A morphological adaptation to drought stress can promote the enlargement of
root diameter and decrease in the root elongation as a response to less permeability of
the dried soil. The contaminants in the soil may be immobilized by certain plant
species through absorption, accumulation, and adsorption onto the roots or precipitation within the root zone [103]. Phytoextraction occurs in the root zone of plants.
The root zone is generally shallow, and bulk of the roots are at shallower regions,
which is a limitation for phytoextraction. The remediation was limited to the top
15 cm of soil when lead-contaminated soil was remediated using Brassica
juncea [104].
For the enhancement in the remediation process, pH plays a significant role in the
soil. Lower pH value increases the mobility for cationic species, while the opposite is
true for anionic species [50]. The mobilization and immobilization of metals and
ligand depend on the pH [105]. To reduce the uptake of lead, the pH of the soil may
be adjusted to neutral 6.5 to 7.0. Soil pH determines the solubility of heavy metals
and nutrients. The increase in pH results in the formation of less soluble forms of
trace elements in the soil and restricts the possibility of their accumulation by plants.
Leaching of biogenic components may be prevented by an increase in pH
value [106].
The addition of biodegradable chelating agents and micronutrients increases the
bioavailability of the metals and the metal uptake by the plants. The heavy metal
uptake capacity of the microorganisms in the soil is stimulated by the chelating
agents and micronutrients. It facilitates the faster uptake of heavy metals, thus
reducing the requirement of remediation time [28]. It has been reported that exposure
of the soil to chelating agents such as EDTA and citric acid could improve the uptake
of metals [107–109]. The presence of a ligand results in the formation of metalligand complexes which influences the leachability of metals from below the root
zone [105].
The conditions of the environment affect the vegetative uptake. The temperature
in the root and above the soil surface affects the plant growth and consequently the
length of the root. Root structure is influenced by environmental conditions.
The seasonal variations in climate create gradients in temperature, light intensity,
and light regime [110]. The uptake of phosphorous and arsenic was found to
fluctuate during the growing season [111–114]. Plant age affects the metal uptake.
Generally, growth of young roots is faster, and they uptake nutrients at higher rates
than older roots [115].
Rofkar and Dwyer [110] have observed that the arsenic uptake by S. pectinata
occurred at a greater rate in summer than in spring, while C. stricta showed similar
uptake in both the seasons. They suggested that to maximize arsenic removal, (1) a
diverse group of warm- and cool-season plant species may be employed to supplement the uptake during seasonal variations, and (2) plants may be allowed to
accumulate arsenic during the growing season, and then older plants which has
enough time to transfer a large portion of the accumulated arsenic to shoots may be
harvested. Plant age was found to affect transfer of arsenic from roots to shoots. In
addition, when metals are bound to the soil, the pH, organic matter content, and
redox potential influence the metals to exist in ionic and plant-available form
58
S. Sophia and V. Shetty Kodialbail
root diameter and decrease in the root elongation as a response to less permeability of
the dried soil. The contaminants in the soil may be immobilized by certain plant
species through absorption, accumulation, and adsorption onto the roots or precipitation within the root zone [103]. Phytoextraction occurs in the root zone of plants.
The root zone is generally shallow, and bulk of the roots are at shallower regions,
which is a limitation for phytoextraction. The remediation was limited to the top
15 cm of soil when lead-contaminated soil was remediated using Brassica
juncea [104].
For the enhancement in the remediation process, pH plays a significant role in the
soil. Lower pH value increases the mobility for cationic species, while the opposite is
true for anionic species [50]. The mobilization and immobilization of metals and
ligand depend on the pH [105]. To reduce the uptake of lead, the pH of the soil may
be adjusted to neutral 6.5 to 7.0. Soil pH determines the solubility of heavy metals
and nutrients. The increase in pH results in the formation of less soluble forms of
trace elements in the soil and restricts the possibility of their accumulation by plants.
Leaching of biogenic components may be prevented by an increase in pH
value [106].
The addition of biodegradable chelating agents and micronutrients increases the
bioavailability of the metals and the metal uptake by the plants. The heavy metal
uptake capacity of the microorganisms in the soil is stimulated by the chelating
agents and micronutrients. It facilitates the faster uptake of heavy metals, thus
reducing the requirement of remediation time [28]. It has been reported that exposure
of the soil to chelating agents such as EDTA and citric acid could improve the uptake
of metals [107–109]. The presence of a ligand results in the formation of metalligand complexes which influences the leachability of metals from below the root
zone [105].
The conditions of the environment affect the vegetative uptake. The temperature
in the root and above the soil surface affects the plant growth and consequently the
length of the root. Root structure is influenced by environmental conditions.
The seasonal variations in climate create gradients in temperature, light intensity,
and light regime [110]. The uptake of phosphorous and arsenic was found to
fluctuate during the growing season [111–114]. Plant age affects the metal uptake.
Generally, growth of young roots is faster, and they uptake nutrients at higher rates
than older roots [115].
Rofkar and Dwyer [110] have observed that the arsenic uptake by S. pectinata
occurred at a greater rate in summer than in spring, while C. stricta showed similar
uptake in both the seasons. They suggested that to maximize arsenic removal, (1) a
diverse group of warm- and cool-season plant species may be employed to supplement the uptake during seasonal variations, and (2) plants may be allowed to
accumulate arsenic during the growing season, and then older plants which has
enough time to transfer a large portion of the accumulated arsenic to shoots may be
harvested. Plant age was found to affect transfer of arsenic from roots to shoots. In
addition, when metals are bound to the soil, the pH, organic matter content, and
redox potential influence the metals to exist in ionic and plant-available form
58
S. Sophia and V. Shetty Kodialbail