Light quality affects diurnal gas exchange but not diurnal deacidification in CAM leaves

Abstract

During the light period, crassulacean acid metabolism (CAM) plants remobilise malic acid from the vacuole and decarboxylate it to supply CO2 for Rubisco behind closed stomata. Whilst it has been well documented that diurnal deacidification depends strongly on light availability, the impact of light quality on daytime leaf gas exchange and deacidification remains poorly understood. The obligate CAM model species Kalanchoe<spacing diaeresis> fedtschenkoi was subjected to either different monochromatic wavelengths (blue, green, red) or modified full-spectrum (white) light spectra with a particular waveband omitted (blue, orange, red, far-red, blue+red). Leaf gas exchange parameters and different biochemical parameters associated with diurnal deacidification [i.e., malate and starch content, in vitro activities of malic enzyme (ME), pyruvate orthophosphate dikinase (PPDK), and Rubisco] were measured at dawn, midday, and late afternoon. Under high light intensities of 300 mu mol m- 2 s- 1, blue wavelengths were found to be a key determinant in promoting high daytime stomatal conductance (Phases II and IV), resulting in improved C3 carboxylation. Furthermore, omitting red wavelengths from the light spectrum led to a substantially higher overall diel CO2 uptake (+300 %) compared to plants subjected to spectra lacking either blue or both blue and red wavelengths. In contrast to the large impact on leaf gas exchange, light spectral composition had only a minor impact on diurnal changes in intrinsic enzyme activities of NAD(P)-ME, PPDK, and Rubisco. Consistent with these observations, the diurnal malate degradation and starch accumulation rates were remarkably similar under different spectral compositions. Our findings reveal that stomatal behavior and gas exchange are primarily influenced by light quality, with a more pronounced role for blue compared to red wavelengths. The core biochemical events associated with diurnal deacidification are mainly influenced by light intensity and rather insensitive to changes in spectral composition.