Introduction

Plants in nature constantly come under threat from abiotic and biotic stresses (Xe et al. 2019). Stresses like drought, salt, and temperature form the crucial environmental factors regulating the geographically diversified distribution of plants in nature, limiting agricultural plant productivity and threatening food security. Soil salinity is a major global issue limiting crop productivity and quality in many arid and semiarid regions (Xe et al. 2019). Drought is yet another significant abiotic stress that harms the majority of cultivated crops, particularly in semiarid and arid places (Naveed et al. 2014; Bodner et al. 2015). Drought stress with climate change is expected to cause critical plant growth problems for more than 50% of arable lands by 2050 (Vinocur and Altman 2005).

Additionally, heavy metal pollution is on the rise in the modern era, causing environmental issues (Ahemad 2012; Etesami 2018). Because heavy metals are difficult to remove from the environment, environmentalists are concerned about their toxicity to various environmental-habitats.

Saline environment and stress

Soil salinity, a major challenge for agriculture, is one of the significant environmental stress worldwide, converting agronomically valuable lands into unproductive areas by 1-2% each year in the arid and semi-arid zones (Rasool et al. 2013). Soil salinization has rendered approximately 7% of the world's land and 20% of the total cultivated area non-arable (Rasool et al. 2013; Etesamia and Maheshwari 2018). When the electric conductivity (EC) of the soil solution reaches 4 dS m-1 (equivalent to 40 mM NaCl), an osmotic pressure of about 0.2 MPa is generated and hence, significantly reduces the yields of several crops (Munns and Tester 2008). A major abiotic constraint on agriculture worldwide is salinity in irrigated water and soils (Colla et al. 2010; Acosta-Motos et al. 2017a, b).

Salinity can limit the growth and development of most plants, resulting in lower plant yield (Xie et al. 2019). Furthermore, salinity causes significant changes in plant growth and metabolism (for example, physiological, morphological, and biochemical changes) (Gupta and Huang 2014). Plants must activate various physiological and biochemical mechanisms in order to cope with the resulting stress under saline conditions (Xie et al. 2019). Changes in morphology, anatomy, water relations, photosynthesis, the hormonal profile, toxic ion distribution, salt compartmentalization, and exclusion (Wang et al. 2016) are examples of such mechanisms, as are biochemical adaptations (such as the antioxidative metabolism response) (Hernandez et al. 2001; Parida and Das 2005; Acosta-Motos et al. 2015; Acosta-Motos 2017b). All enzymatic scavengers interact in plants to overcome salt stress and promote growth and development (Acosta-Motos 2017b). According to several studies, increased expression of enzymatic antioxidants induced by salt treatment suggests an effective way to reduce saline toxicity. However, some studies have found that the salinity extent, exposure time, and plant developmental stage all affect the expression levels of these enzyme genes (Guan et al. 2015; Cunha et al. 2016; Li et al. 2019).

Plants are classified based on their ability to thrive in saline environments (Acosta-Motos et al. 2017b). Glycophytes are unable to grow in the presence of high salt levels; NaCl concentrations of 100-200 mM inhibit or even completely prevent their growth, resulting in plant death (Munns and Termaat 1986). Growth inhibition can occur even in the short term (Hernandez et al. 2002). Halophytes, on the other hand, can survive in the presence of high NaCl concentrations (300-500 mM) as they have developed better salt resistance mechanisms, which are unique to these plants (Parida and Das 2005; Flowers and Colmer 2015). Euhalophytes (plants that grow in salty environments) can deal with the damaging effects of salt stress by developing various resistance mechanisms (Acosta-Motos et al. 2017b). These plants can maintain their salt content in several different manners, such as salt exclusion (prevents salts from entering the vascular system); salt elimination (salt-secreting glands and hairs actively eliminate salts, keeping the salt concentration in the leaves under control); salt succulence (if the storage volume of the cells gradually increases with salt uptake as the cells steadily take up water and can be kept reasonably constant for long periods); salt redistribution (Na⁺ and Cl^- are easily translocated in the phloem, allowing high concentrations in actively transpiring leaves to be redistributed throughout the plant).

Stress due to water unavailability

Drought (water stress) is one of the most severe environmental stress and can occur due to a variety of factors such as high and low temperatures, high light intensity, low rainfall, and salinity (Salehi-Lisar and Bakhshayeshan-Agdam 2016). Drought is an influential environmental stress for plant growth that ultimately results in yield reduction in a warming world, particularly for commercial crops such as rice, wheat, and maize (Furlan et al. 2016). Drought stress harms plant growth and metabolic processes, mainly in field-grown crops, by changing water relations, photosynthetic assimilation, and nutrient uptake (Heffernan 2013). Nonetheless, plants have evolved a variety of strategies to reduce damage during drought conditions (Shah et al. 2017). The sheer number of plant species grown in climatic regions with extreme dry conditions suggests that plants have evolved to withstand drought stress through morphological, physiological, and biochemical adaptations (Farnese et al. 2016).

Drought resistance refers to plant species exhibiting adaptations that enable them to escape, avoid, or tolerate drought stress (Levitt 1980). Drought escape refers to a plant species' ability to complete its life cycle before the onset of drought. These plants do not experience drought stress as they have the ability to modulate their vegetative and reproductive growth in response to water availability, primarily through either of the two ways: rapid phenological development and plasticity in development (Jones et al. 1981). Rapid phenological development includes speedy plant growth and generation of a limited number of seeds before soil water depletes. Plants with developmental plasticity exhibit minimal growth all throughout the dry season, with few flowers and seeds but grow indefinitely during the wet season, yielding plenty of seeds. Drought avoidance refers to plants' ability to keep up (relatively) higher tissue water levels regardless of reduced soil water content (Levitt 1980, Basu et al. 2016). It accomplished by utilizing a variety of adaptive traits involves water loss minimization (water savers) and water uptake optimization (water spenders).

Cold environment as stress

In addition to drought stress, low temperature is another most damaging environmental stress that higher plants face (Theocharis et al. 2012; Zhou et al. 2017). According to the ambient temperature, low-temperature stress classified as chilling stress (20°C) or freezing stress (0°C). In addition to affecting plant growth and development, it significantly impacts on the geographical distribution of the plants (Guo et al. 2017; Liu and Zhou 2018; Shi et al. 2018). Tropical and subtropical plants are susceptible to chilling stress and lack cold acclimation capacity. Temperate plants, on the other hand, can tolerate freezing temperatures after a phase of non-freezing temperature exposure, a process known as cold acclimation (Chinnuswamy et al. 2007). They are resistant to seasonal temperature changes and can withstand cold stress in the early spring and winter.

Cold stress responses in plants are highly complex events that modify the biochemical composition of cells to protect them from damage caused by low temperatures (Xe et al. 2019). Complex signalling cascades come into play to change the expression of cold-responsive genes, enabling vegetation to withstand chilling or even freezing temperatures. Chilling induces oxidative stress resulted in lipid peroxidation, chlorophyll degradation, etc., chilling tolerance is thus mainly associated with antioxidant enzyme activities enhancement (Xe et al. 2019). Furthermore, cold stress has a negative impact on plant morphologies, leading to growth repression and decreased yields.

Heavy metals and toxicity

Heavy metals such as iron (Fe), manganese (Mn), copper (Cu), nickel (Ni), cobalt (Co), cadmium (Cd), zinc (Zn), mercury (Hg), and arsenic (As) have been accumulating in soils for a long time as a result of anthropogenic activities, industrial waste, and sewage disposal (Aydinalp and Marinova 2009). Heavy metal pollution has become such an issue since the industrial revolution, induced an increase in the relevant scientific research (Stankovic et al. 2014). All heavy metals are non-biodegradable, means that they cannot be purged naturally from the environment through any natural means. Some of these are immobile (they cannot move from where the accumulated site), while others are mobile (they can be taken up by plants root systems via diffusion, endocytosis, or phagocytosis or through metal transporters) (Ozturk et al. 2008; Sabir et al. 2015; Burakova et al. 2018). Heavy metals are primarily stored in soil and transported to the food chain via plants that grow in that soil (Etesami 2018). Nonetheless, some metals, such as zinc, copper, and nickel, are necessary micronutrients in trace amounts because they act as cofactors for various enzymes. Additional metals found in pesticides, such as Cd and lead (Pb), play no beneficial role and turn toxic when their concentration rises above a certain threshold (Gough et al. 1979; Sharma and Ali 2011; Ghori et al. 2019). Heavy metals harm heavy metal stressed plants' growth, biomass, and photosynthesis (Nagajyoti et al. 2010; Ali et al. 2015). Heavy metals interfering with the absorption and distribution of essential nutrients in plants cause nutrient deficiency and imbalance in plants subjected to metal stress (Etesami 2018; Sharma and Archana 2016). When the amount of heavy metal is controlled to a usual level, plants demonstrate their ability to avoid the negative effects (Juknys et al. 2012). There is already evidence that high levels of heavy metals impair homeostasis and increase ROS production in plant cells (Shahid et al. 2014). Heavy metals absorbed by plants are involved in several mechanisms due to their redox ability that produce free radicals. Fe, Cu, chromium (Cr) (Valko et al. 2005), and other redox-active elements can take part in a redox-cycling reaction, resulting in the production of toxic hydroxyl radicals that severely damage living cells (Xe et al. 2019).

Surprisingly, plants are natural bio accumulators, meaning that they extract and concentrate various heavy metals from the soil and water that might or might not be required for proper growth (Ozturk et al. 2008). The rate of accumulation and plant tolerance to heavy metals vary by species, with some becoming toxic faster than others. Heavy metal toxicity can be manifested in plants as chlorosis, stunted growth, root browning, decline, and death (Ozturk et al. 2008, 2015); oxidative stress pronounced in the cells, and the production of stress-related proteins and hormones, antioxidants, signalling molecules, and heat-shock proteins generated. Plant’s various stress-resistance mechanisms are responsible for the homeostasis of essential metals (Rihan et al. 2017). These mechanisms also aim to protect plants from heavy metals in the soil or to provide tolerance to the plant by detoxifying the metals. Other mechanisms are specific to each stress and is activated when it encountered. The first line of defense in plants is reduction of metal uptake when stimulated by heavy metal toxicity, which includes the assistance provided by cellular and root exudates that prevent metals from entering the cell. Many plants possess unique systems for sequestering individual metal ions within the compartments to avoid exposure to sensitive cellular components. Detoxification mechanisms that chelate, transport, and sequester these metals are introduced as a second line of defense (Rihan et al. 2017).

Nutrient insufficiency

Mineral nutrient imbalance prevents plant growth and development in soil rich or poor in nutrients (Paul and Lade 2014). Soil salinization, competitive ion absorption, and movement or partitioning of ions inside the plant are just a few of the negative consequences of nutritional imbalances, which can occur when a given nutrient's physiological role is deactivated, increasing in internal plant demand for a specific essential element (Grieve and Grattan 1999). A large proportion of nutrients are unavailable to plants due to the binding of organic and mineral constituents to the soil; and a build-up of insoluble precipitates. Plant fitness is also affected by the essential element imbalances through an impact on plant nutrition and retention of water, as well as detrimental impacts on plant cells (Molnár et al. 2011).

Conclusion

Crop yield reduction, a consequence of abiotic stresses, results in significant economic losses. Because of the adverse impact of various abiotic stresses, food production is decreasing while the world's population is rapidly increasing. As a result, one of the primary goals of plant and crop specialists is to reduce these losses. On the other hand, abiotic stress brought about by climate change and human activities is extremely hard to control and requires the development of stress-tolerant crop genotypes. Furthermore, boosting plant stress resistance is vital for agricultural productivity and environmental sustainability. Non-biological stresses will continually increase in the near future due to global climate change. Thus, it is critical to focus on and conduct in-depth research on these stress factors to come up with solutions that will allow plants to thrive in a changing environment.

Conflict of Interest

The authors declare that there is no conflict of interest.

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