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.