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Interactive Effects of Elevated CO2 and O3 on Soybean Leaf Properties
Heagle, A.S., Miller, J.E. and Booker, F.L.  1998.  Influence of ozone stress on soybean response to carbon dioxide enrichment: I.  Foliar properties.  Crop Science 38: 113-121.

What was done
The authors grew four soybean varieties in pots placed within open-top chambers that received atmospheric CO2 concentrations of ambient, 1.3, 1.6, and 1.9 times ambient in combination with ozone (O3) concentrations of 20, 46, and 76 ppb to determine the interactive effects of these gases on leaf properties.

What was learned
Overall, the effects of elevated CO2 on soybean leaves were dependent upon the degree of stress caused by O3.  When O3 concentrations were low there were no positive effects of elevated CO2 on the foliar properties studied.  However, when O3 concentrations were moderate or high, elevated CO2 positively influenced several leaf characteristics.  At the most stressful O3 concentration of 76 ppb, for example, plants grown in the highest CO2 environment of 715 ppm exhibited less foliar injury and greater chlorophyll contents than plants grown at ambient CO2.  In addition, at 76 ppb O3, foliar starch contents were nearly 6-fold higher in leaves of plants grown at 715 vs. 370 ppm CO2.  Thus, the unfavorable effects caused by O3-induced stress generally decreased with increasing atmospheric CO2 concentrations.

With respect to foliar elements, increasing O3 concentrations tended to increase leaf carbon and nitrogen contents, while elevated CO2 typically decreased them.  Likewise, increasing O3 levels increased the content of sulfur and boron in leaves, while atmospheric CO2 enrichment reduced their concentrations.  However, the implications of such elemental changes remain largely unknown; and in the present study, they did not appear to adversely impact the growth and yield of these four soybean cultivars.

What it means
As the CO2 content of the air increases due to mankind's expanding population and industrial activities, the amount of O3 in the air will likely also increase.  Although indiscriminate uptake of O3 by leaf stomata causes cellular damage and reduces photosynthesis and crop yields, lower stomatal conductances typically induced by greater quantities of CO2 in the air will likely ameliorate the negative effects of O3 on plant productivity.  As far as these soybean cultivars are concerned, leaf chlorophyll and starch concentrations should rise as the atmospheric CO2 and O3 levels continue to increase.

Reviewed 1 July 1999