EVIDENCE OF A REQUIREMENT FOR PHYTOCHROME-Pfr IN THE FLORAL INITIATION OF CHENOPODIUM RUBRUM

Abstract

Floral initiation of four Chenopodium rubrum selections was studied in daily photoperiods between 2 hours and continuous light.At 15 °C, decreasing or increasing the photoperiod from optimum to sub- or supra-optimal lengths resulted in progressively later floral initiation. The optimum photoperiod was shorter if the ratio of the red to far-red spectral energies was increased during the terminal portion or the whole of each photoperiod. A given photoperiod that was optimal with an intermediate R/FR ratio was sub- and supra-optimal with low and high R/FR ratios, respectively.A working hypothesis is presented in which the concentration of a floral hormone is postulated to be dependent on the formation of phytochrome-Pfr so that its level would be greatest in extreme long photoperiods and least in continuous darkness. Sub- and supra-optimal amounts of Pfr-dependent floral hormone would be produced in sub- and supra-optimal photoperiods, respectively, which result in later than optimal floral initiation. This concept provides a correlative scheme for optimal initiation in either short or long photoperiods.At 30 °C, ambiphotoperiodism was shown. Floral initiation in both short and extreme long photoperiods was earlier than in extreme short or intermediate photoperiods. In one selection, the earliest floral initiation was in intermediate photoperiods at 15 °C, but in extreme long photoperiods at 30 °C with a secondary optimum in intermediate photoperiods. Ambiphotoperiodism may be indicative of endogenously controlled rhythms interacting with the phytochrome system.Ability of the C. rubrum selections to flower in a wide range of photoperiods, including extremely long ones, is correlated with latitudinal origin, viz. 62°46′ N. > 60°47′ N. > 50°10′ N. > 34°20′ N. These differences demonstrate a role of phytochrome in ecotypic differentiation.