Fig. 4.

Day-night changes in levels of transcripts for PEPC (ppc) and PEPCK (pepck) in young fully developed leaves of Aechmea ‘Maya’ under control light-dark cycle (LD, white light, 100 μmol m^–2 s^–1) and different monochromatic light-dark cycles (10 μmol m^–2 s^–1), i.e. blue, green, and red. The dark period was from 22.00–06.00h. Ubiquitin (ubq) served as control.

Fig. 5.

Western blots showing the abundance of PEPC (top) and PEPCK (bottom) protein in young fully developed leaves of Aechmea ‘Maya’ under control light-dark cycle (LD, white light, 100 μmol m^–2 s^–1) and different monochromatic light-dark cycles (10 μmol m^–2 s^–1), i.e. blue, green, and red. The dark period was from 22.00–06.00h. Similar amounts of soluble proteins were loaded for each treatment.