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Photosynthetic Characteristics in Wheat Exposed to Elevated O₃ and CO₂

Brazilian National Institute for Space Research, Div. of Remote Sensing, C.P. 515, 12201-970 Sáo José dos Campos, SP, Brazil
Dep. of Agronomy, H.J. Patterson Hall, Univ. of Maryland, College Park, MD 20742

E. Lee, R. Rowland and R. Pausch

USDA, Beltsville Agric. Res. Ctr., Climate Stress Lab., Beltsville, MD 20705

^* Corresponding author (cm48{at}umail.umd.edu).

Tropospheric trace gases such as CO₂ and O₃ have progressively increased over the past century and are predicted to increase to levels at which they may have a significant impact on agricultural production. The effects of CO₂ enrichment and O₃ air pollution on leaf photosynthesis (P_n) and stomatal conductance (g₅) were investigated. Two soft winter wheat (Triticum aestivum L.) cultivars, Massey in 1991 and Saluda in 1992, were studied in field experiments at Beltsville, MD, by means of open-top chambers to mimic atmospheric environments predicted for the first half of the 21st century. Plants were exposed to two levels of O₃ (charcoal filtered air and ambient air + an average of 40 nmol O₃ mol^–1 from Monday–Friday of every week). Ozone treatments were superimposed on two CO₂ treatments (350 µmol CO₂ mol^– and 500 µmol CO₂ mol^–1). Averaged over O₃ treatments, P_n was stimulated during the early and late growing season under enriched CO₂. Averaged over CO₂ treatments, high O₃ exposure had a negative impact on P_n early in the season of 1992 and a major impact late in the season of 1991 and 1992 due to premature senescence. Decreases in g_s occurred under the enriched CO₂ environment and to a lesser extent with high O₃. Interactive effects on P_n and g_s were mostly absent. It is likely that if CO₂ and O₃ concentrations continue to increase, the beneficial effect of CO₂ enrichment on P_n may be partially negated by O₃-induced stress. Conversely, damaging effects of O₃ on P_n may be compensated by elevated atmospheric CO₂.

Received for publication March 6, 1995.