The effective light position for promoting cyclamen flowering

Cyclamen (Cyclamen persicum), recognized as a quantitative long-day plant, demonstrates promoted flowering through night-interruption (NI) treatment. The optimal spacing of light sources is a critical consideration for growers. Over-installing light sources increases production costs, while spacing the light sources too far apart reduces the effectiveness of NI due to reduced photon flux density. We identified the threshold photon flux density of NI for the flowering of four cyclamen cultivars. The NI treatments were applied for 4 h from 10 pm to 2 am, with the following photon flux density: 1) <0.1 μmol m^-2 s^-1, 2) 0.1-1 μmol m^-2 s^-1, 3) 1-3 μmol m^-2 s^-1, 4) 3-5 μmol m^-2 s^-1. In C. persicum ‘Compact’, there was a decrease in the days to visible buds with a higher photon flux density of NI. The number of VB increased with photon flux density above 1 μmol m^-2 s^-1. Thus, C. persicum ‘Compact’ required a sustained photon flux density above 1 μmol m^-2 s^-1 for effective NI. In C. persicum ‘Macro Pastel’, the day of VB and flowering were hastened as the photon flux density of NI increased, with photon flux density above 1 μmol m^-2 s^-1 or more. In the case of cyclamen ‘Laser’ and ‘Swan’, NI had no effect in accelerating flowering, but the number of VBs was significantly increased when NI was treated with photon flux density above 1 μmol m^-2 s^-1. We also investigated the photon flux density on the ground surface at heights of 50, 100, and 150 cm. For optimal light uniformity, it is recommended to position the light source at a height of 150 cm. During NI treatment, the light source should be spaced no more than 1.2 m apart to maintain a photon flux density of 3 μmol m^-2 s^-1 or higher. To maintain a photon flux density of 1 μmol m^-2 s^-1 or higher, the spacing should be reduced to within 1.6 m. Proper adjustment of the light source spacing is essential to achieve the desired light intensity.