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

• The minimum daily light integral (DLI) was determined for an early- and late-flowering R. simsii cultivar.
• Whole-plant photosynthesis measurements resulted in a minimum DLI of 2.1 (early-flowering cultivar) and 1.7 mol m−2 d−1 (late-flowering cultivar).
• Increasing temperature by 6 °C increased minimum DLI by 72% for the late cultivar.
• Developmental stage of the flower bud did not affect the minimum DLI.

Forcing of florist azalea is a relative short, though important step in the production cycle of this ornamental. This phase determines the flowering quality which is important for the economic value of the plant. Forcing is performed inside heated greenhouses (21 °C), and application of supplemental light is increasing in popularity among growers. Optimizing the use of supplemental light is essential to minimize energy costs. Here, the minimum daily light integral (DLI) needed to balance photosynthesis and respiration for two Rhododendron simsii cultivars was determined. Whole-plant net CO2 exchange measurements resulted in a minimum DLI for photosynthesis of 2.1 mol m−2 d−1 for the early-flowering cultivar ‘Nordlicht’ and of 1.7 mol m−2 d−1 for the late-flowering cultivar ‘Thesla’. Increasing the forcing temperature by 6 °C resulted in a 72% increase of the minimum DLI for photosynthesis of ‘Thesla’. The developmental stage of the flower buds had no effect on the minimum DLI. Further, forcing experiments showed that to obtain a high-quality flowering plant, this minimum DLI for photosynthesis must be raised by at least 57%.