Isoprene emission from tropical tree species

Isoprene emission from tropical tree species

Environmental Pollution 135 (2005) 101–109 www.elsevier.com/locate/envpol Isoprene emission from tropical tree species P.K. Padhy*, C.K. Varshney Sch...

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Environmental Pollution 135 (2005) 101–109 www.elsevier.com/locate/envpol

Isoprene emission from tropical tree species P.K. Padhy*, C.K. Varshney School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India Received 20 October 2003; accepted 15 October 2004

Isoprene flux (diurnal and seasonal) from some tropical tree species was estimated and a regional comparison was made. Abstract Foliar emission of isoprene was measured in nine commonly growing tree species of Delhi, India. Dynamic flow enclosure technique was used and gas samples were collected onto Tenax-GC/Carboseive cartridges, which were then attached to the sample injection system in the gas chromatograph (GC). Eluting compounds were analysed using a flame ionisation detector (FID). Out of the nine tree species, isoprene emission was found in six species (Eucalyptus sp., Ficus benghalensis, Ficus religiosa, Mangifera indica, Melia azedarach, and Syzygium jambolanum), whereas, in the remaining three tree species (Alstonia scholaris, Azadirachta indica, and Cassia fistula) no isoprene emission was detected or the levels of emission were negligible or below the detection limit (BDL). Among six tree species, the highest hourly emission (10.2G6.8 mg gÿ1 leaf dry weight, average of five seasons) was observed in Ficus religiosa, while minimum emission was from Melia azedarach (2.2G4.9 mg gÿ1 leaf dry weight, average of five seasons). Isoprene emission (average of six species), over five seasons, was found to vary between 3.9 and 8.5 mg gÿ1 leaf dry weight during the rainy season. In addition, significant diurnal variation in isoprene emission was observed in each species. The preliminary estimate made in this study on the annual biogenic VOC emission from India may probably be the first of its kind from this part of the world. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Biogenic emission; Isoprene; Plant emission; Volatile organic compounds

1. Introduction Volatile organic compounds (VOC) play an important role in atmospheric chemistry. High levels of VOC in the ambient environment of Delhi have been reported (Varshney and Padhy, 1998; Padhy and Varshney, 2000a,b). However, emission of VOC from different sources and their strength remain unknown. Vegetation constitutes a prominent natural source of VOC emission. Regional as well as global scale VOC emission, * Corresponding author. Present address: Centre for Environmental Studies, Institute of Science, Visva-Bharati University, Santiniketan731 235, Birbhum, West Bengal, India. Fax: C91 03463 261268/ 262728. E-mail address: [email protected] (P.K. Padhy). 0269-7491/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2004.10.003

from vegetation, may dominate over anthropogenic sources of emission (Guenther et al., 1995). In India, the VOC emission inventory is available for anthropogenic sources (Varshney and Padhy, 1999) however no data is available on VOC emission from natural sources. Among various VOC species, isoprene is an important component of non-methane hydrocarbons, emitted by leaves (Warneck, 1988). Isoprene emission has been reported from a number of taxonomically unrelated plant species, such as bryophytes, ferns, conifers and in many families of angiosperm (Harley et al., 1999). Estimates based on the studies carried out on temperate plants, show that the isoprene emission constitutes about 40% of all biogenic hydrocarbons emitted into the atmosphere from vegetation (Zimmerman et al., 1978). A significant amount of fixed carbon, typically

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0.5–2% at 30  C in light is immediately lost to the atmosphere as isoprene (Sharkey et al., 1991a; Harley et al., 1994). Isoprene emission rate in live oak tree, Quercus sp. represents 1–2% of photosynthesis (Tingey et al., 1979), whereas, in quaking aspen, Populous tremuloides, it is about 7–8% of photosynthesis, on a carbon basis (Monson and Fall, 1989; Loreto and Sharkey, 1990). This provides a broad idea of the variation in isoprene emission in plants, although environmental conditions greatly modulate the emission process. Isoprene emission from plants is light and temperature dependent. It predominantly occurs in sunlight (Tingey et al., 1979; Lamb et al., 1987). Isoprene emission exhibits strong seasonality in broad-leaved trees, and was found to be restricted during day light hours from April to September (Anastasi et al., 1991). Temperature affects emission rate by altering enzymatic production in the leaf tissue (Kuzma et al., 1995). However, recent studies show that the isoprene emission capacity is not only dependent upon light and temperature but may be greatly influenced by environmental conditions during leaf development, leaf age, phenology and immediate past weather conditions (Kesselmeier and Staudt, 1999; Geron et al., 2001). Modelling studies using isoprene as a surrogate for various VOC species emitted by plants, have shown that VOC emission may play an important role in the production of ozone in the urban and rural areas (Trainer et al., 1987; Chameides et al., 1988) and also in the chemistry of the lower troposphere (Jacob and Wofsy, 1988). The rates of photochemical ozone formation of 85 reactive hydrocarbons and their individual contributions, based on measurements made in the urban areas, have been computed (PORG, 1993; Derwent, 1999). Among non-methane hydrocarbons (NMHC), isoprene is very important as far as its role in ozone formation is concerned. In one case during the summer it was found to be the fourth in order of importance (with 0.05 ppb isoprene concentration and its rate coefficient for OH reactions 101!1012, the amount of ozone production was 0.182 ppb hÿ1) (Derwent, 1999). Studies on isoprene emission from temperate and tropical plant species are available (Guenther et al., 1994, 1999; Steinbrecher, 1997; Harley et al., 1999; Keller and Lerdau, 1999; Geron et al., 2001; Klinger et al., 2002). However, not much information on the pattern of isoprene emission, from tropical and subtropical plant species of India and this part of the world, is available. In this study an attempt has been made to measure the emission pattern of isoprene in some common and widely growing tropical tree species in India. By using the emission factors from this study, it is our intention to prepare the total annual biogenic VOC emission from India.

2. Materials and methods Nine common tropical tree species, viz., Alstonia scholaris R. Br., Azadirachta indica A. Juss., Cassia fistula Linn., Eucalyptus sp., Ficus benghalensis Linn., Ficus religiosa Linn., Mangifera indica Linn., Melia azedarach Linn. and Syzygium jambolanum DC (out of 51 plant species screened for VOC emission), were selected for studying the pattern of isoprene emission, spanning over five different seasons. The selected tree species retain indigeneity in the sense of their widespread availability in India. 2.1. Sampling of isoprene Sampling of isoprene emitted from tree species was done using dynamic flow branch enclosure technique, initially developed by Zimmerman (1979). Subsequently this technique was used for the measurement of emission fluxes of VOC (mainly isoprene and monoterpenes) from vegetation by Cao et al. (1997). A healthy branch of a tree sapling was carefully enclosed in a transparent polythene bag along with two plastic tubes, partly projecting out of the enclosure to serve as the inlet and outlet for air circulation. The bag enclosure was sealed to make it airtight. Of the two tubes, one was relatively longer and extended up to the top of the branch enclosure, while, the other tube was short and was restricted to the base of the enclosure. The free ends of the longer and shorter tubes were connected to an electrified pump (flow rate 2 l minÿ1) and Tenax-GC/ Carboseive (solid adsorbent; obtained from Supelco Inc. Bellefonte, PA) tubes, respectively. The circulating air brought about mixing of air in the incubating chamber. The Tenax-GC/Carboseive in the tube absorbs and retains VOC from the flowing air current. Three different studies of various time spans were undertaken for specific plant species. It was observed that the sample adsorption by the Tenax/Carboseive was well within the breakthrough volume and the retention and recovery of isoprene were perfect. The measurements of temperature (by thermometer) and photosynthetically active radiation (PAR; using Li Cor Quantum Sensor, Model LI-183) were made during the incubation period of this study and the data is given in the Table 1. The PAR sensor was placed on the sun facing side of the branch whereas the temperature sensor was placed away from the sun (i.e. behind the branch). 2.2. Sampling schedule Foliar emission of isoprene from nine tree species (A. scholaris, A. indica, C. fistula, Eucalyptus sp., F. benghalensis, F. religiosa, M. indica, M. azedarach and S. jambolanum) was measured from May 1998 to April 1999, for five different seasons (viz., summer, rainy,

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P.K. Padhy, C.K. Varshney / Environmental Pollution 135 (2005) 101–109 Table 1 Solar intensity and temperature data (both ranges and average) during the study period in different seasons Solar intensity (mmol mÿ2 sÿ1)

Summer

Rainy

Autumn

Winter

Spring

Range Average

98–1783 1168

10–1233 732

3–1616 875

12–1577 637

13–2077 1215

Temperature (  C)

Day

Night

Day

Night

Day

Night

Day

Night

Day

Night

Range Average

30.9–39.5 36.8

30.4–37.9 33.5

28–32 30.9

27.8–30 29.1

16.9–27 23.2

17.3–23.2 20.4

11.8–19.6 16.0

12–16 13.9

19.9–32.6 28.3

20.4–29.8 24.6

autumn, winter, and spring). A graphical representation of the sampling schedule is given in Fig. 1. On one day during each season, three saplings of different tree species were sampled for isoprene. Sampling for each sapling of individual tree species was carried out for a period of 50 min at an interval of 3 h for 24 h in each season. After the sampling was over, the leaves from the saplings were taken for calculating the dry leaf biomass. The leaves were dried up at 80  C for 48 h and weights were taken. In all, there were about 40 samples (8 samples per day!5 different seasons) for each tree species. In one day three tree species were evaluated for isoprene emission and over three consecutive days all nine tree species were evaluated during each season. 2.3. Analysis of isoprene The analysis of isoprene was carried-out using a gas chromatograph (GC, Model 5765, accuracy 99%; manufactured by Nucon Engineers, New Delhi), fitted to a thermal desorption sample injection system and a fused silica capillary column (length 30 m, i.d. 0.53 mm, bonded phase BP-1, equivalent to SE-30, OV-101; obtained from Alltech Associates, Deerfield, IL, USA) was connected to FID. The capillary column was maintained at an isothermal temperature of 120  C. The injection temperature was 140  C and the detection temperature was 240  C. The flow of carrier gas was maintained at 10 ml minÿ1 and flow of hydrogen and oxygen was kept 30 and 300 ml minÿ1, respectively. The Tenax-GC tube (after sampling is over) was inserted into the thermal desorption sample injection system Tree Species I II III I

II III I II III I II III I II III I II III I

1 2 3 4

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

II III I II III

Time of the day (hr)

Fig. 1. Sampling schedule for three different tree species over a 24-h period. Tree species: (first day) I, Eucalyptus sp.; II, Melia azedarach; III, Syzygium jambolanum; (second day) I, Alstonia scholaris; II, Cassia fistula; III, Mangifera indica; (third day) I, Azadirachta indica; II, Ficus benghalensis; III, Ficus religiosa.

attached to the GC. The chromatographic signals were amplified and plotted on chromatographic sheet by Computing Integrator (Series 3300) attached to an Epson LX-800 printer. The isoprene identification was verified by matching its retention time with the spectra of the standard (obtained from E-Merck, Germany), and the quantification was done using a standard calibration graph.

3. Results Six out of nine tree species, viz., Eucalyptus sp., F. benghalensis, F. religiosa, M. indica, M. azedarach and S. jambolanum were found to emit isoprene. No isoprene emission was detected in the other three species, viz., A. scholaris, A. indica and C. fistula. Isoprene emission pattern in the six tree species varied from one to another. The normalised emission rate (at 1000 mmol photons mÿ2 sÿ1 PAR and 30  C temperature) of isoprene is given in Table 2. Among the six tree species, Ficus religiosa emitted a very high quantity of isoprene (10.2G6.8 mg gÿ1 leaf dry weight hÿ1), and the emission ranged from 4.7 to 21.5 mg gÿ1 leaf dry weight hÿ1. On the other hand, in M. azedarach, emission was minimum, i.e. 2.2 mg gÿ1 leaf dry weight hÿ1. F. religiosa is a high emitter of isoprene, whereas Eucalyptus sp., M. indica are found to be moderate emitters, i.e. 9.9, 6.0 mg, respectively; F. benghalensis and S. jambolanum are low emitters, releasing about 4.4 and 2.2 mg gÿ1 leaf dry weight hÿ1, respectively (Table 2). Out of the six tree species, it was observed that only M. azedarach, showed no emission during rainy, autumn, winter and spring seasons (Table 2). The reason for such behaviour of M. azedarach is not known and needs further investigation. In further investigation, CO2 and water exchange measurements should have to be incorporated for determining the physiological status of the plants. The hourly average isoprene emission during different seasons is given in Table 2. The average emission of five seasons for Eucalyptus sp., F. benghalensis, F. religiosa, M. indica, M. azedarach and S. jambolanum has been found to vary between 2.2G4.9 and 10.2G6.8 mg gÿ1 leaf dry weight hÿ1 (Table 2). The highest hourly

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Table 2 Foliar emission of isoprene (in mg gÿ1 leaf dry weight hÿ1) by various tree species of India in different seasons (both ranges and average) Season

E. sp. A

B

A

B

A

B

A

B

A

B

A

B

Summer Rainy Autumn Winter Spring Average SD

5.3–14.3 3.1–6.1 4.8–28.1 BDL–52.5 1.6–11.2

9.3 4.4 11.3 17.4 7.4 9.9 4.8

0.3–4.1 5.0–28.3 BDL–5.4 BDL–0.3 0.3–3.3

1.3 16.0 3.2 0.1 1.4 4.4 6.6

1.4–15.2 BDL–59 BDL–30 BDL–10 BDL–29

4.6 21.4 11.6 4.9 8.4 10.2 6.8

0.9–57.1 0.6–13.8 BDL–6.9 BDL–1.0 3.1–14.6

13.8 6.5 1.9 0.2 7.7 6.0 5.3

2.6–26.3 – – – –

11.1 BDL BDL BDL BDL 2.2 4.9

BDL–0.4 0.8–6.9 BDL–24.9 BDL–3.7 BDL–5.0

0.1 2.8 7.4 1.2 1.9 2.7 2.8

F. b.

F. r.

M. i.

M. a.

S. j.

Average

SD

6.7 8.5 5.9 3.9 4.4 5.9 5.2

5.5 8.3 4.9 6.8 3.7 3.5 1.4

E. sp., Eucalyptus sp.; F. b., Ficus benghalensis; F. r., Ficus religiosa; M. i., Mangifera indica; M. a., Melia azedarach; S. j., Syzygium jambolanum; SD, standard deviation; BDL, below detection limit. (A) Range of variation in isoprene emission; (B) average normalised emissions (4 numbers) during the day. The emission rates in different seasons are normalised to 1000 mmol PAR mÿ2 sÿ1 and 30  C leaf temperature according to Guenther et al. (1995). In every season, for each plant species, there were 8 different samples out of which 4 samples (which are measured during the sampling day and given under (A) as range of variation in isoprene emission) have been taken for isoprene emission and the average of 4 normalised emission rates has been given under (B). All the figures given in Table 2 are normalised emission rates.

average emission per g leaf dry weight was observed in F. religiosa (10.2G6.8 mg) and the minimum (2.2G4.9 mg) in M. azedarach. The isoprene emitting tree species in terms of descending rank order was: F. religiosaOEucalyptus sp.OM. indicaOF. benghalensisOS. jambolanumOM. azedarach.

change in season, i.e. during summer and rainy seasons, the emission started at 7:00 h; during autumn it was from 10:00 h and during winter it was between 13:00 and 14:00 h, and during spring, the emission started early in the morning. However, in case of F. religiosa and M. azedarach, the emission pattern was different (Table 2 and Fig. 2c).

3.1. Diel variation

3.2. Seasonal variation

All the six tree species showed variation in the isoprene emission over a 24 h period. During nighttime there was no isoprene emission from any species. The diel variation in the normalised isoprene emission (at 1000 mmol photons mÿ2 sÿ1 and 30  C temperature), from five tree species (except M. azedarach), is shown in Fig. 2a–e. As the isoprene emission was observed only in one season (out of five), no graphical representation of the same was made in the case of M. azedarach. The emission of isoprene started early in the morning, it increased gradually and reached its peak during noon and afternoon. The percentage of emission, during the peak hour was 44% during 13:00 h in Eucalyptus sp., 39% during 16:00 h in F. benghalensis, 63% at 9:00 h in F. religiosa, 39% during 5:00 h in M. indica, 59% during 17:00 h in M. azedarach and 50% in S. jambolanum at 8:00 h. The pattern of emission varied with species and season (Table 2 and Fig. 2a–e). In the case of Eucalyptus sp., the emission of isoprene started in the morning, increased gradually and reached the peak mostly during afternoon (13:00 h, Fig. 2a). In F. benghalensis, Mangifera indica and S. jambolanum, there was no definite peak during any season, in any specific time. Throughout the daytime the emission peak varied (Fig. 2b,d,e). However, in the case of F. religiosa, the emission started early in the morning, peaking during 9:00 h in most of the season (Fig. 2c). In all the above plants, the initiation of emission shifted with the

Isoprene emission varied considerably from one season to another. The average isoprene emission (by taking the average emission of each plant emission data after normalising to 1000 mmol PAR mÿ2 sÿ1 and 30  C temperature, using Guenther et al., 1995) varied from 3.9G6.8 to 8.5G8.3 mg gÿ1 leaf dry weight hÿ1. The maximum amount of isoprene was emitted in the rainy season (8.5G8.3 mg gÿ1 leaf dry weight hÿ1), followed by summer (6.7G5.5), autumn (5.9G4.9), spring (4.5G3.5) and the lowest emission was during the winter (3.9G6.8 mg gÿ1 leaf dry weight hÿ1). In most of the species, except Eucalyptus sp. and S. jambolanum, the emission was low in winter. However, the emission at its peak (for all 6 species) was not confined to any particular season (Table 2). A comparison between isoprene emission rates, measured under ambient conditions and adjusted to basal conditions, is given in Fig. 3. Except the summer, the isoprene emission, in both normalised and ambient conditions showed similar trend, whereas, from the summer to rainy season they showed a reversal.

4. Discussion Isoprene emission is not a universal phenomenon in plants. It showed significant seasonal fluctuations. On a seasonal basis, the emission was higher during the rainy season (8.5G8.3 mg gÿ1 leaf dry weight hÿ1) as

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a: Eucalyptus sp.

b: Ficus benghalensis

30

60

Summer

Summer 50

Rainy

25

Rainy

Autumn Winter

Winter

40

µg g-1 leaf dw h-1

µg g-1 leaf dw h-1

Autumn Spring 30

20

20

Spring

15

10

5

10

0

0 1

4

7

10

13

16

22

19

1

4

7

10

Time (hour)

c: Ficus religiosa

16

19

22

d: Mangifera indica

60

70 60

Rainy

Rainy Autumn

µg g-1 leaf dw h-1

Autumn

Winter 40

Summer

50

Summer

50

Spring

30

40

Winter Spring

30

20

20 10

10 0

0 0

3

6

9

12

15

18

21

2

5

8

11

14

Time (hour)

Time (hour)

e: Syzygium jambolanum 30

25

µg g-1 leaf dw h-1

µg g-1 leaf dw h-1

13

Time (hour)

Summer Rainy Autumn

20

Winter Spring

15

10

5

0 2

5

8

11

14

17

20

23

Time (hour) Fig. 2. Diel variation in the emission of isoprene in various tree species of India in different seasons.

17

20

23

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Isoprene (µg g-1 leaf dry wt h-1)

25 Ambient

Normalized

20 15 10 5 0 Summer

Rainy

Autumn

Winter

Spring

Season Fig. 3. Isoprene emissions rates (average of six tree species) under ambient and adjusted to normalized conditions.

compared to winter (3.9G6.8 mg gÿ1 leaf dry weight hÿ1). Studies on various plant species, as reported in the literature, reveal that isoprene emission rates vary widely and also some of them were no emitters (Rasmussen, 1972; Zimmerman, 1979; Evans et al., 1982). It is also reported that isoprene is emitted from a variety of plant species (Hewitt et al., 1990; Corchnoy et al., 1992; Khalil and Rasmussen, 1992). A number of deciduous trees have been reported to emit primarily isoprene (Tingey et al., 1979; Monson and Fall, 1989; Loreto and Sharkey, 1990). According to a recent study, more than 122 families out of 426 studied and 306 plant species out of 543 studied (in angiosperms only) were found to emit isoprene (Harley et al., 1999). There was no clear-cut information on why plants emit isoprene and it had been hypothesised that isoprene protects the photosynthetic apparatus of leaves against abiotic stresses. Isoprene formation inside photosynthetic apparatus could serve to prevent light damage by dissipating excessive energy and hence, to the protection of chloroplasts (Sharkey et al., 1991a; Zeidler et al., 1997). Other studies maintain that isoprene production protects the membrane against heat stress (Sharkey and Singsaas, 1995; Singsaas et al., 1997). It has been reported in the recent literature that isoprene has many roles for the benefit of the plants. It can increase the thermo-tolerance so as to reduce the stress of extreme temperature (Sharkey et al., 2001). It also protects the photosynthetic apparatus against ozone damage, quenches ozone products, and reduces lipid peroxidation of cellular membranes (Loreto and Velikova, 2001). It helps in scavenging singlet oxygen (free radical) in the leaves (Affek and Yakir, 2002). The mean rate of isoprene emission varied with tree species and could be divided into 4 categories (high emitters, F. religiosa; moderate emitters, Eucalyptus sp. and M. indica; low emitters, F. benghalensis; M. azedarach and S. jambolanum; and no emitters, A. scholaris, A. indica and C. fistula). The share of isoprene, out of the total VOC emitted (reported elsewhere), is

quite significant, i.e. about 40% (M. azedarach) to 91% (Eucalyptus sp.). Researchers have classified the species into different types, depending upon the emission rates (Evans et al., 1982; Benjamin et al., 1996). However, such differences in isoprene emission could not be merely because of genetic differences but also due to variations in the state of leaf physiology and environmental conditions (Kesselmeier and Staudt, 1999; Geron et al., 2001). Foliar emission of isoprene exhibits a distinct diel pattern. Isoprene emission started in the morning, and it gradually increased with the increase in solar radiation and day temperature, and reaching its peak during the noon or afternoon. This is due to the fact that the emission is directly related to sunlight. Isoprene emission is known to be dependent upon both light and temperature (Tingey et al., 1979; Guenther et al., 1991; Sharkey et al., 1991b). A good correlation was observed between isoprene emission (not normalised emission) and solar radiation in the case of five tree species. The correlation coefficients (r) for Eucalyptus sp., F. benghalensis, F. religiosa, M. indica and S. jambolanum are 0.8284, 0.5699, 0.7357, 0.7303 and 0.6167, respectively. Among the five tree species, Eucalyptus sp. showed a sufficiently high degree of correlation; whereas, F. religiosa, M. indica and S. jambolanum showed a moderate degree of correlation. However, as it is discussed that isoprene emission is dependent upon several factors and varies from species to species, it is not possible to have a good correlation between isoprene emission and temperature. It is already observed that isoprene emission reaches the maximum at light intensities in saturated photosynthesis and increases with the rise in temperature up to certain point and thereafter it rapidly declines. A screening of the VOC emission database of 51 plant species (out of which only 9 commonly growing tree species were taken for detailed study and reported here) was considered and it was observed that 32 out of the 51 species (27 trees and 5 shrubs) emit isoprene. A comparison of isoprene emission was made between this study and other tropical studies of Africa and South America. According to the Central African EXPRESSO study, carried out by Guenther et al. (1999), the percentage of isoprene-emitting foliage varied from 3% (Mopane Savanna, South Africa) to 85% (Gilbertiodendron rainforest, Central Africa). About 30% of the species investigated by Guenther et al. (1996a); Klinger et al. (1998) emit isoprene in Africa, which is also similar to that observed in North American vegetation (Guenther et al., 1994). It is interesting to see that the proportion of isoprene emitting species in the present study was 63% whereas, the same value for other studies (South America) are 29% for semideciduous forests of Panama (Keller and Lerdau, 1999), 29% for subtropical dry forest in Guanica,

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Puerto Rico (Lerdau and Keller, 1997), 28% for subtropical savanna in the Republic of South Africa (Guenther et al., 1996a) and temperate deciduous forests (Guenther, 1997). The emission rate of isoprene varied in Africa from 0.8 mg C mÿ2 hÿ1 (degraded forest/woodland) to 3.3 mg C mÿ2 hÿ1 (dense tropical forest) (Guenther et al., 1999), and from 1.6 mg C mÿ2 hÿ1 (degraded forest/woodland) to 4.9 mg C mÿ2 hÿ1 (woodland/savanna, grass/shrub) (Guenther et al., 1995). The average rate of emission for all isoprene emitting species, in the present study, at the standard conditions of 30  C and 1000 PAR (basal emission rate) was 5.9G3.5 mg gÿ1 leaf dry weight hÿ1 (Table 2). An attempt has been made to compare the isoprene emission rate of the present study with the values reported in the literature in respect of identical species/ genus (Table 3). The emission rate values given for different plant species are based on screening results (i.e. three enclosure runs). The values obtained in the present study were in reasonable agreement with the literature data except Bauhinia varigata, Ficus elastica and

Mangifera indica (species level comparison, Table 3a) and Albizzia sp., Dalbergia sp. and Syzygium sp. (genus level comparison) (Table 3b). In all the above plants the emission rates observed in the present study were found to be lower as compared to the reported data (Klinger et al., 2002) for the same species/genus. Isoprene emission rate measurements of tree species (based on enclosure measurement), reported for Africa (Guenther et al., 1996a; Klinger et al., 1998), range from !0.1 to O100 mg C gÿ1 hÿ1. The same for the present study was BDL to 108.4 mg gÿ1 leaf dry weight hÿ1 (Lantana camara Linn.; not normalised). However, it is difficult to estimate emission capacities from our study since the influence of leaf age and past environmental conditions are not characterised. Guenther et al. (1991) have reported leaf-to-leaf and plant-to-plant variations of about G50% for isoprene because of the errors/uncertainties involved in emission measurements. In view of the above considerations, data reported in the present study and literature values given in Table 3 may be regarded as indicative of the emission characteristics of different plant species.

Table 3 A comparison of the observed isoprene emission rates from different tree species (data based on initial screening) with the values reported in literature Scientific name

Family

Present study (mg gÿ1 leaf dry weight hÿ1)

Literature (mg gÿ1 leaf dry weight hÿ1)

Reference

(a) Species level comparison Acacia auriculoformis Alstonia scholaris Bauhinia variegata Cassia siamea Ficus elastica Mangifera indica Melia azedarach M. azedarach Psidium guajava Terminalia bellirica Ziziphus jujubas

Mimosaceae Apocyanaceae Caesalpiniaceae Caesalpiniaceae Moraceae Anacardiaceae Meliaceae Meliaceae Myrtaceae Combretaceae Rhamnaceae

0.00 0.00 0.00 0.00 0.5 13.8 7.2 7.2 9.5 0.00 0.00

0.00 La Ha La Ma Ha La !0.1 – 0.00 La

Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Guenther et al., 1994 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002

(b) Genus level comparison Acacia sp. Albizzia sp. Bombax sp. Cassia sp. Citrus sp. Dalbergia sp. Eucalyptus globulus

Mimosaceae Mimosaceae Bombaceae Caesalpiniaceae Rutaceae Papilionaceae Myrtaceae

1.1 1.0 0.6 0.00 3.9 6.8 BDL-52 /E. sp.

E. citriodora E. viminalis Ficus sp. Milletia sp. Morus sp. Polyalthia sp. Prunus sp. Pterospermum sp. Syzygium sp.

Myrtaceae Myrtaceae Moraceae Papilionaceae Moraceae Annonaceae Rosaceae Sterculiaceae Myrtaceae

BDL-52 /E. sp. BDL-52 /E. sp. 21.4 0.00 6.2 0.00 0.00 1.0 8.7

0.38ÿMa 26.8 La La La Ha 57 28 20 15–49 Ha 8 0.06–139 0.8 La La La La Hÿ199a

Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Evans et al., 1982 Pio et al., 1996 Simpson et al., 1999 Street et al., 1997 Klinger et al., 2002 Winer et al., 1983 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002 Klinger et al., 2002

a

Isoprene emission potentials (mg C gÿ1 hÿ1): HZ70, MZ14, LZ0.1.

108

P.K. Padhy, C.K. Varshney / Environmental Pollution 135 (2005) 101–109

There are many problems involved in deriving the standard emission factor. Guenther et al. (1996b) report that the standard emission factors derived in branch enclosure technique is underestimated because of the leaf shading and other related factors. Therefore, the present study could be an underestimation of the emission factors as the normalised emission factors are derived from the database on branch enclosure method. The result obtained here however, is consistent with another study done in China (Klinger et al., 2002). The study undertaken by Klinger et al. (2002) is quite exhaustive. Our study is based on a few plant species in Delhi, India and therefore, the results could not be applied to the regional VOC emission model to arrive at total annual biogenic VOC emissions. However, a preliminary estimate has been made by using the emission factor obtained from the present study. To arrive at the VOC emission estimate, average rate of emission for isoprene, a-pinene and other terpenes in different seasons were used. Then VOC emission from forests per day and seasons were calculated. The emission of VOC from different seasons was summed up to arrive at annual VOC emission from the forests. For calculating the VOC emission from crops and grasslands, emission factors from the literature were used. The annual VOC emission from biogenic sources (forests, grasslands and crops) was calculated to be 7 Tg C, which may be an underestimation and suffer from uncertainties. Keller and Lerdau (1999) have reported that the study by Guenther et al. (1995) underestimates the response of tropical plant species under extreme light and temperature. Our study based on normalised formula provided by Guenther et al. (1995), could be also an underestimation. A more detailed and elaborate study is required to ascertain climatic influence on isoprene emission vis-a`vis inherent variation in representative of species from different taxa. In addition, measurements for at least three successive seasons are required to minimise uncertainties in emission rate measurements. Such information will greatly contribute to our understanding of carbon dynamics of tropical plants, particularly in the Indian subcontinent. Notwithstanding the limitations, this study, probably for the first time, from this part of the world, provides baseline data on tropical tree species. It also gives a preliminary idea about the quantum of annual biogenic VOC emission. Hence, this study can be used as a pivotal point for future studies and comparison.

Acknowledgements The fellowship provided to one of the authors (PKP) by the University Grants Commission (UGC), New Delhi during the course of the research work is

gratefully acknowledged. We thank four anonymous reviewers for their helpful comments and suggestions on this manuscript.

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