Is Pfrthe 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.