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Abiotic Stress in Plants

Living on the same planet, plants means a lot to us. No matter being taken, as our food or treated with great commercial significance, plants are so indispensable that we have to learn how to protect, make use of, and most important of all, get on well with them. In the first place, what we all understand is: plants are distinguished from us or other animals by being unable to escape from the surrounding circumstances. Thus, when they are confronted with living-threaten pressures, their only choice is to try their best to adjust to them.


Introduction
In the current scenario, population explosion has emerged as one of the major challenges, especially for sustainable food production, in feeding the growing population . The world population may reach up to 10.9 billion by 2100 and will lead to an increase in demand for food by nearly 50%. To achieve the "Zero Hunger" goal, which is one of the goals of sustainable development of the UN to be achieved by 2030, there is an urgent need for revolutionizing conventional agricultural practices. Such changes can be achieved by employing eco-friendly and sustainable innovations .
Plants are unable to move physically from their location to prevent the consequences of environmental stress such as abiotic stresses. Among different abiotic stresses, heavy metal (HM) contamination, soil salinity, and drought stress are described to limit the crop productivity by multiple orders of magnitude . These changes under abiotic stress trigger perturbations in the metabolism of plants, thereby facilitating reorganization of the metabolic network in order to keep the vital metabolic processes active . Soil pollutants, especially HMs and metalloids such as Cr, Cd, Ni, Zn, As, and Hg, are identified as the most commonly detected contaminants . The increased release of HMs in the terrestrial environment has been documented to severely affect the productivity of cultivated areas .
Furthermore, most of the metal contaminants eventually find their way into the terrestrial and aquatic environment, thereby directly or indirectly affecting human health and the associated ecosystems .
Additionally, there are ample chances of accumulation of HMs in plants exposed to contaminated areas .
Salinity and drought stresses are devastating stresses that are reported to limit the economic yield of several crops via inducing biochemical and physiological perturbations . These stresses confine the plant productivity and growth due to osmotic stress, nutritional imbalance, and oxidative stress . Nanoparticles as soil-improving agents, nano-fertilizers, nano-pesticides, growth stimulators, and nano- In order to obtain in-depth knowledge on field applications of NPs, the present review aimed to discuss the challenges of different abiotic stresses causing substantial changes in crops at the morphological, anatomical, biochemical, and physiological levels and the possible roles and mechanisms of NPs for mitigating the negative consequences of abiotic stresses to improve the agricultural productivity.

Alleviation of Heavy Metal Toxicity in Plants Using Nanoparticles
Excessive release of HMs in the environment by an exponential rise in anthropogenic activities and industrial processing is of great concern . The risks of HM contamination in cultivated fields and aquatic environments due to the indiscriminate addition of various agro-fertilizers are of considerable concern . The abundance of HMs in a given environmental matrix beyond certain limits exerts toxicity because of accumulation and genotoxic, carcinogenic, and mutagenic behaviors .
The HMs and metalloids associated with the environmental and human health concerns include Cu, Zn, Cd, Cr, Pb, As, and Hg . Therefore, it is imperative to develop innovative and economical technologies for the successful elimination of HMs from contaminated sites. However, the characteristic toxicity at low concentrations, slow removal using conventional approaches, and non-biodegradable The contribution of myriads of NPs in overcoming the challenges of HM-induced toxicity has been [2] [55] [56] presented by various researchers worldwide (Table 1). In general, it has been observed that NPs minimize the uptake of HMs by modifying the expression of genes responsible for metal uptake and by reducing HM bioaccumulation. Furthermore, NP treatment improves the physiological and biochemical parameters of the plants such as enhancing the synthesis of defense enzymes (SOD, POX, CAT, APX, etc.); augmenting nutrient uptake; decreasing the loss of electrolytes; improving pigments and soluble proteins; reducing peroxidation; and causing rise in the levels of proline, glutathione, and phyto-chelatins. These attributes are primarily responsible for the overall increase in the tolerance of the crops and may vary slightly according to different plant species.

Alleviation of Salinity Stress in Plant Using Nanoparticles
Salinity has emerged as a global concern due to steady increases in salt-affected land throughout the    The application of Zn NPs to salt-stressed Brassica napus plants alleviated the salinity-induced detrimental impacts by upregulating the antioxidative mechanism, osmolyte biosynthesis, and ionic control . In Solanum lycopersicum, Cu NPs applied to the leaves mitigated salinity stress by improving growth and the Na /K ratio. Moreover, Cu NPs improved the level of glutathione, polyphenols, and vitamin C content as compared to the control. Additionally, the activities of APX, GPX, and SOD were also modulated, thereby improving the overall plant's normal growth and development . In addition, seed priming with ZnO NPs (60 mg/L) ameliorated the detrimental consequence induced by the NaCl treatment in Lupinus termis via increasing the pigments, osmoregulation, and regulation of the contents of stressassociated metabolites. In another study, the seed priming of T. aestivum L. with Ag NPs was also proven to be an adequate salinity stress management strategy .
Recently, a study depicted that the exogenous application of salicylic acid+nano-Fe O to Trachyspermum ammi L. alleviated salinity stress to a considerable extent via increasing K uptake, K /Na ratio; iron content; activities of various antioxidative enzymes viz. SOD, catalase (CAT), peroxidase (POD), and phenol peroxidase (PPO); and the contents of the compatible solutes. These modifications collectively led to the improvement in membrane stability index, leaf water content, pigments, and growth of the plants (Table 2) . Increased chlorophyll content and photosynthetic attributes, protein content, and activities of antioxidative enzymes (POX, SOD, and CAT) in salinity-stressed tomato plants.
Seed priming with TiO positively impacted the germination (germination percentage, germination energy, and seedling vigor index) and seedling growth (lengths of root and shoot, fresh, and dry weight) and reduced the mean emergence time.
Results showed the enhancement in potassium ion concentration, relative water content, contents of total phenolic and proline contents; increased SOD, CAT, and PAL activities; and decreased sodium ion concentration, membrane electrolyte leakage, and MDA content.

Germination and Morphological Responses
Physiological Responses References

Alleviation of Drought Stress in Plants Using Nanoparticles
Drought stress is reported to have severe consequences for crops, including reduced leaf area, reduced growth, limited carboxylation, decreased water potential, hormonal imbalance, and oxidative stress . It is frequently associated with high temperature due to increased water loss through evapotranspiration. Owing to this reason, plants reduce the leaf water and turgor pressure. These consequences are also associated with the stomatal closure, which in turn decelerates the plant's metabolism and ceases vital enzymatic reactions. In addition, the severe water shortages eventually contribute to stunted crop growth and finally death .
An insight into the morphophysiological responses and consequences of plants against drought stress is presented in Figure 1. The key factors under drought stress are the severity and duration of the stress, which could be directly correlated with drought stress-induced loss in crop productivity and economic yield . Furthermore, salinity combined with drought stress led to a decrease in water potential, but the osmotic potential decreased more significantly .
Applications of NPs to drought stress plants have been observed to improve photosynthetic rate, stomatal conductance, relative water content, and ameliorated cell membrane damage by lowering the contents of stress metabolites and electrolyte leakage. Furthermore, increases in osmolyte contents, carotenoid content, chlorophyll content, protein content, and phenolic substances (i.e., rosmarinic acid and chlorogenic acid) and improved activities of antioxidant enzymes such as CAT, SOD, and POX have also been found as a general mechanism in overcoming and mitigating drought stress. [