INTRODUCTION:

For growth and development plants absorb large amounts of elements and only small amounts of toxic elements that could harm them. There are 35 metals that concern because of occupational or residential exposure; 23 of these are the heavy elements or “heavy metals”: Ag, An, As, Au, Bi, Cd, Ce, Cr, Co, Cu, Fe, Ga, Hg, Mn, Ni, Pb, Pt, Te, Tl, Sn, U, V and Zn [1]. In nature land and water are precious natural resources for the sustainability of agriculture and the civilization of mankind. However, rapid industrialization and urbanization shows maximum exploitation and severe pollution in these resources. Land and water pollution by heavy metals is a worldwide issue [2]. Mobilization of heavy metals in environment due to industrial activities is of serious concern due to toxicity of these metals in human and other forms of life [3]. The use of plant species for cleaning polluted soils and waters named as phytoremediation has gained increasing attention since last decade, as an emerging cheaper technology. Literature survey suggests that several aquatic species have been identified and tested for the phytoremediation of heavy metals from the polluted water such as, sharp dock (Polygonum amphibiumL.), duck weed (Lemna minor L.), water hyacinth (Eichhornia crassipes), water lettuce (P. stratiotes), water dropwort [Oenathejavanica(BL) DC], calamus (Lepironia articulate), pennywort (Hydrocotyle umbellate L.) [4].

The ability of aquatic macrophytes to take up heavy metals make them acceptable research applicants especially for the treatment of effluents having medium concentration level pollutants and city sewage waters [5]. Plants play an important role in solar energy transport to bio-energy and can clean the environment in an environmentally friendly manner they would also play an important role in heavy metal remediation. Some of the metals are essential for the growth and development of living organisms. However, many heavy metals are highly toxic when the concentration exceeds certain limits [6]. Heavy metal accumulation in plants depends upon plant species and the efficiency of different plants in absorbing metals is evaluated by either plant uptake or soil to plant transfer factors of the metals [7].

To understand the effects of heavy metals on plants and resistance mechanisms would be helpful for using plants to clean and remediate heavy metal pollution. To study their growth tolerance, change in biomass and also to test their maximum accumulation potential of heavy metal.

MATERIALS AND METHODS:

Experimental setup:

Trapa natans with approximately the same size and weight, 7-8 weeks old were collected from an uncontaminated pond. The plants were washed thoroughly with the tap water followed by de-ionized water prior to the experimentation. All the plants were grown hydroponically for 21 days in modified Hoagland’s nutrient solution. The experiment was conducted in triplets and repeated three times for a period of 21 days each one.

Figure 1 Experimental device located at the University of Allahabad, Allahabad

Heavy metal preparation:

All experimental work was done using deionised water, and all reagents were of analytical grade. Lead stock solution was prepared by dissolving 1.598 mg of Pb (NO₃)₂ in 1000 ml of deionised water which was later diluted as required. Trapa natans which was acclimatized in the laboratory condition, applied to a solution of lead concentration of 1.5, 2.5 and 3.5 mg/l in five plastic tubs of 11 liter capacities. A plant control, i.e. plant grown in tap water and metal control, metal solution without any plants were also established.

Biomass estimation:

Determination of Fresh weight:

After 21 days of treatment each plant sample from control and different concentration of lead where carefully taken and the fresh weight of the plant samples were analyzed using a monobalance. The readings were noted.

Determination of Dry weight:

The fresh plants from both the control and different concentrations (1.5, 2.5 and 3.5 mg/l) were taken and dried in hot air oven at 80⁰C. The dry weights of the samples were analyzed using a monobalance.

RESULTS AND DISCUSSION:

Growth analysis:

In present study plant growth was measured in terms of root length, leaf area and biomass production in terms of fresh and dry weight. The causes of growth reduction differ [8], but it is not clear which mechanisms plants employ to maintain residual growth for short and long term responses.

Root length:

Maximum root length was observed in control (12.9cm) on 21^th day while minimum root length was observed as 6.133 cm at 3.5 mg/l concentration on the initial day (Table 1).In general, most studies reported the higher concentrations of metals in roots than in shoots. Some metals are accumulated in roots, probably due to some physiological barriers against metal transport to the aerial parts, while others are easily transported in plants.

Leaf area:

Maximum average leaf area was 11.376 cm²in control plant 1.5mg/l on the 21 day and the minimum average leaf area was 8.026 cm² on the 21^th day observed at 3.5 mg/l concentration (Table 1). On the 21^th day the leaf area of Trapa natans decreased with the increasing concentrations of Pb(NO₃)₂ (Table 1).From the present results, it is clear that leaf area per plant decreased when exposed to 1.5, 2.0 and 3.5 mg/l concentration of Lead nitrate.

Biomass:

Fresh weight:

It is evident from the results that the fresh weight of Trapa natans sensitive parameter at various concentrations of lead nitrate. Fresh weight of the leaves was maximum with 0.402 g in control set on the 21th day and the minimum fresh weight was0.299 g on 21th day at 3.5 mg/l concentration (Table 1). On the 21th day the fresh weight decreased serially with the increase in concentration of lead. Though the plants showed slight increase in fresh weight on 5th and 10^th day at 1.5, 2.5 mg/l concentration over the control, it was adversely affected at higher dose at the end of treatment (Table 1). The reduction in shoot biomass production by the plant may be due to the chlorosis and necrosis of the leaves that reduce the photosynthetically active area. The decrease in fresh weight of the leaves of Trapa natans might be due toxic nature of Pb(NO₃)₂and the suppression of growth under such stress during the early developmental stages.

Table 1. Effect of lead nitrate (mg/l) on root length, leaf area, fresh weight and dry weight of leaves of Trapa natans (L.)

Duration

Root length (cm)

Duration

Leaf area (cm²)

Control

1.5

2.5

3.5

Control

1.5

2.5

3.5

Initial Day

7.766

(±0.46)

7.266

(±0.72)

6.866

(±0.65)

6.133

(±0.40)

Initial Day

11.376

(±0.93)

12.79

(±0.99)

12.17

(±1.22)

12.37333

(±1.25)

5^th Day

10.433

(±0.95)

10.266

(±1.02)

9.833

(±1.55)

9.233

(±1.20)

5^th Day

10.53

(±0.83)

9.18

(±1.11)

8.846

(±1.02)

8.873

(±1.75)

10^th Day

10.7

(±1.53)

9.866

(±1.12)

9.566

(±2.12)

8.366

(±1.50)

10^th Day

10.8

(±0.63)

8.553

(±1.48)

8.416

(±1.43)

8.28

(±1.28)

15^th Day

12.733

(±2.95)

12.633

(±2.27)

11.633

(±2.21)

10.666

(±2.22)

15^th Day

10.35

(±0.59)

8.286

(±0.28)

8.113

(±0.41)

8.203

(±0.31)

20^th Day

12.9

(±1.21)

11.8

1.276715

10.833

0.702377

10.2

(±0.8)

20^th Day

10.07

(±0.68)

8.193

(±0.37)

7.873

(±0.46)

8.026

(±0.67)

Each value is a mean of ten observations and values in parenthesis indicate standard deviation

Duration

Fresh weight (g)

Duration

Dry weight (g)

Control

1.5

2.5

3.5

Control

1.5

2.5

3.5

Initial Day

0.327

(±0.04)

0.344

(±0.06)

0.336

(±0.02)

0.344

(±0.02)

Initial Day

0.048

(±0.00)

0.044

(±0.00)

0.046

(±0.00)

0.045

(±0.00)

5^th Day

0.351

(±0.01)

0.359

(±0.04)

0.350

(±0.05)

0.321

(±0.06)

5^th Day

0.056

(±0.00)

0.043

(±0.00)

0.036

(±0.00)

0.039

(±0.00)

10^th Day

0.382

(±0.02)

0.366

(±0.06)

0.385

(±0.03)

0.328

(±0.03)

10^th Day

0.050

(±0.00)

0.049

(±0.00)

0.035

(±0.00)

0.034

(±0.00)

15^th Day

0.398

(±0.06)

0.382

(±0.05)

0.340

(±0.04)

0.305

(±0.02)

15^th Day

0.054

(±0.00)

0.053

(±0.00)

0.034

(±0.00)

0.033

(±0.00)

20^th Day

0.402

(±0.03)

0.389

(±0.03)

0.334

(±0.03)

0.299

(±0.07)

20^th Day

0.055

(±0.01)

0.055

(±0.00)

0.031

(±0.00)

0.032

(±0.00)

Each value is a mean of ten observations and values in parenthesis indicate standard deviation

Dry weight:

It is evident from the results that the dry weight of the plants is also another sensitive parameter like fresh weight. Dry weight of the leaves was maximum with 0.056g on the 5th day in control set and minimum dry weight was 0.032 g on 21th day at 3.5 mg/l concentration (Table 1). On the 21th day the dry weight decreased serially with increase in concentration showing a perfect negative correlation. The dry weight decreased with high concentration of Pb(NO₃)₂(Table 1). A decrease in dry weight of the leaves at the highest concentrations of Pb(NO₃)₂ might be due to the inhibition in hydrolysis of reserved foods and their translocation to the growing shoots.

Heavy metal accumulation by Trapa natans:

The amount of accumulation seems to increase with an increase in concentration of lead. Trapa natans proves to be more efficient in accumulating lead. As the concentration increased, the amount of accumulation of heavy metal also increased. The results obtained in the experimental study prove the fact that Trapa natans has the innate capacity for the accumulation of appreciable quantities of lead. Plants may accumulate higher concentration of metals in the roots since roots are usually at the base of the plant and removed from photosynthetic process for their own tolerance [9].Plants show potential to be used in the phytoremediation system which requires plants to be able to accumulate acceptable amount of metals and also survive in the contaminated condition [10]. Plants shows potential to be used in the phytoremediation system which requires plants to be able to accumulate acceptable amount of metals and also survive in the contaminated condition. Figure 2 shows that concentration of nitrate increases in root, the concentrations of lead decreases in shoot.

Figure 2. Accumulation of Lead in Trapa natans after 21 days of exposure

CONCLUSION:

The study have shown though Trapa natans is invasive plants, they can effectively be employed in the phytoremediation of aquatic ecosystem which have been polluted by harmful, toxic heavy metals i.e. lead. We also suggest the need to understand processes that affect metal availability, metal uptake and translocation in Trapa natans. There are two aspects on the interaction of plants and heavy metals. On one hand, heavy metals show negative effects on plants. On the other hand, plants have their own resistance mechanisms against toxic effects and for detoxifying heavy metal pollution.

ACKNOWLEDGEMENTS:

The authors would like to thank all other members of Bhargava Agricultural Laboratory. Authors are also thankful to the Head, Department of Botany, University of Allahabad for providing laboratory facilities.

REFERENCE:

[1] Glanze WD (1996). Mosby Medical Encyclopedia, Revised Edition 1996. St. Louis MO: C.V. Mosby.

[2] Gade, L. H. 2000. Highly polar metal—Metal bonds in “early-late” heterodimetallic complexes. Angewandte Chemie-Internat. Ed. 39:2658–2678.

[3] Vieira, R. H. S. F., B. Volesky, Biosorption: A solution to pollution. Int. Microbiol. 3:17-24.

[4] Prasad, M. N. V. and H. M. D. Freitas. 2003. Metal hyperaccumulation in plants—Biodiversity prospecting for phytoremediation technology. Elect. J. Biotechnol. 93:285-321.

[5] Sood, A., P. L. Uniyal, R. Prasanna and A. S. Ahluwalia. 2012. Phytoremediation potential of aquatic macrophyte, Azolla. AMBIO: J. Human Environ. 41(2):122-137.

[6] M.M. Al-Subu, R. Salim, I. Abu-Shqair, K.M. Swaileh, Removal of dissolved copper from polluted water using plant leaves: I. Effects of acidity and plant species, Rev. Int. Contam. Ambient. 17 (2001) 91-96.

[7] Khan S., Cao, Q., Zheng Y.M., Huang Y.Z. and Zhu Y.G., Health risks of heavy metals in contaminated soils and food crops irrigated with wastewater in Beijing, China. Environmental Pollution, 152, 686-692 (2008).

[8] Munns, R. 2003. Comparative physiology of salt and water stress. Plant Cell Env. 25:239-250.

[9] A. Kamal, A.E. Ghaly, N. Mahmoud and R. Cote, ‘‘Phytoaccumulation of heavy metals by aquatic plants,’’ Environmental International, 29(2004), 1029-1039.

[10] C. Lamai, M. Kruatrachue, P. Pokethitiyook, E. S. Upatham and V. Soonthornsarathool, “Toxicity and accumulation of lead and cadmium in the filamentous green alga Cladophorafracta: A laboratory study,” ScienceAsia, vol. 31, pp. 121-127, February 2005.

Received on 10.03.2014 Modified on 30.03.2014

Asian J. Research Chem. 7(4): April 2014; Page 434-437