Blue radiation signals and saturates photoperiodic flowering of several long-day plants at crop-specific photon flux densities

When the natural photoperiod is short, electric lighting can be used to promote flowering of a wide range of ornamental long-day plants (LDP) grown inside greenhouses. A combination of red (R; 600 -700 nm) and far-red (FR; 700 -800 nm) radiation is effective when delivered at a low intensity (1-2 mu molm(- 2) s(-1)), but recent research shows blue (B; 400 -500 nm) radiation can also be effective. We performed an experiment, replicated in time, that identified the B photon flux density that controlled flowering of four LDPs when delivered as a 4-h night interruption (NI) or 7-hour day extension (DE) during an otherwise 15-h night. Seedlings of four annual bedding plants were initially grown under a 9-h day, and then were transferred to one of seven lighting treatments, where subscripts indicate their photon flux densities: R + FR2-3 NI; B-5, B-15, or B-30 NI; and B-5, B-15, or B-30 DE. At a sufficiently high photon flux density, B radiation delivered as a 4-h NI or 7-h DE promoted flowering of all four LDPs. All species exhibited a dose-response relationship between B photon flux density and flowering time. The threshold B photon flux density above which flowering was promoted varied among the four LDPs, and was 5 mu mol m(-2) s(-1) for coreopsis ( Coreopsis grandiflora) and snapdragon ( Antirrhinum majus) and 15 mu mol m(-2) s(-1) for petunia (Petunia x hybrida) and rudbeckia ( Rudbeckia hind). A B photon flux density of 15 or 30 mu mol m(-2) s(-1) delivered as an NI or DE was usually as effective as 2 -3 mu mol m(-2) s(-1) of R + FR radiation for all LDPs tested. We conclude that while flowering of LDPs is more sensitive to R + FR than B radiation, relatively low B photon flux densities are perceived as a long day when delivered as a DE or NI.