Hormones, Enzymes, and Vitamins in Agriculture and Biology

Plant Hormones (Phytohormones)

i) 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.

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.

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.

ii) 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.

iii) 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.

iv) 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.

iv) 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.

v) 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.

vi) 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.

vii) 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),