Inhibition of Flowering of Chenopodium rubrum by Prolonged Far-Red Radiation

Abstract

1. Chenopodium rubrum seedlings were used in a spectrographic analysis of flower inhibition by prolonged irradiation in the region of 700-800 mμ (farred). The seedlings were photoinductive as early as 2 days after emergence, and five 16-hour nights alternated with 8-hour days were sufficient to induce flowering. Several minutes of radiation in the region of 640 mμ (red) given near the middle of the 16-hour night completely inhibited flowering. When the red was followed immediately by several minutes of farred irradiation, flowering was repromoted. Prolonged irradiation with far-red, however, inhibited flowering, and the degree of inhibition varied with wavelength across the far-red region of the spectrum. 2. Comparison of far-red action-spectrum with phytochrome-absorption curves showed that the inhibitory action on flowering coincides with the weak overlapping of the phytochrome Pr (absorption maximum 660 mμ) absorption curve into the far-red region of the spectrum. A low level of phytochrome Pfr (absorption maximum 730 mμ) is maintained in the presence of continuous far-red radiation and is the effective inhibitor of flowering under prolonged far-red irradiation. 3. Molar absorbencies (ε) of the Pr and Pfr forms of phytochrome can be obtained from the results. Values of (εPr) and (εPfr) at various wavelengths in the region of 693-795 mμ are listed in table 6. They are compared in table 7 with values obtained from published results on control of flowering of Pharbitis nil (11). 4. Phytochrome in Chenopodium rubrum and Pharbitis nil under continued irradiation in the region of 730-790 mμ reaches a photochemical steady state with 1-2% Pfr present. Maintenance of this level for about 60 minutes is adequate to inhibit flowering. The rate of inhibition is independent of greater percentage transformation to Pfr under photochemical steady state.