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

Impact of Ozone on Crops 193
1996). ROS are therefore implicated in most, if not all, stress responses. The
importance of oxidative stress is that ROS are implicated in various deleterious
effects. In plants, ROS have been implicated in wound responses, decreased
photosynthesis, root growth, yield, and senescence (Scandalios, 1994).
Superoxide anion, hydroxyl radicals and singlet oxygen have very few characterized
functions in plant cells, except perhaps in senescence and cell death.
Hydrogen peroxide has many important metabolic roles, since it has been demonstrated
to function on plant defence responses (Mehdy et al., 1996) such as oxidative
cross linking of cell pathogens, direct pathogen killing, activation of host defencerelated
genes, host cell death, and down-regulation of host defence-related genes.
In addition, it is now generally accepted that H 2O 2 is implicated in lignifications
during plant development (Olson and Warner, 1993; Nose et al., 1995).
Nitrogen oxide is a free radical gas with a well-characterized signal role in
mammalian systems. It is now clear that NO is also a major signal molecule in plants
(Durner and Klessig, 1999). NO can be synthesized during environmental stress
responses at the same time as H 2O 2 and it may be that cellular effects reflect
responses of both H 2O 2 and NO. However, the full range of biological functions
for these two signalling molecules has still to be catalogued and determining the
ways in which they interact will need to be elucidated.
The extremely short lifetime of ROS makes their production analysis on plants
a very difficult task. Spin label, advanced molecular genetic techniques, and digital
imaging of ROS are power tools for investigating the development of oxidative stress
in leaves and for the regulation of ROS metabolism.
The functions of calcium, jasmonic acid, salicylic acid, salicylic acid-signalling
and jasmonic acid-signalling pathways on O 3-induced oxidative stress will need to
be elucidated.
6. PHYTOTOXICITY OF REACTIVE OXYGEN SPECIES
The O 3-generated reactive species include OH·, O 2 · – and H 2O 2. The hydroxyl radical
is one of the most reactive species of oxygen and no protective mechanisms
are known, except α-tocopherol (Heath, 1980). The free radical hydroxyl is also
formed by the chemical reaction of H 2O 2 and O 2 · – in the presence of transition metals
(Haber-Weis reaction). The hydroxyl radicals are responsible for a great part of
O 3 phytotoxicity since they react rapidly with protein, lipids, and DNA causing
cell damage (Iqbal et al., 1996). Hydroxyl radicals can modify proteins making them
more susceptible to protein attack (Casano et al., 1994). Once damaged, proteins
can be broken down further by specific endopectidases (Casano et al., 1994).
Hydrogen peroxide can cause DNA breakage and can also inactivate thiol-containing
enzymes such as thioredoxin-modulated enzymes in the chloroplast stroma (Charles
and Halliwell, 1981; Hagar et al., 1996). Although O 2 · – anion and H 2O 2 can inactivate
various macromolecules directly, it is their conversion to hydroxyl radical that
accounts for their main toxicity. Superoxide and H 2O 2 in the presence of transition
metals (Haber-Weis reaction) lead to hydroxyl radical formation. Hydroxyl
radicals can cause damage to all kinds of biological macromolecules. These hydroxyl