Quantifying Kinetics of Sequential Electron Transport Between Photosystem II and Photosystem I Using Light Induced Fluorescence Transient (LIFT) Method.

Kinetic properties of photosynthetic electron transport are generally derived from the relaxation phase of the light induced fluorescence transients. Profiles of fluorescence transients observed during this phase are controlled, to a first order, by the rates of electron transport between PSII and PSI. To quantify these rates the observed fluorescence transients are usually fitted into two-, or tri-exponential decay models. Although efficient numerically, the parameters recovered using this procedure (the time constants and their pre-exponential amplitudes) can only be rigorously interpreted in terms of several parallel pathways of QA^- re-oxidation, which is incorrect. I will present a numerically-equivalent procedure for extracting time constants of sequential electron transport: t1 (QA -->QB), t2 (QB-->PQ Pool) and t3 (PQ pool -->PSI). The presented approach also allows calculation of the oxidized portion of the PQ pool, and the occupancy level of QB prior to the flash, as well as monitoring changes in the levels of QA, QB and PQ pool reduction along the fluorescence transient. A set of experimental data acquired with several phytoplankton species and with higher plants will presented in support of the proposed technique.