2017 ASHS Annual Conference

This study was conducted to determine the effects of relative humidity (RH) and nighttime CO₂ concentration control on the photosynthetic characteristics of young Phalaenopsis orchids. In RH experiment, young Phalaenopsis ‘Blanc Rouge’ clones, with 2-3 fully developed leaves, were grown under three different RH conditions (30, 50, and 70%). In CO₂ experiment, young Doritaenopsis ‘Mantefon’ clones, with 2-3 fully developed leaves, were grown under four different nighttime CO₂ concentration conditions [400 (control), 900, 1500, and 2100 ppm]. Each treatment was maintained for 6 months in environment-controlled growth chambers. In RH experiment, the highest rate of CO₂ uptake was observed in the 70% treatment, followed by the 50 and 30% treatments. The stomatal conductance increased with increasing RH. In CO₂ experiment, the amount of CO₂ that was absorbed during nighttime was the highest under 900 ppm of CO₂ concentration after 2 weeks of treatment. However, the CO₂ uptake of a new leaf, newly developed after treatments, significantly increased with increasing CO₂ concentration. The highest rate of CO₂ uptake of a new leaf was observed in the 2100 ppm treatment, but there were no significant differences among 900, 1500, and 2100 treatments. In CO₂ exchange-CO₂ concentration curve, CO₂ uptake of a new leaf was saturated above 900 ppm in the 900, 1500, and 2100 ppm treatments, while the CO₂ uptake decreased above 1200 ppm in the control treatment. These results indicate that RH and nighttime CO₂ concentration control can improve the photosynthesis of Phalaenopsis orchids. In this study, 70% of RH and 900 ppm of nighttime CO₂ concentration are recommended for Phalaenopsis cultivation.