Effect of low temperature on the photosynthetic metabolism of the C grass Echinochloa crus-galli.

CO curves of photosynthesis and activities of the four C enzymes and Ribulose bisphosphate carboxylase (RUBP) were compared in two populations of the C grass Echinochloa crus-galli from contrasting thermal environments (Québec 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 C pathway are more susceptible to become limiting at low temperatures. Both populations maintained, after chilling, a pattern of CO fixation typical of C plants with photosynthesis saturating at low external CO concentrations. However, the chilling treatment led to reductions in carbon uptake and in the activities of the C enzymes. RUBP activity was not significantly affected by chilling. Reductions in photosynthesis and in C enzyme activities following the chilling treatment were significantly larger for plants of the Mississippi population. The enzyme data suggest that two steps of the C pathway, NADP-malate dehydrogenase and pyruvate P dikinase, are likely to be associated with the reduction of CO uptake in C plants under cool conditions. When the experiment was replicated under enriched atmospheric CO (675 μl l CO), similar differences were observed between the two populations. CO 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-malic 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 CO.