Tolerance to low temperature and paraquat-mediated oxidative stress in twomaize genotypes

Tolerance to low temperature and paraquat-mediated oxidative stress was inv estigated in two Zea mays genotypes, VA36 and A619, grown at 25/22 degrees C and 16/14 degrees C for 50 d after germination. VA36, the tolerant genoty pe, showed an enhanced resistance to paraquat as compared to A619, the sens itive genotype, when grown at low temperature. In VA36, superoxide dismutas e and ascorbate peroxidase activities increased during growth at both 25/22 degrees C or 16/14 degrees C. in A619, superoxide dismutase activity was s imilar in plants grown at both 16/14 degrees C or 25/22 degrees C. Ascorbat e peroxidase activity was always significantly lower in plants grown at low temperature than in plants grown at 25/22 degrees C, The total ascorbate p eroxidase activity was correlated with the cytosolic ascorbate peroxidase p rotein content in all but A619 plants grown at low temperature for 25 d. Th e isozyme pattern of SOD showed a higher abundance of MnSOD in VA36 than in A619 and of FeSOD in A619 compared to VA36. Growth at low temperature enha nced resistance to paraquat infiltration more in VA36 than in A619. SOD and APX activities were generally higher and more stable with the increase of paraquat concentration in VA36 than in A619. Damage indicated by F-v/F-m an d ion leakage after paraquat infiltration were generally higher in plants g rown at 25/22 degrees C than at 16/14 degrees C and higher in A619 than in VA36, However, no causal link is proved between the extent of damage and th e increase of SOD and APX activities alone. It is suggested that tolerance to oxidative stress requires an integrated enhancement of the antioxidant s ystem.