Background
"Vitrified shoots," to quote the author, "are characterized as being small, succulent (i.e., 'glassy' or 'wet' in appearance) and immature, but [are] capable of readily proliferating additional axillary shoots."  Also, he notes that vitrified shoots, of the type that are cultured in vitro, "do not transfer readily into soil well."  Hence, because there is a need for millions of sweetgum seedlings to be planted annually (Lin et al., 1995), it would be advantageous if a technique could be developed to increase the success of transferring tissue-culture-produced vitrified shoots to ex vitro growth in soil.

What was done
In searching for a technique to accomplish this feat, Tisserat first produced sweetgum shoots in an automated plant culture system in which ten times more shoots were produced than in prior plant culture systems, but where vitrification was observed in fully 80% of the shoots.  Then, he documented the effects of ultra-high atmospheric CO2 concentrations on the vitrified shoots when they were transferred to soil and grown in air having CO2 concentrations ranging from 350 to 30,000 ppm.

What was learned
After four weeks of growth in atmospheric CO2 concentrations of 350, 1500, 3000, 10000 and 30000 ppm, survival percentages of 1-cm-long explants were, respectively, 48.6, 56.5, 65.7, 93.1 and 67.1, while corresponding survival percentages of 2-cm-long explants were 61.2, 64.1, 69.2, 93.9 and 64.3.  For these same CO2 concentrations, the numbers of leaves produced per shoot were 4.17, 5.38, 5.85, 6.14 and 4.83, while the numbers of roots produced per shoot were 5.35, 8.58, 9.19, 9.66 and 9.82.  Leaf and shoot lengths were also similarly enhanced by the suite of increased CO2 concentrations.

What it means
Within the context that formed the basis for his study, Tisserat says that the procedures he utilized "will minimize the time and labor involved in sweetgum micropropagation," and that they "can be readily adapted to the micropropagation of other woody and non-woody plants."  Within the contest of the ongoing rise in the air's CO2 concentration, Tisserat's results - as well as the similar results of many others (see Growth Response to Very High CO2 Concentrations in our Subject Index) - suggest that man will never be able to pump enough CO2 into the air to negatively affect the growth and development of sweetgum trees and, by implication, many (if not most) of earth's other plants.  Even in those cases where plant growth responses did decline between 10,000 and 30,000 ppm in Tisserat's study, for example, the responses at 30,000 ppm CO2 were still greater than those at 350 ppm.

Reference
Lin, X., Bergmann, B.A. and Stomp, A.-M.  1995.  Effect of medium physical support, shoot length and genotype on in vitro rooting and plantlet morphology of sweetgum.  Journal of Environmental Horticulture 13: 117-121.