Growth and Development of Potted Rhododendron Cultivars ‘Catawbiense Boursault’ and ‘Old Port’ in Response to Regulated Deficit Irrigation

The aim of this study was to analyse the effect of regulated deficit irrigation (RDI) on growth and development of potted rhododendron ‘Catawbiense Boursault’ and ‘Old Port’ plants and to evaluate the usefulness of this technique for saving water in nursery production and promoting flowering. Plants were grown in 1.9 litre plastic containers in unheated greenhouse and were subjected to six irrigation treatments lasting for 14 weeks from June to mid-September. A drip irrigation system with one 2 dm3·h-1 emitter per container was used. Six treatments of irrigation were applied: T1) 1 ETp (evapotranspiration) (control, well watered plants); T2) 0.75 ETp (moderate deficit irrigation); T3) 0.5 ETp (strong deficit irrigation) during the entire period of the experiment and the others three were: T4) 1 ETp for 5 weeks followed by 0.5 ETp for 4 weeks and 1 ETp for 5 weeks (strong deficit irrigation in phase II); T5) 1 ETp for 5 weeks followed by 0.25 ETp for 4 weeks and 1 ETp for 5 weeks (very strong deficit irrigation in phase II) and T6) 0.5 ETp for 5 weeks followed by 1 ETp for 4 weeks and 0.5 ETp for 5 weeks (strong deficit irrigation in phases I and III and well watered plants in phase II). The results showed that exposing plants to moderate water deficit (0.75 ETp) for 14 weeks had the best effect on quality of Rhododendron ‘Old Port’ plants. In this cultivar the application of very strong water deficit (0.25 ETp) for 4 weeks during floral buds initiation improved significantly floral bud set. Reduction of water supply by 50% during 14 weeks of Rhododendron ‘Catawbiense Boursault’ cultivation resulted in both the enhanced plant quality and increased number of floral buds set. Rhododendrons has adapted to reduced water supplies through stomatal control. At the end of the experiment, chlorophyll fluorescence parameter F_v/F_m (quantum efficiency of photosystem II) did not indicate damage to photosynthetic apparatus and relative chlorophyll content in leaves of plants subjected to all irrigation regimes did not differ significantly between each other.