Photosynthetic characteristics in wheat exposed to elevated O-3 and CO2

Tropospheric trace gases such as CO2 and O-3 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 O-3 air pollution on leaf photosynthesis (P-n) and stomatal conductance (g(s)) were investigated. Two soft red winter wheat (Triticum aestivum L.) cultivars, Massey in 1991 and Saluda in 1992, were studied in field experiments at Beltsville, RID, 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 O-3 (charcoal filtered air and ambient air + an average of 40 nnol O-3 mol(-1) from Monday-Friday of every week). Ozone treatments were superimposed on two CO2 treatments (350 mu mol CO2 mol(-1) and 500 mu mol CO2 mol(-1)). Averaged over O-3 treatments, P-n was stimulated during the early and late growing season under enriched CO2. Averaged over CO2 treatments, high O-3 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 O-3. Interactive effects on P-n, and g(s) were mostly absent. It is likely that if CO2 and O-3 concentrations continue to increase, the beneficial effect of CO2 enrichment on P-n may be partially negated by O-3-induced stress. Conversely, damaging effects of O-3 on P-n may be compensated by elevated atmospheric CO2.