JARS v48n1 - Acid Precipitation Injury to Azaleas

Acid Precipitation Injury to Azaleas
Robert C. Musselman
USDA Forest Service, Fort Collins, Colorado
Patrick M. McCool
Statewide Air Pollution Research Center
University of California Riverside, California 1

Acid rain has been recognized in recent years as a possible disruption to ecosystems in the northeastern United States. Acid rain is generated from the burning of fossil fuels such as oil and coal which produces nitrogen and sulfur oxides. As these pollutants are carried east with the prevailing air masses, the oxides of nitrogen and sulfur react chemically in the air to form nitric and sulfuric acids which eventually come down with rain, fog or snow.
The effects of acidic precipitation on natural and agricultural systems have received considerable interest among scientists in recent years. The effect of acidic rain on lakes is well documented (5) and it has also been suggested that acidic precipitation may be a contributor to recent declines in forest growth in localized areas (6). The effect of acidic precipitation on plant tissue has been studied for many crop species as well as effects of acidic precipitation on forest ecosystems (6).
Acidity is measured on the pH scale, which indicates the hydrogen ion concentration of a solution. The pH scale ranges from pH 0 to 14, where pH 7 is considered neutral, values above pH 7 are alkaline and those below pH 7 are acidic. Each unit change in pH represents a 10-fold change in acidity. For example, change from pH 6 to 5 increases acidity 10 times and change from pH 6 to 4 represents a 100-fold increase in acidity. As a reference for comparison, tomato juice has an acidity of about pH 3.8, vinegar is generally pH 2.8 and lemon juice is about pH 2.3. Most background unpolluted precipitation is somewhat acidic because the normal carbon dioxide in the air reacts in water to form carbonic acid. Other natural compounds in non-polluted air also may add to acidity, such that precipitation acidity recorded for non-poluted areas may range as low as pH 4.5. Acidity in the northeastern U.S. frequently measures about pH 4. However, some rain episodes have been considerably more acid. Fogs in the southwestern U.S. are routinely between pH 2 and 4, with several episodes reported below pH 2.
Research examining the effects of acidic precipitation on crops has found that exposure of plant tissue to acidities above pH 4 results in no negative effects. Exposure to acidities between pH 3 and 4 may cause tissue injury, particularly near pH 3. Acidities below pH 3 often result in leaf injury for most plant species, especially as acidities drop below pH 2.5 (2). Acidic precipitation injury to plants generally occurs as small, irregularly shaped brown or rust colored dead areas on the leaf or flower petal surface. Injury often is found on the lower edges or drip lines of leaves, or in cupped areas of leaves or petals where moisture can collect. Acids concentrate in these areas as the tissue dries. Small, brown dead areas can also appear on the stem of the plant. Most injury occurs on exposed portions of the plant which receive the most precipitation. Little is known about the impact of acidic precipitation on ornamental foliage and flower crops. Simulated acidic rain has been shown to injure flowers of zinnia at pH 2.8 and mature leaves and cotyledons at pH as high as 4.5 (1).

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Several experiments were conducted in southern California to determine the effects of acidic precipitation on azaleas. Acidic fog was selected as a worse case test for azaleas, since fog acidities are more likely than rain to reach levels which might cause injury to azaleas. Fog solutions were formulated to match the background chemistry of fogs which naturally occur in southern California, then adjusted to various pH levels with a mixture of sulfuric and nitric acids. Potted azalea plants were exposed to multiple, two-hour simulated fog episodes at acidity levels as low as pH 1.6. Acidic fog was generated and delivered to plants using a portable apparatus (4) which utilized a commercial fog nozzle to produce artificial fogs from the acidic solutions prepared for testing. The optimum pH level for the root medium of azaleas is 4.5 to 5.8.

Results and Discussion
Results from the simulated fogs indicate that azalea foliage is quite tolerant to exposure to acidic fog (Table 1). No injury was evident after several exposures to acidic fog at pH 3.6 or above. Only small amounts of injury occurred at pH 2.6. Injury to azalea from acidic fog is typical for that seen on other plant species exposed to acidic rain or fog as described above. Some azalea varieties were more sensitive than others. More injury in these experiments occurred on 'Red Wings' than on the other azalea cultivars examined.

Table 1:  Injury to azalea foliage exposed to acidic fog.
Percent Leaf Area Injured
(pH level)
Cultivar 1.6 2.6 3.6 4.6 5.6
'Red Wings' 37 12 0 0 0
'Phoenicia' 27 7 0 0 0
'Mrs. G.G. Gerbing' 27 2 0 0 0
'Fielder's White' 26 2 0 0 0

Injury to azalea flowers occurred at acidity levels of pH 3.6 or below (Table 2). Azalea flowers appeared to be more sensitive to acidic fog than leaves. This is likely due to the capacity of the leaf tissue to partially neutralize acidic precipitation on the surface. The ability of leaves or flowers to neutralize acidity depends on inherent buffering capacity of plants (3). Moisture on the surface of azalea foliage exposed to acidic fog for two hours at pH 3.6 averaged pH 6.0 (Table 3), indicating that leaves were able to neutralize the acidity at that treatment level. However, acidity of moisture on azalea leaves exposed to pH 1.6 was still pH 1.6 after two hours, and these leaves became injured. This data indicates that azalea leaves have only limited capacity to neutralize highly acidic episodes. The neutralizing capacity of flower tissue, unlike that of normal leaves, is particularly low and therefore flower injury may occur at acidity levels which do not injure leaves.

Table 2:  Injury to azalea flowers exposed to acidic fog.
Percent Flower Area Injured
(pH level)
Cultivar 1.8 2.4 3.0 3.6 5.6
'Red Wings' 17 25 12 2 0
'Phoenicia' 21 17 11 8 0
'Mrs. G.G. Gerbing' 15 7 2 2 0
'Fielder's White' 21 14 4 3 0

Table 3:  Acid neutralizing capacity of azalea foliage after exposure to acidic fog.
Fog pH (at spry nozzle) Measure pH of Leaf Moisture
1.6 1.6
2.6 2.9
3.6 6.0
4.6 6.9
5.6 7.0

Fog or rain events are frequently longer than the two hours used in our tests, suggesting that leaves exposed to longer episodes might not be able to continue to neutralize additional acidic input even at higher pH levels. Chemicals used by leaves to neutralize acids can be transferred to the surface from within the leaf. Other chemicals produced at the leaf surface or surface dust deposited on leaves can also neutralize acids. The acid neutralizing capacity is eventually overcome after repeated or prolonged exposure to acidic input or as amount of acidity increases (pH decreases).
It can be concluded that azaleas are mostly tolerant to acidic precipitation and are able to neutralize large quantities of acidic precipitation. However, ambient levels of precipitation in many areas of the United States, particularly those areas with very acidic fog such as the southwestern U.S., are of sufficient acidity to cause some injury to azalea flowers and foliage.

1.  Keever, J. G.; Jacobson, J. S., Simulated acid rain effects on Zinnia as influenced by available nutrients. J. Ameri. Soc. Hort. Sci. 108:80-83; 1983.
2.  McCool, P. M.; Musselman, R. C., Acid fog injures California crops. Calif. Agriculture. 42:6-7; 1988.
3.  Musselman, R. C., Acid neutralizing capacity of leaves exposed to acidic fog. Environ. Exp. Bot. 28:27-32; 1988.
4.  Musselman, R. C.; Sterrett, J. L.; Granett, A. L., A portable fogging apparatus for field or greenhouse use. HortScience 20:1127-1129; 1985.
5.  National Acid Precipitation Assessment Program. Integrated Assessment Report, Aquatic Effects (Section 2.2), pp. 13-44. Office of the Director, NAPAP, Washington, D. C.; 1990.
6.  National Acid Precipitation Assessment Program. Integrated Assessment Report, Terrestrial Effects (Section 2.3), pp. 50-73. Office of the Director, NAPAP, Washington, D. C.; 1990.

We wish to acknowledge the assistance of Dr. Andrew Granett who first proposed this research, and Mr. Jerry Sterrett who coordinated the predawn fogging exposures. We also gratefully acknowledge Milfeld's Nursery, Inc., Monrovia, California, who provided the azaleas for testing, and the American Rhododendron Society for providing financial assistance to Dr. Granett to conduct this research.

1 The authors' addresses are:
Robert C. Musselman, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, 240 West Prospect Rd., Ft. Collins, CO 80526.
Patrick M. McCool, Statewide Air Pollution Research Center, University of California, Riverside, CA 92521.