**5. Heavy metal stresses**

#### **5.1. Cd stress**

Cd toxicity has emerged as one of the major agricultural problems in many soils around the world [110]. It has been shown to interfere with the uptake, transport, and utilization of essential nutrients and water, change enzyme activities, cause symptoms (chlorosis, necrosis), decrease in fresh and dry mass of root and shoot and also their lengths [110, 111, 112].

There are lots of studies to investigate the effects of BRs on Cd stress in plant species [110, 113, 114]. In these studies, results showed that BRs change different parameters such as germina‐ tion, plant dry biomass, protein content, and antioxidant enzyme activities (Table 2). It is proposed that the changes induced by BRs are mediated through the repression and/or derepression of specific genes [58]. Microarray experiments evaluating gene expression changes in *Arabidopsis* roots and shoots under Cd stress were performed [115]. Moreover, studies showed that gene expression in response to Cd mimics a BR increase, and Cd exposure most probably induces an activation of the BR signaling pathway in *Arabidopsis* [116].

### **5.2. Cr stress**

**4. Temperature stress**

In general, a transient elevation in temperature (usually 10–15o

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

heat shock or heat stress [89]. High-temperature effects can be seen at the biochemical and molecular level in plant organs (especially leaves). Heat stress induces decrease in duration of developmental phases, leading to fewer organs, smaller organs, reduce light perception over the shortened life cycle, and finally play an important role in losing the product [90, 91, 92]. High-temperature stress often induces the overproduction of ROS [93] which can cause membrane lipid peroxidation, protein denaturation, and nucleic acid damage [94, 95]. Many studies have demonstrated that ROS scavenging mechanisms play an important role in protecting plants from high-temperature stress [96, 97]. BRs applications decrease ROS levels and increase antioxidant enzyme activities to provide thermotolerance to elevated tempera‐

Chilling and frost stresses affect growth, development, survival, and crop productivity in plants [99, 100, 101]. However, BRs treatments enhance seedling tolerance to chilling stress [101] and increase the height, root length, root biomass, and total biomass of rice under lowtemperature conditions [102, 103]. In another study, Krishna [104] reported the same results in maize. They postulated that treatments with BRs promoted growth recovery of maize

Chilling stress increases the proline, betaine, soluble protein, soluble sugar contents of plants [79, 105]. Studies showed that BRs treatment enhanced proline content and therefore increased

Chilling stress could trigger the production of antioxidant enzymes in plants to prevent the chilling injury [108]. In the previous investigations, it was reported that treatment with BRs further increased the activities of antioxidant enzymes under chilling stress as well [99, 100, 107, 109]. The enhanced activities of the antioxidative enzymes as a result of BRs applications may occur with increasing de novo synthesis or activation of the enzymes, which is mediated

Cd toxicity has emerged as one of the major agricultural problems in many soils around the world [110]. It has been shown to interfere with the uptake, transport, and utilization of essential nutrients and water, change enzyme activities, cause symptoms (chlorosis, necrosis), decrease in fresh and dry mass of root and shoot and also their lengths [110, 111, 112].

plant chilling resistance and cell membrane stability [99, 100, 106, 107].

through transcription and/or translation of specific genes to gain tolerance [57].

C above environment) causes

**4.1. High temperature**

tures [98].

**4.2. Low temperature**

**5. Heavy metal stresses**

**5.1. Cd stress**

seedlings following chilling treatment (0–3°C).

Cr (III or VI) is not required by plants for their normal plant metabolic activities [117]. The entry of Cr into a plant system occurs through roots via using the specialized uptake systems of essential metal ions required for normal plant metabolism [118]. On the contrary, excess of Cr (III or VI) in agricultural soils causes oxidative stress to many crops. Reduced seed germi‐ nation, disturbed nutrient balance, wilting, and plasmolysis in root cells and thus effects on root growth of plants have been documented in plants under Cr stress [118, 119].

Choudhary et al. [120] reported that EBL treatment improved seedlings growth under Cr (VI) stress. Ability of EBL to increase seedling growth under this metal stress could be attributed to the capacity of BRs to regulate cell elongation and divisional activities, by enhancing the activity of cell wall loosening enzymes (xyloglucan transferase/hydrolase, XTH) [121]. Studies also indicated that increment in antioxidant activities as a result of BRs application (Table 2) provide plant tolerance to grow under Cr stress.

#### **5.3. Ni stress**

The heavy metals that affect (either positively or negatively) plants include Fe, Cu, Zn, Mn, Co, Ni, Pb, Cd, and Cr, but out of them, nickel has recently been defined as an essential micronutrient, because of its involvement in urease activity in legumes [122]. Excess Ni causes different problems. These symptoms include the inhibition in root elongation, photosynthesis and respiration, and interveinal chlorosis [123]. Moreover, the toxic concentration of Ni also inhibits enzyme activities and protein metabolism [124]. This metal also accelerates the activities of antioxidative enzymes [125, 126].

BRs effect on Ni stress in plants has been studied to understand the relationship between BRs and this stress (Table 2). One of these studies was carried out by Yusuf et al. [49]. They showed that seed germination and seedling growth were significantly reduced by Ni treatment, but HBL treatment enhanced germination percentage as well as shoot and root lengths in Nistressed seedlings. BRs confer tolerance against heavy metals either by reducing their uptake or by stimulating the antioxidative enzymes in *B. juncea* [127, 128]. The exogenous application of BRs in nickel-stressed *R. sativus* L., and *Triticum aestivum* L. plants enhanced the pool of antioxidant enzyme activity, thus alleviating the toxic effects of this stress [129].

#### **5.4. Cu stress**

Among the pollutants of agricultural soils, Cu has become increasingly hazardous due to its involvement in fungicides, fertilizers, and pesticides [130]. In addition, Cu present in excess has been known to decrease root biomass and alter plant metabolism [131, 132]. Sharma and Bhardwaj [127] demonstrated decrease in growth parameters of *Brassica juncea* grown under Cu stress. The reduction in growth parameters due to the Cu stress occurred as a result of decreasing mitotic activity and cell elongation [133, 134]. Moreover, Chen et al. [130] suggested a different opinion. They concluded that Cu-induced inhibition in root growth of rice seedlings was due to the stiffening of the cell wall. Moreover, excess of Cu ion leads to the generation of harmful ROS via the formation of free radicals [135].

Effects of exogenous application of BRs were studied on *Raphanus sativus* seedlings under Cu stress. It was found that 24-epiBL promoted the shoot and root growth by overcoming the Cu toxicity [136]. The growth-promoting effects of BRs on seedlings under Cu stress may be linked to the general ability of BRs to promote cell elongation and cell cycle progression [137, 138] as well as the stimulation of genes encoding xyloglucanses and expansins [139]. BRs applications also increase antioxidant enzyme activities [140, 141]. Increasing all parameters as a result of BRs application improves plant tolerance against Cu stress, and finally plant development (Table 2).

#### **5.5. Zn stress**

Zn is an essential microelement, the second most abundant transition metal after iron (Fe), and has a role in many metabolic reactions in plants [35, 36]. However, high concentrations of Zn are toxic, induce structural disorders, and cause functional problems in plants. At organism level, Zn stress causes reduced rooting capacity, growth, and at cellular level alters mitotic activity [37, 38]. It induces oxidative stress by promoting ROS production as a result of indirect consequence of Zn toxicity [142].

Application of BRs on plants alleviates Zn stress via increasing protein content and antioxidant enzyme activities (Table 2). Çağ et al. [143] reported that EBL application effectively enhanced the protein content in *Brassica oleraceae* cotyledons. Sharma et al. [144] also reported that presowing treatments of HBL lowered the uptake of metal and enhanced the activities of antiox‐ idative enzymes and protein concentration of *B. juncea* seedlings under Zn stress. Moreover, Ramakrishna and Rao [145] also reported that the application of 24-epiBL significantly alleviated the Zn-induced oxidative stress.



**Table 2.** Effects of BRs on plants subjected to Cd, Cr, Ni, Cu and Zn stresses. Dashes indicate that there are no results in study
