nclusion of natural (glycerol, proline) or synthetic (polyethylene glycol) compatible solutes in the assay medium for phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity at low substrate (phosphoenolpyruvate) levels enhanced the enzymic activity throughout the temperature range tested (11-42 °C). This effect was much more pronounced above 30 °C and, therefore, the optimum for activity was shifted from 27-29 °C to 37-39 °C, a temperature more consistent with the prevailing leaf temperatures during the growing season for C4-plants; there was also a better correlation between activity/temperature profile and the net photosynthetic rate of intact leaves of Cynodon dactylon (L.) Pers., at the temperature range tested.