Exposure to a variety of environmental factors such as salinity, drought, metal toxicity, extreme temperature, air pollutants, ultraviolet-B (UV-B) radiation, pesticides, and pathogen infection leads to subject oxidative stress in plants, which in turn affects multiple biological processes via reactive oxygen species (ROS) generation. ROS include hydroxyl radicals, singlet oxygen, and hydrogen peroxide in the plant cells and activates signaling pathways leading to some changes of physiological, biochemical, and molecular mechanisms in cellular metabolism. Excessive ROS, however, cause oxidative stress, a state of imbalance between the production of ROS and the neutralization of free radicals by antioxidants, resulting in damage of cellular components including lipids, nucleic acids, metabolites, and proteins, which finally leads to the death of cells in plants. Thus, maintaining a physiological level of ROS is crucial for aerobic organisms, which relies on the combined operation of enzymatic and nonenzymatic antioxidants. In order to improve plants’ tolerance towards the harsh environment, it is vital to reinforce the comprehension of oxidative stress and antioxidant systems. In this review, recent findings on the metabolism of ROS as well as the antioxidative defense machinery are briefly updated. The latest findings on differential regulation of antioxidants at multiple levels under adverse environment are also discussed here.

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Melon (Cucumis melo L.)-an important horticultural crop that is often cultivated in simply equipped solar greenhouses in northwestern regions of China-usually suffers under poor water management. Arbuscular mycorrhizal (AM) symbiosis can play a major role in enhancing drought tolerance. Plant growth, physiological, and photosynthetic responses of melon plants inoculated with three Glomus species under two water conditions were investigated. Results show that inoculation with Glomus improves the physiological and photosynthetic parameters of inoculated seedlings compared with non-AM seedlings. Regardless of water conditions, plant height, root length, biomass production, antioxidant enzyme activity, soluble sugar content, net photosynthetic rate, and photosynthetic water use efficiency were elevated in AM seedlings compared to non-AM seedlings. EachGlomus species manifests unique effects under the two watering conditions. We posit that AM symbiosis can protect melon plants against water deficiencies by improving their antioxidant activity, bi-directional transport of carbohydrates, and photosynthetic capacity. In addition, regardless of water conditions, the most efficient fungus for melon (Cucumis melo L.) was Glomus mosseae.