Efficient Water Management for Cymbidium Grown in Coir Dust Using a Soil Moisture Sensor-Based Automated Irrigation System

Efficient long-term management for the production of high-quality Cymbidium plants is required as these orchids generally require 3–4 years of vegetative growth to allow flowering. This study was conducted to investigate the optimal substrate moisture levels to efficiently produce young cymbidium using a soil moisture sensor-based automated irrigation system over 42 weeks of vegetative growth. One-year-old cymbidium “Hoshino Shizuku” plantlets were grown in coir dust substrate at four levels of volumetric water content (0.25, 0.35, 0.45, and 0.55 m3·m−3). At harvest, the numbers of leaves and pseudobulbs, and the chlorophyll content of the cymbidiums did not differ among the four θ threshold treatments. However, plants grown at 0.25 m3·m−3 had significantly smaller leaves, pseudobulbs, and biomass than those at the other θ threshold treatments. Although the lower θ decreased the photosynthetic parameters, such as the net photosynthesis, stomatal conductance, and transpiration, there were no differences in the maximum quantum yield of photosystem II, indicating that the reduction in net photosynthesis is mostly mediated by stomatal closure. Although the net photosynthesis at θ of 0.35 m3·m−3 was also lower than that at 0.55 m3·m−3 treatment, biomass was significantly lower only at 0.25 m3·m−3 treatment, suggesting that a critical growth reduction by the water deficit occurred for the cymbidium at 0.25 m3·m−3. As the θ threshold increased, the total irrigation amount significantly increased, which inversely decreased the water use efficiency. Although the plants grown at 0.25 m3·m−3 had the highest water use efficiency (WUE) and substrate electrical conductivity they showed significantly reduced growth compared to other θ threshold treatments, and thus this was not a reliable θ threshold level for producing high (visual) quality cymbidium. Overall, the 0.35 and 0.45 m3·m−3 threshold treatments provided appropriate moisture levels for high-quality cymbidium production with high water use efficiency.