Photosynthesis in Grass Species Differing in Carbon Dioxide Fixation Pathways: III. OXYGEN RESPONSE AND ENZYME ACTIVITIES OF SPECIES IN THE LAXA GROUP OF PANICUM1

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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% O2. 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% O2. 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% O2. 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 O2.

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