Functions of Phosphorus in

hosphorus is one of 17 essen- Energy Reactions tial for plant growth. Its functions cannot Phosphorus plays a vital role in virtually Pbe performed by any other , and every plant process that involves energy trans- an adequate supply of P is required for opti- fer. High-energy , held as a part of mum growth and reproduction. Phosphorus is the chemical structures of adenosine diphos- classified as a major nutrient, meaning that it phate (ADP) and ATP, is the source of energy is frequently deficient for that drives the multitude of crop production and is Phosphorus (P) is vital to chemical reactions within the required by crops in relative- plant growth and is found in plant. When ADP and ATP ly large amounts. The total P every living plant . It is transfer the high-energy concentration in agricultural involved in several key plant phosphate to other molecules crops generally varies from functions, including energy (termed ), the 0.1 to 0.5 percent. transfer, , stage is set for many essential transformation of processes to occur. Uptake and Transport and starches, nutrient of Phosphorus movement within the plant Photosynthesis Phosphorus enters the and transfer of genetic The most important chem- plant through hairs, root characteristics from one ical reaction in nature is pho- tips, and the outermost layers generation to the next. tosynthesis. It utilizes light of root cells. Uptake is also energy in the presence of facilitated by mycorrhizal fungi that grow in chlorophyll to combine and association with the of many crops. water into simple sugars, with the energy Phosphorus is taken up mostly as the primary being captured in ATP. The ATP is then avail- - orthophosphate (H2PO4 ), but some is able as an energy source for the many other also absorbed as secondary orthophosphate reactions that occur within the plant, and the = (HPO4 ), this latter form increasing as the sugars are used as building blocks to produce pH increases. other cell structural and storage components. Once inside the plant root, P may be stored in the Chlorophyll root or trans- Photosynthesis = Carbon Dioxide + Water + Carbohydrates ported to the Phosphate Energy upper portions of the plant. Through various chemical reac- Genetic Transfer tions, it is incorporated into organic com- Phosphorus is a vital component of the pounds, including nucleic (DNA and substances that are building blocks of genes RNA), phosphoproteins, , and chromosomes. So, it is an essential part of , enzymes, and energy-rich phos- the process of carrying the genetic code from phate compounds...for example, adenosine one generation to the next, providing the triphosphate (ATP). It is in these organic “blueprint” for all aspects of plant growth and forms as well as the inorganic phosphate ion development. that P is moved throughout the plant, where it An adequate supply of P is essential to is available for further reactions. the development of new cells and to the transfer of the genetic code from one cell to

6 Better Crops/Vol. 83 (1999, No. 1) another as new cells are formed. Large quan- and growth and development of the plant to tities of P are found in seeds and fruit where it proceed at a normal pace. is believed essential for seed formation and When P is limiting, the most striking development. effects are a reduction in leaf expansion and Phosphorus is also a component of leaf surface area, as well as the number of phytin, a major storage form of P in seeds. leaves. Shoot growth is more affected than root About 50 percent of the total P in legume growth, which to a decrease in the shoot- seeds and 60 to 70 percent in cereal grains is root dry weight ratio. Nonetheless, root growth stored as phytin or closely related compounds. is also reduced by P deficiency, leading to less An inadequate supply of P can reduce seed root to reach water and nutrients. size, seed number, and viability. Generally, inadequate P slows the processes of carbohydrate utilization, while Nutrient Transport carbohydrate production through photosyn- Plant cells can accumulate nutrients at thesis continues. This results in a buildup of much higher concentrations than are present carbohydrates and the development of a dark in the soil solution that surrounds them. This green leaf color. In some plants, P-deficient allows roots to extract nutrients from the soil leaves develop a purple color, tomatoes and solution where they are present in very low corn being two examples. Since P is readily concentrations. mobilized in the plant, when a deficiency Movement of nutrients within the plant occurs the P is translocated from older tissues depends largely upon transport through cell to active meristematic tissues, resulting in membranes, which requires energy to oppose foliar deficiency symptoms appearing on the the forces of osmosis. Here again, ATP and older (lower) portion of the plant. However, other high energy P compounds provide the such symptoms of P deficiency are seldom needed energy. observed in the field...other than loss of yield. Other effects of P deficiency on plant Phosphorus Deficiency growth include delayed maturity, reduced Adequate P allows the processes quality of forage, fruit, vegetable, and grain described above to operate at optimum rates crops, and decreased disease resistance.

World Production... (continued from page 5)

1975 to 1979 (Figure 1). Consumption reached a high of 798,164 tons of P2O5 in declined to an average of 4.3 million tons 1985 (Figure 1). Consumption declined to yearly during the 1985-1989 , but 637,175 tons in 1991, but has since has since begun increasing, averaging increased to 775,370 tons in 1997. About about 4.5 million tons since 1995. 75 percent of Canada’s phosphate con- Illinois led the U.S. in phosphate con- sumption occurs in the prairie provinces sumption in 1997, followed by Iowa, (Alberta, Manitoba and Saskatchewan). Minnesota and Texas (Table 3). Others in

the top 10 consuming states Canadian consumption, included Nebraska, California, 6,000 1,000 5,000 800 Indiana, Kansas, Missouri, and 4,000 '000 tons 600 North Dakota. These 10 states 3,000

'000 tons 400 accounted for 55 percent of 2,000 1,000 U.S. Canada 200 U.S. phosphate consumption. U.S. consumption, Canadian consumption of 0 0 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 phosphate followed Year similar trends to the U.S., but Figure 1. Consumption of P2O5 in North America.

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