Lycopersicon esculentum seedlings were collected from the field and they were transplanted to soil containing different concentrations of zinc sulphate (100, 200, 300, 400 and 500 ppm). Treatment concentrations of zinc sulphate were selected based on the literature. The plants were grown in triplicates in pots of two kg capacity. They were watered in morning and evening and exposed to normal photoperiod. Plants from each concentration of zinc sulphate were removed at an interval of 15, 30, 45 and 60 days of treatment. The soil has been subjected to AAS analysis for detecting the concentrations of copper, zinc, manganese and iron in the soil.

In the present study, metal extractant capacity of L. esculentum was tested by supplementation of zinc sulphate in soil at different concentrations such as 100, 200, 300, 400 and 500 ppm. At the commencement of the experiment, it was observed that growth of seedlings was not influenced by increasing concentrations of zinc in soil. Metal accumulating ability of L. esculentum was tested by analyzing the soil samples and also the leaf samples. The residual concentrations of zinc in soil exposed to different concentrations of zinc sulphate and treated with L. esculentum are shown in Figure 1. It is observed that the minimum residual concentration of zinc in soil was 4.56 ppm at 100 ppm after 15 days of treatment and maximum concentration was 7.62 ppm at 300 ppm after 45 days of treatment. The residual concentration of zinc in soil was found to increase after 30 and 40 days of treatment, but the residual concentration decreased after 60 days of treatment. Figure 2 illustrates copper concentration in soil exposed to zinc sulphate and treated with L. esculentum. The copper concentration in soil ranged from a minimum value of 1.34 ppm at 400 ppm after 45 days of treatment to a maximum value of 6.4ppm at 30 ppm after 30 days of treatment. The concentration of copper was found to increase after 30 days and decrease after 45 days of treatment.

The concentrations of manganese and iron in soil exposed to different concentrations of zinc sulphate and treatment with L. esculentum are shown in Figure 3 and 4 respectively. The manganese concentration in soil ranged from a minimum value of 1.54 ppm at 400 ppm after 15 days of treatment and a maximum value of 10.48 ppm at 200 ppm after 60 days. The iron concentration in soil ranged from a minimum value of 4.58 ppm at 200 ppm after 15 days of treatment to a maximum value of 10.48 ppm at 200 ppm after 60 days of treatment. The concentration of manganese increased drastically after 60 days of treatment while a slight increase was noticed after 45^th day. The concentration of iron was found to be increasing after 30, 45 and 60 days of treatment (Figure 4).

The uptake of metals in the above ground parts of L. esculentum exposed to various concentrations of zinc sulphate for 60 days is shown in Table 1. The concentration of copper ranged from a minimum value of 85ppm to a maximum of 140ppm. Zinc and iron concentrations were found to be the same in all test concentrations. Manganese was absent in the above ground parts of treated tomato plants. Table 2 divulges the uptake of metals in the below ground parts of L. esculentum exposed to various concentrations of zinc sulphate for 60 days. The concentrations of copper, zinc, manganese and iron were found to be the same in all the test concentrations in the below ground parts of L. esculentum exposed to various concentrations of zinc sulphate for 60 days. In the below ground parts of L. esculentum exposed to various concentrations of zinc and control plants, manganese was absent. Figure 5 shows the percent removal of zinc in soil exposed to various concentrations of zinc and treated with L. esculentum. The percent removal of zinc was found ranging from 75.14 % at 27.52 ppm of zinc after 30 days of treatment to 95.79% at 118.37 ppm of zinc after 45 days of treatment. Variations in chlorophyll content of the leaves of L. esculentum on exposure to various concentrations of zinc sulphate in the soil are shown in Table 3. The chlorophyll content was observed to be diminishing at low and high concentrations of zinc sulphate. Table 4 shows the microbial colonies (bacteria, fungi and actinomycetes) that were isolated from the rhizosphere soil samples in nutrient agar, potato dextrose agar and KenKnight and Munaier’s medium. The microbial colonies were found to be inhibited slightly due to the increased concentration of zinc when compared to control soil which was not exposed to zinc.

Two way analysis of variance (ANOVA) for different tested factors was done at the end of the present study to find out the statistical significance of the variations. The variations in iron concentration in soil treated with L. esculentum due to zinc concentration were statistically not significant at 5% level. The variations in copper concentration in soil treated with L. esculentum due to zinc concentration were statistically significant at 5% level. The variations in percent removal of zinc in soil treated with L. esculentum due to zinc concentration and treatment period were statistically significant at 5% level. The variations in manganese concentration in soil treated with L. esculentum due to zinc concentration were not statistically significant but due to treatment period variations were statistically significant at 5% level. The variations in residual concentration of zinc in soil treated with L. esculentum due to zinc concentration and treatment period were statistically significant at 5% level (Table 5).

The heavy metals like zinc, cadmium, copper, lead and others are subsequently added in to the soil through various human activities. Vast utilization of pesticides and fertilizers in agriculture spoils the nature of soil and ultimately its results in soil contamination. Like that, release of industrial effluents and dumping of waste materials are also the major reasons for soil pollution. These kinds of activities not only affect the nature of soil and also affect the quality of ground water and other surrounding water bodies. Due to the uptake of heavy metals by plants from the contaminated soil at the time of their growth and development, heavy metals enter in to the animals including human beings through the food chain 2425. Based on the wide range of applications in industries as well as in agricultural practices and toxicity level in the local environment, zinc sulphate was selected for the present study among the different types of zinc compounds.

Various methods are employed in order to recover the contaminated soils. Among them, phytoremediation is considered as one of the most economic as well as eco-friendly methods 2627. The present investigation revealed the potential of L. esculentum in the removal of zinc. Even at 500 ppm of zinc sulphate, 95% removal of zinc was observed after 15 days itself. Zinc uptake in both above ground and below ground parts of the tomato plants remained as 90 ppm in all the tested concentrations and control which might be due to the nature of soil, microbes in the soil, presence of other metals and the physiology of the tomato plants. High level of heavy metals tolerance is the most promising feature in hyperaccumulator plants which is provided by hyperaccumulation and vacuolar compartmentalization activities 28. Vogeli and Wagner 29 observed the high level accumulation of Cd and Zn in the vacuoles isolated from tobacco protoplasts. Vazquez and coworkers 30 also noticed similar kind of results with reference to vacuolar compartmentalization for Zn in leaves of the hyperacumulator, Thlaspicaerulescens through electron microscopic studies. Hence, vacuolar compartmentalization may be one of the reasons for accumulation of Zn in L. esculentum.

Plants should possess high level of translocate ability of elements from roots to shoots. Metal concentration in roots is normally more than in the shoots, but in hyperaccumullator plants, shoot metal concentration can exceed root levels 31. Similar kind of result was also observed in the present study where zinc concentration in shoot was more than in root. Similarly, plants such as Brassica juncea, Amaranthusspinosus, Salix nigra, Avena sativa, Datura innoxia and Eichhorniacrassipes are also capable of accumulating zinc along with some other heavy metals in significant level 323334. The plant growth was not disturbed by high level of zinc, which may be due to the occurrence of certain proteins such as phytochelatins and metallothionins. These proteins are inducible in nature when the plants are exposed to heavy metal stress. During that exposure period, free metals are combined with the proteins and kept inside the vacuoles where they are not toxic to the plant. Subsequently, they can be utilized for the normal growth and development of the plant when the stored metal is essential element like zinc for plant s growth. Phytochelatin is responsible for heavy metal detoxification which is synthesized in plants at the time of abundance of heavy metals such zinc and copper. The enzyme phytochelatin synthase is activated by the heavy metals and that enzyme acts on the glutathione substrate to produce phytochelatins. This event persists until all of the free metals are bound with the proteins. 353637. When compared with phytochelatins, metallothionins are more powerful in detoxification of heavy metals. They are small, highly conserved, cysteine-rich metal- binding proteins which are essential for zinc and copper homeostasis. They give the protection against oxidative stress and buffering against toxic heavy metals. Metal exclusion and metal accumulation are the two basic strategies used by certain plants for metal tolerance. The exclusion approach includes avoiding of metal uptake and restriction of metal transport to the shoot which is normally used for phytostabilisation. The metal excluders may alter their membrane permeability, change metal biding capacity of cell walls or exude more chelating substances 3839.

Various factors are responsible for the success of phytoremediation. At the time of accumulation of more metals, the plants must produce adequate amount of biomass. In hyperaccumulator plants, the metals are concentrated in their aerial portions in huge level when compared with the level of metal accumulation in soil. These plants have the potential to accumulate more amounts of contaminants in different parts of the plant. Metal hyperaccumulator as plants contain more than or up to 0.1% of Cu, Cd, Cr, Pb, Ni or 1% of Zn or manganese in dry matter 4041. Metals are initially attached with the cell wall; it is an ion exchange of comparatively low affinity and low selectivity. Uptake of metal ions is likely to take place through secondary transporters such as channel proteins and/or H⁺-coupled carrier proteins. The membrane potential which is negative inside the cell membrane and exceeded -200mV in root epidermal cells might have driven the uptake of cations through secondary transporters 4243. Once inside the plant, most metals are too insoluble to move freely in the vascular system, so they usually form carbonate, sulphate or phosphate precipitates immobilizing them in apoplastic (extracellular) and symptostic (intracellular) compartments 44. The cell walls of the endodermal cell layer act as a barrier for apoplaster diffusion into the vascular system. Symplastic transport of metals may occur in the xylem after they cross the cospacian strip 45. It requires that metal ions pass across the plasma membrane, which has a negative resting potential of about 170mV. This membrane potential provides a strong electrochemical gradient for the onward movement of metal ions. Precipitation, compartmentalization and chelation are the most likely major events that take place in resisting the damaging effects of metals 46. In general, enzymes such as nitroreductases, glycosyl and glutathione transferases, oxidases, phosphatases, nitrilases, and dehalogenases synthesized from plants and microorganisms are involved in detoxification processes. The activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) also take place at the time of phytoremediation depending upon the plant species, nature and concentration of available heavy metals in the environment. In certain occasions, phytochelatins (PCn), small heavy metal binding peptides are also produced from glutathione by phytochelatin synthase (PCS) in higher plants when they are exposed to heavy metals. Some compounds intend to metal complexation which helps to avoid their movement from the root to the different parts of plant body. The choice of mechanism for heavy metal tolerance is mainly based on the genetic nature of the plant and its growth circumstances 474849. Tomato plants exploit diverse mechanisms to neutralize the toxic effects of heavy metals. In addition to that, they accumulate heavy metals usually in their roots than other parts of the body which reveal that they possess distinct mechanisms for heavy metal tolerance to restrict or condense the heavy metals accumulation in stem, leaves and fruits 505152. Further research relevant to accumulation of heavy metals in tomatoes would be very useful for the effective application of tomato plants in phytoremediation since it is an edible plant species. In contrast, it is better to avoid the consumption of the fruits which are developed from the tomato plants used for phytoremediation for the time being as a safety measure since they are very effective in heavy metal removal.