117
Changes in antenna size occur on two physiological time scales: as long-term (days)
adaptation to average growth conditions and short-term (minutes) response to changes in light
intensity and spectral quality (Ley, 1980). The latter response is called a light state transition
and is generally characterized by changes in the functional antenna sizes of PS II and/or PS
I to maintain optimal photosynthetic efficiency. In green algae or higher plants, if light
affecting the turnover of PS I is limiting the rate of electron transport, antenna complexes
coupled to PS II are reversibly dissociated, decreasing the PS II antenna size and rate of PS
II turnover (helping to balance the distribution of light energy between PS I and PS II) (Fork
and Satoh, 1986). The dissociated complexes may become coupled to PS lor may simply
dissipate energy as heat. In either case, excited state energy is being dissipated by processes
that compete with PS II fluorescence. The time scale of light state transitions is on the order
of 1 to 10 minutes. Quenching of fluorescence due to light state transitions (~) has been
identified in higher plants, and several algal classes. Although mechanisms of light state
transitions may differ greatly between classes (Biggins and Bruce, 1989), their contribution
to total 'IN is probably ubiquitous.
Excited state annihilation. In general, the yield of fluorescence emitted by a sample is
linearly related to the intensity of light exciting the fluorescence (equation 5). However, at
very high light intensities, there is a probability that two or more excited states may exist in
the antenna of a reaction center at the same time. Under these conditions, an annihilation
process may occur between two excited states where the energy of one or both excitations is
lost as heat (Campillo and Shapiro, 1978). The threshold for the occurrence of multiple
excitations occurs only at very high, non-physiological light intensities (focused laser
illumination, for example). Annihilation reactions at high light intensity decrease fluorescence
yield in a non-photochemical manner that is independent of the biological reactions regulating
fluorescence yield.
INTERPRETATION OF FLUORESCENCE DATA
Present limitations. The description of processes that regulate the yield of chI fluorescence
in vivo presented in the previous sections is largely derived from studies on higher plant
chloroplasts or green algal suspensions. The interpretation of fluorescence signals from other
Changes in antenna size occur on two physiological time scales: as long-term (days)
adaptation to average growth conditions and short-term (minutes) response to changes in light
intensity and spectral quality (Ley, 1980). The latter response is called a light state transition
and is generally characterized by changes in the functional antenna sizes of PS II and/or PS
I to maintain optimal photosynthetic efficiency. In green algae or higher plants, if light
affecting the turnover of PS I is limiting the rate of electron transport, antenna complexes
coupled to PS II are reversibly dissociated, decreasing the PS II antenna size and rate of PS
II turnover (helping to balance the distribution of light energy between PS I and PS II) (Fork
and Satoh, 1986). The dissociated complexes may become coupled to PS lor may simply
dissipate energy as heat. In either case, excited state energy is being dissipated by processes
that compete with PS II fluorescence. The time scale of light state transitions is on the order
of 1 to 10 minutes. Quenching of fluorescence due to light state transitions (~) has been
identified in higher plants, and several algal classes. Although mechanisms of light state
transitions may differ greatly between classes (Biggins and Bruce, 1989), their contribution
to total 'IN is probably ubiquitous.
Excited state annihilation. In general, the yield of fluorescence emitted by a sample is
linearly related to the intensity of light exciting the fluorescence (equation 5). However, at
very high light intensities, there is a probability that two or more excited states may exist in
the antenna of a reaction center at the same time. Under these conditions, an annihilation
process may occur between two excited states where the energy of one or both excitations is
lost as heat (Campillo and Shapiro, 1978). The threshold for the occurrence of multiple
excitations occurs only at very high, non-physiological light intensities (focused laser
illumination, for example). Annihilation reactions at high light intensity decrease fluorescence
yield in a non-photochemical manner that is independent of the biological reactions regulating
fluorescence yield.
INTERPRETATION OF FLUORESCENCE DATA
Present limitations. The description of processes that regulate the yield of chI fluorescence
in vivo presented in the previous sections is largely derived from studies on higher plant
chloroplasts or green algal suspensions. The interpretation of fluorescence signals from other