**3.1 Influence of water tension on plant adaptation at physiological level**

The ever-changing nature of mercurial environment has forced the higher group of plants to develop a variety of intrinsic tactics at morphological, physiological (**Figure 1**), biochemical, and molecular levels for survival especially at limited water conditions. On the other hand, some plant species avoid water shortage circumstances by finishing their life cycle, for instance, before or after a drought period, while others showed adaptations to increase water absorption and minimise water loss to prevent its negative effects [15]. For example, *Phedimus aizoon* L., which was observed to respond the severity of drought stress by accelerated root system, thickened the waxy layer of leaf surface and closure of stomata for making sure of maximum water retention [16]. Under extreme arid conditions, the xerophyte *Zygophyllum xanthoxylum* is surprisingly found to accumulate ample amounts of Na+ ions, coming from the soils they thrive on. The primary role of accumulated Na+ in *Z. xanthoxylum* has been attributed to their ability to drastically reduce the osmotic potential of leaves, which enhances their ability to absorb water during drought spells [17]. *Reaumuria soongorica* shows specific characteristics during the process of adapting to desertification, such as an incredibly thick cuticle, hollow stomata, specialised leaf shape, deep root system, and efficient physiological mechanisms like a decreased transpiration rate, increased water use efficiency, and maintaining stem vigour to survive desiccation by leaf abscission [18].

**Figure 1.** *Plausible alterations in plant physiognomy under drought stress.*

#### *3.1.1 Root modification*

The soil provides nutrients and water to roots. As a result, the morphological and physiological traits of roots greatly influence the growth of shoots and overall production [19]. Plants attempt to extract water from deeper soil layers when there is a water shortage by strengthening their root architecture. In addition, roots are the primary organ that detects the presence of water, and control key aspects of plant growth and development [20]. In comparison with plants with shallow roots, those with deep root systems and perennial growth patterns demonstrated greater drought tolerance [21]. In addition to increasing the amount of soil that may be investigated for water and the surface area of roots in contact with moisture, roots with small diameters and long specific root lengths also boost hydraulic conductivity by lowering the apoplastic barrier to water entering the xylem. Additionally, decrease in root diameter also attribute the enhancement of water access and increases the productivity of plants under water stress. An examination of the root system of marigolds revealed a sharp decrease in the meta-xylem area (*Tagetes erecta* L.). Reducing the diameter of the meta-xylem vessels reduced embolism risk and improved water flow. Increased meta-xylem area is related to the flow of minerals and water and necessary for the growth of cortical parenchyma [22]. According to reports, the mechanism for drought tolerance in winter wheat, is supported by development of a deep root system, whereas a wellbranched (albeit shallow) root system is found in spring wheat [23].

There are three alternative strategies to confer drought resistivity, *viz*., drought escape, drought avoidance, and drought tolerance. Each of these tactics could develop into a constitutive reaction that happens independently of environmental cues such as water deficit. Drought tolerance and drought avoidance are the major strategies of plants against water deficit stress. The ability of a plant to withstand a dry environment through a variety of physiological processes, such as osmotic adjustment using osmoprotectants, is known as drought tolerance [24]. The continuation of physiological functions including stomata regulation, and root system development even at the period of prolonged dry spell is known as drought avoidance. The ability to adjust short life cycle to avoid drought stress is known as drought resistance [25]. The root system plays a crucial part in the plant's response to drought stress and may be the first organ to detect it. Shorter roots are less suited to drought tolerance than longer roots. Drought stress results in a significant reduction in the number of roots, as shown by *Helichrysum petiolare* [26]. Drought tolerant adaptive characters of plant roots including long roots, high density, and intense root system. Long roots with a high density are necessary for plants to retain performance when water is scarce, especially when the water is deeper. Factually, more roots may come into contact with more water vapours in the soil, and a denser root system absorbs comparatively more water than thinner ones [27].

#### *3.1.2 Leaf modification*

The majority of photosynthetic products are produced primarily in the leaf, which is the main portion of the plant. When *Andrographis paniculate* was subjected to water stress, precocious leaf fall was found [28]. Reduced leaf area due to water stress results in less photosynthesis, which lowers crop output. In order to achieve stability between the water received by roots and the water status in different plant parts, leaf area was found to be decreased in *Petroselinum crispum* and *Stevia rabaudiana* at limited water conditions [29]. Reducing leaf area is a method for avoiding drought

because it reduces the amount of water lost by transpiration. This reduction in leaf area is due to the suppression of leaf growth caused by a decline in cell division, which causes a loss in cell turgidity [30]. Reduced leaf area is probably a fundamental element of the drought resistance strategy used by eucalypts, and it might be more beneficial to survival than any physiological changes that have been observed [31].

The decline in leaf water potential is typically followed by the rolling of the leaves. Reduced leaf rolling, which occurs in plants with high osmotic adjustment, is thought to indicate that the plant is avoiding desiccation to a larger extent through a deep root system [32].

In addition, thick epidermis with large epidermal cells in plants also comes under the potential strategy of plant drought tolerance. Epidermal tissue thickness offers higher resistance of plants to water loss from root surface under arid climate [33].

With the application of the drought hardening treatment, the stomatal density of potato seedling leaves dramatically increased while the leaf area, stomatal size, and stomatal aperture decreased. These changes led to reduced leaf transpiration rate and improved water utilisation efficiency (WUE). The drought resistance of the potato seedlings that had undergone drought hardening was also enhanced by the alterations in leaf microstructure [34].

An intensive study on leaf trichomes in *Caragana korshinskii* has revealed that leaf trichomes are important structures on epidermis which uptake the dew from outer environment that assist in sustaining the leaf hydraulic assimilation system and mitigate the adverse effects of drought stress [35]. The outermost layer of defence against abiotic stress on plants is called cuticular wax. It was found that compared to healthy plants, sunflower genotypes exposed to drought stress had increased wax loads [36].

#### **3.2 Influence of water tension on plant adaptation at biochemical level**

#### *3.2.1 Photosynthesis*

A severe drought results in decrease or suppression of photosynthesis. Increased stomatal closure, reduced leaf area, and consequent reduced leaf cooling by evapotranspiration leading to damages to the photosynthetic apparatus contribute as the major obstacles for photosynthesis [37]. Decline in CO2 conductance via reduced stomatal activity enhances diffusive resistance and other vital metabolic processes [38]. Loss of CO2 uptake, affect Rubisco activity and decrease the function of nitrate reductase and sucrose phosphate synthase and the ability for ribulose bisphosphate (RuBP) production [39]. The closing of stomata, restriction of gas exchange, degraded photosynthetic apparatus, primarily PSI and PSII, and increased metabolite fluxes are all factors that also contribute to reduced photosynthesis [40]. Drought induced water loss affects the activity of photosynthesis-related enzymes, causing the photosynthetic device to malfunction and resulting in the poor execution of metabolic processes [14]. Reduction in photosynthesis attributed to increased metabolite fluxes result in the production reactive oxygen species, which impede cell growth by causing oxidative stress [41]. Extreme water limitations substantially hinder the rate of CO2 uptake and the photosynthetic system in cedar seedlings (*Cedrus atlantica* and *Cedrus libani*). The chlorophyll content, net photosynthesis, potential yield of the photochemical reaction of PSII and stomatal conductance of *Atractylodes lancea* shown persistent negative trends as the length of drought stress treatment increased [42].

#### *3.2.2 Mineral nutrition*

Water deficit situations usually lessen the ion content in various plant tissues by reducing the overall soil nutrient accessibility and root nutrient translocation [43]. Water stress conditions decreased plant potassium (K) uptake [44]. Reduced K mobility, declined transpiration rate and weakened action of root membrane transporters [44, 45]. Decrease in K level in leaves due to disrupted stomatal dynamism as well as irregular guard cell turgidity, also restricts the rate of photosynthesis and, backpedal the plant biomass production [46]. K transporters were inhibited by water stress conditions [47] and inner K channels were stimulated by a protein kinase, CIPK23, which in turn cooperates with calcium sensors (calcineurin B). This K channel was inhibited in roots but activated in leaves of grapevine [48]. K level decreased in *Ocimum basilicum* and *Ocimum americanum* plants subjected to limited water availability [49].

Leaf nitrogen (N) content did not change under drought-stress in *Mentha piperita*, *Salvia lavandulifolia*, *Salvia sclarea* and *Thymus capitatus*, whereas, in *Lavandula latifolia* and *Thymus mastichina* plants, reduced N level were observed. While leaf phosphorus (P) level reduced in all species except *S. sclarea* whose concentration remained the same [50]. Reduced N level and decline in K level in *Thymus daenensis* was considered as the main responsible factor for photosynthesis decline and leaf senescence under water deficit conditions [51]. Water deficit conditions increased the accumulation of manganese (Mn), molybdenum (Mo), P, K, copper (Cu), calcium (Ca) and zinc (Zn) in soybean [52].

#### *3.2.3 Antioxidant defence system*

Plants defensive system prevents the unwanted exposure of extraneous physical and biological agents which harm the plant body. In this context, a prompt, powerful and efficient antioxidant system is of pivotal importance to provide drought tolerance [53]. This system involves enzymatic and non-enzymatic detoxification moieties, which lessen and repair injury triggered by ROS. Antioxidant defence system helps in ROS scavenging that decreases electrolyte leakage and lipid peroxidation, therefore maintaining the vitality and integrity of organelles and cell membrane [54].

It is well established that drought induces oxidative stress by generating ROS, for instance O2 • , hydroxyl radicals (OH• ), singlet oxygen (1 O2) and H2O2 [55]. Numerous studies conducted under water stress conditions found enhanced activities of pivotal antioxidant enzymes, namely CAT, SOD, POD and APX [56]. Usually, an enhanced antioxidant enzymes activity is observed in stress tolerant genotypes as compared to non-tolerant plants.

Antioxidant enzymes like superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) significantly involve in the production of antioxidants such as O2• and H2O2 [57]. Ascorbate peroxidase (APX) also participates as ROS scavenger. APX mainly occurs in the chloroplast and cytoplasm and is a crucial enzyme for scavenging H2O2 in chloroplasts which convert H2O2 to H2O), and its activity is usually elevated under stress conditions. APX mainly occurs in the chloroplast and cytoplasm and is a crucial enzyme for scavenging H2O2 in chloroplasts [58].

Enzymatic activities of SOD, CAT and POD were stimulated by limited water availability in *Vicia faba* [59]. The amount of enzymatic and non-enzymatic antioxidants was improved in drought tolerant plants under mild and moderate water deficit conditions. CAT, SOD, POD and APX activities indicating that improved functioning of these enzymes helps to lower the level of ROS and mitigate the drought generated oxidative stress [60]. Water deficit boosted the levels of SOD and POD levels of these enzymes which stimulate tolerance against drought stress and are vital to reduce its adverse effects [61].

#### *3.2.4 Secondary metabolites*

Plants produce some chemical compounds in response to various environmental stresses, called secondary metabolites [62]. Biosynthesis of secondary metabolites (SMs) is regulated by environmental factors, such as temperature, light regime and nutrient availability. In this context, the drought stress signals induce systemic SM biosynthesis such as terpenes, alkaloids, and phenolic complexes to protect the plant system from oxidative stress [63]. On the order hand, high temperatures can also induce changes in SM biosynthesis. For example, heat stress has shown that isoprene levels increase; this biosynthesis is energetically costly for the plant, but these SM protect the cell membrane against oxidative stress, showing physiological benefits that far outweigh their energetic cost. Improved production of secondary metabolites is usually observed under water deficit conditions, which is caused by reduction in biomass formation and destination of assimilated CO2 to C-based secondary metabolites to avoid sugar-promoted feedback of photosynthesis.
