Metabolic and physiological changes in the sugar beet under salt stress conditions

Salt stress is a major problem in global crop production. Sugar beet which is one of the world’s leading sugar crops has strong salt tolerant features than other crops. To investigate this, the response of sugar beet towards various levels of salt stress, sugar beet was grown hydroponically under 70mM, 140, 210 and 280mM NaCl concentrations with a control of 3mM concentration. 

There was no difference in the dry weight of the aerial parts and leaf area of the plant treated with 70mM salt stress and that of the control conditions. Dry weight of the root and the entire plant treated with 70mM salt conditions was less than that of the controls. The growth was arrested with increase in salt concentrations. 

The tissues of petioles and old leaves were with highest concentrations of Na+ and Cl-, which is nothing but salt stress. With the rise in NaCl concentrations, the content of nitrogen and potassium in the tissues of leaf, petiole and roots were decreased quickly and the content of phosphorus increased. The antioxidant enzymes like superoxide dismutase, catalase, ascorbate peroxidase and glutathione peroxidase showed higher activities in the high salt concentrations. 

As the external salt concentrations increased, osmoprotectants such as free aminoacids and betaine increased in their concentrations. Two organic acids such as malate and citrate that are involved in tricarboxylic acid cycle showed increased contents under the salt stress. It was also found that the activity of Rubisco also reduced with increase in salt stress. The activities of enzymes like NADP-malic enzyme, NADP malate dehydrogenase and phosphoenol pyruvate carboxylase first increased and then decreased. Their activities were seen higher with salinity at 140mM NaCl. 

This research study revealed the mechanisms of physiological and metabolic responses of sugar beet at various levels of salt stress.

Role of macro and micronutrients

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 a 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 sulfate and is present in two amino acids called cysteine and methionine. Sulfur is also the major constituent of coenzymes, ferredoxin, and vitamins.

Ferrous

It is the major constituent of proteins like cytochromes and ferredoxin that are associated with 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 anion-cation balance in the cells.

Symptoms of plants deficient in essential elements

If the essential elements are supplied in limited quantities, the growth of plant will be reduced. The concentration of essential elements below which the growth of the plant is reduced is known as the critical concentration of the element. If the element is present in a 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 from 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.

Sulfur 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 already 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.

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 propagation.

§  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 macronutrients and micronutrients.

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, sulfur, 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 inactivating or inhibiting the enzymes. For example, Mg2+ activates Ribulose bis-phosphate carboxylase oxygenase and phosphoenolpyruvate 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 of the cell. Stomatal opening and closure are influenced by potassium. Minerals show the impact on determining the water potential in the cell.

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.

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.

Transport and uptake of mineral nutrients

The carbon and oxygen for the plant are available from the carbon-dioxide present in the atmosphere. The hydrogen for the plants is obtained from the water and the mineral nutrients are obtained from the soil.

The plant roots cannot absorb mineral nutrients in a passive manner from the soil unlike they take up water. Minerals cannot be taken passively by the plants as they are present as ions or charged particles in the soil. These charged particles cannot get transported across cell membranes. The mineral concentration in the soil is found to be lower compared to that of the root. Hence, the minerals have to enter into the epidermal cells of the root through active absorption which means energy in the form of ATP is essential for the entry of minerals into the plant. The gradient of water potential in roots is caused by the active uptake of ions. Osmotic intake of water is also done by the active process. The charged ions also move into the epidermal cells in a passive manner.

The ions are absorbed from the soil through both the active as well as passive transport. The proteins present in the membrane of the root hair cells pump the ions from the soil into the epidermal cell cytoplasm. The endodermal plasma cell membrane consists of transport proteins that are usually present in all other cells. Some of the solutes cross the membrane transport proteins of the endodermal cells while some solutes are not transported. The screening of solute types that enter into the xylem and the quantity of them is monitored by the endodermal cells. The single direction active transport of ions occurs due to the suberin layer of the root endodermis.

Mechanism of water transport

Plants, unlike animals, do not possess any metabolically active pump, like the heart to carry the fluid in the vascular system. The movement of water in a passive manner occurs by pressure and by the gradient in the chemical potential. Another way of water movement that occurs in plants is called as a cohesion-tension mechanism. Here, the water movement is caused by the absorption and transportation of water bulk, driven by the negative pressure that is created by the transpiration or evaporation of water from the leaves. The forces created by hydrogen bonding are called as “cohesive” and the water movement is due to the cohesive nature of the water movement during transpiration.

The significant tension in the water columns of the plant is sustained by the hydrogen bonds. This tension is considered to be helpful in the movement of water to 100m above the soil surface. The cohesive-tension is generated by transpiration. The evaporation inside the leaves occurs from moist cell wall surfaces surrounded by the airspace network. At the interface of the air and water, menisci are formed. The apoplastic water present in the cell wall capillaries is connected with the air present in the sub-stomatal cavity. The sun’s energy used for breaking the hydrogen bonds between the molecules helps in the evaporation of water from the menisci. The surface tension in the water at menisci pulls away from the water molecules, to substitute the molecules that are lost due to the evaporation. This surface tension or force that is transmitted through the water columns into the roots will stimulate the water influx from the soil. The continuous water transport pathway is otherwise called as Soil Plant Atmosphere Continuum (SPAC) by the scientists.

The water movement in the plants is carried out by cohesive tension mechanism which is primarily suggested by Stephen Hales. The movement of a solute across the semi-permeable membrane is dependent on the water movement as per the chemical potential of water, by the process of osmosis. The water movement between the cells and plant compartments is governed mainly by the osmosis. In the transpiration deficiency, the movement of water into roots is dominated by osmotic forces. The osmotic forces are manifested as guttation and root pressure, which are usually observed in lawn grass. Guttation is the process where the water droplets are accumulated at the leaf margins when the evaporation is low. The root pressure occurs when the solutes are accumulated at higher concentrations in the root xylem than in the other tissues of the root. The root water influx is driven by chemical potential gradient across the root and into the xylem. The plants where the transpiration occurs very rapidly, do not contain root pressure. The root pressure is considered to be playing the main role in filling the non-functional xylem, especially after winter.