Pak. J. Bot., 43(1): 445-452, 2011.
ZIA-UR-REHMAN
FAROOQI, MUHAMMAD ZAFAR IQBAL,
MUHAMMAD
KABIR AND MUHAMMAD SHAFIQ
Department
of Botany, University of Karachi, Karachi, 75270, Pakistan. E-mail: farooqi_bot@yahoo.com;
shafiqeco@yahoo.com
Abstract
Lead
produced significant effects on different growth parameters of Albizia lebbeck (L.) Benth such as root,
shoot, seedling length, leaf area, plant circumference and seedling dry biomass
in natural field conditions. Seedling growth performance of A.
lebbeck showed low level of tolerance with increasing concentrations of
lead treatments from 25 to 125µmol/L. Lead treatments at 25 to 125µmol/L
produced significant effect on shoot, root and seedling length of A.
lebbeck. Number of leaves, leaf area and circumference were significantly
(p<0.05) reduced at all concentrations of lead treatments. Seedling dry
biomass was significantly reduced at 100 and 125µmol/L treatment of lead. Lead
treatment at 25µmol/L showed high percentage of tolerance indices while by
increasing lead levels to 125µmol/L, percentage of tolerance indices was low.
Introduction
All compartments of the biosphere are polluted by a variety
of inorganic and organic pollutants by anthropogenic activities which alter the
normal biogeochemical cycling (Prasad & Freitas, 2003). Most of the toxic
pollutants are discharged in the air by man made activities (Nriagu &
Pacyna, 1988). The continuous discharge of various types of toxic materials,
such as carbon particles, unburned and partially burned hydrocarbons, fuels,
tar materials, heavy metals and other elements in the environment, produced
toxic effects on plants (Qadir & Iqbal, 1991).
Lead is a common heavy-metal pollutant, released from
leaded gasoline and industrial processes (Xiong, 1998). Lead is a toxic element
that is being releasing into the atmosphere for the last 70 years by human
economic activity (Antosiewicz & Wierzbicka, 1999). Lead is generally added
in the environment through automobile exhaust (Lagerwerff & Specht, 1970)
and industrial effluents (Campbell, 1976). Dense traffic releases detrimental
exhaust gases and toxic pollutants like unburnt and partially burnt
hydrocarbons, lead compounds and other elements that are contained in petrol
polluting the city environment (Iqbal et
al., 2001). Lead and cadmium are
the toxic elements of primary importance in ecotoxicology (Breckle &
Kahile, 1992). Exposure to lead (Pb) as well as other heavy metals in the
environment is still a matter of public health concern (Rinderknecht et al., 2005). Lead and cadmium treatments at 10, 30, 50, 70 and 90µmol/L
inhibited the seed germination and seedling growth of Thespesia populnea L. (Kabir et
al., 2008). Lead and cadmium produced significant effects on germination,
root, shoot, seedling lengths and seedling dry biomass of Albizia lebbeck (L.) Benth in lab conditions at 10, 30, 50, 70 and
90µmol/L (Farooqi et al., 2009).
Emission of Pb from petrol driven motor vehicles is an
environmental problem. Yousafzai (1991) found the level of Pb (810-4527 ppm)
and Cd (0.2-4.5 ppm) in the street dust of metropolitan city of Karachi and
concluded that Pb in roadside dust of Karachi city was mostly attributed by
leaded gasoline from vehicular traffic. Inhibition of germination and
retardation of plant growth are commonly observed due to lead toxicity (Morzeck
& Funicelli, 1982; Wierzbicka & Obidzinsca, 1998; Lerda, 1992; Shaukat et al., 1999). Lead produced highly
significant effects on shoot, root lengths and seedling dry biomass of Lythrum salicaria (Juseph et al., 2002). Foliar application of
lead affected growth and yield parameters of wheat was studied by Rashid &
Mukhirji (1993). Effects of lead toxicity on seed germination and seedling
growth of some tree species were carried out (Iqbal & Sidiqui, 1992; Shafiq
& Iqbal, 2005). Germination of two rice (Oryza
sativa L.) cultivars ( Ratna and IR36 ) in the presence of 10 µM Pb
decreased germination percentage, germination index, shoot/root length,
tolerance index and dry mass of shoots and roots (Mishra & Choudhuri,
1998).
Siris (A. lebbeck (L.)
Benth) belongs to a family Mimosaceae is a multipurpose tree for semiarid
regions. A. lebbeck has been widely
distributed around the tropics and mainly planted as a shade tree. This tree is
found on a wide range of soil types including those that are alkaline and
saline (Prinsen, 1986) but not subject to water logging.
The aim of the present investigation was to determine the
tolerance of Albizia lebbeck (L.)
Benth., seedlings to lead treatments in the natural field conditions.
Materials and Methods
The healthy seeds of Albizia lebbeck (L.) Benth were collected randomly from the Karachi University campus. The experiment was conducted in natural field conditions at Department of Botany, University of Karachi. The top ends of the seeds were slightly cut with a clean scissor to remove any possible dormancy. The seeds were sown in large earthen pots having garden soil at 1 cm depth and watered regularly. After two weeks of their germination, uniform size seedlings were transplanted in plastic pots of 7.0 cm in diameter and 9.8 cm in depth containing garden soil. One seedling was transplanted in each pot and was treated with a solution of lead nitrate having 25, 50, 75, 100 and 125 µmol/L concentrations. There were five replicates for each treatment and the experiment was completely randomized. The seedlings were treated with 5 ml of their respective treatment after two days intervals. Pots were reshuffled weekly to avoid light/ shade or any other environmental effect. After 8 weeks seedlings were removed from pots and washed their roots with water. Data on seedling length, root and shoot length, leaf area was obtained. Seedling dry biomass was determined by drying the plant material in an oven at 80oC for 24 hours. Tolerance indices (TI) were determined using the formula given by Iqbal & Rahmati (1992).
The data obtained was statistically analyzed by analysis of
variance (ANOVA) (Steel & Torrie, 1984) and Duncan's Multiple Range Test
(Duncan, 1955) at p<0.05 level.
Results
Lead treatments showed prominent effects on seedling growth
performance of Albizia lebbeck (L.) Benth in
natural field conditions. Reduction in root, shoot and seedling lengths, leaf
number, leaf area and dry biomass was observed for A. lebbeck in all concentrations (25, 50, 75, 100 & 125µmol/L)
of lead treatment as compared to control (Table 1). Shoot length of A. lebbeck was gradually reduced (14.80,
14.60, 14.40,
13.40 & 13.40 cm) with increase in
concentration of lead (25, 50, 75, 100 & 125µmol/L).
Root length of A. lebbeck was suppressed
(7.20, 7.00. 6.80. 6.80 & 6.20 cm) at 25, 50, 75, 100 and 125 µmol/L
concentrations respectively and control showed (7.80 cm) better root growth.
Lead treatments at 25 to 125µmol/L concentrations significantly (p<0.05)
affected seedling length. A significant (p<0.05) reduction was also found in
number of leaves at all treatments of lead. Leaf area showed a prominent
decrease (11.80, 9.78, 7.89, 6.97 and 6.25 cm2) at 25, 50, 75 100,
and 125 µmol/L concentrations of lead. Plant circumference of A. lebbeck was also determined and
showed significant reduction (31.20, 31.20, 29.60, 28.20 and 26.20 cm) at 25,
50, 75, 100 and 125 µmol/L concentrations of lead as compared to control which
showed 34.00 cm circumference. A significant reduction (1.14 and 1.05 g) in
seedling dry biomass was found at 100 and 125 µmol/L of lead treatments,
respectively. Reduction in percentage of different growth variables was
increased by increasing concentrations of lead (Fig. 1).
The seedlings of A.
lebbeck were tested for tolerance to different concentrations of lead (Fig.
2). It is shown that tolerance of A.
lebbeck was gradually decreased by increasing concentrations of lead
treatments. Lead treatment at 25 µmol/L showed high percentage (92.31%) of
tolerance in seedlings of A. lebbeck while
lead treatments at 50, 75 and 100µmol/L showed 89.74, 87.18 and 87.18% of
tolerance, respectively. Lead treatment at 125µmol/L showed lowest percentage
of tolerance (79.49%) in A. lebbeck seedlings
as compared to control.
Discussion
Albizia lebbeck (L.)
Benth showed inhibition in all growth variables at different levels of lead
treatments. Root, shoot and seedling
length were highly affected with increasing concentrations of lead. A
significant reduction was observed in root, shoot and seedling length as
compared to control. This might be due to reductions in both new cell formation
and cell elongation in the extension region of the root and these findings were
similar to the results of Haussling et al.,
(1988). Excessive amounts of toxic elements usually caused reduction in plant
growth (Prodgers & Inskeep, 1981). Seedling growth inhibition by heavy
metals has also been reported by many other workers (Morzek & Funicelli,
1982; Azmat et al., 2005; Shafiq
& Iqbal, 2005). Heavy metals have been widely recognized as highly toxic to
plants. Plants can be affected directly by air pollutants, as well as
indirectly through the contamination of soil and water. At the same time, plant
is a part of food chain and may create a risk for man and animals through
contamination of food supplies (Fargasova, 1994).
Lead is a toxic element that is harmful to growth of A. lebbeck and it showed a significant
reduction in leaf area and seedling dry biomass. This reduction might be due to
accumulation of lead in soil which physically blocks water uptakes from root to
shoot and is related with the rate of photosynthesis, mainly associated with
the water content and CO2 absorption. These results were supported
by Jaja & Odoemena (2004) as they found that leaf area, fresh and dry
biomass of wheat (Roma VF) variety was highly inhibited with increase in lead
levels. In the present investigation, seedling growth performance of A. lebbeck gradually decreased with the
increase in concentration of lead as compared to control. Similar results were
found by applying lead treatments in the lab conditions at 10, 30, 50, 70 and
90 μmol/L concentrations which produced significant (p<0.05) effects on seed
germination and seedling length of A.
lebbeck while lead treatment at 50, 70 and 90 μmol/L significantly affected
root growth and seedling dry biomass as compared to control (Farooqi et al., 2009). The circumference of A.
lebbeck was also reduced with increase in lead which entered into plant
system through stomates and disturbed physiological activities of plants. It
has been shown that the metals induce chromosomal abnormalities and also
decrease the rate of cell division. In general,
effects
Fig. 1. Percentage increase
in reduction of seedling, root and shoot lengths, leaf area, circumference and
seedling dry biomass at increasing concentrations of lead.
Fig. 2. Tolerance indices of Albizia lebbeck at different concentrations of lead as compared to
control.
are more pronounced at higher concentration
and at longer duration of exposure (Monn et
al., 1995). Plants have a range of potential mechanisms at the cellular
level that might be involved in the detoxification and thus tolerance to heavy
metal stress. These all appear to be involved primarily in avoiding the
build-up of toxic concentrations at sensitive sites within the cell and thus
preventing the damaging effects described above, rather than developing
proteins that can resist the heavy metal effects (Hall, 2002). The effects of
heavy metals on plants depend on the amount of toxic substance taken up from
the environment. The seedlings of A.
lebbeck also showed a gradual decrease in seedling dry biomass as
concentrations of lead increased. The toxicity of some metals may be so severe
that plant growth is reduced before large quantities of the element can be
translocated (Haghiri, 1973). Seedling growth of Arabidopsis thaliana was found sensitive to Pb2+
treatment at 1 mM (Li et al., 2005).
Tolerance to lead treatments in A. lebbeck was lower as compared to control. This information can
be considered a contributing step in exploring and finding of tolerance limit
of A. lebbeck at different levels of
treated metal. These findings were similar to the results of Kabir et al., (2008). According to them the
tolerance limit of Thespesia populnea L.
gradually decreased with increasing lead levels. Tolerance to heavy metals in
plants may be defined as the ability to survive in a soil that is manifested by
an interaction between a genotype and its environment (Macnair et al., 2000). Metal hyperaccumulating
plants are thus not only useful in phytoremediation, but also play a
significant role in biogeochemical prospecting and have implications on human
health through food chain and possibly exhibit elemental allelopathy (metallic
compounds leached through plant parts of the hyperaccumulator would supress the
growth of other plants growing in the neighbourhood) and resistance against
fungal pathogens (Boyd et al., 1994).
Further studies on the morphological attributes of the root characteristic such
as root length, root diameter and root hairs are required. These characters
could play a key role in understanding the transport of metal from soil to
aerial parts of the plants which is possible due to presence of the specialized
epidermal cells found in the roots and stem.
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