Chilling of Arabidopsis thaliana (L.) Heynh. callus tissue to 4 degrees C led to conditions of oxidative stress, as indicated by increased levels of the products of peroxidative damage to cell membranes. Cellular H2O2 was also observed to increase initially upon chilling but by day 8 cellular levels had declined to below control levels. Although levels of catalase activity remained similar to those in unchilled tissue, activity of ascorbate peroxidase increased between days 4 and 8 of chilling to 4 degrees C. In callus held at 23 degrees C, levels of reduced glutathione remained static whereas they rose in callus held at 4 degrees C. Levels of oxidised glutathione were initially low but increased significantly by day 4 in the chilled callus. At 23 degrees C, however, levels of oxidised glutathione remained low. Between days 1 and 3 at 4 degrees C, levels of glutathione reductase activity increased but by day 8 glutathione reductase activity was similar to that in cells held at 23 degrees C. Exposure of callus to abscisic acid at 23 degrees C also led to increased activities of ascorbate peroxidase and glutathione reductase.

exposed in order to assess t h e consequences of oxidative stress tolerance cannot reproduce those that will be experienced in field conditions. Only when plants with higher GR levels a n d increased glutathione synthesis capacity are grown in field trials will it be possible to make a full assessment of the benefits of engineering plants with altered glutathione metabolism.

SynopsisAlthough chilling of Arabidopsis thaliana cells to 4°C reduced respiration and superoxide generation, the latter still appears significant. Superoxide may contribute to the observed chilling-induced cellular lipid peroxidation and through processes that may be facilitated by the up-regulation of Cu,Zn-superoxide dismutase activity.