Hormones, Enzymes, and Vitamins in Agriculture and Biology

Plant Hormones (Phytohormones)

• Auxins (IAA, IBA, NAA, 2,4-D):
  • Auxins (IAA):
    • Control the growth of plant tissues, particularly the elongation of cells.
    • Responsible for apical dominance (inhibiting lateral bud growth) and promoting root growth.
    • Discovered by: F.W. Went (1928).
    • Went identified the first auxin, Indole-3-Acetic Acid (IAA), which controls the elongation of cells in plants.
    • Prevent premature fruit drop.

1. • Indole-3-Butyric Acid (IBA):
   • Discovered by: K. Skoog and F. Miller (1939).
   • Stimulates root formation, especially in cuttings during propagation.

• • Naphthalene Acetic Acid (NAA):
    • Discovered by: L.E. Melhuse (1937).
    • Used to prevent fruit drop, particularly in fruit crops like apples and oranges.
    • Acts as a fruit thinner by regulating fruit size and preventing overcrowding.

1. • 2,4-D:
     • At low concentrations (less than 20 ppm), it acts as a fruit setting hormone.
     • At higher concentrations (more than 20 ppm), it functions as a herbicide, killing unwanted plants.

• Cytokinins:
  • Discovered by: F. Skoog and C. Miller (1954).
  • Break dormancy in seeds, buds, and tubers, stimulating their growth.
  • Promote cell division and shoot formation, particularly useful in tissue culture for plant propagation.
  • Help delay leaf senescence (aging).
  • Act in coordination with auxins to regulate plant growth.

• Gibberellins:
  • Discovered by: E. Kurosawa and J. Yabuta (1935).
  • Stimulate cell elongation, causing increased plant height and internodal length.
  • Promote fruit size by stimulating growth of immature fruits.
  • Break seed dormancy and aid in seed germination, especially under unfavorable conditions.
  • Used to improve yields in certain crops such as grapes, tomatoes, and citrus.

• Abscisic Acid (ABA):
  • Inhibits growth and induces dormancy, particularly in seeds and buds.
  • Acts as a stress hormone, helping plants respond to water stress, cold stress, and high salinity by closing stomata to reduce water loss.
  • Plays a key role in leaf senescence (aging) and in fruit ripening.

• Ethylene::
  • Promotes fruit ripening (e.g., bananas, tomatoes).
  • Induces leaf abscission (shedding of leaves) and fruit drop at harvest time.
  • Promotes isodiametric growth in stem and roots.
  • Inhibits elongation and promotes thickening of stems, particularly in certain crops like potatoes.

• Chloromequat (CCC):
  • Regulates plant growth by limiting excessive vertical growth, promoting shorter, stockier plants.
  • Used to control lodging (collapse of plants due to excessive height) in crops like wheat.
  • Helps improve the quality of crops by reducing excessive growth that could lead to weak stems.

• Mallic Hydrazide::
  • A growth retardant used in horticulture to prevent sprouting of onions during storage.
  • Marketed as Sproutstop, it is used in commercial onion storage to maintain quality and prevent unnecessary sprouting.

• Ethephone (Ethrel)::
  • Used to regulate fruit ripening, particularly in bananas, to ensure uniform ripening and reduce wastage.
  • It is a source of ethylene, which promotes ripening and maturation in fruits.

Vitamins

• Vitamins:
  • Definition: Organic compounds essential for normal growth, development, and metabolism in both plants and animals. They are classified into water-soluble and fat-soluble groups.
  • Vitamin Deficiency: Lack of specific vitamins can lead to various diseases. For example, Vitamin A deficiency leads to night blindness, while Vitamin C deficiency leads to scurvy.

• Fat-Soluble Vitamins:
  • Vitamin A:
    • Vital for vision, skin health, and immune function.
    • Found in foods like carrots, sweet potatoes, and spinach.
    • Deficiency can cause night blindness and weakened immunity.
  • Vitamin D:
    • Regulates calcium and phosphorus absorption, crucial for bone health.
    • It is produced in the skin upon exposure to sunlight.
    • Deficiency can lead to rickets in children and osteomalacia in adults.
  • Vitamin E:
    • Acts as an antioxidant, protecting cells from oxidative damage.
    • Important for immune function and skin health.
  • Vitamin K:
    • Vital for blood clotting and wound healing.
    • Found in leafy greens, broccoli, and cabbage.

• Water-Soluble Vitamins:
  • Vitamin C (Ascorbic Acid):
    • Essential for the growth and repair of tissues in the body.
    • Important for collagen formation and iron absorption.
    • Found in citrus fruits, strawberries, and bell peppers.
  • B-Complex Vitamins:
    • B1 (Thiamine): Vital for energy metabolism and nerve function.
    • B2 (Riboflavin): Supports energy production and acts as an antioxidant.
    • B3 (Niacin): Helps in DNA repair and energy production.
    • B6 (Pyridoxine): Important for amino acid metabolism and neurotransmitter synthesis.
    • B12 (Cobalamin): Essential for red blood cell production and neurological function.
    • Folate (Folic Acid): Crucial for red blood cell formation and fetal development during pregnancy.
• Folic Acid:
  • Function: Essential for RBC (red blood cell) maturation and preventing neural tube defects in infants.
  • Sources: Leafy greens, legumes, and citrus fruits.

Enzymes

• Definition:
  • Enzymes are proteins that act as biological catalysts, speeding up chemical reactions in living organisms. They are crucial in various physiological processes such as digestion, metabolism, and cell repair.

• Types of Enzymes:
  • Hydrolases: Involved in breaking down molecules using water (e.g., amylase, lipase).
  • Oxidoreductases: Catalyze oxidation-reduction reactions (e.g., dehydrogenases).
  • Transferases: Transfer functional groups between molecules (e.g., kinases).
  • Ligases: Join two molecules together (e.g., DNA ligase).

• Enzyme Specificity: Enzymes are highly specific in terms of the substrates they act on. This specificity is determined by the enzyme’s active site, where the substrate binds.

• Factors Affecting Enzyme Activity:
  • Temperature: Enzymes function optimally at a specific temperature; extreme temperatures can denature enzymes.
  • pH: Enzymes also have an optimal pH range.
  • Substrate Concentration: Higher substrate concentrations can increase the rate of enzyme activity up to a point, after which the enzyme becomes saturated.

Essential Amino Acids

• Amino Acids:
  • Amino acids are the building blocks of proteins. They are classified as essential (cannot be synthesized by the body) and non-essential (can be synthesized by the body).
  • Essential Amino Acids: Must be obtained from the diet, and they include:
    1. Tryptophan
    2. Valine
    3. Methionine
    4. Isoleucine
    5. Leucine
    6. Lysine
    7. Phenylalanine
    8. Arginine (conditionally essential)
    9. Threonine
    10. Histidine (conditionally essential)

Essential Fatty Acids 

• Fatty Acids:
  1. Fatty acids are important components of lipids (fats), which are crucial for energy storage, membrane structure, and cellular function.
  2. Essential Fatty Acids: Cannot be synthesized by the body and must be obtained from the diet:
     1. Oleic acid
     2. Linoleic acid
     3. Linolenic acid
     4. Arachidonic acid

Functions:

• 1. Oleic acid: Supports heart health by lowering LDL cholesterol levels.
  2. Linoleic acid: Essential for proper skin health and inflammation regulation.
  3. Linolenic acid: Omega-3 fatty acid, important for brain function and reducing inflammation.
  4. Arachidonic acid: Plays a role in inflammatory response and cell signaling.

• Vitamins: The term “vitamin” was introduced by Casimir Funk (1912)
• Kuhne coined the term “enzyme” (1898),