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Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

(Valko et al., 2005), and inhibition antioxidants that cause damage to plants

(Bielen et al., 2013).

Heavy metals often reduce germination rates, reduce root, and shoot

elongation rates, soluble protein content, and dry mass (Wang et al., 2003).

Various heavy metals such as Cd, Hg, Ni, Pb, and Cr have toxic roles that

affect productivity and affect germination (Ghosh & Sethy, 2013). In the case

of HgCl2, Hg reacts with the sulfhydryl groups and alters cell membrane

permeability, therefore, inhibits germination (Bose et al., 1983a, b). Along

with germination, heavy metals also show affinity towards phosphate groups,

essential ions and causes oxidative stress. Plants have developed various

sophisticated mechanisms to overcome heavy metal stress, this involves

physical barriers such as thick cuticles, cell wall, mycorrhizal association,

and trichome tissues that store heavy metals and detoxify by producing

secondary metabolites (Hall, 2002; Harada et al., 2010; Hauser, 2014; Lee

et al., 2002; Wong et al., 2004). Another mechanism used by plants is the

production of proteins called chelators such as spermine, nicotianamine,

phytochelatin, glutathione, organic acid, or cellular exudates such as pheno­

lics, HSPs, phenolics, and flavonoids, and amino acids, i.e., proline and

histamine, and hormones like jasmonic acid and salicylic acid, etc. (Dalvi

& Bhalerao, 2013; Sharma & Dietz, 2006; Viehweger, 2014). However,

hydropriming and halo priming by Mg(NO3)2 and Ca(NO3)2 improve germi­

nation percentage, seed emergence, radicle, and plumule length, α-amylase

activity, and soluble sugar content in endosperm in wheat (Kumar et al.,

2016). Hydropriming enhanced the seed germination of Medicago trun­

catula in in heavy metal contaminated soil. There was upregulation of genes

involved in DNA damage repair and antioxidant defense in hydroprimed

plant material (Forti et al., 2020).

4.3.4 TEMPERATURE STRESS

Temperature is considered to be one of the major factors which play a vital

role in seed germination and various enzymatic and biochemical reactions

required for the germination of seeds (Bewley et al., 2013; Nascimento

et al., 2021) Temperature stress can be categorized into three types, i.e.,

chilling, freezing or high temperature. Temperature stress can develop by

increase or decrease in temperature, duration, growth stage, and the rate

at which temperature changes. Under stress, plant shows growth retarda­

tion, low germination rate and often death. However, plants exhibit various

mechanisms to overcome temperature stress, i.e., molecular mechanism