reproductive behaviour (Hashimoto and Komatsu 2007). Generally tropical plants
are more susceptible to chilling than the temperate plants; for example, Zea mays,
Lycopersicum esculentum, Glycine max show chilling injuries at 10–15
C, there are
some temperate plants which experience chilling injury at 0
C–5
C; for example,
apple (Malus sp.) and Asparagus sp. (Hopkins and Huner 2009).
Plants also acclimatize themselves for chilling injuries. The acclimatization in
plants is associated with changes in several biochemical and physiological processes
such as altered gene expressions dehydrins (LEA, late embryogenesis abundant) are
induced in various plants, changes in hormone levels (increased ABA), increase in
soluble sugars, amino acids, organic acids, increased levels of osmoprotectants, etc.
(Palva et al. 2002).
1.2.1.2 High Temperature Stress
Change in global mean temperature and light conditions have significant impact on
distribution, abundance, phenology, and physiology of various crop species
(Djanaguiraman and Prasad 2014). It has been estimated that climate change
decreases the average suitable cultivable area for many plant species such as
Arachis, Solanum, and Vigna by 63–100%. It is also predicted that some species
might extinct in near future due to reduction in cultivable area (Jarvis et al. 2008).
Soil temperature ranges between 45 and 80
C, where forest soil temperature
remains between 40 and 50
C and in deserts it may reach up to 70–80
C.
Depending on the temperature range, responses exhibited by plants can be
categorized as over temperature, intermediate and under temperature responses.
The high temperature or heat stress damages the plants and causes the altered
phenology, reduced growth and development, scorching of leaves and fruits, sun
scabs, abrasions, etc. (Nahar et al. 2015). At cellular and molecular level, high
temperature affects stability of various proteins, thus prohibiting or altering many
enzymatic activities leading to metabolic disorders, change in membrane and cytoskeleton structure, altered RNA species. Effect of high temperature has been
observed in many crop species such as Oryza sativa (rice), Capsicum annuum
(capsicum), Triticum aestivum (wheat), Hordeum vulgare (barley), Zea mays
(maize), Glycine max (soybean), Abelmoschus esculentus (okra), etc.
(Hasanuzzaman et al. 2013). Also, there is formation of Heat Shock Proteins
(HSPs), whenever temperature is elevated beyond the threshold limits, which varies
from species to species.
1.2.2 Light Stress
Light is an important environmental factor which is essential for CO 2 assimilation
through photosynthesis, but as it crosses a plants tolerance level it becomes a stress
factor and photo inhibition occurs in plants. Plants are exposed to two types of
radiations: UV-A (315–400 nm) and UV-B (280–315 nm). UV-A is photo-oxidative
and UV-B is photo-oxidative as well as causes photo lesions in bio-membranes. UV
damages disulphide bridges in the proteins, dimerization of thymine groups in DNA,
6
P. Baweja and G. Kumar
are more susceptible to chilling than the temperate plants; for example, Zea mays,
Lycopersicum esculentum, Glycine max show chilling injuries at 10–15
C, there are
some temperate plants which experience chilling injury at 0
C–5
C; for example,
apple (Malus sp.) and Asparagus sp. (Hopkins and Huner 2009).
Plants also acclimatize themselves for chilling injuries. The acclimatization in
plants is associated with changes in several biochemical and physiological processes
such as altered gene expressions dehydrins (LEA, late embryogenesis abundant) are
induced in various plants, changes in hormone levels (increased ABA), increase in
soluble sugars, amino acids, organic acids, increased levels of osmoprotectants, etc.
(Palva et al. 2002).
1.2.1.2 High Temperature Stress
Change in global mean temperature and light conditions have significant impact on
distribution, abundance, phenology, and physiology of various crop species
(Djanaguiraman and Prasad 2014). It has been estimated that climate change
decreases the average suitable cultivable area for many plant species such as
Arachis, Solanum, and Vigna by 63–100%. It is also predicted that some species
might extinct in near future due to reduction in cultivable area (Jarvis et al. 2008).
Soil temperature ranges between 45 and 80
C, where forest soil temperature
remains between 40 and 50
C and in deserts it may reach up to 70–80
C.
Depending on the temperature range, responses exhibited by plants can be
categorized as over temperature, intermediate and under temperature responses.
The high temperature or heat stress damages the plants and causes the altered
phenology, reduced growth and development, scorching of leaves and fruits, sun
scabs, abrasions, etc. (Nahar et al. 2015). At cellular and molecular level, high
temperature affects stability of various proteins, thus prohibiting or altering many
enzymatic activities leading to metabolic disorders, change in membrane and cytoskeleton structure, altered RNA species. Effect of high temperature has been
observed in many crop species such as Oryza sativa (rice), Capsicum annuum
(capsicum), Triticum aestivum (wheat), Hordeum vulgare (barley), Zea mays
(maize), Glycine max (soybean), Abelmoschus esculentus (okra), etc.
(Hasanuzzaman et al. 2013). Also, there is formation of Heat Shock Proteins
(HSPs), whenever temperature is elevated beyond the threshold limits, which varies
from species to species.
1.2.2 Light Stress
Light is an important environmental factor which is essential for CO 2 assimilation
through photosynthesis, but as it crosses a plants tolerance level it becomes a stress
factor and photo inhibition occurs in plants. Plants are exposed to two types of
radiations: UV-A (315–400 nm) and UV-B (280–315 nm). UV-A is photo-oxidative
and UV-B is photo-oxidative as well as causes photo lesions in bio-membranes. UV
damages disulphide bridges in the proteins, dimerization of thymine groups in DNA,
6
P. Baweja and G. Kumar