Root
Water stress is primarily sensed by roots, which then transfers this
signal to other organs in a plant. The response of rooting system to
drought influences the impact of the stress on plants (Anderegg, 2012;
Zhou et al., 2018). In response to drought, plants produce a greater
number of deeper and thinner roots increasing the total absorption
surface area favoring increased nutrient and water uptake (Chapman et
al., 2012; Fuentealba et al., 2015; Ma et al., 2018; Zhou et al., 2018).
Physiology (e.g., how root respires), architecture (e.g., depth of
roots), and morphology (e.g., root diameter) (Zhou et al. 2018) of roots
are modified in response to drought.
Water uptake by roots depends on two types of resistance: i) radial, the
resistance experienced by water while travelling from soil to root and
then to the vascular system, and ii) axial, the resistance on water
travelling from root to shoot using vascular root. Rowse & Goodman
(1981) studied the role of axial and radial resistance in absorption of
water from soil and reported that water uptake is mainly dependent on
radial resistance rather than axial resistance. Richards & Passioura
(1989) conducted an experiment to modify plants for drought escape where
they reduced the uptake of water in earlier water-abundant season by
increasing the axial resistance with reduced diameter of xylem. This
xylem modification helped in retaining soil water for later stages of
grain filling leading to increased productivity. The nodal roots played
an important role in this process by extracting maximum soil water from
rainfall.
Reproductive phase
Reproductive stages show continuous adjustments from vegetative stages
with changing environmental conditions (Onyemaobi et al., 2021).
Reproductive phase is more vulnerable and results in fewer flowers with
poor quality fruit and fewer seeds (Seghatoleslami et al., 2008;
Pushpavalli et al., 2014). Processes such as gametogenesis,
embryogenesis, and fertilization are severely affected limiting
development of seeds and impairing yield (Farooq et al., 2009, 2014).
Pollen fertility is seriously affected (Al-Ghzawi et al., 2009) during
stress conditions, disturbing growth of pollen tube and germination (Q.
Fang et al., 2010; Gusmao et al., 2012). It is also observed that under
water stress, amount of non-reducing sugars is increased, and
accumulation of starch is reduced leading to abortion of ovary,
(Andersen et al., 2002). For example, in legumes drought decreased the
seed size, and suppressed production of pods and flowers (Fang et al.,
2010). In Hordeum vulgare, exposer of reproductive stage of
growth to water stress resulted in increased root:shoot ratio and
reduced root thickness (Nosalewicz et al. 2016). Similarly,Astragalus nitidiflorus exhibited increased seed dormancy during
drought conditions (Segura et al., 2015). Drought along with heat stress
has more drastic effect during early phases of reproductive development
as various important biological function are affected such as anthesis,
micro and megasporogenesis, growth of pollen tube, stigmatic function,
fertilization and early embryo development (Prasad et al., 2008). In the
following subsections, we discuss the impact of drought on different
reproductive stages in plants.