Mineral salt absorption

MINERAL SALTS ABSORPTION
Plants solve the problem of absorbing frequently scarce water and mineral elements from the soil by producing large root system. The overall shapes of such system are controlled mainly by genetic rather than environmental factors. Thus grass have fibrous root system near the soil surface while herbaceous dicot have a tape root that extends several meters downwards.Even though root morphology is genetically controlled, soil environments such as mechanical impedance, temperature, aeration, availability of water and mineral salt influence branching pattern.
Most of essential elements are found in soil solution either in the form of inorganic salt or ions which may exist either in free state or bound to clay particles. 
Whenever there is depletion of any free ions from the soil solution, respective ions are released from clay particles into the soil solution to maintain the equilibrium
After dissolving of nutrient into the soil water they are absorbed by the roots into plants.
Roots as Absorbing Surfaces
Plant roots posses several features which ensures maximum absorption of nutrient from the soil. These features include;Cylindrical and filamentous root shape which enable the root to penetrate more soil volume and increase the absorption of water with dissolved mineral salt from the soil to plant.Development of root hairs, these are modified epidermal cells which increase the surface area for absorption of nutrient by the root.
Development of mycorrhizae-This is a symbiotic and mutualistic association between non pathenogenic fungus and living root cell which improve water and mineral salt absorbing property, they also release powerful enzymes into the soil that dissolve hard-to-capture nutrients, such as organic nitrogen, phosphorus, iron and other "tightly bound" soil nutrients

There are two types of mycorrhizae
Ectomycorhizae- the one in which the fungal hyphae form a mantle outside the root and also inside the root in the intercellular space of the epidermis and cortex without intracellular penetration. It is common in trees
Endomycorrhizae –The one in which the fungal hyphae penetrate into the cytosol of epidermal or cortical cells, it is common in monocot and dicot.
Advantages of mycorrhizae to plant
Improved nutrient and water uptake,Improved root growth,Improved plant growth and yield
Reduced transplant shock,Reduced drought stress
Mechanisms of nutrient absorption
Two different mechanisms have been proposed to explain absorption of mineral salts from soil to cell root cells. These mechanisms are
1) Non-mediated or passive uptake
2) Mediated or active uptake
1)Non-mediated or passive uptake of ions- Is the absorption of nutrients in and out of the cell which occur spontaneously down a gradient of electrochemical potential energy. This type of ion absorption does not need metabolic energy.Passive absorption may take place by the following means i) Pressure or mass flow – is the flow of water with nutrients from the area with high pressure potential to low pressure potential area.ii) Diffusion- Is the a absorption of nutrients from an area with high chemical potential to an area with low chemical potential. The solute move diffusion equal concentrations are attained;Note
Diffusion help lateral flow of fluid through short distance such as from cell to cell or uptake of nutrient from the soil by root but long distance movement of fluid such as through xylem and phloem takes place by bulk flow;iii) Ion exchange- In ionic exchange mechanism, anions or cations from within cells are exchanged with anions or cations of equivalent charge of the external solution. Either by contact theory were by under contact between root and soil particles e.g. Mg ion can replace Ca ion Sometime ion exchange takes place by Carbonic acid exchange theory were by H ions formed from reaction of carbondioxide and water are released from the root and exchanged with Potassium ion or Sodium ion from clay into the plant

Co2 +H2o====H2co3 +H ions

Released H ions can be exchanged with K ions or Sodium ion
2) Mediated or active uptake- is the movement of nutrient across a cell membrane against concentration gradient (from low to high concentration) using energy released from cellular respiration.
During this process ATP react with water in presence of enzyme ATPase found in the cell membrane and release energy
ATP + H2O==== ADP + H2PO4 ion +energy + Energy (31.6 kJ)

The energy released in this process cause the cell cytosol to be slightly negatively charged relative to the surroundings and cause the pH inside the cell to rise which favour nutrient absorption into the cell.
Evidence to support active uptake of mineral salts:
(i) The actual rate of absorption of minerals is too rapid in plants than that can be explained by passive absorption.
(ii) Sometimes the absorbed mineral is at a higher concentration within the cell and is at a lower concentration outside (soil)
(iii) The factors like low temperature, deficiency of oxygen, metabolic inhibitors etc. which inhibit metabolic activities like respiration in plants also inhibit absorption of ions.
(iv) The rate and amount of absorption of minerals is directly related to the expenditure of metabolic energy.After mineral absorption by the root they are transported by symplatic or apoplastic pathway to the the xylem elements were are moved upwards along with transpiration stream and get distributed to all other plant parts.
FACTORS THAT CONTROL MINERAL ABSORPTION
Soil texture
The proportion of sand, silt, and clay in the soil affects the ability of the soil to hold nutrients. For example clay soil hold nutrients very strongly and prevent them from dissolving in water while sand soil have large pore spaces, allowing more leaching of nutrients and hence limit nutrient absorption. Therefore loam soil have good capacity of holding nutrient and allow them to dissolve in water hence increase the nutrient absorption capacity of plant.
Similarly soil texture affect soil aeration, As mineral absorption requires energy, poor soil aeration affects the ability of roots to absorb adequate quantities of minerals. Water logged soils or soils with higher content of clay have very little amount of air; under such conditions roots are subjected to anaerobic conditions and the absorption of minerals is drastically affected
Soil pH
The degree of ionization of minerals and other nutrients depends upon the hydrogen ion concentration of the soil solution. The tendency of nutrient to dissolve in the soil solution largely depend on soil pH. For example the micronutrients such as Iron, Zinc, Manganese and cupper tend to precipitate under high pH above 5 to react with Hydroxyl ions and precipitate to form insoluble hydrous metal oxide. Similarly Ph affect the form of ion present in the soil ,for example, most of the phosphate ions, at alkaline pH exist either as bivalent H3PO4 ions or trivalent H3PO4 ions. 
Such ions are not favoured for absorption but neutral pH favours the presence of monovalent ion which is absorbed more easily..  So the soil pH has a significant effect not only on the rate but also the kind of ions uptake.That is why the maintenance of proper soil pH is very important in agriculture.  Too acidic or too alkaline soil is virtually useless for cultivation.  Until and unless the soils are restored in terms of pH, such soils remain as wastelands.
FACTORS THAT CONTROL ABSORPTION
Soil aeration:   In most of the cases, living cells cannot survive without oxygen.
As the roots contain a large number of living cells, they require considerable amount of energy for their metabolic activities and growth.
So oxygen is absolutely essential for generating energy rich components by biological oxidative process. As mineral absorption requires energy, poor soil aeration affects the ability of roots to absorb adequate quantities of minerals. Water logged soils or soils with higher content of clay have very little amount of air; under such conditions roots are subjected to anaerobic conditions and the absorption of minerals is drastically affected. The inhibition of absorption of minerals due to the effect of respiratory poisons on roots clearly suggests that the absorption of minerals is an energy dependent process.
TEMPERATURE:
Soil temperature has a significant effect on roots metabolic activities and also it affects the mobility of ions in soil solution.  If the temperature of the soil is lowered, absorption of minerals will be drastically reduced; but with the increase in temperature, the rate of absorption also increases, but up to certain limits. Drastic variations in the rate of absorption due to changes in the temperature suggest, the process is dependent on protein or enzymatic activity.
pH OF THE SOIL SOLUTION
The degree of ionization of minerals and other nutrients depends upon the hydrogen ion concentration of the soil solution.  For example, most of the phosphate ions, at alkaline pH exist either as bivalent H3PO4 ions or trivalent H3PO4 ions. 
Such ions are not favored for absorption.  On the other hand neutral pH favors the absorption of monovalent ions.  So the soil pH has a significant effect not only on the rate but also the kind of ions uptake.  This property is also due to its effect on cellular components that are involved in absorption, which further suggests that proteins are involved in the ion uptake.  As the protein structure is very sensitive to pH, its function also changes if there is any change in the pH. That is why the maintenance of proper soil pH is very important in agriculture.  Too acidic or too alkaline soil is virtually useless for cultivation. Until and unless the soils are restored in terms of pH, such soils remain as wastelands.

CONCENTRATION OF SOIL SOLUTION
Generally the concentration of minerals and its components found in soil solution is far below the levels of the same found in the cell sap. It means that the absorption of ions takes place against concentration gradient.  The relative concentration of ions found in the cell sap and soil solution gives absorption ratio. Ocean water contains relatively greater amount of salts than that of fresh waters.  The land plants which are adapted to grow in fresh water soils die in marine water, because the marine water is enriched with greater amount of metal ions. Physiologically dry for them.  But marine plant cells which have been adapted to such waters contain much more ionic contents than found in sea water.  Even here, the ions are absorbed against concentration gradient.
The rate of absorption of ions very and depends upon the concentration of the soil solution.  Normally, roots absorb greater amount of ions at a greater rate in dilute solutions than in a relatively high concentration solutions.  How exactly the dilution enhances the rapid uptake is not clear, but it is a fact.
ION-ION INTERACTIONS
Soil solution consists of a wide variety of ions in different concentrations.  While roots absorb inorganic nutrients, the ions of one kind present in the solution, either facilitate or interfere with the uptake of the other kind of ions.  This phenomenon is called ion antagonism.  On the other hand, a particular species of or ions enhance the uptake of another kind of ions.  Such a phenomenon is referred to as ion facilitated uptake.  
Epstein has demonstrated that the absorption of K and Fe is antagonized by the presence of calcium and magnesium bivalent ions. Similarly CaCl2 has been found to inhibit the uptake of Cu2 ions and save the plants from copper toxicity.  On the other hand, sodium chloride has been found to facilitate the uptake a wide variety of ions.Such type of ion interactions leading to antagonistic or facilitated uptake is explained on the basis of carrier molecules.  Different ions have different carriers or transport proteins.  Because of the specific binding site, any ion that competes with the other ion for the same sites results in ion antagonism. On the contrary, a particular ion binding to carrier molecules facilitates the binding of specific ion and enhance uptake of the said ion. So the balanced inorganic nutrient is very important, otherwise roots absorb more of one kind of ions or the absorption f an essential ion may be prevented by the presence of another kind of ion.
IMPORTANT FEATURES OF ABSORPTION
Unequal absorption and specificity of ion:If a mixture of different elements of equal molar concentration in the form of a buffered solution is provided to the root system,it absorbs some ions in greater amounts than other, the rest are absorbed in traces with variations. This indicates the unequal uptake and also specificity. Certain cells, at a particular stage of development, absorb specific ions because they are required for their metabolism. The specificity is demanded by the needs of cells or tissues.  In spite of it, the pH of the external solution remains more or less neutral.
    This is certainly due to exchange of ions.  This can be demonstrated by placing a tomato plant with its roots intact in a dilute solution of NaCl.After a period of time, certain ions like K and Ca2 are found in the external solution, which were not present before. This phenomenon explains the exchange of ions between the external solution and internal sap.Another equally important aspect of unequal uptake or absorption of ions is the dilution effect, where greater the dilution of external solution greater is the rate of uptake.  This behavior is difficult to explain.Furthermore, the preferential uptake clearly suggests the role of specific carriers in the process of absorption of ions.
 Salt Accumulation
Analysis of the concentration of specific nutrients present within the cell sap and the external solution reveals, that the relative concentration of specific components show greater concentration within the cells than in the external solution.  Use of radioactive isotopes as traces also supports the same view, where certain ions are accumulated or taken in against concentration gradient.  If the same is expressed in terms of chemical potential measured as milli volts.  In Nietellas’ cellular cytoplasm, Na+ shows a chemical potential of 72 mV, potassium shows a difference of 40 MV and chorine +237mV.The above observations clearly suggest that the concentration of Na is very high in the external solution. In normal course, it should diffuse into the cell across the membrane by passive process,but to maintain chemical potential gradient and to prevent excess Na toxicity, Na ions are expelled out of the cell. 
On the contrary, the movement of CI is an uphill journey, because the concentration of them is many folds higher inside the cell than outside. The movement of ions inwards and outwards is referred to as ion flux and ion efflux respectively.
     Saturation Effect: 
If a root system is provided with an excess amount of specific ions, initially ions are taken up at a greater rate but later the rate of uptake remains steady and constant.  This observation further suggests that for a given ion there is a fixed number of specific carrier href="http://www.chempapy.blogspot.com/"> sites; if all are loaded with their respective ions, the rate of uptake can not be increased until and unless the number of carriers is increased.
4.      Metabolic Energy:
Energy is required for all metabolic processes. To demonstrate whether metabolic energy is required for ion uptake or not, it is possible to test it by providing respiratory poisons like KCN, DNP, rotenone, etc, to the root system engaged in the absorption of minerals. As soon as the inhibitors are added, the uptake of ions drastically reduces, which suggests that energy is absolutely required for the absorption of ions. The cell mebrane is largely impermeable to free ions. Honert (1937) proposed the carrier concept theory to explain uptake of ions across the cell membrane against the concentration gradient.He is of the opinion that cell membrane contains specific protein moleculcs which behave as carriers to pick up ions from outer "space. At first the carriers are activated.
The activated carrier then combines with the ions of outer space to form an intermediate external carrier-ion-complex. The carrier-ion-complex then moves across the cell membrane.On reaching the innerside of the membrane1the complex breaks down releasing the ions into inner space of the cell and the carrier gets separated.
.      Apparent free space:
 If a plant, with its root system intact, is provided with a known amount of radioactive ions like 35SO4 or 32P for about 30 minutes, it is possible to determine the total amount of ions taken up by the root system by measuring the amount of radioactivity left in the exogenously provided solution.






 

TRANSLOCATION OF MNERAL SALTS

Absorbed mineral salts and other components like cytokinins, and others absorbed by roots, ultimately reach the xylem elements found in the root system i.e xylem vessels.  Most of the minerals are absorbed by meristems than root hairs. The minerals then are transported to the vascular system

The separated carrier again goes back to the outer surface to pick up fresh ions and the process of absorption of ions continues. Once the ions are released into inner space, they can not move out and then they are accumulated there. The metabolic energy in the form of ATP is supposed to be used in one or more steps of the process.
TIn the scheme, X & Y stands for carriers, X' & Y' for precursors of carriers, XK and YA for carrier ion complexes and k' and A for cation and anion respectively. In the process carrier precursors x' and y' are activated to form carrier X and Y which combine with ions to form ion- carrier-complex XK and YA for carrying them to inner space and releasing them over there.
Factors affecting absorption of mineral salts:
Some of the important factors affecting absorption of mineral salts by plants are as follows:
Temperature: The absorption of inorganic salts increases with an increase in temperature, but is confined to a very narrow range.(ii) Oxygen: It greately influences active absorption of salts.
(iii) Light: Light enhance transpiration and effects photosynthesis, so exhibits it influence in salt absorption.(iv) Hydrogen ion concentration: The decrease in the pH of soil solution accelerates the absorption of anions and increase in pH favours the absorption of cations.(v) Presence of other ions: The rate of absorption of one type of ion is influenced by the presence of other ions in the medium.
For example, rate of absorption of KT decreases if Ca++, MgT etc. are present in the medium.
In otherwords the presence of Ca" antagonises the absorption of k*. This phenomenon is called ion-antagonism.(vi) Growth: Active cell division, elogation and developmental processes promote absorption of salt.young and developing xylem elements or they can be transported into mature vascular elements .The xylem elements have xylem parenchyma, which are living and supporting the process of translocation of minerals into upward conducting system. Once the elements are loaded into xylem cells, they are transported upwards along with the transpiration stream and thus they reach the aerial parts of the plant body where the nutrients; first diffuse into apparent free spaces then they are absorbed into living cells by active/facilitated transport. While the minerals are translocated all along the length of the stem, considerable amount of them diffuse laterally and reach the cortical cells. In spite of the process of absorption of mineral salts by root system is highly specific and energy dependent, the upward movement is entirely due to passive process for the dissolved components just move along with the water column due to transpiration pull. Yet one cannot rule out the ability of livings cells to do such  active transportation for dead xylem  are supported by living xylem parenchyma. Even Xylem parenchyma tissue a living tissue has roles in such mineral uptake and transportation. Whichever factor that affects the transpiration pull also affects the translocation of mineral salts in the plant body.



REFERENCES
Lincoln Taiz and Eduardo Zeiger. (2006) Plant Physiology, 4th ed. Sinauer Associates, Inc. Sunderland
Horst Marschner (1986) Mineral Nutrition of Higher Plants, 2nd. Ed. Academic Press, San Diego,CA:

Lincoln Taiz and Eduardo Zeiger. (2006) Plant Physiology, 4th ed. Sinauer Associates, Inc. Sunderland
Horst Marschner (1986) Mineral Nutrition of Higher Plants, 2nd. Ed. Academic Press, San Diego, CA