pines are especially sensitive [53]. A large number of pine trees in some of the
western forests appear to be dying as the result of prolonged exposure to photo
chemical oxidants. The injury threshold for sensitive Plants has been estimated at.
About 0.03 ppm (60 µg/m³) for 8-hr exposure. Damage to the plants cited above
has been observed at concentrations of 0.1 ppm or less for time periods of .] to
8 hr. A concentration of 0.06 ppm for 3 to 4 hr damages alfalfa. The retarding of
citrus growth has also been attributed to ozone.
Peroxyactyl nitrate (PAN) has also been found to affect vegetation. It attacks
the spongy parenchyma cells surrounding the air space into which the stomata
open. The principal visible effect is the silvering or bronzing of the lower leaf area
The threshold concentration for injury is estimated at 0.01 ppm for a 6-hr exprosure,
altough petunias have been affected at 0.005 ppm for an 8-hr exposure. The
youngest leaves in terms of maturity are most sensitive.