Download Why Plants Need Phosphorus (Missouri)

M I S S O U R I
Why Plants Need Phosphorus
By D.G. Blevins
H
ealthy soybean leaves track the sun Photosynthesis
One of the keys to “life on earth” is the
during the day in order to absorb a
maximum amount of light, whereas ability of plants to harvest energy from sunleaves suffering P deficiency may turn their light and trap it in the form of high energy
edges toward the sun in order to absorb mini- phosphate bonds, to ultimately build carbohydrate molecules in the process of photosynthemum amounts of sunlight.
sis. Photosynthesis in plants
Phosphorus is an imporPhosphorus (P) is involved in
involves P in many ways.
tant essential macronutrient
photosynthesis, seed formaSugars made early in this
required by all plants for
tion, and numerous other
process are mainly triose
growth, development and
plant functions.
phosphates and hexose phosreproduction.
phates. Phosphate must also
There are many important biochemicals in plants that contain P. enter the chloroplast in order for triose phosPhospholipids are the primary structural com- phates to get out of the chloropast for use in
ponent of membranes that surround each other parts of the cell and in other plant parts.
plant cell and organelle. Inside the cell, genet- This phosphate/triose phosphate exchange
ic information in the form of DNA and RNA reaction is critical for proper movement of
molecules contains P as an integral structural sugar made in photosynthesis. In fact, in soycomponent. These important molecules form beans that are deficient in P, small sugars canthe genetic information in the plant and guide not exit chloroplasts properly. These sugars
the synthesis of proteins that work inside then accumulate and form large starch crystals which eventually cause structural damage
cells.
to chloroplasts and shut down photosynthesis.
Proteins
Once proteins are made, when and Water Movement
One of the most striking features of plants
where they work may be regulated by events
that again involve P. Much of the metabolism is their ability to move water in the xylem tisinside cells is controlled by phosphorylation sue. Xylem tissue is like an open piping sysor dephosphorylation of certain proteins in an tem where water and nutrient elements move
important class of proteins called enzymes. from roots to leaves. Water flow up the xylem
The addition or removal of phosphate then tissue is very responsive to P and increases
becomes a key signaling mechanism for what with high levels of P nutrition.
The control of the activity of proteins in
is happening inside a plant cell. The source of
phosphate for signaling events is ATP (adeno- plants by phosphorylation or dephosphorylasine triphosphate). Besides this role, ATP is tion is of critical importance in many plant
also the major energy currency in the cell. processes. Several proteins have unique strucThis molecule contains high energy phosphate tures that form gated channels through plant
bonds, which store and supply energy for cel- membranes, and these channels open and
close depending on whether or not they are
lular functions.
Better Crops/Vol. 83 (1999, No. 2)
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Better Crops/Vol. 83 (1999, No. 2)
Photo source: Lauer and Blevins
phosphorylated. These gated
channels control, among other
things, mineral and water transport in plants.
An adequate level of P
nutrition has been found critical
for proper magnesium (Mg) and
calcium (Ca) uptake by roots
and translocation up the xylem
to leaves. The flow of Mg, Ca,
and water up the xylem may be Soybeans grown with adequate P nutrition (left) can rotate their
dependent upon availability of leaves during the day to maximize interception of sunlight. With
the high energy currency, ATP. inadequate P nutrition, soybean plant leaves may have edges
With high levels of ATP, more P turned toward the sun and absorb minimal amounts of sunlight.
would be available for phosphorylation reactions opening Mg, Ca, and water strategies include association with mycorchannels in plant cells. More work needs to be rhizal fungi which attach to plant roots and
done on specific channels in plant roots to develop hyphae that penetrate the soil, extract
determine their responses to the level of phos- P, and then deliver it to the plant root in
phate nutrition and ATP concentration in exchange for sugar. Some plants secrete
organic acids, such as citric acid and malic
cells.
acid, which form complexes with aluminum
(Al) and Fe, releasing P for uptake by roots.
Seeds
During the late stages of plant reproduc- Roots also secrete special enzymes like phostive growth when young seeds are being phatases, which break down organic forms of
formed, P is remobilized from older leaves and P in soil and make the P available for uptake
moves into developing seeds. This makes a lot by plant roots. Some plants have developed
of sense, because seeds will contain important unique root architecture that helps them
genetic information in the form of DNA. ‘mine’ P from the soil. These strategies are a
Phosphorus in seeds is also stored in phytic few of the plant survival techniques that occur
acid molecules. Each molecule of phytic acid under stressful conditions.
contains six carbon atoms and six P atoms,
and each of the P atoms has a negative charge. Deficiency Symptoms
When plants are suffering from P defiTherefore, the entire molecule contains six
negative charges which can attract positively ciency, they have “hidden hunger” or they
charged cations like potassium (K), Mg, Ca, may show obvious visible symptoms. Visible
copper (Cu), zinc (Zn), and iron (Fe). This is an effects of P deficiency are small, dark leaves
effective way for seeds to store P and impor- and in some cases, purple coloration of stems
and leaves. Roots of some plants suffering
tant cations for the next generation of plants.
severe P deficiency may grow longer and skinnier than normal. This is fascinating since P
Availability
Low soil level of plant-available P is a deficient plants would not be equipped to hancommon condition around the world. Even dle a rapid rate of photosynthesis.
though the total soil P may be high, the P is
tightly bound to organic and inorganic soil Dr. Blevins is with Department of Agronomy,
components and is unavailable for uptake by University of Missouri, 1-87 Agriculture Bldg.,
roots. However, plants have developed many Columbia, MO 65211. Phone: 573-882-4819, Fax:
strategies for gaining access to bound P. These 573-882-1469, e-mail: [email protected]