2018 ASHS Annual Conference

Blue (B, 400 to 500 nm) and far-red (FR, 700 to 800 nm) radiation have antagonistic effects on stem elongation; B typically suppresses extension growth while FR promotes it. Although the effects of B and FR radiation on plant growth have been investigated independently, little research has been published on how they interact to regulate extension growth and flowering. We grew seedlings of geranium (Pelargonium ×hortorum), petunia (Petunia ×hybrida), and coleus (Solenostemon scutellariodes) at 20 °C under six sole-source LED lighting treatments with an 18-h photoperiod. All treatments provided a photosynthetic photon flux density of 160 µmol∙m⁻²∙s⁻¹ with the following intensities (subscript in µmol∙m⁻²∙s⁻¹) of B (peak= 447 nm), red (peak= 660 nm), or/and FR radiation (peak= 731 nm): B₈₀R₈₀, B₈₀R₈₀FR₁₀, B₈₀R₈₀FR₈₀, R₁₆₀, R₁₆₀FR₂₀, and R₁₆₀FR₁₆₀. When seedlings were sufficiently large, they were transplanted into 10-cm pots and subsequently grown in a common greenhouse finishing environment at 20 °C with a 16-hour photoperiod. As expected, stem length of all species increased linearly with additions of FR [as the R:FR or estimated phytochrome photoequilibrium (PPE) decreased]. When R was partly substituted with B light (B₈₀R₈₀), stem length of shade-avoiding petunia and geranium also increased linearly with decreasing PPE, but substantially less (55-85%) than under R₁₆₀. In shade-tolerant coleus, there was little to no effect of PPE on stem elongation under B₈₀R₈₀. In geranium, shoot dry weight decreased linearly with increasing PPE similarly under R₁₆₀ and B₈₀R₈₀, while in petunia, similar trends occurred only under R₁₆₀. In the long-day plant petunia, decreasing the PPE promoted subsequent flowering by 7 to 11 d under B₈₀R₈₀ and R₁₆₀. In day-neutral geranium, plants grown under R₁₆₀FR₁₆₀ flowered earlier (by 4 d) than those grown under B₈₀R₈₀. We conclude that a moderately high intensity of B attenuates the effects of FR radiation on extension growth but has no apparent effect on the FR-promotion of flowering promotion in at least some long-day plants.