The dependence of the degrees of sigmoidicities of fluorescence induction curves in spinach chloroplasts on the duration of actinic pulses in pump-probe experiments

TY - JOUR

T1 - The dependence of the degrees of sigmoidicities of fluorescence induction curves in spinach chloroplasts on the duration of actinic pulses in pump-probe experiments

AU - France, Louisa L.

AU - Geacintov, Nicholas

AU - Breton, Jacques

AU - Valkunas, Leonas

PY - 1992/7/6

Y1 - 1992/7/6

N2 - Utilizing a pump-probe double flash method, the shapes of the fluorescence induction curves in spinach chloroplasts (0°C), and the induction ratio R = FM/F0 (F0 and FM are the fluorescence yields when all PS II reaction centers are open, and closed, respectively) were studied as a function of: (1) the duration (t1) of the pump flash, (2) the number of pump flashes, (3) absence or presence of DCMU, and (4) the time interval Δt between the pump and the probe flashes. In the pump-probe technique, an actinic pulse (P1) of different fluences closes a fraction q of the PS II reaction centers. After a variable time interval Δt a second, weak pulse (P2), is used to measure the variable fluorescence yield Fv. The shapes of the fluorescence curves (Fv vs. the fluence of the P1 pulse) were analyzed in terms of the standard eqution Fv = (1 - p)q/(1 - pq), where p is a parameter describing the connectivity between different photosynthetic units (Joliot, A. and Joliot, P. (1964) C. R. Acad. Sci. 13, 4622-4625). The shapes of the Fv curves are found to depend only on the width (t1) of the pump pulses. For t1 ≤ 300 ps, the curves are exponential in shape with p = 0.0 and R < 2.5. However, for pulse durations t1 in the millisecond range, and employing the same probe flash method for measuring the variable fluorescence, the Fv curves assume the same familiar sigmoidal shapes as in the case of conventional steady-state illumination. With t1 in the microsecond range (t1 ≈ 0.7-2 μs), the Fv curves are more nearly exponential than sigmoidal in shape with p values generally around ≈0.3 and R < 3. For t1 >/ 50 μs, the fluorescence induction curves are sigmoidal in shape and, generally, p values of 0.55-0.60 are observed with R > 3. A sequential hit model (Valkunas, L., Geacintov, N.E., France, L. and Breton, J. (1991) Biophys. J. 59, 397-408), in which the PS II reaction centers evolve through different states characterized by differences in fluorescence yields, a process characterized by a dark-time t1 or ≈2-50 μs between two successive hits, can account for these results. This model is consistent with the concept of interunit transfer of excitations. However, the shapes of the fluorescence induction curves cannot provide any information on the value of p within the context of this model.

AB - Utilizing a pump-probe double flash method, the shapes of the fluorescence induction curves in spinach chloroplasts (0°C), and the induction ratio R = FM/F0 (F0 and FM are the fluorescence yields when all PS II reaction centers are open, and closed, respectively) were studied as a function of: (1) the duration (t1) of the pump flash, (2) the number of pump flashes, (3) absence or presence of DCMU, and (4) the time interval Δt between the pump and the probe flashes. In the pump-probe technique, an actinic pulse (P1) of different fluences closes a fraction q of the PS II reaction centers. After a variable time interval Δt a second, weak pulse (P2), is used to measure the variable fluorescence yield Fv. The shapes of the fluorescence curves (Fv vs. the fluence of the P1 pulse) were analyzed in terms of the standard eqution Fv = (1 - p)q/(1 - pq), where p is a parameter describing the connectivity between different photosynthetic units (Joliot, A. and Joliot, P. (1964) C. R. Acad. Sci. 13, 4622-4625). The shapes of the Fv curves are found to depend only on the width (t1) of the pump pulses. For t1 ≤ 300 ps, the curves are exponential in shape with p = 0.0 and R < 2.5. However, for pulse durations t1 in the millisecond range, and employing the same probe flash method for measuring the variable fluorescence, the Fv curves assume the same familiar sigmoidal shapes as in the case of conventional steady-state illumination. With t1 in the microsecond range (t1 ≈ 0.7-2 μs), the Fv curves are more nearly exponential than sigmoidal in shape with p values generally around ≈0.3 and R < 3. For t1 >/ 50 μs, the fluorescence induction curves are sigmoidal in shape and, generally, p values of 0.55-0.60 are observed with R > 3. A sequential hit model (Valkunas, L., Geacintov, N.E., France, L. and Breton, J. (1991) Biophys. J. 59, 397-408), in which the PS II reaction centers evolve through different states characterized by differences in fluorescence yields, a process characterized by a dark-time t1 or ≈2-50 μs between two successive hits, can account for these results. This model is consistent with the concept of interunit transfer of excitations. However, the shapes of the fluorescence induction curves cannot provide any information on the value of p within the context of this model.

KW - (Spinach)

KW - Chloroplast

KW - Fluorescence

KW - Fluorescence induction curve

KW - Photosystem II

KW - Reaction center

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U2 - 10.1016/0167-4838(92)90474-R

DO - 10.1016/0167-4838(92)90474-R

M3 - Article

VL - 1101

JO - Biochimica et Biophysica Acta - Proteins and Proteomics

JF - Biochimica et Biophysica Acta - Proteins and Proteomics

SN - 1570-9639

IS - 1

ER -