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In contrast, fluorescence excitation spectra probe the spectral properties of those pigments that
are capable of transferring absorbed light energy to chI a. Because of the diversity of lightharvesting antenna pigments among the algal classes (Anderson and Barrett, 1986),
fluorescence excitation spectra can be of taxonomic value in evaluating differences between
field samples (Yentsch and Yentsch, 1979; Hinton et al., 1989). In addition, fluorescence
excitation spectra have been used to evaluate photoadaptation (Neori et al., 1984) and to
identify light-harvesting function of pigments that are unique to specific groups or species of
algae (Owens et al., 1986).
Modeling of photosynthetic light reactions. When evaluating the complex processes of the
photosynthetic light reactions, it is useful to have a kinetic model which parameterizes the
component reactions which are competing for excited state energy. Equations 6 and 7, which
express the quantum yields of fluorescence and photochemistry in terms of rate constants for
processes competing for excited state energy in the antenna, grossly oversimplify the
complexity of the photosynthetic light reactions. They are explicitly correct only when each
of the rate constants is an "effective" rate constant integrated over all pigments in the
antenna. For example, ~ in equations 6 and 7 represents the rate at which excited states are
quenched by photochemistry from the entire antenna, not just on the reaction center pigment.
The most widely accepted models describing photochemistry, fluorescence and thermal
emission in PS II have been the bipartite and tripartite models of Butler (Butler, 1978). These
models have been generally successful in predicting changes in the yield of fluorescence and
photochemistry under many physiological conditions. Recently, Holzwarth and co-workers
have presented an alternative model based on ultrafast time-resolved spectroscopy in PS II
(Schatz et al. 1988). None of these models is completely successful at explaining all
phenomena related to the light reactions of photosynthesis. Thus, although Butler's model
remains useful in many situations, the limitations of this or any model should be recognized
in predicting or evaluating physiological phenomena.
TECHNIQUES FOR MEASURING IN VIVO FLUORESCENCE
Measurement of chI fluorescence requires external illumination for two different types of
processes: to generate excited states in the antenna whose decay via fluorescence is directly
In contrast, fluorescence excitation spectra probe the spectral properties of those pigments that
are capable of transferring absorbed light energy to chI a. Because of the diversity of lightharvesting antenna pigments among the algal classes (Anderson and Barrett, 1986),
fluorescence excitation spectra can be of taxonomic value in evaluating differences between
field samples (Yentsch and Yentsch, 1979; Hinton et al., 1989). In addition, fluorescence
excitation spectra have been used to evaluate photoadaptation (Neori et al., 1984) and to
identify light-harvesting function of pigments that are unique to specific groups or species of
algae (Owens et al., 1986).
Modeling of photosynthetic light reactions. When evaluating the complex processes of the
photosynthetic light reactions, it is useful to have a kinetic model which parameterizes the
component reactions which are competing for excited state energy. Equations 6 and 7, which
express the quantum yields of fluorescence and photochemistry in terms of rate constants for
processes competing for excited state energy in the antenna, grossly oversimplify the
complexity of the photosynthetic light reactions. They are explicitly correct only when each
of the rate constants is an "effective" rate constant integrated over all pigments in the
antenna. For example, ~ in equations 6 and 7 represents the rate at which excited states are
quenched by photochemistry from the entire antenna, not just on the reaction center pigment.
The most widely accepted models describing photochemistry, fluorescence and thermal
emission in PS II have been the bipartite and tripartite models of Butler (Butler, 1978). These
models have been generally successful in predicting changes in the yield of fluorescence and
photochemistry under many physiological conditions. Recently, Holzwarth and co-workers
have presented an alternative model based on ultrafast time-resolved spectroscopy in PS II
(Schatz et al. 1988). None of these models is completely successful at explaining all
phenomena related to the light reactions of photosynthesis. Thus, although Butler's model
remains useful in many situations, the limitations of this or any model should be recognized
in predicting or evaluating physiological phenomena.
TECHNIQUES FOR MEASURING IN VIVO FLUORESCENCE
Measurement of chI fluorescence requires external illumination for two different types of
processes: to generate excited states in the antenna whose decay via fluorescence is directly