1. Nanoparticles in Salt-Stress Tolerance
Global-warming-driven water scarcity also forces irrigation with saline water in agricultural lands all over the world, which leads to enhanced salt content in the soil. Salinity (the buildup of excessive salt in the soil) is one of the main challenges to modern agriculture, and it eventually stunts and impairs plant growth and development, ending in plant mortality
[1][2]. Most plants die when the NaCl content is higher than 200 mM. Salinity has a significant impact on every stage of the plant’s life cycle, including seed germination, seedling development, vegetative growth and blooming
[3]. Numerous horticultural crops, such as fruits, vegetables and spices, are impacted by salinity. In addition to causing osmotic stress, water stress, oxidative stress, nutritional stress and reduced cell division, salt stress imbalances ionic strength, which has an impact on a number of biochemical, physiological and metabolic processes
[4][5]. The response to various abiotic stress has been illustrated in
Figure 13.
Figure 13.
Nanoparticles involved in combating abiotic stress.
According to Zulfiqar and Ashraf
[6], the application of nanoparticles, such as Zn NPs, Ag NPs, SiO
2 NPs, Cu NPs, Fe NPs, Mn NPs, C NPs, Ti NPs, Ce NPs and K NPs, was effective in mitigating the toxic effects of salt stress in various plants. El-Sharkawy et al.
[7] found that the foliar application of K NPs in salt-sensitive
Medicago sativa improved salt tolerance by reducing electrolyte leakage and enhancing the proline and antioxidant-enzyme content, such as that of catalase. Similarly, reduced oxidative stress was evident in the lower MDA and ROS levels and higher antioxidant activity in AgNPs-treated pearl millet plants, which may have been caused by a decrease in Na
+ absorption in the leaves
[8]. Cerium-oxide nanoparticles were discovered to be beneficial in increasing photosynthetic activity in
Brassica napus by altering the root cells and thus improving the mineral uptake
[8][9]. Increasingly prevalent data suggest that applying nanoparticles to plants can considerably reduce the detrimental impacts of salt stress, and thus also control plant adaptations.
2. Nanoparticles in Drought-Stress Tolerance
Drought is regarded as the most detrimental environmental stress, reducing crop yield more than any other. According to the Intergovernmental Panel on Climate Change (IPCC), the average temperature will rise by 1.8 to 4.0 °C by 2100, and drought will affect vast areas of the world
[10]. Drought affects agriculture when plants have insufficient moisture to develop normally and complete their life cycles. The severity of drought is further increased by a continuous decline in precipitation and increase in evapotranspiration demand
[11]. For instance, drought stress prevents plant development, because water is required for cell turgor, which is the pressure that a contained liquid exerts on cell walls, causing cells to expand
[12]. The principal effects of drought on crop plants include slower rates of cell division and growth, smaller leaves, longer stems and roots, disordered stomatal oscillations, altered water and nutrient relationships with lower crop output and inefficient water usage
[11].
As per previous studies, NPs cause a variety of morphological, physiological and biochemical changes in plants as they increase their resistance to drought stress by increasing plant root hydraulic conductance and water uptake and demonstrate a differential abundance of proteins involved in oxidation-reduction, ROS detoxification, stress signaling and hormone pathways
[13]. The foliar application of metal-oxide nanoparticles, such as titanium dioxide (TiO
2), zinc oxide (ZnO) and iron oxide (Fe
3O
4), were found to be effective in enhancing the plant’s physiological and metabolic activities under drought stress
[14]. When Si NPs were applied to drought-stressed pomegranate plants, additional improvements were made to their photosynthetic pigments, nutrient status, physical and chemical parameters (especially those related to fruit cracking), phenolic content and concentrations of osmolytes, antioxidant enzymes and abscisic acid
[15]. El-Zohri et al.
[16] suggested that green ZnO-NPs administered topically at lower concentrations could successfully boost tomato tolerance to drought stress. In addition to nanofertilizers, green synthesized Fe
3O
4 NPs were also found to be effective in reducing the impact of drought stress on fenugreek plants
[17]. However, a study by Potter et al.
[18] indicates that the potential benefits of using NPs in enhancing plant drought resistance only actualize under specific environmental circumstances.
3. Nanoparticles in Cold-Stress Tolerance
Global climate change also contributes to cold or low-temperature stress, which harms plant growth and development. Plants often experience two types of low-temperature stress: chilling and freezing. Chilling temperatures for plants range from 0 to 15 °C, depending on the species and tolerance level of the plant. The air temperature and wind speed during exposure are other factors that affect chilling temperatures. In contrast to its response to chilling temperatures, the plant will battle against freezing temperatures (below 0 °C)
[19]. Crop species can be hurt or killed by low and nonfreezing temperatures, which can have an impact on their productivity, survival and ecological dispersion
[20]. As enzyme and other-protein activity are reduced at colder temperatures, cold stress slows down plant growth
[12]. Numerous processes in these plants are impacted by low temperatures, including those involved in secondary metabolism, respiration, defense and protein and nucleic acid production
[19].
Chitosan nanoparticles and TiO
2 NPs have been used extensively in a variety of studies for their efficiency in cold-stress tolerance. The application of Ti NPs was found to be effective in improving electrolyte leakage, photosynthetic activity and membrane damage under cold-stress conditions in chickpea plants using transcriptional regulation
[21][22][23]. Hasanpour et al.
[24] suggest that when TiO
2 NPs are applied to plants, the tolerance of chickpea plants to cold stress may develop by controlling the pressure of the temperature drop injury and altered metabolism for plant growth. The deleterious effects of cold stress are reduced and glycyrrhizin content is enhanced when using TiO
2 NPs in licorice plants
[25]. The use of chitosan nanoparticles was found to be effective in reducing the ROS with the accumulation of osmoprotectants in banana plants under cold-stress conditions
[26]. Furthermore, in rice plants, the foliar application of ZnO NPs may reduce chilling stress through the antioxidative system and transcription factors involved in the chilling response
[27]. Similarly, the use of SiNPs can also improve the photosynthetic ability of sugarcane plants under chilling stress
[28].
4. Nanoparticles in Heavy-Metal-Stress Tolerance
Heavy-metal (HM) stress is one of the deleterious factors that reduces crop productivity in the modern day. Human activities, such as industrialization and urbanization, have resulted in HM pollution all over the world
[29]. Enhanced implementation of modern agricultural tools, such as chemical pesticides and fertilizers, has also contributed to HM stress in crop plants. Heavy metals such as Hg, Pb, Cd, Ni, Co, Cr and Ag have deleterious impacts on plants
[30]. Since plants reside at the baseline of trophic systems, the chances of bioaccumulation of these HMs via the food chain are high, and this eventually leads to chronic health impairments, such as kidney and liver damage, in humans and other animals. In addition, HMs have a direct impact on plants, such as through morphological and physiological abnormalities and impaired metabolic pathways
[31]. These affect the quality and quantity of plant-based products, especially in agricultural crops and medicinal plants.
A number of studies on the use of nanoparticles to alleviate HM stress have been conducted. Nanoparticles applied to the soil can absorb and transform the HMs in soil, thereby reducing the bioaccumulation and mobility of HMs. The Cd metal availability in soil has been reduced by the application of Fe
3O
4 NPs
[32]. The hydroxyapatite NPs can reduce the toxic effects of metals in soil and can maintain the soil pH by releasing phosphate ions
[33]. NPs also induce the formation of apoplast barriers, which reduce the heavy-metal content in the root. Furthermore, heavy metals can be intercepted by the regulation of metal transporter genes in plants using specific NPs, which can deter the translocation of HMs by forming complexes with them
[34]. NPs such as SiNPs have endorsed the production of organic acids that curtail the damage of HM stress
[35][36]. NPs also activate the antioxidant system, thereby reducing the stress caused by ROS
[36].
5. Nanoparticles in Flooding-Stress Tolerance
Most plants are sensitive to flooding as a result of excessive water clogging in soil. Flooding is caused either by excessive rainfall, poor soil drainage or irrigation practices. The complete submersion of plants in floodwater can be disastrous for crops. Flooding is thus one of many abiotic-stress factors that affect food availability and countries’ economies. It influences the plants grown in different ecosystems, such as floodplains, riparian zones, salt marshes, tidal zones and wetlands. Plants grown in different ecosystems show varied responses to flooding stress; wetland plant species show tolerance to shoot submergence and soil water logging, while dry-land species are sensitive to flooding stress. Excessive water logging in air spaces delays the exchange and diffusion of gas between the roots (rhizosphere) and the atmosphere, thereby inhibiting respiration due to a lack of oxygen leading to hypoxia and ultimately leading to anoxia in plants. Under flooding stress, soil pH and redox potential will be affected, the carbon-dioxide content increases and the mobilization of phytotoxins increases, affecting the root metabolism, nutrient uptake and overall plant growth
[37].
Nanoparticles have been reported to alleviate flooding stress in plants. In soybean plants under flooding stress, silver NPs helped to alleviate stress conditions by regulating amino-acid synthesis, proteins, glycolysis and wax formation, and NPs enhanced the growth of soybean plants despite stress
[38][39]. Another study has been conducted into soybean plants under flooding stress, where Al
2O
3 NPs were applied to ameliorate the growth impairment induced by flood stress. The Al
2O
3 NPs increased root length, including that of the hypocotyl, suppressed the proteins involved in glycolysis, arbitrated the cells involved in the scavenging of ROS by upregulating the ascorbate/glutathione pathway (AsA/GSH) and increased the ribosomal proteins
[40].
6. Nanoparticles in Heat-Stress Tolerance
High temperatures can cause heat stress. In recent decades, global warming has worsened this trend. The rise in temperature above a critical limit for a longer time sufficient to permanently harm plant development is often understood to constitute heat stress
[41]. Extreme changes may damage the intermolecular connections required for optimal growth during hot summers, which would hinder plant development and fruit set
[42]. In general, heat stress decreases the effectiveness of photosynthetic processes, shortening the plant life cycle and lowering productivity
[43]. Heat stress may become a significant issue restricting field-crop productivity in tropical and subtropical areas.
The application of Se NPs in sorghum plants exposed to high temperatures was found to be helpful in ameliorating negative impacts, such as membrane damage and reduced pollen germination and crop yields, by activating the antioxidant defense system
[44]. The application of AgNPs shielded wheat plants from heat stress by enhancing morphological growth
[45]. Similar to this, Zn nanoparticles were discovered to be helpful in improving wheat’s ability to withstand heat stress by increasing the production of antioxidant enzymes and decreasing lipid peroxidation
[46]. The foliar application of nanoparticles on tomato leaves becomes activated when the temperature exceeds certain limits and protects plants from heat stress. Si NPs are also said to be helpful in coping with heat stress
[47].