On the intermediate steps between the O and P steps in chlorophyll α fluorescence rise measured at different intensities of exciting light

It is known that the characteristic O-I-P transient of the fast chlorophyll a fluorescence rise (FR) detected under low intensity of exciting light, changes to the O-J-I-P (O-I-1-I-2-P) transient under higher intensity of exciting light. In this work, we extend an application of the mathematical model of FR [Lazar et al. (1997) Pesticide Biochemistry and Physiology 57, 200-210], involving photosystem II (PSII) heterogeneity and simulating the O-J-I-P transient, for modeling of FR under different intensities of exciting light. Our simulations qualitatively agree well with experimental FR curves obtained with wheat leaves. The simulations demonstrate that the first step after the O fluorescence level (the I step and the J step under lower and higher intensities of exciting light, respectively) is caused mainly by the accumulation of reduced Q(A) electron acceptors without reduction of subsequent electron acceptors. The first step appears at shorter times with increasing intensity of exciting light as reported previously [Strasser et al. (1995) Photochemistry and Photobiology 61, 32-42]. Our simulations also demonstrate, in accordance with previous publications, that the O-I phase in FR detected under low intensity of exciting light, reflects mainly the accumulation of Q(A)(-) in the Q(B)-non-reducing PSII centers and partly also the accumulation of Q(A)(-) in the Q(B)-reducing PSII centers. In our model, the accumulation of Q(A)- in the Q(B)-non-reducing PSII centers gradually shifts with increasing intensity of exciting light to shorter times and participates in the formation of the O-J phase; this is also supported by measurements of FR with wheat leaves treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU). Under high intensity of exciting light, the O-J phase in FR also markedly reflects the accumulation of Q(A)(-) in the Q(B)-reducing PSII centers. Our simulations also support the previously reported suggestion of Strasser's group that the I step in FR obtained under high intensity of exciting light (in the position of the I step detected under lower intensity of exciting light) appears mainly owing to the accumulation of the Q(A)-Q(B)(2-) and Q(A)(-)Q(B)H(2) forms.