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

Thirty-three grass species were examined in two experiments in an attempt to locate plants with photosynthetic responses to O(2), CO(2) compensation concentrations, and leaf anatomy intermediate to those of C(3) and C(4) 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 O(2)-sensitive photosynthesis typical of C(3) plants showed more than 37% increase in apparent photosynthesis at 2% O(2) compared to 21% O(2) at 25 C and 335 microliters per liter CO(2), 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 O(2). Species with O(2) responses characteristic of C(3) plants exhibited CO(2) compensation concentrations of 44 microliters per liter or higher at 21% O(2) and 25 to 27.5 C and species characterized as O(2)-insensitive had values of microliters per liter or less. The CO(2) compensation concentration (capital GHE, Cyrillic) 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 O(2) response and capital GHE, Cyrillic values of either C(3) (P. laxum Sw., P. hylaeicum Mez., and P. rivulare Trin.) or C(4) (P. prionitis Griseb.) plants. Leaves of species with O(2) response and CO(2) compensation values typical of C(3) plants had poorly developed or nearly empty bundle sheath cells, and much larger distances and mesophyll cell numbers between veins than did the O(2)-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 C(3) grasses, two O(2)-sensitive species of Dactylis had vein spacings similar to these Panicums and veins in Glyceria striata, another O(2)-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 C(3) grasses.