Growth and CO 2 exchange in young Phalaenopsis orchids grown under different levels of humidity during the vegetative period

Phalaenopsis plants are distributed throughout tropical regions with high humidity, which is generally necessary for their cultivation. This study was conducted to determine the effects of different relative humidity (RH) conditions on the growth and photosynthetic characteristics of young Phalaenopsis plants. Three-month-old clones of Phalaenopsis ‘Blanc Rouge’ and Doritaenopsis ‘Mantefon’ plants were grown under 30 ± 10, 50 ± 10, 70 ± 10, or 90 ± 10% RH conditions. The RH treatments were maintained for approximately 6 months in environment-controlled growth chambers. The temperature and photoperiod were maintained at 28 °C and 12/12 h, respectively. At the start of the RH treatments, the mean leaf span was 14.2 and 16.7 cm in ‘Blanc Rouge’ and ‘Mantefon’ plants, respectively. In both cultivars, plants grown under 70% RH showed the longest leaf span, which was 25.3 and 20.6 cm in ‘Blanc Rouge’ and ‘Mantefon’, respectively. High-humidity conditions (i.e. 70 and 90% RH) generally promoted the length of leaf span, but the growth rate of leaf span was slightly lower at 90% RH than at 70% RH. CO2 exchange, stomatal conductance, and transpiration rate of the uppermost mature leaf were observed in ‘Blanc Rouge’ plants grown at 30, 50, and 70% RH. The highest rate of CO2 uptake was observed in the plants grown at 70% RH, followed by 50 and 30% RH. The stomatal conductance increased and transpiration decreased with increasing RH. The maximum quantum yield of photosystem II (Fv/Fm) of plants grown under 70 and 90% RH slightly decreased after 9 weeks of treatments, but there were no differences among the treatment groups at the conclusion of the study. These results indicated that high RH (e.g. 70%) slightly increases CO2 uptake and leaf span of young Phalaenopsis plants.