Effect of Ozone on Physiological and Biochemical Processes of Plants

Abstract

The oxidizing nature of O3 is responsible for its phytotoxic effects on plants. O3 enters the plants through stomata and dissolves in the aqueous phase of the substomatal cavity to generate reactive oxygen species (ROS) such as superoxide anions (O2°-), hydrogen peroxide (H2O2), hydroxyl radicals (OH°) and singlet oxygen (1O2). Although ROS are an inevitable part of normal cellular metabolism and are continuously produced in the subcellular compartments like mitochondria, peroxisomes, chloroplasts, etc., O3 exposure stimulates the overproduction of ROS which exceeds the scavenging capacity of the cells intrinsic defense machinery. Plants have incorporated a constitutive antioxidative system which operates to scavenge the ROS generated under normal as well as stress conditions. The defense mechanism of plants has both enzymatic as well as non enzymatic components and works towards annihilating the ROS generated in apoplast as well as symplast. Excess of ROS that are not scavenged by apoplastic antioxidants, target the membrane permeability via the lipid peroxidation of the bilipid layer of the membranes. O3 also brings about alterations in the physiological process by affecting the biochemistry of photosynthetic machinery, disrupting the chlorophyll fluorescence kinetics and light as well as dark reactions of photosynthesis. O3 also alters the biophysical parameters like stomatal conductance and internal CO2 concentration which directly affects the rate of photosynthesis. In addition to this, excess of ROS stimulates the enhanced biosynthesis of cellular antioxidants. O3 stress also brings about changes in allocation of photosynthates, as more biomass is utilized in O3 injury repair rather than being converted to storage sugar, starch. O3 also affects the enzymes of nitrogen metabolism, thus influencing the biosynthesis of amino acids. This chapter investigates the role O3 in ROS generation and stimulation of antioxidant production. Effect of O3 on physiological processes, metabolite contents and nitrogen metabolism is also discussed.