In many highly populated areas, such as New Jersey, trees are periodically exposed to elevated ozone (O3) levels during the growing season. More recently, evidence has been found for increased contamination of the environment by cadmium (Cd), a heavy metal which is emitted by various industrial processes, by vehicular traffic, and by the application of sewage sludge to agricultural and forest soils (Beavington 1975; Lagerwerff and Specht 1970; Sidle and Kardos 1977; Sommers 1977). Cadmium per se is also phytotoxic at certain concentrations (Page et al. 1972). Since two pollutants can act synergistically, as in the case of O3 and sulfur dioxide (Reinert et al. 1975), we examined the possibility for an interaction between Cd and O3 on trees. Quaking aspen, Populus tremuloides Michx., was chosen as the test plant because it is one of the most sensitive tree species to O3 (Karnosky 1976).
Experimental Method
Cuttings of quaking aspen clone 6, rooted as described by Karnosky (1976), were obtained from the Cary Arboretum, Millbrook, New York 12545. Rooted ramets, 7 to 10 cm in length, were planted in 5 L plastic pots containing washed sand on 4 August. Plants received dilute nutrient solution (Shive and Robbins, 1937) for four weeks. On 5 September plants were separated into four treatment groups, each containing eighteen randomly selected individuals and placed outdoors in New Brunswick, New Jersey, exposed to whatever pollutants existed in ambient air. Ambient temperatures ranged from 20-29°C during the day and 12-23°C at night. The oxidant concentration of the air was measured continuously with a Mast Sensor (Clarke et al. 1978). Symptoms of Cd toxicity and O3 injury were under daily surveillance. Each group received one of the following treatments: (1) complete nutrient solution, (2) complete nutrient solution amended with 1.0 ug Cd/ml as CdCl2, (3) complete nutrient solution amended with 5.0 ug Cd/ml as CdCl2 and (4) complete nutrient solution amended with 10 ug Cd/ml as CdCl2. Solutions were poured on the sand daily for 30 days, in volumes sufficient to attain field capacity (750-1000 mis).
Plants were ozonated, after 30 days of Cd treatment, in a 6m3 glass enclosed fumigation chamber located within a greenhouse. Ozone was produced by passing pure dry O3 through a commercial O3 generator and mixed with a stream of charcoal filtered air. A complete air change occurred every 45 seconds (Leone et al. 1966). Chamber temperatures were maintained between 18-23°C and relative humidity at 75-80%. Uniform exposure to the O3-air mixture was ensured by placing plants on a rotating table. Ozone levels in the chamber were monitored with a Mast Ozone Meter (Mast Development Co., Davenport, Iowa 52802) and calibrated by the neutral buffered Kl method (Jacobs, 1960). Plants were watered to field capacity ½ h before fumigation. Two O3 exposures were included in this experiment. The lower O3 fumigation was conducted utilizing 0.2 ppm O3 from 1130 hr to 1400 hr on 6 October and the higher exposure with 0.3 ppm O3 from 1100 hr to 1400 hr on 7 October. Each fumigation contained five replicates per treatment. Control plants were placed in an identical chamber under similar conditions in the absence of O3.
Ozone toxicity symptoms on aspen foliage were recorded 48 hr after fumigation. Plants were rated according to a foliar injury index (Table 2). Only leaves greater than 2 cm long were scored. Trees were separated into leaf, stem and root fractions and weighed. Tissue was dried in a forced air oven at 70°C, reweighed, and ground through a 40 mesh screen in a Wiley Mill. One gram samples were ashed at 550°C in a muffle furnace for 8 hr, digested in 3 N nitric acid and analyzed for Cd by means of a Perkin-Elmer Model 303 Atomic Absorption Spectrophotometer. All data were statistically analyzed using an ANOVA and Tukey’s HSD Multiple Comparison Test (Steel and Torrie 1960).
Cd tissue concentrations and Cd symptoms
As shown in Table 1, tissues harvested from plants receiving nutrient solution devoid of Cd contained about 1.0 ppm Cd. When the Cd concentration in the sand was increased, the Cd concentration in all tissue fractions increased significantly. Generally, the Cd gradient was roots > leaves > stems. This is a rather common distribution when Cd is administered via the roots.
Aspens treated with 1 ppm Cd developed a bifacial interveinal chlorosis which became increasingly more severe at the higher Cd treatments (Fig. 1). Actually, the 1 ppm Cd treatment had some beneficial effects; the trees had a greater number of leaves (Fig. 2) and the stem length was increased (Fig. 3).
Cd and O3 Interaction
During the 30 days that the quaking aspens grew outdoors the oxidant concentration of ambient air ranged from 0.01 to 0.08 ppm, relatively low for the fall months in New Jersey. Trees treated with 10 ppm Cd exhibited significantly more oxidant injury on the foliage than trees receiving 0, 1, or 5 ppm Cd (Fig. 4).
When the aspen trees were subjected to a low O3 exposure in a fumigation chamber (0.20 ppm O3 for 2.5 hr), the results were comparable to that obtained in ambient air. Again, O3 phytotoxicity was more severe on the trees receiving 10 ppm (Fig. 5). Plants receiving no cadmium increments exhibited a light stipple of the adaxial leaf surface, but the tree treated with 10 ppm Cd had blotches along green veins and injury extending through to the abaxial surface (Fig. 6 and 7).
When aspen trees were subjected to a higher O3 dosage (0.30 ppm O3 for 3 hr) in a controlled chamber, all the individuals were severely injured and there was no opportunity to observe a Cd-O3 interaction (Fig. 8).
Conclusion
Relatively low levels of Cd, while not adversely affecting the growth of aspen, significantly intensified the degree of foliar injury resulting from low O3 exposures. This is the first time this phenomenon has been demonstrated with a tree species, although it has been shown at higher Cd levels with some herbaceous plants in controlled fumigations (Czuba and Ormrod 1974). We believe that the Cd levels selected in this study are realistic since Cd concentrations as high as 1700 ppm have been reported in heavily polluted soils (Buchauer 1971).
This interaction may have practical importance when trees are growing near a point source of Cd such as a zinc or nickel smelter, in close proximity to a heavily traveled highway, or on soil amended with Cd contaminated sewage sludge. Further studies are needed to determine how long term O3 and Cd exposures will effect tree physiology and productivity in the field.
Footnotes
↵1 This research was supported by McIntire-Stennis funds at the New Jersey Agricultural Experiment Station, Cook College, Rutgers University, New Brunswick, New Jersey 08903.
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