HIGH NIGHT TEMPERATURES DELAY FLOWERING, PRODUCE ABNORMAL FLOWERS AND RETARD STEM GROWTH OF CUT-FLOWER CHRYSANTHEMUMS

The responses of chrysanthemum (Dendranthema grandiflora Tzvelev) cultivars 'Polaris' and 'White Marble' to night temperatures of 32 degrees C depended on the duration of exposure and the time of transfer to the high night temperature. Transfer for 7 nights at the start of short-day treatment (SD) delayed the onset of flower bud initiation, while transfer for longer, delayed the completion of flower bud formation. When transfer was made for 42 nights from the start of SD, no florets were initiated at the extreme tip of the receptacle in either cultivar throughout the period of transfer. Transfers that delayed flower bud initiation also delayed anthesis, though the effect was generally less. The greatest delay in anthesis occurred when the high night temperature was given at the onset of floret initiation, i.e. between 14 and 21 SD. Transfer at this time also caused the production of more disc florets in 'Polaris' and the formation of bracts among the disc florets of 'White Marble'. Transfer to high night temperature at any stage induced foliar chlorosis and retarded stem extension, though final stem length was not significantly shortened. Attention is drawn to the need to reduce night temperature under black-out covers in summer, especially between 14 and 21 SD, and to some methods of achieving this.