A scheme to account quantitatively for the action of phytochrome in etiolated and light‐grown plants

Abstract

Abstract A scheme to account for the phytochrome responses of dark grown and green plants is presented. In contrast to some other models (e.g. Smith, 1970; Schäfer, 1975) it is proposed that both Pfr and the photoconversions of phytochrome are effectors of phytochrome action and interact to promote the observed responses. Under conditions where Ptot is not strongly wavelength dependent (e.g. with short pulses of light and in plants grown for prolonged periods in the light), responses will be predominantly related to Pfr concentration. Where Ptot is the major wavelength dependent variable (e.g. in the ‘high irradiance reaction’ of etiolated seedlings) it is proposed that the photoconversion process itself predominates in controlling the ultimate response. Some responses predicted by the scheme are compared with relevant published data.