Responses of Some Vegetable Crops to Toxic Heavy Metals

Abstract

Some common green vegetables (Brassica juncea, Brassica rapa, Lactuca sativa, Lepidium sativum and Daucas carota) of Kathmandu valley have been investigated for the morphological changes after growing on different concentrations of heavy metals (CdCl2, CuCl2, Pb(NO3)2 and ZnCl2). Morphological parameters such as root length (RL), shoot length (SL), fresh weight (FW), dry weight (DW), dry weight % and leaf size (leaf area/length) have been examined in potted plants grown on soil, artificially contaminated with heavy metal. Bioaccumulations of non-supplied micronutrients (Cu, Fe, Mn and Zn) after treatment with heavy metals were also observed in these vegetables. On the basis of FW of whole plant (matured), threshold concentration for Cd in B. juncea, B. rapa, L. sativum, L. sativa and D. carota were ascertained to be as 2.75, 0.25, 3.22, 1.25 and 1.0 mg kg-1 DW, respectively; for Cu were 18, 17.75, 14.75, 17.25 and 10.75 mg kg-1 DW; for Pb to be 46.5, 34.5, 52.7, 81.0 and 31.5 mg kg-1 DW; and for Zn were 173.5, 192, 172.5, 93.0, and 55.5 mg kg-1, respectively. Among the studied vegetables, D. carota showed lowest accumulation of all heavy metals in roots and shoots. Highest accumulation of Cd was observed in the shoot of L. sativum; Pb in the root of B. rapa, Cu in both root and shoot of B. juncea and Zn in the shoot of B. rapa. Reduction in FW, SL and RL in B. rapa, L. sativa and D. carota have been recorded as symptoms of Cd toxicity; and reduction in FW and RL in B. juncea, B. rapa and L. sativa as symptom of Cu-toxicity. Similarly, reduction in FW in B. rapa, L. sativum and RL in B. rapa, D. carota have been observed as Pb-toxicity, reduction in FW and RL in B. juncea and B. rapa, and SL in B. rapa, L. sativum and L. sativa as symptoms of Zn- toxicity.

Impact of different concentrations of CdCl2, CuCl2, Pb(NO3)2 and ZnCl2 on chlorophylls after the increased uptake of Cd, Cu, Pb and Zn in B. juncea, B. rapa, L. sativa, L. sativum and D. carota were studied. Significant loss of Chl–a, Chl-b and total chlorophyll were observed in B. juncea and L. sativum with depletion of Cu, Zn and/or Mn after increased Cd uptake. Significant loss in total-chl after increased Cu uptake was observed in B. juncea and D. carota with depletion of essential Zn, Fe (in B. juncea) and or Mn (in D. carota). Total chlorophyll increased significantly in B. juncea up to 500 mg Pb(NO3)2 kg-1 soil treatment but it decreased insignificantly in D. carota only at 1000 mg kg-1 soil treatment. There was moderate increased in Fe accumulation in all vegetables (except L. sativum) when grown on Pb(NO3)2 treated soil; but Zn, Cu and Mn depleted in D. carota grown on Pb(NO3)2 treatments. Insignificant change in chlorophyll was observed in Zn-treated vegetables. Present studies suggest that depletion of essential micronutrients like Zn and Fe; or Zn and Cu; or Fe and Mn in the plant body after heavy metal accumulation may be one of the causes for chlorophyll loss. Evaluation of heavy metal in soil and vegetables grown from agricultural fields of Kathmandu valley was conducted in the present study. Highest accumulations of Cd (2 mg kg-1), Cu (65.5 mg kg-1) and Pb (46.75 mg kg-1) from Shankhamul; Ni (29.25 mg kg-1) from Nakhu; Co (15.25 mg kg-1) and Mn (675 mg kg-1) from Balkhu; Cr (73.75 mg kg-1) from Banasthali, Zn (162 mg kg-1) and Fe (75636 mg kg-1) from Khusibun were recorded in soils of agricultural fields. On the basis of mean values, highest accumulation of Cu, Co and Zn were recorded in spinach and Pb in red radish; Ni, Cr and Fe were recorded in broad leaf mustard and Mn in cress leaf. Concentration of non –essential but toxic metal like Pb was higher than the normal plant value (0.1-10 mg kg-1) in all the vegetables (except potato) collected from different sampling sites. Potato accumulated fewer amounts of heavy metals among the others.

As vegetables are one of the importance dietary items, it is cultivated widely in Kathmandu valley. The daily intake of heavy metals (Cd, Cu, Co, Cr, Fe, Mn, Ni, Pb and Zn) via vegetables was estimated in the present study. Intake estimation was based on vegetables availability data and analysis of vegetables contaminants. The mean daily intake of potentially toxic metals ranged about 33.4 % (for Cd), 40.38 % (for Ni) and 251.6 % (for Pb) of the provisional tolerable daily intakes (PTDI) for adults. Vegetables were found to contribute significantly to the recommended daily intake of essential elements such as Cr (97.4-24.35 %), Cu (15.45-7.7 %), Fe (41 %), Mn (65.71-26.28 %), and Zn (21.23 %).

To understand the phytoextraction of heavy metals from heavy metal contaminated soil, 18 different vegetable were grown on soil artificially contaminated with 300 mg CuCl2 kg-1, 500 mg Pb(NO3)2 kg-1, 800 mg ZnCl2 kg-1 or their mixed metal (1600 mg kg-1). From the single metal contaminated soil, lowest Cu accumulation was recorded in B. juncea and highest in V. faba; lowest Pb accumulation was recorded in S. tuberosum, and highest in A. fistulosum; and lowest Zn accumulation was recorded in A. fistulosum and S. tuberosum and highest in V. faba. From mixed metal soil treatments, lowest accumulation of Cu, Pb and Zn was recorded in B. juncea, B. caulorapa and A. fistulosum, respectively, and highest accumulation of Cu and Pb in S. oleracea (Deshi), and Zn in S. oleracea (Patane) was recorded. Remedial measures using cow dung and lime at different treatments showed that Cu and Pb accumulation in vegetables are higher in lime treatments than in 20 % cow dung treatments. Zinc accumulation increased in B. rapa and L. sativum in both cow dung and lime than in control. Among the tested vegetables for remedial measures, accumulation of Cu, Pb and Zn from mixed metal treatment was highest in L. sativum (treated with lime 9 g for Cu and Pb, and 20 % cow dung for Zn). Morphological changes such as FW, DW, SL and RL (except RL in B. rapa) increased significantly (P≤0.01) in all vegetables in Zn and cow dung treated soil, but not significant changes were observed with lime treatment. FW and DW increased only in L. sativum grown in 3 g lime treatment, whereas DW % increased significantly in B. rapa of both the doses of lime. Immobilization of Cu, Pb and Zn in both single and mixed metal treatments was found to be high in cow dung amended soil. But in single metal salts such as CuCl2 or Pb(NO3)2 and lime treated soil, the concentration of Cu and Pb retained in the soil was lower than in control, indicating their free mobility in the plants. From this it can be ascertained that 20 % cow dung treatment is suitable for immobilization of supplied metals than lime treatment.