Symptoms of plants deficient in essential elements

If the essential elements are supplied in limited qunatities, the growth of plant will be reduced. The concentration of essential elements below which the growth of plant is reduced is known as critical concentration of the element. If the element is present in lower concentration than its critical concentration then it is said to be deficient.

If every element has a specific structural and functional role in plants, the plants show morphological changes in the absence of that particular element. These morphological changes identify certain deficiencies of elements and are called as deficiency symptoms. The deficiency symptoms differ for one element to another and they vanish when the mineral element that is deficient is supplied to the plant. If the deficiency of that element continues then it would lead to the death of the plant. The plant parts that show the deficiency symptoms depends on the mobility of the element in the plant. If the element is actively mobilized inside the plant and is exported to the younger parts of the plant tissue, the deficiency symptoms are seen earlier in older tissues.

For example, the deficiency symptoms of potassium, nitrogen, and magnesium are seen initially in senescent leaves. Older leaves get separated making these elements mobilized to younger leaves. The young tissues are initially made to experience the deficiency symptoms when the element is immobile and not transported out of the mature organs.

Sulphur and calcium are the elements that are part of the structural component of the cell and so are not released from the organs. This aspect of the mineral nutrition of the plants is of higher significance and importance to agriculture and horticulture.

The deficiency symptoms that are generally appearing in plants include necrosis, chlorosis, premature fall of buds and leaves, and ceasing of cell division. Chlorosis is the chlorophyll loss leading to yellowing of leaves, which is caused from the deficiency of elements like Fe, S, Mn, Mg, Zn, Mo, N,  and K. Necrosis is another symptom of the leaf that occurs due to the deficiency in Mg, Ca, K and Cu. Reduction in the levels of N, Mo, S and K stops the division of cells. Some elements such as Mo, N and S delay the flowering process when their levels in plants are low. Therefore, deficiency of elements can lead to several symptoms caused by the deficiency of various different elements. Same symptoms might be caused due to the deficiency of a single element out of many. To learn about the deficient element, the symptoms developed in all parts of the plant have to be studied and compared with the standard symptoms that are aleady given in the standard tables. It is also vital to know that different parts of the plant will show different symptoms in response to the same element.

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Role of macro and micro nutrients

Based on the criteria of essentiality, some of the elements are considered to be very much essential for the growth and metabolism of the plant. These elements are further classified into two groups. They are micronutrients and macronutrients.

Essential elements participate in the metabolic processes of the plant cell like regulating the osmotic concentration of the cell sap, permeability of the cell membrane, buffering action, electron transport systems, and enzymatic activity. These elements form major part of the macromolecules and co-enzymes.

The various functions of the mineral elements are given here:

Nitrogen

It is absorbed as nitrate, nitrite and ammonia. Nitrogen is taken by all parts of the plant including the meristematic tissues and metabolically active cells. Nitrogen is present in nucleic acids, proteins, hormones and vitamins.

Phosphorus

It is absorbed in the form of phosphate ions. Phosphorus is part of the cell membranes, nucleic acids, proteins, nucleotides and is required in all the phosphorylation reactions.

Potassium

It is absorbed as potassium ions and is utilized mostly in meristematic tissues, leaves, root tips, and buds. The anion-cation balance is maintained in the cells by the potassium. It is used in the protein synthesis, closure and opening of stomata, in the maintenance of cell turgidity and in the activation of enzymes.

Calcium

It is absorbed in the form of calcium ions. It is utilized by the meristematic and differentiating tissues. Calcium is used in the cell wall synthesis in the form of calcium pectate in the middle lamella. It is also used in the formation of the mitotic spindle. Calcium is used in the normal functioning of the cell membranes. It helps in the activation of the enzymes and plays an important role in maintaining the metabolic activities.

Magnesium

This divalent cation is involved in the activation of the enzymes of respiration, photosynthesis and in the synthesis of DNA and RNA. Magnesium is part of the chlorophyll structure and it helps in the maintenance of the structure of the ribosome.

Sulphur

It is absorbed in the form of sulphate and is present in two amino acids called cysteine and methionine. Sulphur is also the major constituent of coenzymes, ferredoxin and vitamins.

Ferrous

It is the major constituent of proteins like cytochromes and ferridoxin that are associated in the transfer of electrons. It is used in the chlorophyll synthesis.

Manganese

It is absorbed in the form of manganous ions. It activates the enzymes associated with respiration, photosynthesis and nitrogen metabolism. Manganese is involved in the water splitting to liberate oxygen at the time of photosynthesis.

Zinc

It is absorbed as zinc ions. It stimulates various enzymes like carboxylases. This enzyme is required in the auxin synthesis.

Copper

It is absorbed as cupric ions. It is associated with the plant metabolism. Iron is involved in the enzymes of the redox reactions.

Boron

It is absorbed as borate ion. Boron is necessary for the uptake of calcium and its utilization, pollen germination, cell differentiation, cell elongation and carbohydrate translocation.

Molybdenum

Plants absorb it in the form of molybdate ions. It is associated with the functioning of nitrogenase and nitrate reductase which participates in nitrogen metabolism.

Chlorine

It is absorbed in the form of chloride. It helps in the determination of solute concentration and anioncation balance in the cells.

 

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Types of Roots

Roots are helpful for the plants as they provide support to the stems as well as absorb water and minerals from the soil. Roots also store food materials. Root system is present in two types. They are primary root and secondary root.

Primary root originates from the embryo and is also called as tap root system. The branches in the tap root system are fibrous and are known as secondary roots. The primary root system of plant lives for short period and it is followed by secondary root system. The secondary roots are permanent roots in monocots like grasses. The roots from rhizomes, bulbs, corms and tubers are adventitious roots. They also arise from the aerial stems as well as from the stem cuttings and root cuttings. The plants like Ficus, Rhizophora and Pandanus sps., extend down the adventitious roots such that these roots act as support to the horizontal branches.

Root hairs are specialized extensions of the cells that can penetrate into the soil particles. The outer wall of the cell is expanded and forms an outgrowth tubular in shape. This outgrowth will make contact with the soil and absorb minerals and water from the soil. The mineral nutrients and water move from the soil into the root hairs followed by their entry into the rootlets and then into main root which transports water and minerals into the stem and leaves. More number of root hairs are formed at the tip of the roots and they are short-lived in most of the plants. The transplantation of plants from one place to another has to be done carefully to avoid the loss of these root hairs which help in the proper absorption of water and minerals.

Tap Roots

They are primary roots which give rise to several lateral roots or secondary roots. Tap roots divide, become fleshy and travel deep into the soil. Tap roots are seen to be present in perennial herbs like Quercus sps., Citrus sps., Daucus carota, and Taraxacum officinale, so on.

Fibrous Roots

These roots do not have a separate primary root. They have numerous small thread like roots that are similar in size and originate from the stem base. Fibrous roots spread on the soil surface instead of entering deep and straight into the soil. Fibrous roots are seen in grasses and in some shrubs.

Fleshy Roots

The fleshy roots are the storage places for food. They store excess food during the winter and adverse conditions such that the plant can make use of the food for its regrowth process. Some of the plants that come under this category are carrot (Daucus carota), turnips (Brassica rapa), beet root (beta vulgaris). These plants have tap root or main root storing the food. The secondary roots are transformed into tubers in the case of plants like sweet potatoes (Ipomea batatas) and Dahlia sps.

Aerial Roots

The aerial roots are formed in many of the land plants as well as in water plants. They are formed in the moist atmosphere. The aerial roots allow the climbers like Philodendron sps., to take the support of the host. The aerial roots in the epiphytes like orchids not only take the support of the host plants but, will absorb water from the air. The aerial roots are fleshy in many cases as they store water transforming into reservoirs.

Knees

The pneumatophores are developed in the plants like Taxodium distichum,ImageImageImageImage also called as Bald Cypress. The ground or water, cuts off the air from the roots and make the trees to develop knees. These knees are protruding on the surface, allowing the plant to take in air.

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Essential mineral elements required by plants

Minerals enter into plants through roots. Plants were found to be consisting of more than 60 elements. Selenium and gold are also accumulated by some plants. Plants surviving near nuclear test sites will absorb even radioactive strontium. How to check whether the plant is really in need of all the minerals. How to know about the essential minerals for the plants.

Criteria for essentiality

The reasons for the elements to be considered essential for the plant growth are mentioned below.

  • The normal growth and reproduction of the plant is supported and the plants depend on the elements to complete their life cycle and prepare the seeds for further propogation.
  • The requirement of the element is specific and cannot be replaced by another element.
  • This element is involved in the plant metabolism directly.

Based on the above mentioned features, a few elements were found to be completely essential for the plant metabolism and growth. These elements are further widely divided into two categories, like macro nutrients and micro nutrients.

Macronutrients

These are generally present in the plant tissues in large amounts, like 10mmole per kg of dry matter. The macronutrients include oxygen, hydrogen, nitrogen, carbon, sulphur, calcium, potassium, magnesium and phosphorus. Of all these elements, carbon, oxygen and hydrogen are mainly obtained from water and carbon-di-oxide, while the other elements are taken from the soil as mineral nutrition.

Micronutrients

These are called trace elements as they are necessary in small amounts. These nutrients include copper, manganese, iron, molybdenum, nickel, boron, chlorine and zinc.

Apart from the above mentioned nutrients, there are some more useful elements such as  selenium, sodium, cobalt and silicon which are used by the higher plants. Based on the relevant functions of the essential elements, they are classified into four categories. They are:

  • Essential elements that form part of the biomolecules and that are seen as structural part of the cell, like carbon, hydrogen, nitrogen and oxygen.
  • Essential elements that are part of the components of the chemical substances in plants, like phosphorus in ATP and magnesium in chlorophyll.
  • Essential elements that act as catalysts in activating or inhibiting the emzymes. For example, Mg2+ activates Ribulose bis-phosphate carboxylase oxygenase and phosphoenol pyruvate carboxylase, which are critical enzymes in photosynthetic carbon fixation processes. Nitrogenase is activated by molybdenum, alcohol dehydrogenase is activated by zinc during the process of nitrogen fixation. There are several other biochemical pathways that involve minerals in the process.
  • Essential elements can also change the osmotic potential in the cell. Stomatal opening and closure are influenced by potassium. Minerals show impact on determining the water potential in the cell.


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MINERAL NUTRITION OF PLANTS

The basic requirements of all the plants are macro molecules, such as carbohydrates, proteins, fats, minerals and water. These elements help in the growth and development of plants. This article discusses the various methods involved in the growth and development of the plants and the conditions that aid in monitoring the necessity of these methods.

Methods to analyze the mineral requirements of plants

A well-known botanist in 1860 called Julius Von Sachs could demonstrate that plants can be grown in a well-defined nutritious environment in the absence of the soil. This technique of growing plants in a nutrient solution is called “Hydroponics”. From then onwards various methods were developed and improved to find out the essential mineral nutrients for the plant growth. All these methods focused basically on growing the plants in a defined mineral solution in the complete absence of soil. Mineral nutrient salts and purified water were essential for running these methods.

Series of experiments were conducted in which plant roots were immersed in nutrient solution. Each of the elements were added and removed alternately to check the necessity of that particular element in the growth of the plant. Gradually, a mineral solution that is appropriate for the optimal plant growth was designed and developed. The essential elements for the plant were recognized by this method. Hydroponics is a technique that is used commercially in the production of vegetables, like lettuce, tomato and seedless cucumber. It has also been found that nutrient solutions have to be aerated to support optimal growth of plant.

Hydroponic plant production

Hydroponic plant production

Typical nutrient solution culture

Typical nutrient solution culture

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Pressure flow or mass flow hypothesis

The process of translocation of sugars from the source to the sink is known as a pressure flow hypothesis. Glucose is synthesized by photosynthesis and it is changed into sucrose. Sucrose moves into the companion cells and then into the sieve tube cells through the active transport process. The increase of sugar content in the source region makes the phloem to become hypertonic. The water moves from the adjacent xylem into the phloem through the process of osmosis. Once the osmotic pressure is increasing in the source region, the phloem sap moves to the region of lower osmotic pressure. The osmotic pressure of the sink has to be lower than that of the source.

The phloem sap moves out of the phloem into the cells that make use of the sugars by the active transport process. The sugars at the sink region are converted into energy, cellulose and starch. When the sugars are moved out from the source, the osmotic pressure reduces at that region and hence the water moves out of the phloem.

To conclude, the sugars move from the phloem after they are loaded into the sieve tubes by the active transport. The phloem is loaded to create the water potential gradient that can facilitate the movement of sap.

phloem transport

The phloem comprises of long sieve tubes that have sieve plates at the end of each cell. The sieve plates possess small holes. The cytoplasmic strands traverse through the sieve plate holes and form continuity in those filaments. The pressure flow starts when the hydrostatic pressure in the sieve tubes increases and the movement of sap occurs in the phloem. At the sink region, the sugars in the phloem are actively transported out of it, which get transformed into complex carbohydrates. The solute removal creates a higher water potential in the phloem, while the water moves out of the phloem into the nearest xylem vessels.

An experiment called ‘girdling’ helps in identifying the plant tissues that are involved in the transport of food materials. The bark that exists till the phloem layer towards inside, in the trunk region is removed. This stops the food movement downwards. After some time, it can be observed that the part of the stem that is above the removed region will be swollen. This simple experiment illustrates that the phloem is responsible for the transport of food and the food transport occurs in one direction itself, down towards the roots. This experiment can be easily performed by everyone.

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Phloem Transport – Flow from Source to Sink

The primary food source commonly called as sucrose is transported by the vascular tissue phloem from the source to the sink. The source is the plant part which can synthesize the food, like leaves. Sink is the plant part that requires the food and stores the food. The source and sink are reversed based on the season or the needs of the plant. The sugars stored in the roots would be made to move as the source of food in the spring while the tree buds act as sink receiving the food from the roots. The photosynthetic apparatus development and growth of the plant parts occurs by the energy generated from the food that is received by the sink. The positions of source and sink are variable and hence the movement of food through phloem can be bi-directional (upwards or downwards). The movement of water in the xylem is always in upward direction. The flow of water in transpiration is towards up and the movement of phloem sap occurs in both the directions, depending on the source of the sugar and sink that can make use, store and remove the sugars. The major constituents of phloem sap are water and sucrose while the hormones, amino acids and sugars are also translocated through the phloem.

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