The effects of day temperature (DT), night temperature (NT) and photosynthetic photon flux (PPF), on rate of development and flower size were studied in chrysanthemum (Dendranthema grandiflora Tzvelev. cultivar 'Bright Golden Anne'). DT and NT ranged from 10 to 30°C and PPF from 1.8 to 21.6 mol day-1m-2. Flower initiation did not occur after 100 short days (SD) at low PPF levels (1.8 mol day-1m-2) in combination with high DT or NT (30°C). The number of days to flower varied from 58 to 140 days among plants grown under environmental conditions allowing flower initiation within 100 SD. The time to flower from start of SD decreased nonlinearly as PPF increased. Increasing PPF by 9.9 mol day-1m-2 at 20°C accelerated flowering 20 days when the initial PPF was 1.8 mol day-1m-2, but only 10 days when the initial PPF was 11.7 mol day-1m-2. The DT and NT for most rapid flower development were estimated from a model predicting time to flower. Independent of PPF in the range from 2 to 20 mol day-1m-2, the optimum DT was 17°C and the optimum NT was 18°C. Total flower area per plant varied from 14 to 310 cm2. The flower size increased linearly as PPF increased from 1.8 to 21.6 mol day-1m-2 at a constant temperature of 20°C. The optimum DT NT combination for largest flower size changed from 21 14° to 20 18°C as PPF increased from 5 to 20 mol day-1m-2.
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