**2.2 Drought**

Pigeonpea is a rainfed crop grown during the kharif season. Because of its deep root structure, it is considered a drought-tolerant legume [25]. It can suffer from early and terminal drought stress due to its deep and broad root structure [26]. The germplasm has a wide range of Osmotic adjustment variation (0.2–1.6 MPa), with some accessions reaching 5.0 MPa. Some varieties, such as Bahar, BSMR 853, and ICP 84031, have demonstrated increased osmotic adjustment under drought conditions [35]. In response to drought conditions, relative water content (RWC) of leaves and dehydration tolerance are crucial (**Figure 1**). Drought resistant breeding should be done under true moisture-deficit conditions using agronomic traits such as pods per plant, seeds per pod, seed size, and seed production per plant. Therefore, physiological interactions, as well as high mean seed yield, should be employed to identify superior genotypes for low-soil-moisture situations [23, 36].

To understand the molecular mechanism for drought response in pigeonpea, a study has been performed on ICP151, ICPL8755 and ICPL227, where 51 genes were selected using Hidden Markov Model (HMM) to identify protein domain responsible for stress-responsive genes. Ten genes of U-box proteins, H+ antiporter proteins, and universal stress proteins were studied out of 51 drought genes (*AuspA*). These genes offer the way for molecular research into drought resistance [37]. The identified genes can also be validated at the sequence level in various genetic backgrounds to identify

**Figure 1.** *Abiotic stresses in pigeonpea with their effects.*

*Perspective Chapter: An Insight into Abiotic Stresses in Pigeonpea – Effects and Tolerance DOI: http://dx.doi.org/10.5772/intechopen.110368*

the presence of sequence variations for the formation of gene-based markers for crop improvement and the development of breeding lines and hybrids that are more tolerant through genomics-assisted breeding [38, 39]. As drought stress stagnates the food security over the globe, it is important to develop new varieties to achieve a proper amount of yield with maintained quality under such climatic perturbation. Strategies should develop where pigeonpea could enhance physicochemical capability of their cells to continue metabolism at low leaf water status [40].

#### **2.3 Soil salinity**

Salt stress is a significant constraint to the productivity of the nutritional rich pigeonpea. India accounts for more than 85% of global production and consumption of this legume crop. Excess Na<sup>+</sup> accumulation during salt stress interferes with hydrogen bonding and polar interactions, causing protein and nucleic acid structure to be disrupted. Thus, the total soluble protein content of stressed pigeonpea plants was found to be significantly lower [24]. The moisture content and succulence of *C. cajan* decreased dramatically as salinity increased, indicating a loss of turgor. When subjected to increasing salinity, this crop reduces water content in order to reach low osmotic potential. Salinity was performed to extend the 50% flowering stage by 1–2 weeks while also delaying the peak flowering stage. It increases floral shedding, lowering the effective quantity and weight of pods, and lastly lowering the number of seeds, lowering production [41].

Previous studies observed that the salt tolerance gene, *CcCYP*, is responsible for upregulated salt tolerance in root, whereas *CcCDR* was upregulated in shoot [42]. To make this legume crop resilient to salt stress, a better understanding of the molecular networks, in particular epigenetic regulation of gene expression, would be beneficial [23]. The potential of producing salt-tolerant lines of pigeon pea through genetic engineering has not been thoroughly studied. There is only one occurrence where transgenic pigeon pea plants were given salt tolerance through overexpression of the mutant 1-pyrroline-5-carboxylate synthetase gene (P5CSF129A) from *Vigna aconitifolia* [43]. These lines are notably salt tolerant. The identification of novel molecular targets that can be exploited by transgenic technologies would undoubtedly benefit from genome-wide association studies (GWAS) that uncover gene expression profiles in salt-stressed pigeon pea. It is also possible to use genomics-assisted intensive breeding to find quantitative trait loci and potential markers in salt-tolerant pigeon pea cultivars [44].

#### **2.4 Metal toxicity**

Changes in the environment are most likely to have a significant impact on how plants evolve, mostly through interfering with the process through mutations, gene flow, and evolution. Heavy metals are the major environmental changes/pollutants and their toxicity is a growing concern for ecological, evolutionary, nutritional and environmental reasons. These contaminants have a negative impact on the environment, impair agricultural output, and pose serious health risks to living organisms [45]. Metals exerts several effects on legume crop generated by elements such as Cd, Cu, Al, Hg, Pb and As, among others.

Cadmium (Cd), the most dangerous heavy metals because of their great mobility, non-degradability, and toxicity to plants as well as animals [46]. Excessive Cd2+ accumulation in plants can result in severe phytotoxicity as well as a variety of

physiological, morphological, and biochemical toxic effects on plant attributes such as pigment destruction, photosynthetic and respiration process inhibition, reduced nutrient uptake, overproduction of reactive oxygen species (ROS), enzyme and gene suppression, growth inhibition, and even plant death [47, 48].

Copper (Cu) is a vital element for plants since it helps with several physiological processes such as mitochondrial respiration, photosynthetic electron transport, and cell wall metabolism [49]. However, due to its redox characteristics, it is harmful to plants in large quantities (**Figure 2**). Excessive amounts impede plant growth, interfere with photosynthetic and respiratory activities, reduce nutrient uptake, target the membrane transport system, and produce excessive amounts of ROS [50]. Copper concentrations in the soil gradually lowered pigeonpea secondary metabolite biosynthesis (phenolics and flavonoids). Under Cu stress, pigeonpea had severe oxidative damage, as evidenced by higher levels of MDA (Malondialdehyde contents), hydrogen peroxide, and electrolyte leakage. Antioxidant enzymes (Superoxide dismutase, Peroxidase dismutase, Catalase and Glutathione peroxidase) and proline content were considerably increased with increasing Cu concentration to reduce oxidative damage [51].

Mercury contamination has emerged as a critical modern environmental issue. Its treatment highly reduced seed germination. Mercury chloride was found to be very harmful to seedling growth of legume crops. Plants grown at various levels of cadmium revealed a considerable drop in the length of shoots and roots, yellowing and ultrastructural abnormalities of the leaves, and a significant decrease in the essential oil content [52]. This metal exists in both organic and inorganic forms, and both are extremely dangerous. Its concentration in soil and water is an issue due to the widespread use of mercury-containing chemicals, fungicides, algaecide, paper pulp

#### **Figure 2.**

*Toxic effect of different heavy metals on pigeonpea.*

*Perspective Chapter: An Insight into Abiotic Stresses in Pigeonpea – Effects and Tolerance DOI: http://dx.doi.org/10.5772/intechopen.110368*

industries, and agriculture. Mercury released into the near environment may penetrate pigeonpea and other crops that humans eat, affecting human health. Therefore, it is critical to reduce the use of mercury in industries, as well as mercury-containing insecticides and fungicides [53].

Aluminum is the third most prevalent element in the earth's crust (after oxygen and silicon). The presence of poisonous Al3+ cations in acidic soils (pH 5.0) is a major constraint to agricultural productivity worldwide. The excess of Al is a major soil limitation to food and biomass production [54]. The suppression of root extension is the first sign of Al toxicity, which has been postulated to be produced by a variety of mechanisms, including Al interactions with the plasma membrane or the symplast. Aluminum poisoning has a negative impact on root growth and interferes with water and mineral nutrient intake [54, 55]. Pigeonpea plants cultivated in Al-challenged soil have lower nodulation. However, the use of 24-EBL inhibited the effect of Al on nodulation. Rhizobium multiplication and nodule development were reported to be more sensitive aspects of the symbiotic interaction to excess Al. Al poisoning caused a significant decrease in chlorophyll concentration. The use of 24-EBL on *C. cajan* plants significantly boosted photosynthetic pigments and counteracted the negative effects of Al+3 stress [18, 23, 56]. Plants have evolved various strategies to minimize metal-induced damage, including metal exclusion, compartmentalization, chelation, and a wide range of ROSscavenging mechanisms, including antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione reductase (GR), ascorbate peroxidase (APX), as well as non-enzymatic antioxidants such as phenol [18, 51].

#### **2.5 Temperature**

#### *2.5.1 Cold stress*

During the winter season in northern India, pigeonpea suffers from low temperature stress (December–January). If the minimum temperature goes below 5°C, stress impacts plant growth, survival, and reproductive capacity [57]. At freezing temperatures, intracellular water condenses into ice, causing cell contraction within the plant, resulting in wilting and plant death [18, 58]. Initial research at IIPR Kanpur also revealed genotypic differences in cold tolerance in pigeonpea. Because knowledge on cold stress and its impact on the pigeonpea crop is scarce, screening a large number of pigeonpea genotypes for low temperature tolerance under controlled temperature conditions is still required to confirm and generate precise genetic information [59].

#### *2.5.2 Heat stress (HS)*

Pigeonpea is a grain legume that is resistant to climate change. Though the ideal temperature for produced is 25–35°C, wild relatives grow at temperatures ranging from 18 to 45°C [60]. Heat Stress (HS) is the most serious abiotic threat to all legume crops. It reduces plant biomass build-up, resulting in lower yield, particularly in tropical and subtropical environments [61]. A prior analysis stated that a 1°C increase in maximum temperature during crop season could result in a 20.8% decrease in pigeonpea output. HS causes critical protein complexes to dissociate and the production of Reactive Oxygen Species (ROS) [62]. Plants tend to up-regulate the genes encoding molecular chaperones and signaling molecules in response to HS, thereby regulating a chain of events that lead to HS responses [63].

## **3. Mitigate the climatic change for** *C. cajan* **production**

Food production, security and climate change are all interconnected and hence affecting living systems. Long-term changes cause the entire weather pattern to alter, and also increase temperature, unpredictable rainfall, floods and a rise in sea level. India and other developing nations struggle to produce enough food to feed their expanding populations [23, 63]. Legumes, especially pulses make up the majority of the food on an Indian meal platter. Over a few decades, Pigeonpea in India has transitioned from being an orphan crop to a cash crop. Its production as a main crop is constrained by its lengthy maturation period and low yield [64]. This crop can withstand prolonged periods of drought and are well adapted to rain-fed conditions. They require little soil moisture to maintain themselves and generate a respectable amount of yield. However, this legume crop is sensitive to high temperatures and waterlogging. The effects of shifting climatic conditions on arhar that are rainfed are significant [64, 65]. According to reports, pulses are especially susceptible to heat stress during the bloom stage; just a few days of exposure to high temperatures ((30–35°C) can result in significant yield losses due to flower drop or pod damage. The crop's ability to grow in a larger range of latitudes and altitudes has been constrained by shifting rainfall patterns, rising yearly temperatures, and irregular climatic trends. However, there is no denying that the crop has the potential to support food security, nutrition, forage, and income production [66, 67].

Indian farmers have long waited for early-maturing pigeonpea cultivars that are compatible with their farming practices and produce higher yields with little inputs. The super-early varieties (ICPL 11255, ICPL 20340, and ICPL 20338) that ICRISAT's pigeonpea breeding team recently created are luring farmers from numerous states, including Maharashtra, Odisha, Karnataka, Telangana, and Andhra Pradesh [https:// www.icrisat.org/]. Given their photo- and thermo-insensitivity and capacity to grow in a larger range of latitudes (30°N) and altitude (1250 msl), such as in Uttarakhand, Rajasthan, Odisha, and Punjab, these cultivars have the potential to flourish in varied agro-ecologies. Creating short-lived variants has an added benefit. They may be cultivated with minimal inputs post-rainy season or off-season, giving farmers in dryland areas of India an extra source of income [68].

Gene mining for abiotic stress tolerance, restructuring plant types for climatevulnerable regions, changing cropping patterns, effective nutrient and water management, seed banks for alternative legume crops, watershed management, and micro-irrigation facilities are some of the better options to address climate change-related issues [42, 67, 69]. Furthermore, crop improvement strategies could be enhanced to mitigate climate changes by developing climate resilient varieties, reducing crop duration, adopting diversification in practices, improving crop specific practices, reducing greenhouse gas emission and use of biofertilizers. Therefore, more effective agronomic techniques have a huge potential to counteract the negative effects of climate change on arhar production. Adopting suggested management measures helps agriculture not only conserve soil and water, but also increases soil organic carbon levels and lessens the effects of climate change [70].

### **4. Genetic enhancement in abiotic stress tolerance in pigeonpea**

Genomics is concerned with the physical integrity of the genome, with the purpose of identifying, diagnosing, and regulating genetic traits throughout the chromosomes. We are now considering certain genetic advances to better understand abiotic stress tolerance in Pigeonpea. Specific trait markers for blooming, fertility, and resistance to sterility mosaic disease. QTL mapping, association mapping for candidate genes, transcriptome assembly, and genome sequencing technologies can be used to identify yield factors [71].
