Is P fr the active form of phytochrome?

Abstract

A selective but critical assessment of the published relations between spectrophotometric measurements of phytochrome parameters in vivo and physiological responses is presented. Although a number of correlations between response and the apparent concentration of P fr (i.e. [P fr ]) have been reported, these are counterbalanced by authenticated cases of lack of correlation. The reported relations between [P fr ] and response are not uniform, nor are they subject to ready interpretation. It seems useful to consider the responses of dark-grown and light-grown plants to be in principle different, with partly de-etiolated plants exhibiting intermediate responses. Dark-grown plants show high sensitivity to small changes in [P fr ] at low [P fr ], whereas light-grown plants show high sensitivity at high [P fr ]. Current data are not easily interpretable in terms of P fr being the only active component of the phytochrome system.