Effects of temperature and CO enrichment on kinetic properties of phospho-enol-pyruvate carboxylase in two ecotypes of Echinochloa crus-galli (L.) Beauv., a C weed grass species.

Two populations of Echinochloa crus-galli (Québec, Mississippi) were grown at the Duke University Phytotron under 2 thermoperiods (28°/22°C, 21°/15°C day/night) and 2 CO regimes (350 and 675 μl l). Thermostability, energy of activation (E ),K (PEP), K (Mg), and specific activity of phospho-enol-pyruvate carboxylase (PEP) were analyzed in partially purified enzyme preparations of plants grown for 5 weeks. Thermostability of PEP from extracts (in vitro) and leaves (in situ) was significantly higher in Mississippi plants. In vitro denaturation was not appreciably modified by thermal acclimation but CO enrichment elicited higher thermostability of PEP. In situ thermostability was significantly higher than that of in vitro assays and was higher in Mississippi plants acclimated at 28°/22°C and in plants of the two ecotypes grown at 675 μl l CO. E (Q 30°/20°C) for PEP was significantly lower in Québec plants as compared to Mississippi and no acclimatory shifts were observed. Significantly higher K 's (PEP) in 20°C assays were obtained for Mississippi as compared to Québec plants but values were similar at 30°C and 40°C assays. K (Mg) decreased at higher assay temperatures and were significantly lower for PEP of the Québec ecotype. No significant changes in K (Mg) values were associated with modifications in temperature on CO regimes. PEP activity measured at 30°C was significantly higher for Québec plants when measured on a leaf fresh weight, leaf area or protein basis but not on a chlorophyll basis. Significantly higher PEP activity for both genotypes was observed for plants acclimated at 21°/15°C or grown at 675 μl l CO. Net photosynthesis (Ps) and net assimilation rates (NAR) were higher in Québec plants and were enhanced by CO enrichment. NAR was higher in plants acclimated at low temperature, while an opposite trend was observed for Ps. PEP activities were always in excess of the amounts required to support observed rates of CO assimilation.