Seneviratne, et al. 2017. Published in Environmental
Geochemistry and Health, 41(4): 1813-1831.
Heavy metal-induced oxidative stress on seed germination
and seedling development: a critical review
.
Mihiri Seneviratne
.
Nishanta Rajakaruna
.
Muhammad Rizwan
H. M. S. P. Madawala
.
Yong Sik Ok
.
Meththika Vithanage
Abstract Heavy metal contamination in soils can
influence plants and animals, often leading to toxico-
sis. Heavy metals can impact various biochemical
processes in plants, including enzyme and antioxidant
production, protein mobilization and photosynthesis.
Hydrolyzing enzymes play a major role in seed
germination. Enzymes such as acid phosphatases,
proteases and a-amylases are known to facilitate both
seed germination and seedling growth via mobilizing
nutrients in the endosperm. In the presence of heavy
metals, starch is immobilized and nutrient sources
become limited. Moreover, a reduction in proteolytic
enzyme activity and an increase in protein and amino
M. Seneviratne
Department of Botany, Faculty of Natural Sciences, Open
University of Sri Lanka, Nawala, Nugegoda, Sri Lanka
N. Rajakaruna
Unit for Environmental Sciences and Management,
North-West University, Potchefstroom, South Africa
N. Rajakaruna
Biological Sciences Department, California Polytechnic
State University, San Luis Obispo, CA 93407, USA
M. Rizwan
Department of Environmental Sciences and Engineering,
Government College University, Allama Iqbal Road,
Faisalabad 38000, Pakistan
H. M. S. P. Madawala
Department of Botany, University of Peradeniya,
Peradeniya, Sri Lanka
acid content can be observed under heavy metal stress.
Proline, is an amino acid which is essential for cellular
metabolism. Numerous studies have shown an
increase in proline content under oxidative stress in
higher plants. Furthermore, heat shock protein pro-
duction has also been observed under heavy metal
stress. The chloroplast small heat shock proteins (Hsp)
reduce photosynthesis damage, rather than repair or
help to recover from heavy metal-induced damage.
Heavy metals are destructive substances for photo-
synthesis. They are involved in destabilizing enzymes,
oxidizing photosystem II (PS II) and disrupting the
electron transport chain and mineral metabolism.
Y. S. Ok (&)
Korea Biochar Research Center & School of Natural
Resources and Environmental Science, Kangwon National
University, Chuncheon 24341, Korea
e-mail: soilok@kangwon.ac.kr
M. Vithanage (&)
Environmental Chemodynamics Project, National
Institute of Fundamental Studies, Kandy, Sri Lanka
e-mail: meththikavithanage@gmail.com
M. Vithanage
Office of the Dean, Faculty of Applied Sciences,
University of Sri Jayewardenepura, Nugegoda, Sri Lanka
Although the physiological effects of Cd have been
investigated thoroughly, other metals such as As, Cr,
Hg, Cu and Pb have received relatively little attention.
Among agricultural plants, rice has been studied
extensively; additional studies are needed to charac-
terize toxicities of different heavy metals on other
crops. This review summarizes the current state of our
understanding of the effects of heavy metal stress on
seed germination and seedling development and
highlights informational gaps and areas for future
research.
Keywords Antioxidant system Starch
mobilization Proline Protein degradation
Chlorophyll Heavy metal stress
Introduction
Chemical waste resulting from industrial activities
and agricultural effluents such as fertilizer, herbi-
cides and pesticide has contributed to the increased
accumulation of heavy metals in soil (Fu et al. 2014;
Hu et al. 2013;Nicholsonetal. 2003;Neilson and
Rajakaruna 2015). Since heavy metals are non-
degradable, they can cause long-term deleterious
effects on ecosystem health. Copper (Cu), As and Cd
are the major heavy metals present in agrochemicals
whileHg, Cr,PbandAs arerichinindustrially
contaminated soils (Wuana and Okieimen 2011).
Cadmium (Cd), Hg, Pb, Cu and As are some of the
most toxic heavy metals or metalloids found in soil
(Dago et al. 2014;Clemens 2006;Hadietal. 2013).
Cadmium, for example, is nonessential for plant
metabolic activities and a known phytotoxicant
(Grata
˜
oetal. 2005). The long biological half-life of
Cd contributes to its mutagenic, cytotoxic and
carcinogenic properties (Sa
´
nchez-Virosta et al.
2015; Waalkes 2003). Ore smelting, coal burning,
industrial effluents, and As-containing herbicides and
insecticides release As, a toxic metalloid, to the
environment(Linetal. 2016). Mercury (Hg), which
is one of the most potent toxic heavy metals, is
released mainly from mining activities (Lacerda
1997; De Lacerda and Salomons 2012); however,
low amounts can also be found in agrochemicals
(Saueia et al. 2013; Falkowska et al. 2013). Copper is
released from several industrial activities and by the
excess usage of fungicides. Copper-containing fungi-
cides and bactericides are used extensively for
disease control in numerous crops. Lead (Pb), a
metal that can remain in the soil for thousands of
years (Kumar et al. 1995), can be released to the
environment via mining activities (Bakırdere et al.
2016) and other industrial processes (Be
´
nard et al.
2014). Although some heavy metals are considered
as micronutrients (Cu, Zn, Mn, etc.), other metals and
metalloids are highly toxic (Hg, Pb, Cd, As, etc.) and
play no role in plant nutrition and their presence in
high concentrations is highly toxic (Nagajyoti et al.
2010;
Boyd and Rajakaruna 2013).
The organic matter and clay content and pH are the
major factors governing the availability of heavy
metals in soils (Zeng et al. 2011; Spurgeon and Hopkin
1996). A high proportion of humified organic matter
(OM), i.e., mature compost, can also decrease the
bioavailability of heavy metals, while increasing the
residual fraction of heavy metals in the soils (Castaldi
et al. 2005), which is mainly attributed to the
precipitation caused by the complex formation (Bolan
and Duraisamy 2003). Moreover, the large specific
surface area of clay minerals contributes to higher
immobilization of heavy metals in soil (Prost and
Yaron 2001; Usman et al. 2005). As the soil pH is
increased, the immobilization is also increased for
many heavy metals, especially for those metals present
as cations (i.e., Pb, Cd and Cr). Biochar application to
soil, which increases the soil pH, has also demon-
strated the effect of pH in heavy metal immobilization
(Herath et al. 2015).
High concentrations of heavy metals in soil cause a
number of deleterious effects on plants (Khan et al.
2000), such as growth retardation, destruction of
chlorophyll, disorders in biochemical activities, muta-
tions and reproductive disorders (Schu
¨
tzendu
¨
bel and
Polle 2002; Sharma and Agrawal 2005; Gall and
Rajakaruna 2013). Seed germination, one of the most
significant stages in a plant’s life, is sensitive to
chemical and physical conditions of the rhizosphere
(Bewley 1997). Although the seed coat can act as a
principle barrier limiting harmful effects of heavy
metals, most seeds and seedlings show a decline in
germination and vigorin response to heavy metal
stress (Adrees et al. 2015), causing a major concern for
agricultural and forestry practices. Therefore, the
effects of heavy metals on germination and seedling
growth are an important research area deserving
extensive study. Recent studies have documented that
via inhibition of storage food mobilization, reduction
in radical formation, disruption of cellular osmoreg-
ulation and the degradation of proteolytic activities,
heavy metals cause inhibition of seed germination and
seedling development (Adrees et al. 2015; Barcelo
´
and
Poschenrieder 1990; Perfus-Barbeoch et al. 2002;
Karmous et al. 2015; Baszyn
´
ski 2014) (Fig. 1).
However, several management practices have been
utilized to minimize heavy metal stress on plants
grown in contaminated soils. While physical and
chemical technologies are useful in remediation of
heavy metal contaminated soils (Dermont et al. 2008;
Wu et al. 2010), the use of different amendments is one
of the low-cost and efficient methods utilized to
minimize the plant heavy metal uptake in contami-
nated soils; biochar, crab shells and organic matter are
a few such amendments (Uchimiya et al. 2010).
Amendments are used to immobilize the contami-
nants, thereby limiting the bioavailable fraction
(Bandara et al. 2016, 2017; Herath et al. 2017;
Kumarathilaka and Vithanage 2017). In addition to
amendments, there are also internal mechanisms in
place, which can limit the interaction of heavy metals
in plant metabolism. Phytochelatins and metalloth-
ioneins are such proteins that may play vital roles in
heavy metal detoxicity in microorganisms and plants
(Shen et al. 2010).
This review focuses on the influence of heavy
metals and metalloids (especially, Cd, Hg, Ag and As)
on seed germination and seedling development in
plants, focusing particularly on physiological and
biochemical effects on germinating seeds and devel-
oping seedlings.
Effect of heavy metals on seed germination
and seedling growth
Heavy metals are known to negatively influence seed
germination and seedling development (Table 1).
Cadmium leads to germination inhibition as docu-
mented by several studies. For example, when
Sorghum bicolor (Poaceae) seeds were treated with a
Cd concentration series (0.0, 0.5, 1.0, 2.0 and
3.0 mM), a significant reduction in germination was
observed in the presence of the metal (Barcelo
´
and
Poschenrieder 1990). The decrease in germination
appears to result from the inhibition of physiological
and metabolic activities of the seed. Since Cd is able to
decrease the water stress tolerance of plants (Barcelo
et al. 1986), it can cause a loss of turgor pressure at a
higher relative water content compared to that of non-
treated plants. The effect of Cd on plant water status
has been examined with germinating rice seeds
(Barcelo
´
and Poschenrieder 1990), showing that the
water status of germinating rice seeds is highly
Fig. 1 Different effects of heavy metals on seed germination and seedling development
Table 1 A summary of studies examining the effects of heavy metals on seed germination and seedling development
Heavy metal Species Findings References
Al Zea mays Reduction in seed germination was
observed beyond 50 mg L
-1
Nasr (2013)
As Brassica oleracea, Amaranthus
sp., Raphanus sativus,
Reduction in germination was observed at
10 mg L
-1
Dutta et al. (2014)
Daucus carota
Cd Triticum aestivum Cd showed toxicity at 5 mg L
-1
in root Ahmad et al. (2012)
and shoot growth. Seed germination and
germination energy were affected at
20 mg L
-1
which is aggravated by
further addition of Cd from 50 to
80 mg L
-1
Oryza sativa Cd shows a significant reduction in rice
He et al. (2014)
seed germination index, vigor index,
root and shoot lengths as well as fresh
weight compared to control at 100 lM
Picea omorika 1 mM concentration inhibited Prodanovic et al. (2016)
germination
Solanum nigrum Germination rate was dramatically
reduced at 200–300 lmol L
-1
Cd
Liu et al. (2012)
Triticum aestivum A decrease in germination was recorded
from 20 mg Cd L
-1
Ahmad et al. (2013)
Helianthus annuus Gradual reduction in germination
(2–10 mg L
-1
)
Imran et al. (2013)
Triticum aestivum Seed germination was stimulated at low
concentrations (0–2.5 mg L
-1
) while a
Mahdieh et al. (2013)
reduction in germination was observed
under high concentrations
(5–30 mg L
-1
)
Vigna radiata and Glycine max 1mgkg
-1
As addition stimulated seed Wan et al. (2013)
germination and increased about 12% of
the germination weight, but the seed
germination was significantly
suppressed when with As addition was
over 5 mg kg
-1
Festuca rubra No germination inhibition was observed
at 25 mg L
-1
while a reduction in
Va
´
zquez de Aldana et al.
(2014)
germination was observed at 25 and
50 mg L
-1
Brassica juncea No significant effect was observed in seed Srivastava et al. (2013)
germination up to 250 lM
Cd, Pb, Cu Arachishypogeae Concentrations of 75 and 100 mg L
-1
, Abraham et al. (2013)
Cd and Pb, respectively, decreased seed
germination while Cu condensed seed
germination at 100 mg L
-1
Cr Cucumis melo Reduction in seed germination as the Akinci and Akinci (2010)
concentration increased
Co, Cr Phaseolus vulgaris Reduction in seed germination at high
concentrations (10
-2
M)
(Zeid 2001)
Table 1 continued
Heavy metal Species Findings References
Cu Oryza sativa
Triticum aestivum
Zea mays
Triticum aestivum and Oryza
sativa
Medicago sativa
Eruca sativa
Hg Vignaradiata
Albizia lebbeck
Cajanus cajan
Brassica napus
Ni Medicago sativa
Zea mays
Pb Triticum aestivum
Lens culinaris
The germination of seeds decreased as the
concentration increased (0.2–1.5 mM)
The germination of seeds decreased from
5mgL
-1
Increased seed germination at 0.1 mM
Reduced germination by more than 35
and 60%, respectively, with 10 lM
Inhibit seed germination by 39% with
40 mg L
-1
0.3–1.2 mM did not decrease seed
germination while lower Cu
concentration (\0.7 mM) increased
seed germination
No significant reduction in seed
germination was shown at 1 mM;
however, beyond 3 mM, a reduction in
germination was observed
A significant gradual reduction in seed
germination was observed beyond
1mM
Reduction in seed germination was
observed beyond 5 mg L
-1
Reduction in seed germination was
observed beyond 1 mM
40 ppm inhibited seed germination by
24%
Reduction in seed germination was
observed beyond 50 mg L
-1
Increased germination rate and root mass
with Pb concentration less than
50 mg L
-1
A significant reduction in germination
was observed from 0.5 mM
Ahsan et al. (2007a)
Singh et al. (2007)
Bashmakov et al. (2005)
Mahmood et al. (2007)
Aydinalp and Marinova
(2009)
Zhi et al. (2015)
Muhammad et al. (2015)
Iqbal et al. (2014)
Patnaik and Mohanty
(2013)
Rezaei et al. (2013)
Aydinalp and Marinova
(2009)
Nasr (2013)
Kang et al. (2009)
Cokkizgin and Cokkizgin
(2015)
affected by Cd. Since water imbibition is a major
requirement for seed germination, the negative influ-
ence of Cd on the water content of the seed is
significant (Barcelo
´
and Poschenrieder 1990). Simi-
larly, an inhibition of root and coleoptile growth has
been observed in seedlings under Cd stress (Kuriakose
and Prasad 2008). Cadmium appears to cause an
inhibition of carbohydrate hydrolysis and the translo-
cation of hydrolyzed sugars, resulting in a reduction in
seedling growth. Mercury (Hg) is also known to
negatively influence seed germination and seedling
growth (Muhammad et al. 2015). As Vigna radiata
(Fabaceae) was treated with a Hg concentration series
(1, 3, 5 and 7 mM), there was no significant reduction
under 1 mM; however, the 7-mM treatment showed
the highest reduction among all treatments, resulting
in 42, 70, 66 and 47% reduction in seed germination,
seedling length, root length and seedling dry weight,
respectively. Though silver (Ag) has often being used
in nanofertilizers, only a handful of studies have been
conducted to assess the effects of Ag with respect to
seedling growth. A study examining the effects of
polyvinylpyrrolidone-coated silver nanoparticles
(PVP-AgNP) on 11 wetland plants (Loliummultiflo-
rum, Panicumvirgatum, Carexlurida, C. scoparia, C.
vulpinoidea, C. crinita, Eupatorium fistulosum, Phy-
tolacca americana, Scirpus cyperinus, Lobelia cardi-
nalis and Juncus effusus) showed that Ag
