Comparison of Photoacoustic and Chlorophyll Fluorescence Signatures of Green Leaves
The photoacoustic (PA) signals of tobacco and beech leaves are compared with measurements of the chlorophyll fluorescence, the CO2-assimilation and the pigment content of the leaves to show that the PA-method is a valuable tool to obtain additional information about the photosynthetic apparatus. The PA-signals are shown as excitation spectra in the visible range and as induction kinetics excited by a He/Ne-laser.
The PA-signals are determined by the light-induced heat but are superimposed on the photosynthetically produced oxygen pulses at low chopping frequencies (150 Hz).
During the autumnal chlorophyll breakdown the PA-signal declines in the red-light part of the spectrum due to the loss of chlorophyll. In the blue-light part the PA-signal remains at a constant height, indicating that the light-energy absorbed by the carotenoids can no longer be used in photosynthesis and is dissipated as heat.
During the light-induced induction kinetics of the green leaves the heat signal (PA-signal at 238 Hz) and the chlorophyll-fluorescence signal decrease after a yery fast initial increase, whereas the PA-signal at 22 Hz increases furthermore after the fast increase at the onset of illumination. The PA-signals at 22 and 238 Hz and the net CO2-assimilation reach the steady state after 20 min, whereas the chlorophyll fluorescence is much faster and already reaches the steady state after 4 min. The increase of the CO2-assimilation seems to be related more closely to the decrease of the heat signal than to the decrease of the fluorescence signal.
If continuous saturating white light is added to the chopped excitation laser light (238 Hz) at the steady state of the kinetic, the heat signal increases and remains at a constant height during the illumination, whereas the fluorescence signal increases and then declines. This may indicate that the heat signal arises from both photosystems, whereas the chlorophyll-fluorescence emission at room temperature primarily emanates from photosystem II. The height of the increase of the PA-signal reflects the photosynthetic activity of the leaves.
Key Wordsaurea mutant chlorophyll fluorescence CO2-assimilation PN heat fluorescence induction kinetics photoacoustic signais photosynthesis sun and shade leaves
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