**1. Introduction**

The ability to produce enough food for an endlessly expanding population is a key issue for mankind in the twenty-first century [1]. Currently, the scenario has been more difficult by the loss of arable farmland brought on by human habitation, the deterioration of the soil, and a range of environmental conditions, such as flooding, drought, salinity, temperature, and heavy metal pollution [2]. Eventually, accumulation of osmolytes at cellular level, modification of water flow, and scavenging of reactive oxygen species are some of the most frequent and well-documented adaptations that plants, which are sessile, have developed to recognize and respond to stress situations [3].

Environmental stress known as abiotic stress restricts plant growth and metabolism. Abiotic stressors are thought to diminish major food and cash crop yields and output by more than 50% [4]. Abiotic pressures may be divided into two categories: above- and below-ground abiotic stresses. Abiotic strains that are atmospherically produced come from the atmosphere, whereas abiotic stresses that are edaphic come from the soil [5]. In regions where climatic variability and precipitation patterns alter with extended periods of drought interspersed with spells of copious rainfall, abiotic stressors of atmospheric origin are prevalent [6]. On the other hand, anthropogenic activities such as the use of brackish water and sewage water for irrigation, sewage sludge for fertilisation, and inorganic chemicals for fumigation may result in abiotic pressures of edaphic origin. This issue is frequently made worse by inadequate waste management procedures, the weathering of local rocks, and subpar cultural practises that have rendered vast tracts of land unsuitable for agricultural development [7].

Aridity has a significant impact on community structure as well as ecological exploits, including primary productivity and nutrient cycling, by acting as a powerful environmental filter for plant survival, growth, and development [8]. For instance, plants with tiny leaves benefit from aridity as part of their strategy for adjusting the soil to water shortfalls and nutrient constraints. Plants adapt to their environment and develop an ideal phenotype, producing a set of adaptation tactics at both the collective and individual levels.

### **2. Relevance of plant-water kinship in agriculture**

Water is the most abundant material in any living entities across the globe. The weight of water contained in a plant is usually four to five times the total weight of dry matter [9]. Inside a plant body, about 80–90% of cell mass is comprised of water. Plants absorb water from soil through their roots and other parts in the way of vascular system. Xylem tissues of plant vascular system play a crucial role in the movement of water containing essential elements from roots to the shoot. Water supply through cells by diffusion alone is not enough to maintain the hydration of a perspiring canopy plant. The necessity for a vascular system becomes more apparent while studying the hydraulic dynamic of a tree on a hot day, which requires a massive flow of water. Water transport through xylem is over a million times more efficient than water transport through plasmodesmata of parenchyma. Several theories have been proposed to explain the mechanism of movement of water into xylem against the concentration gradient. The cohesion-tension theory, proposed by Boehm, Dixon and Joly (1894) in the late 19th century is thought to be the most appropriate tenet to explain the mechanism of upward movement of water. According to this theory, the water evaporated from leaf surface establishes a tensile strength in the xylem, where the hydrogen bonds provide a continuous intermolecular attraction (cohesion) between the water molecules from the leaf to the root. Thus, the water column in the xylem lumen is driven out of a region with a higher water potential, *i.e.,* from the root and the stem, to a region with a lower water potential, as the leaves, and finally toward the air that can reach very low water potential. Once water reaches the xylem, it enters conducting elements of either conifer tracheid or angiosperm vessels, and flows upwards through the stem to the leaves. The conduit diameter of xylem gets smaller and tapered with plant height, indicating the widening aspect of xylem anatomy from apex to the base of plant. Plants that have an increased number of xylem conduits per cross-sectional area can maintain hydraulic conductance by reducing effects of path length [10].

#### *Molecular Basis of Plant Adaptation against Aridity DOI: http://dx.doi.org/10.5772/intechopen.110593*

Droughts can be classified as meteorological, agricultural, hydrological, or socioeconomic, according to the American Meteorological Society (1997). Precipitation deficits can be used to categories meteorological droughts, and these crises can develop in other categories of droughts. Agricultural drought focusing on precipitation shortages, discrepancies between actual and potential evapotranspiration, inadequate soil water, and lower reservoir levels. A lack of water in the hydrological system is referred to as a "hydrological drought," which is characterised by reduced river flow as well as declining dam, lake, and subsurface water levels on a basin-scale. Economic, social, and environmental harm brought on by many sorts of droughts refers to what is meant by socioeconomic droughts [11]. There is limited clarity over the metrics that better reflect the effects of drought on the environment and society.

Agriculture is a key activity of human being since it provides basic needs and water is a critical input for agriculture production. Several factors pose significant risk to farms leading to yield reduction like limited water condition. A limited water availability leading to drought, increased diseases and pest incidence and extreme weather events at local to regional scale. Limited water availability accounts for about 30–70% loss of productivity. It also results in abnormal metabolism that may reduce plant growth or cause the death of plant. Water stress is one of the most detrimental factors seriously affecting the growth and production of many plants mostly during the flowering phases. Under the exposure of severe water crisis, significant diminution in the major growth attributing characters including number of leaves, leaf area, stem length is very often in various plants. Furthermore, the crop yield and productivity are also found to be affected severely under water stress. The damaging effects on plants are associated with oxidative damage in the plant cells are commonly realised by elevated lipid peroxidation, reactive oxygen species (ROS) accumulation, and electrolyte leakage. Under usual conditions, ROS exist in plant organelles, mainly mitochondria, chloroplasts, and peroxisomes, while under stressful conditions such as drought, ROS levels increase resulting in lipid peroxidation and proteins degradation [12]. Also, biological yield and physiological characters such as stem length, number of leaves, leaf area, relative water content, and chlorophyll concentration as well as overall biological yield are decreased under stress condition in many plants [13]. Drought during blossoming is frequently associated with infertility [14], owing to a reduction in assimilating flow to the developing ear. Drought stress can significantly reduce production in important field crops by prolonging the anthesis period and delaying grain filling [15]. Numerous factors could explain the decline in yield, including decreased photosynthesis, inefficient flag leaf formation, uneven assimilate portioning, and a depleted pool of critical biosynthesis enzymes such as starch synthase, sucrose synthase, starch enzymes, and α-amylase.
