The significance of the kinetic analysis of fluorescence induction in DCMU‐inhibited chloroplasts in terms of photosystem 2 connectivity and heterogeneity

A study has been made on the slow component (β max ) of chlorophyll fluorescence induction curves exhibited by DCMU‐inhibited pea thylakoids. In the absence of high levels of screening cations, β max is high and the fluorescence yield low, while the addition of K + , Mg 2+ or Tris(ethylenediamine) cobaltic(III) cation (TEC 3+ ) decreases β max and increases fluorescence yield. These changes are inhibited when the thylakoids are fixed with glutaraldehyde. By comparing cation‐ and light‐harvesting chlorophyll a/b ‐protein (LHCP) phosphorylation‐induced changes it can be seen that β max correlates with changes in the overall kinetics of photosystem (PS) 2 photoreduction, as indicated from the normalised area over the induction curve (Anorm). When the plastoquinone (PQ) pool is chemically reduced by dithionite, prior to the initiation of the curve, both F o and F m are increased and the slow component removed. These observations are used to question the concept of two structurally distinct PS2 centres [Arch. Biochem. Biophys. (1978) 190, 523–530] and to discuss the use of β max in monitoring PS2 organisation.