Interactive Effects of Elevated Ozone plus Carbon Dioxide on Duckweeds Exposed in Open-Top Chambers

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The response of Lemna minor L. andSpirodelapolyrhiza (L.) Schleiden to projected future ambient levels of O3 and CO2 was studied under field conditions. The two duckweed species were treated with either charcoal-filtered air (CF), ambient O3 (1XO3), twice ambient O (2XO ), twice ambient CO2 plus twice ambient O3 (2XCO2+2XO3), or chamberless open-air (OA). Two experiments were conducted. In Experiment I, L. minor was treated for 15 d with a cumulative O exposure of 14.4 ppm* h. No O effects were observed during Experiment I. Dry weight of individual fronds and photosynthesis per frond increased inZ. minor exposed to 2XCO2+2XO^- air. In Experiment II after 25 d of treatment (cumulative O3 exposure of 16.2 ppmh), negative effects of 2XO on the photosynthetic and growth rates of L. minor were observed. Dark respiration of Z. minor significantly increased in 2XO3-air compared with controls, but declined significantly in 2XCO2+2XO3-air compared to those grown in 2XO3-air. Photosynthesis and dry weight per frond increased in 2XCO2+2XO3-air when compared with all other treatments. Measurement of A/C. (assimilation versus intercellular CO2 concentration) curves in Z. minor showed a significant reduction in carboxylation efficiency and maximum rates of photosynthesis in 2XCO2+2XO3-air compared with other treatments when expressed per weight. No differences in carboxylation efficiency were detected between treatments when expressed per frond.


Author Institution: Biology Department, Denison University ; USDA Forest Service



The Ohio Journal of Science. v99, n2 (April, 1999), 19-25