**5. Indices to estimate hyperaccumulation potential**

The hyperaccumulation potential of macrophytesare determined primarily based on two indices *viz*.,bio concentration factor (BCF) and translocation factor (TF). BCF is defined as the ability of a plant to accumulate a particular metal in its plant part with respect to its concentration in the soil substrate while TF is the ratio of metal concentration in shoot to that in the root. BCF more than oneindicates that the plant is an accumulator while less than one, means the plant is an excluder. Hyperaccumulators are plants that contain more than 10,000 mg kg−1 of Zn and Mn; 1000 mgkg−1 of Cu, Cr, Pb, Ni, Co and 100 mg kg−1 of Cd and other rare metals, in the dry matter [75].

A high value for TF indicate the efficiency of the plant to translocate metals from the root to shoot and such plants (TF > 1) are referred as hyperaccumulators. They possess the phytoextraction ability to remove contaminants from the growth medium to the above ground portions and the biomass can be uprooted and removed. Aquaticmacrophytes, especially floating macrophytes, have the potential to concentrate metals more in the roots. Based on BCF and TF, the hyperaccumulation potential of *E. crassipes*and *A. philorexoides*for Cd has been proved beyond doubt, whereas higher BCF and lower TF is an indication of phytostabilisation effect eg. *L. flava* and *C. dactylon.*

## **6. Mechanisms of heavy metal tolerance by macrophytes**

Accumulation of heavy metals inside the plant body results in certain physiological changes and synthesis of certain enzymes to tolerate the metal stress. Major changes that occur inside the plant cell to activate metal absorption include enhancement in the bioavailability of metal in the rhizosphere region leading to an increased uptake of metal towards the plasma membrane. Inside the cell wall, chelation of metal may occur by binding with various proteins like phytochelatin or, metallothionein or form a bond with the cell wall or get sequestered into the cell vacuole [76, 77].

Acidification of rhizosphere by the action of plasma membrane proton pumps and secretion of ligands capable of chelating the metal helps in desorption of metals from the soil matrix. Soluble metals can enter into the root symplast by crossing the plasma membrane of the root endodermal cells or they can enter the root apoplast through the space between cells. Excluder plants survive by enhancing specificity for the essential element or pumping the toxic metal back out of the plant. On reaching the xylem, the metal will get transported alongwith xylem sap towards the leaves and get deposited there. The cell tissue where the metal get deposited, vary with the hyperaccumulator species as shown by *T. caerulescens* and *Arabidopses halleri* - *T. caerulescens* has preferential adsorption for Zn in the epidermis over mesophyll cells while the reverse for *Arabidopses halleri* [78].

At any point along the pathway, the metal could be converted to a less toxic form by chemical conversion or complexation. Various oxidation states of toxic elements have very different uptake, transport, and sequestration or toxicity characteristics in plants. Two major chelating peptides present in plants include metallothioneins and phytochelatins. Sequestration of metals in sites away from where the cellular processes are likely to be get disrupted will result in their deposition. The most prominent site is cell vacuole, for that metal or metal- ligand complex must cross the vacuolar membrane. Metal ions may also get bonded with negative charges on cell wall leading to their sequestration in the cell wall.

#### **7. Conclusions**

It is high time that the water bodies be conserved for ecological sustenance and well-being of the future generation. Aquatic plants can play a vital role in the purification of contaminated lakes, rivers and ponds, which make them fit for human consumption and irrigation purposes. The nature and extent of amelioration varies with particular plant species. They are specifically adapted to tolerate heavy/ toxic metal concentration in their ecosystems.

*Aquatic Plants as Bioremediators in Pollution Abatement of Heavy Metals DOI: http://dx.doi.org/10.5772/intechopen.99627*
