Antioxidant defence and damage in senescing lupin nodules

Dark-induced and natural nodule senescence (ageing) were studied in lupin (Lupinus albus L. cv. Multolupa) plants. Continuous darkness caused loss of nitrogen fixation. After 2 d of treatment the nitrogenase (Nase, EC 1.18.6.1) activity decreased by 90%, and after 4 d it was completely abolished. Elevated catalytic iron content was detected in the nodule cytosol after 2-7 d of dark and after 7-9 weeks of nodule development versus mature 5-week-old nodules. Accumulation of ferritin, a protein involved in the storage of iron, occurred after 4-7 d of dark. The ascorbate content of nodules declined after 2-7 d of dark as well as in 8 weeks-old plants. The concentration of reduced thiols was lowered with darkness but augmented during ageing; however, the oxidized/reduced thiol ratio was enhanced in both circumstances. There were significant increases in the activity of superoxide dismutases (SOD, EC 1.15.1.1), glutathione reductase (GR, EC 1.6.4.2) and peroxidases in senescing nodules, and decreases in the catalase (EC 1.11.1.6), ascorbate peroxidase (APX, EC and monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) activities. The dehydroascorbate reductase (DHAR, EC 1.8.5.1.) activity also increased in dark stress but decreased during ageing. The content of leghaemoglobin decreased and protein carbonyl groups rose similarly at advanced stages of dark-induced and natural senescence. However, enhanced levels of malondialdehyde (MDA) only were detected during ageing. Overall, these data support the idea that nodule senescence is related to an increase in oxidative stress. Concerning the ultrastructural alterations, a strong degree of similarity has been found between dark-induced and natural senescence. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.