**3.1 Water stress**

*Drought*: Under scarcity of water, the seed germination and early seedling establishment get hampered due to depletion of seed reserves as well as mechanical obstruction made by the hard soil, which are followed by restricted vegetative growth.

### **Figure 2.** *Influence of abiotic stresses on plant.*

#### *DOI: http://dx.doi.org/10.5772/intechopen.106045 Influence of Abiotic Stresses on Seed Production and Quality*

Some other major impact of drought includes reductions of leaf area, chlorophyll a, b, carotenoids, stomatal conductance, rubisco activity leading to poor photosynthesis and translocation of photo-assimilates from source to sink. It impairs photosystem (PS) I and II as well as reduces starch biosynthesis through hampering ribulose phosphatase activity. Consequently, dry matter production and development of reproductive structures are negatively impacted. Under drought stress, concentrations of cell solutes increase and pose toxic effects on plants. This stress further increases the intra- and inter-competitions for water among crop plants and between crop and weeds. It increases sucrose viscosity leading to restricted movement of sap inside plant. Under drought stress availability of nutrients are variably altered. For instance, availability of nitrogen increases, while phosphorus availability decrease and no distinct effect is seen on potassium under drought stress in root vicinity, resulting in variability in uptake by the plant and consequently, metabolism of nutrients in cell is affected [13]. Assimilation of ammonia to organic form is restricted as activities of nitrate reductase, glutamine synthetase etc. are reduced. C4 plants are more sensitive to drought stress than C3 plants due to closure of stomata which ultimately reduces photosynthesis [14]. Drought restricts mineral uptakes and nitrogen fixation ability in various leguminous crops.

*Flood*: Flood generally creates deficiency of oxygen/hypoxia as anaerobic situation is formed through either water logging or submergence. Flood reduces the movement of oxygen and other gases in root zone of plant and ethylene diffusion from plant, which causes chlorosis of plant leaves. Negative impact on plants due to flood includes wilting of shoot, loss of chlorophyll pigments, decay and death of leaves/aerial parts, abscission, epinasty, lenticel formation, build-up of toxins under hypoxic environment, less root respiration, root proliferation and other physiological disorders.

#### **3.2 Salinity stress**

Salinity stress possess two major primary effects on plants: osmotic stress and ion toxicity. As already mentioned earlier, this stress can induce the drought stress by restricting the uptake of water by plants from the soil through exerting higher osmotic pressure to root cell (osmotic pressure of soil solution> osmotic pressure inside plant cell sap). Even if water uptake occurs, water also carries lots of salts (Na+ , Cl− etc.) inside the plant along with it and these excess salt ions can pose injury at the cellular level of the plant by hampering some essential enzyme activities. Production of reactive oxygen species (ROS) owing from oxidative stress results in detrimental effect on protein, nucleic acid and certain enzymes of plant [15]. Under excess salts, plant can show burnt like appearance in leaves and experiences deficiency of some essential elements such as calcium (Ca2+), potassium (K+ ), magnesium (Mg2+), nitrate (NO3 − ) etc. and abundance of Na+ near root zone. Calcium (Ca2+), potassium (K+ ), magnesium (Mg2+) and nitrate (NO3 − ) play major role in photosynthesis and less uptake due to salinity leads to reduction of photosynthesis and translocation of assimilates from source to sink. It causes poor leaf expansion, stunted growth and less dry weight of plant, sterility of florets and loss of pollen viability in plant. Under excessive salt situation, there is an increase in epidermal thickness, mesophyll thickness, palisade cell length and diameter and spongy cell diameter and reduction of intercellular space in leaves. Salinity stress specifically, increases Na+ /K+ ratio of the cell. Imbalance supply of potassium reduces cell turgidity, enzyme activity and membrane potential of plant. It further induces some secondary effects on plants such as reductions of

cell expansion as well as division, membrane stability and cytosolic metabolism as dominance of Na+ in cell creates restriction of some essential enzyme activities. Under salinity stress, closure of stomata partially or completely reduces transpiration and cell division. Consequently, there is an increased rate of reduction of plant growth, high degree of defoliation as well as senescence and plant's death [16].
