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).