STEADY-STATE AND INDUCTION KINETICS OF THE PHOTOSYNTHETIC ELECTRON-TRANSPORT RELATED TO DONOR SIDE OXIDATION AND ACCEPTOR SIDE REDUCTION OFPHOTOSYSTEM-1 IN SUNFLOWER LEAVES

The photosynthetic CO2 exchange rate, chlorophyll fluorescence and P70 0 oxidation (absorption at 830 nm) were recorded in attached leaves of sunflower plants grown in soil by irradiance 460 mumol m-2 s-1. Durin g the photosynthesis in 510 cm3 m-3 CO2, 1 % O2, photosystem 1 (PS 1) efficiency was almost totally determined by the donor side oxidation u nder all irradiances. Fluorescence data showed that this control was e xercised mostly by DELTApH-dependent plastoquinol oxidation. At 1.5 mu M intracellular [CO2], the PS 1 reduction level on the acceptor side b ecame significant and increased as [CO2] --> 0. DELTApH controlled the electron flow when the rate exceeded the value of 50 mumol(e-) m-2 s- 1, which was close to the rate supporting photorespiration and CO2 rea ssimilation at CO2 compensation concentration in 21 % O2. The DELTApH ability to control the electron transport rate adequately prevented el ectron carriers reduction at PS 1 acceptor side, with stomata closed u nder stress. When the low CO2 or carbon reduction enzymes inactivation restricted the electron transport downstream of PS 1, open PS 1 centr es (oxidised on acceptor sides) percentage declined in proportion to C O2 uptake rate. This makes us doubt the common belief that the rapid e lectron transport in photosynthesis involves interconnected, mobile el ectron carrier pools of plastocyanin and ferredoxin (Fd). Rather, the implications are explored in terms of supercomplexes involving cytochr ome b6/f, PC, PS 1, Fd and Fd-NADP reductase.