PHOTOSYNTHETIC CHARACTERISTICS IN WHEAT EXPOSED TO ELEVATED O-3 AND CO2

Tropospheric trace gases such as CO2 and O-3 have progressively increa sed over the past century and are predicted to increase to levels at w hich they may have a significant impact on agricultural production. Th e effects of CO2 enrichment and O-3 air pollution on leaf photosynthes is (P-n) and stomatal conductance (g(s)) were investigated. Two soft r ed winter wheat (Triticum aestivum L.) cultivars, Massey in 1991 and S aluda in 1992, were studied in field experiments at Beltsville, RID, b y means of open-top chambers to mimic atmospheric environments predict ed 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 4 0 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 5 00 mu mol CO2 mol(-1)). Averaged over O-3 treatments, P-n was stimulat ed during the early and late growing season under enriched CO2. Averag ed 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 like ly that if CO2 and O-3 concentrations continue to increase, the benefi cial effect of CO2 enrichment on P-n may be partially negated by O-3-i nduced stress. Conversely, damaging effects of O-3 on P-n may be compe nsated by elevated atmospheric CO2.