Plant Mineral Nutrition & Hydroponics

• Carbon, hydrogen, and oxygen are essential for carbohydrates, fats, and proteins
• Besides carbon, hydrogen, and oxygen, plants require other elements for growth.
• Plants absorb minerals from the soil through their roots, mainly as inorganic ions

Mineral Nutrition

• Absorption and utilization of mineral elements for growth and development

Soilless Culture / Hydroponics

• Plants are cultivated by directly placing roots in nutrient solution
• Julius Sachs (1860) demonstrated that plants can mature in a defined nutrient solution without soil
• Experiments involve adding, removing, or varying element concentrations to obtain suitable mineral solutions for plant growth
• Regular purification of water and nutrient salts is required, and nutrient solutions must be aerated for optimal growth
• Essential elements can be identified and deficiency symptoms can be discovered via hydroponics
• Used for commercial production of vegetables like tomatoes, seedless cucumbers, and lettuce
• Vegetables and flowers can be cultivated year-round, using hydroponics
• Gericke developed hydroponics in 1940

Criteria for Essentiality of an Element

• Plants absorb minerals from the soil, with over 60 of 105 elements found in different plants
• Essentiality is based on water culture experiments proposed by Arnon and Stout in 1939
• Elements must be necessary for normal growth and reproduction
• Element requirements must be specific and not replaceable
• Elements must be directly involved in metabolism or biological processes

Examples

• Magnesium is essential and irreplaceable because it is a chlorophyll constituent for photosynthesis
• Magnesium is a cofactor for enzymes in cellular respiration and metabolic pathways
• Iron is essential, irreplaceable and a constituent of cytochromes

Notes:

• Elements satisfying all three criteria are essential
• Elements not satisfying the above criteria are inert or non-essential

Essential and Non-Essential Elements

• About 20 elements have been found to be essential, 17 are listed
• Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N)
• Phosphorus (P), Potassium (K), Calcium (Ca)
• Magnesium (Mg), Sulphur (S), Zinc (Zn)
• Iron (Fe), Copper (Cu), Manganese (Mn), Molybdenum (Mo)
• Boron (B), Chlorine (Cl), Nickel (Ni)
• Some additional elements have been found to be essential for some plants
• Cobalt (Co), Silicon (Si), Vanadium (V)
• Sodium (Na), Selenium (Se), Aluminium (Al)
• Silicon is required by most grasses and cereals.
• Sodium is involved in membrane permeability
• Functional elements or non-essential functional elements include silicon/sodium
• Sodium (Na) and iodine (I) are essential for animals, but the majority of plants do not need them

Macro-elements and Micro-elements

• Essential elements are differentiated on the basis of the quantitative proportion of elements found in plants
• Macroelements
• Microelements

Macroelements (Macronutrients)

• Essential elements at easily detectable quantities, exceeding 10 m mole kg⁻¹ of dry matter
• The 9 macroelements are:
• Carbon
• Hydrogen
• Nitrogen
• Oxygen
• Phosphorous
• Sulphur
• Potassium
• Calcium
• Magnesium

Microelements (Micronutrients)

• Found only in traces in plants, i.e., less than 10 m mole kg⁻¹ of dry matter
• The 8 microelements are:
• Iron
• Manganese
• Zinc
• Copper
• Molybdenum
• Boron
• Chlorine
• Nickle
• Macroelements are involved in organic molecule synthesis and osmotic potential
• Carbon, hydrogen, and oxygen constitute about 96% of the dry matter in plants
• Microelements function mostly in enzymes as cofactors or metal activators
• Besides the 17 essential elements, sodium, silicon, cobalt, and selenium are specially required by higher plants

Types of Essential Elements

• Grouped into four broad categories based on diverse functions
• Structural elements: C, H, O, N are found in biomolecules
• Components of energy-related compounds: Mg and P are part of chlorophyll and ATP/ADP
• Activators or inhibitors of enzymes:
• Molybdenum which is an activator of nitrogenase enzyme
• Zinc is an activator of alcohol dehydrogenase enzyme
• Magnesium which is an activator for RUBP carboxylase-oxygenase and PEP carboxylase enzymes
• Osmotic potential maintaining elements
• Potassium is important for opening and closing stomata
• Chlorine which plays an important part
• Sodium which is important for some plants

Sources of Essential Elements for Plants

• Elements enter plants via the atmosphere, water, and soil
• Soil is a reservoir, rich in ions, inorganic salts, air, water, and microbes
• Plants obtain carbon from atmospheric carbon dioxide
• Plants obtain hydrogen mainly from water
• Plants obtain oxygen from air or water, often as inorganic ions
• Plants absorb nitrogen in compound state from the soil
• Plants derive other needed elements from the parent rocks by disintegration and weathering

Non-Mineral and Mineral Elements

• Essential elements obtained by the plants from the soil are called mineral elements, most of the elements are mineral elements
• The essential elements (such as carbon, hydrogen, and oxygen), obtained by the plants from air or water are known as non-mineral elements
• Nitrogen is considered a unique element, as it is derived from both mineral and non-mineral sources
• The required elements for various metabolic activities like regulation of cell membrane permeability, osmotic pressure of cell sap are replenished by fertilizers
• Nitrogen, phosphorous, and potassium fertilizers are key for plants

Critical Concentration

• The concentration of the essential element below which growth in the plant is stunted
• Elements at or below this concentration becomes deficient
• Abnormalities in plants are called deficiency symptoms, caused by a deficiency of essential elements, also known as hunger signs
• Deficiency symptoms disappear when the deficient mineral nutrient is provided
• Prolonged deficiency can cause death

Role and Deficiency of Macro Nutrients in Plants

• Nitrogen (N2)
• Absorbed as: Mainly as NO3- also taken up as NO2-and NH4+
• Functions: Part of proteins, chlorophyll, cytochromes, phytochromes, hormones (auxins, cytokinins), nucleic acids (DNA, RNA), NAD etc
• Serves as enzymes and promotes vegetative growth
• Deficiency Symptoms: Stunted growth, Chlorosis, dormancy of causal buds
• Phosphorus (P)
• Absorbed as: Phosphate ions (H2PO4-or HPO42-)
• Functions: Part of RNA, DNA, NADP, ATP, phospholipids, etc. Important in energy transfer reactions in cell metabolism and a constituent of the cell membrane
• Deficiency Symptoms: Poor growth, leaves are dull green, seed germination delays, purple or red spots on leaves appear, and premature leaf fall occurs
• Sulphur, absorbed as Sulphate SO42-
• Part of proteins (amino acids-cysteine, methionine), vitamins (biotin, thiamine, Coenzyme A) and ferredoxin
• Is related with: Chlorosis of younger leaves and stunted growth
• Potassium (K)
• Absorbed as potassium ions (K+)
• Functions: Helps maintain anion-cation balance in cells, involved in protein synthesis, opening and closing of stomata, enzyme activation, and turgidity of cells, electro-osmotic flow of sucrose across sieve plates
• Deficiency Symptoms: Stunted growth, yellow leaves edges, mottled appearance of leaves, premature death
• Calcium (Ca)
• Absorbed as calcium ions (Ca 2+)
• Participates in cell wall synthesis (middle lamella), activator of amylase, ATPase, phospholipase, mitotic spindle during cell division, normal membrane functioning
• Relates to Stunted growth and chlorosis of young leaves
• magnesium (Mg)
• Absorbed as Divalent (Mg2+)
• Activates enzymes in phosphate metabolism, a constituent of chlorophyll, maintains ribosome structure
• deficiency Symptoms: Chlorosis between the leaf veins, narcosis purple colours spots on older leaves

Role and Deficiency of Micro Nutrients in Plants

• Iron (Fe)
• Obtained as Ferric ions (Fe3+)
• Functions as a constituent of Ferredoxin and cytochrome and is needed for the synthesis of chlorophyll.,
• Deficiency Symptoms: Chlorosis of leaves
• Manganese (Mn)
• Obtained as Manganous ions (Mn2+)
• Functions: Activates certain enzymes involved in photosynthesis, respiration and nitrogen metabolism and it has photolysis of water in photosynthesis,
• Deficiency Symptoms: Chlorosis and grey spots on leaves
• Zinc (Zn)
• Obtained as Zn2+
• Functions: Activates various enzymes like carboxylases Required for the synthesis of auxins
• Deficiency Symptoms: Malformation of leaves
• Copper (Cu)
• Obtained as Cu2+
• Functions as Activates certain enzymes and is essential for overall metabolism
• Deficiency Symptoms: Stunted growth, inter-veinal chlorosis in leaves, necrosis of the tip of young leaves, die back of shoot
• Boron (B)
• Obtained as BO33–, B4O72–
• It's required for uptake of water and Calcium (Ca), for membrane functioning, pollen germination, cell elongation and carbohydrate translocation
• deficiency Symptoms are death of stem and root apex, loss of a folical dominance, abscission of flowers, small size of fruits
• Molybdenum (Mo)
• Obtained as MOO2 2+ (molybdate ions)
• Functions: Activates certain metabolism.
• Deficiency Symptoms; Nitrogen deficiency interveinal chlorosis, retardation of growth
• Chlorine (Cl)
• Obtained as Cl–
• Functions: Maintains solute concentration along with Sodium (Na+) & Potassium (K+), It also maintains anion-cation balance in cells and it is essential for oxygen evolution in photosynthesis.
• Deficiency Symptoms: Wilted leaves, stunted root growth and reduced fruiting

Appearance of Deficiency Symptoms in Plants

• Depend on the mobility of the elements
• Actively mobilized elements: Deficiency first appears in older parts
• E.g. nitrogen, potassium, magnesium
• Immobile elements: Deficiency appears first in younger tissue
• E.g. sulphur and calcium

Symptoms Caused by the Deficiency of Minerals at a Glance

• Chlorosis or yellowing, can result from a deficiency of N, K, Mg, S, Fe, Mn, Zn, Mo.
• Stunted growth can be related to N, K, Ca, S, Zn, B, Mo, Cl.
• Interveinal chlorosis can be due to an iron deficiency.
• Purple coloration can be a result of a deficiency of N, P, S, Mg, or Mo.
• Necrosis, or cell death, can be the result of a shortage of Ca, Cu, K, or Mg.
• Premature leaf and bud fall can be symptoms of a P, Mg, or Cu deficiency.
• Inhibition of cell division can result from a deficiency of N, S, Mo, K.
• Wrinkling of cereal grains indicates a possible need for N, S, or Mo.
• Dormancy of lateral buds indicates a need for a N, S, or Mo supplement.
• Delayed flowering indicates potential issues from N, S, or Mo deficiencies.
• Dieback of stem and leaves can happen due to an imbalance of K, or Cu.
• Wilted leaves indicate the possible deficiency of Cl.
• Death of shoot and root tip is shown with B levels.
• Bushy habit of stem may appear with K deficiency
• Scorched leaf tips may appear with K levels

Toxicity of Micronutrients

• An excessive amount of a mineral element can be toxic
• Minerals have a narrow range of requirements, becoming toxic at low concentrations
• If the concentration of a mineral reduces the dry weight of tissue by 10%, it is considered toxic
• Critical toxicity concentration varies for different micronutrients
• For example, manganese is toxic for the soya bean at 600 µg g-1. For sunflower to be toxic it has to exceed 5300 µg g-1
• Manganese competes for uptake with iron and magnesium and for binding with enzymes, it competes with magnesium
• Manganese inhibits calcium translocation towards shoot.
• Prominent features of manganese toxicity are brown spots bordered by chlorotic veins
• Excess manganese can cause deficiency of iron, magnesium, and calcium

Mechanism of Absorption of Mineral Elements

• Involves two phases: initial and metabolic
• Initial Phase: Rapid and passive uptake of ions into the intracellular or free spaces of cells (apoplast)
• Metabolic Phase: Slow movement of ions into the cell cytoplasm and vacuole (symplast)
• It's an active process and the requires metabolic energy

Translocation of Solute

• Transportation of minerals is through the xylem, along with ascending water
• Transpiration drives mineral movement

Metabolism of Nitrogen and the Nitrogen Cycle

• Molecular nitrogen needs to be fixed, that is, combined with other elements for utilization.
• Nitrogen cycle: Conversion of nitrogen (N2) to ammonia is termed as nitrogen fixation
• Occurs via: physical and biological processes
• Physical Nitrogen Fixation: Can be natural, or Industrial
• Natural nitrogen fixation
• Atmospheric nitrogen combines with oxygen during electrical discharge resulting in nitric oxide (NO)
• Nitric oxide is oxidized to nitrogen peroxide (NO2) that combines with rainwater to form nitrous and nitric acids.
• The acids react with alkaline radicals in the ground to form water-soluble nitrates and nitrites

Industrial Nitrogen Fixation

• Ammonia is produced through direct combination of N2 with H2 at high pressures and temperatures
• Converted to fertilizers like urea

Ammonification

• Decomposes organic nitrogen from dead plants and animals into ammonia
• Carried out by Bacillus ramosus, Bacillus vulgaris and Bacillus mycoides

Nitrification

• Ammonia is converted into nitrate by soil bacteria via these steps:
• Ammonia is oxidized to nitrite by Nitrosomonas and Nitrococcus
• Nitrite is further oxidized to nitrate by Nitrobacter
• Nitrosomonas. Nitrococcus, and Nitrobacter are nitrifying bacteria and chemoautotrophs
• Nitrate is absorbed by plants and translocated to the leaves. In leaves, it is reduced to ammonia that forms an amine group

Denitrification

• Nitrate in the soil is reduced to nitrogen by Pseudomonas and Thiobacillus, and is mixed and released into the atmosphere.
• Nitrogen is fourth prevalent element in living organisms. Plant compete with microbes for the limited nitrogen

Biological Nitrogen Fixation

• Reduction of nitrogen to ammonia carried out by prokaryotes called N2-fixers
• Fixers use Molybdenum
• Two methods of Nitrogen Fixation
• Asymbiotic
• Symbiotic

Asymbiotic Nitrogen Fixers

• Aerobic: Azotobacter, Beijerinckia, Derxia
• Anaerobic: Rhodospirillum, Clostridium, Bacillus
• Cyanobacteria: Nostoc, Anabaena, Aulosira, Cylindrospermum, Trichodesmium
• Bacteria: Pullularia, yeasts; Rhodopseudomonas, Chlorobium, Rhodospirillum
• Chemosynthetic: Desulfovibrio

Symbiotic Nitrogen Fixers

• Frankia produces nodules in non-leguminous angiosperms (Casuarina, Alnus) and gymnosperms (Cycas, Podocarpus)
• Cyanobacteria establish symbiotic relationships
• Nostoc is a liverwort Anthoceros symbiont, and Anabaena is within Azolla
• Cycas's coralloid roots harbour blue-green algae, like Nostoc, Anabaena

Mechanism of Biological Nitrogen Fixation

• Dinitrogen is reduced with hydrogen that involves:
• A strong reducing agent like NADPH2, FMNH2
• A source of energy like ATP
• Nitrogenase and compounds for trapping ammonia formed
• Dependent upon type of nitrogen fixer, ATP and reducing agents are provided by either respiration or photosynthetic metabolism
• Nitrogenase has Molybdenum and Iron and weakens bonds between the two atoms of the latter

= The weakened molecule of nitrogen is reduced via Reducing agent (NADPH, FMNH2)

• This produces Diazene (N2H2) and Ammonia
• Ammonia is toxic. Nitrogen fixers protect themselves by providing organic acids which react with ammonia
• nod genes and bacterial nod, nif and fix control nitrogen fixation

Root Nodule Formation

• Small, irregular outgrowths are called root nodules
• Bacteria are able to colonise and mutiply
• Chemical substances cause haircurling, creating an infection thread in cells
• Bacteroids of bacteroids do not transform and faciliate infection
• Vascular connections for mutal exchange are produced
• Genes on cluster for nod and fix determine nitrogen fixation

Root Nodule

• Appears pink/red because of containing pigment called leghaemaglobin
• Combines with Oxygen and works as scaveneger
• Nitrogen fixing happens under anaerobic condition protected by leghemaglobin

Nitrate Assimilation in Plants

• Is the most important source of Nitrogen
• Plants can also reduce into ammonia before incorporating it for organic compounds, this process id completed via reduction of nitrate in different steps
• It goes to reduction where the Reduction occurs and is reduced via enzyme called 'nitrate reductase'
• At the nitrite end, nitrite reduces, doesn't require molybendum and goes through the cells
• Lastly, it has a fate to form Ammonium which transaminates to then create amino acids
• Reductive Amination: Ammonia reacts with carboxylic acids to form glutamic acid.
• Transamination: Transfer of amino group done catalysed by the Enzyme Transaminase.

Amides

• Amydes Transports
• Are catalysed, reacted via enzyme called Transaminase.
• Transmination happens through Xylem Vessels