Vegetative phase
Plants are severely affected by changes in environmental conditions. Plants require optimum level of water in the soil beyond which uptake of nutrients is inhibited, cell enlargement is affected, cell wall extensibility is reduced leading to loss of turgor and ultimately end of plant growth and development (Seleiman et al., 2021). On perception of any abiotic stress, a signal cascade is activated that triggers cell cycle checkpoints, which slows down the process of DNA replication, impairment of G1 to S transition and delay mitosis (Qi & Zhang, 2020). Drought stress also limits water imbibition and decreases seed vigor by disturbing osmotic balance, increasing ROS production, reducing respiration rate thus affecting seed germination (Farooq et al., 2009; Hussain et al., 2018).
Plants show immediate response to drought by decreasing rate of transpiration by reducing leaf area and stomatal activity. For instance, in stay-green (Stg) sorghum, size of upper leaves is reduced, leaf area is decreased at anthesis and lower tillering occurs in response to drought (Borrell, Mullet, et al., 2014; Varshney et al., 2021). In Atriplex hortensis water scarcity causes reduced fresh and dry weight of root and shoot, decreased germination and reduced length of hypocotyl (Franco et al., 2011; Kachout et al., 2021). In the following subsections we discuss the impact of drought with respect to different vegetative stages in plants.
Seedling stage
Drought can affect plants at different stages and of those germination of seeds and seedling emergence are the most critical vegetative stages (Kızılgeçi et al., 2017). Plant survival and growth are highly influenced by processes of germination and growth of seedlings (Kachout et al., 2021). Water scarcity decreases hydraulic conductivity leading to impairment of various metabolic and physiological processes impacting germination (Bareke, 2018; Marthandan et al., 2020b). Studies on various field crops such as Pisum sativum (pea), Oryza sativa (rice), Medicago sativa (alfalfa) has reported decreased potential of germination and reduced hypocotyl length in water stress conditions (Okcu et al., 2005; Manickavelu et al., 2006; Zeid and Shedeed, 2006). Kızılgeçi et al. (2017) reported drastic decrease in rate of germination in wheat seeds when treated with PEG 6000 solution, Islam et al (2018) also conducted similar kind of experiment and reported that rice seedlings also showed reduced germination on increasing water stress. Due to dehydration, osmotic imbalance occurs in plants leading to decreased meristematic activity impairing cell elongation and thereby impairing length of root and shoot. Less water supply towards seed also slows down the process of hydrolyzation of stored carbohydrate reserves, which ultimately affects the transport of food to developing embryo, restricting radicle emergence from seed coat, hence reducing germination (Channaoui et al., 2019). In cotton, root morphology has been shown to play significant role in conferring tolerance to drought especially at seedling stage (Singh et al., 2018; Mahmood et al., 2022). Pace et al. (1999) observed that cotton seedlings showed increased thickness and length of roots when subjected to drought stress as compared to the control seedlings.
Leaf
The leaf also undergoes drastic changes when plants counter water scarcity. The plants which are adapted to avoid drought achieve it through a string of measures such as reduction in number of metaxylem vessels, reduction in stomatal area and density, and increase in thickness of leaf (Mansoor et al., 2019). These plants also have specialized tissue to help such as thick epidermis (Mansoor et al. 2019) and well-developed bulliform cells (Balsamo et al., 2006; Hameed et al., 2009). The bulliform cells help the leaf to roll when needed thus reducing transpiration (Alvarez et al., 2008; Mansoor et al., 2019). The tissues like cortical and mesophyll parenchymatous cells help leaf attaining succulence, a well-known adaptation under water scarcity (Abdel and Al-Rawi, 2011). Another such adaptation is sclerification (Mansoor et al., 2019; Vendramini et al., 2002) which not only provides mechanical support (Moulia et al., 2006) but also prevent collapse of cells (Mansoor et al., 2019; Turner, 1994). Granier et al., (2000) has reported that in maize and wheat duration of cell cycle is increased under water stress condition and length of meristem is decreased in leaves. Crop yield is also reduced during drought stress due to reduction in plant height and leaf growth which ultimately reduce photosynthesis rate (Aslam et al., 2015). Delayed spikelet development and pollen abortion, has also been reported in maize plants during water stress (Lu et al., 2011; Edmeades, 2013).
A combination of drought and heat stress have drastic impact on leaves causing wilting, severe chlorosis, damage of membrane (Awasthi et al., 2014), impairment of assimilate production and photosynthesis (Roohi et al., 2013; Sehgal et al., 2017; Sita et al., 2017). Hairy leaves (Boulard et al., 2017) and trichomes on either side allow plants to tolerate water scarce conditions as they help in reducing temperature of leaf by increasing the rate by which light reflection occurs and thereby decreasing transpiration and water loss (Seleiman et al., 2021; Tiwari et al., 2021). In sorghum staygreen (Stg) near-isogenic lines, reduced leaf area allowed the plant to retain more water for later stages of grain filling and thereby resulting in increased yield (Borrell, van Oosterom, et al., 2014; Varshney et al., 2021).