**2.2 Abiotic stress**

Physical or chemical environment puts adverse effect on the living organisms and thus, creates abiotic stress. In agriculture, abiotic stress results in impairment of normal growth and development of plants and thereby, crop yield, quality and farmers' income get hampered. It is further noted that in environment, several stresses together put combined effect on plant and thereby, deteriorate crop productivity and quality in greater extent as compared to their individual effect. It adversely affects the crop from morphological to physiological, biochemical or molecular levels. Abiotic stress includes water stress (drought and flood), salinity stress, temperature stress

**Figure 1.** *Various types of stress.*

(heat and cold), heavy metal toxicity, nutrients and pesticides toxicities, light stress (high and low). Some other types of abiotic stresses are shade, UV exposure, photoinhibition, air pollution, wind velocity etc. In following section, descriptions of these stresses are briefly mentioned.

#### *2.2.1 Water stress*

Water is the precious input for any life and obviously, for the crop. Plant's photosynthesis, transpiration, nutrient uptakes, translocation of assimilates etc. depend on water and thereby, its imbalance in plant can cause serious damage to plant. The uneven distribution of water is resulted from climate change scenario which the globe is experiencing now. Generally, water stress occurs in the form of drought and flood.

*Drought*: In meteorology, drought is defined as the period of deficiency of rainfall from the normal in an area. In hydrology, drought arises from low river and stream flows and reduced ground water table. In agriculture, drought indicates the dry period during the crop growth period which critically hampers the crop growth and yield. Sometimes, plant is unable to uptake soil moisture even after its availability due to high salt concentrations in soil solution. This phenomenon is known as apparent drought. Drought can also be resulted from high and low temperature. High temperature induces evapotranspiration loss of water and thus, creates drought like situation. In case of cold condition like at freezing temperature, ice crystals are formed in the extracellular spaces of plant, resulting in loss of the water potential which finally causes intracellular water efflux. Drought stress can lead to loss of plant vitality and alter in plant's normal functioning.

*Flood*: Flood is the opposite situation of drought. It is the situation of excess water in an area for a period of time. Flood when occurs from the sudden outburst of cloud and results in excessive rainfall in a short spell of time is known as flash flood. Flash flood lasts for a short period of time generally up to few weeks. Another type of flood is deep water flood which last for a longer period of time.

#### *2.2.2 Salinity stress*

Salinity is a major problem in any part of the world especially in arid and semi-arid areas where potential evapotranspiration is higher than the rainfall and poor leaching of salts beyond root zone is seen due to insufficient rainfall. Salinity can be defined as presence of excess quantity of salts in the soil, which negatively impacts on the crop [9]. Salinity is measured using electrical conductivity (EC). In general, soil having EC >4 dS/m, exchangeable sodium percentage (ESP) < 15.0 and pH. <8.5 is considered as saline soil [10]. Saline soil contains chloride, sulfate salts of sodium, magnesium and calcium ions. Salinity in soil can be developed by both natural and anthropogenic means. Weathering of rocks, flooding and intrusion of sea water to agricultural land, seepage of saline water, wind blow etc. are the natural cause of build-up of salts in the soil. Human induced causes of soil salinity are poor water quality of irrigation, deforestation, overgrazing, intensive cropping etc. Excessive soil salinity leads to deterioration of soil health through changing cation exchange capacity, damaging soil physical structure through deflocculation and reduction of hydraulic conductivity and hampering soil microbial activity.

#### *2.2.3 Temperature stress*

Abrupt change in temperature in response to global climate change is a great concern. Increase of temperature due to global warming and anthropogenic causes can change biodiversity, crop ecosystem and thus, limit the crop production to a high extent. High temperature or heat stress induces higher respiration over photosynthesis and thereby, leads to starvation injury through loss of food reserves in plants. Plants are differentiated based on their degree of high temperature tolerance: psychrophiles (up to 15–20°C), mesophiles (up to 35–45°C) and thermophiles (up to 45–100°C) [11]. Low temperature or cold stress is the opposite phenomena of heat stress and is mostly experienced in temperate areas. It is of two types: chilling stress and freezing stress. Chilling and freezing stresses affect the crop's physiological, biochemical functions and thereby, hamper its growth and productivity. According to the tolerance to cold temperature, plants are classified into 3 categories: Chilling sensitive (Plants are extremely sensitive above 0°C and below 15°C), chilling resistant (plants tolerate low temperature but adversely affected under formation of ice crystals in extracellular spaces) and frost resistant (plants tolerate extremely low temperature).
