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

Impact of Ozone on Crops 197
al., 1997; Dizengremel, 2001). The fate of O 3 upon entry into the leaf is not well
known. It has been demonstrated that toxic ROS are formed in the cell wall. The
phytotoxicity of O 3 is due to its high oxidant capacity (redox potential = +2.07 V)
and its capacity to generate other toxic species such as OH·, O 2 · – , and H 2O 2. These
toxic substances attack the composition, structure and function of the plasma
membrane giving place to the primary effects of O 3. H 2O 2 can be transported through
membranes and can induce the generation of other toxic species and signal chain
with messenger molecules. H 2O 2, salicylic acid, jasmonic acid, and calcium have
been proposed in signal pathways for plant defence reaction and may act as second
messengers. The mechanism interactions of these molecules are not yet clear.
12. METHODOLOGY FOR STUDYING IMPACT ON
AGRICULTURAL CROPS
In order to estimate the impact of ambient O 3 on agricultural crops, several methods
have been used. These methods include controlled environment exposure, field
exposure, and field plot systems. Sufficient details may be obtained from Manning
(1999) and EPA USA (1997) reviews and references therein. The most used method
is the open-top chamber (Heagle et al., 1973). These cylindrical open-top chambers
consist of a metallic frame, plastic film panel, and a set-up to sample the air inside
the chamber uniformly. The incoming air is circulated with a fan to reduce the
temperature increase at midday and the entrance of ambient air above the OTC.
Sometimes, a baffle (fustrum) is added to the top portion of the OTC to reduce downdraft.
The growth environment within the OTC will differ in terms of temperature,
relative humidity, irradiance, and wind speed and these parameters are usually
measured inside and outside. The OTC technique is used with a range of concentrations
above and below the ambient air.
Several dose levels of O 3 are used to formulate the dose-response relationships
for yield. Linear polynomial and non-linear Weibull models are normally used to
quantify the dose-response relationships (Kickert and Kruppa, 1991).
The difficulty of transferring results gain under partially controlled conditions
to a more complex situation in the field is well known. Concern about the relevance
of plant responses to plant exposures from plants grown in chambers has
existed for many years (Lewis and Brennan, 1977). However, OTC are considered
the best compromise in simulating natural systems because the experimental
unit is replicable, a range of treatments are available, control of exogenous factors
is possible, and the simulation of field losses is possible. Also, the simulating of
field losses due to O 3 is relatively accurate (US EPA, 1996).
One major concern about OTC has been their modification of microclimatic
conditions. In OTC, daytime temperatures can increase, while photosynthetic active
radiation and wind speed can decrease (Heagle et al., 1988). However, modification
of microclimatic conditions does not appear to affect relative plant responses
to O 3 (US EPA, 1996).
In addition to the aforementioned concerns, constant air flow and turbulence
within the chamber have been criticized (Kimball et al., 1997) and thus, it is