Although many metal elements are essential for the growth of plants in low concentrations, their excessive amounts in soil above threshold values can result in toxicity. This detrimental effect varies with the nature of an element as well as plant species. Heavy metal toxicity in plants depends on the bioavailability of these elements in soil solution, which is a function of pH, organic matter and cation exchange capacity of the soil. Nonessential metals/metalloids such as Hg, Cd, Cr, Pb, As, and Sb are toxic both in their chemically combined or elemental forms, and plants responses to these elements vary across a broad spectrum from tolerance to toxicity. For example, the bioaccumulation of heavy metals in excessive concentrations may replace essential metals in pigments or enzymes disrupting their function and causing oxidative stress. Heavy metal toxicity hinders the growth process of the underground and aboveground plant parts and the activity of the photosynthetic apparatus, which is often correlated with progress in senescence. To avoid the toxicity, plants have developed specific mechanisms by which toxic elements are excluded, retained at root level, or transformed into physiologically tolerant forms. In this chapter, we have discussed the toxic effects of heavy metals on plant growth and their detoxification mechanisms that enable them to tolerate high levels of metals in the soil environment.

A pot experiment was conducted to evaluate the effects of the industrial effluent of the Hudiara drain on the growth and element accumulation by Eucalyptus camaldulensis at early growth stage. Plants were irrigated for 18 months with effluent diluted with tap water at 0% (T0), 25% (T1), 50% (T2), 75% (T3), and 100% effluent (T4). Results showed that the maximum growth in terms of stem height (260 cm), number of branches (29), stem fresh weight (436.67 g), stem dry weight (203.33 g), total seedling length (344 cm), number of leaves (825), leaf fresh weight (195 g), and leaf dry weight (100 g) were recorded in plants treated with T2. However, maximum seedling collar diameter (2.25 cm), root fresh weight (230 g), and root dry weight (103.33 g) were observed in T3 treated plants. On the other hand, seedlings attained maximum root length (100.67 cm) at T1 treatment. Chlorophyll a, chlorophyll b and total chlorophyll increased up to T2, declining beyond that treatment. The accumulation of Na, Cd, and Cr in tissues increased with increasing concentrations of the effluent. However, the increase in effluent concentration decreased K and P in roots, and increased Fe in roots and stems, while T1 and T2 increased Mg in stems. The results suggest that mixing the wastewater of the Hudiara drain with tap water (50:50v/v) benefits the growth of E. camaldulensis.