Many plant crops need more phosphorus than is dissolved in the soil to grow optimally. In addition, crops are usually harvested and removed — leaving no decaying vegetation to replace phosphorus. Many farmers replenish phosphorus through the use of phosphate fertilisers. The phosphorus is obtained by mining deposits of rock phosphate.
Locally produced sulfuric acid is used to convert the insoluble rock phosphate into a more soluble and usable form — a fertiliser product called superphosphate. Adjusting the pH of the soil for efficient plant uptake of phosphate should be done prior to fertilisation. For example, adding lime reduces soil acidity, which provides an environment where phosphate becomes more available to plants.
When fields are overfertilised through commercial fertilisers or manure , phosphate not utilised by plants can be lost from the soil through leaching and water run-off. This phosphate ends up in waterways, lakes and estuaries. Excess phosphate causes excessive growth of plants in waterways, lakes and estuaries leading to eutrophication.
Steps are being taken in agriculture to reduce phosphate losses in order to maximise the efficiency of fertiliser and effluent applications. Notify me of new comments via email. Notify me of new posts via email. Struvite, a phosphorus fertilizer made from reclaimed wastewater.
Credit: Don Flaten Plants get phosphorus from the soil. Historical sources of phosphorus Bones Even though they did not know that bones were rich in phosphorus, farmers in places such as China and Wales recognized the benefits of using bones as a source of fertilizer many centuries ago.
Guano Another important but short-lived source of phosphorus fertilizer for Europe and North America in the mids was guano. Mining of phosphate rock in the island-country of Nauru, As of , the island is no longer mined, but the environment was greatly impacted. Photo Lorrie Graham retrieved from Flickr In the mids, chemists discovered that rock phosphate could be turned into an effective fertilizer. The future of phosphorus — reduce losses, recover and recycle!
Wastewater treatment infrastructure, such as this anaerobic digester, can be leveraged to capture and recycle phosphorus, a limited essential nutrient. Image credit Michael Northrop In addition, we must do more to recover phosphorus from wastewater and food waste and recycle it back into the food production system. The discovery and general uses of phosphorus Why is phosphorus needed on farms? Ten things we all can do to manage phosphorus better To receive notices about future blogs, be sure to subscribe to Soils Matter by clicking on the Follow button on the upper right!
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However, it does not seem to increase above about 25 mm Lee et al. When the supply of Pi is limited, plants grow more roots, increase the rate of uptake by roots from the soil, retranslocate Pi from older leaves, and deplete the vacuolar stores of Pi. In addition, mycorrhizal fungi may more extensively colonize the roots. Conversely, when plants have an adequate supply of Pi and are absorbing it at rates that exceed demand, a number of processes act to prevent the accumulation of toxic Pi concentrations.
These processes include the conversion of Pi into organic storage compounds e. Any or all of these processes may be strategies for the maintenance of intracellular Pi homeostasis. It is clear from both kinetic and molecular studies that the capacity to transport Pi across cellular membranes involves several different transporters and is in some way regulated by the external supply of Pi.
Furihata et al. The expression of certain members of the putative plasma membrane or tonoplast phosphate-transporter gene family increases during periods of Pi starvation. In Arabidopsis at least three genes encoding phosphate transporters are expressed in roots and are up-regulated by Pi starvation. Similarly, in potato one gene was specifically induced in roots and stolons by starving the plants of Pi, whereas a second gene was expressed throughout the plant under conditions of high or low phosphate.
Changes in Pi-transport activity and phosphate-transporter gene expression show that plant cells respond to changes in the Pi concentration of the external medium or in the vacuole.
However, the intracellular signals and the factors that modify gene expression in the nucleus while cytoplasmic concentrations of Pi remain relatively constant are unknown.
Progress at the molecular level may eventually provide insight into the processes that regulate phosphate uptake through the isolation of genes encoding proteins that interact and regulate phosphate-transport mechanisms.
Recent studies Mimura et al. In P-sufficient plants most of the Pi absorbed by the roots is transported in the xylem to the younger leaves. There is also significant retranslocation of Pi in the phloem from older leaves to the growing shoots and from the shoots to the roots. In Pi-deficient plants the restricted supply of Pi to the shoots from the roots via the xylem is supplemented by increased mobilization of stored P in the older leaves and retranslocation to both the younger leaves and growing roots.
This process involves both the depletion of Pi stores and the breakdown of organic P in the older leaves. A curious feature of P-starved plants is that approximately one-half of the Pi translocated from the shoots to the roots in the phloem is then transferred to the xylem and recycled back to the shoots Jeschke et al.
In the xylem P is transported almost solely as Pi, whereas significant amounts of organic P are found in the phloem. A number of mutants that show altered Pi accumulation in leaves have been identified. These may help us to understand the processes controlling the allocation of Pi within the plant. One Arabidopsis mutant pho1 was isolated based on reduced total phosphate concentrations in the leaf tissue Poirier et al.
In the pho1 mutant, it is not known whether a gene encoding a transporter or regulatory molecule has been mutated; however, the phosphate-transporter genes that have been cloned do not map to the pho1 or pho2 locus. This mutation highlights the importance of specialized mechanisms for the transfer of Pi to the xylem. Another Arabidopsis mutant, pho2 , accumulates P in its leaves to toxic concentrations, which is indicative of a defect in the regulation of Pi concentrations in shoots Delhaize and Randall, and illustrates the significance of regulating intracellular concentrations.
There is a general perception that Pi uptake by plants occurs as a direct consequence of uptake from the soil by root cells. In these plants the fungal hyphae play an important role in the acquisition of P for the plant Bolan, ; Smith and Read, Mycorrhizae can be divided into two main categories: ectomycorrhizae and endomycorrhizae, of which vesicular arbuscular mycorrhizae are the most widespread in the plant kingdom Smith and Read, The mycorrhizal symbiosis is founded on the mutualistic exchange of C from the plant in return for P and other mineral nutrients from the fungus.
The few published studies of the kinetics of Pi uptake indicate that mycorrhizal roots and isolated hyphae have P-uptake systems with characteristics similar to those found in nonmycorrhizal roots and other fungi Thomson et al. A number of factors may contribute to the increased rate of Pi uptake measured in mycorrhizal plants Smith and Read, An extensive network of hyphae extends from the root, enabling the plant to explore a greater volume of soil, thereby overcoming limitations imposed by the slow diffusion of Pi in the soil.
Several studies have shown that the depletion zone around plant roots, which is caused by plant uptake and the immobile nature of Pi, is larger in mycorrhizal than in nonmycorrhizal plants Bolan, Mycorrhizal fungi may also be able to scavenge Pi from the soil solution more effectively than other soil fungi because C which may be limiting in the soil is provided to the fungus by the plant.
Mycorrhizal fungi may also be able to acquire P from organic sources that are not available directly to the plant e. Little is known about the transport of P compounds within mycorrhizae or the mechanism of P efflux from the fungus. Pi and organic P such as polyphosphate could be carried within the fungus by cytoplasmic streaming or by bulk flow to the plant root from external hyphae located in the soil. The current view is that Pi is the major form effluxed by the fungus across the interfacial membranes.
However, there is also evidence in higher plants that phosphocholine can be broken down outside cells to release Pi. Since it is known that the phosphate transporter cloned from Glomus versiforme GvPT is not expressed in fungal structures inside the plant, it cannot be a candidate for the fungal P efflux mechanism. Efflux of P must depend on a different transporter of unknown structure. The role of P in the regulation of symbiosis is still poorly understood, in part because of conflicting experimental results.
In mycorrhizal roots demand for P by the plant may regulate the activity of P transporters in the fungus, with efflux from the fungus being the limiting step. The mycorrhizal plants did not accumulate Pi in the vacuoles, which suggests that the fungus Hebeloma arenosa may be able to limit the efflux of P to the plant.
Mycorrhizal roots are able to take up Pi from solutions containing up to mm Pi Smith and Read, , concentrations far above that likely to be encountered in the soil. High external Pi concentrations up to 16 mm had little adverse effect on germination and growth of germ tubes in the vesicular arbuscular mycorrhizal fungus G. These results suggest that the low levels of colonization seen in plants growing in soils with high P status may not be the result of direct regulation of the activity of the fungus by soil Pi, but, rather, that specific signals from the plant regulate the activity of the fungus.
Considering that P is an essential and often limiting nutrient for plant growth, it is surprising that many aspects of P uptake and transport in plants are not thoroughly understood.
Perhaps the next important leap in our conceptual understanding in this area will come from the integration of these techniques to provide a comprehensive picture of the function of phosphate transporters and how the control of their spatial and temporal expression allows the plant to cope with changing environmental conditions.
A final issue to raise is that the soil Pi concentration has often been ignored by plant physiologists. It is common to find experiments in which plants were grown in 1 mm Pi, which may be fold higher than the Pi concentrations plants encounter in agricultural or natural ecosystems. To fully understand how plants acquire Pi from soils and regulate internal Pi concentrations, future studies on Pi uptake by plants must more closely mimic soil conditions, in which the concentration of Pi is always low and soil microflora influence both acquisition and mobilization.
We thank Professors F. Smith for their critical comments and discussions. We apologize to the colleagues whose papers were not directly cited because of space limitations. Eur J Biochem Google Scholar. Bieleski RL Phosphate pools, phosphate transport, and phosphate availability. Annu Rev Plant Physiol 24 Bolan NS A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants. Plant Soil Mol Cell Biol 11 Curr Genet 29 Plant Physiol Drew MC Saker LR Uptake and long distance transport of phosphate, potassium and chloride in relation to internal ion concentrations in barley: evidence of non-allosteric regulation.
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