Keywords

25.1 Introduction

Plants encounter with various abiotic (natural or anthropogenic stress) and biotic stresses throughout their course of life. Among abiotic stresses, temperature stress (heat and cold stress), water stress (drought stress and flooding), and salt stress are major threats for sustainable crop production (Fig. 25.1) (Zhang et al. 2020). These stresses modulate plants at morphological, physiological, anatomical, biochemical, and molecular level temporarily or permanently which hamper plant growth and development and ultimately reduce crop yield. Plants exposed to salt stress experience osmotic stress (water deficit) during initial stage of stress followed by hyperionic stress (high concentration of ions Na+ and Cl− in cytosol) as well as oxidative stress (production of reactive oxygen species) at later stage of salt stress. The adverse effect of osmotic stress (physiologically dry soil) due to salinity on plants is similar to drought stress (physical dry soil). Oxidative stress occurs due to production of reactive oxygen species (superoxide radicals (O2˙−), singlet oxygen (1O2), hydrogen peroxide (H2O2), hydroxyl radical (OH˙−), hydroperoxyl radical (HO2˙), alkoxy radical (RO˙), peroxyl radicals (ROO˙), and excited carbonyl (RO)) which leads to disruption of cell membrane and ultimately cell death (Hasanuzzaman et al. 2013). Ionic stress leads to accumulation of sodium ions (Na+) in cytosol, which disturbs ion homeostasis by disturbing uptake of potassium (K+) and calcium ions (Ca+) which are essential macroelements for plant growth and development. It also leads to premature leaf fall and affects crop yield. Drought stress (low moisture content in soil) causes drastic loss in crop yield in arid regions. It negatively affects plant growth and development, one of them is reduction in leaf expansion (decrease in photosynthetic area) due to decrease in water content of leaf. Other effects include production of reactive oxygen species, decrease in leaf area, and in severe conditions wilting and ultimately plant death. Plants either fight with these stresses or avoid it to nullify the negative effects of stresses on themselves. The plants which fight with stresses are called stress tolerant plants, and the ability of plants to fight with stresses is known as their tolerance. The plants having low tolerance for stresses are known as stress sensitive plants. The plants which avoid stresses are called escapers. Halophytes have in-built mechanisms to fight with salt stress (tolerance mechanism). These are called salt stress tolerant plants. Ephemeral plants complete their life cycle before the onset of dry period (avoidance mechanism). These are known as drought escapers.

Fig. 25.1
A classification chart exhibits plant stress factors. Stress is categorized into abiotic and biotic. Abiotic stress is further divided into temperature, water, and salt stress. Temperature stress is classified into heat and cold, while water stress is categorized into drought and flooding.

Major stresses faced by plants during its life

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The various strategies are sought and adopted to cope with stresses or to improve plant tolerance against stresses (Fig. 25.2). One of them is tailoring of stress tolerant genotypes with high grain yield followed by their screening under natural stress conditions in fields. The development of tolerant varieties is time consuming and labour-intensive with limited success. The screening of these genotypes for their tolerance against stress is extra burden for scientists and researchers. The degree of tolerance varies among different genotypes. So alternative approaches are sought, and applied for mitigation of these stresses, and maximize yield potential of crops. The most prevalent approach is upgradation of plant tolerance mechanism against stresses by exogenous application of plant growth regulators, osmoprotectants, biofertilizers, inorganic and organic compounds as well as nanoparticles. The utilization of nanotechnology in agriculture to cope with stress as well as for sustainable crop production drew attention of scientists, researchers, agriculturists, and farmers (Heidarabadi 2022).

Fig. 25.2
A chart of approaches in mitigation of stress. It includes tailoring stress-tolerant genotypes, exogenous application of plant growth regulators, exogenous application of inorganic and organic compounds, exogenous application of nanoparticles, and molecular approaches.

Approaches in mitigation of drought and salt stress

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25.2 What Are Nanoparticles?

The term nanotechnology was first earned by scientist Norio Taniguichi, a professor at Tokyo University of Science, in 1974, and this technology came into limelight during twentieth century (Khan and Rizvi 2014) and attained prominent position in field of science, engineering, medical, agriculture, and industries in the twenty-first century. The nanomaterials (NMs) are proved as Kohinoor gems in the field of agriculture. These are used in the form of plant growth promoters or stimulants, pesticides, herbicides, fertilizers, pesticides carriers, and plant growth regulators. Plant growth stimulants nanoparticles are employed for alleviation of negative effects of stress by modulation of their tolerance mechanisms at physiological, biochemical as well as at molecular level. The dimensions of NMs range from 1 nm to 100 nm with large surface-to-volume ratio, and these are available in one dimension (surface films), two dimensions (strands and films), and three dimensions (nanoparticles (NPs)). On the basis of mode of synthesis, the NPs are of three types: natural, incidental, and synthetic (engineered). The synthetic NPs are synthesized either from bulk chemicals (top-down approach) or from atoms or ions (bottom-up approach) by physical, chemical, and biological methods (using microorganisms and plants (green factories)). The NPs prepared using plant extracts are called green nanoparticles, and this is cost-effective and eco-friendly approach for synthesis of NPs. The synthesis of nanoparticles using pant extract is called green synthesis or phytosynthesis (not confused with photosynthesis), e.g. synthesis of selenium NPs using stem extract of Leucas lavandulifolia (Kirupagaran et al. 2016). On the basis of their nature, NPs are of two types: (a) inorganic NPs and (b) organic NPs. a) Inorganic NPs are further divided into two groups:

  1. 1.

    Metallic and metallic oxides NPs (Au NPs, Ag NPs, CuO, ZnO, Fe2O3)

  2. 2.

    Metalloid and metalloid oxides NPs (SiO2 NPs)

b) Carbon nanoparticles (quantum dots, carbon nanotubes, Fullerenol NPs (C6O(OH)24)) are involved in the category of organic NPs (Taran et al. 2017).

25.3 Inorganic Nanoparticles

25.3.1 Silver NPs (Ag NPs)

Hojjat (2016) observed positive effect of AgNPs in lentils exposed to drought stress. The application of Ag NPs enhanced germination rate, germination percentage, root length, root fresh, and dry biomass of lentils under drought stress.

25.3.2 Gold NPs (Au NPs)

The gold nanoparticles are included in the category of metal nanoparticles. The potential role of Au NPs in inducing tolerance against salinity was observed in wheat by Wahid et al. (2022). These NPs minimized oxidative stress by modulating activities of antioxidant enzymes. The improvement in plant growth attributes under saline conditions with application of Au NPs was also reported (Wahid et al. 2022).

25.3.3 Zinc Oxide NPs (ZnO NPs)

Zinc is included in the category of essential microelement. In tomato, the role of these NPs to cope with salt stress was observed by Faizan et al. (2021). The tomato seedlings were subjected to salt stress (150 mM), and foliar application of ZnO NPs at concentration of 10, 50, and 100 mg/L was given at 25 DAS (days after sowing). The application improved tomato plant growth in terms of shoot length, root length as well as fresh and dry biomass and leaf area under salt stress. The physiological attributes (photosynthesis rate, chlorophyll content, and carotenoid content) as well as biochemical parameters (antioxidant enzymes and protein content) also improved with foliar spray of them. Semida et al. (2021) also studied the role of ZnO NPs in stress (water deficit) tolerance in eggplant. The foliar application of NPs alleviated the negative effects of drought stress by improving plant growth, membrane stability, photosynthetic rate, water productivity as well as yield. Application of 50 and 100 ppm ZnO NPs improved fruit yield by 12.2% and 22.6%, respectively of plants exposed to water stress, in comparison with plants grown in fully irrigated soil.

25.3.4 Copper Oxide NPs (CuO NPs)

Copper is one of the microelements in plants. CuO NPs are included in the category of inorganic metal oxide nanoparticles. These NPs amplified drought tolerance in maize when 12-day old seedlings were treated with it. The grain yield reduced significantly under water deficit conditions but application of CuO NPs increased grain yield as well as seed number. It ameliorated the negative effects of water deficient stress on maize yield by enhancing chlorophyll and carotenoid content, which directly increased the photosynthesis rate and grain yield. Significant increase in level of antioxidant enzymes under drought stress with application of CuONPs was also reported. These enzymes fight with reactive oxygen species and minimize the effects of drought stress on plants (Van Nguyen et al. 2022).

25.3.5 Titanium Oxide NPs (TiO2 NPs)

Titanium oxide is one of the inorganic nanoparticles. Various studies reported the application of TiO2 NPs in mitigation of salt and drought stress by improving their growth, and enhanced yield. Dawood et al. (2019) fertigated the water deficit soil with these NPs and assessed the performance of four wheat cultivars (Sohag 3, Benisuif 5, Sakha 93, and Sed 12). Under control conditions (water deficit soil), wheat growth in terms of leaf traits is negatively affected. TiO2 NPs promoted photosynthetic rate, improved leaf health, increased leaf chlorophyll content, LAI (leaf area index) as well as leaf growth (thicker and heavier leaves) and decreased leaf aging.

In other study, the positive effect of foliar application of these NPs under water deficit condition was reported in Dragon head (Dracocephalum moldavica L.) (Mohammadi et al. 2016). Water deficit conditions damage cell membrane and lead to oxidative stress. TiO2 NPs application at different concentrations (10 and 40 ppm) ameliorated effect of water stress by increasing proline content and reducing hydrogen peroxide and MDA content (MDA content is indicator of degree of membrane damage). The stabilization of membrane was also reported by Sompornpailin and Chayaprasert (2020) in Nicotiana tabacum. The increase in level of enzymatic and nonenzymatic antioxidants in Vicia faba was reported (Khan et al. 2020). Shariatzadeh Bami et al. (2021) reported that application of these NPs at different concentration 10, 20, and 30 ppm modulated molecular pathway in Artemisia absinthium L. (a herb), grown under salt stress conditions. The degree of expression of two genes ADS and DBR2 (key gene in biosynthesis pathway of artemisinin) was noted under salt stress with application of NPs. The expression of ADS gene recorded maximum in plants, sprayed with 30 ppm NPs followed by 20 and 10 ppm with salinity stress 50 ppm. While expression of DBR gene showed reverse trend. The maximum expression was observed in plants grown under control conditions (no application of NPs) followed by plants with NPs application. The alternation in biochemical pathways for salt stress tolerance with NPs was reported in Artemisia absinthium L. With application of TiO2 NPs, a significant increase in antioxidant enzymes catalase, peroxidase, superoxide dismutase, polyphenol oxidase, and guaiacol as well as protein concentration was reported as compared to control. These enzymes engulf reactive oxygen species synthesized during later phase of salt stress (Shariatzadeh Bami et al. 2021).

25.3.6 Iron Oxide NPs (Fe2O3 NPs)

Iron is one of the micronutrients in plants. It acts as cofactors for various enzymes. It exists in two forms: Fe2+ and Fe3+ form. The maghemite (yttrium doping-stabilized γ-Fe2O3NPs) are used as fertilizers and improve plant growth and development, but their role in drought stress tolerance using maghemite as nanozyme was studied by Palmqvist et al. (2017) in rapeseed. The increase in catalase activity (decrease in H2O2 content) as well as membrane stability (decrease in lipid peroxidation) with application of NPs contributed for stress tolerance in these plants. The role of Fe2O3 NPs as nanozyme against salt stress tolerance by improving their growth parameters as well as modulating their gene activity in Eucalyptus tereticornis was studied by Singh et al. (2022).

25.3.7 Cerium Oxide NPs (Ce2O3 NPs)

Cerium oxide NPs act as weapon to fight with stress. The application of these NPs in soil at 500 mg/kg soil, equipped the roots of Brassica napus with large apoplastic barriers which led to reduction in transport of Na+ ions in shoot as well as their accumulation. This provided physiological tolerance to plants against salt stress (Rossi et al. 2017).

25.4 Organic Nanoparticles

25.4.1 Chitosan Nanoparticles (CSNPs)

Chitosan polysaccharide is deacetylated form of chitin, hydrophilic in nature. The use of chitosan NPs in soil, in hydroponics and through foliar spray equipped the plants with protective mechanisms against stress. In periwinkle (Catharanthus roseus), 1 g/L chitosan NPs improvised plant tolerance against drought stress (50% FC) (Ali et al. 2021). The drought mediated oxidative stress was also minimized by increasing the activities of enzymes catalase and ascorbate peroxidase. Membrane disruption during salt stress and drought stress leads to leakage of ion. This leakage is indication of membrane stability. The malondialdehyde (MDA) content also indicates membrane stability. In chitosan NPs treated plants, MDA content decreased which is indication of membrane integrity under stress.

Hassan et al. (2021) exposed the same plant to salt stress (NaCl 150 mM), and applied 1% CSNPs as foliar spray. The application of NPs helped the plants to cope with salt stress by modulating their antioxidant enzyme activities (catalase, glutathione reductase, and ascorbate peroxidase) as well as gene expression of MAPK3 (mitogen-activated protein kinases), GS (geissoschizine synthase), and ORCA3 (octadecanoid-derivative responsive AP2-domain). These genes are related to biosynthesis of alkaloids which improve plant tolerance against stress.

25.4.2 Nanoparticles in Alleviation of Salt and Drought Stress

NPs are used as weapons to fight with stress, and these nullify or minimize the deteriorative effects of stresses on plants. These are employed either in soil or through foliar spray. From soil, these particles enter inside the plants through root and from leaves these enter through stomata.

In the plants, NPs alter their morphological, physiological, and biochemical as well as molecular states positively and negatively. Most of the studies reported positive modulation in their tolerance mechanisms against drought and salt stress and improved plant growth and development and ultimately production (Ali et al. 2021; Zulfiqar and Ashraf 2021).

25.4.3 Nanoparticles in Alleviation of Salt Stress

The protective role of ZnO NPs in tomato against salt stress was reported by Hosseinpour et al. (2020). In other study, salt stress negatively affected growth parameters (plant height, number of leaves, fresh and dry biomass of root and shoot) of Moringa peregrina but application of Hoagland solution containing ZnO and Fe3O4 NPs improved growth attributes as well as biochemical parameters of plants under normal and saline conditions (Soliman et al. 2015). Various reports on the exogenous application of NPs to mitigate salt stress are depicted in Table 25.1.

Table 25.1 List of nanoparticles used in amelioration of salt stress in plants
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25.4.4 Nanoparticles in Alleviation of Drought Stress

Yang et al. (2017), Borišev et al. (2016), and Mohammadi et al. (2016) also reported the protective role of CuO and ZnO NPs in Triticum aestivum, Fullerenol nanoparticles in Beta vulgaris L., and TiO2 NPs in Dracocephalum moldavica L. against drought stress. In wheat, ZnO and CuO NPs interacted with microorganisms in rhizosphere and improved plant growth and development under drought stress. Various reports on the exogenous application of NPs to mitigate drought stress are depicted in Table 25.2.

Table 25.2 List of nanoparticles used in amelioration of drought stress in plants
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