Memory of prolonged Winter cold inhibits flowering and increases long-day leaf number in the chrysanthemum cultivar ‘Nagano Queen’

SummaryChrysanthemum (Chrysanthemum morifolium Ramat.) is a short-day plant; however, flowering is eventually initiated under non-inductive long-day or night-break conditions. When using Summer-to-Autumn-flowering cultivars, such flowering is undesirable for stable horticultural production, particularly for Summer cropping, because of the short length of the cut flower, the formation of a crown bud, and the development of a branched spray of flowers. Under non-inductive conditions, flowering depends on the number of leaves that form below the terminal flower bud (i.e., the long-day leaf number). Long-day leaf numbers varied among cultivars in this study. ‘Nagano Queen’ showed a seasonal change in long-day leaf number, being high in Spring and decreasing in Summer. In addition, rooted cuttings under night-break conditions initiated very early flowering. Here, we show how exposure to prolonged Winter cold influenced long-day leaf number, and its inhibitory effect on flowering. Cold pre-treatments inhibited flower initiation under subsequent short-day conditions, with noticeable genetic variation among cultivars. For instance, flower initiation in ‘Nagano Queen’ was strongly inhibited in Spring, but the inhibition decreased in Summer. This result indicated that the inhibitory effect of Winter cold caused a delay in Spring flowering, which gradually declined from Spring to Summer. Exposure to prolonged Winter cold also inhibited flower initiation under long-day and night-break conditions, and increased long-day leaf number in Spring. These results indicate that this seasonal change in the inhibitory effect of Winter cold influences long-day leaf number in ‘Nagano Queen’.