**2. Water stress**

Water stress is characterized by a reduction of water content and leaf water potential, closure of stomata, and decreased growth. Severe water stress may result in the arrest of photosyn‐ thesis, disturbance of metabolism, and finally the death of plant [4]. This water loss causes a loss of turgor pressure that may be accompanied by a decrease in cell volume depending on the hardness of the cell wall [5]. The cells of the root must activate processes to limit water loss

Salinity also affects plant growth, activity of major cytosolic enzymes by disturbing intracel‐ lular potassium homeostasis, causing oxidative stress and programmed cell death, reducing nutrient uptake, genetic and epigenetic effects, metabolic toxicity, inhibition of photosynthesis, decreasing CO2 assimilation, and reducing root respiration [6, 7, 8, 9]. Salt stress affects the root in all developmental zones. Cell division decreases in the meristematic zone and cell expansion attenuates in the elongation zone, resulting in reduced overall growth [10]. Cells also expand radially in the elongation zone [11], and root hair outgrowth suppresses in the differentiation zone [12]. Salt stress additionally results in agravitropic growth [13] as well as reduced lateral root number under high-salt conditions and enhanced lateral root number under moderate-salt conditions [14, 15]. Salt stress developes from excessive concentrations of salt, especially sodium chloride (NaCl) in soil. Root is the primary organ of exposure and

High temperature increases the permeability of plasma membrane [20], and also reduces water availability [21]. Moreover, low temperature (chilling and frost stress) is also a major limiting factor for productivity of plant indigenous to tropical and subtropical climates [22]. Chilling stress has a direct impact on the photosynthetic apparatus, essentially by disrupting the thylakoid electron transport, carbon reduction cycle, and stomatal control of CO2 supply, together with an increased accumulation of sugars, peroxidation of lipids, and disturbance of

Heavy metal contamination in soil could result in inhibition of plant growth and yield reduction and even poses a great threat to human health via food chain [24]. Among heavy metals, Cadmium (Cd) in particular causes increasingly international concern [25]. Cdcontaminated soil results in considerable accumulation of Cd in edible parts of crops, and then it enters the food chain through the translocation and accumulation by plants [26, 27]. Another metal, chromium (Cr III or VI), is not required by plants for their normal plant metabolic activities. On the contrary, excess of Cr (III or VI) in agricultural soils causes oxidative stress for many crops. Reactive oxygen species (ROS), like hydrogen peroxide (H2O2), hydroxyl

cause oxidative damages to DNA, RNA, proteins, and pigments [28, 29]. Nickel (Ni) is one of the most abundant heavy metal contaminants of the environment due to its release from mining and smelting practices. It is classified as an essential element for plant growth [30]. However, at higher concentrations, nickel is an important environmental pollutant. Ni2+ ions bind to proteins and lipids such as specific subsequences of histones [31] and induce oxidative damage. Copper (Cu) is also an essential micronutrient for most biological organisms. It is a cofactor for a large array of proteins involved in diverse physiological processes, such as

−

/K+ ratio in the root that leads

) generated under Cr-stress, are highly reactive and

hence responds rapidly [16]. Salt stress is known to increase Na+

to cell dehydration and ion imbalance [17, 18, 19].

374 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

), and superoxide radical (O2

and mitigate its harmful effects.

water balance [23].

radical (OH<sup>⋅</sup>

Water shortage is predicted as one of the most important environmental problem for the 21st century that limits crop production [54]. Although drought stress inhibites the plant– water relations, exogenous application of BRs maintaines tissue–water status [55] by stimulating the proton pumping [56], activating nucleic acid and protein synthesis [57] and regulation of genes expressions [58]. It has been shown that 24-epibrassinolide (24-epiBL) treated *Arabidopsis* and *B. napus* seedlings had a higher survival rate when subjected to drought [59], and in another study BR-treated sorghum (*Sorghum vulgare*) showed in‐ creased germination and seedling growth under osmotic stress [60].

Root nodulation is a fundamental developmental event in leguminous crops, and is sensitive to water shortage [61, 62, 63]. As endogenous hormones play an important role in the orga‐ nogenesis and initial growth of nodules in roots, attempts have been made to increase root nodulation by growth regulator treatments [64, 65]. The potential of BRs in the improvement of root nodulation and yield have been reported in groundnut [66]. Upreti and Murti [67] also studied the effects of two BRs, epibrassinolide (EBL) and homobrassinolide (HBL), on root nodulation and yield in *Phaseolus vulgaris* L. cv. Arka Suvidha under water stress. They concluded that water stress negatively influenced nodulated root, but BRs increased tolerance to water stress and EBL was relatively more effective than HBL.

Several researchers have found that increased proline levels can protect plants from water stress. BR treatment increased the contents of proline and protein under water stress [68]. Zhang et al. [69] also indicated that BR treatment promoted the accumulation of osmoprotec‐ tants, such as soluble sugars and proline. It may be due to the fact that BRs activated the enzymes of proline biosynthesis, which caused an additive effect on the proline content [70].

Drought stress causes increment in H2O2 due to decrease in antioxidative enzyme activities [71]. Plants have improved various defense mechanisms to respond and adapt to water stress [72]. Vardhini et al. [73] studied with sorghum seedlings grown under PEG-imposed water stress and investigated the effects of HBL and 24-epiBL on the activities of four oxidizing enzymes: superoxide dismutase (SOD), glutathione reductase (GR), IAA oxidase, and poly‐ phenol oxidase (PPO). They found that supplementation of both the BRs resulted in enhanced SOD and GR but lowered IAA oxidase and PPO. Li and Feng [68] also reported that treatment of brassinolide significantly increased peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) activities of seedlings under normal water and mild water stress. Therefore, increment in enzyme activities provided tolerance of *Xanthoceras sorbifolia* seedlings to drought stress. It has been found that BRs can induce the expression of some antioxidant genes and enhance the activities of antioxidant enzymes such as SOD, POD, CAT, and APX [74, 75].
