5.2.4 Metals and Salt Metals
Metals, at high concentrations, act as stress agents to plants, therefore, they can
induce changes in the secondary metabolism causing an elicitation effect. Exposure
of plants to metals, such as Ni, Ag, Fe, and Co, has shown increased production of
secondary metabolites in a variety of plants (Zhao et al. 2001). For instance, cadmium (Cd
2+ ) and copper (Cu
2+ ) are known for their toxicity and for not having any
value for plants (Das et al. 1997). However, Cd and Cu treatments resulted in
enhanced phenolic accumulation on the medicinal plant Gynura procumbens
(Ibrahim et al. 2017). Several factors influence the response of plants to metal
exposure, mainly depending on the chemical metal species and concentration, the
plant species, climate conditions, growth stage, among others (Lajayer et al. 2017).
The use of nonfood crops with the capacity of absorbing and accumulating
heavy metals is an alternative for remediation of contaminated environments. It has
been shown in certain medicinal and aromatic plants that this practice can lead to
the accumulation of secondary metabolites, which can be phytoextracted to obtain
high-value compounds (Lajayer et al. 2017). Metabolic changes by the action of
heavy metals can lead to inhibition of enzymes involved in the production of
photosynthetic pigments, sugars, proteins, and nonprotein thiols (Naik and
Al-Khayri 2016; Nasim and Dhir 2010). To date, many studies have shown
increases in medicinal plant performance following exposure to heavy metal stress.
For example, in a study where garden mint (Mentha crispa L., Lamiaceae) was used
for phytoaccumulation of lead (Pb), the chemical composition of the essential oil of
the plant was affected by improving the production of carvone, a major component
of essential oils (Sá et al. 2015).
Heavy metals have also shown to have a role in stress amelioration through
changes in antioxidant balance which often comes hand in hand with increased secondary metabolites. A study subjecting Camellia sinensis (L) plants to drought stress
was performed to understand the role of Zn in modulating stress conditions. Results
showed decreases in hydrogen peroxide (H 2 O 2 ) and lipid peroxidation, and at the
same time increases in phenolics content and differential expression of antioxidant
enzymes, such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POX),
polyphenol peroxidase (PPO), glutathione reductase (GR), and ascorbate peroxidase
(APX) (Upadhyaya et al. 2013). Similar results were obtained in Brassica napus
exposed to cadmium (Cd) stress, exogenous application of low concentrations of
selenium (Se) increased the tolerance of plants meanwhile concentrations of ascorbic
acid and reduced glutathione were increased (Hasanuzzaman et al. 2012).
Metallic salts have also shown enhanced production of secondary metabolites
during in vitro root cultures treatments such as two tropane alkaloids, scopolamine
and hyoscyamine, by eliciting with silver nitrate (AgNO 3 ) and cadmium chloride
(CdCl 2 ) in Brugmansia candida (Angelova et al. 2006), increases in tanshinone
contents using AgNO 3 in Perovskia abrotanoides (Zaker et al. 2015) and
sesquiterpenoid–defensive compounds using cadmium salts in Datura stramonium
(Furze et al. 1991).
172
H. Aguirre-Becerra et al.
Metals, at high concentrations, act as stress agents to plants, therefore, they can
induce changes in the secondary metabolism causing an elicitation effect. Exposure
of plants to metals, such as Ni, Ag, Fe, and Co, has shown increased production of
secondary metabolites in a variety of plants (Zhao et al. 2001). For instance, cadmium (Cd
2+ ) and copper (Cu
2+ ) are known for their toxicity and for not having any
value for plants (Das et al. 1997). However, Cd and Cu treatments resulted in
enhanced phenolic accumulation on the medicinal plant Gynura procumbens
(Ibrahim et al. 2017). Several factors influence the response of plants to metal
exposure, mainly depending on the chemical metal species and concentration, the
plant species, climate conditions, growth stage, among others (Lajayer et al. 2017).
The use of nonfood crops with the capacity of absorbing and accumulating
heavy metals is an alternative for remediation of contaminated environments. It has
been shown in certain medicinal and aromatic plants that this practice can lead to
the accumulation of secondary metabolites, which can be phytoextracted to obtain
high-value compounds (Lajayer et al. 2017). Metabolic changes by the action of
heavy metals can lead to inhibition of enzymes involved in the production of
photosynthetic pigments, sugars, proteins, and nonprotein thiols (Naik and
Al-Khayri 2016; Nasim and Dhir 2010). To date, many studies have shown
increases in medicinal plant performance following exposure to heavy metal stress.
For example, in a study where garden mint (Mentha crispa L., Lamiaceae) was used
for phytoaccumulation of lead (Pb), the chemical composition of the essential oil of
the plant was affected by improving the production of carvone, a major component
of essential oils (Sá et al. 2015).
Heavy metals have also shown to have a role in stress amelioration through
changes in antioxidant balance which often comes hand in hand with increased secondary metabolites. A study subjecting Camellia sinensis (L) plants to drought stress
was performed to understand the role of Zn in modulating stress conditions. Results
showed decreases in hydrogen peroxide (H 2 O 2 ) and lipid peroxidation, and at the
same time increases in phenolics content and differential expression of antioxidant
enzymes, such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POX),
polyphenol peroxidase (PPO), glutathione reductase (GR), and ascorbate peroxidase
(APX) (Upadhyaya et al. 2013). Similar results were obtained in Brassica napus
exposed to cadmium (Cd) stress, exogenous application of low concentrations of
selenium (Se) increased the tolerance of plants meanwhile concentrations of ascorbic
acid and reduced glutathione were increased (Hasanuzzaman et al. 2012).
Metallic salts have also shown enhanced production of secondary metabolites
during in vitro root cultures treatments such as two tropane alkaloids, scopolamine
and hyoscyamine, by eliciting with silver nitrate (AgNO 3 ) and cadmium chloride
(CdCl 2 ) in Brugmansia candida (Angelova et al. 2006), increases in tanshinone
contents using AgNO 3 in Perovskia abrotanoides (Zaker et al. 2015) and
sesquiterpenoid–defensive compounds using cadmium salts in Datura stramonium
(Furze et al. 1991).
172
H. Aguirre-Becerra et al.