Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/1999
Publication Date: N/A
Citation: Interpretive Summary: Atmospheric concentrations of carbon dioxide (CO2) and ozone (O3) are increasing. Variations in CO2 concentrations have generally increased crop growth and yields. Chronic exposure to high ozone causes leaf injury and generally reduces crop growth and yields. Moderate drought reduces gas exchange and may reduce crop sensitivity to ozone. Crop responses to these gases have been studied individually. The main objective of this study was to evaluate the combined effects of soil moisture, CO2, and ozone on physiological characteristics of soft red winter wheat. Wheat was grown in large open top chambers with two soil moisture levels and four air quality treatments (ambient and elevated CO2 and/or ozone). The addition of CO2 reduced stomatal conductance and increased leaf photosynthesis under high ozone. High CO2 also increased water use efficiency. The increase in photosynthesis to CO2 under well watered conditions likely caused the increase in water use efficiency. The avoidance mechanism of partial stomatal closure appears to be an opportunistic strategy for protecting the plant against ozone and taking advantage of high levels of CO2.
Technical Abstract: Crop responses to enhanced CO2 or O3 have not been characterized under different soil moisture levels. Field studies were conducted in 1995-1997 to investigate the interactive effects of soil moisture, CO2, and O3 on the physiology of two soft red winter wheat cultivars. The plants were grown in 3-m diameter open top chambers near Beltsville, MD. Two soil moisture regimes were applied: well watered (WW) and restricted moisture (RM). The four air quality treatments (whole plots) were as follows: charcoal filtered air (CF); CF with addition of approx. 150 uL CO2 L-1 (CF+CO2); non-filtered air with addition of 35 + or - 5 nL 03 L-1 (NF+O3); and NF with addition of both CO2 and O3 (NF+CO2+O3). The addition of CO2 increased leaf photosynthetic rates (Pn) from 11 to 39% under high O3. Elevated CO2 reduced stomatal conductance (gs), especially prior to flowering. These lower gs values were accompanied by higher leaf temperatures when both CO2 and O3 were present, but did not significantly affect transpiration. Intercellular CO2 concentrations (Ci) were not affected by O3, but showed much greater increases from pre to post flowering under elevated CO2 treatments. Both cultivars exhibited increased water use efficiency (WUE) in response to elevated CO2 under WW conditions and exhibited a trend for such under high O3 after flowering when grown under RM conditions. The increase in Pn in response to CO2 under WW conditions likely caused the increase in WUE. The avoidance mechanism of partial stomatal closure appears to be an opportunistic strategy for protecting the plant against O3 and taking advantage of high levels of CO2.