Effects of Ambient Levels of NO, on Navel Oranges C. Ray Thompson,l Gerrit Kats, and Earl G. Hensel Statewide Air Pollution Research Center, University of California, Riverside, Calif. 92502 ~~
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Mature navel orange trees, enclosed in plastic-covered greenhouses, were exposed to ambient and 2 times ambient air levels of NO, for 8 months from blooming to picking time t o find out whether this pollutant is causing injury t o citrus. There was n o visible evidence of injury. Leaf drop was greater and yield of fruit was less in ambient air containing photochemical smog than in trees which received carbon-filtered air or carbon-filtered air to which ambient or 2 times ambient levels of NO2 were added. The addition of either of the two levels of NO, had no statistical effect on leaf drop or yield. Ambient levels of NO, which occur in the Los Angeles Basin are probably without effect on citrus.
F
ossil fuel combustion contributes significantly to the amounts of nitrogen oxides that occur in the air of densely populated areas, especially where motor vehicles are a major mode of travel, Los Angeles County alone will have an estimated 730 tons of NO, emitted during 1970 from auto traffic (Fuller, 1969). Other sources will add to this amount. Previous workers observed that the continuous exposure of tobacco for several days to peak concentration levels of NO2 which have occurred in Los Angeles caused marked chlorosis of basal leaves (Bush et al., 1962). Taylor and Eaton (1966) exposed tobacco and beans t o 2 t o 10 ppm of NO2 (4.108 t o 20.54 mg/m3) for 3 days or less and compared the injury caused with that from ozone. Tissue of the same physiological age was affected but the symptoms were visibly different. In previous work with citrus where nitric oxide was added to atmospheres around trees to react with ozone in the photochemical smog complex, thus producing NOs, no change in apparent photosynthesis, water use, leaf drop, or yield of fruit was observed (Thompson et al., 1967; Thompson and Taylor, 1969). In a subsequent direct fumigation study, continuous exposure of navel orange trees to 0.5 and 1.0 ppm of NO, (1.027 and 2.054 mg/m3) caused severe defoliation and chlorosis in 35 days. Exposure t o 0.25 ppm, and possibly less, caused increased leaf drop and reduced fruit yield (Thompson, et al., 1970). The present study was a continuation of this effort to find out whether ambient levels of NO2 are causing injury to navel oranges and if so t o assess the degree of damage.
and 1.5 m from the trunk of the tree, supplementary water was provided. Fertilization and pest control followed commercial practice. Biological control of aphids and scale insects was successful, but red citrus mites required spraying with a mixture of chlorobenzilate and ovotran twice during the study. Trees for the various treatments were randomly selected to minimize effects of the previous experimental treatments (Cochran and Cox, 1957) which was a study of effects of successive levels of NO1. This randomization was used because of the large expense involved in moving these structures and the attendant experiment to another location. The previous experiment had shown minimal effects on the trees, and randomization was considered adequate to remove the inffuence of previous treatments. The trees were divided into four experimental groups of 7, 7, 7, and 6 enclosed trees, respectively, with four outside check trees (Table I). The experimental treatments were ambient air, carbon-filtered air, carbon-filtered air plus ambient lecels of NOn, and carbon-filtered air plus 2 X ambienf leuels o j NOn. The ambient air was the atmosphere that occurred in the experimental location. Carbon-filtered air was obtained by bringing in ambient air through activated carbon filters. Carbon-filtered air plus ambient lecel of NOt was prepared by adding the ambient amount of NO, to the incoming, carbon-filtered air from a cylinder (Thompson et al., 1970), but on a one-day delayed basis as was done previously with HF (Thompson and Ivie, 1965). This was accomplished by monitoring the ambient levels of NOr on any given day. Then these 24 hourly levels were set on a master timer equipped with 10 solenoid valves and needle valves. This timer and valve system then metered out the required levels of NO.> through a stainless steel tubing manifold system and provided each tree with the levels of NO2 which occurred in the atmosphere one day previously. Carbon-filtered air plus 2 X ambient lecel oj' NO2 received twice this amount of pollutant which the previously described treatment gave. Details of the procedure for diluting the NO, and injecting it into the incoming air for the trees were published previously (Thompson et al., 1970). Flowmeters were installed on the greenhouses which received the diluted mixture of NOs-air. These were checked several times daily to ensure that the intended levels of pollutant were being delivered to the trees. The amounts of NOi in the outside air were recorded continuously. T o check on the dispensing system, a n NO-NOI
Method
Twenty-seven navel orange trees enclosed in ventilated plastic-covered greenhouses (Thompson and Taylor, 1969) and located near Upland, Calif., were used for this study. The trees were 15 years old and in good condition. The area has a climate and photochemical smog levels typical of the plain areas of the Los Angeles Basin 30-40 air miles from the Pacific. The trees were irrigated on a 14-day schedule, but if soil suction near the trees exceeded 50 centibars a t a depth of 0.5 m
Table I. Experimental Design of Field Study Treatment
No. of trees
Ambient air Carbon-filtered air Carbon-filtered air plus ambient lecel o j ' N 0 , Carbon-fi[tered air plus 2 X urnbient lecel o f NO? Outside checks
7 7 7 6 4
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Table 11. Comparison of Amounts of NOz Theoretically Dispensed and Measured in Greenhousea Dispensed Recorded Date PPhm pphm 10/21 3.3 5.0 10122 7.92 8.1 10/28 16.1 13.9 10/29 10.2 8.1 10/30 3.3 1.4 11/4 12.2 14.0 1l/5 8.5 6.3 11/7 6.4 8.0 11/12 6.5 7.9 11/13 6.5 8.0 11/14 6.4 16.2 Av. 1 . 5 4 7.86 N.S. a Carbon-filtered air plus 2 X ambient NOa.
analyzer (Atlas Electric Devices, Chicago, Ill.) was used to sample the air from the outlets of the houses. Typical data showing the amounts theoretically dispensed and the amounts actually recorded in parts per hundred million (pphm) are given in Table 11.The average amounts recorded in the carbonfiltered air plus 2 X ambient NOa were slightly higher than those theoretically dispensed but was not significantly greater statistically. The ambient levels of NO, in this location varied from 0 t o 20 pphm (0-0.41 mg/m3).The monthly average concentrations plus the maximum and minimum levels are shown in Figure 1. The average levels in May, June, and November were lower than during the heavy summer smog season and probably result from better ventilation of automotive exhaust from the Los Angeles Basin. The increased level in December could be the result of space heating during winter. Leaf drop was recorded in two ways. Total drop consisted of raking up the fallen leaves, air drying, and weighing. Leaf drop from selected branches was recorded by counting the number of newly set leaves on five branches o n each quadrant of the tree. These 20 branches per tree were tagged o n May 27, 1969, and the number of leaves remaining were counted at six successive times during the remaining season. Results
No visible changes occurred in the behavior of the orange trees which could be correlated with the treatments. All leaves which dropped were gathered and weighed at 4-week intervals.
Ambient air 3118 3647 3854 2988 3511 3236 3313 Av. 3381 a
2oE--l
May
June
July
Aug
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Nov
Dec
Figure 1. Average monthly ambient concentrations of NOI at Upland, Calif.
The total weights are shown in Table 111. Statistical evaluation of these results showed that the amount of leaves which dropped from both outside checks and ambient air trees was significantly greater (1,OZ level) than those in the three carbon-filtered treatments. Although, the weights dropped from the trees receiving NOz were greater numerically than those from the trees receiving carbon-filtered air, this trend was not statistically significant. The leaf drop from the selected 20 branches on each tree was a bit more definitive (Table IV). These results showed that the drop in ambient air was significantly greater than in carbon-filtered air, carbon-filtered air plus ambient NOa, or carbon-filtered air plus 2 X ambient NOZ, a t the 0.1, 0.1, and 0.5 % levels of probability, respectively. The leaf drop in carbon-filtered air plus ambient NOz was greater numerically than carbon-filtered air but was not significantly greater statistically. However, twice this amount of NO2 (carbonfiltered air plus 2 X ambient NO,) was marginally greater ( l o x level) and was about double numerically. The outside
Table 111. Total Leaf Drop From Naval Orange Treesa Carbon-Jittered Carbon-Jltered Carbon-filtered air plus 2 X ambient air air plus ambient NO, NOn 842 1939 1346 2637 2590 3158 2019 1834 1846 1925 2150 2393 1156 2750 2374 1398 1613 1899 . . . 1871 3403 1692 2325 2169
Grams per tree.
Sept
MONTHS 1969
Outside
checks 3836 2740 4921 3140 ... 3659
Table IV. Average Number of Leaves Dropped per Tree from 20 Selected Branches Treatment 5127-718 7/84/18 8118-9/30 9130-1 2118 12118 ~ 2 / 1 0 Ambient air 1.6 1.9 5.6 10.0 9.4 Carbon-filtered air 0.6 0.6 0.6 2.8 2.6 Carbon-filtered air plus ambient NOz 1.1 0.1 0.8 2.9 6.4 Carbon-filtered air plus 2 X ambient NOz 0.9 1. o 0.7 5.0 7.8 Outside checks 15.0 2.5 7.0 6.3 17.3 a
Total 28.5 7.2 11.3 15.4 48.1
Omitted five branches from northeast corner of all trees because of possible wind damage.
checks were not strictly comparable because of lower humidity and temperature and the effect of whipping of branches by wind. The fruit was picked January 19,1970 and recorded as both numbers of fruit and total weight (Table V). As found in previous studies (Thompson and Taylor, 1969 ; Thompson et al., 1967) both number and weight of fruit were statistically greater in carbon-filtered air than in ambient air (1 level). The addition of NOe at both ambient and 2 X ambient lecels had no statistical effect on yield over the trees receiving carbon-filtered air. The outside check trees were not comparable in environmental conditions but gave results statistically the same as the trees receiving ambient air and less than three treatments which got carbon-filtered air.
The effect of NOs on the trees receiving carbon-filtered air indicated only a trend toward greater leaf drop. With both ambient and 2 X ambient levels, the total weight of leaves dropped and the actual number of leaves lost from selected branches was greater than without the pollutant. Thus, it would seem that the amount of NO2 which would cause premature leaf drop was being approached in the study but was not being exceeded. This corroborates a prediction made by Thomas (1961) that ambient levels of oxides of nitrogen will probably not be injurious to vegetation. This conclusion also adds further weight t o the hypothesis made previously (Thompson et al., 1970) indicating that NO2 levels in the Los Angeles Basin are not causing significant injury to citrus.
Discussion
Acknowledgment
These results show the same effect of photochemical smog in ainbient air o n leaf drop and yield of navel oranges as observed previously (Thompson and Taylor, 1969). Total leaf drop in ambient air was 2 times that in filtered air and the drop from selected branches was 4 times greater. This difference in weight is caused by a preponderance of old leaves in the total leaf samples which tend to “even out” the selective effect of smog which causes younger leaves to drop. The counting of young leaves on selected branches thus allows a better measure of the time which leaves are retained by the tree. The difference in yield of fruit in these two treatments was more than twofold.
The authors wish to acknowledge the technical advice of 0. C. Taylor.
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Table V. Fruit Yield From Navel Orange Trees Receiving N O r No. of fruit, Weight, Treatment av./tree kg/tree Ambient air 136 20 2 Carbon-filtered air 366 56 6 Carbon-filtered air plus ambient NO? 41 3 63 0 Carbon-filtered air plus 2 X ambient NO2 284 48 2 Outside checks 176 27 6 -
Literature Cited
Bush, A. F., Glater, R. A. Dyer, J., Richards, G . , Dept. Eng. Rept. no. 62-63, Univ. of California, Los Angeles, Calif., 1962, p p 1-35. Cochran, W. G., Cox, G. M., “Experimental Designs,” 2nd ed., Wiley, New York, N. Y . ,1957, pp 447-449. Fuller, L. J., “Profile of air pollution control in Los Angeles County,” Los Angeles Air Pollution Control District, January 1969. Taylor, 0.C., Eaton, F. M., PlantPhys. 41 (l),132-135 (1966). Thomas, M. D., World Health Org. Monogram Series no. 46, 233-278 (1961). Thompson, C. R., Hensel, E. G., Kats, G., Taylor, 0. C., Atmos. Encir. 4, 349-355 (1970). Thompson, C. R., Ivie, J. O., Int. J . Air WaterPollut. 9, 799805 (1965). Thompson, C. R., Taylor, 0. C., ENVIR.Scr. TECHNOL. 3, 934-940 (1969). Thompson, C. R . , Taylor, 0. C., Thomas, M. D., Ivie, J. O., ENVIR.SCI.TECHNOL. 1,644-650 (1967). Receiced for reuiew August 10, 1970. Accepted March I , 1971. This incestigation was supported in part by grant no. AP-00270 from the Air Pollution Control Office and by Kaiser Steel Corp., Sunkist, Inc., San Bernardino County Air Pollution Control District, and other donors to the Agricultural Air Research Program.
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