Low temperature diminishes the photoperiodic flowering response of three petunia cultivars

Many common petunia (Petunia xhybrida) cultivars have a quantitative long-day response, so when the natural day length is short, commercial growers often deliver low-intensity lighting to accelerate flowering. We grew three petunia cultivars in controlled-environment greenhouses at 16 or 21 degrees C under a short day (SD, 10 h d(-1)) or long day (LD, 16 h d(-1)), each with the same photosynthetic daily light integral, to determine whether temperature influenced the photoperiodic flowering response. Flowering was incomplete when 'Wave Purple Classic' ('WPC') was grown under SDs whereas essentially all 'Easy Wave Neon Rose' ('EWNR') and 'Wave Purple Improved' ('WPI') plants flowered in all treatments. Petunia 'EWNR' and 'WPI' flowered 31 or 59 d earlier, respectively, when grown at 21 degrees C with LDs compared to 16 degrees C with SDs. Temperature had a greater effect on flowering time under LDs than SDs, and the LD promotion of flowering was less at 16 degrees C than at 21 degrees C. For example, growing plants at 16 versus 21 degrees C delayed flowering by 18% under SDs and 95% under LDs in 'EWNR', and in 'WPI', flowering was delayed by 15 or 110% under SDs or LDs, respectively. These results indicate that the LD promotion of flowering is diminished when petunia is grown at a low temperature, at least for the three cultivars studied.