The activities of PEP carboxylase and the C4 acid decarboxylases are little changed by drought stress in three C4 grasses of different subtypes

The C-4 photosynthetic pathway involves the assimilation of CO2 by phosphoenolpyruvate carboxylase (PEPC) and the subsequent decarboxylation of C-4 acids. The enzymes of the CO2 concentrating mechanism could be affected under water deficit and limit C-4 photosynthesis. Three different C-4 grasses were submitted to gradually induced drought stress conditions: Paspalum dilatatum (NADP-malic enzyme, NADP-ME), Cynodon dactylon (NAD-malic enzyme, NAD-ME) and Zoysia japonica (PEP carboxykinase, PEPCK). Moderate leaf dehydration affected the activity and regulation of PEPC in a similar manner in the three grasses but had species-specific effects on the C-4 acid decarboxylases, NADP-ME, NAD-ME and PEPCK, although changes in the C-4 enzyme activities were small. In all three species, the PEPC phosphorylation state, judged by the inhibitory effect of L-malate on PEPC activity, increased with water deficit and could promote increased assimilation of CO2 by the enzyme under stress conditions. Appreciable activity of PEPCK was observed in all three species suggesting that this enzyme may act as a supplementary decarboxylase to NADP-ME and NAD-ME in addition to its role in other metabolic pathways.