Blue light associated with low phytochrome activity can promote flowering: a comparison with red light in four bedding plant species

Our previous study indicates that blue light associated with low phytochrome activity can promote elongation growth in four bedding plant species during vegetative growth stage, when compared to red light. To clarify the flowering response to blue light associated with varying phytochrome activity, the flowering traits of petunia (Petunia×hybrida 'Duvet Red'), calibrachoa (Calibrachoa × hybrida 'Kabloom Deep Blue'), geranium (Pelargonium × hortorum 'Pinto Premium Salmon'), and marigold (Tagetes erecta 'Antigua Orange') were investigated under six light quality treatments: 1) R, pure red light (660 nm); 2) B, pure blue light (455 nm); 3) BRF0; 4) BRF2; 5) BRF4; and 6) BRF6, unpure blue light created by mixing B with low-level (6%) R, and further adding far-red light of 0, 2, 4, and 6 μmol m^‑2 s^‑1, respectively. Plants received continuous (24-h) light-emitting diode lighting with a 100 μmol m^‑2 s^‑1 photosynthetic photon flux density and were kept at ≈23°C. B and BRF6 promoted flowering compared to R or BRF0, as demonstrated by an earlier flowering time for petunia and calibrachoa, and a greater flowering index, more visible flower buds, and more opened flowers for geranium and marigold. The promotion effect of unpure blue light on these traits increased following the order of BRF0, BRF2, BRF4 and BRF6, which varied in sensitivity among plant species. Also, a similar pattern of promotion by blue light was found in the flower size for calibrachoa, and in the canopy size for all the species. The calculated phytochrome photostationary state, which indicates phytochrome activity, was higher for R (0.89) than B (0.49), and decreased gradually for unpure blue light treatments: BRF0 (0.68), BRF2 (0.65), BRF4 (0.63), and BRF6 (0.60). This suggests that blue light associated with lower phytochrome activity can promote flowering, although there is a varying level of sensitivity among species.