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).