High photosynthetic photon flux and high CO2 concentration under increased number of air exchanges promote growth and photosynthesis of four kinds of orchid plantlets in vitro

In vitro plantlets of Phalaenopsis 'Happy Valentine, Neofinetia falcate Hu, Cymbidium kanran Makino, and Cymbidium, goeringii Reichb. f. were grown under photoautotrophic [high photosynthetic photon flux (PPF), high CO2 concentration, and increased number of air exchanges] and heterotrophic (low PPF, low CO2 concentration, no air exchanges) culture conditions. After 40 d of culture, a significant difference in plantlet growth was observed between the two cultures. Total fresh and dry mass were on average 1.5 times greater in photoautotrophic culture than in heterotrophic culture. Higher net photosynthetic rates, were also, observed for Phalaenopsis in photoautotrophic culture. In photoautotrophic culture, little difference was observed in air temperature between the inside and outside of the culture vessel, whereas in heterotrophic culture, air temperature inside the culture vessel was 1-2 degreesC higher than that outside the culture vessel. Relative humidity inside the culture vessel was remarkably different between the two cultures: 83-85% in photoautotrophic culture and 97-99% in heterotrophic culture. These results indicated that growth and net photosynthetic rate of in vitro orchid plantlets were susceptible to the culture environments such as PPF, CO2 concentration, relative humidity (RH), and the number of air exchanges, which would allow a more efficient micropropagation system for these orchid plants.