The effect of redox potential on the kinetics of fluorescence induction in Photosystem II particles from Phormidium laminosum. Sigmoidicity,energy transfer and the slow phase

Fluorescence induction curves in 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-inhibited Photosystem (PS) II particles isolated from the blue-green alga Phormidium laminosum have been analysed as a function of redox potential. Redox titration of the initial fluorescence indicated a single component with E_m,7.5 = +30 mV (n = 1) (Bowes, J., Horton, P. and Bendall, D.S. (1981) FEBS Lett. 135, 261–264). Despite this simplified electron acceptor system and the small number of chlorophylls per reaction centre, a sigmoidal induction curve was nevertheless seen. Sigmoidicity decreased as Q was reduced potentiometrically prior to induction such that the induction was exponential when the ratio $\text{F}{}_{\mathrm{<mtext>i</mtext>}}\text{F}{}_{\mathrm{<mtext>m</mtext>}}\text{= 0.64}$. These particles also showed a slow (β) phase of induction which titrated with an E_m value slightly more positive than that of the major quencher. It is concluded that the sigmoidal shape of the fluorescence induction curve observed in Phormidium PS II particles is not a consequence of a requirement for two photons to close the PS II reaction centre, but is generated as a result of energy transfer between photosynthetic units comprising one reaction centre per approx. 50 chlorophylls. Also, the existence of PS II heterogeneity (PS II_α, PS II_β centres) does not require a structurally differentiated chloroplast, but may only indicate the extent of aggregation of PS II centres.