Effects of atmospheric CO2 concentration and defoliation on the growth of Themeda triandra

The effects of elevated atmospheric carbon dioxide (CO₂) concentration (700 μmol mol^?1) on defoliated (three clippings at 3‐week intervals) and undefoliated plants were determined for the C₄ grass Themeda triandra, Forsk. The elevated CO₂ concentration significantly increased leaf regrowth following defoliation, and total leaf production was greatest in this treatment. Shoot biomass of undefoliated plants was also increased under the elevated CO₂ concentration treatment. The primary effect of the elevated CO₂ concentration in both defoliated and undefoliated plants was an increase in individual leaf length and mass of dry matter, linked to a higher leaf water content and increased photosynthetic rates at the canopy level. Photosynthetic down‐regulation at the leaf level occurred, but this was compensated for by increased assimilation rates and greater canopy leaf area at the elevated CO₂ concentration. Increases in leaf and sheath growth of defoliated plants in the elevated CO₂ concentration treatment were lost following a final 3‐week reversion to ambient CO₂ concentration, but occurred in plants exposed to the elevated CO₂ concentration for the final 3‐week period only. In conclusion, elevated atmospheric CO₂ concentration increases shoot growth via increased leaf extension, which is directly dependent on stimulation of concurrent photosynthesis. CO₂ responsiveness is sustained following moderate defoliation but is reduced when plants experience reduced vigour as a result of maturation or high frequency of defoliation.