%20Benth.PNG)
Pak. J. Bot., 41(1): 27-33, 2009.
Z.R. FAROOQI, M. ZAFAR IQBAL, M. KABIR AND M. SHAFIQ
Department of Botany, University of Karachi, Karachi, 75270, Pakistan
E-mail:
farooqi_bot@yahoo.com; shafiqeco@yahoo.com
Abstract
The effects of lead and cadmium on seed
germination, seedling, root, shoot length and seedling dry biomass of Albizia lebbeck was evaluated under
laboratory conditions with and without lead and cadmium treatments. Lead and
cadmium treatments at 10, 30, 50, 70 and 90 µmol/L affected seed germination
and seedling growth of A. lebbeck as
compared to control. Lead treatments at 10, 30, 50, 70 and 90 µmol/L
concentrations produced significant (p<0.05) effects on seed germination and
seedling length of A. lebbeck while
lead treatment at 50 µmol/L significantly affected root growth and seedling dry
biomass as compared to control. Similarly, cadmium treatments from 10 to 90
µmol/L affected the seed germination, root, shoot length and seedling dry
biomass of A. lebbeck as compared to
control. Seedlings vigor index of A.
lebbeck showed gradual decrease with increase in concentration of lead and
cadmium. Cadmium treatments showed adverse effects on seedlings of A. lebbeck as compared to lead. Lead and
cadmium treatments at 90 µmol/L exhibited lowest percentage of tolerance in
seedlings of A. lebbeck as compared
to control.
Introduction
The toxicity of heavy metals is a problem for ecological,
evolutionary and environmental reasons (Nagajyoti et al., 2008). Heavy metals such as lead and cadmium are highly
toxic pollutants as they are added in the environment through automobile
exhausts (Lagerwerff & Specht, 1970). Inhibition of germination and
retardation of plant growth are commonly observed due to lead toxicity (Morzeck
& Funicelli, 1982; Wierzbicka & Obidzinsca, 1998; Lerda, 1992;
Antosiewicz & Wierzbicka, 1999; Shaukat et
al., 1999; Iqbal & Shazia, 2004). Negative effects of lead toxicity on
seed germination and seedling growth of some tree species were examined (Iqbal
& Siddiqui, 1992; Shafiq & Iqbal, 2005). Lead contents in soil and
wheat tissues along with the roads were decreased with increase in distance
from the roads (Lone et al., 2006).
Foliar application of lead affected growth and yield of wheat (Rashid &
Mukhirji, 1993). Lead produced highly significant effects on shoot, root
lengths and seedling dry biomass of Lythrum
salicaria (Juseph et al., 2002).
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).
The increasing influx of heavy metals into water bodies
from industrial, agricultural, and domestic activities is of global concern
because of their well documented negative effects on human and ecosystem (Mataka
et al., 2006). Cadmium is a heavy metal with high toxicity and has an
elimination half-life of 10-30 years (Jan, et
al., 1999). People are exposed to cadmium by intake of contaminated food or
by inhalation of tobacco smoke or polluted air (Järup et al., 1998). High concentrations of cadmium in soils represent a
potential threat to human health because it is incorporated in the food chain
mainly by plant uptake (Alvarez-Ayuso, 2008). Influence of cadmium toxicity on
germination and growth of some common trees were investigated by Iqbal &
Mehmood (1991).
The aim of the present research was to investigate the
effects of lead and cadmium on seed germination and seedling growth of Albizia lebbeck (L.) Benth.
Materials and Methods
The healthy seeds of Albizia
lebbeck were collected randomly from the Karachi University Campus. The top
ends of seeds were slightly cut with a clean scissor to remove any possible
dormancy. The seeds were surface sterilized with dilute solution of Sodium-hypo
chloride to prevent any fungal contamination. Ten seeds were placed in Petri
dishes (90 mm diameter) on filter paper (Whatman No. 42). Metal treatments of
Pb and Cd were prepared using lead nitrate and cadmium nitrate with
concentrations of 10, 30, 50, 70 and 90 µmol/L respectively. At the start of
experiment, 3 ml of respective treatment was added to each set of Petri dish
and at every 3rd day, the old solution was sucked out and replaced
with 2 ml of new solution. The control received only 2 ml of distilled water.
There were five replicates per treatment and the Petri dishes were kept at room
temperature (20±2°C) with 3 hourly light period provided by 200 watt bulb and
the experiment lasted for 12 days. The experiment was completely randomized.
Germination was recorded and seedling dry biomass was determined by placing the
seedling in oven at 80°C for 24 hours. The number of germinated seeds were
counted after 12 days of treatment. Seedling dry biomass was measured with
electrical balance. Maximum root, shoot and seedling length were also obtained.
Seedling vigor index (S.V.I) was determined as per the formula given by Bewly
& Black (1982) and Tolerance indices (TI) were determined by the use of the
formula given by Iqbal & Rahmati (1992). The seed germination and seedling
growth data were statistically analyzed by Analysis of Variance (ANOVA) (Steel
& Torrie, 1984) and Duncan's Multiple Range Test (DMRT) (Duncan, 1955) to
determine the level of significance at p<0.05.
Results
Seed germination, root, shoot and seedling length, root shoot
ratio and dry biomass of Albizia lebbeck
L., were highly decreased with the treatment of Pb and Cd at 10, 30, 50, 70 and
90 µmol/L as compared to control (Tables 1&2, Fig. 1). Lead treatments at
10 µmol/L concentration produced significant (p<0.05) effects on seed
germination and seedling length as compared to control. Increase in
concentration of lead to 30µmol/L significantly affected seedling dry biomass
of A. lebbeck as compared to control.
Further increase in the concentration of lead upto 50 µmol/L produced toxic
effects on root growth. Similarly, cadmium treatment at low concentration of 10
µmol/L significantly (p<0.05) decreased seed germination, seedling and root
length as compared to control (Table 2). Cadmium treatment at 30 µmol/L produced
significant (p<0.05) effects on root length of A. lebbeck as compared with control (Table 2). Seedling vigor index
for A. lebbeck was highest in control
seedling and gradually declined with the increase in concentration of lead and
cadmium treatment from 10 to 90 µmol/L. The tolerance of A. lebbeck seedlings to lead and cadmium gradually decreased with
the increasing concentrations of lead and cadmium as compared to control (Fig.
2). Lead treatments at 10, 30, 50, 70 and 90 µmol/L produced 94.12, 82.35, 74.51,
72.56 and 27.45% of tolerance in A.
lebbeck, respectively. Cadmium treatment at similar range of treatments
produced 92.15, 80.39, 64.7, 41.18 and 11.76% of tolerance in A. lebbeck respectively. According to
tolerance indices, lead and cadmium treatments at 90 µmol/L showed lowest
percentage of tolerance in A. lebbeck seedlings
as compared to control. Cadmium
treatment produced more toxic effects on A.
lebbeck seedlings than lead treatment at all concentrations.
%20Benth.PNG)
%20Benth.PNG)
Fig. 1. Percentage decrease
in seed germination, seedling length, root length and seedling dry biomass of Albizia lebbeck at different
concentrations of lead and cadmium as compared to control.
Discussion
Siris (A. lebbeck
(L.) Benth) belonging to family Mimosaceae is a multipurpose tree for semi-arid
regions. Siris has been widely distributed around the tropics and mainly
planted as a shade tree. Siris is found on a wide range of soil types including
those that are alkaline and saline (Prinsen, 1986) but not subject to water
logging. 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 (Farga Šová, 1994).
Fig. 2. Tolerance indices of Albizia lebbeck at different
concentrations of lead and cadmium as compared to control.
Lead and cadmium toxicity have become an important issue
due to their constant increase in the environment. In the present
investigation, seed germination and seedling vigor index of A. lebbeck gradually decreased with the
increase in concentration of lead and cadmium. Lead and cadmium treatments
significantly (p<0.05) decreased seed germination as compared to control.
Seed germination and seedling growth inhibition by heavy metals has also been
reported by many other workers (Morzek & Funicelli, 1982; Al-Helal, 1995;
Azmat et al., 2005; Shafiq &
Iqbal, 2005). The decrease in seed germination of A. lebbeck can be attributed to the accelerated breakdown of stored
food material in seed by the application of lead and cadmium. (Kalimuthu &
Siva, 1990) found reduction in seed germination in maize treated with 20, 50,
100 and 200 µg/ml lead acetate and mercuric chloride. Excessive amounts of
toxic elements usually caused reduction in plant growth (Prodgers &
Inskeep, 1981). Some elements such as Cu, Co, Fe, Mo, Mn, Ni and Zn are
essential mineral nutrients. Others, such as Cd and Pb, however have no known
physiological activity (Lasat, 2002).
Significant reduction in root growth of A. lebbeck with the increase
in concentration of cadmium treatment was also observed as compared to control.
Cadmium is a highly toxic contaminant that affects many plant metabolic
processes (Li et al., 2008). Cadmium
can also affect root metabolism, which shows sensitivity to Cd2+
toxicity by a reduction in lateral root size (Wójcik &Tukendorf, 1999).
This is due to reductions in both new cell formation and cell elongation in the
extension region of the root (Prasad, 1995; Liu, et al., 2004).
The effects of heavy metals on plant depend on the amount
of toxic substance taken up from a given environment. The seedlings of A. lebbeck also showed a gradual
decrease in seedling vigor and dry biomass as concentrations of lead and
cadmium increased. Similar observations in crops had been observed by Hailing et al., (1991). 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).
According to the tolerance test, tolerance to lead and
cadmium treatments in A. lebbeck was
lower as compared with control. This information can be considered a
contributing step in exploring and finding of the tolerance limit of A. lebbeck at different concentrations
of treated metal. Cadmium is found highly toxic to seedling growth of A. lebbeck as compared to lead. Results
of the findings can be useful indicator of metal tolerance to some extent for
plantation of this species in metal contaminated area. How fairly low amounts
of lead and cadmium absorbed over many years could lead to extinction of such
an important plants species is unknown. In the metal contaminated areas,
further research is needed to determine different levels of metals in the
environment and various parts of the plants.
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(Received for publication 26 March 2008)