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IV. Environmental Factors Influencing Susceptibility and Sensitivity
IRWIN P. TING AND ROBERT L. HEATH
in turn alters the chronological age of susceptibility. In a variety of reported
studies, the peak age of susceptibility varied from that at the maximum
growth or expansion rate of a leaf to that just after maximum expansion.
The light factors, quality, quantity, and duration, appear to be very important in governing plant sensitivity to oxidants. First, and perhaps foremost, light affects stomatal opening. The role of stomatas is discussed in
detail in Section V, and only those light effects apparently independent
of stomatal aperture will be mentioned here.
Early studies of Dugger et al. (1963a) showed that, in order for bean
leaves to be injured by PAN, plants must be kept in the light before, during,
and after fumigation. Plants kept in the dark before or after exposure do
not show PAN injury. The explanation for this response is not evident;
however, there are sufficient data to implicate sulfhydryls in PAN injury
and to show a photodependent production of sulfhydryls in bean plants
(Dugger and Ting, 1968). Thus, the light may act to maintain sulfur
groups in a more reduced state. Bean plants kept in the dark for 24 hours,
exposed to light for 30 minutes to open stomata, and fumigated with either
PAN or ozone, show no PAN injury, but significant ozone damage. This
observation rules out stomatal participation in PAN-induced injury.
Juhren et al. (1957) showed that Poa annua grown on short photoperiods were more PAN sensitive than control plants. MacDowell (1965)
and Heck and Dunning (1967) demonstrated the same phenomenon with
tobacco plants. Recent studies with tobacco substantiate these results. Besides the influence of photoperiod, there exists a diurnal fluctuation in sensitivity. When cotton plants grown on 12-hour photoperiods are transferred
to light at the end of the dark period, the stomata open within 15 to 30
minutes, but the plants remain insensitive to ozone for several hours. (Ting
and Dugger, 1968). Studies by Hull and Went (1952) and Koritz and
Went (1953) using ozonated hexene showed that plants were more sensitive at midday or toward the afternoon. Heck and Dunning (1967) also
reported more ozone injury to pinto beans at 1 1 :00 AM and 1 :00 P M
than at either 9 :00 AM or 3 :00 PM.
These studies clearly indicate that light duration and time of light exposure relative to oxidant exposure strongly govern sensitivity. Light is required for the occurrence of PAN injury and plants are generally, though
less decisively, more sensitive to ozone after several hours in the light.
RESPONSES OF PLANTS TO AIR POLLUTANT OXIDANTS
2 . Quantity
A variety of studies have shown that high light treatment tends to protect
plants against ozone injury. Hence, shade tobacco is more sensitive to
ozone than field-grown tobacco. Bean plants on 8-hour photoperiods are
less sensitive to ozone at 3000 ft-c than at 2000 ft-c (Heck and Dunning,
1967), and cotton shows an inverse sensitivity curve with the least amount
of injury for plants grown in full sunlight in comparison with shade-grown
plants (Ting and Dugger, 1968).
Exactly why plants grown under high light are less sensitive to oxidant
injury is not clear, but it may be due in part to increased sugar levels
producing energy for protection (Dugger et al., 1962b).
There is some circumstantial evidence that plant sensitivity to ozone is
influenced by light quality (Heck, 1968). In the case of PAN, however,
it is clear that light quality is an important factor. Dugger et al. (1963b)
constructed an action spectrum for PAN injury of pinto beans that implicated the importance of carotene and chlorophyll-type compounds in the
injury process. There are data to indicate that 660 nm light acts like white
light in enhancing PAN injury, whereas treatment with 700 nm light mimics
the protective dark treatment (Dugger and Ting, 1968). It was suggested
that 660 nm light enhanced sensitivity by driving total electron flow of
the photosynthesis, thus supplying reducing potential for perhaps keeping
the sulfhydryls reduced. The 700 nm light or dark conditions would tend
to oxidize the electron transport chain.
Heck (1968) discussed in detail the temperature sensitivity response,
and we will only indicate here that the response depends upon the individual temperature-growth responses of plants. Furthermore, temperature also
interacts with water status and thus will influence sensitivity indirectly. In
general, temperature-stressed plants will not be sensitive to oxidants, perhaps owing largely to stomata1 closure.
C . INORGANIC
Studies of nutritional effects on foliar injury by ozone have been limited.
Recent results were discussed by Heck (1968), who concluded that known
IRWIN P. TING AND ROBERT L. HEATH
experimental results were inconsistent, inconclusive, and required further
By and large, it appears that plants given adequate nitrogen are more
sensitive to ozone and oxidants than those with deficient or surplus nitrogen
(MacDowell, 1965; Brewer et al., 1961; Leone et al., 1966). In our own
study with nitrogen nutrition of young bean seedlings, we noticed that an
N deficiency shifted the physiological age to a lower chronological age and
reduced overall sensitivity to ozone.
Brewer et al. (1961) studied the effect of potassium and phosphorus
on oxidant injury to spinach and mangels. Results indicated that increased
phosphorus decreased leaf weight production and reduced oxidant injury.
Potassium had no such effect on leaf production but did increase injury
when available phosphorus was low, but not when phosphorus was high.
When nitrogen was high, potassium reduced injury. As seen in general nutrient studies, there are significant interactions among the elements.
Since nutritional status alters growth rates and time of maturity, it also
probably alters the chronological age at which plants are ozone susceptible.
However, like Heck (1968), we must conclude that further studies are
necessary to sort out the inconsistencies presently in the literature.
Oertli (1959) studied the effects of soil salinity on the response of sunflowers to natural Los Angeles Basin smog and found that increased soil
salinity decreased injury. He attributed this effect to stomata1 closure resulting from a salt-induced water stress. In a later study, Maas et al. (1973)
and Hoffman et al. (1973) reported that salinity reduced both growth and
ozone injury to pinto beans. In the same study, leaf resistance to gas transfer was shown to be greater at -4.0 bars than at -2.0 or -0.4, and
it was concluded that greater ozone tolerance at -4.0 bars was related
to the lower expected uptake of ozone. However, other salinity effects on
ozone injury were not ruled out.
Stolzy et al. (1964) studied the effect of root aeration on ozone injury
to tomato. They found that low soil oxygen resulted in a reduced carbohydrate level of the leaves and a significant reduction in ozone injury.
There are a few reports in the literature concerning the effect of oxidants
on plants infected with pathogens (Darley and Middleton, 1966). Yarwood
and Middleton (1954) observed that bean leaves infected with the rust,
Uromyces phaseoli, were not as injured by natural Los Angeles Basin smog
RESPONSES OF PLANTS T O AIR POLLUTANT OXIDANTS
as were healthy leaves, Resh and Runeckles (1973) found that bean leaves
infected with U . phuseoli and uninfected leaves did not differ in their response to low ozone levels. Pucciniu helcuntki infected wheat leaves were
less injured by ozone than healthy leaves (Heagle and Key, 1973).
The protection afforded by pathogens seems to be greatly specific.
Mesophyll cells directly below stomata with visible appressoria as well as
mesophyll cells adjacent to inoculated areas were protected. For these reasons, Heagle and Key (1973) suggested the cells were protected by a diffusible material from the fungus.
Brennan and Leone (1969) observed reduced ozone injury to tobacco
infected with tobacco mosaic virus. They suggested that infection may alter
susceptibility by hastening maturation or by directly affecting cellular metabolism changing susceptibility and sensitivity.
That diseased plants have an altered, reduced response to oxidants is
consistent with the general rule that any factor which interferes with optimum growth tends to reduce oxidant injury.
F. WATERAND WATERSTATUS
In general, high moisture stress results in greater tolerance to oxidants
as a careful study by Leone and Brennan (1969) fully substantiates. Further, Ting and Dugger (1971) using resistant and susceptible varieties of
tobacco, showed that a sensitive cultivar generally had a more favorable
water status under usual growth conditions as estimated by water potential,
transpiration, and leaf resistance measurements.
Since water stress is usually accompanied by stomatal closure, it is sometimes difficult to sort out leaf resistance increases and other effects of water
stress. Overall, those factors which create stress or poor growth seem to
also lessen oxidant injury to plants. As discussed in Section V, the effects
of specific factors on stomatal function must be carefully considered.
The Role of Stomata
The role of stomates has been recognized since the beginning of air pollution studies with plants. Many research papers and review articles have
been produced on the topic, most recently a detailed review by Mansfield
( 1973). Several important questions deserve discussion and perhaps further study. First, what is the role of stomata in regulating ozone uptake?
Second, what is the effect of ozone on stomata? And third, to what extent
does the stomatal apparatus impart ozone resistance to the plant?
IRWIN P. T l N G A N D ROBERT L. HEATH
Stomata1 governing of gaseous uptake can be evaluated using the gas
transfer equations of Penman and Schofield ( 195 1 ) . Simply, the expression
+ R , 4- Rm)
will describe ozone uptake, where Q = ozone uptake in nig cm-2 sec-l;
D = diffusion coefficient for ozone in mg ~ m mbar-l;
Av = ozone pressure
difference between source (atmosphere) and sink (reacting) surface of tissue
in millibars; R, = boundary layer resistance in sec cm-'; R , = leaf or stomatal resistance in sec cm-'; R , = residual resistance to ozone transfer beyond
Ra and R,, also in sec cm-l.
An additional resistance, R,., defined as the resistance to gas transfer
of the nonstomatal surface of the leaf, can usually be ignored since for
water vapor and CO, its value is quite high and constant. We know little
about the R,. for ozone, but Rich et al. (1970) estimated it at 19 sec cm-l
for bean plants. D,the diffusion coefficient for ozone in air is not known,
but would be an exponential function of temperature.
The rate of ozone uptake, therefore, can be increased by a variety of
means including ( a ) temperature increases that directly affect D, ( b )
increases in the amount of ozone in the atmosphere, which would increase
Av, or (c) reductions in any of the resistances, R.
The boundary layer resistance, R,, is a partial function of air movement
as well as the surface characteristics of the leaf and leaf canopy. R,, is
usually in the range of 0.1 to 1.0 sec cm-' and in still air was estimated to
be 0.6 sec cm-' for ozone by Rich et al. (1970). Hill and Littlefield
(1969) reported a severalfold increase in the rate of ozone uptake by
oat leaves with air velocities increased from near 0 to 3.0 miles per hour.
The latter conditions probably operated to reduce R,.
R,, or the stomatal resistance, is most important for our discussion since
it is the variable resistance by which gas transfer is regulated. The study by
Rich et al. (1970) strongly suggests that R , is the major resistance regulating ozone uptake by leaves since resistances for water loss and ozone
uptake are nearly the same. Furthermore, Lee (1965) constructed a curve
showing the expected ozone injury response as a function of stomatal
opening in sensitive bean tissue. This curve is exactly predicted from the
gas transfer equation. At low stomatal apertures, and hence high R,, R ,
becomes the limiting resistance and a near-linear response to gas uptake
and stomatal aperture is seen. At wider stomatal apertures and consequent
lower R , values, other resistances (i.e., the constant R, and Rm)predominate, and further increases in stomatal aperture have minimal effects. This
is why only small correlations may exist between stomatal aperture and