Journal
GRASS AND FORAGE SCIENCE
Volume 59, Issue 3, Pages 215-226Publisher
WILEY
DOI: 10.1111/j.1365-2494.2004.00420.x
Keywords
C-4 grass; leaf length; leaf mass; photosynthetic gas exchange; defoliation; elevated CO2 concentration
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The effects of elevated atmospheric carbon dioxide (CO2) concentration (700 mumol mol(-1)) on defoliated (three clippings at 3-week intervals) and undefoliated plants were determined for the C-4 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.
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