Abstract
Chrysanthemums (Dendranthema indicum cultivars) are grown worldwide as a year round cut flower crop. Since the 1960's photoperiodic lighting and darkening can program plants very precisely. Recently, there has been an increase in the use of supplementary lighting for chrysanthemums with HPS lamps being also used for night break lighting. The traditionally used incandescent lighting system provides a photosynthetically active photon integral of 7200-8600 μmol m-2 per night. From experiments with a gradient of lighting from a photosynthetic photon flux (PPF) of 0.02 μmol m-2 s-1 to 3.5 μmol m-2 s-1 it was clear that depending on the season 1000 to 4000 μmol m-2 per night was enough when the night break was given after 8 hours of darkness. In order to provide the same amount of light with an HPS system as the cyclic incandescent system, 8 minutes was determined to be sufficient. This was tested and validated at a commercial chrysanthemum propagator, particularly with cultivars having a high longday leaf number.
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