Our recent research has revealed that most chlorophyllous explants/plants in vitro including cotyledonary stage somatic embryos have the ability to grow photoautotrophically (without sugar in the culture medium), and that the low or negative net photosynthetic rate of plants in vitro is not due to the poor photosynthetic ability, but because of the low CO₂ concentration in the air-tight culture vessel during the photoperiod. Furthermore, we have shown that the photoautotrophic growth in vitro of many plant species can be significantly promoted by increasing the CO₂ concentration and light intensity in the vessel, by decreasing the relative humidity in the vessel, and by using a fibrous or porous supporting material with high air porosity instead of gelling agents such as agar. Recently, a photoautotrophic or sugar-free medium micropropagation system (PAM) consisting of five large culture vessels (volume: 120 l each) with a forced ventilation unit for supplying CO₂-enriched air was developed and applied for the commercial production of calla lily (Zantedeschia elliottiana) and China fir (Cunningghamia lanceolata) plantlets in China. The culture period of calla lily plantlets in the PAM was reduced by 50%, compared with that in a conventional, photomixotrophic micropropagation system (PMM) using small vessels each containing a sugar-containing medium. Percent survival ex vitro of calla lily plantlets from the PAM was 95%, while that from the PMM was 60%. The production cost of calla lily in the PAM was reduced by about 40%, compared with that in the PMM, and the initial investment per plantlet for the PAM was about 10% lower than that for the PMM. The sales price of ex vitro acclimatized calla lily plantlet was increased by 25% due to its higher quality, compared with plantlets produced in the PMM.

Key words: CO₂ enrichment, Cunningghamia lanceolata, forced ventilation, natural ventilation, production cost, porous supporting material, sugar-free medium, Zantedeschia elliottiana.