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Encyclopedia of Physical Science and Technology EN012C-562 July 26, 2001 15:30

12 Photoacoustic Spectroscopy

d. Photosynthetic oxygen evolution. In photo-
chemical reactions of photosynthesis, water is the primary
electrondonorandNADPistheultimateelectronacceptor.
During the process of water oxidation, oxygen is released
as a by-product of photosynthesis. In the low-frequency
range, PAS detects this oxygen produced during photo-
synthesis. By recording the signal and normalizing it with
the heat signal, one can study the relative quantum yield
of photosynthetic evolution.
By measuring energy storage, oxygen evolution, pho-
tosynthetic transients, light distribution between PS I and
PS II , low light state adaptations, and independent activity
of PS I in vivo have been examined.
Several environmental stress effects on plants have been
investigated using PAS. PA methods have been used to
monitor the effects of water stress, high-light stress, low-
and high-temperature stress, and chemical pollutant stress
on plants. As the technique facilitates understanding the
FIGURE 11 Photoacoustic spectrum of pea leaf recorded at
photochemical activity of intact leaves, several studies 40 Hz in the absence (solid line) and presence (broken line) of
have been made to investigate drought stress- and high background light.
light stress-induced changes in the photochemical activ-
ity of the leaves. PAS also helps one to understand the light
distribution and balanced excitation of photosytems dur- conditions resemble those of absorption spectra (Fig. 11).
ing stress conditions. Under high light conditions, heat PA spectra have been used to identify and characterize
emission has been considered a protective mechanism. pigments and to analyze the depth profiles of pigments in
PAS was used to measure the heat emission in high light- intact leaves. The spectra have also been used to under-
treated plants, and it was demonstrated that plants dis- stand the physiological intactness of plants under environ-
sipate the excess light energy in the form of heat. In mental stress conditions, to understand the energy transfer
shade-adapted sugar maple plants, an instantaneous heat process among pigment molecules, and to determine the
emission upon exposure to high light was demonstrated. relative quantum yield of photochemistry in spectral range
Using PAS, the mechanism of action of bisulfite in pea of photosynthetically active radiation.
leaves was investigated. Using oxygen free-radical scav-
engers, it was demonstrated that the Calvin cycle is a
G. Environmental Sciences
valid candidate for the primary site of action of bisulfite in
plants. Using PA and fluorescence techniques, it was also LaserPAShasbeenusedtomonitorairsamples.Minimum
demonstrated that abscisic acid offers protection against detectable concentrations of gaseous pollutants are often
the inhibition of photosynthesis by bisulfite. Interestingly, in the parts per billion (ppb) or sub-ppb range depending
the stimulatory effect of sulfur dioxide on maple leaf pho- on the molecular absorption cross section and on possible
tochemical activity was also reported using PAS. Thus absorption interferences. Most studies have been devoted
PAS has proved to be a versatile technique for studying to investigations on collected air samples of different ori-
environmental stress effects on plants. gin. A CO laser-based PAS has been used for analysis of
vehicular exhausts. A number of compounds such as ni-
tric oxide, nitrogen dioxide, H 2 O vapor, and other volatile
3. Spectral Properties
organic compounds such as alkenes, aromatic hydrocar-
As mentioned above, there is oxygen contribution to the bons, and aldehydes have been reported. A mobile PAS
PA signal in the low-frequency range. The amplitude of system has been developed for analysis in the field.
the photochemistry depends on the wavelength of excita- Similarly, analysis and time-resolved monitoring of
tion light. In order to avoid the spectral deformities due to stack emissions from power plants, incineration plants,
modulated photochemistry, PA spectra of intact leaves are or industrial plants are also of considerable interest. Air
recorded in the presence of nonmodulated, strong, back- samples have been analyzed using the mobile PAS system
ground white light, which closes the reaction centers and in urban and rural areas. Thus, PA monitoring has not been
allows all the absorbed modulated light to escape as modu- restricted to laboratory measurements, but is performed in
lated heat. PA spectra of green leaves recorded under such the field.