et al. 1991). It implies that HS indirectly affects the root by translocation of shoot
carbon to root or change in root water relations driven by shoot water demand
(Jordan and Nobel 1984). In severe HS condition reduction in mass, number, and
growth of root leads to limit the supply of nutrients and water to the plants (Wahid
et al. 2007; Huang et al. 2012). Nagel et al. (2009) in maize found that root zone
temperature below 25
C manifest 46% decrease in the primary root length as
compare to controlled condition. However, above threshold temperature showed a
decline in root growth since further growth in root tip required a balance between cell
division and cell elongation in the meristem (Beemster and Baskin 1998; Beemster
et al. 2003; Fiorani and Beemster 2006), the later decline in root growth was due to
insufficient division of new cells in apical meristem to recoup the enhanced cell
elongation rate. In wheat growth pattern of crown roots were found to be inhibited by
lower temperature (10
C), while the growth of seminal axes was restricted by HT
(30
C) (Huang et al. 1991). Nagel et al. (2009), studied the branching pattern of
lateral roots in maize. They reported an increase in temperature widens the angle
between a main and lateral root thus more volume of the substrate can be accessed by
a root system in HS. Elevated root zone temperature above air temperature by up to
6
C to 8
C caused shorter stature, reduced leaf area as well as increased carbon
allocation to reproductive structures as compared to plants grown in a controlled
environment (Monje et al. 2007).
3.2.1.2 Reproductive Development
The reproductive stage was found to be the most sensitive and crucial stage affected
in wheat (Farooq et al. 2011; Nawaz et al. 2013; Dwivedi et al. 2017) and maize by
HS (Cairns et al. 2012). One degree rise in temperature during the reproductive stage
caused severe yield loss in both wheat and maize (Lobell et al. 2011; Bennett et al.
2012; Yu et al. 2014). HT impede pollen tube germination, pollen viability, ovule
viability, an anomaly in position of stigmatic and style, number of pollens per silk
during fertilization, poor growth of endosperm, and development of the barren
embryo (Giorno et al. 2013). HS hinder the cell division process leads to abruption
in micro and mega sporogenesis process in wheat and maize (Saini et al. 1983, 1984;
Zaidi et al. 2016) responsible for the production of a sterile plant due to absent in
flower or fruit at the reproductive stage (Yun-Ying et al. 2008). However, the
sensitivity for HT is more in male than female gametophytic tissue (Devasirvatham
et al. 2012, 2013).
HT (>30
C) at the tetrad stage of pollen development in maize (Begcy et al.
2019) and wheat decreases the viability of pollen grains (Begcy et al. 2018), pollen
tube growth (Hedhly et al. 2009; Yang et al. 2013). Begcy et al. (2019) in maize
examined the closer look of pollen and found that heat-stressed pollen contained
fewer starch granules which can be considered as one of the reasons for pollen
development and did not develop a functional pollen tube. Cossani and Reynolds
(2012) exposed wheat crop at HT (30
C) for 3 days led to abnormalities both
structurally and functionally in anther (80% of florets). The increased temperature at
mid-anthesis period was also reported to be sensitive to HS in spring wheat (Ferris
et al. 1998) and maize (Sanchez et al. 2014). Minimum temperature (Tmin),
3 Plant Morphological, Physiological Traits Associated with Adaptation Against. . .
55
carbon to root or change in root water relations driven by shoot water demand
(Jordan and Nobel 1984). In severe HS condition reduction in mass, number, and
growth of root leads to limit the supply of nutrients and water to the plants (Wahid
et al. 2007; Huang et al. 2012). Nagel et al. (2009) in maize found that root zone
temperature below 25
C manifest 46% decrease in the primary root length as
compare to controlled condition. However, above threshold temperature showed a
decline in root growth since further growth in root tip required a balance between cell
division and cell elongation in the meristem (Beemster and Baskin 1998; Beemster
et al. 2003; Fiorani and Beemster 2006), the later decline in root growth was due to
insufficient division of new cells in apical meristem to recoup the enhanced cell
elongation rate. In wheat growth pattern of crown roots were found to be inhibited by
lower temperature (10
C), while the growth of seminal axes was restricted by HT
(30
C) (Huang et al. 1991). Nagel et al. (2009), studied the branching pattern of
lateral roots in maize. They reported an increase in temperature widens the angle
between a main and lateral root thus more volume of the substrate can be accessed by
a root system in HS. Elevated root zone temperature above air temperature by up to
6
C to 8
C caused shorter stature, reduced leaf area as well as increased carbon
allocation to reproductive structures as compared to plants grown in a controlled
environment (Monje et al. 2007).
3.2.1.2 Reproductive Development
The reproductive stage was found to be the most sensitive and crucial stage affected
in wheat (Farooq et al. 2011; Nawaz et al. 2013; Dwivedi et al. 2017) and maize by
HS (Cairns et al. 2012). One degree rise in temperature during the reproductive stage
caused severe yield loss in both wheat and maize (Lobell et al. 2011; Bennett et al.
2012; Yu et al. 2014). HT impede pollen tube germination, pollen viability, ovule
viability, an anomaly in position of stigmatic and style, number of pollens per silk
during fertilization, poor growth of endosperm, and development of the barren
embryo (Giorno et al. 2013). HS hinder the cell division process leads to abruption
in micro and mega sporogenesis process in wheat and maize (Saini et al. 1983, 1984;
Zaidi et al. 2016) responsible for the production of a sterile plant due to absent in
flower or fruit at the reproductive stage (Yun-Ying et al. 2008). However, the
sensitivity for HT is more in male than female gametophytic tissue (Devasirvatham
et al. 2012, 2013).
HT (>30
C) at the tetrad stage of pollen development in maize (Begcy et al.
2019) and wheat decreases the viability of pollen grains (Begcy et al. 2018), pollen
tube growth (Hedhly et al. 2009; Yang et al. 2013). Begcy et al. (2019) in maize
examined the closer look of pollen and found that heat-stressed pollen contained
fewer starch granules which can be considered as one of the reasons for pollen
development and did not develop a functional pollen tube. Cossani and Reynolds
(2012) exposed wheat crop at HT (30
C) for 3 days led to abnormalities both
structurally and functionally in anther (80% of florets). The increased temperature at
mid-anthesis period was also reported to be sensitive to HS in spring wheat (Ferris
et al. 1998) and maize (Sanchez et al. 2014). Minimum temperature (Tmin),
3 Plant Morphological, Physiological Traits Associated with Adaptation Against. . .
55