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Environmental stress generates an increased synthesis of Reactive Oxygen Species (ROS). The accumulation of ROS in the large amount causes the occurrence of oxidative stress but also stimulates the activation of an antioxidative defense system. The interaction of responses towards double stresses in the plant could promote negative effect because there was an accumulation of negative influences from both stresses, or contrarily increase the resistance of the plant toward stress. Hydrogen peroxide (H2O2) is the main molecule of ROS that is the most stable, which plays the role in the control of acclimatization and defense against stress. This research aimed to examine the effect of the application of exogenous H2O2 to the induction of antioxidative defense system on the soybean plants that get or that did not get drought stress and to define the optimum concentration of H2O2 that can induce maximum oxidative defense system. This research was conducted with Completely Randomized Designs (CRD) with two factors, including the concentration of H2O2 (0 mM, 0.5 mM, 1 mM, and 2 mM, and water supply (watering and drought stress), with five replication for each treatment unit. The result concluded that the application of exogenous H2O2induced the activation of oxidative defense system through the increased activity of enzymes i.e.superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR) and total phenol content. Soybean plants became more resistant to drought stress after the application of exogenous H2O2, shown by the unwithered leaves. The optimum concentration of H2O2 that can induce the oxidative defense system on the soybean cv. Grobogan was 1 mM. [ABSTRACT FROM AUTHOR]

Plants respond to environment stresses in various ways, namely by Reactive Oxygen Species synthesizing in abundance that can trigger the activation of the antioxidant defense system and increase of Phenylalanine Ammonia-lyase enzyme activity that catalyses secondary metabolites formation such as phenolic compounds. Phenol can act as an antioxidant and as allelochemicals which can inhibit plant competitors growth in the surrounding vegetation. This research was carried out using Completely Randomized Designs (CRD) with two factor, i.e. purple nutsedge interference (control, three and six purple nutsedge) and drought (control, mild and severe). The aims of the research was to assess the effect of the multiple stresses of purple nutsedge interference and drought against ROS accumulation in the form of O2.-and H2O2, PAL activity and phenolic compounds composition. The results showed that purple nutsedge interference and drought interact in determining the increase of ROS accumulation, PAL activity and accumulation of phenolic compounds in the leaves of soybean, but the composition of phenolic compounds showed increased accumulation occurs in vanillic acid, 2,5-dihydroxybenzoic acid, caffeic acid, syringic acid, trans-cinnamic acid and salicylic acid, while the composition of 4-hydroxybenzoic acid, trans-ferulic acid and coumaric acid show variation in different treatments. [ABSTRACT FROM AUTHOR]