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Role of Nanoparticles in Abiotic Stress

Nanotechnology is currently seeking much attention of researchers because of their wide applications in diverse sectors including agriculture. The influence of nanoparticles on physiological state of plants at the different levels of their organization, beginning from molecular, has been studied at various plants. It is known that nanoparticles in different concentrations can impact both positive and negative biological effects. Nanomaterials confer profound uses for sustainable crop production, reducing loss of nutrients, suppression of diseases and thereby enhancing the yields. Concerning the role of nanomaterials in alleviating the damage of plant abiotic stresses or in inhibiting plant growth and its toxicity, further studies are essential under different levels including plant molecular and cellular levels. A wide variety of research has been conducted to study plant responses to waterlogging stress that include various disciplines like molecular, biochemical, and physiological, anatomical and morphological examinations. Nano technological implications for curbing water-logged conditions recently came into limelight and have drawn much attention in the last few years. Nanotechnology is defined as the systems and processes which operate at a scale of 100 nm or less. Nanotechnology has many applications in the field of agriculture. There are majority of nano-materials which are known for its plant growth promoting effects. Nanoparticles have unique physiochemical properties such as high reactivity, particle morphology, and large surface area. They also boost the plant metabolism.


Introduction
Population explosion during the last few decades has led to increased pressure on the agriculture sector by an upsurge of continuously increasing food demand. Natural resources of the world are continuously diminishing at a much faster pace than their renewal and the agriculture sector is no exception to this presently prevailing scenario. Sustainability issues due to population explosion, climate change, urbanization, habitat loss assisted by environmental issues are some of the global challenges faced by the green plants including the agriculture sector [1]. Plants, the vital component of our planet remain always exposed to different environmental variations and numerous stress factors throughout their life. Unlike animals, plants are deprived of motility

Abiotic stress in crops and current scenario
Plants are constantly exposed to various stress factors throughout their life span. As per the data available, the relative decreases in potential maximum yields associated with abiotic stress factors vary between 54 and 82% [17]. Crops confront various types of abiotic stress and it has been well documented as well that among stresses, extreme temperatures (freezing, cold, heat), water availability (drought, flooding), and ion toxicity (salinity, heavy metals) are the major causes which adversely affect the plant growth and productivity worldwide [18][19][20][21]. These abiotic stresses are interconnected to osmotic stress that results in the disruption of ion

Role of nanotechnology in abiotic stress
Nanotechnology is a platform for developing tools and technology for the improvement of the bio system [25]. Nanoparticles (NPs) are small molecular aggregates with dimensions of 1-100 nm [26]. NPs have been investigated to improve plant growth, development, and productivity and thus, proving their use to overcome various abiotic and biotic stress of crops [27]. In the last decade, the science of nanotechnology has attained, a promising position to mitigate the constraints associated with the aforementioned stresses to achieve a secure future of agriculture worldwide and nano technological findings possess immense potential to open up numerous ways in the field of biotechnology and agriculture [28,29]. Some of them are discussed here in detail:

Heat stress
The negative impacts of heat stress (also Thermal stress) on plants are substantial, detrimental, and often account for reduced crop yield and productivity as well. Adverse thermal environments pose a great challenge for crop plants to sustain and survive. In addition to it, another major concern remains the global climatic change i.e., an overall increase in the average global temperature of the earth that had led to increased thermal stress on plants and other organisms along with altered patterns of precipitation. Leaving aside all these problems, defining and quantifying heat stress remains a daunting task. In general, heat stress is categorized relative to some estimate of an optimal thermal range that is characteristic of each species in question.
Heat stress involves elevated temperature at such a harsh level for a long enough time that could result in irreparable loss to the development of plants [30,31]. Heat stress enhances the Reactive Oxygen Species (ROS) generation and causes oxidative stress, as a result of which membrane lipid degeneration and leakage of membrane ion occur which led to degradation of the protein [32][33][34][35], in addition to decreased rate of photosynthesis and chlorophyll content [36]. Several studies have been conducted by many workers from time to time to access the applicability of nanotechnology to minimize heat stress. Selenium nanoparticle application in the low concentration found reducing the effect of heat stress by increasing hydration ability, chlorophyll content, and development of plant [37]. Also, Selenium nanoparticles at low concentrations exhibit antioxidative properties to plants, while oxidative stress had been induced by the high concentration of Se nanoparticles [38,39]. Plants synthesize several heat shock proteins and molecular chaperones during the period of heat stress [40]. Other proteins are assisted by heat shock proteins in sustaining their fidelity in stress conditions [30] and are involved in heat stress resistance. It was already in reports that multiwall carbon nanotubes could upregulate gene expression of heat shock proteins viz. HSP90 [41]. Also, maize plants exposed to CeO2 nanoparticles depicted excessive generation of H2O2 and upregulation of HSP70 [42]. Furthermore, TiO2 nanoparticles treatment reduced the effect of heat stress by stomatal opening regulation [43].

Salinity
Salinity, a major type of abiotic stress factor, limits the production of food and deteriorates the quality of ever-increasing growth in food crops. For scientific communities, increased salinity remains a major constraint to attain sustainable crop production. Worldwide, 20% of cultivated land is facing salinity stress and the amount is increasing day by day. The majority of crop plants species belong to the category of glycophytes, which are highly susceptible to salt stress hence are the most critical environmental abiotic stress that can ruin crop production [44,45]. Most salinity problems arise due to excess sodium chloride (NaCl) which is widely distributed along with coastal and arid region soils and water supplies. Higher levels of NaCl impose at least three types of problems for higher plants. These include: (i) the osmotic pressure in the external solution can exceed the osmotic pressure in the plant cells and therefore require an osmotic adjustment by the cells to avoid desiccation; (ii) sodium, in excess, can disrupt the uptake and transport of nutritional ions such as K and Ca; and (iii) both Na and Cl can exert direct toxic effects on membranes and enzyme systems [46]. Besides the aforementioned problems, lowering of soil osmotic potential, creation of nutritional imbalance, enhancing specific ionic toxicity (salt stress), or one or more combination of these factors, are some of the common implications that salinity stress exerts on crop plants. Most vital processes of plants like photosynthesis, protein synthesis, and lipid metabolisms, etc. are badly affected by salinity stress [47]. Salt stress is associated with oxidative stress too. However, to confront salt stress-induced oxidative stress, plants are very well equipped with a defense system of various antioxidant enzymes that include Role of Nanoparticles in Abiotic Stress DOI: http://dx.doi.org/10.5772/intechopen.99928 superoxide dismutase (SOD) and peroxidase (POD). The SOD constitutes the first line of defense against ROS [48] and dismutase superoxide radicals to H2O2, whereas POD reorganizes H2O2 into water and oxygen [49]. Besides oxidative stress, salt stress also creates osmotic stress, which reduces the ability of plants to take up water and minerals [50]. Also, plants have been found to abide by osmotic stress by the provision of accumulation of osmolytes, such as proline (Pro) and Glycine Betaine (GB) [51]. Application of nano-fertilizers is a quite hopeful method that can potentially increase plant resource use efficiency and help in reducing environmental toxicity due to the accumulation of unused chemical fertilizers and pesticides in the soil. Therefore, the application of nano-fertilizers could serve as an alternative approach to overcome soil toxicity issues and other associated stresses.
Adverse effects of salinity stress on crop plants have been extensively studied by many workers from time to time. Hussein and Abou-Baker [52] conducted experiments to study the foliar application of nano zinc to mitigate the adverse effect of salinity and confirmed that diluted seawater could be used in the irrigation of the cotton plant. They reported that increasing the application rate of nano-Zn may reduce phosphorous (P) absorption and translocation to leaves and consequently reduce the P/Zn ratio. They suggested that an additional dose of P-fertilizer with nano-Zn could be used to avoid the P/Zn imbalance. Avestan et al., [53] in their investigations proposed that salinity stress treatments were detrimental to morphological and physiological parameters of strawberry plants. They found that nSiO2 treatments suppressed the negative effects of salinity, possibly by improving the Epicuticular Wax Layer (EWL); and nSiO2 treatments enabled salt-stressed plants to better maintain their chlorophyll content and leaf relative water content (RWC) and relative water protection (RWP) relative to controls (no SiO2). They concluded their findings by suggesting three possible directions for future research: (1) Further exploring how variation in the timing of silicon treatments influences EWL deposition by testing EWL at multiple plant developmental stages; (2) investigation of whether there is genetic variation for EWL deposition in strawberry; and (3) testing to distinguish the benefit of greater EWL deposition in saline conditions relative to the benefit of the other signaling and physiological changes that are linked to increased silicon uptake.
Khan [54] in his studies investigated the effect of nano TiO2 in several plant developmental processes including defense against environmental stresses. They concluded that the cumulative effect of the parameters under consideration contributed to improved growth and yield of tomato plants. Therefore, based on the assessment of results it was propounded that nano-TiO2 at the rate of 20 mg/l proved best in enhancing the growth, yield, and quality of tomatoes. In one more study, conducted by Yassen et al., [55] on the cucumber (Cucumis sativa) effect of silicon dioxide nanoparticles was assessed where the results indicated an increase in nitrogen and phosphorus, content and uptake and decrease in Na content and uptake when adding SiO2 nano fertilizer. The findings of the study suggested that silicon dioxide nano fertilizer can exert a positive effect on the growth and yield of cucumber.

Heavy metal stress
Nano biotechnology growing as a technology that could make the environment cleans. Nanoparticles, often regarded as particles having a significant amount of surface area with unique physical and chemical properties and having applications in reducing the negative effects of heavy metals on the natural wealth [56,57]. Some workers have exploited nanotechnology to explore plant phytotoxicity caused by heavy metals in various environments. Although nanoparticles are cost-effective in reducing heavy metal toxicity in plants [58], mitigation of heavy metal-induced root growth inhibition and oxidative stress in the plant has been barely studied [58,59].
Heavy metal ions were productively adsorbed by magnetic nanoparticles (Fe3O4) [57]. In addition, Nanoscale zero-valent iron (nZVI) nanomaterials are core-shell structures that are in use for decreasing metal toxicity. Ronavari et al. [60] reported that nZVI nanoparticles are for immobilizing heavy metal ions due to their distinct structure. Also, Fajardo et al. [61] found that lead and zinc mobility and availability decreased when soils were treated with nZVI. The addition of nZVI and active carbon efficiently immobilized copper, lead, cadmium, and chromium in sediments, thus, decreasing the bioavailability and toxicity of heavy metals [62].
Nano hydroxyapatite (nHAp) particles are also in use to remediate metal toxicity. nHAp have been successfully applied to remediate soils contaminated by metals and to purify wastewater due to their outstanding ability to absorb heavy metals like copper (II), zinc (II), lead (II), and cadmium (II) [63]. Zhang et al. [64] found that nHAp effectively decreased the exchangeable fractions of Pb and Cd in contaminated sediments, especially for Pb, and dramatically decreased the metal(loid) ion concentration in pore water.

Drought stress
Plants have been always combating water stress for millions of years, ever since they first left the water bodies and conquered and colonized dry land. When drought strikes, higher plants are the first victims that have always been obliged to endure it or to adjust their life cycles to avoid it. Thus, a major means of propulsion behind the evolution and emergence of land plants has been their need to search for water, to absorb it, to transport it, and retain it. Even so, drought is still the major constraint to crop production [46,70,71]. The term 'Drought' does not merely represent lack of rainfall instead for plant physiologists, it is a concurrence of various environmental stresses that includes: (i) low soil moisture availability; (ii) high evaporative load, (iii) high temperature, (iv) high solar irradiance, (v) increased soil hardness, (vi) unavailability of nutrients and (vii) accumulation of salts in the topsoil region.
Taran et al., [72], in their studies have shown that Cu-Zn-nanoparticles reduced the negative effect of drought action upon plants of steppe ecotype Acveduc. In particular, increased activity of antioxidative enzymes reduced the level of accumulation of Thiobarbituric Acid Reactive Substances (TBARS) and stabilized the content of photosynthetic pigments, and increased relative water content in leaves. Colloidal solution of Cu-Zn-nanoparticles had a less significant influence on these indexes in seedlings of the Stolichna variety under drought. They studied the use of binary compositions of nanoparticles in agro-technologies to enhance the biological productivity of agriculture systems. Ashkavand et al., [73] studied the effect of SiO2 nanoparticles on drought resistance in hawthorn seedlings and concluded that silicon nanoparticles (SNPs) can increase plant resistance to drought stress. It could be explained by the improvement of photosynthesis rate and stomatal conductance by SNPs pretreatments. Application of silicon on two sorghums (Sorghum bicolor (L.) Moench) cultivars possessing different drought susceptibility exhibited improved drought tolerance irrespective of their drought susceptibility by lowering shoot to root (S/R) ratio, which perhaps could be an indicator of improved root growth and the maintenance of the photosynthetic rate. These findings could be attributed to improving the drought tolerance of sorghum via the augmenting water uptake efficiency of plants [2,74]. Applications of silver nanoparticles Role of Nanoparticles in Abiotic Stress DOI: http://dx.doi.org /10.5772/intechopen.99928 (AgNPs) has also been appreciated in diminishing negative effects of drought stress on lentil (Lens culinaris Medic). Significant effects of different concentrations of Polyethylene glycol (PEG) and silver nanoparticles on germination rate and germination percentage, root length, root fresh, and dry weight in lentil seeds were reported [75]. In a study, conducted by Sedghi et.al [76], it was observed that nano zinc oxide has the potential to enhance seed germination percentage thereby, overcoming water stress.

Water logging
Over irrigation, prolonged periods of precipitation coupled with poor soil drainage system gives rise to a condition called 'water logging'. Both, natural vegetation and agriculture crops are equally affected by this worldwide occurring condition of waterlogging. The waterlogged soils offers/presents an unpleasant and uneasy environment for normal growth and development of plants because: (i) air spaces occupied by water delays the exchange and diffusion of gases between the roots (rhizosphere) and atmosphere [77]; (ii) levels of dissolved oxygen are depleted from soil solution by respiration of soil inhabitants and roots [78] and (iii) flooding of fields is often associated with the release of toxic compounds and obnoxious gases. Depending upon the height of the water column produced, flooding can be classified as (i) waterlogging, when it is superficial and encase only the roots, and (ii) submergence, when water completely covers the aerial plant tissues [79]. In both types of flooding, the movement of oxygen from the air to plant tissues is highly disrupted [80], producing a natural condition known as hypoxia (<21% O2) [79].
Depending upon certain parameters like temperature, microbial respiration activity, frequency, and duration of soil saturation, the depletion of dissolved oxygen in waterlogged soils leads to conditions called 'hypoxia 'and 'anoxia' within few hours to days. In recent years, flooding stress and its subordinates like submergence, waterlogging, hypoxia, and anoxia, were investigated extensively in plants, especially in Arabidopsis and rice, to pinpoint molecular elements that may play a vital role in flood tolerance. Roots of the plants remain the first victims that are worst hit by flooding. Plant roots facing waterlogging stress follow glucose metabolism according to the classical scheme of alcoholic fermentation in an oxygen deficit medium (anaerobiosis), where self-poisoning of tissues takes place as a result of the formation of end products of fermentation mainly ethanol. Maintenance of an appropriate oxygen supply and energy balance is paramount for the survival of the root system to waterlogging stress.
Nanotechnology has provided new discernment to the problems arising in plants and food science (post-harvest products) and offers novel approaches to the rational selection of raw materials. Silver Nano Particles (SNPs) are the most commonly used nanomaterials in the field of nanotechnology after carbon nano-tubes that every day is added in its application to the nano-world. In this sense, nanoparticles are useful tools as an excessive water supply induces hypoxia in plants [80], increases the vulnerability to pathogen attack [81], and limits the flow of light to the plant [82].
During recovery after a flooding event, plants experience oxidative stress [83] and must remobilize nutrients to achieve a normal homeostatic state [84]. Concerning the protection of plants against oxidative stress, nanomaterials are found to mimic the role of first-line defense antioxidative enzymes like peroxidase, superoxide dismutase, and catalase, which are supposed to form the antioxidant defense grid [85]. Also, plants respond to flooding and the associated stress by changes in gene expression that are finely regulated at a multilevel scale from epigenetics [86] to transcriptional [80,87] and translational regulation [88]. Rezvani et al., [89] conducted experiments to study the effect of Nano silver ions (as an ethylene inhibitor on the growth of Saffron (Crocus sativus) under flooding conditions. Corms of saffron were soaked with different concentrations of nanosilver ranging from 0 to 120 ppm (0, 40, 80, and 120) and planted under flooding stress or non-flooding stress conditions and the results of the investigations showed that the number of roots, root length, fresh and dry weight of roots and leaves were reduced by 10-day flooding stress. Soaking the saffron corms with 40 or 80 ppm concentration of Nano silver rewarded the effect of flooding stress on the root number by increasing it. Also, it was found that 40 ppm of nano-silver increased the root length in stress. 80 ppm concentration of nano-silver was found to increase leaves dry weight. In another study conducted on the same plant (C. sativus) under flooding stress, foliar application of Nano silver was accessed by Sorooshzadeh et al., [90]. Results of the investigations showed that flooding stress led to a significant reduction in weight and height of the plant and the number of corms per plant was increased by increasing the concentration of nano-silver. In all, they concluded that flooding stress and Nano silver had a significant interaction effect on all parameters under consideration of the study.

Mechanism of abiotic stress control by nanoparticles (NPs)
Developing technology for improving food production, minimizing crop productivity loss is the prerequisite for obtaining sustainability in the field of agriculture. Abiotic stress of plants is considered a major emerging problem in the field of agriculture, its diverse types include salinity drought, waterlogging, submergence, heavy metal stresses, and mineral and metal toxicity/deficiencies that minimize crop growth and productivity [91][92][93]. A decrease in productivity is mainly attributed to these factors. Plant throughout their lifespan has to face various types of abiotic stress and has to come up with strong defense mechanisms to cope up with them. Investigation on NPs has reported that they help plants to overcome abiotic stress by their concentration-dependent impact on plant growth and development [73,[94][95][96]. It is also reported that various antioxidant enzymes like catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) were found to enhance their activity using NPs [97]. Depending upon their chemical composition, size, surface covering, reactivity, NPs interact with plants in various ways causing many morphological and physiological changes and play a very vital role in improving crop plants. NPs has both positive and negative effect on plant growth and development [98]. Some NPs along with their possible effect (negative and positive) on a plant are depicted in Figure 2.

Conclusion
Nanotechnology, a multi-disciplinary approach, has emerged out as a powerful discipline in the last few years and is revolutionizing various fields like medicine, agriculture, industrial, environment, electronics, etc. Nanotechnology is emerging as a tool for agriculture by empowering it with tools to conquer nutritional poverty and food scarcity. Nanoparticles are proven beneficial to boost plant growth, development, and increase yield capacity and help to overcome biotic and abiotic stress. The use of nanotechnology will lay a strong platform and will permit a secure future towards sustainability, crop productivity, and overcome abiotic stresses, where loss can be minimized and yield could be enhanced. The most effective way for understanding the action of the mechanisms of NPs applications is to apply the present knowledge by collaborating with various disciplines that may include molecular biology, plant physiology, plant breeding, cytology, soil physics along nanotechnology. Such associations could be helpful for the encouragement of multi-disciplinary projects that may be carried worldwide. Nanotechnology promises new insights into the mechanism of various abiotic stress tolerance in plants to complement physiological studies. Also, there is a need to detangle various factors responsible for abiotic stress. The implementation of action mechanisms of NPs will require information and expertise from the aforementioned disciplines to combat various stress effects. The applicability of nanotechnology needs to be commercialized from laboratory to agricultural fields.