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Temperature is an important factor for growth, development, productivity and geographical distribution of many plants (Nahar et al. 2015). Chilling stress is a major abiotic stress of crop production in Northeast China. Chilling stress exposure has been shown to enhance production of reactive oxygen species (ROS) and oxidative stress occurs (Nahar et al. 2015). The ROS, which include superoxide radical (O2*-), hydrogen peroxide (H2O2), hydroxyl radical (.OH), and singlet oxygen (1O2), cause damage to structural proteins, nucleic acids, enzymes, cell membranes, and other essential molecules involved in plant metabolism (Sharma et al. 2012, Nahar et al. 2015). Plants have developed mechanisms to tolerate environmental stress conditions through various physiological adaptations, including non-enzymatic and enzymatic ROS scavenging pathways (Hossain et al. 2010, Sharma et al. 2012, Nahar et al. 2015). Non-enzymic components of the antioxidative defense system include reduced ascorbate (AsA) and reduced glutathione (GSH) as well as osmotic adjustment substances as proline, soluble sugars, and soluble proteins which protect membrane integrity and cellular components from dehydration (Özlem and Ekmekçi 2011). The enzymatic antioxidants comprise superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), etc. (Gill and Tuteja 2010, Hossain et al. 2010). These enzymes, through step-by-step reaction, scavenge ROS with AsA and GSH as electron acceptors (Gill and Tuteja 2010). Nahar et al. (2015) has established that low temperature stress increased H2O2 and MDA content. Exogenous spermidine (Spd) in low temperature treatment increases the content of AsA and GSH, decreases the content of oxidized ascorbate (DHA) and oxidized glutathione GSSG, and improves activity of APX and GR. Various strategies are being employed in order to minimize the adverse effects of environmental stresses in plants. Exogenously applied plant growth regulators (PGRs) is an effective, facile, and practical technique to enhance tolerance of crops, and this approach has been used widely in recent years. One of the PGRs is 5-aminolevulinic acid, or 5-amino-4-oxo-pentanoic acid (ALA), which has a relative molecular mass of 131, and it is an essential precursor of tetrapyrrole compounds including chlorophyll, heme, and phytochrome (Balestrasse et al. 2010) and its formation may be the rate limiting step. Hotta et al. (1997a,b) observed that low dosage of ALA has plant growth regulator properties, such as promoting chlorophyll biosynthesis and enhancing photosynthesis (Memon et al. 2009), responding to environmental stresses (Korkmaz and Korkmaz 2009, Korkmaz et al. 2010, Zhang et al. 2012, Dan et al. 2013, Fu et al. 2016), and promoting recovery of growth after herbicide applications (Zhang et al. 2008). High dosages of ALA can cause the accumulation of several chlorophyll synthesis intermediates, but also production of ROS leading to oxidative stress (Balestrasse et al. 2010). Materials and methods Plants and chilling stress: Mung be |
