Plant growth regulators (PGRs) are chemicals farmers spray on crops to control growth, yield, ripening and weeds. For NEET, remember 5 key uses: gibberellin lengthens sugarcane stems (higher sugar yield), 2,4-D (an auxin) kills dicot weeds as a herbicide, ethylene ripens fruits and sets pineapple flowers, cytokinin delays leaf ageing, and ABA closes stomata in stress. Memory hook: "Grow-Kill-Ripen" — Gibberellin grows sugarcane, 2,4-D kills weeds, Ethylene ripens fruit.
Five PGRs and their key agricultural uses: gibberellin (sugarcane yield, malting), 2,4-D auxin (weed killer), ethylene (fruit ripening, pineapple flowering), cytokinin (delays leaf ageing), and ABA (stomatal closure, seed dormancy).
Your doubts, answered
Which plant growth regulator is sprayed on sugarcane to increase yield?
Gibberellin (GA3). Sugarcane stores sugar in its stem, so spraying gibberellin increases the length of the stem (internodes). A longer stem stores more sugar, raising yield by as much as 20 tonnes per acre. This is a very common NEET question, so link it firmly: sugarcane = gibberellin = stem elongation = more sugar.
Why is 2,4-D used as a weed killer but gibberellin is not?
2,4-D is a synthetic auxin. In high doses it kills broad-leaved dicot weeds but does not harm mature monocots like grasses. So a farmer can spray it on a lawn or wheat field to remove dicot weeds without killing the crop. Gibberellin only promotes elongation and does not kill plants, so it is never used as a herbicide.
Which plant growth regulator ripens fruits?
Ethylene, a gaseous PGR. It speeds up fruit ripening (as in tomatoes, apples and bananas) and raises the respiration rate during ripening. Ethephon is the common commercial source that releases ethylene slowly. Ethylene is also used to set pineapple flowers, induce mango flowering, and thin cotton and cherry.
Which PGR is used in the brewing industry?
GA3 (gibberellic acid). It speeds up the malting process in brewing. This exact pairing (GA3 = brewing industry) appears in NEET match-the-column questions, so memorise it along with 2,4-D = herbicide, kinetin (cytokinin) = nutrient mobilisation, and ABA = stomatal closure.
Which PGR delays ageing and keeps vegetables fresh?
Cytokinin (e.g. kinetin). It delays senescence (ageing) of leaves, so it helps keep leafy vegetables and cut flowers fresh for longer. It also promotes cell division and helps break apical dominance. Do not confuse this with ABA, which promotes ageing and dormancy.
⚠️ The NEET trap ✗ Auxin or ethylene is sprayed on sugarcane to increase yield. ✓ Gibberellin (GA3) is sprayed on sugarcane. It elongates the stem (internodes), so more sugar is stored, raising yield by up to 20 tonnes per acre. Auxin and ethylene do not give this stem-elongation yield effect. 🧠 Sugarcane yield trap
Real NEET questions
NEET 2024
Spraying sugarcane crop with which of the following plant growth regulators, increases the length of stem, thus, increasing the yield?
A · Gibberellin ✓
B · Cytokinin
C · Abscisic acid
D · Auxin
Solution: Gibberellins increase the length of the axis. Spraying sugarcane with gibberellin lengthens the stem and raises sugar yield by up to 20 tonnes per acre. Cytokinin (cell division), ABA (growth inhibitor) and auxin (rooting, apical dominance) do not produce this stem-elongation effect.
NEET 2021
The plant hormone used to destroy weeds in a field is
A · 2, 4-D ✓
B · IBA
C · IAA
D · NAA
Solution: 2,4-D is a synthetic auxin widely used as a herbicide to kill dicot weeds without harming mature monocots. IBA, IAA and NAA are auxins used mainly for rooting and growth control, not as the standard weed killer.
NEET 2022
The gaseous plant growth regulator is used in plants to:
A · Speed up the malting process
B · Promote root growth and root hair formation to increase the absorption surface ✓
C · Help overcome apical dominance
D · Kill dicotyledonous weeds in the fields
Solution: The gaseous PGR is ethylene. It promotes root growth and root hair formation, increasing the absorption surface. Malting is a GA3 effect, overcoming apical dominance is a cytokinin effect, and killing dicot weeds is a 2,4-D (auxin) effect.
Solved Plant Growth and Development NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the main agricultural uses of PGRs for NEET?
Gibberellin lengthens sugarcane stems (yield) and speeds malting in brewing; 2,4-D kills dicot weeds (herbicide); ethylene ripens fruit and sets pineapple flowers; cytokinin delays leaf ageing; ABA closes stomata under stress and induces seed dormancy.
Does 2,4-D harm grass in a lawn?
No. 2,4-D kills broad-leaved dicot weeds but does not affect mature monocots like grasses, so it removes weeds from a lawn or cereal crop without killing the crop.
What is the commercial source of ethylene used by farmers?
Ethephon. In an aqueous solution it is absorbed and transported in the plant and slowly releases ethylene, hastening ripening in tomatoes and apples and causing abscission (thinning) of cotton, cherry and walnut.
Which PGR increases sugarcane yield and by how much?
Gibberellin (GA3). By elongating the stem it can increase yield by as much as 20 tonnes per acre, a figure NEET expects you to remember.
Which PGR is called the stress hormone and how is it useful?
Abscisic acid (ABA). It closes stomata to reduce water loss, increases stress tolerance, and induces seed dormancy so seeds can be stored and survive unfavourable conditions.
