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Articles

Photosynthesis in Grass Species Differing in Carbon Dioxide Fixation Pathways

III. OXYGEN RESPONSE AND ENZYME ACTIVITIES OF SPECIES IN THE LAXA GROUP OF PANICUM¹

Bonnie J. Reger

Department of Agronomy, University of Georgia, Athens, Georgia 30602, Richard Russell Research Center, Science and Education Administration, Athens, Georgia 30604, United States Department of Agriculture, Science and Education Administration, Athens, Georgia 30604

Measurements of CO₂ exchange at varying O₂ concentrations in seven grass species of the Laxa group of Panicum and activities of five photosynthetic enzymes were compared to values obtained for these characters in a cool season C₃ grass, tall fescue (Festuca arundinacea Schreb.) and a C₄ grass, P. maximum Jacq. Plants were divided into three groups on the basis of the inhibition of apparent photosynthesis by 21% O_2. Rates of apparent photosynthesis in P. prionitis Griseb. and P. maximum were virtually unaffected by changes in O₂ concentration. In another group consisting of P. hylaeicum Mez., P. rivulare Trin., P. laxum Sw., and tall fescue apparent photosynthesis was inhibited by 28.2 to 36.0% at 21% O_2. An intermediate inhibition of 20.6 to 23.3% at 21% O₂ was exhibited by P. milioides Nees ex Trin., P. schenckii Hack., and P. decipiens Nees ex Trin. The CO₂ compensation concentration for P. prionitis and P. maximum was low (6 microliters per liter CO₂ at 21% O₂) and affected little by O₂, whereas values for P. hylaeicum, P. rivulare, P. laxum, and tall fescue were much greater, and increased almost linearly from 2 to 48% O_2. Values for P. milioides, P. schenckii, and P. decipiens were intermediate to the other two groups. The effect of O₂ on total leaf conductance to CO₂ was similar to the C₃ grasses and the intermediate Panicums. However, estimates of photorespiration in the intermediate species were low and changed little with O₂ in comparison to estimates for the C₃ species which were higher and increased greatly with increased O_2.

Activities of phosphoenolpyruvate carboxylase were greatest in P. maximum and P. prionitis and one-fourth or less in the remaining species. Activity of ribulose bisphosphate carboxylase was 548 micromoles per mg chlorophyll per hour in tall fescue; activity in the remaining species was approximately one-fourth or less of that in tall fescue, with the exception of P. rivulare, in which it was 440 micromoles per milligram chlorphyll per hour. High activities of two C₄ decarboxylating enzymes, phosphoenolpyruvate carboxykinase and NADP-malic enzyme, were observed in P. maximum (1,988 micromoles per milligram chlorophyll per hour) and P. prionitis (125 micromoles per milligram chlorophyll per hour), respectively. Only minimal activities of decarboxylating enzymes were detected in the remaining species.

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