Growth and Stomatal Conductance of Prosopis cineraria (Ghaff Tree) Exposed to Sulphur Dioxide
Salim H. Al-Rawahy,
The present study aimed to investigate the sensitivity of an indigenous leguminous plant species of Oman, Prosopis cineraria (ghaff tree), to SO2 pollutant. Plants were exposed to 0, 25, 50, 100 and 150 ppb SO2 for 30 min daily for the period of ten weeks, under light and dark conditions. The formation of marginal necrotic areas on leaflets was seen as the first symptom of SO2 injury in P. cineraria plants. Leaf senescence was highly significant (p<0.01) in plants exposed to SO2 in light conditions and significant (p<0.05) in plants exposed to SO2 in dark conditions compared with control plants. There was significant (p<0.05) decrease in Relative Growth Rate per week in plants exposed to SO2 in both light and dark treatments compared with control plants, but more pronounced reduction in light conditions. Stomatal conductance was significantly (p<0.01) reduced after SO2 exposure in both light and dark treatments. These results became the first record of Oman indigenous plant species showing confirmed injuries as a result of exposure of SO2 concentrations.
Sulphur dioxide (SO2) is an industrial air pollutant associated with fossil fuel combustion and refining. It is one of the several pollutants released in automobile exhaust fumes, industrial smokestacks and metal smelting effluent. Together, natural and anthropogenic global SO2 sources emit an estimated 194 million tones annually, of which about 83% is anthropogenic from fossil fuel combustion (Botkin and Keller, 2005). Once emitted, SO2 is deposited onto surfaces by diffusion at various rates according to meteorological conditions, or may undergo a number of chemical reactions before dry or wet deposition. Among S gases, SO2 is considered to be the most important phytotoxic molecule (Jacobson, 2002) and its adverse effects on plants were recognized long before the effects of the other air pollutants (Winner et al., 1985). Recently, Swanepoel et al. (2007) reported that increased uptake of SO2 causes toxicity and reduced growth and productivity in plants due to accumulation of sulphite and sulphate within cells.
In some areas of Oman, especially in the vicinity of industrial areas, the
visible foliar injury was observed in some indigenous plant species, e.g., Prosopis
cineraria. Plants growing within these areas are exposed to greater SO2
concentrations as well as other pollutants, such as ozone (O3) and
nitrogen dioxide (NO2). Hence there is considerable concern about
the interactive and synergistic effects of exposure to mixtures of SO2
and other pollutants (Ashmoor, 2002). However, it is important as a first step
that the effects of each pollutant be understood independently, that can help
identify similarities in the ways in which environmental pollutants effect vegetation
and contribute to our general understanding of plant responses to combination
of pollutants. To evaluate the impact of contamination in an ecosystem, it is
necessary to first establish the background level of the contaminants. The background
level may be interpreted as a natural level, that is the average conditions
of an area where there may be human activity, but which is in a good state of
conservation (Conti and Cecchetti, 2001). From continuous air quality monitoring
in Oman during the past 5 years (unpublished data), the background concentration
of SO2 in rural area has been recorded as 11 ppb, while the average
background concentration in cities is 22 ppb. Frequently (average once per week),
however, the SO2 concentration in Muscat (capital) area exceeds 50
ppm and occasionally (average once per two months) exceeds 100 ppb. Once the
background level had been established, the contamination factor may be used
to evaluate the state of conservation or degradation (Conti and Cecchetti, 2001).
The majority of past researches on the effects of SO2 on plants were
focused on plants of economic value. Recent concern about the natural environment,
however, has focused more interest on the ecological value of ecosystems. This
study aimed to investigate the effects of SO2 exposure with special
attention to growth and stomatal conductance in indigenous leguminous plant
species of Oman, Prosospis cineraria. The species is an excellent multi-purpose
tree for local people, particularly in providing fodder, fuel-wood and shade
protection, as well as creating microenvironment that supports various wildlife
MATERIALS AND METHODS
Seeds of P. cineraria (L.) Druce collected from wild trees when they
became available in the summer of 2004 and 2005. Seeds were germinated in pots
containing uniform soil compost. Seedlings were grown under growth room conditions
(14 h photoperiod, flux density of 110 μmoles m-2 sec-2
at 28±2°C and 70±5% RH) and irrigated daily with tap water
for 4 months. Fifty plants were then selected randomly and exposed to 0 (control)
50, 100 and 150 ppb SO2 gas (from MEGS Specialty Gases, Inc.) for
30 min daily for 10 weeks in a 150x100x65 cm perforated glass chamber. SO2
flow rate was manually adjusted using SUPERIORTM Gas Sulphonator
and the concentration within the chamber was measured using a Tetra Crowcon
SO2 gas detector. The experiment was repeated under light and dark
conditions. In dark treatment light was switched off 30 min before the exposure
to SO2 and switched on immediately after the exposure. Stomatal conductance
of six plants per treatment selected randomly was measured on the first day
of the experiment starting 1 h before the exposure to SO2 and 5 h
after the exposure (-60, 0, 60, 120 180, 240 and 300 min) by using LI-COR, LI-6200
Portable Photosynthesis System and LI-6250 Gas Analyser. In order to understand
the stomatal behaviour of P. cineraria in field conditions, 10 wild plants
were randomly selected at various locations between 23°20N-57°45E
and 22°40N-58°35N. Stomatal conductance of each plant was
measured at least 2 different days during the month of August 2005 at interval
of 1 h between 6.00 and 18.00 h. Experimental plants were observed for any foliar
injuries and these were assessed in percentages (No. of injured leaves/total
No. of leaves x 100). Observation of minute necrotic spots was made using the
light microscope. At the end of the experiment, plants were harvested and the
Relative Growth Rate (RGR) per week of each plant was calculated:
RGR/week = (ln (Dry Weight)-ln(Σ Initial Dry Weight))/10.
Initial dry weight is the average dry weight of 25 plants selected randomly harvested just prior to the first exposure of SO2 to above experimental plants. Data were analysed using a factorial design Analysis of variance (ANOVA) using SPSS package and presented as means±standard error.
The formation of marginal necrotic areas on leaflets was seen as the first symptom of SO2 injury ( Table 1). These marginal necrotic areas started as dark green color and eventually became dry and changed to brown color. At the end of the experiment, 89 and 41% of the total number of leaves observed had SO2 damage symptoms under light treatment and dark treatment, respectively.
The difference in the number of leaves injured was highly significant (p<0.01)
between different SO2 concentrations under light conditions and significant
(p<0.05) under dark conditions. Minute necrotic spots around or near stomata
were observed under the light microscope, which were significantly more numerous
in plants exposed to SO2 under light conditions. There were no observed
foliar injuries in control plants.
|| The effects of fumigated SO2 on the growth of
P. cineraria plants
|*Only one set of control treatment, grown under growth room
conditions, NS: Non significant
|| Stomata conductance (mmol m-2 sec-1)
of P. cineraria as affected by exposure of different concentration
of fumigated SO2 for 30 min. Stomata conductance measurements
of control plants under light condition were made only once
|*60 min before SO2 exposure, +Measurements
start immediately after SO2 exposure and last for 30 min, NS:
|| Stomata conductance of 10 wild P. cineraria plants
measured during the month of August 2005 at interval of 2 h
|*n = 2
Leaf senescence was mostly observed in plants exposed to SO2 under
light conditions. Compared to the control plants, there was a highly significant
(p<0.01) decrease in RGR/week in plants exposed to SO2 under light
treatment and a significant (p<0.05) decrease under the dark treatment compared
with control plants. Stem diameter was significantly (p<0.05) reduced in
exposed plants under both light and dark conditions. Stomatal conductance was
substantially reduced after SO2 exposure under light conditions compared
to SO2 exposure under dark conditions ( Table 2).
These responses were transient in dark treatment and maintained for several
hours after SO2 exposure under light treatment. Stomatal conductance
in wild plants was greater during the early and late hours of the day ( Table
In this study, the symptoms of SO2 injury were first seen on the
leaves of P. cineraria. Almost 75% of the overall population exposed
to SO2 has showed visible symptoms and incidence of injury was greater
for plants exposed to SO2 under light condition. Descriptions of
visible injuries and susceptibility of many plant species to SO2
have been reported by a number of investigators (Bell and Mudd, 1976; Ayazloo
and Bell, 1981; Keller, 1981; Black, 1982; Krupa, 1996; Legge and Krupa, 2002;
Moraes et al., 2002; Raziuddin et al., 1999). Chlorosis and marginal
necrotic areas on leaves are the prominent phenomenon of SO2 phytotoxicity
and are derived from the breakdown of photosynthetic pigments in mesophyll tissues
(Garsed, 1985). The necrotic areas in this study ranged in color from dark green
to reddish-brown to brown. This is supported by SO2-induced damage
reported by Legge and Krupa (2002). Dry deposition of SO2 involves
the transfer of SO2 from the air stream to the canopy (Rennenberg
and Polle, 1994). Once in the canopy, SO2 may penetrate the boundary
layer by a diffusion process. SO2 molecules that move through boundary
layer of P. cineraria canopy will most probably enter the leaves through
stomatal openings. Garsed (1985) reported that the pathway of SO2
to unwetted foliage of Vicia faba is largely through open stomata and
less than 10% of the total flux was actually accounted for by adsorption on
the cuticle. The absorption to the surface of the leaves characterized by a
thick cuticle layer could be very much reduced. The diffusion flux of SO2
molecules to the site of assimilation or damage in the leaf mesophyll is determined
by the concentration gradient and leaf diffusion resistance (Heldt, 1996). Following
the absorption of SO2 through the stomatal pores the gas is dissolved
in mesophyll spaces near the stomata to form hydrated SO2 (SO2.H2O),
which act as a strong acid, dissociating to HSO3−
and SO3− in proportions determined by
the pH of the water films bounding the mesophyll epidermal apoplast (Legge and
Krupa, 2002). Peiser and Yang (1985) reported that SO2 is rapidly
hydrated, forming bisulfite and sulphite. These byproducts have been implicated
in SO2 toxicity and if the uptake exceeds the capacity of cells to
detoxify sulfites, minute necrotic spots are formed near or around stomata.
In this study the foliar SO2 injury symptoms were mostly seen on leaves at a full stage of development. Similar SO2 injury symptoms were observed in other plant species including Alsike clover (Trifolium hybridum) and prickly rose (Rosa acicularis) (Legge and Krupa, 2002). Various studies showed that there is a strong correlation between leaf senescence in plants and SO2 exposure (Heldt, 1996).
The results presented here showed highly significant difference in leaf senescence
in plants exposed to SO2 under light conditions and significant in
plants exposed to SO2 in dark conditions compared with control plants.
If the rate of leaf senescence becomes faster than the rate at which new leaves
grow, the photosynthetic leaves will decrease and this in turn will reduce net
assimilation rates and relative growth rates. SO2 can also affect
photosynthesis by altering stomatal conductance (Marshall, 2002) or by changing
the metabolic capacity of mesophyll cells (Winner et al., 1985). Mansfield
and Pearson (1996) reported that short-term exposure to SO2, particularly
at concentration <50 ppb, often causes wider stomatal opening, while long-term
exposure with higher concentrations usually causes partial stomatal closure.
Present results with P. cineraria showed that stomatal closure can occur
within minutes after the SO2 exposure of 50, 100 and 150 ppm under
light treatment and this response is maintained for a few hours. In field measurements
of 10 wild P. cineraria plants, it was found that stomatal conductance
was greater during early and late hours of the photoperiod. The morning hour
peak and late evening peak of SO2 levels during most days in the
vicinity of industrial areas and urban areas may directly alter natural stomatal
behavior in this plant species. Photosynthetic decline in P. cineraria
in the presence of SO2 pollution could therefore be directly related
to reduced stomatal conductance. Moreover, the proportion of photosynthetic
inhibition due to non-stomatal factors, including physiological and biochemical
damage, increase as greater quantities of SO2 are absorbed into the
leaf (Krupa, 1996). The advantage of stomatal closure is the reduction or alteration
in the quantity of SO2 that enters the plant and arrives at metabolic
sites. The disadvantage, however, is the depression in photosynthetic carbon
dioxide uptake (Raziuddin et al., 1999). The level of leaf physiological
responses of decreased photosynthetic capacity that lead to altered carbon allocation
pattern may ultimately influence growth characteristics such as shoot or root
growth (Al-Rawahy, 2000). Decreased growth may affect plants ability to
acquire essential resources from the environment (Novak et al., 2003).
In response to many stresses, shoots are affected more than roots (Al-Rawahy, 2000). This was also the case of P. cineraria seedlings, where there was no significant reduction in root dry weight after SO2 exposure compared with control seedlings in both light and dark treatments. Cheesman (1993) suggested that increase root/shoot ratio in stressed plants help to reduce the demand for photosynthetic products to shoot while maintain the root size so the absorption of water and mineral is not affected. However, the cost for this change is the reduced ability to supply products of photosynthesis to the growing apices (Al-Rawahy et al., 2003). In the long term exposure to stress the growth is likely to be strongly reduced even in roots.
The current study was successful in validating SO2-induced foliar injury and reduction in stomatal conductance in P. cineraria plant species. These results became the first record of Oman indigenous plant species showing confirmed injuries as a result of exposure of SO2 concentrations. Several genera and species need to be investigated in order to establish which are the more sensitive species that may serve as bioindicators of SO2 pollutants in the field. Use of native plant species as bioindicators would have a higher significance for the characterization of the air pollution impact on the ecosystem than those from studies with exotic plant species (Moraes et al., 2002).
The Authors would like to thank Sultan Qaboos University for supporting research and providing facilities. The Ministry of Regional Municipalities, Environment and Water Resources is very much thanked for various support and help throughout the project. Dr. M. Kokkin, Department of Biology, SQU, is thanked for reading the manuscript.
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