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-degrees-C temperature range at intervals of 6-degrees-C. Plants of neither cultivar survived at 5-degrees-C, and growth was very slow at 11-degrees-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-degrees-C to a maximum of 0.229 leaves day-1 at 32-degrees-C. A linear function (leaves day-1 = -0.1130 + 0.01148.T) approximated the curvilinear relationship in the range from 10-30-degrees-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-degrees-C. To unfold one leaf, 87 degrees-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-degrees-C and leaf unfolding was calculated at 30-degrees-C at temperatures greater-than-or-equal-to 30-degrees-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.