Control of Flowering in Petunia by Photoperiod and Irradiance

Abstract

Petunia `Midnight Madness' plants were grown for 4 weeks starting 3 weeks after seeding, at 8 or 16 hours photoperiod and 3, 7.5, or 12 mol·d–1·m–2. The temperature was 20 ± 1°C throughout the study. The plants were allowed to flower following the 4 weeks photoperiod treatment at 16 hours of 6 mol·d–1·m–2. Petunias grown at long days flowered (first open flower) faster than those exposed to 8 hours day length for 4 weeks. Plants grown at short days required 8 to 10 more days for flowering compared to plants grown at the same irradiance delivered during a 16-hour day. Flowering was first observed 61 ± 0.9 days from seeding for the plants at long days and 12 mol·d–1·m–2. Plants grown at 8 hours and 3 mol·d–1·m–2 required on average 84 ± 0.8 days from seeding to reach flowering. The number of flowers and flower buds (10 ± 1.8 flower buds) was lower for plants grown at 12 mol·d–1·m–2 independent of day length. There were no significant differences in the number of flower buds (16 ± 2.6 flower buds) at termination of the experiment for the plants grown at the two lower irradiance levels for either 8 or 16 hours day length.