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nents resulting largely from the combustion of high-sulfur fossil fuels. The

second type of smog is comprised of oxidizing compounds, primarily ozone

and oxides of nitrogen. Peroxyacetyl nitrate (PAN) is the major harmful

nitrogenous component, but quantitatively PAN is much less important

than ozone. Photochemical smog containing ozone and NO,/NO occurs

in many major cities of the world, but because of the concentration of

people and the peculiar atmospheric conditions of the Los Angeles Basin,

it is most prevalent in Southern California.



Most photochemical smog arises from the incomplete combustion of

fuels by the internal combustion engine. High temperatures and insufficient

combustion yield hydrocarbon fragments, which act as catalysts in oxidation reactions, and the oxides of nitrogen, largely NO,. The NO, is photochemically cleaved to NO and nascent oxygen, 0:

N O 1 4 NO

+ 0:

The resultant oxygen radical quickly reacts with molecular oxygen to form




0 2 4 0 3

the concentration of which can build up to as high as 1.0 ppm (part per

million, v/v, or 2000 pg/m3 at STP). The kinetics of these reactions are

complex since they are both reversible and dependent upon other atmospheric components, e.g., metals, hydrocarbons, and particulates (Stephens, 1969; Leh and Lak, 1974; Pitts et al., 1975). Furthermore, the hydrocarbon fragments, NOJNO,, and ozone react in a complex manner to

yield PAN and higher homologs of peroxyacyl nitrates. Thus, the photochemically produced N O and NO, are directly responsible for both ozone

and the peroxyacyl nitrate series.

The makings of an air pollution episode are extremely variable throughout the world, but they include the presence of specific types of air pollutants and high temperatures and depend upon relative humidity, time of

day, solar radiation, and condition of the local air mass (for further discussion, see Snodderly, 1974). It is the proper combination of these environmental factors that gives rise to biologically injurious air pollution.

The present United States federal government air quality standard for

total oxidants (both ozone and PAN) is 0.08 ppm for 1 hour. This standard is frequently exceeded in urban atmospheres around the world, especially during summer periods when air inversions or other atmospheric



peculiarities produce stagnant air masses, which allow the pollutants to

build up to high levels. The amount of ozone present in the Los Angeles

Basin ranges from less than 0.1 ppm to well over 0.8 ppm.


The severity of oxidant damage to plants varies from species to species

but depends upon the total physiological state, including nutritional state,

developmental age, and water status (see following sections). Extreme

levels of air pollutants nearly completely destroy leaves; however, moderate

or slight levels induce leaf injury which is characteristic of the particular

pollutant (Darley et af., 1966). Ozone injury is characterized by adaxial

or upper surface necrosis, “silvering,” collapse of specific regions of cells,

waterlogging, and, later, chlorosis of the palisade cells near the stomates.

PAN causes a bronzing of the lower surface in the spongy mesophyll cells,

again adjacent to the stomata1 spaces. Other air pollutants are associated

with different symptoms; e.g., fluoride induces a burn or browning on leaf

margins while SO, causes interveinal burns (see reviews by Thomas, 1951;

Middleton, 1961 ; Rich, 1964; Darley and Middleton, 1966; Heck, 1968).

Traditionally, pinto beans, tobacco, and soybeans have been used to

study acute ozone injury in the laboratory, although many other types of

plants have been occasionally researched. Figure 1A shows ozone-injured

pinto bean leaves. Note that the vein regions are still healthy. Higher concentrations of ozone induce greater injury. Figure 1B shows the typical

symptoms of PAN-injured pinto bean. Here, the upper surfaces of the

leaves are not injured while the lower surfaces have the characteristic

“bronzing,” or “silvering,” as illustrated in the black and white photograph.

Figure 1C shows the pronounced effect of light on air pollution injury induced by ozone, PAN, and ozonated/hexene vapors (an older method for

producing “smoglike” injury). PAN injury is light-dependent in contrast

to ozone.

Although most symptomology-type research has been done with leaf

effects, we know that root growth may be reduced (Tingey et af., 1971),

root nodules of legume are affected (Tingey and Blum, 1973), and in the

case of agricultural production, yields may be drastically reduced (Taylor,


Recently, Feder ( 1970) reported on low-level chronic effects of oxidants

on plant growth.

This review will focus principally upon the historical and current literature dealing with ozone-induced injury to plant systems and will emphasize

the physiological and biochemical symptoms of oxidant injury. Although,

as compared to ozone, investigative interest in PAN has been considerably

FIG.1. Injury t o plant leaves by oxidants. ( a ) Typical variations of ozone injury

to pinto bean. Pinto beans (grown on vermiculite for 12 days) were subjected to

ozone (0.30 ppm) for 1 hour. Leaf on left shows more extreme interveinal injury

(exposed to light prior to fumigation) than leaf on right (injury localized near vein;

held in dark prior to fumigation).

( b ) Typical peroxyacetyl nitrate ( P A N ) injury t o pinto bean. Bean plants were

grown as described in ( a ) and subjected to P A N (0.030 ppm) for 2 hours. Exarnination of lower leaf surface 2-3 days later shows the typical bronzing pattern (right

leaf). Sister leaf's (left) upper surface shows no injury.

(c) Effect of light on oxidant injury. Petunia cuttings (14 days old, grown in

greenhouse) were fumigated for 30 minutes with either ( 1 ) PAN at 0.60 ppm, ( 2 )

O:, at 0.4 ppm, or ( 3 ) 5 pprn O:, 30 pprn gaseous hexene. T h e front row of plants

had the normal light period before and during exposure (about 4 hours) while the

back row of plants had been held in darkness for 26 hours, prefurnigation. Ozone

injury ( 2 ) takes place in light or dark, while P A N injury ( I ) does not occur if

the plant was not exposed to light. Ozonated hexene ( 3 ) does not give same pattern

of injury of ozone but rather produces an interval flecking for both light to dark

prefumigation plants. [All the above figures (a-c) by courtesy of Dr. 0. C. Taylor,

California Statewide Air Pollution Research Station, UCR.]




less, we will present certain results obtained with PAN in an effort to develop a coherent picture of oxidant injury to plants.

There have been a variety of reviews of air pollution (see references),

and two comprehensive books on air pollution and plant life are available

(Dugger, 1974; Mudd, 1975). For the more interested reader, a monthly

summary of air pollution papers is published by the United States Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1 .


Biochemical and Physiological Effects




Ozone is often used as an analytical tool in organic chemistry for precise,

stoichiometric analysis through bond cleavage. Using NADH as a model

system to study the interaction of ozone with biochemicals, Mudd et af.

(1974) showed that the amount of NADH altered by ozone (through a

cleavage of the nicotinamide ring) was stoichiometric with the dose (over

a wide range of ozone concentrations). Likewise, certain amino acids (e.g.,

tryptophan) react stoichiometrically with respect to ozone dose (Mudd,


The ozone molecule is very reactive with a standard redox potential of

approximately $2.1 V (Thorp, 1954). The active species is thought to

be an ionic form of a bent array of oxygen atoms (one resonance form



is: 0 - 0 = 0 ) .Although very dependent upon total atmospheric conditions, it appears that both decomposition and back reactions ultimately

limit the final ozone concentration in the atmosphere (Stephens, 1969;

Pitts et af., 1975).

The solubility of ozone in water, or Bunsen coefficient, is known for

some conditions, notably in acidic media in which ozone is relatively stable.

Ozone is more soluble than oxygen and at 25OC has a Bunsen coefficient

of approximately 0.25 (milliliters of 0, dissolved per milliliter of H,O)

(Hoather, 1948). The solubility of ozone obeys Henry’s law up to at least

several percent (Kashtanov and Oleshchuk, 1937). In alkaline solutions

ozone rapidly decomposes releasing molecular oxygen (Alder and Hill,

1950; Kilpatrick et al., 1956).

In biological studies, the ease with which the ozone molecule penetrates

the cell may be affected by its solubility in the membrane and the cellular

pH as well as other factors. It is not known whether the aqueous layer

that surrounds the cell wall inside a plant is acidic or basic (for a discussion

of cell pH, see Small, 1955) ; however, the zwitterionic form of ozone might

be stabilized by bonding near the cell wall/membrane (see Somers, 1973,



for other types of stabilization). Water in the cell wall is highly involved

in the gel structure of matrix material and charged species (notably Ca2+)

are thought to neutralize the acidic wall constituents (Northcote, 1972).

The reactions of PAN, on the other hand, are less well understood; however, in alkaline solutions of water it decomposes to mixed carbon products, oxygen, and nitrate (Leh and Lak, 1974). The oxygen is thought

to be produced initially in the singlet state and therefore is very reactive

(Stephens, 1969). On the basis of its reactivity with sulfhydryls, PAN has

been postulated to be less oxidizing than ozone (Dugger and Ting,

1970a,b). Furthermore, its solubility in water is also higher than oxygen.



I . Model Systems

Unicellular or chemical systems (Mudd, 1973) suspended in aqueous

media have been used as “models” to study the mode of action of ozone

or PAN. The ozone or PAN is usually bubbled into the liquid medium

for a short time (1-10 minutes) at concentrations that generally exceed

100 ppm. Use of these concentrations is frequently criticized since they

are 100-200 times greater than the highest levels in urban atmospheres.

However, it can be shown that nearly all the ozone that passed through

such systems emerges unreacted (Heath et al., 1974; Frederick and Heath,

1975). Only with certain reactive biochemicals (e.g., fatty acids or sulfhydry1 reagents) does a significantly high proportion of the ozone react

(Mudd et al., 1971a,b; Heath et al., 1974). Using the Bunsen coefficient,

it can be shown that 100 ppm ozone in air above an aqueous medium

effectively amounts to about 1 pM ozone concentration in the water solution or

mole of 0, per 55 moles of water (0.02 ppm, in terms of

water) at room temperature (Thorp, 1954). This type of quantitation is

not available for PAN. Unfortunately, if the water at the cellular surfaces

within a leaf has the same chemical potential as the bulk water, then the

use of 100 ppm in liquid media may still be too high [see Kuiper (1972)

for arguments that bulk water properties differ from water properties near


2. Sulfhydryls

Due to their high reaction rates in chemical systems, sulfhydryls have

long been postulated to be primary sites of ozone and PAN attack in biological systems (Mudd, 1973). Not only are sulfhydryls (-SH) oxidized reversibly to disulfides (S-S) by PAN and to sulfenic acid groups ( S O r H)

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