The differences in the effects of low temperature and day length on the flowering within seed propagated Dianthus spp. and cultivars

Abstract

Seed-propagated annual dianthuses, D. x Mikadonadeshiko, D. barbalus, D. superbus var longicalicinus, and D. japonicus, are grown for cut flowers in Japan. Investigations on the flowering behavior in responses to chilling and to day length in these dianthuses were carried out. D. x Mikadonadeshiko and D. barbatus have a cold requirement for flowering as their representative cultivars Miss Biwako and Kurokawawase, and required exposure to natural chilling till mid December for flower bud formation and stem clongation. Plants grown above 14 °C remained vegetative, exhibiting no vernalizing effect. Long day length (LD) promoted flowering and stem clongation for the suboptimally or optimally vernalized plants, LD had no effect on flower bud initiation in non-vernalized plants. Non vernalized plants initiated flower buds under natural short days during the winter. On the other hand, D. superbus var longicalicinus and D. japonicus showed no cold requirement and were day neutral for flowering. Rosetted growth in winter was caused simply by low temperature which regresses stem elongation. Dianthus spp. which require low temperature possess a juvenile phase lasting 9 weeks from seeding by which time 9 to 10 leaf pairs are produced on the main stem. Seed vernalization was ineffective for these cultivars. In the youngest plants capable of being stimulated by low temperature, only the main stem bolted, the lateral shoots which were immature during vernalization remained vegetative even after vernalization. This demonstrated that the action of cold on flowering was limited to mature shoots. The differences in cold requirement among the six D. barbatus cultivars was also observed. The ever flowering cv Kyakko required no chilling, whereas others, such as cv Arima, required 9 or more weeks of chilling at 5°C to flower.