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

The effect of night temperature on the ̄owering of heat-tolerant and heat-sensitive cultivars of potted chrysanthemum (Chrysanthemum xgrandi ̄orum) 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-tōower (TTF) between heat tolerance classi®cations were less than anticipated. TTF was affected the most in `Iridon', a heat-tolerant cultivar, decreasing by an average of 4.2 days/8C as mean diurnal temperatures (MT) decreased from about 268C to about 238C. TTF was affected the least in `Coral Charm', a heat-sensitive cultivar, decreasing by an average of 2.8 days/8C over the same range. In ̄orescence 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 classi®ed based on consistency of ̄ower quality rather than TTF. # 2000 Elsevier Science B.V. All rights reserved.