Plants can prepare carbohydrates, fats, proteins and vitamins in their body. However they cannot generate the mineral ions. Thus for plants, the constant supply of minerals is very essential. The study of how plants obtain mineral elements, either through water, air or soil and utilize them for their growth and development is called mineral nutrition.
Mineral nutrients are elements such as nitrogen, phosphorus, and potassium acquired primarily in the form of inorganic ions from the soil. Although mineral nutrients continually cycle through all organisms, they enter the biosphere predominantly through the root systems of plants, so in a sense plants act as the “miners” of Earth’s crust. The large surface area of roots and their ability to absorb inorganic ions at low concentrations from the soil solution make mineral absorption by plants a very effective process. After being absorbed by the roots, the mineral elements are translocated to the various parts of the plants, where they are utilized in numerous biological functions.
Other organisms, such as mycorrhizal fungi and nitrogen-fixing bacteria, often participate with roots in the acquisition of nutrients.
The study of how plants obtain and use mineral nutrients is called mineral nutrition. This area of research is central to modern agriculture and environmental protection. High agricultural yields depend strongly on fertilization with mineral nutrients.
In this section, we will discuss the nutritional needs of plants, the symptoms of specific nutritional deficiencies etc. then we will examine how soil structure (the arrangement of solid, liquid, and gaseous component), root morphology influence the transfer of mineral nutrients from the environment into a plant and factors affecting mineral nutrient acquisition by plant. Finally, we will introduce the topic of mycorrhizal symbiotic associations.
MINERAL NUTRITION IN PLANTS
The study of how plants obtain, distribute, metabolize and utilize mineral nutrients. “MINERAL”: An inorganic element acquired mostly in the form of inorganic ions from the soil. “NUTRIENT”: A substance needed to survive or necessary for the synthesis of organic compounds.
Only certain elements have been determined to be essential for plants. An essential element is defined as one that is an intrinsic component in the structure or metabolism of a plant or whose absence causes severe abnormalities in plant growth, development, or reproduction. If plants are given these essential elements, as well as water and energy from sunlight, they can synthesize all the compounds they need for normal growth.
The elements listed in table, are considered to be essential for most, if not all, higher plants. The first three elements- hydrogen, carbon, and oxygen- are not considered mineral nutrients because they are obtained primarily from water or carbon dioxide.
CLASSIFICATION OF MINERAL ELEMENTS:
Essential mineral elements are usually classified as macronutrients or macronutrients according to their relative concentration in plant tissue.
MECHANISM OF MINERAL ABSORPTION
The penetration and accumulation of ions and molecules into the living cells or tissue from the surrounding medium, crossing the membrane, is called absorption.
Various theories have been put forward to explain the mechanism of mineral absorption. They are broadly categorized into two groups based on the involvement of metabolic energy in the process. Those groups are given below:
- Passive mechanisms. Those which believe that metabolic energy is not involved in the mineral absorption.
- Active mechanisms. Those which believe that metabolic energy is involved in the mineral absorption.
MECHANISM OF PASSIVE ABSORPTION
Absorption of ions and molecules propelled by physical driving forces is called passive absorption. The various modes of passive absorption are given below:
- Concept of outer free space
Absorption of mineral salts takes place through the roots which are in close contact with the soil particles or soil solution. It has been observed that when a plant growing in the medium of low salt concentration is placed in the medium of high salt concentration, an initial rapid uptake of ions takes place. The same plant when returned to pure water, some of the ions diffuse out. This indicate that a part of cell or tissue is open to free diffusion and exchange of ions. This is termed as “the outer free space”.
The movement of ions and molecules from the region of its higher to lower chemical potential along the concentration gradient is called diffusion. The driving force in the process of such diffusion is chemical potential gradient.
- Facilitated diffusion (Carrier mediated diffusion)
The passive absorption of solute mediated by a carrier is called facilitated diffusion. It has been proposed that certain carriers-probably some proteins, act as shuttle for a passive transport of ions or molecules across the membrane. The carrier selects out and binds certain molecules to form solute-carrier complex. The complex then diffuses across the membrane and finally releases the solute towards the other side without involving the metabolic energy.
- Ion exchange
Ion exchange mechanism offers a greater opportunity for absorption of ions from the external medium. The ion exchange mechanism could be explained by two widely accepted theories, which are-
- The contact exchange theory. This theory is based on the ion exchange from one absorbent to another without the participation of free electrolytes. An ion, which is adsorbed electrostatically to a solid particle, is not tightly bound but oscillates within a small volume of space. This is termed “oscillation volume”. Therefore, the cations or anions which are adsorbed on the surface of root cell membrane or clay particles oscillate in a limited area.
- The carbonic acid exchange theory. According to this theory the soil solution plays an important role by providing medium for ion exchange. Respiration occurring in a root cells results in the interaction of carbon dioxide, which forms carbonic acid when dissolved in water. The carbonic acid dissociates into hydrogen ions and bicarbonate ions. These ions may then changed for similar charged ions of the soil solution.
- Donnan equilibrium
This theory suggests that the cell membrane allows only selected ions and particles to pass through it. Some ions get accumulated on the inner side of membrane. These ions are fixed. In this situation, to maintain electrochemical equilibrium, additional mobile ions enter the cell membrane and come inside and counteract the charges of fixed ions. Due to this movement, the product of anions and cations in the internal medium become equal to the product of anions and cations in outside medium.
- Mass flow of ions
According to this theory, an increase in the rate of transpiration causes increase in the salt uptake. Some investigators believe that transpiration plays its role indirectly by removing ions after they have been released into the xylem duct. Mass flow of ions may also occur due to transpiration pull.
The absorption of solute linked with some metabolic reactions involving expenditure of energy is called active absorption or active transport.
- Any fertile soil contains at least some clay particles within its structure.
- Clay particles carry a negative electrical charge to which the mineral ions (K+, Na+, Ca2+) attach.
- This attachment effectively prevents the leaching of the mineral ions from the soil.
- Unlike animal cell there are no potassium-sodium pumps in the cell membranes of plant cells. Rather there are proton pumps which pump protons (H+) outside of the cell. This creates an electro-negative charge within the cell.
- When the root cells secrete protons into the surrounding soil water the hydrogen ions displace the mineral ions from the clay particle, freeing them into solution.
- The mineral ions in the soil water are free to be absorbed by various pathways.
Absorption of mineral ions.
The plasma membrane of the plant cell can bring about the absorption of mineral by two different energy demanding processes:
- Indirect method in which proton pumps (hydrogen pumps) establish electrochemical gradients.
- Direct method in which membranes actively transport a particular mineral.
Proton (hydrogen) pumps in the plasma membrane pump out hydrogen ions (H+) this has a number of effects which are covered in the model below.
- The cations such as K+ which are free and in solution in the soil water can be taken up actively by active transport membrane pumps.
- Specific membrane pumps exist for the different cations.
- Experiments that metabolically poison the root (stop ATP production) causes all mineral absorption to stop.
FACTORS AFFECTING MINERAL NUTRIENT ABSORPTION
Absorption of mineral salts is affected by the number of external and internal factors. Some of them are listed below:
- External factors
- Hydrogen ion concentration (pH).
- Internal factors-
- Growth and morpho-physiological status.
- Temperature. Absorption of mineral salt is affected by change in temperature. In general, an increase in temperature results increase in the absorption of salts up to a certain optimum level. At very high temperature the absorption is considerably inhibited. The inhibition might be due to denaturation of proteins which are directly or indirectly involved in mineral salt absorption. The change in temperature also affects the process of diffusion. The rate of diffusion depends upon the kinetic energy of diffusing molecules or ions which, in turn, dependent upon temperature.
- Hydrogen ion concentration (pH). Change in the hydrogen ion concentration (pH) of the soil solution affects the availability of ions to the plants. In general, decrease in the pH of soil solution accelerates the absorption of anions. For example, boron is taken up as the undissociated acid, H3BO3 as the H2BO3?ions. It is absorbed at lower pH. In contrast to the anions, increase in pH will favour the absorption of cations. However, pH values across the physiological range may damage the plant tissue and inhibit the salt absorption.
- Light. Light has no direct effect, but indirectly by transpiration and photosynthesis, influences salt absorption.
- Oxygen. The active salt absorption is inhibited by the absence of oxygen.
- Interaction. The absorption of one ion is influenced by the presence of other ions in the medium. For example, Viets (1944) demonstrated that the absorption of potassium is affected by the presence of calcium, magnesium and other polyvalent cations in the soil solution. Epstein (1978) demonstrated the interaction of several ions (K, Cs, Li, Rb and Na) as competitive for binding sites on carriers. For example, K, Rb and Cs compete with one another for the same binding sites. Li and Na, on the other hand, are not competitive because they have different binding sites.
- Growth. Active cell division, elongation and developmental processes promote the absorption of mineral salts.
- Aging. As the root matures it increases the surface area which is favourable for salt absorption, but due to heavy suberization the mineral salt uptake is greatly reduced.
FACTORS AFFECTING MINERAL NUTRIENTS ACQUISITION BY PLANTS
The word “acquisition” emphasizes that more is involved in getting inorganic nutrients into plants than ion transport across cell membrane.
Adaptation and evolution of terrestrial plants depend, to a large extent, on their ability to acquire nutrients. This is a modern and integrative treatment of the mechanisms controlling the plant nutrient uptake and how plants respond to changes in the environment. Some factors are given below:
- Soil nutrients bio-availability
- Root responses to variations in nutrients supply
- Nitrogen fixation
- Regulation of nutrient uptake by internal plant demand
- Root characteristics
- Kinetics of nutrient uptake
- Response to climate change
This integrated view of nutrient uptake helps us to understand the mechanisms that govern present-day plant communities and is indispensable in models designed to predict the response of plants to a changing climate.
THE SOIL AFFECTS NUTRIENT ABSORPTION:
- pH affects the growth of plant roots and soil microbes.
- Root growth favors a pH of 5.5 to 6.5.
- Acidic conditions weathers rock and releases potassium, magnesium, calcium, and manganese.
- The decomposition of organic material lowers soil pH.
- Rainfall leaches ions through soil to form alkaline conditions.
- Negatively charged soil particles affect the absorption of mineral nutrients.
- Cation exchange occurs on the surface of the soil particle.
- Cations (+ve charged ions) bind to soil as it is negative charged.
- If potassium binds to the soil it can displace calcium from the soil particle and make it available for uptake by the root.
PLANT ROOT- the primary route for mineral nutrient acquisition
- pp05080Meristematic zone
- Elongation zone
- Maturation zone
Different areas of root absorb different mineral ions
- Apical region
- Apical region (barley) or entire root (corn)
- Potassium, nitrate, ammonium, and phosphate
- All locations of root surface.
- In corn, elongation zone has max K accumulation and nitrate absorption.
- In corn and rice, root apex absorbs ammonium faster than the elongation zone does.
- In several species, root hairs are the most active phosphate absorbers.
Root tips are the primary site for the nutrient uptake because of following reasons:
- There are the tissues with greatest need for nutrients.
- The cell elongation requires Potassium, nitrate, and chlorine to increase osmotic pressure within the wall.
- Ammonium is a good nitrogen source for cell division in meristem.
- Root apex grows into fresh soil and finds fresh supplies of nutrients.
- Nutrients are carried via bulk flow with water, and water enters near tips.
- It maintains concentration gradients for mineral nutrient transport and uptake.
Root uptake soon depletes nutrients near the roots
- The formation of a nutrient depletion zone in the region of the soil near the plant root.
- It forms when rate of nutrient uptake exceeds rate of replacement in soil by diffusion in the water column.
- The root associations with Mycorrhizal fungi help the plant overcome this problem.
- Mycorrhizae are not unusual; in fact, they are widespread under natural conditions.
- 83% of dicots, 79% of monocots and all gymnosperms regularly form mycorrhizal associations.
Ectotrophic Mycorrhizal fungi
- Form a thick sheath around root. Some mycelium penetrates the cortex cells of the root
- Root cortex cells are not penetrated, surrounded by a zone of hyphae called Hartig net.
- The capacity of the root system to absorb nutrients improved by this association – the fungal hyphae are finer than root hairs and can reach beyond nutrient-depleted zones in the soil near the root.
Vesicular arbuscular mycorrhizal fungi
- Its hyphae grow in dense arrangement, both within the root itself and extending out from the root into the soil.
- After entering root, either by root hair or through epidermis hyphae move through regions between cells and penetrate individual cortex cells.
- Within cells form oval structures – vesicles – and branched structures – arbuscules (site of nutrient transfer).
- pp05110P, Cu, & Zn absorption improved by hyphae reaching beyond the nutrient-depleted zones in the soil near the root.
Nutrients move from fungi to the root cells
- Occurs by simple diffusion from the hyphae in the hartig net to the root cells.
- Vesicular arbuscular mycorrhizal fungi
- Occurs by simple diffusion from the arbuscules to the root cells.
- Also, as arbuscules are degenerating as new ones are forming; the nutrients may be released directly into the host cell.
Plant roots often form associations with mycorrhizal fungi. The fine hyphae of mycorrhizae extend the reach of roots into the surrounding soil and facilitate the acquisition of mineral elements, particularly those like phosphorus that are relatively immobile in the soil. In return, plants provide carbohydrates to the mycorrhizae. Plants tend to suppress mycorrhizal associations under conditions of high nutrient availability.
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