Pretreatment with nitrogen dioxide modifies plant response to ozone

ma4 -69R I/87 $3.00 + 0.w 6: 1987 Pergamon Jovrnals Ltd

Amospheric Environment Vol. 21. No. 3, pp. 717-719. 1987 Printed in Great Britain

SHORT COMMUNICATION PRETREATMENT

WITH NITROGEN DIOXIDE RESPONSE TO OZONE

MODIFIES

PLANT

V. C. RUNECKLES and K. PALMER Dcpartmcnt of Plant Science, University of British Columbia, Vancouver, B.C., Canada V&f 2A2 (Firsr received 25 April 1986 and infinolform 20 June 1986) Abstract-Plant growth inhibition by ozone is significantly affected by previous exposure to nitrogen dioxide. Experiments on the early growtb of four crop speciesshowed that daily pretr~tmcnt with NO2 (0.08-0.10 ppm for 3 h) immediately prior to exposure to O3 (0.08-O. 10 ppm for 6 h) increased the inhibition of radish and wheat growth, decreasedthe inhibition of bush bean growth, but had no effect on the growth of mint. The magnitudes of the interactive cffcetsindicate that in regions where rcfatively high concentrations of Oa are produced by photochemical processes,for example, downwind from urban ccntres,assessmentsof the impact of Oj on vegetation based on knowledge of response to O3 alone may be seriously flawed. Key word index: Air pollutants, sequential exposure, ozone, nitrogen dioxide, plant growth.

1NTRODlJCIION There is abundant evidence that Oa may cause injury and economic loss to crops and vegetation (Heck et al., i984; Roberts, 1984). In air basins where phytotoxic levels of O3 occur as a result of photoehcmieal reactions involving anthropogcnic sources of NO, and hydrocarbon (NC) precursors, there is a wclbestablished sequencein the build-up of the constituents of ‘smog’during the daylight hours, in which the final peak of O3 is preceded by a peak in the level of NO* (Nationat Academy of Seicnecs. 1977). although situations also exist in which higher than normal O3 levels are not prcecded by higher than normal NO2 concentrations as a result of long-distance transport of 0, (Pratt er al., 1983; Lefohn and Tingcy, 1984) There is considerable information available about the effeots of various mixtures of pollutants on plants (Runccklcs, 1984), including a few investigations of NOI+ mixtures (for example see NIES, 1984a, b), but there is little on the et%ets of pollutant sequencesin spite of their real-world importance, and none on the NOL-OS sequence in particular, in spite of its ubiquity. Lefohn and Ormrod (1984) and Lefohn and Tingey (1984) have pointed out that greater emphasis should be placed on sequencesof pollutants rather than on mixtures in designing experiments to assessthe effects of pollutant combinations on plants. The lack of information about thecffeets of the N02-O3 sequence led to the studies de&bed here.

METHODS AND MATERIALS We subjected radish (Raphams sarious L. cv Cherry Belle), wheat (Triricum sufivum i. cv Sun), bush bean (Phase&s vulgoris L. cv Pure Gold wax) and mint ( Menrhaninerim L.) plants, grown from seed (or from rooted cuttings’in the case of mint) in pots in a standardized potting soil, to tr~tment with O,, with or without pretreatment with NOI. Treatments were applied daily from the time of sowing (or transplanting in the cast of mint) in compartmented plastic greenhouses through which c~rcoal-alters air was passedcontinuously toprovidconcairchange mu-’ witha wind speed > 1 m s- I at the plant canopy surface. Gas treatments were randomly assigned to compartments. Sinn only three compartments

717

wercavailablc,one was maintained with filtered air alonc,as I control, one received NO, (0.08~.10 ppm for 3 h between 09:OOand 12:~)andoner~eiv~ 0~ (O.OS~.lO ppm for 6 h between 12:OO and 18:oO). The sequential treatment was achieved by moving pots from the NO1 to the OJ compartments and back carefully to minimize their disturbance. At the time that pots were moved to achieve the NO*-O1 sequential treatment, pots of all other treatments were carefully rc-randomized within each treatment in order to subject all plants to the same amount of handling. NO, was delivered from gas cylinders and was monitored usinga Mast Development Co. meter; O3 was produced by Wellsbach RB4 generators and was monitored using a Monitorlabs Inc. chemilumincseence meter. Both monitors were calibrated using standard procedures in the British Columbia Ministry of Environment’s calibration laboratory. No NO2 could be detected in the O3 treatments. At weekly intervafs the treatments were re-randomized among the chambers. From time to time, series of stomata1 conductance measurements were made on each species,using a LiCor diffusive resistance porometer. Forty plants (IO in the ease of mint) were harvested after approximately 40days. Total and component dry weights were determined. Additional wheat plants were maintained in the treatments and harvested after 280 days. Three experiments were carried out with radish, two with bean, and one each with wheat and mint. The data from the radish and bean experiments were subjeoted to analysis of variance, with each individual experiment treated as a block. With neither specieswas a sign&ant block ctTcetfound and the Student-Newman-Keuls Multioic Rannc Test was anplied to the pooled data (Oshima and Ben&, 1979). In the wheat and mint experiments, there was no true replication of the treatments and therefore no statistical analysis was possible, recourse being made to the descriptive statistics: means and standard errors.

RESULTS AND DLSCUSSION The concentration of NOI used was well below that reported to cause visible injury to any species,while the level of 0, was sufficient to cause the appearance of typical visible symptoms of chronic injury on radish, wheat and bean

Short Communi~tion

718

Table 1. Elfects of NO1, 0, and NO*-0, sequence on total dry matter accumulation (g plant - ’ ) during the early growth of radish, wheat, bush bean and mint plants

Species

Days from start of treatment

Radish (3 expts) Wheat

40

Bean (2 expts) Mint

40

38

56

Treatment Control 0.73 b* I.01 + 004 2.69 b 41.18 +2.13

NO2

OJ

1.41 a

0.66 b

1.07 * 0.06 3.35 a 42.69 + I.27

N&-O,

0.80 + 0.0s 0.53 d

0.54 c 0.50 + 0.03 0.84 c

40.55 * 0.90

43.77 2 1.29

Means and standard errors for wheat and mint; for radish, wheat and bean, n = 40 p~nts/tr~tment/ex~riment; for mint, n = 10. l Student-Newman-Keuls Multiple Range Test: means in the same row followed by the same letter are not significantly different at P 0.05.

leaves-diffuse bleaching or bronzing, with progressivechlorosis. No visible symptoms of acute injury were observed on any species. Chronic injury symptoms were only observed on plantsexposed to the O3 or N02-O~ treatments. Withineach species the visible symptoms were identical, regardless of whether or not the plants received the NO2 pretreatment. Visible effects on growth were apparent as early as IO days after germination. The overall elkts on growth are presented in Table 1. In the cases of radish and bean, NO2 alone caused a significant stimulation of overall growth, presumably reflecting these specks’ability to avail themselves of the gas as an additional source of nitrogen. 0, treatment significantly reduced the overall growth of wheat and bean; in these experiments, suppression of the growth of radish at the levels of Oa used just failed to reach statistical significance although radish has been shown to be relatively sensitive to 0, (Tingey et ai., 1971). 0, preceded by NOI resulted in significant reductions in the overall growth of radish, wheat and bean, when compared to the control. The growth of radish and wheat was inhibited more severely by the sequence than by O\ alone. In these species, ozone- re&oed total dry weight accumulation 10 and 20% resuectivelv. _. relative to the controls, while the sequence caused 26 and 51 Y0reductions. On the other hand, NOa-pretreatment significantly reduced the harmful effects of Oj on bean growth from 80 to 69 “/, relative to the controls. No signi~~nt treatment effects occurred on total dry matter accumulation of mint, although there was the suggestion that pretreatment counteracted the negative elfect of 03. These observations illustrate two types of response. In radish and wheat, NO*-pretr~tment sensitized the plants to subsequent 0s. whereas in beans (and possibly in mint), pretreatment reduced the effect of 0,. However, the growth responses of radish and wheat differed significantly in several respects. In radish, the stimulatory e&et of NO2 was eomptetety overcome by the subsequent 0, treatment. In contrast wheat growth was unaffected when treated with NO1 alone, but the sequential treatment resulted in an enhancement of the inhibition caused by 0, alone. Thus radish exhibited what may be termed a ‘quasi-synergistic’ effect in that the adverse effect of the sequence ( - 26%) was greater than both the effect of OJ alone (- 10 %) and the net effect of the independent NO2 and 0, treatments (+ 93 % - 10% = + 83 %; Table I). since NO2 by itself was stimulatory. Wheat, on the other hand, showed a more typical synergistic response, with the sequence resulting in a 51% decrease in dry weight compared with the net de&ease of 15% calculated from the individual treatments. With beans, the effects of the two gases were clearly antagonistic: more growth occurred in

the sequential treatment than in 0, alone, but the amount was less than the net effect of the independent treatments. Analysis of variance confirmed the significance of these interactions at the P 0.05 level. The treatments resulted in numerous signifi~nt effects on the growth of different parts of the plants which generally reflected the effects on the whole plants. Thus with respect to effects on root weight, Table 2 shows that NO2-pretreatment increased the inhibition caused by O3 alone in the cases of radish and wheat, but reduced the inhibition in both bean and mint. In wheat, both O:, alone and the Not-O, sequence significantly increased the numbers of tillers plant-‘: from 6.3 + 0.3 in the control to 8.6 + 0.6 and 9.4 + 0.6, respectively, after 38 days. This response was sustained: after 280 days there were 27.0 + 0.8 tillers/p~nt in the controls, vs 31.9 + 0.8 and 30.8 k 0.6, respectively~in the O3 and NO1 treatments. The lonn-term (28sday) treatment ofwheat revealed no effect on headweight’as a result of 0, alone (12.6 rt:0.5 g plant- 1vs l3.4* 0.5) but a significant reduction was caused by the sequential treatment (10.8 + 0.4). The observed growth suppressions caused by 03 alone are typical of those reported for a range of species (Roberts, 1984). Studies involving NO1 at comparable concentrations have demonstrated both harmful and beneficial elkcts (Spierings, 1971; Capron and Mansfield, 1976; Troiano and

Table 2. Effects of NOa, O3 and NO&r sequences on hypoeotyl orroot growth (gdrymatter plant-‘)ofradish, wheat, bush bean and mint Treatment Species

Radish (hypocotyl) (3 expts) Wheat slp,:pts) Mint

_ NOZ -03

NO2

0,

0.27 b

0.85 a

0.21 b

O.lSc

0.22 kO.01 0.72 a

0.18 k 0.01 0.73 a

0.19 * 0.02 0.17c

0.08 + 0.01 0.22 b

5.52 + 0.57

5.21 * 0.41

3.19 f 0.22

6.17 + 0.75

Control

Details as for Table I.

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Short Communication Leone, 1977; Ashenden and Mansfield, 1978).The particular significance of the results of our study concerns the variety and magnjtude of the changes caused by NO*-pretr~tment to the e&sets of OJ alone. Although the nature of the experimental design and the facilities avaifable required the daily movement of the plants involved in all treatments, careful handling minimized the likelihood of any effect induced solely by moving the plants. Stomata1 conductance measurements made on each species in the controls did not reveal any changes attributable to plant handling per se. Hence, the variation in the effects of NO2 on response to OJ must reflect differences among the species with respect to biochemical or biophys~i effects of NOz. These preliminary studies have admitt~ly taken a simplistic approach, with no overlap of the gas treatments, no involvement of NO, the first component of the photochemical sequence, and daily repetition of the pollutant treatments, albeit at naturally occurring concentrations (Lefohn and Ormrod, 1984). Nevertheless, they show that, in situations where the NOz-O3 photo-oxidant generation sequence occurs, plant response to the latter pollutant may be modified anoreciablv. They suooort the view (Runeckles. 1984: Lefohn anb Ormr& 1984; gfohn and Tingey, 1984) that &quenceS of pollutant exposures are important determinants of plant response and may be as important as exposures to mixtures of pollutants or more so. An awareness of these erects is essential in the establishment ofambient air quality standards for 0, if they are to have relevance to areas in which the typical pattern of events in photochemical 0, production occurs. A detailed examination of the effects of the NO,-0, sequence on the dynamics of the growth of these species will be reported elsewhere. acknowledgement-This work was supported by an operating grant to V.C.R. from the Natural Sciences and Engineering Research Council of Canada.

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