It is well established that phytochrome exists as a dimer in vitro. A comparison of the relative photoequilibrium concentrations of P^Pr, P^Pfr and PfrPfr. witli the relative sizes of the Pf^-pools which undergo dark reversion in the intact plant, leads to the hypothesis that phytochrome also exists as a dimer in vivo. This hypothesis is in accordance with kinetic properties of the phytochrome system under continuous irradiation. Additional support for this view is provided by the observation that Pf^-destruetion after a red light flash, which should favour the for-mation of PfPfr dirners, is paralleled by a decay of P^, even if the presence of P, cycled through P,, can be excluded. Preliminary observations could indicate an interaction of the subunits of a phytochr-ome dimer during the process of phototransformation.
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