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

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 phosphoenol-pyruvate 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.