Phosphorus is utilized in the fully oxidized and hydrated form as orthophosphate. Plants typically absorb either H2PO4- or HPO42-, depending on the pH of the growing medium. However, under certain conditions plants might absorb soluble organic phosphates, including nucleic acids. A portion of absorbed inorganic phosphorus is quickly combined into organic molecules upon entry into the roots or after it is transported into the shoot.
Phosphate is a trivalent resonating tetraoxyanion that serves as a linkage or binding site and is generally resistant to polarization and nucleophilic attack except in metal-enzyme complexes (9). Orthophosphate can be condensed to form oxygen-linked polyphosphates. These unique properties of phosphate produce water-stable anhydrides and esters that are important in energy storage and transfer in plant biochemical processes. Most notable are adenosine diphosphate and triphosphate (ADP and ATP). Energy is released when a terminal phosphate is split from ADP or ATP.
Total phosphorus in plant tissue ranges from about 0.1 to 1%. Bieleski (10) suggests that a typical plant might contain approximately 0.004% P as deoxyribonucleic acid (DNA), 0.04% P as ribonucleic acid (RNA), 0.03% as lipid P, 0.02 % as ester P, and 0.13% as inorganic P.
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