Light‐quality effects on Arabidopsis development. Red, blue and far‐red regulation of flowering and morphology

Arabidopsis thaliana(L.) Heynh. race Columbia plants were grown in red. blue, red + far‐red, blue + far‐red and various light mixtures of red + blue + far‐red light under 14 h light/10 h dark photoperiods. Each single light source and light mixture maintained a constant irradiance (50 μmol m−2 s−1) and the mixtures of red + blue + far‐red maintained a constant ratio of red/far‐red light, but varied in the ratio of blue to red + far‐red light. Depending on the method used for calculation, values of the fraction of phytochrome in the far‐red absorbing form (Pfr/Ptot) for these light mixtures were either constant or decreased slightly with increasing percentage of blue light in the mixtures. Arabidopsis flowered early (20 days) in blue, blue + far‐red and red + far‐red light and late (55 days) in red light. In mixtures of red + blue + far‐red light, each of which established a nearly constant Pfr/Ptot flowering was in direct relation to time and irradiance level of blue light. Leaf area and petiole length were also correlated with blue light irradiance levels.