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Mineral Uptake by Plants

Introduction

Uptake of minerals by plants has been studied under axenic, experimental and field conditions. The general concepts developed indicate a complex set of factors determining plant uptake of minerals. The presence of a mycorrhizal fungus in this equation extends the complications, though underlying principles remain.

Root Uptake of Minerals

Critical factors determining the rate of uptake of minerals by plants include:

  1. Availability of the mineral. The mineral must be in a form that can be absorbed by the root.
  2. Concentration in soil. The higher the concentration, the greater the likelihood of uptake.
  3. Rate of mass flow in soil. The supply of minerals to the root is determined by the movement of the mineral in solution.
  4. Rate of root elongation. The root tip grows into soil containing fresh supplies of the mineral.
  5. Surface area of the root. The larger the contact area available for potential uptake, the greater the uptake.

The factors of critical importance for each mineral differ. P, N and K are required by plants in large amounts. P is required for ATP, DNA, phospholipids and other important biochemicals.

In general, Australian soils are deficient in phosphate because the soils are old and highly leached. Further, though P is present in bedrock, P becomes available very slowly. In soils of low and high pH, P is relatively unavailable to plants because it is fixed in mineral complexes. The mass flow of phosphate in soil is very slow, because the phosphate molecule when hydrated is large, and has a high charge. While P is contained in organic material, specific enzymes are required to extract P from these sources. Thus in Australian soils, available P is removed rapidly by roots from the soil, forming a depletion zone around the root. Thus plants require mechanisms to deal with P deficiency to enable survival and grow in most Australian soils.

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Plant Mechanisms to Maximise Efficiency of Use

The plant may use one or more mechanism to deal with P deficiency. These mechanisms may be related to use of P within the plant or uptake from soil.

P Use

cartoon P uptake
Cartoon showing various plant mechanisms to manage P.

P is actively mobilised and moved around the plant. Old tissues tend to have lower concentrations than freshly formed tissues. Plant parts that are shed regularly, such as leaves and bark, tend to have very little P in them. Australian native plants also tend to use P very efficiently, achieving maximum rates of growth at lower plant concentrations than plants adapted to P rich soils. In addition, rates of growth of plants from P deficient soils tends to be slower than those adapted to P sufficient soil. Plants may store P in perennial tissues such as the primary root during periods when P is relatively more available, and use the store during periods when P availability is reduced.

P Uptake

Concentrations in the soil are commonly only a small proportion of the concentrations in plants indicating that the plant expends energy to absorb P and hold it against the diffusion gradient with the soil. A few plants have specialised mechanisms, such as cluster roots, which increase the surface area of contact, and these roots are commonly formed in mineral-rich litter layers. Plants may also excrete acids which mobilise P from recalcitrant sources. Finally, plants commonly use a fungal symbiont to aid uptake of P, and other immobile minerals.

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AM Fungi - Uptake of P

AM fungi grow within roots and subtend hyphae from the root surface into the surrounding soil, especially beyond the P depletion zone. The quantity of fungus in soil varies, but up to 1 m hyphae for each 1 cm of colonised root has been measured in agricultural soils. This is equivalent to approximately 0.2 m of hyphae per g of soil. In undisturbed soil, the density of hyphae may be considerably greater.

AM fungi develop extensive mycelium. Hyphae have been found 10 cm from the root in experimental conditions, though the pattern of soil exploration differs markedly between species of AM fungi. In soil, hyphae will link adjacent plant roots creating a network of hyphae through the soil not occupied by roots. Thus the exploration of soil increases markedly in plants with AM, enabling the the plant to utilise P from beyond the depletion zone. 

The fungus and plant access the same pool of available P. In comparison with plants, however, more of the 3 dimensional profile of the soil is available to the fungi. The fungi ramify over and within soil aggregates, accessing pools of P physically separated from and unavailable to the plant root. Although enzymic degradation of organic P in litter has been suggested, the evidence for such a mechanism in AM fungi is poor. However, hyphae of AM fungi may access P released from organic sources when they associate with P solubilising saprotrophic fungi in organic matter.

The energy supplied by the host to support the hyphal network is thought to be between 6 and 20% of the total photosynthate of the host. The energetic cost of AM is less than an equivalent increase in root mass necessary for similar P uptake.

AM fungi also influence the plant uptake directly. Once colonisation has commenced, one family of plant P transporters is down regulated, and another family is upregulated, meaning that much of of the plant P is delivered via the AM fungi connected to the plant. Indeed, in many plants the outer layer of the cortex becomes sealed from the soil matrix following suberisation of the surface of exodermal passage cells. Thus the cortex might be thought of as an exchange region: the AM fungus is regulating plant uptake of P, and the plant has responded by reducing loss of organic compounds to the soil, while at the same time is regulating the quantity of carbon lost by preventing fungal expansion.

The effect of AM on plant growth is determined by several factors, of which P concentration in soil is the most important. The consequence of AM on plant uptake of P is dramatic in soils with low available P: plant growth and P concentrations in the plants is increased. For a given concentration of P in soil, mycorrhizal plants are markedly larger than nonmycorrhizal plants.

As the P concentration in soil increases, the difference between mycorrhizal and nonmycorrhizal plants declines. At very high concentrations, the increased uptake of P probably has no effect on plant growth rate and the loss of organic carbon to the fungus causes a slight decline in relative plant growth.

The effect of AM on plant growth differs with stage of growth. Initial growth of seedlings is determined by seed reserves. Beyond approximately 5 to 10 days though, the seedlings becomes more responsive to the association. Note that in the case of non-mycorrhizal Banksia, seed reserves enable seedling growth for up to 12 months. Seed reserves of minerals may be an important further determinant of seedling response to mycorrhiza.

If colonisation remains low, the rate of growth of the seedling is slower. If colonisation is high, seedlings have the potential to grow faster. In mature crop plants, presence of AM probably has no influence on the plant once seed reserves have been stored, and indeed, colonisation of roots in mature annual plants is often much less than during phases of rapid growth.

The quantity of hyphae supported by the plant varies with the concentration of P in the plant. At extremely low available P, the plant does not support adequate amounts of colonisation. At high plant P, the proportion of the roots colonised is also reduced. In between, however, the plant can have most of the fine root colonised by AM fungi.

Thus the cost of the symbiont is reduced in conditions where the response to the association is less.

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AM and Uptake of Other Minerals

If the rate of movement of a mineral through soil is slow, then when that mineral is deficient, AM may aid plant uptake. Thus increases in plant uptake of K, NH4, Cu and Zn are commonly recorded. Just as for P uptake, the fungi increase the volume of soil from which minerals are extracted, and the association becomes important in soils where the mineral is relatively deficient.

Where a mineral diffuses rapidly through soil, the presence of AM will normally have no influence on plant uptake. Nitrate diffuses rapidly, and instances of increased uptake of nitrate by mycorrhizal plants are unusual. Here, the interaction with water status might be important. As soils dry, the water connection from root surface to soil water is lost. AM fungi may provide a bridge between soil stores and the root. Increased uptake of nitrate may be due to the fungus connecting the root to moist nitrate-rich compartments in dry soil.

The exception to this general trend is Mn. The availability of Mn is indirectly influenced by fungal hyphae. In soils with high available Mn, hyphae reduce available organic carbon in soil around the root, which influences which bacteria are active in the soil. Mn becomes less available, thus reducing fungal and plant uptake. It is clear that the role of AM in plant mineral nutrition is complex.

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Conclusion

The uptake of minerals by roots is supplemented by mycorrhizal fungi. AM fungi increase minerals that are poorly available, immobile in soil, and depleted around the root. The fungi also influence availability by changing the movement of available organic energy from the root into the soil.

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References

Hodge A, Helgason T & Fitter AH. 2010 Nutritional ecology of arbuscular mycorrhizal fungi. Fungal Ecology 3: 267-273.

Marschner H. & Dell B. in Robson AD, Abbott LK & Malajczuk N 1994 Management of Mycorrhizas in Agriculture, Horticulture and Forestry. Kluwer. Ch 9.

Smith SE & Read DJ 1997 Mycorrhizal Symbiosis. Academic Press.

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