1. Prologue
  2. Plant growth regulators (PGRs)
    1. Auxins
    2. Gibberellins (GA)
    3. Cytokinins
    4. Ethylene
    5. Ethephon
    6. Abscisic acid (ABA)
    7. Essential elements
  3. Macronutrients vs Micronutrients
    1. Chlorosis vs Necrosis
    2. Nitrogen
  4. Misc. Terms
    3. Biennial Plants
    4. Plasticity
    5. Hydroponics
  5. Possible MCQs


  • To provide level playing field to “Only-IFoS” aspirants, UPSC had asked tough technical questions from environment and agriculture last time. For an ordinary “only-CSE” aspirant, the cost:Benefit not that good in pursuing those things.
  • However, some of the content provided in class 11-12 biology textbooks in connection with Botany and agriculture, could prove to be useful.
  • So, this article contains revision note out of Biology Class11: Chapter 12 (essential nutrients) and 15 (growth regulators).

Plant growth regulators (PGRs)

  • Growth means an irreversible permanent increase in size of an organ /cell.
  • water, oxygen and nutrients=> Metabolism (Anabolic/catabolic)=>Growth.
  • Plant growth regulators are indole compounds and have two subtypes:
    1. growth promoters (auxins, gibberellins and cytokinins)
    2. growth inhibitor (e.g. Absicis acid)

Ethylene fits in either group, but largely growth inhibitor.


  • Auxins (Greek word “to grow”). Chemically they’re indole-3-acetic acid (IAA).
  • first isolated from human urine
  • Auxins used for Parthenocarpy in tomatoes i.e. the development of a fruit without fertilization or seeds.
  • Assertion: Auxins used as herbicides. Correct Reason: They can kill dicotyledonous weeds without harming mature monocotyledonous plants.
  • Auxins help to initiate rooting in stem cuttings- hence used for plant propagation.
  • Auxins prevent fruit and leaf from dropping at early stages but promote the abscission of older mature leaves and fruits.
  • Auxins promote flowering in pineapples.

Gibberellins (GA)

  • Gibberellins (1) increase length of grapes stalks (2) They elongate and improve shape of Apples.

    They delay senescence, so fruits can be left on the tree longer so as to extend the market period.

  • In brewing industry, GA 3 is used to fasten the malting process
  • Sugarcane stores carbohydrate as sugar in their stems. GA spray will increase stem’s length & thereby yield to 20 tonnes per acre.
  • GA spray will hasten maturity of juvenile conifers and lead to early seed production.
  • In beet, cabbages – GA promotes bolting (internode elongation just prior to flowering).
  • All GAs are acidic in Nature.


  • They’re produced in in regions where rapid cell division occurs- e.g. root apices, developing shoot buds, young fruits etc.
  • They produce new leaves, chloroplasts in leaves and growth of shoot.
  • They promote nutrient mobilisation to delay leaf senescence
  • Cytokinins help overcome the apical dominance.
  • Apical dominance: In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds. Therefore, in plantations, hedge-making- shoot tips are remoted (decapitation) to make lateral buds grow.


  • Ethylene is a simple gaseous Plant growth regulators (PGRs)
  • It is synthesized in tissues undergoing senescence and ripening fruits
  • In leaves and flowers, Ethylene promotes following
    • Senescence: process of growing older and showing the effects of increasing age
    • Abscission: Shedding of flowers, leaves and fruit.
    • respiratory climactic: Ethylene enhances the respiration rate during ripening of the fruits. Thus helps in fruit ripening.
  • Ethylene breaks seed and bud dormancy, peanut seeds germination, potato tubers’ sprouting,
  • Ethylene promotes elongation in deep water rice plants so leaves can remain above water.
  • Ethylene also promotes root hair growth to increase absorption surface
  • Ethylene is used to synchronise fruit-set in pineapples and flowering in mango

    Since ethylene regulates so many physiological processes, it is one of the most widely used PGR in agriculture


  • Ethephon is most widely used source of Ethylene. It is an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly.
  • Ethephon hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut). It promotes female flowers in cucumbers thereby increasing the yield.

Abscisic acid (ABA)

  • It acts as a general plant growth inhibitor, metabolism inhibitor, seed germination inhibitor.
  • ABA induces dormancy, helps seeds to withstand desiccation.
  • Assertion: Abscisic acid is also known as stress hormone. Correct Reason:  It increases the tolerance of plants to various kinds of stresses.

Essential elements

  • weathering and breakdown of rocks=> essential minerals => root=> xylem tissue=> plant
  • Thus, Most of the essential minerals are derived from the rock minerals. Therefore, plant nutrition is called “mineral nutrition”.
  • 60+ elements found in different plants- including gold, selenium and even radioactive strontium (if plant growing in nuke site)
  • So, the question is- how can we know if given element is essential for plant or not?

Well, Essential elements have 3 characteristics:

  • If they’re absent, plant will not complete its lifecycle.
  • They’re not replaceable by other elements.
  • They’re directly involved in plant-metabolism.

Total 17 Essential elements further divided into two groups:

Macronutrients vs Micronutrients
Macronutrients Micronutrients
Present in plant tissues in large amounts (>10 mmole Kg –1 of dry matter). Less amount / trace elements.
  • From soil: nitrogen, phosphorous (ATP), sulphur, potassium (stomata opening-closing), calcium and magnesium (Chlorophyll)
  • From air:  carbon, hydrogen, oxygen
iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel
Among above, carbon, hydrogen, oxygen and nitrogen are components of biomolecules hence called “structural elements”
If nitrogen, potassium and magnesium deficiency, then it’ll be first visible in the senescent leaves.

Chlorosis vs Necrosis

Chlorosis Necrosis
Leaves turn yellow due to loss of Chlorophyll. Death of leaf tissue
deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo deficiency of Ca, Mg, Cu, K.


Nitrogen Cycle Agriculture Chemistry

  • Plants need this element in the greatest amount, because of
    • Its role in meristematic tissues and the metabolically active cells.
    • Nitrogen is one of the major constituents of proteins, nucleic acids, vitamins and hormones.
  • Artificial: Industrial combustions, forest fires, automobile exhausts and power-generating stations.
  • Natural: Decomposition of organic material => Ammonification => Nitrogen fixing bacteria Nitrosomonas and/or Nitrococcus => Nitrite=>oxidation by Nitrobacter.
  • nitrogen-fixing microbes could be free-living or symbiotic.
  • Azotobacter is a free-living nitrogen-fixing aerobic microbes.
  • Assertion: nitrogen is a limiting nutrient for both natural and agricultural eco-systems. Correct-Reason: Plants compete with microbes for the limited nitrogen in the soil.

Misc. Terms


  • Plant-flowering depends on combination of light exposure and its duration. This is known as photoperiodism.
  • Day-neutral plants: They have no correlation between exposure to light duration and flowering response.


Vernalisation: When flowering is dependent on exposure to low temperature. Example- Wheat, barley and rye- they have two varieties:

Winter Spring
Planted in autumn normally planted in the spring and come to flower and produce grain before the end of the growing season.
If winter variety of planted in spring, they’ll fail to flower / they’ll fail to produce mature grain within flowering season.

Biennial Plants

  • Examples: Sugerbeet, cabbages, carrots.
  • They normally flower and die in the second season.
  • They are monocarpic plants i.e. Dying after bearing fruit only once.


  • It means Plants follow different pathways in response to environment to form different kinds of structures. This ability is called e.g., heterophylly in cotton, coriander and larkspur.
  • In such plants, juvenile plant’s leaves are different in shape than mature plants.


  • German Botanist Julius von Sachs showed plants could be grown without soil, in a nutrient solution. This technique is called hydroponics.
  • Hydroponics is used in commercial cultivation of tomato, seedless cucumber and lettuce.

Possible MCQs

  1. Which of the following statements are correct about (1) plant growth regulators (2) photoperiodism (3) vernalisation (4) Biennial plants (4) Hydropphonics (5) essential elements’ characteristics (6) Nitrogen cycle
  2. Which of the macronutrients are available from soil / air?
  3. Which of the following are examples of micronutrients?
  4. How does Gibberellins spray increases the yield in sugar cane?
  5. Which of the following effects on flower/fruits are caused by ethylene?
  6. Why is Abscisic acid called a “stress hormone”?
  7. Which of the following are examples of biennial plants?
  8. What is Parthenocarpy? Why is Auxin used in it?
  9. Difference between Plasticity and apical dominance?
  10. Match the following: plant growth regulator vs. impacts, essential element versus its importance