EFFECT OF LOW-TEMPERATURES ON THE ACTIVITY OF OXYGEN-SCAVENGING ENZYMES IN 2 POPULATIONS OF THE C-4 GRASS ECHINOCHLOA-CRUS-GALLI

To discriminate among possible mechanisms responsible for the differen tial response to cold temperatures among ecotypes of the C-4 grass wee d species Echinochloa crusgalli (L.) Beauv., the specific activities o f five oxygen-scavenging enzymes responsible for the elimination or re duction of free radicals and hydrogen peroxide during cold-induced pho toinhibition were determined in 5-week-old plants of two populations o f the species collected from sites of contrasting climates, Quebec (QU E) and Mississippi (MISS). Enzyme activities were measured at temperat ures ranging from 5 to 30 degrees C. The specific activities of ascorb ate peroxidase, monodehydroascorbate reductase, dehydroascorbate reduc tase and glutathione reductase were significantly higher in cold-adapt ed QUE plants at low assay temperatures than in warm-adapted MISS plan ts at the same temperature. The specific activities of superoxide dism utase assayed at 5 and 25 degrees C were similar among plants of the t wo E. crus-galli populations. Ascorbate concentrations were not differ ent among plants of the two populations, suggesting that the observed differences in the specific activities of ascorbate peroxidase assayed at 5 degrees C, truly reflect a better capacity of the QUE enzyme to reduce H2O2 to water at temperature conditions associated with the pho toinhibitory process. The enhanced specific activity of four of the fi ve oxygen-scavenging enzymes measured in the cold-adapted QUE populati on at low assay temperatures correlates with the syndrome of cold-adap ted features reported for plants of this population in earlier studies .