Plants from two populations of the C-4 grass Echinochloa crus-galli (L.) Beauv. from Quebec (QUE) and Mississippi (MIss), were acclimated under controlled conditions at 28/22 degrees C, 21/15 degrees C and 14/8 degrees C (day/night). The apparent energy of activation (E-a), Michaelis-Menten constant (K-m for malate), V-max, V-max/K-m ratios and specific activity of NADP(+)-malic enzyme (E.C. 1.1.1.40) were investigated. Electrophoresis disclosed one molecular form for the enzyme with a molecular weight of 280,000 daltons and similar electrophoretic mobility for the two populations. E-a values obtained for NADP(+)-ME from QUE plants acclimated to 14/8 degrees C were significantly lower than those of MISS plants in the 15-30 degrees C assay temperatures but, no such differences were found under higher acclimatory conditions. K-m values decreased as a function of assay temperature and, except for 40 degrees C assays, there were no significant differences between populations or between thermoperiods for this parameter. V-max values and V-max/K-m ratios were higher for the enzyme of plants of both populations grown at 14/8 degrees C and significantly higher in QUE plants as compared to MISS plants acclimated to 14/8 degrees C. NADP(+)-ME activities were significantly higher in QUE plants as compared to MISS plants acclimated to 28/22 degrees C or 21/15 degrees C but not at 14/8 degrees C. For both populations, enzyme activities were significantly reduced at 14/8 degrees C as compared to those measured at higher thermoperiods. The modifications in kinetic properties of NADP(+)-ME among the two Echinochloa populations do not explain the chilling sensitivity of this chloroplastic enzyme in MISS plants and suggest that the cause might be associated to chloroplast disfunction.