Comparative analysis of phytochrome-mediated growth responses in internodes of dwarf and tall pea plants

Responses of dark-grown pea seedlings of a dwarf (Progress No. 9) and a tall (Alaska) cultivar to red light have been compared 24 hours after irradiation. Identical percentage inhibition of elongation in 10-mm sub-apical segments of intact third internodes, identical dose-response curves and similar reversal of the inhibition by far-red light were observed. The time courses of the growth inhibition were quantitatively dissimilar in that seedlings of Alaska showed greater inhibition soon after light treatment, but greater recovery at later stages.Small varietal differences in phytochrome concentrations in comparable parts of the apical region correlate with early varietal differences in the growth response. Seedlings of Progress showed considerable far-red reversal of the red-light effect even with an extended time lapse between red and far-red irradiations, while decay of far-red reversibility was comparatively rapid in Alaska seedlings. It is concluded that there is no difference in energies required to activate the phytochrome system in the two varieties, but that the Pfr form of phytochrome probably remains stable and active in Progress but decays rapidly in Alaska seedlings.