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

190 S. del Valle-Tascon and J. L. Carrasco-Rodriguez
2. THE TROPOSPHERICAL OZONE
For many years experts thought that tropospherical O 3 was formed exclusively in
the stratosphere. It was assumed that O 3 was transported into the troposphere until
it was destroyed through deposition (Junge, 1963). Experimental data have demonstrated
that the photochemical production of O 3 from nitrogen oxides and volatile
organic compounds is the major source of tropospherical O 3 (Gunderian, 1985).
The overall reactions of the photochemical theory are well known (Stockwell et
al., 1997). However, the exact relationship between precursor levels and O 3 concentrations
is highly complex and not yet completely understood. Knowledge of this
relationship is important to develop effective strategies for controlling ambient O 3
reductions. The production of O 3 depends on light intensity, whereas decomposition
of O 3 also takes place in darkness. In winter, anthropogenic emission of NO
and hydrocarbons are able to destroy O 3, whereas in summer, due to intensive
sunshine anthropogenic O 3 is formed. Air polluted with O 3 is not confined to urban
localities but also affects many rural locations.
Tropospheric O 3 levels are dependent upon the hour of day, season, geographical
location and meteorological conditions (US EPA, 1986). At low elevation
sites, a clear 24-hour periodicity is usually observed, with low concentration at night
and maximum levels before midday. At high elevation sites, diurnal variation is
not observed. Average daytime concentrations ranges from 35 to 55 ppb in summer,
but at peak episodes of severe air pollution concentrations from 400 to 500 ppb have
been measured.
Many O 3 dose exposures have been applied (Lefhon, 1992; Musselman et al.,
1994). Examples of exposure indices are 7 or 8-hour seasonal average, the sum of
all hourly mean concentrations above a threshold of 30, 40 ppb over a defined period,
SUM06, SUM08 (sum of all mean concentrations above 60, 80 ppb), and AOT40
(sum of all hourly average O 3 concentrations above 40 ppb, for a defined period).
A limit AOT40 value of 5.3 ppm has been proposed for crop plants, calculated
for a period of 3 months at day-light hours (Fuhrer et al., 1997).
3. OZONE UPTAKE
Ozone is transported to the surface of plants by turbulent atmosphere. In leaves,
cuticles represent an impermeable barrier (Kerstein and Ledzian, 1989) and thus,
the main route of O 3 entry into the leaf is via the stomata. Stomatal apertures change
in response to a multiplicity of environmental and internal factors in order to achieve
fine adjustment of the gaseous conductance of leaves. It is difficult to generalize
about the stomatal apertures caused by O 3 because the same concentration sometimes,
can, in different circumstances, cause stomata to open or to close, and
dose-response relationships may be erratic. Reduced stomatal conductance is
commonly observed after O 3 exposition (Mansfield and Freer-Smith, 1984; Winner
et al., 1988; Mansfield and Pearson, 1996). However, the control of stomatal aperture
by O 3 has still to be elucidated. The flux of O 3 from the troposphere into leaves
depends on different resistances at various levels. High O 3 concentrations are usually