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

200 S. del Valle-Tascon and J. L. Carrasco-Rodriguez
air pollutants (US EPA, 1998). This difficulty is mainly due to two facts: (a) O 3 is
not emitted directly into the atmosphere by specific sources. Ozone-forming volatile
organic compounds include gasoline vapours, chemical solvents, combustion fuels
and consumer products. (b) The O 3-precursos can be transported hundreds of km
by atmosphere winds. Unfortunately, up to now the control of O 3-precursors has
not been successful.
16. AMELIORATION OF O 3-INDUCED INJURY
16.1. Chemical protective agents
Diverse groups of chemical compounds have been utilized to protect plants against
O 3 injury. These groups include antioxidants, antitranspirants, growth regulators,
fungicides, herbicides, and antisenescence agents (for a recent review see Manning,
1999). Many reports suggest that the chemical agents protect against O 3, but more
experimental data are necessary to demonstrate the proposed reduction of injury
symptoms clearly. The most frequently used are antioxidant ethylene diurea, the
systemic fungicide benomyl, and the antioxidant ascorbic acid.
Ethylene diurea is a protective chemical that prevents the onset of foliar injury
and estimated production loss by O 3. This chemical compound is applied as a
foliar spray, a soil drench or by direct injection (Weidensaul, 1980; Kostka-Rick
and Manning, 1992). The delay of senescence is the most usual response. When
properly used, ethylene diurea is a good tool for detecting the phytotoxicity O 3
concentrations and loss of yield. Ascorbic acid is an antioxidant in plants (Horemans
et al., 2000). Application of ascorbic acid to leaves increases the concentration of
ascorbic acid in cell walls and results in considerable protection from O 3 injury
(Freebairn and Taylor, 1960). The systemic fungicide, benomyl, shows antiozonant
and antisenescence in plants (Manning et al., 1979). However, separation of
fungicides from O 3 protection action is necessary to avoid confusion.
The use of protective chemicals in commercial agriculture requires a toxicological
study prior to determining the dose-response relationship required to determine
appropriate treatments in the field. The advantages of the utilization of chemical
protectives are low cost, good reproducibility of experiments, and no chambers
are required. The disadvantages of this method are that ambient O 3 concentrations
and environmental conditions must be measured, and finally, it is not possible to
calculate the dose-response relationship.
16.2. Elevated CO 2 ameliorates the impact of O 3
In C 3 species, the photosynthetic process is not CO 2-saturated under present-day conditions
because the atmospheric concentrations of ca 370 ppb is well below the
saturation point of ca 670 ppb. In short-term experiments, increased atmospheric
CO 2 may increase photosynthetic rates in C 3 species, provided that sufficient water
and nutrients are available (Long et al., 1993). In long-term experiments, some