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Articles

Photosynthesis in Grass Species Differing in Carbon Dioxide Fixation Pathways

II. A Search for Species with Intermediate Gas Exchange and Anatomical Characteristics ¹

^a Department of Agronomy, University of Georgia, Athens, Georgia 30602

Thirty-three grass species were examined in two experiments in an attempt to locate plants with photosynthetic responses to O₂, CO₂ compensation concentrations, and leaf anatomy intermediate to those of C₃ and C₄ species. Species examined included seven from the Laxa group in the Panicum genus, one of which, P. milioides Nees ex Trin., has been reported earlier to have intermediate characteristics. The species with O₂-sensitive photosynthesis typical of C₃ plants showed more than 37% increase in apparent photosynthesis at 2% O₂ compared to 21% O₂ at 25 C and 335 microliters per liter CO₂, whereas in Panicum milioides, P. schenckii Hack., and P. decipiens Nees ex Trin., members of the Laxa group of Panicum, increases ranged from 25 to 30%. The remainder of the species did not respond to O₂. Species with O₂ responses characteristic of C₃ plants exhibited CO₂ compensation concentrations of 44 microliters per liter or higher at 21% O₂ and 25 to 27.5 C and species characterized as O₂-insensitive had values of microliters per liter or less. The CO₂ compensation concentration () values of P. milioides, P. schenckii, and P. decipiens ranged from 10.3 to 23.3 microliters per liter. Other species of the Laxa group of Panicum exhibited O₂ response and values of either C₃ (P. laxum Sw., P. hylaeicum Mez., and P. rivulare Trin.) or C₄ (P. prionitis Griseb.) plants. Leaves of species with O₂ response and CO₂ compensation values typical of C₃ plants had poorly developed or nearly empty bundle sheath cells, and much larger distances and mesophyll cell numbers between veins than did the O₂-insensitive ones. Vein spacings in P. milioides, P. schenckii, and P. decipiens ranged from 0.18 to 0.28 millimeter and mesophyll cell number between veins from 5.2 to 7.8. While these vein spacings are closer than those of most C₃ grasses, two O₂-sensitive species of Dactylis had vein spacings similar to these Panicums and veins in Glyceria striata, another O₂-sensitive plant, were separated by only four mesophyll cells and 0.12 millimeter. Bundle sheath cells of the three intermediate Panicums contained greater quantities of organelles than are typical for C₃ grasses.

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