Ecophysiology and stress responses of aquatic macrophytes under metal/metalloid toxicity

Many metal elements are essential for plant growth at low concentrations but their excessive levels in the rhizosphere may cause phytotoxicity depending upon the fact that metals are easily absorbed and translocated in soil-plant systems. Nonessential metals/metalloids, i.e., Pb, Cr, Cd, As, Hg, etc., initiate a series of consecutive and/or parallel events at morphological, physiological, and molecular levels in aquatic plants depending on the nature of element and plant species. This chapter emphasizes the responses of aquatic macrophytes to metals/metalloids, with possible implementation in phytoremediation techniques. Metal-triggered growth inhibition, alterations in enzyme activities, inhibition of photosynthesis, changes in nutrient acquisition and metabolism, and the formation of free radicals are the major components reviewed in this book chapter. Discussion about the metal toxicity avoidance strategies like fluctuations in rhizospheric environments, plasma membrane exclusion, cell wall immobilization, phytochelatin-based sequestration and compartmentalization processes, stress proteins, and metallothioneins is also within the scope of this chapter. Metal tolerance in aquatic plants is more likely involved in an integrated network of multiple response processes generally described as “fan-shaped response” rather than several isolated functions described above. Plant tolerance to metals/metalloids is mainly determined from its transport across plasma membrane and tonoplast in plant. The appropriate understanding of metal-triggered ecophysiological responses of aquatic plants may make it promising to use them for treatment of metal polluted waters and soils.