Prying into the green black-box.

Life-long efforts of the Tartu photosynthesis research group have been summarized. The measurements were facilitated by self-designed instruments, distinct in multifunctionality and fastresponse time. The black-box type kinetical analysis on intact leaves has revealed several physiologically significant features of leaf photosynthesis. Rubisco studies reflected competition for the active site between the substrates and products, linearizing in vivo kinetics compared with the low-K in vitro responses. Rubisco Activase usually activates only a small part of the Rubisco, making the rest of it a storage protein. Precisely quantifying absorbed photons and the responding transmittance changes, electron flow rates through cytochrome bf, plastocyanin and photosystem I were measured, revealing competition between the proton-uncoupled cyclic electron flow from PSI to Cyt bf to P700 and the proton-coupled linear flow from PSII to Cyt bf to P700. Analyzing responses of O evolution and Chl fluorescence to ms-length light pulses we concluded that explanation of the sigmoidal fluorescence induction by excitonic connectivity between PSII units is a misconception. Each PSII processes excitation from its own antenna, but the sigmoidicity is caused by rise of the fluorescence yield of the Q-reduced PSII units after their Q site becomes occupied by reduced plastoquinone (or diuron). Unlike respiration, photosynthetic electrons must prepare their acceptor by coupled synthesis of 3ATP/4e. Feedback regulation of this ratio leads to oscillations under saturating light and CO, when the rate is P-limited. The slow oscillations (period 60s) indicate that the magnitudes of the deflections in the 3ATP/4e ratio, corrected by regulating cyclic and alternative electron flow (including the Mehler type O reduction), are only a fraction of a per cent. The P limitation causes slip in the ATP synthase, slightly increasing the basic 12H/3ATP requirement.