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

196 S. del Valle-Tascon and J. L. Carrasco-Rodriguez
aminoethoxyvinyl glycine to inhibit stress ethylene biosynthesis makes plants less
sensitive (Mehlhorn et al., 1991). Ozone may react with volatile compounds emitted
by plants into the protoplast. Interactions of O 3 with ethylene or other hydrocarbons
are thought to be part of the mechanism leading to injury. Ozone-sensitive
species were found to produce more ethylene during O 3 treatment than ozone-tolerant
plants (Wellburn and Wellburn, 1996). In summary, stress ethylene is considered
to be the major cause of accelerated senescence and leaf abscission (Mehlhorn et
al., 1991).
9. LEAF VISUAL INJURY
Visual injury to crop plants caused by O 3 can range from severe necrosis and death
of much or all the exposed tissue, to mild chlorosis (Heagle et al., 1973; Deveaou
et al., 1987; Heagle et al., 1987; Mulchi et al.; 1988, Heggestad, 1997). Acute injury
usually involves necrosis, varying from general large die-back areas, to small areas,
stipples or flecks; such injuries occurring as a result of fumigation with high doses
of O 3. Chronic injury results from exposures to sub-acute dosage. Chlorosis and
death of isolated cells are generally observed in these conditions. However, visible
injury is not dependent upon O 3 concentration or exposure time (Heck et al., 1966).
In the last few decades, the effects of O 3 on crops have been made by visual procedures.
The principal drawbacks of this procedure are subjectivity, inconsistency,
and low specificity.
10. PHYSIOLOGICAL EFFECTS
Elevated atmospheric O 3 frequently has harmful effects on the photosyntheticperformance
of agricultural crops (Heck et al., 1983, Amundson et al., 1987; Lehnherr
et al., 1988; Violin, 1998; Donnelly et al., 1998, 2000). Reduction in chlorophyll
content (Köllner and Krauss, 2000) and carboxylation efficiency (Reid and Fiscus,
1988; Farage et al., 1991) contribute to the observed decrease in net photosynthesis
under elevated O 3. The loss of Rubisco activity induced by elevated O 3
apparently results from a decrease in the quantity of Rubisco present (Pell et al.,
1992; Baker et al., 1994). Moreover, exposure to O 3 typically reduces effective
leaf area, thus decreasing light interception and the quantity of assimilate available
to support the growth of economic yield component. O 3-induced disruption
of cellular metabolism may also promote a loss of chlorophyll and increase
senescence (Grandjean and Furher, 1989; Fangmeier et al., 1994; Djanperä et al.,
1998).
11. MECHANISM OF OZONE ACTION
The precise mechanism of O 3-induced injury in plants is poorly understood (Pell
et al., 1992; Heath, 1994; Schaudner et al., 1997; Heath and Taylor, 1997; Pell et