Phosphorus (P) - Macronutrients
In soil, phosphorus occurs almost exclusively in the form of orthophosphate. Substantial amount of P is associated with the soil organic matter. The major P-containing ions in soil solutions are HPO2-4 and H2PO4. Roots are capable of absorbing phosphate from solution low in phosphate content. The phosphate content of roots and xylem sap is about 100-1000 fold higher than that of the soil solution. This shows that phosphate is absorbed by plants against a very steep concentration gradient. Phosphate is highly mobile in the plant and can be translocated in upward or downward direction. The inorganic form of phosphate found in plants are orthophosphate and to a minor extent, pyrophosphate. The organic form of phosphates are compounds in which the orthophosphate is esterified with hydroxyl groups of sugars and alcohols or bound through a pyrophosphate bond to another phosphate group. You know that phosphorylated sugars and alcohols are the major intermediary compounds of metabolism.
Phosphate is also present in phospholipids. The nucleotide phosphates - ATP, UTP, GTP, CTP supply energy to various endergonic processes including active ion uptake and the synthesis of various organic compounds. Another important phosphorus containing compound is phytin which is mainly found in seeds. It is Ca or Mg salt of Phytic acid and is formed during seed formation. Phytic acid is the hexaphosphoric ester of inositol. Immediately after pollination there is an increase in P transport towards the developing seeds. Phosphorus in the phytin of seeds is regarded as a P reserve. During seed germination phytin is mobilised and converted into other phosphate forms that are needed in the metabolism of young plants. Plants suffering from P deficiency are retarded in growth. In cereals tillering is reduced. Generally, the symptoms of P deficiency appear in the older leaves which become darkish green in colour. The stems of many annual plant species suffering from P deficiency develop reddish colour due to enhanced formation of anthocyanin pigments.