Redox State of Photosynthetic Ferredoxin Under Heat and Light Stress

The kinetics of oxidation/reduction of the primary donor of photosystem 1 (PS1) P700, plastocyanin (PC), and ferredoxin (Fd) in the first leaves of barley seedlings under exposure to high-intensity light (2000 μmol∙quanta∙m^–2·s^–1, 30 min) and elevated temperature (40°C, 3 h) were studied using differential absorption photometry. Exposure to high-intensity light increased the accumulation of the oxidized form of PS1 reaction center P700⁺, oxidized PC, and reduced Fd. Reduced Fd was not reoxidized under the same conditions on exposure to red and far-red light. The accumulation of P700⁺, PC⁺, and Fd ^– increased in barley seedlings exposed to elevated temperatures. Also, reoxidation of leaf Fd accelerated under red light. Oxidized Fd did not accumulate under far-red light. It was concluded that photo-independent electron flow through Fd under light stress and alternative electron flows involving the plastoquinone pool under heat stress were activated.