Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J–I–P rise

“If we study the history of science we see produced two phenomena which are, so to speak the inverse of the other. Sometimes it is simplicity which is hidden under what is apparently complex; sometimes, on the contrary, it is simplicity which is apparent, and which conceals complex realities”— Henri Poincare (1854—1912); see Science and Hypothesis, Dover Publications, New York, 1952

Abstract

The fast (up to 1 s) chlorophyll (Chl) a fluorescence induction (FI) curve, measured under saturating continuous light, has a photochemical phase, the O–J rise, related mainly to the reduction of Q_A, the primary electron acceptor plastoquinone of Photosystem II (PSII); here, the fluorescence rise depends strongly on the number of photons absorbed. This is followed by a thermal phase, the J–I–P rise, which disappears at subfreezing temperatures. According to the mainstream interpretation of the fast FI, the variable fluorescence originates from PSII antenna, and the oxidized Q_A is the most important quencher influencing the O–J–I–P curve. As the reaction centers of PSII are gradually closed by the photochemical reduction of Q_A, Chl fluorescence, F, rises from the O level (the minimal level) to the P level (the peak); yet, the relationship between F and [Q_A ⁻] is not linear, due to the presence of other quenchers and modifiers. Several alternative theories have been proposed, which give different interpretations of the O–J–I–P transient. The main idea in these alternative theories is that in saturating light, Q_A is almost completely reduced already at the end of the photochemical phase O–J, but the fluorescence yield is lower than its maximum value due to the presence of either a second quencher besides Q_A, or there is an another process quenching the fluorescence; in the second quencher hypothesis, this quencher is consumed (or the process of quenching the fluorescence is reversed) during the thermal phase J–I–P. In this review, we discuss these theories. Based on our critical examination, that includes pros and cons of each theory, as well mathematical modeling, we conclude that the mainstream interpretation of the O–J–I–P transient is the most credible one, as none of the alternative ideas provide adequate explanation or experimental proof for the almost complete reduction of Q_A at the end of the O–J phase, and for the origin of the fluorescence rise during the thermal phase. However, we suggest that some of the factors influencing the fluorescence yield that have been proposed in these newer theories, as e.g., the membrane potential ΔΨ, as suggested by Vredenberg and his associates, can potentially contribute to modulate the O–J–I–P transient in parallel with the reduction of Q_A, through changes at the PSII antenna and/or at the reaction center, or, possibly, through the control of the oxidation–reduction of the PQ-pool, including proton transfer into the lumen, as suggested by Rubin and his associates. We present in this review our personal perspective mainly on our understanding of the thermal phase, the J–I–P rise during Chl a FI in plants and algae.