INTRODUCTION
Environmental stresses affect plant growth in multiple ways. Plants
recognize and respond to these stress conditions including drought with
a variety of biological signals at various stages of its growth
(Takahashi and Shinozaki 2019; Takahashi et al. 2020). From 2005 to 2015
drought in developing countries caused agricultural losses of more than
USD 29 billion (FAO, 2021). Drought also occurs with other stresses such
as heat, nutrient deficiency, and salinity in the environment (Sehgal et
al., 2018). Drought stress is often accompanied by heat stress in the
field (Sehgal et al., 2018). Drought impairs transport, partitioning and
uptake of nutrients, hence suppressing plant reproduction and growth (Hu
and Schmidhalter 2005; Gessler et al. 2017). Due to low moisture content
in soil, rate of mineralization and diffusion of nutrients towards
surface of root is reduced thereby affecting plant nutrient uptake (Alam
1999; Luo et al. 2018). For example, drought stress reduced
concentration of phosphorous (P) and nitrogen (N) in plant tissue as
mass flow of nutrients from roots to above ground tissue has been
impaired (He and Dijkstra 2014; Bista et al. 2018). During concurrent
salinity stress, roots accumulate more ions such as Na+ and Cl-,
reducing the hydraulic conductance in roots, thereby triggering drought
conditions (Raja et al., 2020). Factors such as moisture storing
capacity of soil, distribution of rainfall, and rate of evaporation are
mainly responsible for determining the extent of drought (Wery et al.
1993; Hussain et al. 2016). With changing climatic conditions, plants
with higher drought resilience are preferred in agricultural systems.
The drought stress induces various kinds of responses in plants
including morphological, physiological, and biochemical changes which
helps them to thrive under drought stress. The first thing that plant
senses during drought stress is the decreasing water potential
(Christmann, Grill, and Huang 2013) because of which the roots are no
longer able to absorb water from soil. This restriction of water flow
through xylem and other parts of plants inhibits elongation of cell
(Nonami 1998; Hussain et al. 2016). Availability of water impacts
metabolic activity and, reactive oxygen species (ROS) production is
increased which can alter DNA, RNA, and protein,. This also affect ATP
production and disturb balance of osmotic pressure (Priestley 1986;
Hussain et al 2018). Another primary response of plants during drought
stress is stomata closure to prevent loss of water. Due to closure of
stomata, diffusion of CO2 does not occur in leaves thus
reducing the rate of photosynthesis (Raja et al. 2020).
Generally, three types of responses are observed: drought avoidance,
drought escape and drought tolerance (Figure 1) (Kooyers, 2015;
Fang et al., 2017). Drought avoidance refers to morphological and
physiological adaptations including reduced number of stomata, minimized
leaf area, increased root growth, thick old leaves, cuticle wax
synthesis, rolling of leaves and protection from osmotic shock under
stress conditions (Lee & Suh, 2013; Y. Liu et al., 2017). Drought
escapers are the ones that employ strategies such as precocious
flowering, increased photosynthetic ability, increased level of
nitrogen, and fast growth in order to complete its life cycle before
beginning of drought stress conditions (Kooyers, 2015; Marthandan et
al., 2020). In normal conditions crop cycle of longer duration is
generally preferred as it enhances the absorption of sunlight but under
drought conditions longer crop cycle decreases plant fitness as soil
water is depleted before cycle is completed (Tardieu et al., 2018;
Varshney et al., 2021). Drought tolerant ones accumulate various
osmolytes such as proline and glycine and triggering phenylpropanoid
pathway increasing lignin biosynthesis which helps to maintain structure
of membranes and production of antimicrobial compounds such as
phytoalexin (Nadeem et al., 2019; Sharma et al., 2019; Marthanandan et
al., 2020; Sheoran et al., 2022).