Plant Growth

What nutrients do plants require?

Despite this underlying similarity, plants produce a vast array of chemical compounds with unusual properties which they use to cope with their environment. Pigments are used by plants to absorb or detect light, and are extracted by humans for use in dyes. Other plant products may be used for the manufacture of commercially important rubber or bio-fuel. Perhaps the most celebrated compounds from plants are those with pharmacological activity, such as salicylic acid or aspirin, morphine, and digitalis. Drug companies spend billions of pounds each year researching plant compounds for potential medicinal benefits

Plants require some nutrients, such as carbon and nitrogen, in large quantities to survive. Such nutrients are termed macro-nutrients (as mentioned in Soil Science section), where the prefix macro- (large) refers to the quantity needed, not the size of the nutrient particles themselves. Other nutrients, called micro-nutrients, are required only in trace amounts for plants to remain healthy. Such micro-nutrients are usually absorbed as ions dissolved in water taken from the soil, though carnivorous plants acquire some of their micro-nutrients from captured prey.



Carbon forms the backbone of many plants bio-molecules, including starches and cellulose. Carbon is fixed through photosynthesis from the carbon dioxide in the air and is a part of the carbohydrates that store energy in the plant.


Hydrogen also is necessary for building sugars and building the plant. It is obtained almost entirely from water. Hydrogen ions are imperative for a proton gradient to help drive the electron transport chain in photosynthesis and for respiration.


Oxygen is necessary for cellular respiration. Cellular respiration is the process of generating energy-rich adenosine triphosphate (ATP) via the consumption of sugars made in photosynthesis. Plants produce oxygen gas during photosynthesis to produce glucose but then require oxygen to undergo aerobic cellular respiration and break down this glucose and produce ATP.


Phosphorus is important in plant bioenergetics. As a component of ATP, phosphorus is needed for the conversion of light energy to chemical energy (ATP) during photosynthesis. Phosphorus can also be used to modify the activity of various enzymes by phosphorylation, and can be used for cell signaling. Since ATP can be used for the biosynthesis of many plant bio-molecules, phosphorus is important for plant growth and flower and seed formation. Phosphate esters make up DNA, RNA, and phospholipids. Most common in the form of polyprotic phosphoric acid (H3PO4) in soil, but it is taken up most readily in the form of H2PO4. Phosphorus is limited in most soils because it is released very slowly from insoluble phosphates. Under most environmental conditions it is the limiting element because of its small concentration in soil and high demand by plants and microorganisms. Plants can increase phosphorus uptake by a mutualism with mycorrhiza. A Phosphorus deficiency in plants is characterized by an intense green coloration in leaves. If the plant is experiencing high phosphorus deficiencies the leaves may become denatured and show signs of necrosis. Occasionally the leaves may appear purple from an accumulation of anthocyanin. Because phosphorus is a mobile nutrient, older leaves will show the first signs of deficiency. It is useful to apply a high phosphorus content fertilizer, such as bone meal, to perennials to help with successful root formation.


Potassium regulates the opening and closing of the stomata by a potassium ion pump. Since stomata are important in water regulation, potassium reduces water loss from the leaves and increases drought tolerance. Potassium deficiency may cause necrosis or interveinal chlorosis. K+ is highly mobile and can aid in balancing the anion charges within the plant. It also has high solubility in water and leaches out of soils that rocky or sandy that can result in potassium deficiency. It serves as an activator of enzymes used in photosynthesis and respiration. Potassium is used to build cellulose and aids in photosynthesis by the formation of a chlorophyll precursor. Potassium deficiency may result in higher risk of pathogens, wilting, chlorosis, brown spotting, and higher chances of damage from frost and heat.


Nitrogen is an essential component of all proteins. Nitrogen deficiency most often results in stunted growth, slow growth, and chlorosis. Nitrogen deficient plants will also exhibit a purple appearance on the stems, petioles and underside of leaves from an accumulation of anthocyanin pigments. Most of the nitrogen taken up by plants is from the soil in the forms of NO3. Amino acids and proteins can only be built from NH4+ so NO3 must be reduced. Under many agricultural settings, nitrogen is the limiting nutrient of high growth. Some plants require more nitrogen than others, such as corn (Zea mays). Because nitrogen is mobile, the older leaves exhibit chlorosis and necrosis earlier than the younger leaves. Soluble forms of nitrogen are transported as amines and amides.


Sulphur is a structural component of some amino acids and vitamins, and is essential in the manufacturing of chloroplasts. Sulphur is also found in the Iron Sulphur complexes of the electron transport chains in photosynthesis. It is immobile and deficiency therefore affects younger tissues first. Symptoms of deficiency include yellowing of leaves and stunted growth.


Calcium regulates transport of other nutrients into the plant and is also involved in the activation of certain plant enzymes. Calcium deficiency results in stunting.


Magnesium is an important part of chlorophyll, a critical plant pigment important in photosynthesis. It is important in the production of ATP through its role as an enzyme co-factor. There are many other biological roles for magnesium. Magnesium deficiency can result in interveinal chlorosis.


In plants, silicon strengthens cell walls, improving plant strength, health, and productivity. Other benefits of silicon to plants include improved drought and frost resistance, decreased lodging potential and boosting the plant's natural pest and disease fighting systems. Silicon has also been shown to improve plant vigour and physiology by improving root mass and density, and increasing above ground plant biomass and crop yields. Although not considered an essential element for plant growth and development (except for specific plant species - sugarcane and members of the horsetail family), silicon is considered a beneficial element in many countries throughout the world due to its many benefits to numerous plant species when under abiotic or biotic stresses. Silicon is currently under consideration by the Association of American Plant Food Control Officials (AAPFCO) for elevation to the status of a plant beneficial substance. Silicon is the second most abundant element in earth’s crust. Higher plants differ characteristically in their capacity to take up silicon. Depending on their SiO2 content they can be divided into three major groups:

  • Wetland graminae-wetland rice, horse tail (10-15%)

  • Dryland graminae-sugar cane, most of the cereal species and few dicotyledons species (1-3%)

  • Most of dicotyledons especially legumes (<0.5%)

  • The long distance transport of Si in plants is confined to the xylem. Its distribution within the shoot organ is therefore determined by transpiration rate in the organs

  • The epidermal cell walls are impregnated with a film layer of silicon and effective barrier against water loss, cuticular transpiration rate in the organs.

Si can stimulate growth and yield by several indirect actions. These include decreasing mutual shading by improving leaf erectness, decreasing susceptibility to lodging, preventing Mn and Fe toxicity.


Some elements are directly involved in plant metabolism. However, this principle does not account for the so-called beneficial elements, whose presence, while not required, has clear positive effects on plant growth. Mineral elements which either stimulate growth but are not essential or which are essential only for certain plant species, or under given conditions, are usually defined as beneficial elements.


Iron is necessary for photosynthesis and is present as an enzyme co-factor in plants. Iron deficiency can result in interveinal chlorosis and necrosis. Iron is not the structural part of chlorophyll but very much essential for its synthesis.


Molybdenum is a co-factor to enzymes important in building amino acids. Involved in Nitrogen metabolism. Mo is part of Nitrate reductase enzyme.


Boron is important for binding of pectins in the rhamnogalacturonan II region of the primary cell wall, secondary roles may be in sugar transport, cell division, and synthesising certain enzymes. Boron deficiency causes necrosis in young leaves and stunting.


Copper is important for photosynthesis. Symptoms for copper deficiency include chlorosis. Involved in many enzyme processes. Necessary for proper photosythesis. Involved in the manufacture of lignin (cell walls). Involved in grain production.


Manganese is necessary for building the chloroplasts. Manganese deficiency may result in coloration abnormalities, such as discoloured spots on the foliage.


Sodium is involved in the regeneration of phosphoenolpyruvate in CAM and C4 plants. It can also substitute for potassium in some circumstances.

  • Essential for C4 plants rather C3

  • Substitution of K by Na: Plants can be classified into four groups:

Group A- a high proportion of K can be replaced by Na and stimulate the growth, which cannot be achieved by the application of K

Group B-specific growth responses to Na are observed but they are much less distinct Group C-Only minor substitution is possible and Na has no effect

Group D- No substitution is occurred

  • Stimulate the growth- increase leaf area, stomata, improve the water balance

  • Na functions in metabolism

  • C4 metabolism

  • Impair the conversion of pyruvate to phosphoenol-pyruva

  • Reduce the photosystem II activity and ultrastructural changes in mesophyll chloroplast

  • Replacing K functions

  • Internal osmoticum

  • Stomatal function

  • Photosynthesis

  • Counteraction in long distance transport

  • Enzyme activation

  • Improves the crop quality e.g. improve the taste of carrots by increasing sucrose


Zinc is required in a large number of enzymes and plays an essential role in DNA transcription. A typical symptom of zinc deficiency is the stunted growth of leaves, commonly known as "little leaf" and is caused by the oxidative degradation of the growth hormone auxin.


In higher plants, Nickel is absorbed by plants in the form of Ni+2 ion . Nickel is essential for activation of urease, an enzyme involved with nitrogen metabolism that is required to process urea. Without Nickel, toxic levels of urea accumulate, leading to the formation of necrotic lesions. In lower plants, Nickel activates several enzymes involved in a variety of processes, and can substitute for Zinc and Iron as a cofactor in some enzymes.


Chlorine is necessary for osmosis and ionic balance; it also plays a role in photosynthesis.


Cobalt has proven to be beneficial to at least some plants, but is essential in others, such as legumes where it is required for nitrogen fixation for the symbiotic relationship it has with nitrogen-fixing bacteria. Vanadium may be required by some plants, but at very low concentrations. It may also be substituting for molybdenum. Selenium and sodium may also be beneficial. Sodium can replace potassium's regulation of stomatal opening and closing.

  • The requirement of Co for N2 fixation in legumes and non-legumes have been documented clearly

  • Protein synthesis of Rhizobium is impaired due to Co deficiency

  • It is still not clear whether Co has direct effect on higher plant


  • Tea has a high tolerance for Al toxicity and the growth is stimulated by Al application. The possible reason is the prevention of Cu, Mn or P toxicity effects.

  • There have been reports that Al may serve as fungicide against certain types of root rot.

Essential nitrogen pathways in plants

    • Nitrogen is a vital macronutrient for all life (2 to 6% plant DW).

    • Classes of N molecules

      1. amino acids

        • building blocks of proteins

        • precursors of other secondary metabolites

      2. Proteins

        • involved in metabolic process

        • serve regulatory roles

        • structural and storage roles

      3. Nucleotides

        • purines, pyrimidines

        • genetic information

        • energy carriers (e.g. ATP)

      4. Other essential compounds

        • alkaloids

        • glycosides

        • polyamines

        • chlorophyll, etc.

    • N occurs in various oxidation state from -3 to +5

o The nitrate uptake system in plant must be versatile and robust because

1. Plants have to transport sufficient nitrate to satisfy the total demand for nitrogen in the face of external nitrate concentrations that can vary by five orders of magnitude.

2. Plants must compete for N in the soil with abiotic and biotic processes such as erosion, leaching and microbial competition.

3. To function efficiently and the face of such environmental variation, plants have evolved 3 transport systems that are:

§ active

§ regulated

§ multiphasic

· Plant have distinct transport systems with different affinities for nitrate.

1. Constitutive high-affinity transport system (CHATS)

o Responsible for nitrate uptake at low concentrations (below ~1 mM).

o Saturation kinetics, with Km values below 300 µM.

2. Inducible high-affinity transport system (IHATS)

o Induced by NO3- taken up by CHATS.

o Increases overall NO3- uptake transiently.

3. Low affinity transport system (LATS)

o Uptake of nitrate at high concentration (> 0.2 mM).

o Like the high affinity systems, the low affinity systems are electrogenic and involve proton cotransport.

o Show linear rather than saturation kinetics.

o Not saturated even at 50 mM NO3-.

· Studies on nitrate uptake

· Measuring NO3- uptake has been hampered by the lack of a suitable radioactive tracer.

1. 13N has a half-life of only 10 min.

2. 15N is not radioactive.

3. When NO3- is supplied to cells it is metabolized and effluxed as equilibrium is established with pre-existing pools.

· Chlorate (ClO3-)

· An alternative substrate of the NO3- uptake mechanism.

· 36Cl can be used as the radioactive isotope to study nitrate uptake properties.

· ClO3- inhibits NO3- uptake and vice versa.

· ClO3- is not assimilated.

· ClO3- uptake into roots is extremely sensitive to the inhibitor FCCP, an uncoupler of energy-dependent ion transport.

· Utilizing ion-specific electrodes it has been suggested that there is a cotransport of NO3-/H+ across the plasmalemma of maize root cells and that the cytoplasmic level of nitrate is kept relatively constant by storage of nitrate in the vacuole.

· In NO3--starved roots, uptake is dominated by a constitutively expressed, low activity, high affinity system for NO3- uptake in barley and maize roots.

· When N-starved plants are treated with NO3-, the root develops a high rate of NO3- uptake with a greater affinity for NO3-.

· 13NO3- influx displays typical Menten kinetics

· The rate of uptake is depending upon the length of the previous treatment with nitrate, thus suggesting that a nitrate transport system is induced by nitrate.

· The induction of mRNA of the IHATS transporter is longer and greater at 1 mM (a) than 10 mM (b) NO

· Nitrate is a signal for developmental changes in the physiology of the plant.

· The primary responses include:

1. Induction of genes for nitrate and nitrite reduction.

2. Nitrate uptake and translocation systems.

3. DNA regulatory proteins required for expression of the secondary response gene system.

· The secondary response include more complex phenomena such as

1. Proliferation of the root system.

2. Enhancement of respiration.

3. Other changes in the physiology of the plant.

· A constitutive 'NO3- sensor' protein would detect the presence of environmental NO3-, then NO3- induction regulatory protein(s) would be activated which would act to initiate transcription of the primary response genes by RNA polymerase, resulting in NR, NiR, NO3- transporters, NO3- translocaters, ammonia assimilation enzymes

· The fate of NO3- taken up by a root epidermal cell.

· Once transported into an epidermal cell, NO3- has one of four fates:

1. It may undergo efflux to the apoplastic and soil environment.

2. It may enter the vacuole and by stored.

3. It may be reduced to ammonium by the combined action of NR and NiR.

4. It may be translocated via the symplast to the xylem.

· Nitrate Reduction

· Virtually all biologically important N-compounds contain N in a reduced form.

· The principal inorganic forms of N in the environment are in an oxidized state. Thus, the entry of N into organisms depends on the reduction of oxidized organic forms (N2 and NO3-) to NH4+.

· The reactions involving inorganic N-compounds occur only in microorganisms and green plants. Animals acquire their N from the catabolism of organic N-compounds mainly proteins, obtained in the diet.

· Nitrate is reduced to ammonia by a two-step process catalyzed by the enzymes nitrate reductase (NR) and nitrite reductase (NiR) mentioned previously.

· Nitrate and nitrite reductase

o NO3- + 2H+ + 2e- NO2- + H2O

o NO2- + 8H+ + 6e- NH4+ + 2H2O

· 4-1. Nitrate reductase (NR)

o Located primarily in the cytosol of root epidermal and cortical cells and shoot mesophyll cells.

o Transfers 2 e- from NAD (P) H to nitrate via three redox centres composed of two prosthetic groups. It also has a molybdenum cofactor (MoCo), a complex of molybdate and pterin, which catalyzes the actual nitrate reduction.