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Title: RESPONSE OF C4 PHOTOSYNTHESIS IN SORGHUM TO GROWTH UNDER FREE AIR CARBON DIOXIDE ENRICHMENT (FACE): YOUNG LEAVES EXHIBIT HIGHER RATES OF PHOTORESPIRATION AND DECREASED ENERGY USE EFFICIENCY

Author
item COUSINS, A - ARIZONA STATE UNIV
item Adam, Neal
item Wall, Gerard - Gary
item Kimball, Bruce
item Pinter Jr, Paul
item LEAVITT, S - UNIV OF ARIZONA
item La Morte, Robert
item MATTHIAS, A - UNIV OF ARIZONA
item OTTMAN, M - UNIV OF ARIZONA
item THOMPSON, T - UNIV OF ARIZONA

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2001
Publication Date: 2/15/2001
Citation: Cousins, A.B., Adam, N.R., Wall, G.W., Kimball, B.A., Pinter Jr, P.J., Leavitt, S.W., La Morte, R.L., Matthias, A.D., Ottman, M.J., Thompson, T.L. 2001. Response of c4 photosynthesis in sorghum to growth under free air carbon dioxide enrichment (face): young leaves exhibit higher rates of photorespiration and decreased energy use efficiency. Plant Physiology. 150(2):275-284.

Interpretive Summary: The onset of the industrial revolution and the exponential growth of the human population have caused rapid changes to our environment at unprecedented rates. Continued human activities are predicted to cause the Earth's atmospheric carbon dioxide concentration to double by the end of the 21st century. This may affect future precipitation patterns and soil water supply. Since elevated atmospheric carbon dioxide plays an important role in plant water and nutrient use, as well as crop yields, a study was conducted to determine its effects on a sorghum crop. Sorghum is one of the world's foremost grain sources in developing countries. Carbon uptake and light energy utilization were measured at various stages of plant development. We determined that, in younger plants, elevated carbon dioxide increased the productivity of sorghum by increasing the rate and energy efficiency of carbon dioxide uptake. In later growth stages, similar carbon dioxide-based enhancement was not observed. This research provides important insight into how sorghum will respond to global changes and will aid researchers in modeling future nutrient and water requirements, as well as predict trends in future crop yield. This information will be beneficial to both producers and consumers.

Technical Abstract: Effects of increased atmospheric CO2 concentration (Ca) on photosynthesis of Sorghum bicolor (DK54) grown in Free Air Carbon Dioxide Enrichment (FACE) has been examined. Simultaneous measurements of chlorophyll a-fluorescence and CO2 assimilation (A) were made on the uppermost fully expanded leaves at various days after planting. The A of the second leaf of plants grown in FACE conditions was 37% (p=0.027) greater than of plants grown in ambient conditions. Reducing O2 partial pressure from 21% to 2% stimulated A by 16% in ambient-grown plants indicating significant rates of photorespiration. In leaves 3 and 5, the ratio of the quantum yield of PSII to the quantum yield of CO2 fixation, PSII/CO2, remained relatively constant over a range of Ca values, indicating A was the primary sink for linear electron transport. In the second leaf, PSII/CO2 was larger than expected at low Ca, possibly due to overcycling of the C4 pump and increased leakiness of CO2 from the bundle sheath cells. As Ca was increased, the PSII/CO2 ratio decreased, as a greater proportion of energy derived from linear electron transfer was utilized by the C3 cycle. These results demonstrate that stimulation of C4 photosynthesis at elevated Ca in young leaves was partially due to suppressed photorespiration. Elevated Ca also enhanced energy use efficiency in young leaves, possibly by decreasing CO2 leakage from bundler sheath cells and by decreasing overcycling of the C4 pump.