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ELS EV I ER Scientia Horticulturae 58 (1994) 67-76
SClENTIA HORTICULI'UI~
Growth responses of tomato plants to low concentrations of sulphur dioxide and nitrogen
dioxide
Jitendra Pandey*, M. Agrawal Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi-221005, India
(Accepted 28 November 1993)
Abstract
Forty-five-day-old plants of Lycopersicon esculentum were exposed to 0.1 ppm SO2, 0.2 ppm NO2 and 0.1 ppm SO2+0.2 ppm NO2 for 4 h daily for 50 days. Plant height, number of leaves, total leaf area, total leaf biomass, total shoot biomass and total plant biomass were increased by these low dosages. When exposed to either NO2 or SO2 alone, relative growth rate, net assimilation rate, leaf production rate and specific leaf area increased initially but declined after exposure for longer times. Both SO2 and NO2 reduced root growth and resulted in a low root:shoot ratio. Number of leaves, total leaf area, total leaf biomass and leaf area duration increased up to 30 days of both SO2 and NO2 exposure, indicating that the tomato plants in response to SO2 and NO2 allocated a greater propor- tion of its photoassimilates for growth and development of photosynthetic organs. Chlo- rophyll concentration increased initially but declined after exposure for longer periods. Foliar N and SO 2--S content increased in plants exposed to NO2 and SO2, respectively. P content was reduced following exposure to SO2 and NO2 alone or in combination. The study suggests that low dosages of pollutants are fertilizing the plants, promoting vegeta- tive growth at the expense of reproductive growth.
Keywords: Growth indices; Lycopersicon esculentum; Nitrogen dioxide; Pollutant; Sulphur dioxide
*Corresponding author. Abbreviations: RGR, relative growth rate; NAR, net assimilation rate; LPR, leaf production rate; RPR, root production rate; LAD, leaf area duration; SLA, specific leaf area; LAR, leaf area ratio; LWR, leaf weight ratio; RSR, root:shoot ratio; TI, SO2 exposed plants; T2, NO2 exposed plants; Ta,SO2 + NO2 exposed plants
0304-4238/94/$07.00 © 1994 Elsevier Science BN. All fights reserved SSD10304-4238 (93)00619-D
68 J. Pandey, M. Agrawal / Scientia Horticulturae 58 (1994) 6 7- 76
1. Introduction
Sulphur dioxide (SO2) and nitrogen dioxide (NO2) are most frequently cited as primary factors responsible for plant damage in urban industrial areas (Mans- field and Freer-Smith, 1981 ). The injurious effects of these pollutants on agricul- ture and native vegetation have been investigated by many workers (Elkiey and Ormrod, 1981; Mansfield and Freer-Smith, 1981; Okano et al., 1985; Martin et al., 1988; Fangmeier, 1989; Murray and Wilson, 1991 ). Low concentrations of these pollutants stimulate plant growth, but at higher concentrations they cause visible injury to leaves and affect the rates of photosynthesis, respiration and transpiration (Koziol and Whatley, 1984; Mansfield et al., 1986; Darrall, 1989 ). Both SO2 and NO2 often alter general growth patterns and the pattern of photo- synthate allocation in plants (Freer-Smith, 1985; Gould and Mansfield, 1988; Pandey and Agrawal, 1991 ).
Different parts of the plant do not respond identically to these pollutants. SO2 consistently lowers the root:shoot ratio (RSR) (Mejstrik, 1980; Jones and Mans- field, 1982). However, the effects of NO2 on this ratio are not consistent. The RSR of several grasses remained unaltered even when growth of the whole plant was significantly reduced (Whitmore and Mansfield, 1983), but for sunflowers exposed to 1.0 ppm NO2 for 2 weeks the ratio was significantly reduced (Okano et al., 1985). SO2 and NO2 can induce compensatory growth by an increase in leaf area ratio (LAR) (Jones and Mansfield, 1982; Okano et al., 1985). The ob- jective of the present study was to investigate the effects of low concentrations of SO2 and NO2, singly and in combination, on growth performance, chlorophyll and nutrient concentrations and photoassimilate partitioning of tomato plants.
2. Materials and methods
2.1. Experimental design
Twenty-day-old tomato (Lycopersicon esculentum Mill. cultivar 'Pusa Ruby') plants were transplanted 20 cm apart into 12 separate well-manured plots each 1.5 m × 1.5 m. The soil was sandy loam of a pale brown colour, pH 7.2, organic carbon 0.86%, total N 0.09%, available P 0.005%, exchangeable K 0.1%, and cat- ion exchange capacity 15.4 mEq (%). Plants were allowed to stabilize for about 3 weeks before fumigation. Plants were kept in a well-watered condition through- out the experiment.
Fumigation of plants with 0.1 ppm SOe (T~), 0.2 ppm NO2 (Te) or 0.1 ppm SO2 + 0.2 ppm NO2 (T3) began when the plants were 45 days old and was applied daily for 4 h, between 08:00 and 12:00 h, up to a plant age of 95 days. Fumigation was done in a 1.5 m 3 iron frame covered with a 0.25 mm thick transparent poly- thene chamber. For the uniform distribution of gaseous pollutants each chamber was supplied with perforated PVC pipes (2.5 cm diameter, with 1.0 mm holes 10 cm apart) arranged at the bottom of the chamber. Each chamber was also fed with an additional flow of air ( 100 1 min- ~ ). A small battery-operated fan was
J. Pandey, M. Agrawal / Scientia Horticulturae 58 (1994) 67-76 69
used to ensure uniform distribution of pollutants within the chamber. The con- trol plants were enclosed in a similar chamber but without pollutants during fu- migation. During the exposure period, the plants were maintained under natural light. Temperature and relative humidity of the exposure and control chambers were 24_+ 3 °C and 65-70%, respectively.
SO2 was generated continuously by bubbling air (0.5-1.01 rain-~ ) through 1% aqueous sodium metabisulphite solution. NO2 was generated continuously by the action of dilute nitric acid on 0.1% sodium nitrite solution (Prasad and Rao, 1979).
To monitor the concentrations of these pollutants, a small portion of air from each chamber was continuously passed through impingers containing sodium te- trachloromercurate solution and sodium hydroxide + sodium arsenite solution. These solutions were later analysed colorimetrically for SO2 (West and Gaeke, 1956) and NO2 (Merryman et al., 1973), respectively. Concentrations of SO2 and NO2 were 0.1 _+ 0.02 p.p.m, and 0.2 _+ 0.02 p.p.m., respectively.
2.2. Plant sampling and analysis
Samples were collected from the control and each set of treatment plots at 10 day intervals, beginning on Day 45; however, only the data for Days 55 and 95 are presented. For each treatment, three replicate plots were maintained. Three plants from each set were harvested at each sampling date. Plant height and num- ber of leaves, flowers and fruits were recorded. Leaf area was determined with a LI-COR leaf area meter (LI-3000; LI-COR, Lincoln, NE, USA).
Chlorophyll pigments were extracted in 80% acetone. The optical densities of the extracts were measured at 645 and 663 nm using a Spectronic 1001 spectro- photometer (Milton, Roy, Rochester, NY, USA). Chlorophyll concentrations were calculated using the formula given by Maclachlan and Zalik ( 1963 ).
For biomass determination, plants were separated into leaf, stem and root and oven-dried separately at 80°C to constant weight. Dry powdered samples were used for the determination of SO 2--S, N and P concentrations. Total N was de- termined by the micro-Kjeldahl technique (Misra, 1968). For P and SO42--S measurements, tissue was ashed at 480°C and dissolved in 0.1 N nitric acid. P was determined by the phosphomolybdic blue colorimetric method (Jackson, 1958 ) and SO2--S by the turbidimetric method of Rossum and Villarruz ( 1961 ).
Relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR), leaf production rate (LPR), etc., were computed from formulae given by Hunt (1982).
At 105 days, yield was determined as number of fruits per plant and dry weight of fruits per plant.
Three-way analysis of variance and the t-test were performed to test the level of significant difference.
70 J. Pandey, M. Agrawal / Scientia Horticulturae 58 (1994) 6 7- 76
3. Results
Plant height increased throughout the experimental period in both control and treated plants (Table 1 ). Compared with the control, the number of leaves in- creased in T 2 plants. In T1 plants, the number of leaves increased initially but declined after exposure for longer periods. However, the number of leaves was always less in T 3 plants compared with the control. In all the treatments, the num- bers of flowers and fruits with respect to the control plant decreased.
Total leaf area and total leaf biomass increased in T2 plants throughout the study (Table 1 ). Tt and T3 plants showed reduced leaf area and leafbiomass after a slight increase up to 10 days of exposure. Total biomass was reduced in T1 and
Table 1 Effects of 0.1 ppm SO, and 0.2 ppm NO, on development of tomato plants
Measurement 55 days 95 days
Control SO2 NO2 SO2+NO2 Control SO2 NO2 SO2 + NO2
Height (cm) 44 47 50 48 67 86** 118"** 72 Leaves per plant (No.) 18 22 24** 18 38 35 66** 32 Leaves per plant (g DW) 1.68 2.0 2.25* 1.76 3.55 3.20 4.70* 2.90 Flower per plant (No.) 4 - - 46 34* 36* 24** Shoots per plant (gDW) 4.90 5.10 5.63* 5.19 8.39 7.85 12.18" 7.20 Roots per plant (gDW) 1.10 1.00 0.92 0.96 2.26 2.10 1.98 2.15 Fruits per plant (No.) - - - 16 10* 8** 6** Fruits per plant (gDW) - - - 36.2 20.7* 16.50"** 12.6 Total leaf area (cm 2) 605 760*** 800*** 635* 960 908* 1410"** 837*** Biomass per plant (gDW) 6.0 6.10 6.55 6.15 10.65 9.95 14.16" 9.35
Fumigation was begun on 45-day-old plants and was applied for 4 h daily thereafter. Data are for 55- and 95-day- old plants. DW, dry weight. Level of significance: *P< 0.05; **P< 0.01; ***P< 0.001.
Table 2 Mean square estimates of chlorophyll and biomass significantly affected by SO2 and NO2 exposure
Source d.f. Mean of squares
Chlorophyll Biomass
SO2 1 1.1368*** 10.3072*** NO2 1 0.8354"** 16.3819"** Age 5 0.0739*** 60.8726*** Replicate 2 0.0019NS 0.3437NS SO2 X NO2 1 0.0491" 3.1732*** SO2 X Age 5 0.0132NS 10.2758"** NO2 X Age 5 0.0020NS 6.8137*** SO2 × NO2 × Age 5 0.0307** 2.0018*** Error 48 0.0104 0.2814
***, **, *, NS: significant at P < 0.005, 0.025, 0.100 and not significant, respectively. d.f., degrees of freedom.
Tab
le 3
E
ffec
ts o
f 0.1
ppm
SO
2 an
d 0.
2 pp
m N
O2
on g
row
th c
hara
cter
istic
s (d
ry w
eigh
t) o
f tom
ato
plan
ts
Mea
sure
men
t 55
day
s 95
day
s
Con
trol
SO
2 N
O2
SO2
+ N
O2
Con
trol
SO
2 N
O2
S
O 2
"t- N
O2
Rel
ativ
e gr
owth
ra
te (
gg -I
day
-l)
0.03
2 0
.06
7*
**
0.
072*
**
0.06
5***
0.
014
0.00
9 N
et a
ssim
ilat
ion
rate
(m
scm
-2 d
ay-t
) 0.
542
0.51
0 0.
573
0.56
1 0.
154
-0.0
99**
* L
eaf p
rodu
ctio
n ra
te (
mg
g-I
day
-l )
6.
70
9.30
* 11
.16"
* 7.
30
-0.2
2
-0.2
3
Roo
t pro
duct
ion
rate
(m
s g
- l d
ay-
t )
3.72
2.
94
1.49
"*
1.92
" 1.
32
1.15
L
eaf a
rea
dura
tion
(m
2 da
y -1
) 5.
10
5.91
6.
09
4.31
9.
40
9.24
Sp
ecif
ic le
af a
rea
(mg
cra -
2)
2.78
2.
63
2.81
2.
77
3.69
3.
50
Lea
f are
a ra
tio
(cm
2g -1
) 1
00
.08
12
4.60
" 12
2.10
10
3.30
90
.10
91.0
0 L
eaf w
eigh
t rat
io
(g 8
- m
) 0.
28
0.32
8 0.
344
0.28
6 0.
330
0.32
2 R
oot:
shoo
t ra
tio
0.22
0.
20
0.16
0.
18
0.27
0.
27
0.01
1
0.10
9***
0.16
"
0.70
*
13.9
5*
3.33
99.6
0.32
2 0.
16"
0.00
8
-0.0
83**
*
0.00
1.20
8.46
3.46
89.5
0
0.31
0.
30
g~
oo
',0
',0
O~
Fum
igat
ion
was
beg
un o
n 45
-day
-old
pla
nts
and
was
app
lied
for
4 h
dail
y th
erea
fter
. D
ata
arc
for
55- a
nd 9
5-da
y ol
d pl
ants
. L
evel
of s
igni
fica
nce:
*P
< 0
.05;
**P
< 0
.01;
***
P<
0.0
01.
72 J. Pandey, M. Agrawal / Scientia Horticulturae 58 (1994) 6 7- 76
T 3 plants after an initial increase up to 30 days and 10 days of exposure, respectively.
In T2 plants, biomass remained higher throughout the exposure than the con- trol. Variations in total biomass were significant with respect to different treat- ments, plant age and their interactions (Table 2). Yield components were signif- icantly reduced by the SO2, NO2 or SO2 + NO2 treatments. Yield reduction was maximum in T 3 plants followed by T2 and T~ plants. In T2 plants, the RGR re- mained higher up to 30 days of exposure compared with the control, whereas in T1 and T 3 plants the RGR increased only up to 10 days (Table 3). The NAR of T2 plants increased with respect to the control up to 30 days of exposure and then declined. The LPR, in general, showed a stimulatory effect up to 10 days in all the treatments (Table 3 ). Maximum leaf production was observed for T2 plants. The root production rate (RPR) was higher in control plants. The magnitude of reduction in root production was maximum in T2 plants. Leaf area duration (LAD) increased in T2 plants but declined in T3 plants. In T~ plants, it increased up to 10 days of exposure then declined.
Specific leaf area (SLA) of T~ and T 3 plants was lower compared with the con- trol at most of the sampling dates (Table 3). LAP, increased in T] and T2 plants compared with the control. In T 3 plants, LAR slightly increased up to 10 days
Table 4 Effects of 0.1 ppm SO2 and 0.2 ppm NO2 on chlorophyll, SOd 2- -S, total N and P concentrations in tomato plants
Measurement 55 days 95 days
Control SO2 NO2 SO2+NO2 Control SO2 NO2 SO2 + NO2
Chlorophyll a (mgg -~ drywt.) 4.05 4.45 4.62 4.10 3.10 2.26 2.55 2.10"
Chlorophyll b (mgg- td rywt . ) 2.93 2.95 3.03 2.95 2.15 1.72 1.95 1.70
Total chlorophyll (mgg -~ drywt.) 6.98 7.40 7.65 7.05 5.25 3.98 4.50 3.80
S0~4--S ( mg g - I dry wt. ) Root 1.44 1.50 1.40 1.40 1.48 1.90 1.30 1.84 Shoot 1.29 1.34 1.30 3.34 1.30 1.55 1.10 1.45 Leaf 1.75 1.90 1.69 1.80 1.86 3.20** 1.80 2.80** TotalN (mgg -~ dry wt.) Root 9.8 9.7 9.9 9.8 9.4 8.4 10.2" 9.5 Shoot 11.8 11.5 12.2 12.0 10.2 9.8 15.0"* 15.4"* Leaf 25.4 24.9 27.5 25.7 21.5 18.2 30.4* 23.6 P ( m g g - t drywt.) Root 3.25 3.25 3.05 3.10 3.20 3.45 2.90 3.10 Shoot 4.40 4.30 4.15 4.10 3.80 3.90 4.30 3.50 Leaf 3.86 3.90 3.86 3.85 4.00 3.50 3.35 3.45
Fumigation was begun on 45-day-old plants and was applied for 4 h daily thereafter. Data are for 55- and 95-day-old plants. Level of significance: *P< 0.05; **P< 0.01; ***P< 0.00 I.
J. Pandey, M. Agrawal / Scientia Horticulturae 58 (1994) 67-76 73
and then declined. Leaf weight ratio (LWR) increased up to 30 days of exposure; however, no definite trend was observed at later stages. The RSR decreased in treated plants and the magnitude of reduction was maximum in T2 plants fol- lowed by T1 and then T3 plants.
Concentrations of chlorophyll a, b and total chlorophyll increased up to 10 days of exposure in T~ and T3 plants and up to 20 days in T2 plants. Thereafter chlorophyll concentrations gradually decreased (Table 4). The magnitude of re- duction was maximum for chlorophyll a. Variations in total chlorophyll were sig- nificant with respect to treatments and plant age (Table 2). Interaction of SO 2 X NO2 (P< 0.1 ) and SO2 × NO2 )< age (P< 0.025 ) were also significant.
SO42--S decreased in T2 plants in comparison with the control (Table 4). Con- versely, TI and T3 plants accumulated SO2--S with maximum accumulation in Tl plants. N concentration declined in T~ plants compared with the control. T2 leaves showed maximum N accumulation (Table 4). P content decreased in all the treatments compared with the control and the maximum decline was ob- served in leaves.
4. Discussion
The beneficial effects of NO2 on plant growth at lower concentration observed here are consistent with reports for rice (Fujiwara, 1973 ), tomato (Troiano and Leone, 1977 ), cucumber (Yoneyama et al., 1980), sunflower and maize (Okano et al., 1985 ), soybean (Sabaratnam et al., 1988 ) and black turtle bean (Sandhu and Gupta, 1989 ). Exposure of plants to NO2 increased plant height, number of leaves and biomass. SO2 also showed stimulatory effects at early stages of fumi- gation, as has been reported previously (Singh and Rat), 1980; Clarke and Mur- ray, 1990). Analysis of variance showed significant differences in biomass accu- mulation of plants with combinations of SO2 and NO2 (P< 0.005). Ashenden and Mansfield (1978 ) reported synergistic effects of SO2 and NO2 mixtures on grasses.
Flowering was delayed by all of the treatments, indicating that exposure of plants to these pollutants stimulates vegetative growth, but leads to slower transition from vegetative to reproductive phase. Since flowering puts an upper limit on vegetative growth of tomato of determinate type, potential capacity for the de- velopment of assimilatory organs should increase if flowering is delayed. In T2 plants, yield was decreased by 50% in spite of a net gain of 25% in total plant biomass after 50 days of exposure. T2 plants produced fewer flowers and fruits, which suggests that less photoassimilate was diverted to these reproductive sinks than to the assimilatory organs, which resulted in a net increase in dry matter production.
Increases in leaf area, leaf dry weight, LAR, LWR and LPR at the initial stage of fumigation also show that the plants respond to these pollutants by allocating a greater proportion of its photoassimilates for growth and development of pho- tosynthctic organs. This effect was most pronounced in T2 plants. Whitmore and
74 J. Pandey, M. Agrawal / Scientia Horticulturae 58 (1994) 67-76
Mansfield ( 1983 ) and Pandey and Agrawal ( 1991 ) have suggested that exposure of plants to SO2 and NO2 alters photosynthate partitioning in favour of leaf pro- duction. The SLA of treated plants was lower when plants were treated for longer times. This indicates decreased photosynthetic efficiency and poor biochemical status of plants.
The stimulatory effect of SO2 on RGR, despite reduced NAR, may be ascribed to an increase in LAR. As RGR is the product of NAR and LAR, a reduction in NAR may be compensated for by an increase in LAR. The inhibitory effect on RGR at later stages may be attributed to a significant reduction in NAR. Al- though LAR was always higher in T1 and T2 plants, this increase in assimilatory area was not enough to compensate for the decrease in photosynthetic efficiency (NAR). Reduction in NAR from exposure to higher concentrations of pollutants has been reported (Freer-Smith, 1985; Okano et al., 1985).
Both the root biomass and the RPR decreased maximally in T2 exposed plants. RSR was also minimum in T2 plants, indicating that exposure of plants to NO2 stimulates shoot growth, but affects the root growth adversely. Plants grown in excess N have been shown to bear excess foliage with a poorly developed root system and therefore a low RSR (Salisbury and Ross, 1986). Relatively higher RSR in plants exposed to a SO2 + NO2 mixture indicates that in the case of a combination of pollutants the inhibitory effect is uniform for both above- and below-ground plant parts.
Low concentrations of SO2 and NO2 increased chlorophyll a and b initially. A low concentration of SO2 has been shown to stimulate chlorophyll production to some extent by supplementing the S requirement of plants (Singh and Rao, 1980). Low dosages of NO2 also induce chlorophyll production (Prasad and Rao, 1979) as it acts as a source of N nutrition in plants. Chlorophyll reduction in Tl plants at later stages of fumigation may be ascribed to the oxidation of the chlorophyll pigments by SO2 induced cytotoxic free radicals (Shimazaki et al., 1980). Accu- mulation of toxic NO2- ions may lead to destruction of chlorophyll molecules in NO2 exposed plants (Wellburn et al., 1981 ).
The increases in SO~--S content in Tl and T2 plants observed here are consis- tent with the reports of Elkiey and Ormrod (1981), Agrawal et al. (1985) and Fangmeier (1989). Reduction in S concentration ofT2 plants is possibly a result of its effective utilization by actively growing vegetative parts. N concentration in plants was significantly elevated by exposure to NO2. Atmospheric NO2 is known to increase the N concentration in plants (Troiano and Leone, 1977; Sa- baratnam et al., 1988 ). However, Elkiey and Ormrod ( 1981 ) observed that the total N content in NO2 exposed petunia leaves was reduced in companson with control plants. Roots of treated plants retained more P with age, indicating re- duced translocation of P from roots to flower and fruits. As the flowers and fruits (higher in number in control plants) act as a strong sink, more root P might have translocated to these parts resulting in a reduced P level in the control plant roots. In T2 plants, root P declined at a faster rate at the initial growth stage of the plants because of its effective utilization by actively growing leaves. Leaf P content in T2 plants equalled that of control plants at the initial stages of fumigation with a consequent reduction in root growth.
J. Pandey, M. Agrawal / Scientia Horticulturae 58 (1994) 67-76 75
This study indicates that the low dosages of NO2 and SO2 are fertilizing the plants, promoting vegetative growth at the expense of reproductive growth. To- mato plants in response to SO2 and NO2 allocated a greater proportion of its photoassimilate for growth and development of photosynthetic organs. Similar adaptive responses have been observed in NO2 exposed sunflower (Okano et at., 1985), SO2 exposed Phleum pratense (Jones and Mansfield, 1982) and 03 ex- posed cotton (Oshima et at., 1978 ) and Dactylis glomerata, Loliurn perenne and Phalaris aquatica plants (Horsman et at., 1980).
Acknowledgements
We wish to acknowledge the University Grants Commission for financial sup- port.
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