Production Practices and Quality Assessment of Food Crops. Vol. 1

Impact of Ozone on Crops 195
Figure 2. The ascorbate-glutathione (Halliwell-Asada) cycle (after Halliwell and Gutteridge, 1989;
Asada, 1994b).
Gutteindge, 1989; Asada, 1994), two enzymes are involved in the detoxification
of reactive oxygen species. The primary scavenger is the enzyme superoxide
dismutase (SOD), which converts superoxide radical. to H 2O 2. Hydrogen peroxide
is eliminated by catalases and peroxidases. Ascorbate peroxidase (APX) is
considered to be the most important scavenger. The product of SOD is eliminated
by ascorbate peroxidase, at the expense of oxidizing ascorbate to monodehydroascorbate.
The enzyme uses ascorbic acid as a substrate and forms part of the
ascorbate-glutathione (Halliwell-Asada) cycle. Other enzymes of the ascorbateglutathione
cycle are monodehydroascorbate reductase (MDAR), dehydroascorbate
reductase (DHAR), and glutathione reductase (GR).
8. ETHYLENE AND PLANT RESPONSES TO O 3
Ozone phytotoxicity has been related to ethylene emission (Tingey et al., 1976).
When plants are subjected to a variety of stresses they often exhibit symptoms of
exposure to ethylene. Exposure of plants to O 3 leads to stress ethylene production.
It is generally accepted that there is a broad correlation between stress
ethylene formations and sensitivity to O 3 (Wang et al., 1990). Exposure of plants
to exogenous ethylene enhances their sensitivity to O 3, whereas pre-treatment with