The effect of night temperature on chrysanthemum flowering: heat-tolerant versus heat-sensitive cultivars

The effect of night temperature on the flowering of heat-tolerant and heat-sensitive cultivars of potted chrysanthemum (Chrysanthemum xgrandiflorum) was examined in four experiments over a period of 4 years. Temperature reductions were imposed only while the plants were under black cloth using a combination of air-conditioning and under-cloth ventilation. The two heat-sensitive cultivars tested were 'Yellow Mandalay' and 'Coral Charm' and the two heat-tolerant cultivars were 'Iridon' and 'Dark Bronze Charm'. Differences in time-to-flower (TTF) between heat tolerance classifications were less than anticipated. TTF was affected the most in 'Iridon', a heat-tolerant cultivar, decreasing by an average of 4.2 days/degrees C as mean diurnal temperatures (MT) decreased from about 26 degrees C to about 23 degrees C. TTF was affected the least in 'Coral Charm', a heat-sensitive cultivar, decreasing by an average of 2.8 days/degrees C over the same range. Inflorescence diameter, on the other hand, increased by as much as 9% in the two heat-sensitive cultivars but by only about 4% in the heat-tolerant cultivars. The results suggest that the heat-tolerant cultivars tested here may have been classified based on consistency of flower quality rather than TTF. (C) 2000 Elsevier Science B.V. All rights reserved.