M. Kabir, M. Zafar Iqbal and M. Shafiq,: Effects of Lead on Seedling Growth of Thespesia Populnea l, Adv. Environ. Biol., 3(2): 184-190, 2009.
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Abstract
The effects of lead on root, shoot and seedling length, leaf area, number of leaves, plant circumference, seedling dry weight, root/shoot and leaf area ratios of Thespesia populnea L. were determined in green house under natural environmental conditions with and without phytotoxic metal ions at 5, 10, 15, 20, and 25 µ mol/L. Lead treatments have strong influence on the growth and development of T. populnea by reducing significantly (P<0.05) all the above parameters. Lead treatment at 5-25 µ mol/L produced significant (P<0.05) effects on seedling and root length while, lead treatment at 10-25 µ mol/L produced significant (P<0.05) effects on shoot length, number of leaves and leaf area as compared to control. A significant (P<0.05) reduction in plant circumference and seedling dry weight of T. populnea was observed with increasing concentrations of lead from 5 to 25 µ mol/L as compared to control. Tolerance in T. populnea seedling at 25 µ mol/L of lead treatment was lowest as compared to all other treatments.
Keywords
Introduction
Heavy metals play an important but dual role in plant metabolism. On one hand, some of them are essential micronutrients acting, for example, as cofactors of key metabolic enzymes. On the other hand, when exceeding their critical concentrations, the same metals become the most toxic pollutants in the soil [30]. Pollution by toxic elements is a world wide problem. Toxic pollutants are discharged in the air by man made activities[21]. Mining operations, metal smelting, electroplating, gas exhaust, energy and fuel production power lines, intense agriculture are some of the numerous human activities that contain quantities of toxic metals[13]. Once released into the environment they are not broken-down into harmless components. The main sources of heavy metals in plants are their growth media, nutrients, agro inputs and soil. Other sources may include pesticides and fertilizers[26]. The hazard and fast industrial growth is causing an enormous environmental pollution affecting plants. Vegetation differs from area to area depending on climatic factors and soil characteristics. Polluted soil can alter plant growth and quality, and the effects are often destructive[24]. In industrial city like Karachi, vehicular emission is a major source of atmospheric pollution[22]. 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[10]. These air pollutants may interfere with the biological processes relating to general metabolism, photosynthetic activities, mitochondrial respiration and stomatal clogging of plants[17,2,1]. Among the heavy metals Pb is highly toxic pollutants. It is generally added in the environment through automobile exhaust[15], fertilizer impurities [31] and through industrial effluents[4]. Lead is the heaviest of the non radioactive metals that also naturally occur in substantial quantities in the earth’s surface. It is present in all soils, rivers, lakes and sea water. It is also a component of dust, rubber, paints, metal products (steel and brass) and lead batteries. Plants absorb lead through soil, water and air. Besides the uptake from the soil and water, Pb may enter the plant surface through aerial parts including leaf surface. Deposition of lead on the vegetation growing along the roads not only effects growth and germination but also causes significant reduction in seed and fruit production of plants[20]. Foliar application of lead nitrate solution resulted in a reduction in various indices and yield parameters of wheat[23]. Lead is a toxic environmental contaminant that induces many biochemical and structural changes in biological systems[18]. T. populnea is a member of family Malvaceae with basically tropical and subtropical world wide distribution. It is used as a shade tree and as a wind break in order to control the soil erosion. In view of the destructive role of heavy metals derived from the vehicular emission and metal processing industries, an investigation was carried out to determine the toxic effects of lead on the seedling growth of Thespesia populnea.
Materials and Methods
The experiment was conducted in green house under the uniform natural environmental conditions at the Department of Botany, University of Karachi during March and April, 2007. Healthy and uniform size seeds of T. populnea were collected from the University Campus. The top ends of seeds were slightly cut with the clean scissor to break dormancy and were sown in garden soil at 1 cm depth in large pots and watered regularly. After two weeks of their germina tion, uniform sized seedlings were transplanted in pots of 20 cm diameter and 9.8 cm in depth containing the same garden soil in which they were germinated. A metal treatment of Pb was applied with lead nitrate at 0, 5, 10, 15, 20 and 25µ mol/L, respectively. There were five replicates for each treatment. The experiment was completely randomized. Only one seedling was grown in each pot and the plants were regularly treated with 3 ml of their respective treatment. Every week reshuffling of pots were also carried to avoid light/ shade or any other green house effects. Daily climatic data, as average atmospheric temperature, atmosphere relative humidity, sun shine and weather outlook were recorded. Seedling height, number of leaves, leaf area and plant circumference were noted after every week. After eight weeks seedlings were taken out from pots, washed their roots with water and measured root length (cm), shoot length (cm), seedlings length (which included length of root and shoot) (cm), plant circumference, number of leaves and leaf area (sq cm). Seedlings were dried in an oven at 80 °C for 24 hours and oven dry weights (g) of seedlings were determined. There is a tendency to assume that plants are growing as fast as they are given the resources such as climatic factors which include humidity, temperature sun shine and weather outlook available to them. So all these factor were also recorded during proceeding periods of an experiment. Tolerance indices (T.I.) were determined with the following formula as given by Iqbal and Rahmati [9]. T.I. = Mean root length in metal solution / Mean × root length in distilled water 100.
Results and Discussion
Seedling growth of Thespesia populnea decreased with phytotoxic metal ions treatments (Pb) at 5, 10, 15, 20 and 25 µ mol/L as compared to control. Data record on seedling growth of T. populnea exposed to different concentrations of Pb is presented in Table 1. The results clearly indicate that all the growth parameters were suppressed gradually with the increase in concentrations of lead. A marked inhibition in seedling length (17.32 cm) became evident at 25 µ mol/L concentration of Pb. Application of Pb at 5, 10, 15 and 20 µ mol/L suppressed the seedling length (19.48, 18.54, 18.14, 17.68 cm, respectively) as compared to control. The assessment of early growth in terms of root and shoot length followed the similar pattern as observed for seedling growth in various applied concentrations of Pb. Root length was consistently reduced with increasing concentrations of Pb at 20 µ mol/L but high reduction was observed at 25 µ mol/L as compared to control and other treatments. Similarly, a gradual decline in shoot length was recorded with enhanced Pb concentration. However, maximum and statistically significant (P<0.05) depression in shoot length (6.52 cm) was evident at the highest concentration of Pb at 25 µ mol/L (Table 1). Number of leaves, leaf area and circumference of plant were also reduced in different concentrations of lead as compared to control. The data record on seedling dry weight revealed that treatment of Pb at 5, 10, 15 and 20 µ mol/L concentrations suppressed the seedling dry weight significantly to 1.56, 1.45, 1.27 and 1.05 g respectively as compared to control (1.75 g). Seedling dry weight (0.77 g) at 25 µmol/L of Pb was significantly decreased as compared to rest of all concentrations. Root / shoot ratio was lowered in treatment as compared to control and it decreased continuously with increase in concentration of Pb (Table 1). It was noted that inhibitory effect of Pb was greater at 25 µ mol/L concentration of Pb than other treatments. Percentage decrease in T. populnea was also calculated in different parameters which increased along with increased concentrations of Pb. (Fig. 1). Climatic data was also recorded during experiment in order to evaluate it effects on different growth parameters. For March, 2007 average temperature was recorded from 21°C up to 27°C along with 30 to 45% atmospheric relative humidity, eleven to twelve hours sunshine and fair / hazy to hot and dry weather outlook. These climatic factors were changed for the next month comprising average temperature up to 31°C, atmospheric relative humidity up to 61% and sunshine from twelve to thirteen hours along with warm and humid weather outlook. The seedlings of T. populnea were tested for tolerance to heavy metals, using different concentrations of lead (Fig. 2). Increased concentration of lead (5-25 µ mol/L) gradually decreased tolerance of T. populnea. Lead treatment at 5, 10, 15, 20 and 25 µ mol/L produced 98.73, 94.14, 92.39, 91.44 and 85.90 % tolerance in T. populnea, respectively.
Discussion
Heavy metal toxicity in the environment is of great concern because of its toxic effects on plants growth. The plants under stress conditions are most likely to be adversely affected by high concentration of heavy metals. Lead toxicity has become important because of its constant increase in the environment. High concentration of Pb may lead to the reduction in root hair development and stunted growth due to reduced photosynthetic rate in plants, which is the result of stomata closure by the deposition of Pb [25]. Lead exerts deleterious effects on morphology, growth and photosynthetic processes of plants and causes inhibition of enzyme activities, water imbalance, alterations in membrane permeability and disturbs mineral nutrition[29,28]. Sharma and R.S. Dubey, Lead toxicity in plants, Brazilian Journal of Plant Physiology 17(2005), pp. 35–52. View Record in Scopus | Cited By in Scopus (22)Israr & Sahi, 2008). Seed germination inhibitions by heavy metals have been reported by some workers[3,19]. Seedling length of T. populnea showed reduction with increase concentrations of lead. The reduction in plant height could be attributed to the adverse affect of metal treatments on the cell elongation and cell expansion. The effects of toxic substances on plants are dependent on the amount of toxic substance taken up from a given environment. The toxicity of some metal may be so high that plant growth is retarded before a large quantity of an element can be translocated[7]. The inhibition in seedling growth of T. populnea appeared to be due to toxicity of Pb solution. Heavy metals had also inhibited the seedling growth of Phaseolus aureus[27]. The reason of reduced shoot and seedling length in metal treatments could be the reduction in meristematic cells present in this region and some enzyme contained in cotyledon and endosperm. Cells become active and begin to digest and store food which is converted into the soluble form and transported to the radicle and plumule tips e.g. enzyme amylase convert starch into sugar and protease act on protein. So when the activities of different enzymes were affected, food did not reach to the radicle and plumule and in this way shoot and seedling length were affected. Pb treatment caused toxic effects on the root growth for seed collected from the polluted and clean areas. The reason for reduced root length in metal treatments could be due to reduction in mitotic cells in meristematic zone of root as suggested by Lerda[16] on Allium cepa. He observed that 50, 100 and 200 ppm of Pb stopped the growing processes in plants after 24 hours. These findings confirm that Pb reduced the frequency of mitotic cell in meristematic zone and Pb causes inhibition of root growth. Cho & Wang[5] showed that percentage length of infected roots and dry biomass of maize plant were reduced by the addition of heavy metals. Dalal & Bairgi[6] have found reduction in seed germination, root, shoot and seedling length of jute verities, Corchorus olitorius cv. JRO 524 and Capsular corchorus JRC 321 at different levels of Pb, particularly at 20 mg/l. The reduction in root length in T. populnea was more prominent in different concentrations of Pb treatments as compared to shoot and seedling length. The influence of increasing concentrations of lead on 14 day-old seedlings of Triticum aestivum cv. Vergina was observed[12]. They found that plants grown in 1/10 strength Rorison's nutrient solution plus increasing concentrations of lead depressed shoot growth and the most evident symptoms were found on roots These effects demonstrate the diverse modes of metal action resulting in different degrees of toxicity. Mineral nutrients are important for the normal growth of plants. Presence of unbalance nutrients in soil can cause disturbance in the uptake of certain elements, which are necessary for plant growth[9]. Some elements such as Cu, Fe, Mn etc are essential mineral nutrients. Others, such as Cd and Pb, however, have no known physiological activity [14]. Plants differ in the level of tolerance to different elements for growth but excessive amount can lead to toxicity. These differences may be attributed to variable ion translocation to the aerial parts of the plants. Dry weight is an important indicator of adaptations mechanism. The seedling of T. populnea showed a gradual decrease in dry weight with increase concentration of lead. The lead treatments showed large effects on seedling dry weight which is evident from the poor growth of root and aerial parts.
Root / shoot and leaf area ratios indicate that increased concentration of Pb produce more toxicity, reducing root / shoot ratio and other parameters of growth. The percent decrease in seedling length, root length, shoot length, number of leaves, leaf area, plant circumference and seedling dry weight was calculated. These findings confirmed that increased concentrations of Pb showed more percentage reductions indicating its toxicity. Tolerance to Pb in T. populnea was relatively low. The reason for reduced tolerance to Pb may be changes in the physiological association of the tolerance mechanism and seedling growth of plant. Results of the present finding appear to be useful indicator establishing the toxic nature and tolerance indices for Pb in T. populnea. Plants differ in the level of tolerance to metal stresses. This information can be considred a contributing step in exploring and finding the tolerance limit of T. populnea at different concentrations of lead treatments.
Conclusion:
Different concentrations of Pb significantly (P<0.05) inhibited growth of Thespesia populnea. All the growth parameters were gradually declined with the increase in concentrations from 5 to 25 µmol/L. Seedling length and seedling dry weight showed more sensitivity to Pb. This suggest that T. populnea should only be planted at low lead contaminated areas.
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