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

In vitro plantlets of Phalaenopsis 'Happy Valentine, Neofinetia falcate Hu, Cymbidium kanran Makino, and Cymbidium, goeringii Reichb. f. were grown un der photoautotrophic [high photosynthetic photon flux (PPF), high CO2 conce ntration, and increased number of air exchanges] and heterotrophic (low PPF , low CO2 concentration, no air exchanges) culture conditions. After 40 d o f culture, a significant difference in plantlet growth was observed between the two cultures. Total fresh and dry mass were on average 1.5 times great er in photoautotrophic culture than in heterotrophic culture. Higher net ph otosynthetic rates, were also, observed for Phalaenopsis in photoautotrophi c culture. In photoautotrophic culture, little difference was observed in a ir temperature between the inside and outside of the culture vessel, wherea s 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. T hese 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 orchi d plants.