Abstract
The relationship between the critical day length and the most effective night-break (NB) time on the depression of flower bud initiation in chrysanthemum was investigated using five cultivars of summer-autumn (SA) flowering types: ‘Floral Yuka’, ‘Iwanohakusen’, ‘Southern Grape’, ‘Southern Chelsea’, and ‘Southern Pegasus’, and five of autumn (A) flowering types: ‘Jinba’, ‘Sanyo Ougon’, ‘Yukihime’, ‘Hakusui’, and ‘Shuhouno-chikara’, in Japan. The critical day length (night length) of the SA flowering types was found to be 15 (9)–16 (8) h during the August flowering and cropping period. The duration from dusk to NBmax, which is the most effective NB time, was 6.5–8.5 h when one hour of NB was included. On the other hand, the critical day length (night length) of the A cultivars was 13 (11) h during the December flowering and cropping period. The duration from dusk to NBmax was 9.0–10.5 h when 40 min of NB was included. Similarly, the SA cultivars with a short critical dark length had a shorter dusk-NBmax period compared to the A cultivars with a similar critical dark length and dusk-NBmax period. The results suggest that the dusk-NBmax duration is associated with the critical night length. The present study demonstrates that the critical day length (night length) is closely associated with the dusk-NBmax duration, thereby suggesting that estimation of the most effective night-break time for each cultivar is possible if the critical day length is ascertained.
