Page 213 - Soil Degradation, Conservation and Remediation

202 6 Soil Pollution

6.2.12.3 Uptake of Heavy Metals by Plants

Plants absorb heavy metals through roots from soil and by leaves from air. Once
absorbed, there are a variety of mechanisms to distribute metals within the plant
body. A portion of absorbed metals may be retained in roots, barred by membranes,
precipitated and accumulated in vacuoles, or translocated to shoots where they

perform physiological functions if they are nutrients or beneficial elements or inter-
fere with normal physiological functions leading to toxicities. Some plants prevent
metal toxicity, by metal compartmentalization and binding to intracellular ligands. In
addition to the organic acids, plants have phytochelatins and metallothioneins.
Phytochelatins are a family of peptides, while metallothioneins are similar to
phytochelatins in being Cys-rich, metal-complexing ligands.
According to Cho et al.( 2003 ), the sensitivity of plants to heavy metals depends
on the (1) uptake and accumulation of metals through binding to extracellular

exudates and cell wall constituents; (2) efflux of heavy metals from cytoplasm to
extranuclear compartments, including vacuoles; (3) complexation of heavy metal
ions inside the cell by various substances, for example, organic acids, amino acids,
phytochelatins, and metallothioneins; (4) accumulation of osmolytes and osmopro-
tectants and induction of antioxidative enzymes; and (5) activation or modifi cation
of plant metabolism to allow adequate functioning of metabolic pathways and rapid
repair of damaged cell structures. Yet, metals are accumulated in plants and become
toxic; and for these reasons, the yield of crop is reduced and the crop is rendered
unsafe if grown in a contaminated soil.

Arsenic Uptake

Gulz ( 2002 ) performed a series of pot culture greenhouse experiments in connec-
tion with her Ph.D. work in Dipl.-Geogr., University of Munich on the uptake of As
by maize, ryegrass, rape, and sunflower from arsenic-contaminated soils. Most As

was accumulated in the roots of plants. Although accumulation in aboveground
biomass remained much lower, As concentrations in stems, leaves, and seeds
reached values above the Swiss tolerance limits for food or fodder crops (0.2 and
− 1
4 mg As kg , respectively), except for maize. Results suggested that besides As
solubility, phosphorous availability as well as phosphorous demand of the plants
has to be taken into account to predict As uptake of crops. Due to the high As trans-

location of sunflower from roots to shoots, further experiments were performed by

using this plant species. In batch as well as growth chamber experiments, the effects

of phosphorous (P) fertilization on P and As availability in the soil, As uptake by
sunflower, and biomass production were investigated. Already the addition of a

− 1
base P fertilization (56 mg P kg ) led to a significant increase of soluble P and As

concentrations in the soil. P addition further tended to increase As uptake in the
roots and shoots of sunflower at low soluble As concentrations. The P effect on

soluble As was lower in the silty sand than in the sandy loam, indicating that a high
fixation capacity of the soil lessened the mobilizing effect of P.