Light-induced rapid absorption changes during photosynthesis

Transient absorption changes at 430 mμ are induced in aged chloroplasts by red light flashes, The absorption change occurs in 10⁻⁴ sec or less and has a half life of approx. 10⁻² sec. The enhancement of the reaction by ascorbate and the abolishment by ferricyanide suggest that an oxidation reaction is responsible for the absorption change. In the presence of 3-(3,4-dichlorophenyl)-I,I-dimethylurea the absorption change is not observed, but the signal can be restored completely by adding ascrobate.

Available evidence from the decay kinetics of the 430-mμ transient absorption changes in the presence of ascorbate suggests that dichlorophenolindophenol at all concentrations reacts with cytochrome while phenazine methosulfate at low concentrations reacts with cytochrome and at concentrations greater than 3.10⁻⁵ M with P700 only. This in accord with the difference spectra observed in the blue region (where also cytochrome changes are observed) and also with the concominant and similarly decaying absorption changes at 430 and 703 mμ.

Light-intensity dependency of the complex reaction in aged chloroplast containing ascorbate and trace amount of phenazine methosulfate showed that at low intensity only the oxidation of the chlorophyll complex takes place. The coupling reaction between the pigment complex and cytochrome takes place only at higher light intensities.