Phytoremediation is a cost-effective and environment-friendly technology that uses plants to clean up environment from different types of pollutants including heavy metals. The technology is now established in the temperate regions and industrialized countries, however its use in the tropics and developing countries is very limited. The aim of the present study was to evaluate growth and phytoremediation potential of some crop plants for Cadmium (Cd), Copper (Cu) and Nickel (Ni). The crop cultivars of three major crops Zea mays L. (H-919, Rifhan-2995, Vaxy), Helianthus annuus L. (Hysun-33, Hysun-38, NK-278) and Triticum aestivum L. (Bhakar-02, Fareed-06, Inqlab-91, Sahar-06 and Shafaq-06) were exposed to varying concentrations of metals i.e., 0, 25, 50 and 100mg kg-1 of soil. The experiments were conducted in pots placed in a wire netting green house at the Botanic Gardens, Bahauddin Zakariya University, Multan, Pakistan in a Complete Randomized manner. The growth potential was evaluated using morphological and biochemical attributes as predictors while phytoremediation potential was appraised through metal accumulation and uptake by below and above ground plant tissues. The results for morphological attributes clearly depicted reduction in biomass of root and shoot, plant height and leaf area. The crop cultivars also exhibited a decline in photosynthetic pigments (chlorophyll a, b and total). However, leaf carotenoids showed a marked increase in response to Nickel levels in Z. mays and H. annuus cultivars. The metal contents increased in a dose dependent manner consistently in both roots and shoots but metal accumulation was more profound in roots than shoots of the plants. The results suggested that presence of cadmium, copper and nickel in the growth medium had caused a major reduction in all parameters studied for three crops and their cultivars responded differentially to heavy metals. Moreover, accumulation of metals in roots and its subsequent translocation to shoots also indicated potential of crops to extract metal contents from the soil without influencing their early establishment. A cultivar of Carthamus tinctorius L. (US- 10) was used as model cultivar to reveal antioxidant potential of crops to heavy metal stress that depicted a drastic increase in catalase (CAT), super oxidase dismutase (SOD) and peroxidase (POD) levels. As far as activity of antioxidant enzymes (catalase, peroxidase and superoxide dismutase) is concerned it became evident that oxidative defense strategy to mitigate toxic effects of metals is present in the species. The performance of the species appeared in an order maize>sunflower>wheat. However among the cultivars of three crops Vaxy for maize, Hysun-33 for sunflower and Shafaq-06 for wheat showed a better threshold for metal toxicity. Nevertheless, intraspecific response also varied widely and no consistency can be ascertained for attributes, metals, levels, crops and cultivars except for maize which can be a crop of choice for metal contaminated soil. The study further necessitated to address insights of the molecular mechanisms that exist in juvenile plants because only tolerant juveniles are able to produce tolerant adults and vegetative success always leads to reproductive success. Moreover, the inter and intraspecific responses of crop plants thus can provide useful information for the economic utilization of metal contaminated sites as well as their exploitation for phytoremediation technologies without affecting their growth potential and early establishment.
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