Photosynthesis in Grass Species Differing in Carbon Dioxide Fixation Pathways: II. A Search for Species with Intermediate Gas Exchange and Anatomical Characteristics 1

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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 (Г) 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 Г 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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