ABSTRACT
Energy-generating biochemical reactions favorably use oxygen and produce inevitable reactive oxygen species in a controlled fashion. Inadequate energy dissipation in photosynthetic/respiratory process and chemical modification turns reactive oxygen species (ROS) into more toxic and reactive species. They act as secondary messengers in a concentration-dependent manner and regulate the signaling pathway of the growth and development process in a normal situation. Exposure to extreme environmental conditions causes abnormal production of ROS, disturbs the equilibrium of ROS metabolism and imposes oxidative stress to biomolecules. To avoid cellular damage and to maintain homeostasis, activation and regulation of the antioxidant defense system (first-line defense) are critically required. The mitigating response of both component antioxidant systems (enzymatic and non-enzymatic) completely depends upon the intensity, duration and stress tolerance capability of the plant. Phytohormones are a growth regulator but also worked as an antioxidant modulator through the up-regulation of antioxidant enzymes to enhance stress tolerance.
