Blue radiation attenuates the effects of the red to far-red ratio on extension growth but not on flowering

Abstract

Phytochromes, which primarily absorb red (R; 600–700 nm) and far-red (FR; 700–800 nm) radiation, and cryptochromes, which primarily absorb blue (B; 400–500 nm) radiation, coordinately control plant morphological and developmental traits, including extension growth and flowering. However, the effects of B and the R:FR on plant responses have usually been investigated independently, and how they interact to regulate plant traits has not been well characterized. We postulated that B radiation would attenuate the effect of the R:FR on extension growth, and with FR, promote flowering. We grew seedlings of petunia (Petunia ×hybrida), geranium (Pelargonium ×hortorum), and coleus (Solenostemon scutellariodes) at 20 °C under six sole-source light-emitting diode treatments with an 18-h photoperiod. The treatments delivered the following photon flux densities (subscript in μmol·m−2·s−1) of B (peak = 447 nm), R (peak = 660 nm), and/or FR radiation (peak = 731 nm): B80R80, B80R80FR10, B80R80FR80, R160, R160FR20, and R160FR160. Seedlings were then transplanted and subsequently grown in a common greenhouse environment at 20 °C with a 16-h photoperiod. Seedling height 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 radiation (B80R80), seedling height of all species also increased linearly with decreasing PPE, but substantially less (55–86%) than under R160. In all species, shoot dry weight decreased linearly with increasing PPE similarly under R160 and B80R80. In the long-day plant petunia, decreasing the PPE promoted subsequent flowering by 7–11 d, regardless of the presence of B radiation. We conclude that a moderately high B photon flux density attenuates the effects of the R:FR on extension growth but has no apparent effect on the FR promotion of flowering in a long-day plant. This suggests that cryptochrome-mediated B signaling dominates phytochrome-mediated FR signaling in the regulation of extension growth, but not flowering.