Effects of interrupted lighting on the spray formation of summer-toautumn-flowering small-flowered spray-type chrysanthemum cultivars 'Haruka' and 'Subaru'

Summer-to-autumn-flowering small-flowered spray-type (SAFS) chrysanthemums (Chrysanthemum morifolium Ramat.) are commercially important in Japan, and there is an occasional increased demand for these flowers. However, under current cultivation methods, the quality of spray formations is compromised in order to maintain a sufficient production rate at times of peak demand. This study investigated the effect of interrupted lighting (IL), the intercalation of a period of long days (by night interruption) into the natural day length (NDL) period of growing, in order to regulate spray formations in SAFS chrysanthemum cultivars 'Haruka' and 'Subaru', in which lighting can markedly delay flowering. The effects of the NDL period before IL and those of the IL period were investigated. First, we investigated the effect of 2–12 days of NDL followed by 12 days of IL. Plants subjected to 2–6 days of NDL had longer flower clusters and a greater number of flower buds on the upper lateral flower stems and developed a broom-like spray. Next, we investigated the effect of 4– 20 days of IL after 4 days of NDL. Plants subjected to 8–20 days of IL had longer flower clusters, more flower buds on the upper lateral flower stems, and broom-like sprays. These changes were more pronounced when the IL periods were prolonged. Our results showed that spray formation in SAFS chrysanthemum cultivars 'Haruka' and 'Subaru' can be regulated by controlling the timing and period of IL.