Does “blue“ light invariably cause plant compactness? Not really: a comparison with red light in four bedding plant species during the transplant stage

To clarify plant elongation response to blue light associated with varying phytochrome activity, the growth and morphology traits of petunia, calibrachoa, geranium, and marigold transplants were compared under six light quality treatments: 1) R, pure red light (660 nm); 2) B, pure blue light (455 nm); and 3) BRF0; 4) BRF2; 5) BRF4; and 6) BRF6, unpure blue light created by mixing B with low-level (6%) R, and further adding 0, 2, 4, and 6 μmol m^‑2 s^‑1 of flar-red light (735 nm), respectively. Plants received continuous (24-h) light-emitting diode lighting with a 100-μmol m^‑2 s^‑1 photosynthetic photon flux density. After 22-35 d of light treatments, B, compared to R, promoted elongation growth for petunia and calibrachoa, as demonstrated by a greater stem extension rate, hypocotyl length, and petiole length. BRF0 showed similar or inhibitory effects on these traits relative to R, while BRF6 exhibited similar effects as B in most cases. The calculated phytochrome photostationary state (PPS) value, indicating phytochrome activity, gradually decreased in the order of R (0.89), BRF0 (0.69), BRF2 (0.65), BRF4 (0.63), BRF6 (0.60), and B (0.50). Generally, with the decreasing PPS values, the elongation growth under “blue” (pure and unpure) lights increased, showing a higher sensitivity for petunia and calibrachoa than geranium and marigold. However, the elongation promotion by “blue” light gradually became saturated once the PPS values decreased below 0.60, which indicated a switch to deactivated phytochrome. This suggests that for bedding plant seedlings, “blue” light promotion of elongation growth is related to the lower phytochrome activity levels, with varying sensitivity among species, and that deactivated phytochrome contributes to the maximum elongation promotion by “blue” light.