Auxin (IAA) is a growth-promoting plant hormone made at the growing tips of shoots and roots. Its main functions are: starting roots in stem cuttings, causing apical dominance, inducing seedless fruits (parthenocarpy, e.g. tomato), promoting flowering in pineapple, preventing early fruit and leaf drop, killing dicot weeds (2,4-D), and controlling xylem differentiation and cell division. Memory hook: "Auxin ROPES the plant" — Rooting, Overcoming buds (apical dominance), Parthenocarpy, Eliminating weeds, Stopping fruit drop.
Auxin (IAA) is made at shoot and root tips and controls many processes: rooting, apical dominance, parthenocarpy, weed killing (2,4-D) and flowering in pineapple. Note the age-dependent effect on drop — it prevents early fruit/leaf drop but promotes abscission of older organs.
Your doubts, answered
Is 2,4-D an auxin or just a herbicide?
2,4-D (2,4-dichlorophenoxyacetic acid) is a synthetic auxin. Being used as a herbicide is one of its functions, not a separate hormone group. NCERT lists NAA and 2,4-D as synthetic auxins, while IAA and IBA are natural auxins isolated from plants. So in a match/statement question, 2,4-D falls under Auxin.
Does auxin prevent fruit drop or cause it?
Auxin does both, depending on the age of the organ. It prevents fruit and leaf drop at EARLY stages, but it promotes the abscission (dropping) of OLDER, mature leaves and fruits. NTA loves this two-sided point, so remember: early stage = prevents drop, old/mature stage = promotes drop.
What is the difference between natural and synthetic auxins?
Natural auxins are made by the plant itself — IAA (indole-3-acetic acid) and IBA (indole butyric acid) were isolated from plants. Synthetic auxins are man-made compounds with the same growth-regulating action — NAA (naphthalene acetic acid) and 2,4-D. Both types are used in farming and gardening.
Does auxin from the shoot tip help or block side buds?
Auxin made by the apical (top) bud BLOCKS the growth of lateral (axillary) buds. This is called apical dominance. Because auxin travels down and suppresses side buds, removing the shoot tip (decapitation) lets the side buds grow — used in tea plantations and hedge-making. So auxin inhibits, it does not promote, lateral bud growth.
Which auxin is used to root stem cuttings?
IBA (indole butyric acid) and NAA are commonly used to initiate rooting in stem cuttings for plant propagation. The general NCERT point is simply that 'auxins help to initiate rooting in stem cuttings'. For NEET, knowing that auxins cause rooting is enough.
Why does spraying auxin (2,4-D) on a lawn kill weeds but not the grass?
2,4-D kills dicotyledonous weeds but does NOT affect mature monocotyledonous plants. Grasses are monocots, so the lawn stays safe while broad-leaved dicot weeds die. This exact logic was asked in NEET 2024.
⚠️ The NEET trap ✗ Auxin is only a growth promoter, so it always prevents leaf and fruit fall. ✓ Auxin prevents fruit and leaf drop at EARLY stages but promotes abscission of OLDER mature leaves and fruits. It is age-dependent, not one-directional. 🧠 Young = hold on, Old = let go. Auxin changes its role with the age of the organ.
Real NEET questions
NEET 2024
Auxin is used by gardeners to prepare weed-free lawns. But no damage is caused to grass as auxin
A · promotes abscission of mature leaves only
B · does not affect mature monocotyledon plants ✓
C · can help in cell division in grasses, to produce growth
D · promotes apical dominance
Solution: The auxin herbicide 2,4-D kills dicotyledonous weeds but does not affect mature monocotyledonous plants. Grasses are monocots, so the lawn is unharmed while dicot weeds die. The other options are not the reason the grass survives.
NEET 2025
Read the following statements on plant growth and development. A. Parthenocarpy can be induced by auxins. B. Plant growth regulators can be involved in promotion as well as inhibition of growth. C. Dedifferentiation is a pre-requisite for re-differentiation. D. Abscisic acid is a plant growth promoter. E. Apical dominance promotes the growth of lateral buds. Choose the option with all correct statements.
A · A, D, E only
B · B, D, E only
C · A, B, C only ✓
D · A, C, E only
Solution: A is correct: auxins induce parthenocarpy (e.g. tomato). B is correct: PGRs both promote and inhibit growth. C is correct. D is wrong: ABA is an inhibitor. E is wrong: apical dominance inhibits, not promotes, lateral bud growth. So only A, B, C are correct.
NEET 2019
It takes very long time for pineapple plants to produce flowers. Which combination of hormones can be applied to artificially induce flowering in pineapple plants throughout the year to increase yield?
A · Auxin and Ethylene ✓
B · Gibberellin and Cytokinin
C · Gibberellin and Abscisic acid
D · Cytokinin and Abscisic acid
Solution: Both auxin and ethylene induce flowering in pineapple, so this pair is used to force year-round flowering and raise yield. Gibberellin, cytokinin and abscisic acid are not the pineapple flowering inducers, so the other combinations are wrong.
Solved Plant Growth and Development NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
IAA stands for indole-3-acetic acid. Yes, it is the main natural auxin. The word 'auxin' comes from the Greek 'auxein', meaning 'to grow', and it was first isolated from human urine.
Where is auxin produced in the plant?
Auxins are generally produced by the growing apices (tips) of stems and roots, from where they migrate to the regions where they act.
What is parthenocarpy and how does auxin cause it?
Parthenocarpy is the formation of fruit without fertilisation, giving seedless fruit. Auxins can induce parthenocarpy, for example in tomatoes.
Name two natural and two synthetic auxins.
Natural auxins: IAA (indole-3-acetic acid) and IBA (indole butyric acid). Synthetic auxins: NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid).
Does auxin have any role in vascular tissue?
Yes. Auxin controls xylem differentiation and also helps in cell division, both important NCERT one-line functions for NEET.
