Determining the minimum daily light integral for forcing of azalea (Rhododendron simsii)

Forcing of florist azalea is a relative short, though important step in the production cycle of this orna-mental. This phase determines the flowering quality which is important for the economic value of theplant. Forcing is performed inside heated greenhouses (21◦C), and application of supplemental lightis increasing in popularity among growers. Optimizing the use of supplemental light is essential tominimize energy costs. Here, the minimum daily light integral (DLI) needed to balance photosynthesisand respiration for two Rhododendron simsii cultivars was determined. Whole-plant net CO2exchangemeasurements resulted in a minimum DLI for photosynthesis of 2.1 mol m−2d−1for the early-floweringcultivar ‘Nordlicht’ and of 1.7 mol m−2d−1for the late-flowering cultivar ‘Thesla’. Increasing the forcingtemperature by 6◦C resulted in a 72% increase of the minimum DLI for photosynthesis of ‘Thesla’. Thedevelopmental stage of the flower buds had no effect on the minimum DLI. Further, forcing experimentsshowed that to obtain a high-quality flowering plant, this minimum DLI for photosynthesis must be raisedby at least 57%.