Toxicity And Tolerance In Samanea Saman (Jacq.) Merr. To Some Metals (Pb, Cd, Cu AND Zn)

Pak. J. Bot., 43(4): 1909-1914, 2011

M. KABIR, M. ZAFAR IQBAL AND M. SHAFIQ 

Department of Botany, University of Karachi,

 Karachi, 75270, Pakistan 

Abstract 

            In this study toxicity and tolerance of Samanea saman (Jacq.) Merr. was investigated for Pb, Cd, Cu and Zn under lab conditions. Germination rate of S. saman showed that increased concentration of different metals from 25 to 100 ppm, significantly (p<0.05) reduced germination which was more prominent for Pb treatments. Seedling growth variables i.e. root and shoot length, seedling size, root/shoot ratio, seedling fresh and dry weights also declined significantly (p<0.05). Seedlings growth of S. saman gradually reduced with increasing in concentrations of metals especially Pb and Cd compared to Cu and Zn. The inhibitory effects of metals had the following order Pb>Cd>Cu>Zn of sequence at different concentrations. Tolerance indices determined for different metal illustrated that increasing concentrations of metals reduced the tolerance of S. saman but this reduction was more prominent for Pb and Cd as compared to Cu and Zn treatments. 

Introduction

               Increasing level of heavy metals in the environment of Karachi city is mainly due to solid refuse and domestic fuel burning. Transportation and industrial activities are the additional source of metals pollution in the environment. The indiscriminate discharge of pollutants in air, water and soil affects the growth performance of flora of the region.

        Many studies have been conducted to identify plant species capable of accumulating undesirable toxic compounds such as heavy metals (Peralta-Videa et al., 2004). Reeves & Baker, (2000) compiled an exhaustive list of plant species that hyperaccumulated Cd, Cr, Ni, Pb, Se and Zn.

       High levels of heavy metals were investigated in soil samples from various polluted areas of Karachi city (Khalid et al., 1996). Iqbal et al., (1998) carried out a survey of vegetation and trace metals (Cu, Zn and Pb) in soils along the super highway near Karachi city. Toxic metal ions enter cells by means of the same uptake processes that move essential micronutrient metal ions (Patra, et al., 2004). Plant and soils near the roadside had a higher concentration of Pb than at a distance of 4-6 meters. A good correlation existed between traffic volumes; total and extractable soil Pb and Pb content of roots and shoots of grass (Cynodon dactylon) in roadside of Delhi (Dutta & Mookerjee, 1981). Lead concentrations up to several thousands parts per million in street dirt and soils are frequently found in urban areas and near some industries (Barltrop et al., 1974). The major sources of lead available to plants has been the soil usually derived from weathered bedrock, parents material from lead mine, smelting operations, use of lead arsenate, use of tetraethyl and tetra methyl lead as antiknocks additive to petrol (Foy et al., 1978). Roadside trees in the city are under pressure and lost due to vehicular-traffic infrastructure and other community needs such as industrial products (Jim, 1998). Deposition of lead on the vegetation growing along the roads not only affected growth and germination but also caused a significant reduction in seed and fruit production of plants (Nasralla & Ali 1985; Ahmad et al., 2009). Foliar application of lead nitrate solution resulted in a reduction in various indices and yield parameters of wheat (Rashid & Mukherji 1993). Lead induces many biochemical and structural changes in biological systems (Minaii et al., 2008). Seedlings of tomato plants grown inHigh levels of heavy metals were investigated in soil samples from various polluted areas of Karachi city (Khalid et al., 1996). Iqbal et al., (1998) carried out a survey of vegetation and trace metals (Cu, Zn and Pb) in soils along the super highway near Karachi city. Toxic metal ions enter cells by means of the same uptake processes that move essential micronutrient metal ions (Patra, et al., 2004). Plant and soils near the roadside had a higher concentration of Pb than at a distance of 4-6 meters. A good correlation existed between traffic volumes; total and extractable soil Pb and Pb content of roots and shoots of grass (Cynodon dactylon) in roadside of Delhi (Dutta & Mookerjee, 1981). Lead concentrations up to several thousands parts per million in street dirt and soils are frequently found in urban areas and near some industries (Barltrop et al., 1974). The major sources of lead available to plants has been the soil usually derived from weathered bedrock, parents material from lead mine, smelting operations, use of lead arsenate, use of tetraethyl and tetra methyl lead as antiknocks additive to petrol (Foy et al., 1978). Roadside trees in the city are under pressure and lost due to vehicular-traffic infrastructure and other community needs such as industrial products (Jim, 1998). Deposition of lead on the vegetation growing along the roads not only affected growth and germination but also caused a significant reduction in seed and fruit production of plants (Nasralla & Ali 1985; Ahmad et al., 2009). Foliar application of lead nitrate solution resulted in a reduction in various indices and yield parameters of wheat (Rashid & Mukherji 1993). Lead induces many biochemical and structural changes in biological systems (Minaii et al., 2008). Seedlings of tomato plants grown in pots, treated with lead nitrate solution at 500 and 1000 ppm showed pronounced effect in the root system as compared to leaves and stem (Jaffer et al., 1999).

      Cadmium is strongly phytotoxic causing growth inhibition and even plant death, alterations in activated oxygen metabolism and cell disturbances (Sandalio et al., 2001). Plants grown under high levels of Cu normally showed reduced biomass and chlorotic symptoms (Yruela, 2005). Excess of Cu affects the aerial part as well as root growth, inhibiting cellular elongation due to the increase in plasmalemma permeability and cell wall lignification (Arduini et al., 1995). Inhibition of root growth is recognized as one of the most conspicuous symptoms of Cu toxicity (Kukkola et al., 2000), in which lateral development and elongation are more sensitive than root initiation (Kahle, 1993; Woolhouse, 1983). They reported that the toxicity and tolerance in plants is a response to heavy metals including cobalt and zinc. Higher concentrations of zinc reduced the germination and growth of Pennisetum americanum and Parkinsonea aculeata. High concentrations of Zn have been reported in some plant species growing along the Super Highway near Karachi (Iqbal & Shafiq, 1999).

       Adaptation is a change in the structure or functioning of an organism that makes it better appropriate to environmental stresses (Jules & Shaw, 1994). Lead (Pb) and copper (Cu) mines produce tons of tailings (wastes) containing excess of elements that are toxic for living organisms, even at low concentrations. However, certain plants as bent grass (Agrotis tenuis), that grows in mine wastes, evolved tolerance to heavy metals in 400 years of mining (Jules & Shaw, 1994). Plants have both constitutive (present in most phenotypes) and adaptive (present only in tolerant types) mechanisms for coping with elevated metal concentrations (Mehrag, 1994). Metal tolerance is an evolutionary phenomenon that can be demonstrated by comparing the growth of mine plants with non-mine plants in non-contaminated soil (Haque et al., 2009). 

       Samanea saman (Jacq.) Merr. belongs to family Leguminosae and sub-family Mimosoideae, normally known as rain tree commonly having 25 m height with rough, fissured bark, bipinnately compound, alternate leaves and commonly planted for shade and wood. In Fiji, it is found from near sea level to an elevation of 700 m, cultivated and sometimes abundantly naturalized along roadsides, on river banks and in forests (Smith, 1985).

    The purpose of this study was to examine the effect of heavy metals on seed germination, seedling growth and seedling dry weight under normal and stress conditions in S. saman.

Materials and Methods

   Healthy seeds of Samanea saman were collected randomly from Karachi University Campus. The top ends of seeds were slightly cut with a clean scissor to remove any possible seed coat dormancy. Then seeds were surface sterilized with 30% dilute solution of sodium-hypo chlorite to prevent any fungal contamination. Petri dishes and filter papers were also sterilized in an autoclave to reduce the chances of fungal growth. Ten seeds were placed on filter paper (Whatman No. 42) in petri dishes (90 mm diameter). Metal treatments were prepared using lead nitrate, cadmium nitrate, copper sulfate and zinc nitrate having 25, 50, 75 and 100 ppm concentrations.

     At the start of the experiment, 3 ml of distilled water and 3 ml of each metal solution of lead nitrate, cadmium nitrate, copper sulfate and zinc nitrate were added to each set of respective treatment. Later on every alternate day fresh treatment of 2 ml solution was given to each set of petri dishes while, distilled water was applied to control treatment before removing the old solution in order to avoid turgidity of seed. The experiment was completely randomized based on 3-replicates. Petri dishes were kept at room temperature (20±2°C) with 250 lux light intensity and the experiment lasted for 12 days. Seed germination, root, shoot, seedling lengths, seedling fresh weight were recorded and dry biomass was determined by placing the seedling in an oven at 80°C for 24 hours. Seedling fresh and dry biomass was measured by electrical balance. Seedling vigor index (S.V.I) was determined as per the formula given by Bewly & Black (1982). Tolerance indices (T.I.) were determined as mentioned by Iqbal & Rahmati (1992). The seed germination and seedling growth were statistically analyzed at p<0.05 level of significance on personal computers SPSS version 13.

Results

       Metals treatments reduced seed germination and seedling growth of S. saman at different concentrations. At 100 ppm of lead, cadmium, copper and zinc treatments, the rate of germination percentage was 53 %, 56%, 60% and 66%. Seedling size (root + shoot length), seedling fresh and dry weight showed reduction when treated with increased concentration of lead from 25 to 100 ppm (Table 1 & Fig. 1). The seed germination of S. saman was significantly (p<0.05) reduced at 25 ppm lead treatment while, reduction was more prominent at 100 ppm of lead treatment. Similar reductions were also noted for root dry weight and seedling dry weight of S. saman. Seedling length and dry weight at 100 ppm lead treatment was 5.76 cm and 262 mg, respectively.

        Cadmium, copper and zinc treatments with different concentrations also reduced seedling growth parameters. Significant differences in seed germination and seedling growth were noted for Cd treatment at 25 ppm in S. saman as compared to control (Table 2). Cd concentrations at 50 ppm markedly decreased shoot growth, root/shoot ratio and seedling growth as compared to control. Cd treatment at 75 ppm found responsible for significantly (p<0.05) reduction in roots growth. It was noted that with an application of Cd at 100 ppm, 45 % reduction in seedling fresh and dry weights was observed as compared to control. 

     Toxicity of copper treatment for S. saman is summarized in Table 3. In control, root length was recorded as 5.46 cm which was significantly (p<0.05) reduced to 3.40 and 2.60 cm at 75 and 100 ppm Cu treatments, respectively. Similarly, shoot length, seedling size, root/shoot ratio, seedling fresh and dry weights also declined when concentration of Cu was increased from 25 to 100 ppm.

         Significant (p<0.05) reduction in seed germination and seedling growth of S. saman were noted with 25 ppm Zn as compared to control. An increase in zinc concentration up to 50 ppm markedly decreased shoot growth (Table 4). Further increase in Zn treatment up to 75 ppm found responsible for reduction in roots growth (4.18 cm). Results illustrated that application of Zn at 100 ppm high percentage of reduction (58 %) in seedling dry weight was observed as compared to others metal treatment with same concentration. 

       Seedling vigor index (SVI) is the potential of seed germination and seedling size against the toxicity and tolerance of metals. Results indicated that S. saman showed reduction in SVI with increasing concentration of different metal treatments but this reduction was more prominent for Pb and Cd as compared to Cu and Zn. Increased concentrations of Pb and Cd from 25 to 100 ppm caused high reduction in SVI of S. saman. Seedling of S. saman showed low percentage of tolerance to Pb, Cd, Cu and Zn treatment as compared to control. The order of decrease in tolerance indices was recorded as Pb>Cd>Cu>Zn (Fig. 2).

Discussion

     Excess level of heavy metals in the environment can produce toxic effects to plants. Lead, cadmium, copper and zinc toxicity has become important due to their constant increase in the environment. Present study revealed that metal treatments produced toxic effects on seed germination and seedling growth of S. saman.

     The plant species under stress conditions are most likely to be adversely affected by metals. Metal sensitivity and toxicity are influenced by the concentration range of the toxicant, the length of the exposure period, and the life-stage or biological process (Ernst & Nelissen, 2000). It is illustrated from the results that S. saman showed higher sensitivity to Pb toxicity as compared to other metals treatment (Tables 1-4). This sensitivity might be due to low tolerance to Pb and more accumulation of the substrate. Our study is also supported by the work of some others workers as increasing concentrations have deleterious affects on plant growth. Farooqi et al., (2009) reported that 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 Albizia lebbeck while lead treatment at 50 μmol-L significantly affected root growth and seedling dry biomass as compared to control.

Fig. 1. Effects of lead, Cadmium, Copper and Zinc on seed germination and seedling growth of Samanea saman at different concentrations in lab conditions. 

Fig. 2. Tolerance indices (%) in Samanea saman at different concentration of Pb, Cd, Cu and Zn.

Nriagu & Pacgana (1988) reported that about 220,00 tones of cadmium is globally discharged every year in to the soil. Cadmium is one of the most toxic elements and it affects plant growth adversely. Our results illustrated that Cd tretment adversely affected the germination and growth of S. saman. Cadmium is easily taken up by plants and translocated to different plant parts due to which it causes significant reduction in different growth parameters. In an investigation, Cd treatment at 10 μmol-L concentration produced toxic effects on seed germination, seedling and root growth. Increase in cadmium concentration up to 50 μmol-L produced a significant reduction in seedling dry weight of Thespesia populnea as compared to control (Kabir et al., 2008). Some other investigations were carried out by a number of workers for Cd uptake and toxicity which are in support of our findings. Patterson (1977) reported that plant species and even genotypes differ greatly in their ability to take up, transport and accumulate Cd within the plant. Muramoto et al., (1990) reported that root and shoot weights of rice were reduced 32% and 21% by 100 mg Cd kg-1. Genotypical variations in tolerance to Cd toxicity are well documented in the literature (Grant et al., 1998; Ozturk et al., 2003). Results of similar nature were also found on Brassica juncea, genotypes (Varuna and DHR-9504) in relation to the effect of cadmium (Cd) on growth, yield and levels of cadmium (Cd) in different plant parts. Increased Cd supply in the form of CdCl2 @ 0, 20, 40, 60, 80 and 100 mg Cd kg-1 soil showed Cd phytotoxicity by growth retardation of Varuna and DHR-9504. Varuna showed greater sensitivity to Cd toxicity than DHR-9504 (Chaturvedi, 2004). Marschner et al., 1988 also demonstrated Cd toxicity by growth retardation and leaf chlorosis in Chinese cabbage. The reduction observed in dry matter yield by Cd application is in agreement with the findings of Lehoczky et al., (1996). Yield reductions in mustard plants have been attributed to the direct effect of higher Cd concentrations in plant tissue and not through an indirectly induced deficiency of other nutrients (Wilson, 1992). Reduction in shoot and root growth of S. saman might be due to low tolerance limit to cadmium stress. Marschner et al., (1988) and Ozturk et al. (2003) also found reduction in shoot and root dry matter to a similar extent due to Cd treatment.

          S. saman also showed reduction in seed germination and seedling growth due to toxicity of copper treatments. Phototoxic responses to Cu have been observed for many plant species (Woolhouse & Walker 1981) and the toxic symptoms and yield reductions observed are thought to be a consequence of tissue damage, alteration of membrane permeability, peroxidation of chloroplast membrane lipids and inhibition of photosynthetic electron transport, and disruptions to carbohydrate metabolism and protein synthesis (Droppa & Horvath, 1990; Woolhouse, 1983). Zinc is an important element for plant growth. Excess Zn reaching the shoot may also be excreted via the salt glands on the leaves (MacFarlane & Burchett, 1999). Seed germination and seedling growth responses of S. saman with excess Zn were also observed. High tolerance to zinc treatment in seedling of S. saman indicated that it has better absorbing ability than lead, cadmium and copper. 

            

           Results indicated that overall, more reduction was observed in the seed germination and seedling growth of S. saman when treated with Pb and Cd as compared to Cu and Zn. Root length was found more susceptible to all the metal treatments as compared to shoot. Results illustrated that tolerance mechanism of S. saman to different metals decreased as Zn>Cu>Cd>Pb. Reduced seedling vigor index was possessed by S. saman with increasing concentration of different metals from 25 to 100 ppm probably due to less tolerance. 

Conclusion

     Heavy metal stress is one of the major problems affecting productivity of plants. Natural flora show relative differences in their heavy metal tolerance capacity. Some plants grow well in a soil enriched with toxic levels of heavy metals while others could not grow (Yadev, 2010). Plants have proven particularly affective for both bioremediation of metal contaminated sites and for studying the uptake, transport and toxicity of metals in plants (Tug et al., 2010; Hogan, 2011). It is concluded that Pb, Cd, Cu and Zn treatment was strongly phytotoxic to seed germination and seedling growth of S. saman. High concentrations of metal treatment found responsible for decreasing the percentage of tolerance indices in seedling of S. saman. The level of plant tolerance to heavy metals is related to the balance between the rate at which metal ions are taken up and the efficiency with which they are detoxified within the plant (Antosiewicz, 2005; Azmat et al., 2009; Malik et al., 2010). Metal uptake and translocation to shoots might be the cause of reduction in seedling growth and biomass production.

Acknowledgements

      We are highly grateful to Higher Education Commission (HEC) of Pakistan for funding this study which is a part of project No. 20-1073/07. The assistance of Mr. Zia-ur-Rehman Farooqi for collection and analysis of data is also acknowledged.  

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(Received for publication 16 June 2010)