*6.2.1 Ion homeostasis*

In plant growth, some ions must be inside of cell. Although ions such as nitrogen, potassium, calcium are present in soil, they cannot enter the plant cells as a result of competition with other ions with high concentrations. Presence of salt with high

concentration in soil complicates the intake of these ions which are effective in plant development. Plants make some responses to ensure that this negative effect is eliminated and for the continuity of low ion concentration. Dividing the ions to be added to plants instead of intaking them inside of cells as one time is considered as important action for plant growth and development [51]. In maintaining the low concentration of ions, in transportation to plants the cell membrane is important [67]. Thanks to proteins, channel proteins and semptomers, ions can be moved to the plant [68]. Antiporters are also used in transportation. These transporters are located in vacuolar membranes. V-ATPase are known as channels needed for the continuity of plant under salt stress [69]. Due to the excessive accumulation in the soil, when Na+ ion enters the cytoplasm, it wants to move to vacuoles and this transport is carried out by Na+ /H+ antiporters. In the cell metabolism of plant, the other role is cytoplasmic K+ homeostasis. Under salinity, K+ concentration undergoes a severe decrease [64]. K+ ions, which can be transmittable to cells by K+ transporters and membrane channels, have low concentration under salinity stress. There is an important factor in cell recruitment. When the extracellular K+ concentration is low, K+ transporters mediating high affinity of K+ uptake mechanisms allow affinity if extracellular K+ concentration is high. As a result of this, concentration of Na+ ion increases under salinity and with this increase Na+ competes with K+ and reduces K+ uptake into the cell [4, 51]. More K+ retention of roots in plants such as wheat, maize, beans have been observed as one of the mechanisms applied by plants to withstand salt stress [51–64]. Accumulation of K+ ions in cell increases under salinity stress [51, 70].

#### *6.2.2 Biosynthesis of osmoprotectants*

Osmoprotectants are high rated soluble compounds [71]. Certain organic compounds, which are sugars, amino acids, proline and osmoprotectants that are interchangeable compounds, are synthesized by plants under stress conditions according to their stress levels. Osmolytes are in charge of providing adaptation of plant to salt stress [71]. Quaternary ammonium compounds as betalain betanin which is synthesized only by a few members of *Plumbaginacease* family [72]. Amino acids like proline present in plant types are present in different type of plants from ammonium compounds like betalain betanin present in plant types. These osmoprotectant compounds can replace each other [73]. While accumulation of compounds varies with salt stress, most of the osmolytes try to make it easier to maintain osmotic balance and structure of plant cells with an uninterrupted water flow [8].

#### *6.2.2.1 Amino acids*

Free amino acids take part in reducing osmotic stress caused by high concentrations of salt [74]. Amino acids such as arginine, glycine, alanine, proline, leucine, valine, serine take part in regulation of cell [75]. Accumulation of these amino acids is done for solving the problems that the plant creates against salt stress. But amino acids such as methionine, arginine, which make up the majority of amino acids, decreases under stress condition unlike proline, which increases under salinity [51].

#### *6.2.2.2 Proline*

Proline, which has wide usage, is one of the most common osmolytes [76]. In high plants, which are found to be abundant, even under salt stress this content does not decrease, conversely it tends to increase [57]. Accumulation of proline

### *Plant Responses to Salt Stress DOI: http://dx.doi.org/10.5772/intechopen.93920*

is accepted as an important precaution to avoid salinity stress [76–77]. The effect of proline accumulation under salinity conditions varies from species to species. Increasing the proline content in plant cells is a positive factor in preventing negative effects occurring at cell level [76]. Even in negative conditions, the accumulation of proline helps the plant to grow [76].
