In vitro growth and single-leaf photosynthetic response of Cymbidium plantlets to super-elevated CO2 under cold cathode fluorescent lamps

To examine the effectiveness of super-elevated (10,000 μmol mol⁻¹) CO₂ enrichment under cold cathode fluorescent lamps (CCFL) for the clonal propagation of Cymbidium, plantlets were cultured on modified Vacin and Went (VW) medium under 0, 3,000 and 10,000 μmol mol⁻¹ CO₂ enrichment and two levels of photosynthetic photon flux density (PPFD, 45 and 75 μmol m⁻² s⁻¹). Under high PPFD, 10,000 μmol mol⁻¹ CO₂ increased root dry weight and promoted shoot growth. In addition, a decrease in photosynthetic capacity and chlorosis at leaf tips were observed. Rubisco activity and stomatal conductance of these plantlets were lower than those of plantlets at 3,000 μmol mol⁻¹ CO₂ under high PPFD, which had a higher photosynthetic capacity. On the other hand, plantlets on Kyoto medium grown in 10,000 μmol mol⁻¹ CO₂ under high PPFD had a higher photosynthetic rate than those on modified VW medium; no chlorosis was observed. Furthermore, growth of plantlets, in particular the roots, was remarkably enhanced. This result indicates that a negative response to super-elevated CO₂ under high PPFD could be improved by altering medium components. Super-elevated CO₂ enrichment of in vitro-cultured Cymbidium could positively affect the efficiency and quality of commercial production of clonal orchid plantlets.