Biochemical Reactions of Ozone in Plants 1

Plants react biochemically to ozone in three phases: with constitutive chemicals in the apoplastic fluid and cell membranes; by forming messenger molecules by the affected constitutive materials (ethylene); and by responding to the messenger molecules with pathogenic RNAs and proteins. For instance, plant reactions with ozone result in constitutive molecules such as the ozonolysis of ethylene in the gas spaces of the leaf, and the reaction with ascorbic acid in the apoplastic fluid. Formation of messenger molecules include the stimulation of ethylene production. Responses to the messenger molecules include the formation of the pathogen related proteins and their mRNAs. Reactions of ozone with biological molecules also frequently result in classical ozonolysis of double bonds, with the production of various aldehydes and peroxides, such as ethylene, isoprene, fatty acids, tryptophan, and some phenylpropenoic acids. Some reactions of ozone with biological molecules do not fit the classical ozonolysis mechanism, such as the oxidation of methionine and some phenylpropenoic acids. Introduction Although ozone has toxic effects on both plants and animals, the mechanisms by which the toxic effects are elicited are inadequately understood. However, plants react biochemically to ozone in three phases: with constitutive cells and materials; by generating messenger molecules; and by responding to the messenger molecules. Reactions with constitutive cells and materials result in substances in the gas phase, (e.g., ethylene) that are dissolved in the apoplastic fluid, (e.g., ascorbic acid) and that react with the lipids and proteins of the plasma membrane of cells surrounding the sub-stomatal cavity. The concentration of ozone in the leaf spaces inside the leaf is zero (Laisk and others 1969). This indicates that ozone passing through the stomata is immediately consumed by oxidizable substances lining the leaf spaces. Because lesions characteristic of ozone damage occur in the palisade parenchyma, palisade cells are part of the sink for ozone. Reactions of messenger molecules cause stimulation of the synthesis of molecules such as ethylene, while the response of messenger molecules results in the formation of messenger RNA and the proteins for which they provide the genetic code. However, because these three stages of response to ozone are simply an analysis of the problem, a practical method of protecting plants needs to be developed. This paper discusses the biochemical reactions of plants to ozone, focusing on the reactions of constitutive cell materials, the synthesis of messenger molecules, the responses of the messenger molecules, and various methods to protect plants from ozone damage. Discussion Ozone Reaction with Constitutive Materials Ethylene Ethylene has been known as a “ripening hormone.” The arrival of the ripening is coincident with a “climacteric” in ethylene production. In addition to this 1 An abbreviated version of this paper was presented at the International Symposium on Air Pollution and Climate Change Effects on Forest Ecosystems, February 5-9, 1996, Riverside, California. 2 Professor Emeritus, Department of Botany, University of California, Riverside, CA 92521, U.S.A. email: [email protected] J. Brian Mudd2