**2. Biochemical and physiological functions of phosphorus**

Phosphorus is a major essential nutrient element for plants that plays a crucial role in every developmental stage of life from very early to adult. This element improves crop yield and quality, and the functions of P cannot be performed by any other nutrient [14]. A plant can neither complete its normal reproductive process nor reach its highest yield potential without an adequate supply of P because this macronutrient has a major contribution to multiple cellular functions. As a structural component of nucleic acids, sugars, and lipids, P takes part in all the growth and developmental processes at both cellular and whole plant levels as, for example, cell division/elongation, membrane structure maintenance, enzyme activation, or inactivation, biomolecule synthesis, photosynthesis, respiration, high-energy molecules formation such

*Sustainable Management of Phosphorus in Agriculture for Environmental Conservation DOI: http://dx.doi.org/10.5772/intechopen.113086*

carbohydrate metabolism, nitrogen fixation, seed germination, seedling establishment, root and shoot development, flower and seed formation, improves disease and stress resistance etc. (**Figure 1**).

Various biochemical functions of P are discussed in the following sections.

#### **2.1 Structural component**

The total P-concentration determines P compartmentalization within the plant cells. P makes up about 0.2% of a plant's dry weight [16, 17]. The optimum concentration for most of the crops is below 4.0 mg P g−1 dry weight [18–20]. Two forms of P function in plant tissues such as free inorganic orthophosphate form (Pi) and organic phosphate esters. Pi is metabolically active and is located in the cytoplasm. When P is excess, it is stored in the vacuole in esterified forms, and from this organelle P is supplied to cytoplasm on cellular demand. Nucleic acids, phospholipids, phosphorylated metabolites, and proteins are some of the esterified forms of P.

#### **Figure 1.**

*Schematic representation of the major physiological and biochemical functions of phosphorus in a typical legume plant. P has a significant contribution to photosynthesis, carbohydrate metabolism, energy release, transfer of nutrients and genetic materials, resistance to stresses and diseases, cell division and development, formation of plant organs such as flower, fruit, and seed, development of roots, nodules, etc., and thus being an indispensable element in plants life from germination up to maturity.*

### *2.1.1 Sugar phosphates*

Sugar phosphates, most of which are Pi esters of monosaccharides, are intermediate compounds of carbohydrate metabolism. Phosphorylation of carbohydrates followed by reaction with ATP forms sugar phosphates, some of which include phytic acid, glucose-6-phosphate, and dihydroxyacetone phosphate. As constituents of glycolysis and respiratory reactions, these phosphorylated compounds play significant roles in photosynthesis as well as the synthesis and breakdown of carbohydrates.
