TRANSPORT OF MINERALS(Translocation of Solutes)
The absorbed mineral elements move laterally from the epiblema to xylem through the cortex, endodermis, and pericycle. Finally, they are transported upwards through xylem along with the transpiration stream. This has been demonstrated by ringing experiment that upward transport of minerals continues even when the phloem tissues were removed. Analysis of xylem sap also shows the presence of large amount of dissolved salts.
These include some organic compounds of nitrogen and phosphorus which are largely responsible for the movement of nitrogen and phosphorus.Some of the minerals are transferred radially from xylem to the phloem mainly through parenchymatous cells. They are also transported to other tissues where they accumulate in the living cells. Several evidences suggest that minerals are transported through phloem also. The translocation in phloem mainly occurs from the leaves just before the abscission.
ION TRAFFIC INTO THE ROOT
It is generally believed that mineral nutrients absorbed by the roots finally reach to the xylem after passing through the cortex, endodermis and pericycle. There are two mechanisms of the movement of minerals from the soil into dead xylem cells – apoplast and symplast.
The apoplastic pathway involves movement of minerals by simple diffusion from cell to cell through their primary cell walls (which is made up of polysaccharides). Minerals move through apoplast in cortex and pericycle but not in endodermis. The cell walls of endodermal cells possess a conspicuous waxy thickening, the Casparian strip, which blocks the passage of solutes from one side of the endodermis to the other via this cell wall route. Therefore, minerals can enter the pericycle and xylem by passing through the protoplast of endodermal cell.
Thus the movement of solutes is apoplasticin cortex and pericycle but symplastic in the endodermis.The symplastic pathway involves movement of minerals, absorbed actively through the plasma membrane of cells of the outer parts of the root (i.e., epidermis), through the protoplast of the cortex, endodermis and pericycle and finally released into the xylem. The cell-to-cell movement of ions in cortex takes place through plasmodesmata (protoplasmic connections between cells). The term “symplast” refers to a system of cells whose protoplasts are interconnected.Transport of mineral
NITROGEN NUTRITION IN PLANTS
The most widely used term ‘nitrogen’ correctly refers to the nitrogen atom (N). The molecular nitrogen (N₂ to N = N) is correctly termed as ‘dinitrogen’. However, in the present text we shall conveniently use the term nitrogen to denote molecular nitrogen or dinitrogen.The ultimate source of nitrogen is the nitrogen gas present in the atmosphere.
The atmospheric air around the earth comprises about 78% by volume of nitrogen molecules. The molecular nitrogen is a highly inert gas and it is energetically difficult for most of the living organisms, including the higher plants, to obtain it directly for their use. It must be fixed (i.e., combined with other elements such as carbon, hydrogen and oxygen) to form compounds (such as nitrates, nitrites, ammonium salts, etc.) before it is absorbed and utilised by the plants.
Higher plants generally utilize the oxidised forms such as nitrate (NO3) and nitrite (NO) or the reduced form (NH) of nitrogen which is made available by a variety of nitrogen fixers.Transport of mineral
The phenomenon of conversion of free nitrogen into nitrogenous salts to make it available for absorption by plants is called nitrogen fixation. On the basis of agency through which the nitrogen is fixed, the fixation of nitrogen is divided into two types-1. Physical nitrogen fixation and 2. Biological nitrogen fixation.
1. Physical Nitrogen Fixation
Out of total nitrogen fixed by natural agencies, approximately 10% of this occurs due to physical processes such as lightning (i.e., electric discharge), thunderstorms and atmospheric pollution. The – lightning and ultra-violet radiations in the atmosphere favours a combination of gaseous nitrogen and oxygen to form nitric oxide (NO).
The nitric oxides are again oxidized with oxygen to form nitrogen peroxide (NO2).the nitrogen peroxide can combine with water during rains to form nitrous acid and nitric acid. The acids fall on the ground along with rain water and react with alkaline radicals to produce. water-soluble nitrates (NO3) and nitrites
nitrates are soluble in water and are directly absorbed by the plants.
2. Biological Nitrogen Fixation:
The conversion of atmospheric nitrogen into inorganic or organic usable forms through the agency of living organism is called biological nitrogen fixation. The process is carried by two main types of micro-organisms: those which are “free-living” or a symbiotic and those which live in close symbiotic association with other plants. However, a third category of microbes which fix nitrogen in association with roots of cereals and grasses has been recognised. It is called associative symbiotic nitrogen fixation.
(a) Asymbiotic biological nitrogen fixation.
A large number of free-living bacteria and cyanobacteria fix atmospheric nitrogen into usabic form. These microorganisms can be categorized as follows-
(1) Free living aerobic nitrogen-fixing bacteria (eg, Azotobacter, Beijerinckia, Dexia, etc.).
(ii) Pree living anaerobic nitrogen-fixing bacteria (eg Clostridium pasteurianum, Bacillus, etc.)
(iii) Free living photoautotrophic nitrogen-fixing bacteria (e.g Chlorobium, Rhodopseudomonas. Rhodospirellum, etc.)
(iv) Free living chemosyn- thetic nitrogen fixing bacteria (e.g., Desulfovibrio)
(v) Free living nitrogen fixing Cyanobacteria – A large number of heterocystous Cyanobacteria (or Blue-green algae) are known to fix atmospheric nitrogen. A few of them are – Nostoc, Anabaena, Autosira, Cylindrospermum, Trichodesmium, Calothrix, etc. Some of the free-living cyanobacteria are nitrogen fixers of rice fields (Aulosira fertilis- sima). Cylindrospermum is an active nitrogen fixer of Sugar cane and Maize fields.
(b) Symbiotic biological nitrogen fixation.
Many bacteria and cyanobacteria fix atmospheric nitrogen in symbiotic association with other plants. Some common examples are
(i) Bacteria Rhizobium in association with roots of leguminous plants;
(ii) Anthoceros (a Bryophyte) in association with Nostoc;
(iii) Azolla (a water fern) in association with Anabaena azollae;
(iv) Coralloid roots of Cycas (a gymnosperm) in association with Anabaena and Nostoc; etc.
Symbiotic Nitrogen Fixation In Leguminous Plants
One of the most important nitrogen-fixing bacteria – Rhizobium fixes nitrogen in symbiotic association with roots of leguminous plants (e.g., Pea, Beans, Alfa-alfa, Clover, Lupines, etc.) Initially the bacteria grow in the soil near the roots of higher plants where they fail to fix nitrogen. These bacteria, when come in contact with the roots of leguminous plants, interact chemically and enter into roots through root hairs. Entry of bacteria into the root hair depends on various chemical signals sent from the root hair.
The enzymes from the bacteria degrade the parts of root hair cell wall which produces a thread like structure called infection thread. The bacteria multiply and invade the infection thread. Finally the invading bacteria reach up to the inner cortex where they enter into cells (particularly tetraploid cells) and stimulate them to divide. The proliferating cells form a knob-like protuberance called root-nodule The bacteria multiply and colonize inside the tetraploid cells until the available cytosol is filled,
They then become dormant. Each enlarged non-motile bacterium is called a bacteroid A typical root nodule cell contains several thousand bacteroids. Usually the bacteroids occur inside the cytoplasm in groups. Each group of bacteroids is surrounded by a membrane called a peribacteroid membrane. The space sur- rounded by the peribacteroid membrane is called peribacteroid space. A red pigment- Leghaemoglobin is filled outside the peribacteroid
space in the cytosol of nodule cells. The pigment leghaemoglobin is similar to hemoglobin of red- blood cells. It has the ability to combine very rapidly with oxygen and thus acts as a very efficient avenger.Transport of mineral
Mechanism of Biological Nitrogen Fixation
The basic requirements of nitrogen fixation are-
(1) Presence of enzyme nitrogenase and hydrogenase;
(2) A protective mechanism for the enzyme nitrogenase against O2;
(3) A non-heme iron protein-ferredoxin as an electron carrior;
(4) The hydrogen donating system (viz., pyruvate, hydrogen, sucrose, glucose, etc.);
(5) A constant supply of ATP:
(6) Presence of thiamine = pyrophosphate (TPP), coenzyme-A, inorganic phosphate and Mg++ as co-factors;
(7) Presence of cobalt and molybdenum; and
(8) A carbon com- pound for trapping released ammonia.
In the process of biological nitrogen fixation by free-living and symbiotic nitrogen fixers, the dinitrogen molecule (NN) is progressively reduced step-by-step to ammonia (NH) by the addition of pairs of hydrogen atoms. Pyruvic acid serves as an electron donor (hydrogen donor) in most of cases. However, other electron donating systems such as hydrogen, sucrose, glucose, etc., have also been shown to operate in different systems. In leguminous plants, the glucose-6-phosphate molecule probably acts as a substrate for donating hydrogen. The overall process occurs in presence of enzyme- Nitrogenase, which is active in anaerobic condition.Transport of mineral
The enzyme nitrogenase consists of two sub-units-
(i) a non-heme iron protein commonly called Fe protein (or dinitrogen reduc- tase) and
(ii) an iron-molybdenum protein called MoFe-protein (or dinitrogenase).
Both the sub- units are needed for the activity of enzyme. The Fe- protein component reacts with ATP and reduces Mo Fe protein which then reduces N₂ to ammonia. The overall biochemical pathway of electron transport in nitrogen fixation The product of nitrogen fixation is ammonia which is toxic to plants. The ammonium ions are, however, safely assimilated by higher plants. In most of the cases, the ammonium ions are transformed into amino acids and then translocated-
In case of legumes, the ammonia is taken up by host and immediately assimilated into organic form (i.e., amino acids, amides or ureides) and then translocated through vascular tissues to other parts of the plant.Most of the ammonia produced from free living nitrogen fixers is either directly metabolized by the microorganisms, absorbed by the higher plants or converted to nitrates by the process known as nitrification.Transport of mineral
Ammonia is oxidised to nitrite and nitrate ions by a group of soil-inhabiting nitrifying bacteria e.g., Nitrosomonas and Nitrobacter. The nitrifying bacteria obtain their energy from the oxidation of ammonium and nitrite ions. The Nitrosomonas group oxidizes ammonia to nitrite – The nitrates (NO3), produced by nitrification, are absorbed by higher plants and assimilated by the process called nitrate assimilation.Transport of mineral
Nitrate Assimilation in Plants
Absorption of nitrate by plant roots from the soil is a carrier-mediated, active and energy-dependent process. The nitrates absorbed by plant roots get converted to amino acids and amides before incorporating into proteins and other macromolecules. The reduction of nitrate into ammonia is called nitrogen assimilation. Transport of mineral
In most plants the reduction of nitrate occurs in the root tissues and then transported to shoot through xylem whereas in others, the process occurs in leaves and stems. The overall summary equation of reduction of nitrate to ammonia is as follows -NO3 + 8 electrons + 10 H—–>NH + 3H2O
This process consists of the following two distinct enzymic steps –
(1) First step is the conversion of nitrate to nitrite.
The reaction is catalyzed by enzyme-nitrate reductase (sulfhydryl containing molydoflavohemoprotein). This step occurs in the cytosol outside any organelle and required NADH (or in few species NADPH) as an electron donor, FAD a prosthetic group, as Cytochrome b557 as an electron carrier and molybdenum (Mo) as an activator of enzyme.Transport of mineral
2) The second step is the reduction of nitrite to ammonium ion.
The reaction is catalysed by enzyme nitrite reductase. The most probable electron donor in the reaction appears to be reduced ferredoxin (an iron-containing, nonheme protein of low molecular weight). The ferredoxin is reduced inthe light reaction of photosynthesis in green leaves. The nature of electron donor in the nongreen tissue (e.g., roots) is not known. In the nongreen tissues some unknown electron carrier , generated in respiratory metabolism donates electrons.Transport of mineral
Bio synthesis of amino acid
Nitrogen assimilation results in the formation of ammonia which is used for the synthesis of amino acids. There are two ways through which amino acids are synthesized-
1. Reductive amination and
1. Reductive amination:
The organic acid- a-keto glutaric acid plays a key role in the synthesis of amino acid. The ammonia, formed by nitrogen assimilation (i.e., reduction of nitrates), reacts with a-ketoglutaric acid to form the amino acid- glutamic acid. In this process, a-ketoglutaric acid comes from krebs cycle and hydrogen is donated by the coenzyme NADH or NADPH. The reaction occurs in presence of enzyme glutamic dehydrogenase. The reaction occurs in two steps as summerized below:Transport of mineral
Once the glutamic acid is synthesized by reductive amination, other amino acids are synthesized by the transfer of its amino group to other carbon skeletons. Therefore, glutamic acid is used as a starting material for the synthesis of other amino acids. Such a transfer of an amino group (NH) from an amino donor compound to the carbonyl position (CO) of an amino acceptor compound is called transamination.
More than 17 amino acids may be formed from glutamic acid by the process of transmination. The reaction occurs in presence of enzyme-transaminase and coenzyme pyridoxal phosphate (a derivative of vitamin B, or pyridoxine).The most common example of transamination reaction in which glutamic acid combines with oxalacetic acid (a member of krebs cycle) to form the amino acid-aspartic acid and release a- ketoglutaric acid is given below:Transport of mineral
Assimilation of ammonia in most of the higher plants involves the formation of amides. Amides are generally formed by combination of ammonia and amino acids. The two most common amides found in plants are glutamine and asparagine. Glutamine is formed by combination of ammonia and glutamic acid In this reaction, the hydroxyl part of glutamic acid is replaced by another NH, radicle. The reaction occurs inpresence of enzyme-glutamine synthetase.Transport of mineral