Environmental Effects on Photorespiration of C3-C4 Species

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Abstract
The possibility of altering CO2 exchange of C3-C4 species by growing them under various CO2 and O2 concentrations was examined. Growth under CO2 concentrations of 100, 350, and 750 micromoles per mole had no significant effect on CO2 exchange characteristics or leaf anatomy of Flaveria pringlei (C3), Flaveria floridana (C3-C4), or Flaveria trinervia (C4). Carboxylation efficiency and CO2 compensation concentrations in leaves of F. floridana developed under the different CO2 concentrations were intermediate to F. pringlei and F. trinervia. When grown for 12 days at an O2 concentration of 20 millimoles per mole, apparent photosynthesis was strongly inhibited in Panicum milioides (C3-C4) and to a lesser degree in Panicum laxum (C3). In P. milioides, acute starch buildup was observed microscopically in both mesophyll and bundle sheath cells. Even after only 4 days exposure to 20 millimoles per mole O2, the presence of starch was more pronounced in leaf cross-sections of P. milioides compared to those at 100 and 210 millimoles per mole. Even though this observation suggests that P. milioides has a different response to low O2 with respect to translocation of photosynthate or sink activity than C3 species, the concentration of total available carbohydrate increased in shoots of all species by 33% or more when grown at low O2. This accumulation occurred even though relative growth rates of Festuca arundinacea (C3) and P. milioides grown for 4 days at 210 millimoles per mole O2, were inhibited 83 and 37%, respectively, when compared to plants grown at 20 millimoles per mole O2.