Role of Defense Enzymes in Plant Responses to Light Stress

Abstract

Light is a major environmental determinant of plant growth, photosynthesis, development, and stress acclimation. Both insufficient and excessive irradiance disrupt metabolism and may reduce productivity. Low light restricts carbon assimilation, biomass accumulation, leaf development, and resource transport, whereas excess light can exceed photosynthetic capacity, promote photoinhibition, impair photosystem II, and increase the formation of reactive oxygen species. Uncontrolled accumulation of superoxide radicals, hydrogen peroxide, and related oxidants damages membranes, proteins, pigments, and other cellular components. Plants counter light-induced oxidative stress through coordinated adjustments involving chloroplast movement, stomatal regulation, pigment composition, non-enzymatic antioxidants, and antioxidant enzymes. This review examines the principal defence enzymes associated with plant responses to light stress, with emphasis on superoxide dismutase, catalase, ascorbate peroxidase, class III peroxidase, glutathione peroxidase, and glutathione reductase. These enzymes function as an interconnected redox network rather than as isolated components: superoxide dismutase converts superoxide to hydrogen peroxide; catalase and peroxidases remove hydrogen peroxide; and glutathione-dependent pathways maintain reducing capacity and redox homeostasis. Evidence across species and light regimes indicates that enzyme responses vary with genotype, light intensity, spectral quality, exposure duration, tissue, developmental stage, and accompanying stresses. Increased enzyme activity does not necessarily indicate greater tolerance because it may also reflect stronger oxidative pressure. Integrated interpretation of enzyme activity, reactive oxygen species accumulation, redox status, membrane damage, and photosynthetic performance is therefore required to identify mechanisms that genuinely support light-stress resilience.