M.G. Karlsson

Irradiance and temperature effects on time of development and flower size in chrysanthemum

Abstract 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 −1 m −2 . Flower initiation did not occur after 100 short days (SD) at low PPF levels (1.8 mol day −1 m −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 −1 m −2 at 20°C accelerated flowering 20 days when the initial PPF was 1.8 mol day −1 m −2 , but only 10 days when the initial PPF was 11.7 mol day −1 m −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 −1 m −2 , the optimum DT was 17°C and the optimum NT was 18°C. Total flower area per plant varied from 14 to 310 cm 2 . The flower size increased linearly as PPF increased from 1.8 to 21.6 mol day −1 m −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 −1 m −2 .

Temperature driven leaf unfolding rate in Hibiscus rosa-sinensis

Leaf unfolding rate of Hibiscus rosa-sinensis cultivars ‘Brilliant Red’ and ‘Pink Versicolor’ was determined in the 5–35°C temperature range at intervals of 6°C. Plants of neither cultivar survived at 5°C, and growth was very slow at 11°C. The rate of leaf unfolding was similar for the two cultivars in the studied temperature range. Daily leaf unfolding rate for hibiscus was described by a cubic polynomial function of temperature (T) where leaves day−1 = 0.06289 − 0.02026 · T + 0.001750 · T2 − 0.00002983 · T3. Based on this function, predicted leaf unfolding varied from 0.012 leaves day−1 at 11°C to a maximum of 0.229 leaves day−1 at 32°C. A linear function (leaves day−1 = − 0.1130 + 0.01148·T) approximated the curvilinear relationship in the range from 10–30°C. A degree-day relationship was calculated to 0.0115 leaves per degree day using the linear function with a base temperature of 9.8°C. To unfold one leaf, 87 degree-days were required as determined by the developed linear model. A linear model was developed from the linear function where leaf unfolding was 0 at temperatures < 10°C and leaf unfolding was calculated at 30°C at temperatures ≥ 30°C. The cubic function and the linear model predicted a similar leaf unfolding rate based on hourly average temperatures recorded in a Florida commercial greenhouse during two times of the year. During a 78-day period from 25 February to 14 May, 9.95 and 9.98 leaves were predicted to unfold by the linear and cubic models respectively. During a 65-day period from 21 July to 24 September, 12.28 and 12.04 leaves were predicted to unfold by the linear and cubic models respectively. The predicted leaf unfolding rate was compared with actual leaf unfolding for the nine cultivars ‘Aloha Pink’, ‘Brilliant Red’, ‘Euterpe’, ‘Florida Sunset’, ‘Painted Lady’, ‘Pink Versicolor’, ‘Sundance’, ‘Tawny’, and ‘Vista’. Predicted leaf unfolding was within 0.9 leaves of observed leaf unfolding for all cultivars over a 7-week period after pinching. After about 7 weeks, the models progressively overpredicted leaf unfolding. Over-prediction was correlated with the appearance of flower buds.