22.2 Heavy Metal(loid)-Induced Phytotoxicity in Crop
Plants
The incessant spread and increasing levels of HMs in agricultural soils have caused
severe impairment of crops which not only results in reduced yield but also a serious
toxic threat to the crop consumers. Plants, being immobile, are unable to escape from
this stressful environment and uptake bioavailable non-essential HM cations into
their plant cells along with essential soil nutrients. These HMs, upon surpassing
certain threshold levels, impose severe cellular damages with various unusual
morphological manifestations. The threshold level of HMs to induce phytotoxicity
highly depends on plant species or even a particular cultivar. The uptake, translocation and cellular compartmentalisation of heavy metals may be governed by perhaps
only one or just a few genes (Ernst 1996). Moreover, this also depends on the
cationic forms of HMs. The observable external changes include reduction of seed
germination, changes in root-shoot length and changes in root-shoot fresh and dry
weight that ultimately decrease plant biomass (Table 22.1). As the root is directly
exposed to the soil HMs, the root is the first organ encountered by toxic HMs, and
the toxic effects follow into the shoots and other aerial parts of the plants. Affected
root growth results in the poor acquisition of essential nutrients, and thereby an
insufficient supply of nutrients to the photosynthetic cells in the aerial parts. To date,
the members of Poaceae are the most studied crops on which the phytotoxic effects
of different HMs have been investigated (Fig. 22.1). The phytotoxic consequences of
all the ten HMs (viz. arsenic, cadmium, chromium, cobalt, copper, lead, manganese,
mercury, nickel and zinc) discussed here have been studied on Poaceae (Fig. 22.1).
After Poaceae, the HM-phytotoxicity studies have focused mainly on members of
Fabaceae, Solanaceae and Brassicaceae, as predominant crops (Fig. 22.1). The lessstudied families in the context with HM phytotoxicity are Amaryllidaceae,
Euphorbiaceae, Amaranthaceae, Rosaceae, Linaceae, Malvaceae, Asteraceae and
Cucurbitaceae (Fig. 22.1).
Among HMs, arsenic (As) is considered as an analog of phosphate (P) that
competes with P-transporters in the root plasma membrane (Meharg and Macnair
1992). Although As-tolerance has been identified in a number of plant species
(Meharg and Macnair 1992), elevated As-level has been found to negatively affect
rice, maize, black gram, soybean, mung bean, cucumber, sorghum, barley, mustard,
broccoli, pea and Chinese cabbage (Table 22.1). Biochemical changes identified in
these crops include a reduction in photosynthetic pigments (chlorophyll, carotenoids), increased accumulation of reactive oxygen species (ROS), membrane lipid
peroxidation, inhibition of ATP formation, enhanced proline and protein content and
increased abscisic acid (ABA) synthesis (Table 22.1). Furthermore, altered activities
of various cellular enzymes including RuBisCO, amylase, protease, catalase, peroxidase and other antioxidant enzymes are evident (Stoeva et al. 2005; Srivastava et al.
2017; Ghosh et al. 2018; Dong et al. 2020; Chauhan et al. 2020). Besides,
As-mediated induction of cell death in root tips, proteomic alteration and disruption
444
K. Pramanik et al.
Plants
The incessant spread and increasing levels of HMs in agricultural soils have caused
severe impairment of crops which not only results in reduced yield but also a serious
toxic threat to the crop consumers. Plants, being immobile, are unable to escape from
this stressful environment and uptake bioavailable non-essential HM cations into
their plant cells along with essential soil nutrients. These HMs, upon surpassing
certain threshold levels, impose severe cellular damages with various unusual
morphological manifestations. The threshold level of HMs to induce phytotoxicity
highly depends on plant species or even a particular cultivar. The uptake, translocation and cellular compartmentalisation of heavy metals may be governed by perhaps
only one or just a few genes (Ernst 1996). Moreover, this also depends on the
cationic forms of HMs. The observable external changes include reduction of seed
germination, changes in root-shoot length and changes in root-shoot fresh and dry
weight that ultimately decrease plant biomass (Table 22.1). As the root is directly
exposed to the soil HMs, the root is the first organ encountered by toxic HMs, and
the toxic effects follow into the shoots and other aerial parts of the plants. Affected
root growth results in the poor acquisition of essential nutrients, and thereby an
insufficient supply of nutrients to the photosynthetic cells in the aerial parts. To date,
the members of Poaceae are the most studied crops on which the phytotoxic effects
of different HMs have been investigated (Fig. 22.1). The phytotoxic consequences of
all the ten HMs (viz. arsenic, cadmium, chromium, cobalt, copper, lead, manganese,
mercury, nickel and zinc) discussed here have been studied on Poaceae (Fig. 22.1).
After Poaceae, the HM-phytotoxicity studies have focused mainly on members of
Fabaceae, Solanaceae and Brassicaceae, as predominant crops (Fig. 22.1). The lessstudied families in the context with HM phytotoxicity are Amaryllidaceae,
Euphorbiaceae, Amaranthaceae, Rosaceae, Linaceae, Malvaceae, Asteraceae and
Cucurbitaceae (Fig. 22.1).
Among HMs, arsenic (As) is considered as an analog of phosphate (P) that
competes with P-transporters in the root plasma membrane (Meharg and Macnair
1992). Although As-tolerance has been identified in a number of plant species
(Meharg and Macnair 1992), elevated As-level has been found to negatively affect
rice, maize, black gram, soybean, mung bean, cucumber, sorghum, barley, mustard,
broccoli, pea and Chinese cabbage (Table 22.1). Biochemical changes identified in
these crops include a reduction in photosynthetic pigments (chlorophyll, carotenoids), increased accumulation of reactive oxygen species (ROS), membrane lipid
peroxidation, inhibition of ATP formation, enhanced proline and protein content and
increased abscisic acid (ABA) synthesis (Table 22.1). Furthermore, altered activities
of various cellular enzymes including RuBisCO, amylase, protease, catalase, peroxidase and other antioxidant enzymes are evident (Stoeva et al. 2005; Srivastava et al.
2017; Ghosh et al. 2018; Dong et al. 2020; Chauhan et al. 2020). Besides,
As-mediated induction of cell death in root tips, proteomic alteration and disruption
444
K. Pramanik et al.