Fluorescence induction studies in isolated chloroplasts I. Kinetic analysis of the fluorescence intensity dependence on time

• 1.

  1. Fluorescence induction curves (intensity versus time) obtained by irradiation of isolated chloroplasts were observed and subjected to a detailed kinetic analysis. This analysis was based on a photosystem II model consisting of independent units, each of which contains one pigment aggregate connected to one electron-transport chain. During irradiation, two electron carriers, which exist initially in the oxidized state, are reduced in a light reaction followed by a dark reaction, both being first order:

  , where Q is the primary oxidant of photosystem II. Q and P are present in a 1:1 ratio¹. The experimental curves agree with the calculated ones over a wide range of light intensity and temperature.

• 2.

  2. Using the analysis outlined above, it was possible to estimate the first-order rate constant of the reaction between Q⁻ and P as $\text{k = 30–40}\text{sec}{}^{\mathrm{-1}}\text{at}\text{22°}\text{and}\text{2–2.5}\text{sec}{}^{\mathrm{-1}}\text{at}\text{0°}$. Comparison with the saturation rate of the Hill reaction shows that the above reaction between Q⁻ and P may be rate limiting in the Hill reaction.