Bottalico et al. (2016) examined the potential of green urban forests to remove
atmospheric PM 10 in the city of Florence. Annual removal of PM 10 was estimated to
be 0.0204 t/ha by conifers, 0.0176 t/ha by evergreen broadleaves, 0.0152 t/ha by
deciduous broadleaves, and 0.0247 t/ha by remaining mixed forests. Nowak (1994)
concluded that plants in urban areas removed approximately 215,000 tons of PM 10 /y
in the USA.
Plants are commonly used as bioindicator to detect the presence of metals/heavy
metals in the atmosphere and soil (García-Florentino et al. 2018; Kłos et al. 2018;
Naderizadeh et al. 2016; Sawidis et al. 2011). Generally, most of the heavy metal
(loid)s present in the soil system are taken up by plants through their root system, but
plants are also capable to absorb heavy metal(loid)s through aerial organs directly
from the atmosphere (Shahid et al. 2017b). Less data is available on the absorption of
heavy metal(loid)s except for some essential metals like Fe, Cu, Mg, Zn, Ni, and Si
(Al-Khlaifat and Al-Khashman 2007; Hong et al. 2016; Kumar et al. 2016; Wang
et al. 2016). Moreover, majority of the studies on foliar heavy metal(loid) uptake are
not recent, and the atmosphere-plant transfer pathways and mechanisms reported
remained unclear (Little 1978; Salim et al. 1993a).
It is proposed that metal-PM deposits on adaxial and abaxial surfaces of the
leaves and is trapped in cuticle of the leaves (Mo et al. 2015; Shahid et al. 2017b). It
is also reported that the foliar entrance of metal-PM mainly depends on the size of
PM. Uzu et al. (2011b) used micro-X-ray fluorescence mappings and reported that
coarse Pb-rich spots were located in necrotic zones of lettuce leaves. These spots
were mainly concentrated at the base of the central nervure. Using scanning electron
microscopy coupled with energy dispersive X-ray, it was shown that fine particles
were mainly found beneath the leaf surface. Xiong et al. (2014a) carried out
SEM-EDX analysis of Pb-PM-exposed spinach and cabbage leaves and reported
that metals were found all over the leaf surfaces and the coverage rate for Pb-PM on
the leaf surface was about 2%.
In addition, fine PMs were found inside stomatal openings. Similarly, Schreck
et al. (2012b) reported that highest Pb concentration attached with fine PM (<1 μm)
was present on the surface and in necrotic zones of lettuce leaves. In ryegrass leaves,
they reported that highest Pb concentrations were found on the leaf surface as well as
plain tissue. This shows that foliar uptake and accumulation also vary with plant type
and PM size. Fewer studies have focused on the intra- and intercellular pathway of
heavy metal(loid) and PM movement after foliar uptake.
Studies have shown that plants growing near industries/smelters have high
concentrations of heavy metal(loid)s in their foliar tissues (Celik et al. 2005;
Gajbhiye et al. 2016b; Tomašević et al. 2004; Uzu et al. 2010) (Table 5). Birbaum
et al. (2010) reported that smaller particles can enter the leaves, while large particles
make aggregates and are trapped in the waxy cuticle layer (Fig. 2). Bondada et al.
(2004) reported that metals can pass through the waxy layer and eventually are
absorbed by the underlying cells of the leaves. Leaf structure plays the main role
in the adsorption of heavy metal(loid)s from the atmosphere. Uptake of heavy
metal(loid)s can occur through foliar surfaces of leaves such as stomata, aqueous
pores, ectodesmata, lenticels, and cuticular cracks (Fernández and Brown 2013;
Winner and Atkinson 1986).
Ecotoxicology of Heavy Metal(loid)-Enriched Particulate Matter: Foliar. . .
81
atmospheric PM 10 in the city of Florence. Annual removal of PM 10 was estimated to
be 0.0204 t/ha by conifers, 0.0176 t/ha by evergreen broadleaves, 0.0152 t/ha by
deciduous broadleaves, and 0.0247 t/ha by remaining mixed forests. Nowak (1994)
concluded that plants in urban areas removed approximately 215,000 tons of PM 10 /y
in the USA.
Plants are commonly used as bioindicator to detect the presence of metals/heavy
metals in the atmosphere and soil (García-Florentino et al. 2018; Kłos et al. 2018;
Naderizadeh et al. 2016; Sawidis et al. 2011). Generally, most of the heavy metal
(loid)s present in the soil system are taken up by plants through their root system, but
plants are also capable to absorb heavy metal(loid)s through aerial organs directly
from the atmosphere (Shahid et al. 2017b). Less data is available on the absorption of
heavy metal(loid)s except for some essential metals like Fe, Cu, Mg, Zn, Ni, and Si
(Al-Khlaifat and Al-Khashman 2007; Hong et al. 2016; Kumar et al. 2016; Wang
et al. 2016). Moreover, majority of the studies on foliar heavy metal(loid) uptake are
not recent, and the atmosphere-plant transfer pathways and mechanisms reported
remained unclear (Little 1978; Salim et al. 1993a).
It is proposed that metal-PM deposits on adaxial and abaxial surfaces of the
leaves and is trapped in cuticle of the leaves (Mo et al. 2015; Shahid et al. 2017b). It
is also reported that the foliar entrance of metal-PM mainly depends on the size of
PM. Uzu et al. (2011b) used micro-X-ray fluorescence mappings and reported that
coarse Pb-rich spots were located in necrotic zones of lettuce leaves. These spots
were mainly concentrated at the base of the central nervure. Using scanning electron
microscopy coupled with energy dispersive X-ray, it was shown that fine particles
were mainly found beneath the leaf surface. Xiong et al. (2014a) carried out
SEM-EDX analysis of Pb-PM-exposed spinach and cabbage leaves and reported
that metals were found all over the leaf surfaces and the coverage rate for Pb-PM on
the leaf surface was about 2%.
In addition, fine PMs were found inside stomatal openings. Similarly, Schreck
et al. (2012b) reported that highest Pb concentration attached with fine PM (<1 μm)
was present on the surface and in necrotic zones of lettuce leaves. In ryegrass leaves,
they reported that highest Pb concentrations were found on the leaf surface as well as
plain tissue. This shows that foliar uptake and accumulation also vary with plant type
and PM size. Fewer studies have focused on the intra- and intercellular pathway of
heavy metal(loid) and PM movement after foliar uptake.
Studies have shown that plants growing near industries/smelters have high
concentrations of heavy metal(loid)s in their foliar tissues (Celik et al. 2005;
Gajbhiye et al. 2016b; Tomašević et al. 2004; Uzu et al. 2010) (Table 5). Birbaum
et al. (2010) reported that smaller particles can enter the leaves, while large particles
make aggregates and are trapped in the waxy cuticle layer (Fig. 2). Bondada et al.
(2004) reported that metals can pass through the waxy layer and eventually are
absorbed by the underlying cells of the leaves. Leaf structure plays the main role
in the adsorption of heavy metal(loid)s from the atmosphere. Uptake of heavy
metal(loid)s can occur through foliar surfaces of leaves such as stomata, aqueous
pores, ectodesmata, lenticels, and cuticular cracks (Fernández and Brown 2013;
Winner and Atkinson 1986).
Ecotoxicology of Heavy Metal(loid)-Enriched Particulate Matter: Foliar. . .
81