Interactive effects of elevated ozone plus carbon dioxide on duckweeds exposed in open-top chambers

The response of Lemna minor L. and Spirodela polyrhiza (L.) Schleiden to projected future ambient levels of O-3 and CO2 was studied under field conditions. The two duckweed species were treated with either charcoal-filtered air (CF), ambient O-3 (lXO(3)), tn ice ambient O-3 (2XO(3)), twice ambient CO2 plus twice ambient O-3 (2XCO(2)+2XO(3)), or chamberless open-air (OA). Two experiments were conducted. In Experiment I, L. minor was treated for 15 d with a cumulative O-3 exposure of 14.4 ppm.h. No O-3 effects were observed during Experiment I. Dry weight of individual fronds and photosynthesis per frond increased in L minor exposed to 2XCO(2)+2XO(3)(-) air. In Experiment II after 25 d of treatment (cumulative O-3 exposure of 16.2 ppm h), negative effects of 2XO(3) on the photosynthetic and growth rates of L. minor were observed. Dark respiration of L minor significantly increased in 2XO(3)-air compared with controls, but declined significantly in 2XCO(2)+2XO(3)-air compared to those grown in 2XO(3)-air. Photosynthesis and drg weight per frond increased in 2XCO(2)+2XO(3)-air when compared with all other treatments. Measurement of A/C-i (assimilation versus intercellular CO2 concentration) curves in L. minor showed a significant reduction in carboxylation efficiency and maximum rates of photosynthesis in 2XCO(2)+2XO(3)-air compared with other treatments when expressed per weight. No differences in carboxylation efficiency were detected between treatments when expressed per frond. After 25 d of treatment, photosynthesis (per frond) and dry weight of S. polyrhiza were reduced in 2XO(3)-air, but final frond number was unaffected. Dark respiration of S. polyrhiza was unaffected in 2XO(3)(-) air, but when exposed to 2XCO(2)+2XO(3)-air, it declined significantly. Although S. polyrhiza photosynthesis per frond increased in 2XCO(2)+2XO(3)-air, dry weight was unaffected when compared with all other treatments. Only when comparisons were made between S. polyrhiza grown in 2XCO(2)+2XO(3)-air and 2XO(3)-air, were significant increases in dry weight observed. The addition of 2XCO(2) to 2XO(3)-air resulted in amelioration of negative O-3 effects for most responses for both duckweed species.