The effects of temperature on photophosphorylation and on the two forms of cytochrome 559 in subchloroplast particles

Two forms of cytochrome 559 have been demonstrated in chloroplasts, one of which is reducible by ascorbate (the A-form) and one of which cannot be reduced by ascorbate (the D-form). Analysis of the effects of temperature on the reduction of the cytochromes by light showed that the D-form was only observed between 20 and 40 °C. Below this range, only the A-form of the cytochrome was affected by light; the D-form remained oxidized. Also both electron transport and PMS⁺-photophosphorylation rates showed a sharp temperature transition at about 20 °C. Both electron transport and photophosphorylation are most sensitive to the inhibitor, CMU, at temperatures above 20 °C, and this coincides with the reduction of cytochrome 559 (D) which is extremely sensitive to CMU. Also, a transition temperature at 18 °C for fluorescent quenching was demonstrated.