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Agriculture (Species - Sorghum: Photosynthesis) -- Summary
Many laboratory and field experiments have demonstrated a pervasive positive influence of elevated levels of atmospheric CO2 on the rate of foliar net photosynthesis in the C4 crop sorghum (Sorghum bicolor (L.) Moench), as can be verified by perusing the results we have posted within the Plant Growth (Photosynthesis) section of our website, which can be accessed via the Data portal located near the top of our home page.  Here, we briefly summarize the findings of the subset of those studies for which we have produced Journal Reviews.

Cousins et al. (2001) grew sorghum at atmospheric CO2 concentrations of 370 and 570 ppm in a FACE experiment near Phoenix, Arizona, USA.  Six days after planting, photosynthetic rates of the second leaves of the CO2-enriched plants were 37% greater than those of the second leaves of the plants grown in ambient air.  This CO2-induced enhancement slowly declined over the growing season, however; but it stabilized at about 15% between 23 and 60 days after planting.  Also, when measuring photosynthetic rates at a reduced oxygen concentration of 2%, the researchers observed increases of 16 and 9% in photosynthesis for ambient and CO2-enriched plants, respectively, indicative of the fact that elevated CO2 was reducing photorespiratory carbon losses.  Even so, this phenomenon could not fully account for the CO2-induced stimulation of photosynthesis.  Consequently, they decided that the elevated CO2 may have decreased CO2 leakage from specialized bundle sheath cells, which concentrate CO2 to promote photosynthetic carboxylation reactions by the enzyme rubisco.

The following year, in an analysis of the findings of all sorghum FACE studies conducted to that point in time, Kimball et al. (2002) determined that the net photosynthetic enhancement at ample water and nitrogen was only 9% for a 200-ppm increase in atmospheric CO2 concentration, but that when water was limiting to growth, the CO2-induced stimulation rose to 23%.  One year later, Bunce (2003) reported the results he obtained when he grew sorghum under normal field conditions within open-top chambers exposed to ambient and twice-ambient atmospheric CO2 concentrations for two years, in a study designed to determine how high vapor pressure deficits (VPDs) might influence leaf photosynthetic response to elevated CO2.  Irrespective of air VPD, he found that the doubled CO2 significantly increased the rate of net assimilation in sorghum by about 13%, which corresponds to an enhancement of about 7% for a 200-ppm increase in the air's CO2 concentration.  Last of all, in another review of the pertinent scientific literature, Ainsworth and Long (2005) determined that a 200-ppm increase in the atmosphere's CO2 concentration enhanced sorghum net photosynthesis by approximately 5%.

In light of these several observations, it would appear that under normal field conditions of adequate water and soil fertility, a 200-ppm increase in the air's CO2 concentration could be expected to boost net photosynthesis in sorghum by somewhere between 5 and 15%, but that under conditions of water stress it could boost it by as much as 23%.

References
Ainsworth, E.A. and Long, S.P.  2005.  What have we learned from 15 years of free-air CO2 enrichment (FACE)?  A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2New Phytologist 165: 351-372.

Bunce, J.A.  2003.  Effects of water vapor pressure difference on leaf gas exchange in potato and sorghum at ambient and elevated carbon dioxide under field conditions.  Field Crops Research 82: 37-47.

Cousins, A.B., Adam, N.R., Wall, G.W., Kimball, B.A., Pinter Jr., P.J., Leavitt, S.W., LaMorte, R.L., Matthias, A.D., Ottman, M.J., Thompson, T.L. and Webber, A.N.  2001.  Reduced photorespiration and increased energy-use efficiency in young CO2-enriched sorghum leaves.  New Phytologist 150: 275-284.

Kimball, B.A., Kobayashi, K. and Bindi, M.  2002.  Responses of agricultural crops to free-air CO2 enrichment.  Advances in Agronomy 77: 293-368.

Last updated 1 February 2006