The Activity of the Antioxidant Defense System of the Weed Species Senna obtusifolia L. and its Resistance to Allelochemical Stress.

Senna obtusifolia L., a common weed in the tropical and subtropical regions of the world, is able to germinate under adverse environmental conditions, suggesting that this species has efficient stress-adaptation strategies. The aims of the present work were to examine the energy metabolism and the antioxidant defense system of the Senna obtusifolia L. during seed germination and initial growth, and the responses to allelochemical-induced stress. Respiratory activity, the activities of alcohol dehydrogenase (ADH), superoxide dismutase (SOD), catalase (CAT),guaicol peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), lipoxygenase (LOX) and the content of malondialdehyde (MDA) and glutathione (GSSG and GSH) were measured. Shortly after seed imbibition, mitochondrial respiratory activity was active and the presence of SOD, CAT, GR and LOX activity in embryos, along with significant KCN-insensitive respiration, indicated that the production of reactive oxygen species (ROS) is initiated as soon as mitochondrial respiration resumes. Among the fourteen allelochemicals assayed, only coumarin significantly supressed the growth of S. obtusifolia seedlings. Although coumarin reduced the activities of CAT, POD and APX, the GSH, GSSG and MDA levels were not altered. Alpha-pinene, quercetin and ferulic acid did not modify the activity of the antioxidant enzymes or the contents of GSH, GSSH and MDA. Thus the antioxidant defense system of S. obstusifolia may be effective in counteracting the harmful effects of ROS generated during seed germination and initial growth in the presence of toxic allelochemicals.

ß-Pinene inhibited germination and early growth involves membrane peroxidation.

ß-Pinene, an oxygenated monoterpene, is abundantly found in the environment and widely occurring in plants as a constituent of essential oils. We investigated the phytotoxicity of ß-pinene against two grassy (Phalaris minor, Echinochloa crus-galli) and one broad-leaved (Cassia occidentalis) weeds in terms of germination and root and shoot growth. ß-Pinene (0.02-0.80 mg/ml) inhibited the germination, root length, and shoot length of test weeds in a dose-response manner. The inhibitory effect of ß-pinene was greater in grassy weeds and on root growth than on shoot growth. ß-Pinene (0.04-0.80 mg/ml) reduced the root length in P. minor, E. crus-galli, and C. occidentalis over that in the control by 58-60, 44-92, and 26-85 %, respectively. In contrast, shoot length was reduced over the control by 45-97 % in P. minor, 48-78 % in E. crus-galli, and 11-75 % in C. occidentalis at similar concentrations. Further, we examined the impact of ß-pinene on membrane integrity in P. minor as one of the possible mechanisms of action. Membrane integrity was evaluated in terms of lipid peroxidation, conjugated diene content, electrolyte leakage, and the activity of lipoxygenases (LOX). ß-Pinene (≥0.04 mg/ml) enhanced electrolyte leakage by 23-80 %, malondialdehyde content by 15-67 %, hydrogen peroxide content by 9-39 %, and lipoxygenases activity by 38-383 % over that in the control. It indicated membrane peroxidation and loss of membrane integrity that could be the primary target of ß-pinene. Even the enhanced (9-62 %) activity of protecting enzymes, peroxidases (POX), was not able to protect the membranes from ß-pinene (0.04-0.20 mg/ml)-induced toxicity. In conclusion, our results show that ß-pinene inhibits root growth of the tested weed species through disruption of membrane integrity as indicated by enhanced peroxidation, electrolyte leakage, and LOX activity despite the upregulation of POX activity.

Peroxidase-mediated transformation of hydroxy-9,10-anthraquinones.

A peroxidase (EC 1.11.1.7) has been isolated and purified from Senna angustifolia. The enzyme was purified by ion-exchange chromatography on high Q and high S columns. SDS-PAGE electrophoresis showed that the protein has a molecular mass of approximately 70 kDa. Hydroxy-anthraquinones and hydroxy-anthracenones were evaluated as substrate of S. angustifolia and horseradish peroxidases. Both peroxidases catalyzed the oxidation of alizarin and purpurin anthraquinones to the corresponding 3,3'-bializarin and the new compound 3,3'-bipurpurin, respectively, as well as the formation of 2,2'-biquinizarin from quinizarin anthracenone. The K(Mapp) and V(max) values for alizarin and purpurin were 97 and 95 microM, and 1.5 and 2.1 microM min(-1) mg prot(-1), respectively. The results suggest that peroxidase may participate in the biogenesis of anthraquinones.