Carbon dioxide, a natural acidic gas, may be ušed for in vivo titration of the buffers in the compartments of cells that contain carbonic anhydíase in intact leaves. A gas systém for the measurement of the CO2 capacity (total inorganic carbon content) of leaves under different extemal CO2 concentrations has been developed. The systém consists of a leaf chamber and of two open (flow-through) channels, one for pre- loading the leaf with CO2 and the other for measurement of the desolubilized CO2. CO2 concentration in the loading channel may be adjusted from 0.03 to 20 %, while CO2 free air is flowing in the burst (desolubilization) channel. The leaf chamber (4.3 X 4.3 X 0.3 cm^) is switched either into the loading channel or into the burst channel. After loading the leaf with high CO2 in the dark the .chamber is switched into the burst channel and the desolubilized CO2 is measured. Preliminary measurements show that the CO2 capacity of a sunflower leaf decreases when the CO2 concentration is increased from 0.03 to 15 %, because of saturation of buffers. The systém can be ušed for direct, non-destructive measurements of pH and buffer capacity in leaf chloroplasts in order to investigate the influence of pH on photosynthesis, the operation of proton pumps and other pH-stabilising mechanisms.
The photosynthetic CO2 exchange rate, chlorophyll fluorescence and P700 oxidation (absorption at 830 nm) were recorded in attached leaves of sunflower plants grown in soil by irradiance 460 pmol m'2 s‘i. Ehiring the photosynthesis in 510 cm^ m*3 CO2, 1 % O2, photosystem 1 (PS 1) effíciency was ahnost totally determined by the donor side oxidation under all irradiances. Fluorescence data showed that this control was exercised mostly by ApH-dependent plastoquinol oxidation. At 1.5 pM intracellular [CO2], the PS 1 reduction level on the acceptor side became signifícant and increased as [CO2] -> 0. ApH controlled the electron flow when the rate exceeded the value of 50 pmol(e‘) m'2 s‘i, which was close to the rate supporting photorespiration and CO2 reassimilation at CO2 compensation concentration in 21 % O2. The ApH ability to control the electron transport rate adequately prevented electron carriers reduction at PS 1 acceptor side, with stomata dosed under stress. When the low CO2 oř carbon reduction enzymes inactivation restricted the electron transport downstream of PS 1, open PS 1 centres (oxidised on acceptor sides) percentage declined in proportion to CO2 uptake rate. This makes us doubt the common belief that the rapid electron transport in photosynthesis involves interconnected, mobile electron carrier pools of plastocyanin and ferredoxin (Fd). Rather, the implications are explored in terms of supercomplexes involving cytochrome b(Jf, PC, PS 1, Fd and Fd-NADP reductase.