Phosphorus is utilized in the fully oxidized and hydrated form as orthophosphate. Plants typically
absorb either H2
, 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.
The transfer of phosphate
molecules to ATP from energy-transforming processes and from ATP to energy-requiring
processes in the plants is known as phosphorylation. A portion of the energy derived from photosynthesis
is conserved by phosphorylation of ADP to yield ATP in a process called photophosphorylation.
Energy released during respiration is similarly harnessed in a process called oxidative phosphorylation.
Beyond their role in energy-transferring processes, phosphate bonds serve as important linkage
groups. Phosphate is a structural component of phospholipids, nucleic acids, nucleotides, coenzymes,
and phosphoproteins. Phospholipids are important in membrane structure. Nucleic acids of genes and
chromosomes carry genetic material from cell to cell. As a monoester, phosphorus provides an essential
ligand in enzymatic catalysis. Phytic acid, the hexaphosphate ester of myo-inositol phosphate, is
the most common phosphorus reserve in seeds. Inorganic and organic phosphates in plants also serve
as buffers in the maintenance of cellular pH.
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.