112
The exceptional case in emission from accessory pigments occurs in phycobilisome antennae.
The phycobilisome is a large aggregate of phycobilin pigments. The lifetime of the excited
state in the phycobilisome may be comparable to or greater than its lifetime in the chI a
antenna (Bryant, 1986) with the result that significant emission from the bilins can be detected
in many cases.
The final consideration in the source of in vivo fluorescence is the relative importance of
antenna chI a versus P680. The reaction center pigment is frequently referred to as a "trap"
for excitation energy in the antenna. In a sense this is correct because the major process
leading to loss of excitations in PS II is photochemistry in the reaction center. Efficient
photochemistry is the main factor that limits the lifetime of the excited state in any antenna
system. However, the concept of a trap is a misnomer because once the excitation reach the
reaction center, it is not immediately destined to be used in photochemistry. Rather, the
excitation may migrate back into the antenna only to return to the reaction center many more
times prior to being utilized in photochemistry. In PS II, the excited state is essentially in a
dynamic equilibrium between all antenna chI a including the reaction center (Schatz et al.,
1988). Because there are hundreds of antenna chI a per reaction center, the total emission is
dominated by the antenna chI a. Thus, with the exception of algae with phycobilisome
antennae, the ultimate source of most fluorescence from photosynthetic samples is the chI a
pigments in the PS II antenna.
PROCFSSFS AFFECTING FLUORESCENCE YIELD IN VIVO
When a dark-adapted photosynthetic sample is suddenly exposed to continuous (constant
intensity) light, the yield of fluorescence passes through a series of maxima and minima
before reaching a steady-state level (Lavorel and Etienne, 1977). This induction phase of chI
fluorescence (the Kautsky effect, Figure 7) is characterized by an instantaneous rise to the
initial (0 or F J level followed by a complex rise to a maximum P and a decline to a
minimum S. These fast changes are completed in a few seconds and may be followed by a
slower (minutes) rise to a new maximum M and finally a decline to the steady-state level T.
The time scale of these changes and the relative amplitudes of the minima and maxima will
depend on the illumination intensity, on the species, and on the previous illumination history
The exceptional case in emission from accessory pigments occurs in phycobilisome antennae.
The phycobilisome is a large aggregate of phycobilin pigments. The lifetime of the excited
state in the phycobilisome may be comparable to or greater than its lifetime in the chI a
antenna (Bryant, 1986) with the result that significant emission from the bilins can be detected
in many cases.
The final consideration in the source of in vivo fluorescence is the relative importance of
antenna chI a versus P680. The reaction center pigment is frequently referred to as a "trap"
for excitation energy in the antenna. In a sense this is correct because the major process
leading to loss of excitations in PS II is photochemistry in the reaction center. Efficient
photochemistry is the main factor that limits the lifetime of the excited state in any antenna
system. However, the concept of a trap is a misnomer because once the excitation reach the
reaction center, it is not immediately destined to be used in photochemistry. Rather, the
excitation may migrate back into the antenna only to return to the reaction center many more
times prior to being utilized in photochemistry. In PS II, the excited state is essentially in a
dynamic equilibrium between all antenna chI a including the reaction center (Schatz et al.,
1988). Because there are hundreds of antenna chI a per reaction center, the total emission is
dominated by the antenna chI a. Thus, with the exception of algae with phycobilisome
antennae, the ultimate source of most fluorescence from photosynthetic samples is the chI a
pigments in the PS II antenna.
PROCFSSFS AFFECTING FLUORESCENCE YIELD IN VIVO
When a dark-adapted photosynthetic sample is suddenly exposed to continuous (constant
intensity) light, the yield of fluorescence passes through a series of maxima and minima
before reaching a steady-state level (Lavorel and Etienne, 1977). This induction phase of chI
fluorescence (the Kautsky effect, Figure 7) is characterized by an instantaneous rise to the
initial (0 or F J level followed by a complex rise to a maximum P and a decline to a
minimum S. These fast changes are completed in a few seconds and may be followed by a
slower (minutes) rise to a new maximum M and finally a decline to the steady-state level T.
The time scale of these changes and the relative amplitudes of the minima and maxima will
depend on the illumination intensity, on the species, and on the previous illumination history