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4 Remediation Mechanisms of Tropical Plants for Lead-Contaminated Environment 63

increased concentrations of sulfide are present. Under anaerobic conditions a
volatile organolead (tetramethyl lead) can be formed due to microbial alkylation
(Ground-Water Remediation Technologies Analysis Center 1997).

4.2.1 Lead Phytotoxicity

Lead is known to negatively affect some of the most classical end points of plant
toxicity like seed germination rate, seedling growth, dry mass of roots and shoots,
photosynthesis, plant water status, mineral nutrition, and enzymatic activities
(Munzuroglu and Geckil 2002). In general, effects are more pronounced at higher
concentrations and continuance. In some cases, lower concentrations can stimulate
metabolic processes and the enzymes involved in those processes (Gomes 2011).
These negative effects can be expressed as symptoms in the form of chlorotic spots,
necrotic lesions in leaf surface, senescence of the leaf, and stunted growth. Germi-
nation of seeds is drastically affected at higher concentrations. Development and
growth of root and shoot in seedling stage are also affected. Pb negatively
influences growth by reducing the uptake and transport of nutrients in plants,
such as Ca, Fe, Mg, Mn, P, and Zn, and by blocking the entry or binding of the
ions to ion carriers making them unavailable for uptake and transport from roots to
leaves (Xiong 1997). Thus, Pb interferes with several physiological and biochemi-
cal processes of plant (Gomes 2011). Many European countries have adopted a
bioavailability-based rationale to improve the reliability of assessments of metal
uptake (Prueb 1997). Current legislation in most countries still uses total soil metal
concentration as a simple index of hazard in contaminated soils, even though this
approach does not take into account of soil characteristics which influence the
bioavailability of metallic pollutants in contaminated soil (Datta and Young 2005).

4.2.2 Lead Uptake by Plants

With the exception of the special conditions that exist for plants cultivated near
metal recycling industries (Uzu et al. 2010), the main pathway by which plants
accumulate metals is through root uptake from soils (Fig. 4.1) (Sharma and Dubey
2005; Uzu et al. 2009). Part of the lead present in the soil solution is adsorbed onto
the roots, and then becomes bound to carboxyl groups of mucilage uronic acid, or
directly to the polysaccharides of the rhizoderm cell surface (Seregin and Ivanov
2001). Lead adsorption onto roots has been documented to occur in several plant
species: Vigna unguiculata (Kopittke et al. 2008), Festuca rubra (Ginn et al. 2008),
Brassica juncea (Meyers et al. 2008), Lactuca sativa (Uzu et al. 2009), and Funaria
hygrometrica (Krzesłowska et al. 2009, 2010). Once adsorbed onto the rhizoderm
root surface, lead may enter the roots passively and follow translocating water
streams. However, lead absorption is not uniform along plant roots as a lead