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In Vitro and Ex Vitro Growth of an Epiphytic CAM Orchid
Yoon, Y.-J., Mobin, M., Hahn, E.-J. and Paek, K.-Y. 2009. Impact of in vitro CO2 enrichment and sugar deprivation on acclamatory responses of Phalaenopsis plantlets to ex vitro conditions. Environmental and Experimental Botany 65: 183-188.

Micropropagation, to quote the authors, "is an established technique for [the] rapid propagation of uniform plants," but they note that leaves formed during in vitro growth "may never attain photosynthetic competence." Consequently, to maintain a positive carbon balance over the in vitro to ex vitro transition, they say "sugar is supplemented as the source of carbon." But they go on to state that "addition of sugar to the culture media [has been] negatively correlated with growth, photosynthesis and expression of enzymes of the carbon assimilation pathway," so that the standard cure for the problem is partially self-defeating. So we have to ask ourselves: Can CO2 do anything to help?

What was done
In investigating this possibility, Yoon et al. grew plantlets of Phalaenopsis "Amaglade" -- a monopodial epiphytic CAM orchid with succulent leaves -- derived from flower stalk cultures and multiplied on a hyponex medium either with or without the addition of 30 g sugar per liter of medium, in air of either 400 or 1000 ppm CO2, for a period of 90 days in vitro followed by 40 days ex vitro, in order "to identify the more suitable treatments in vitro for the best acclimatization of Phalaenopsis plantlets to ex vitro conditions," which they did by monitoring a number of plant morphological traits, photosynthetic attributes and intrinsic elements of the CAM pathway.

What was learned
The extra CO2 eliminated the need for any sugar to be supplied to the plantlets, with shoot dry weight in the CO2-enriched air exceeding that in the ambient air by 120% in the non-sweetened treatment at the end of the in vitro period, and with root dry weight in the CO2-enriched air exceeding that in the ambient air by 350%. Likewise, at the end of the ex vitro period, the CO2-induced shoot and root dry weight increases in the non-sweetened treatment were 55% and 86%, respectively.

What it means
The four Korean researchers concluded that "plantlets grown under CO2 enrichment" -- and without the help of any applied sugar -- "developed completely viable photosynthetic apparatus ready to be efficiently transferred to ex vitro conditions," a fact that they say has "far-reaching implications" for the micropropagation of Phalaenopsis, and possibly other plants as well.

Reviewed 18 March 2009