Protective system against photoreduced species of dioxygen in two populations of the C4grassEchinochloa crus-galli(barnyard grass; Poaceae) originating from contrasting climatic regions
Authors: Hakam Nadia , Simon Jean-Pierre 
Published: April 2011

The comparative effects of cold treatments upon the activities of five enzymes responsible for the elimination or reduction of toxic oxygen species were analyzed in two ecotypes of the C4grass weed species Echinochloa crus-galli (L.) Beauv. from sites of contrasting climates in Quebec and Mississippi. Specific activities of the enzymes extracted from 4-week-old plants were measured daily for 10 consecutive days upon exposure to 14 °C light (L): 8 °C dark (D) and compared with those of corresponding control plants acclimated at 26 °C L: 20 °C D. Activities of superoxide dismutase were not substantially modified by the cold treatment. Activities of monodehydroascorbate reductase, expressed as percentages of the activities of control plants, increased significantly during the cold-treatment period and were significantly higher in Mississippi plants. Activities of glutathione reductase from Mississippi plants increased up to 200% during the cold-treatment period, while those from Quebec plants remained similar to those of corresponding control plants. The pattern of activity of ascorbate peroxidase in Mississippi plants was erratic but was reduced to about 50% that of Quebec plants during the last 2 days of the cold treatment, while in Quebec plants, ascorbate peroxidase activity was more constant over time and remained similar to control plants over the entire cold-treatment period. Dehydroascorbate reductase was the enzyme most affected by the cold treatment but, while the enzyme extracted from Mississippi plants was completely deactivated by day 4, residual activities were still recorded for the Quebec enzyme by day 9 of the cold-treatment period. The ascorbate contents of cold-acclimated Quebec plants were significantly higher than those of Mississippi plants with higher and similar values, respectively, when compared with control plants throughout the cold-temperature treatment. The glutathione content and reduced glutathione to oxidized glutathione ratios were significantly higher in cold-treated Mississippi plants compared with Quebec plants, although values in Quebec plants were never below those of control plants. The complex pattern of modifications in the activities of the oxygen-scavenging enzymes extracted from Quebec and Mississippi plants suggests that both weak adaptive and acclimatory processes are at play to counter, at least in part, the potential photoinhibitory effects imposed by the cold temperature treatment. Modifications of an acclimatory nature, which may benefit the enzyme performance of both Quebec and Mississippi plants, are shown by monodehydroascorbate reductase and, in particular, glutathione reductase, while higher ascorbate reductase and dehydroascorbate reductase activities in Quebec plants subjected to the cold-photoinhibitory treatment at the end of the treatment period would suggest that these enzymes may have been modified by natural selection to perform under cooler climatic conditions more likely to be associated with this cold-adapted ecotype. Key words: photoinhibition, cold temperatures, enzyme activities, superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, Echinochloa crus-galli, barnyard grass.