Phytochrome intermediates in freeze-dried tissue

A new terminology for intermediates observed in the phototransformation of phytochrome (P) is proposed. At 0°C freeze dried tissue of Pisum sativum seedlings containing P in the red absorbing form (Pr) does not undergo complete photoconversion to the far red absorbing form (Pfr) but forms intermediates which revert to Pr in darkness. Use of low temperature techniques and comparison with reactions in fresh tissue allow identification of the intermediates with those observed earlier. They consist of a mixture of the first photoproduct, lumi-R and its dark relaxation product, meta-Ra. Similarly, Pfr forms a mixture of its first photoproduct, lumi-F and its dark relaxation product, meta-Fa. The dark reversion of lumi-R to Pr and of lumi-F to Pfr have also been observed in freeze-dried tissue. Reactions occurring in this material at 0°C are proposed to be essentially restricted to photoreactions of the chromophore and relaxations of the products. Before complete phototransformation of Pr to Pfr and vice versa is possible an aqueous matrix is required for protein/chromophore conformational relaxations to occur.