phosphorus in plants

Phosphorus is one of 17 nutrients essen-tial for plant growth. Its functions cannotbe performed by any other nutrient, and

an adequate supply of P is required for opti-mum growth and reproduction. Phosphorus isclassified as a major nutrient, meaning that itis frequently deficient forcrop production and isrequired by crops in relative-ly large amounts. The total Pconcentration in agriculturalcrops generally varies from0.1 to 0.5 percent.

Uptake and Transport of Phosphorus

Phosphorus enters theplant through root hairs, roottips, and the outermost layersof root cells. Uptake is alsofacilitated by mycorrhizal fungi that grow inassociation with the roots of many crops.Phosphorus is taken up mostly as the primaryorthophosphate ion (H2PO4

-), but some is also absorbed as secondary orthophosphate(HPO4

=), this latter form increasing as the soilpH increases.

Once inside the plant root, P may bestored in theroot or trans-ported to theupper portionsof the plant. Through various chemical reac-tions, it is incorporated into organic com-pounds, including nucleic acids (DNA andRNA), phosphoproteins, phospholipids, sugarphosphates, enzymes, and energy-rich phos-phate compounds...for example, adenosinetriphosphate (ATP). It is in these organicforms as well as the inorganic phosphate ionthat P is moved throughout the plant, where itis available for further reactions.

Plant Energy ReactionsPhosphorus plays a vital role in virtually

every plant process that involves energy trans-fer. High-energy phosphate, held as a part ofthe chemical structures of adenosine diphos-phate (ADP) and ATP, is the source of energy

that drives the multitude ofchemical reactions within theplant. When ADP and ATPtransfer the high-energyphosphate to other molecules(termed phosphorylation), thestage is set for many essentialprocesses to occur.

PhotosynthesisThe most important chem-

ical reaction in nature is pho-tosynthesis. It utilizes lightenergy in the presence of

chlorophyll to combine carbon dioxide andwater into simple sugars, with the energybeing captured in ATP. The ATP is then avail-able as an energy source for the many otherreactions that occur within the plant, and thesugars are used as building blocks to produceother cell structural and storage components.

Genetic TransferPhosphorus is a vital component of the

substances that are building blocks of genesand chromosomes. So, it is an essential part ofthe process of carrying the genetic code fromone generation to the next, providing the“blueprint” for all aspects of plant growth anddevelopment.

An adequate supply of P is essential tothe development of new cells and to the transfer of the genetic code from one cell to

6 Better Crops/Vol. 83 (1999, No. 1)

Functions of Phosphorus in Plants

Phosphorus (P) is vital toplant growth and is found inevery living plant cell. It isinvolved in several key plantfunctions, including energytransfer, photosynthesis,transformation of sugarsand starches, nutrientmovement within the plantand transfer of geneticcharacteristics from onegeneration to the next.

ChlorophyllPhotosynthesis = Carbon Dioxide + Water Sunlight Oxygen + Carbohydrates

Phosphate Energy

another as new cells are formed. Large quan-tities of P are found in seeds and fruit where itis believed essential for seed formation anddevelopment.

Phosphorus is also a component ofphytin, a major storage form of P in seeds.About 50 percent of the total P in legumeseeds and 60 to 70 percent in cereal grains isstored as phytin or closely related compounds.An inadequate supply of P can reduce seedsize, seed number, and viability.

Nutrient TransportPlant cells can accumulate nutrients at

much higher concentrations than are presentin the soil solution that surrounds them. Thisallows roots to extract nutrients from the soilsolution where they are present in very lowconcentrations.

Movement of nutrients within the plantdepends largely upon transport through cellmembranes, which requires energy to opposethe forces of osmosis. Here again, ATP andother high energy P compounds provide theneeded energy.

Phosphorus DeficiencyAdequate P allows the processes

described above to operate at optimum rates

and growth and development of the plant toproceed at a normal pace.

When P is limiting, the most strikingeffects are a reduction in leaf expansion andleaf surface area, as well as the number ofleaves. Shoot growth is more affected than rootgrowth, which leads to a decrease in the shoot-root dry weight ratio. Nonetheless, root growthis also reduced by P deficiency, leading to lessroot mass to reach water and nutrients.

Generally, inadequate P slows theprocesses of carbohydrate utilization, whilecarbohydrate production through photosyn-thesis continues. This results in a buildup ofcarbohydrates and the development of a darkgreen leaf color. In some plants, P-deficientleaves develop a purple color, tomatoes andcorn being two examples. Since P is readilymobilized in the plant, when a deficiencyoccurs the P is translocated from older tissuesto active meristematic tissues, resulting infoliar deficiency symptoms appearing on theolder (lower) portion of the plant. However,such symptoms of P deficiency are seldomobserved in the field...other than loss of yield.

Other effects of P deficiency on plantgrowth include delayed maturity, reducedquality of forage, fruit, vegetable, and graincrops, and decreased disease resistance.

Better Crops/Vol. 83 (1999, No. 1) 7

1975 to 1979 (Figure 1). Consumptiondeclined to an average of 4.3 million tonsyearly during the 1985-1989 period, buthas since begun increasing, averagingabout 4.5 million tons since 1995.

Illinois led the U.S. in phosphate con-sumption in 1997, followed by Iowa,Minnesota and Texas (Table 3). Others inthe top 10 consuming statesincluded Nebraska, California,Indiana, Kansas, Missouri, andNorth Dakota. These 10 statesaccounted for 55 percent ofU.S. phosphate consumption.

Canadian consumption offertilizer phosphate followedsimilar trends to the U.S., but

reached a high of 798,164 tons of P2O5 in1985 (Figure 1). Consumption declined to637,175 tons in 1991, but has sinceincreased to 775,370 tons in 1997. About75 percent of Canada’s phosphate con-sumption occurs in the prairie provinces(Alberta, Manitoba and Saskatchewan).

6,0005,0004,0003,0002,0001,000

0

1,000

800

600

400

200

01960 1964 1968 1972 1976 1980 1984 1988 1992 1996

U.S

. con

sum

ptio

n,

'000

tons

Canadian consumption,

'000 tons

U.S. Canada

Year

Figure 1. Consumption of P2O5 in North America.

World Production... (continued from page 5)