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JXB Advance Access originally published online on July 1, 2003
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Journal of Experimental Botany, Vol. 54, No. 389, pp. 1969-1975, August 1, 2003
© 2003 Oxford University Press

Development of C4 photosynthesis in sorghum leaves grown under free-air CO2 enrichment (FACE)

Received 12 April 2003; Accepted 22 April 2003

A. B. Cousins1, N. R. Adam1,2, G. W. Wall2, B. A. Kimball2, P. J. Pinter Jr2, M. J. Ottman3, S. W. Leavitt4 and A. N. Webber*,1

1 Department of Plant Biology and Center for the Study of Early Events in Photosynthesis, Arizona Sate University, PO Box 871601, Tempe, AZ 85287–1601, USA
2 USDA, Agricultural Research Service, US Water Conservation Laboratory, Phoenix, AZ 85040, USA
3 Department of Plant Science, University of Arizona, Tucson, AZ 85721, USA
4 Laboratory of Tree Ring Research, University of Arizona, Tucson, AZ 85721, USA

* To whom correspondence should be addressed. Fax: +1 480 965 6899. E-mail: andrew.webber{at}asu.edu

The developmental pattern of C4 expression has been well characterized in maize and other C4 plants. However, few reports have explored the possibility that the development of this pathway may be sensitive to changes in atmospheric CO2 concentrations. Therefore, both the structural and biochemical development of leaf tissue in the fifth leaf of Sorghum bicolor plants grown at elevated CO2 have been characterized. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) activities accumulate rapidly as the leaf tissue differentiates and emerges from the surrounding whorl. Rubisco was not expressed in a cell-specific manner in the youngest tissue at the base of the leaf, but did accumulate before PEPC was detected. This suggests that the youngest leaf tissue utilizes a C3-like pathway for carbon fixation. However, this tissue was in a region of the leaf receiving very low light and so significant rates of photosynthesis were not likely. Older leaf tissue that had emerged from the surrounding whorl into full sunlight showed the normal C4 syndrome. Elevated CO2 had no effect on the cell-specific localization of Rubisco or PEPC at any stage of leaf development, and the relative ratios of Rubisco to PEPC remained constant during leaf development. However, in the oldest tissue at the tip of the leaf, the total activities of Rubisco and PEPC were decreased under elevated CO2 implying that C4 photosynthetic tissue may acclimate to growth under elevated CO2.

Key words: C4 expression, elevated CO2, leaf tissue. Sorghum bicolor, structural and biochemical development.


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