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Title: THE GROWTH RESPONSE OF C4 PLANTS TO RISING ATMOSPHERIC CO2 CONCENTRATIONS: A REASSESSMENT

Author
item GHANNOUM, OULA - UNIV.WESTERN SYDNEY,AUST.
item VON CAEMMERER, SUSANNE - AUST. NAT'L UNIV., AUST.
item Ziska, Lewis
item CONROY, JANN - UNIV.WESTERN SYDNEY,AUST.

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Plants with C4 photosynthesis include some of the world's most important crops (corn, sugarcane) and noxious weeds (crabgrass, nutsedge, pigweed). Although C4 plants only represent a small proportion of the world's plant species (5%), they contribute about 18-21% of global productivity because of the high productivity of C4 grasslands. Based solely on the biochemistry yof photosynthesis, it has been suggested that C4 plants will not respond t the ongoing increase in atmospheric CO2 concentration. However, a number of recent studies have shown that the response of C4 plants may have been underestimated. Due to the importance of C4 grasslands in global carbon sequestration, recognition and understanding of the direct impact of rising atmospheric CO2 concentration remains a crucial area of interest. In this review, we examine the current state of knowledge regarding the response of C4 plants to the ongoing increase in atmospheric CO2 concentration and propose that rising CO2 levels can directly and indirectly stimulate the growth of C4 species. Overall, recent data strongly suggest that rising carbon dioxide may directly influence the global primary productivity of C4 grasslands with a subsequent increase in terrestrial carbon sequestration.

Technical Abstract: Despite mounting evidence that C4 plants accumulate more biomass at elevated CO2, the underlying mechanisms of this response are still largely unclear. In this opinion, we review the current state of knowledge regarding the response of C4 plants to elevated CO2 and propose a general hypothesis of the likely mechanisms, relating the various possibilities. Elevated CO2 stimulates the growth of both well-watered and water-stressed C4 plants through two main routes. First, it enhanced leaf CO2 assimilation rates due to increased intercellular CO2. Secondly, it reduced stomatal conductance and, consequently, leaf transpiration rates. Reduced transpiration rates stimulate growth due to its effect on leaf temperature, conservation of soil water, and improved shoot water relations. We argue that bundle sheath leakiness, direct CO2 fixation in the bundle sheath or the immaturity of C4 photosynthesis in young C4 leaves are unlikely explanations for the CO2 responsiveness of C4 photosynthesis. The interaction between elevated CO2, temperature, and shoot water relations on the growth and photosynthesis of C4 plants are identified as key areas needing urgent research. Overall, available data indicate that C4 plants can show significant increases in biomass accumulation with rising atmospheric CO2. This, in turn, may have a considerable impact on terrestrial carbon sequestration because C4 grasslands contribute substantially to global primary productivity.