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CO2, Plant Hormones and Growth
Reference
Teng, N., Wang, J., Chen, T., Wu, X., Wang, Y. and Lin, J. 2006. Elevated CO2 induces physiological, biochemical and structural changes in leaves of Arabidopsis thaliana. New Phytologist 172: 92-103.

What was done
The authors grew well watered and fertilized thale cress (Arabidopsis thaliana (L.) Heynh.) plants from seed to commencement of bolting in pots within controlled-environment chambers maintained at atmospheric CO2 concentrations of either 370 or 700 ppm, while measuring a large number of plant properties and processes.

What was learned
Among other things, Teng et al. determined that the elevated CO2 increased the biomass production of the plants by 29%, leaf total non-structural carbohydrates by 76%, and with respect to plant hormones that it "significantly increased the IAA [indole-3-acetic acid, by 13.7%], GA3 [gibberellic acid, by 55.4%], ZR [zeatin riboside, by 15.6%], DHZR [dihydrozeatin ribosidem, by 55.9%] and iPA [isopentenyladenosine, by 74.6%] contents of leaves, but significantly reduced the ABA [abscisic acid, by 15.2%] content."

What it means
The six Chinese researchers write that plant hormones "can enhance plant growth and development by stimulating cell division, cell elongation and protein synthesis (Yong et al., 2000), whereas ABA is considered an inhibitor of leaf growth (Zhang and Davies, 1990)." In addition, they note that "plant hormone metabolism is dependent on the supply of carbohydrates (Taiz and Zeiger, 1998)." Therefore, based on what they learned from their experiment, they proposed that "higher carbohydrate production," such as that induced by atmospheric CO2 enrichment, "may result in higher hormone concentrations, which in turn may enhance plant growth," a phenomenon that has also been observed by Jitla et al. (1997) and Li et al. (2002).

References
Jitla, D.S., Rogers, G.S., Seneweera, S.P., Basra, A.S., Oldfield, R.J. and Conroy, J.P. 1997. Accelerated early growth of rice at elevated CO2: is it related to developmental changes in the shoot apex? Plant Physiology 115: 15-22.

Li, C.R., Gan, I.J., Xia, K., Zhou, X. and Hew, C.S. 2002. Responses of carboxylating enzymes, sucrose metabolizing enzymes and plant hormones in a tropical epiphytic CAM orchid to CO2 enrichment. Plant, Cell and Environment 25: 369-737.

Taiz, L. and Zeiger, E. 1998. Plant Physiology. Sinauer Associates, Inc., Sunderland, Massachusetts, USA.

Yong, J.W.H., Wong, S.C., Letham, D.S., Hocart, C.H. and Farquhar, G.D. 2000. Effects of elevated CO2 and nitrogen nutrition on cytokinins in the xylem sap and leaves of cotton. Plant Physiology 124: 769-779.

Zhang, J. and Davies, W.J. 1990. Changes in the concentration of ABA in xylem sap as a function of changing soil-water status can account for changes in leaf conductance and growth. Plant, Cell and Environment 13: 277-285.

Reviewed 17 January 2007