Abstract
Factors involved in delayed flowering and the formation of abnormal capitula in four summer-to-autumn flowering cultivars of Chrysanthemum morifolium Ramat. were investigated. Anomalous involucral bracts, i.e. bracts formed among ray and disc florets within the capitulum, were more frequent in ‘Floral Yuka’ plants grown at day/night temperatures of 20/12.5°C than at 20/20°C under a 12 h day length or natural day length of 14–15 h. The number of involucral bracts in the capitulum of plants grown at either 35/25°C or 25/15°C increased compared with that of plants grown at 30/20°C under a 12 h day length. A natural long day length suppressed flower bud development and promoted the formation of many involucral bracts in the capitulum, compared with inflorescences of plants grown under a 12 h day length. Long day length of 14–15 h induced anomalous involucral bract formation in ‘Floral Yuka’ and ‘Iwa no Hakusen’. In contrast, high temperature promoted involucral bract formation in the capitulum, primarily in ‘Seiun’, and neither day length nor temperature affected involucral bract formation in ‘Natsubiyori’.
