CO 2 curves of photosynthesis and activities of the four C4 enzymes and Ribulose bisphosphate carboxylase (RUBPc) were compared in two populations of the C4 grass Echinochloa crus-galli from contrasting thermal environments (Qurbec and Mississippi). Analyses were conducted both before and after 14 h of chilling at 7 ~ C under high light conditions. This comparison provides the opportunity to assess which steps of the C4 pathway are more susceptible to become limiting at low temperatures. Both populations maintained, after chilling, a pattern of C02 fixation typical of C4 plants with photosynthesis saturating at low external C02 concentrations. However, the chilling treatment led to reductions in carbon uptake and in the activities of the C4 enzymes. RUBPc activity was not significantly affected by chilling. Reductions in photosynthesis and in C4 enzyme activities following the chilling treatment were significantly larger for plants of the Mississippi population. The enzyme data suggest that two steps of the C4 pathway, NADP+-malate dehydrogenase and pyruvate P~ dikinase, are likely to be associated with the reduction of C02 uptake in C4 plants under cool conditions. When the experiment was replicated under enriched atmospheric C02 (675 lal 1-1 C02), similar differences were observed between the two populations. C02 enrichment resulted in an increase of activity for phospho-enol-pyruvate carboxylase and NADP+-malate dehydrogenase while activities of phospho-enol-pyruvate carboxylase and NADP ยง enzyme were less reduced following chilling. Such an interaction was not observed for gas exchange parameters but net photosynthesis was lower when plants were grown under enriched C02.
Although increasing attention was recently given to the response of C4 plants to low temperature (Long 1983;Pearcy and Ehleringer 1984), the causes for the exclusion of C4 species from cool areas are not yet perfectly understood. Long (1983) suggested that the extended metabolic pathway of carbon assimilation in C4 plants increases potential sites for low temperature limitation thus reducing the probability of evolving fully cold-tolerant genotypes.