113
of the sample. However, the general shape of the induction curve is seen in most organisms.
The changes in yield during induction are the result of several regulatory processes in
photosynthesis (Sivak and Walker, 1985) and have been widely used to evaluate the
relationship between photosynthetic physiology and fluorescence yield.
Source 0/ variable fluorescence: photochemical quenching. In photosynthetic systems, the
dominant process utilizing excited-state energy is photochemistry. However, the possibility
of photochemistry depends on the open or closed state of the PS II reaction center. In
practice, electron transport from secondary donors to P680+ is much faster than from the
primary electron acceptor to subsequent acceptors, such that a closed reaction center most
frequently occurs when the primary electron acceptor is reduced (Diner, 1986). Thus, the
yield of fluorescence will change depending on the fraction (ex) of reaction centers in the open
state according to the equation:
(6)
This dependence of fluorescence yield on the redox state of an electron transport component
was originally proposed by Duysens and Sweers in their Q-hypothesis (Lavorel and Etienne,
1977). They suggested the presence of a specific fluorescence quencher Q. In its oxidized
state, Q is a strong quencher of excitation energy and thus of fluorescence, but when Q is
reduced, its quenching is blocked. Subsequent experiments identified the quencher Q as the
primary quinone acceptor of PS II (also called ~, Figure 5). When ~ is oxidized,
photochemistry is possible and fluorescence is low; when ~ is reduced, the reaction center
is closed and fluorescence is high. This effect largely accounts for the fluorescence rise from
Fo (all traps open in dark adapted sample) to Fp (some fraction of traps closed) during
fluorescence induction (Figure 7). If fluorescence induction is measured on a sample which
has been treated with an inhibitor such as DCMU (inhibits electron transport after ~), the
fluorescence rapidly rises to a constant, maximum level Fm (or FmaJ(Figure 7). The difference
between Fmax and Fo is called the maximum variable fluorescence, or Fv. Since the
rate-limiting step of photosynthetic electron transport is the oxidation of reduced
plastoquinone, and ~ is in rapid redox equilibrium with plastoquinone, some variable
fluorescence rise is observed even at low light in the absence of inhibitors. This dependence
of fluorescence yield on the photochemical state of the PS II reaction center has recently been
termed photochemical fluorescence quenching (qp). In the presence of DCMU, the rise in
of the sample. However, the general shape of the induction curve is seen in most organisms.
The changes in yield during induction are the result of several regulatory processes in
photosynthesis (Sivak and Walker, 1985) and have been widely used to evaluate the
relationship between photosynthetic physiology and fluorescence yield.
Source 0/ variable fluorescence: photochemical quenching. In photosynthetic systems, the
dominant process utilizing excited-state energy is photochemistry. However, the possibility
of photochemistry depends on the open or closed state of the PS II reaction center. In
practice, electron transport from secondary donors to P680+ is much faster than from the
primary electron acceptor to subsequent acceptors, such that a closed reaction center most
frequently occurs when the primary electron acceptor is reduced (Diner, 1986). Thus, the
yield of fluorescence will change depending on the fraction (ex) of reaction centers in the open
state according to the equation:
(6)
This dependence of fluorescence yield on the redox state of an electron transport component
was originally proposed by Duysens and Sweers in their Q-hypothesis (Lavorel and Etienne,
1977). They suggested the presence of a specific fluorescence quencher Q. In its oxidized
state, Q is a strong quencher of excitation energy and thus of fluorescence, but when Q is
reduced, its quenching is blocked. Subsequent experiments identified the quencher Q as the
primary quinone acceptor of PS II (also called ~, Figure 5). When ~ is oxidized,
photochemistry is possible and fluorescence is low; when ~ is reduced, the reaction center
is closed and fluorescence is high. This effect largely accounts for the fluorescence rise from
Fo (all traps open in dark adapted sample) to Fp (some fraction of traps closed) during
fluorescence induction (Figure 7). If fluorescence induction is measured on a sample which
has been treated with an inhibitor such as DCMU (inhibits electron transport after ~), the
fluorescence rapidly rises to a constant, maximum level Fm (or FmaJ(Figure 7). The difference
between Fmax and Fo is called the maximum variable fluorescence, or Fv. Since the
rate-limiting step of photosynthetic electron transport is the oxidation of reduced
plastoquinone, and ~ is in rapid redox equilibrium with plastoquinone, some variable
fluorescence rise is observed even at low light in the absence of inhibitors. This dependence
of fluorescence yield on the photochemical state of the PS II reaction center has recently been
termed photochemical fluorescence quenching (qp). In the presence of DCMU, the rise in