**4.2 Enzymatic and non-enzymatic antioxidants**

There are several componentsutilized by plantsto cope up with oxidative stress, which are involved in ROS homeostasis modulation [54]. Plants produces various reactive oxygen species (ROS) continuously as bi-products of various metabolic pathways in different cellular compartments like chloroplast, mitochondria, and peroxisome. ROS have partially reduced forms of atmospheric oxygen and under normal conditions, their production in plant cells is balanced by their effective scavenging through enzymatic and non-enzymatic cascade (**Figure 6**). ROS can cause damage to different biomolecules namely DNA, proteins and lipids, and therefore by creating oxidative injury; it leads to a reduction in plant growth and development [56]. The equilibrium between the production and the scavenging of ROS may be perturbed by various stress factors. Thus, the disturbances of cellular homeostasis resulted in a sudden rise in intracellular levels of ROS leading to oxidative stress which in turn can cause substantial damage to cell structure and membrane integrity. To mask themselves from these toxic oxygen intermediates, plant cells contain both enzymatic and non-enzymatic components. Among them enzymatic antioxidantsare superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR) and ascorbate (AsA), glutathione (GSH), carotenoids, glycine betaine, proline, α-tocopherol and flavonoids are the non-enzymatic antioxidants [51, 57]. Hence, stress induced oxidative damage of ROSs can only be counteracted by increased level of enzymatic and nonenzymatic antioxidants [54].

**Figure 6.** *ROS scavenging mechanism by antioxidant defense system in different stress conditions. Source: [55].*
