RANGE OF PHOTOSYNTHETIC CONTROL OF POSTILLUMINATION P700(+) REDUCTION RATE IN SUNFLOWER LEAVES

The kinetics of the postillumination reduction of P700(+) which reflects the rate constant for plastoquinol (PQH(2)) oxidation was recorded in sunflower leaves at different photon absorption densities (PAD), CO2 and O-2 concentrations. The P700 oxidation state was calculated from the leaf transmittance at 830 nm logged at 50 mu s intervals. The P700(+) dark reduction kinetics were fitted with two exponents with time constants of 6.5 and about 45 ms at atmospheric CO2 and O-2 concentrations. The time constant of the fast component, which is the major contributor to the linear electron transport rate (ETR), did not change over the range of PADs of 14.5 to 134 nmol cm(-2) s(-1) in 21% O-2, but it increased up to 40 ms under severe limitation of ETR at low O-2 and CO2. The acceptor side of Photosystem I (PS I) became reduced in correlation with the downregulation of the PQH(2) oxidation rate constant. It is concluded that thylakoid pH-related downregulation of the PQH(2) oxidation rate constant (photosynthetic control) is not present under normal atmospheric conditions but appears under severe limitation of the availability of electron accepters. The measured range of photosynthetic control fits with the maximum variation of ETR under natural stress in C-3 plants. Increasing the carboxylase/oxygenase specificity would lead to higher reduction of the PS I acceptor side under stress.