Plant growth regulators - Growth, Flowering and Fruiting.pdf

Plant growth regulators (PGRs) are natural or synthetic compounds that influence the physiological processes of plants, including growth, flowering, and fruiting. In horticultural crops, PGRs such as auxins, gibberellins, cytokinins, ethylene, and abscisic acid play vital roles. Auxins promote cell elongation, root initiation, and fruit development. Gibberellins stimulate stem elongation, flowering, and seed germination. Cytokinins encourage cell division and delay leaf senescence. Ethylene regulates fruit ripening, flowering, and abscission, while abscisic acid is crucial for stress responses and seed dormancy. These regulators are extensively used in horticulture to enhance crop yield, improve fruit quality, and synchronize flowering and harvesting.

1. Plant Growth Regulators
22CHOR11 - Fundamentals of Horticulture 2(1+1)
Dr. M. Kumaresan
(Hort.)
Department of Horticulture
Vels Institute of Science, Technology &
Advanced Studies (VISTAS)
Pallavaram, Chennai, Tamil Nadu -600117

2. Plant Growth Regulators
Plant growth regulators may be defined as the synthetic organic chemical substances
other than nutrients and vitamins which regulate (or) modify the physiological
process of the plant.
Plant hormones are naturally occurring substance sin low concentration in the plants
having same actions and move from the site of production to the site of action.

5. Auxin
Cell Elongation: Auxin promotes cell elongation by loosening the cell wall, enabling the
plant to grow taller and stronger.
Apical Dominance: It suppresses the growth of lateral buds, ensuring the main shoot grows
dominantly. This is essential for shaping ornamental plants or training fruit trees.
Root Development: Auxin stimulates the formation of lateral and adventitious roots, which
is crucial in vegetative propagation, such as cuttings.
Fruit Development: Auxin contributes to fruit set and growth, often used to induce
parthenocarpy in crops like tomatoes and cucumbers, resulting in seedless fruits.
Abscission Prevention: Auxin delays leaf and fruit drop by maintaining cell adhesion,
helping improve crop retention until harvest.
Phototropism and Gravitropism: Auxin distribution influences plant responses to light and
gravity, aiding in optimal plant orientation and resource use.
Tissue Culture: Auxin is widely used in tissue culture media to promote callus formation,
organogenesis, and root induction.

6. Gibberellins
Stimulating Stem Elongation: Gibberellins promote cell elongation and division, leading to
increased plant height and better vegetative growth in crops like grapes and cucumbers.
Breaking Dormancy: GAs break seed, tuber, and bud dormancy, facilitating germination and
sprouting in crops like potatoes and apples.
Enhancing Fruit Set: Gibberellins improve fruit set in crops where pollination may be poor,
such as parthenocarpic fruit production in seedless grapes and cucumbers.
Inducing Flowering: GAs influence flowering in long-day and biennial plants, aiding in
uniform bloom production in crops like lettuce and carrots.
Improving Fruit Quality: Gibberellins are used to increase fruit size, reduce seed content,
and enhance fruit shape in crops such as citrus, apples, and grapes.
Delaying Senescence: They delay leaf and fruit senescence, extending shelf life and
improving marketability.

7. Cytokinins
Cell Division and Growth: Cytokinins promote cell division in meristematic tissues, aiding
in the growth of roots, shoots, and leaves.
Delay of Senescence: They delay leaf senescence (aging) by maintaining chlorophyll content,
improving photosynthesis, and prolonging the functional lifespan of leaves.
Shoot Growth and Bud Development: Cytokinins stimulate shoot formation, lateral bud
development, and overcome apical dominance, leading to more balanced plant growth.
Nutrient Mobilization: They enhance the movement of nutrients and assimilates to actively
growing tissues, ensuring better resource allocation.
Flowering and Fruit Development: Cytokinins influence flower initiation, improve fruit set,
and enhance fruit quality in several crops.
Stress Resistance: They help plants cope with abiotic stresses like drought and salinity by
regulating stomatal function and maintaining water balance.
Tissue Culture Applications: Cytokinins are widely used in micropropagation to induce
shoot proliferation and regeneration in tissue culture practices.

8. Ethylene
Fruit Ripening: Ethylene is a key regulator of climacteric fruit ripening, promoting color
change, softening, aroma production, and sugar accumulation in fruits like bananas,
tomatoes, and apples.
Senescence and Abscission: It accelerates the senescence of flowers and leaves and facilitates
the abscission of leaves, fruits, and flowers, which is essential for harvest timing and pruning.
Flowering Induction: In some plants (e.g., pineapples), ethylene stimulates flowering. It is
also involved in synchronizing flowering for uniform crop production.
Response to Stress: Ethylene mediates plant responses to biotic and abiotic stresses, such as
pathogen attack, drought, or flooding, by modulating defense mechanisms.
Growth Regulation: It regulates cell elongation and root hair formation, influencing plant
architecture.
Breaking Dormancy: Ethylene can break seed and bud dormancy, promoting uniform
germination and growth.

9. Abscisic Acid (ABA)
Regulation of Stomatal Closure: ABA helps in water conservation during drought stress by
inducing stomatal closure, reducing water loss through transpiration.
Fruit Ripening and Dormancy: ABA promotes fruit ripening in crops like grapes and
tomatoes and induces seed and bud dormancy to protect plants during unfavorable
conditions.
Stress Tolerance: It is a key mediator in abiotic stress responses, such as salinity, heat, and
cold, by activating stress-responsive genes and physiological adaptations.
Leaf Senescence and Abscission: ABA contributes to leaf aging and the natural shedding of
leaves, flowers, and fruits.
Seed Development: It regulates seed maturation, inhibits premature germination, and
enhances seed desiccation tolerance.
Flowering Regulation: ABA modulates flowering time and floral development, particularly
under stress conditions.

10. Brassinosteroids (BRs)
Growth Promotion: BRs enhance cell elongation and division, promoting stem elongation,
root growth, and leaf expansion, which are essential for the vegetative development of
plants.
Flower and Fruit Development: They influence flowering, fruit set, and the growth of fruits,
contributing to higher yields and improved fruit quality in horticultural crops.
Stress Tolerance: BRs help plants cope with abiotic stresses such as drought, salinity, and
temperature extremes by modulating antioxidant systems and improving water-use
efficiency.
Disease Resistance: They enhance plant resistance to pathogens by inducing the expression
of defense-related genes.
Nutrient Uptake: BRs improve the efficiency of nutrient uptake and utilization, promoting
better plant health and productivity.
Ripening and Senescence: In fruit crops, BRs regulate the ripening process and delay
senescence, extending the shelf life of produce.

11. Jasmonic acid (JA)
Stress Response: JA plays a critical role in defending plants against biotic stresses, such as
herbivory and pathogen attacks, by activating defense-related genes and producing
secondary metabolites.
Wound Healing: It is a key signal in response to mechanical damage, aiding in wound repair
and minimizing further tissue damage.
Growth Regulation: JA influences plant growth processes, including inhibition of root
elongation, promotion of senescence, and regulation of leaf abscission.
Reproductive Development: It impacts flower development, pollen viability, and fruit
ripening, ensuring better reproductive success and crop yield.
Secondary Metabolism: JA stimulates the production of secondary metabolites like alkaloids,
flavonoids, and terpenoids, which are crucial for plant defense and have medicinal and
commercial value.
Abiotic Stress Tolerance: JA enhances tolerance to drought, salinity, and temperature
extremes by modulating antioxidant systems and stress-responsive genes.

12. Salicylic acid (SA)
Stress Tolerance: SA enhances plant tolerance to abiotic stresses such as drought, salinity,
and extreme temperatures by modulating antioxidant enzyme activity and reducing
oxidative damage.
Disease Resistance: It plays a crucial role in inducing systemic acquired resistance (SAR),
helping plants defend against pathogens by activating defense-related genes.
Growth and Development: SA influences germination, root elongation, flowering, and fruit
ripening, thereby improving overall plant productivity.
Postharvest Quality: Application of SA extends the shelf life of fruits and vegetables by
delaying senescence and reducing postharvest diseases.
Photosynthesis and Metabolism: SA enhances photosynthetic efficiency by improving
chlorophyll content and regulating stomatal conductance.

13. Propagation of plants
Propagation of plants: The most common use of plant regulators in horticulture is
to induce rooting in stem cuttings and in air and soil layers.
Rooting of cuttings: Certain kind of plants may not successfully root under normal
condition and with the aid of plant regulators; they can be easily made to induce rooting.
The most commonly employed growth regulators for rooting are auxins like IBA, IAA, IPA
and NAA. Among these chemicals IBA is most ideally used since, it is the most effective one.
Concentrations ranging from100-500ppm are used for long dip method of
treatment of cuttings for 12-24 hours and high concentrations of 10,000 to 20,000
for quick dip method for a few seconds. The concentrations differ according to
the type of cutting i.e. herbaceous, Semi-hard wood and hard wood cuttings.
Applications in the form of dust as talcum preparation or in the form of a paste
in lanolin are also used.

14. Control of flowering
Plant growth regulators are used for the regulation of flowering in certain
crops.
In pineapple flowering is irregular and harvesting becomes a problem
and hence to regulate flower production, plant regulators are used. The
treatment generally consists of pouring a required quantity of (50ml),
the solution containing 0.25 to 0.5 mg of the chemical of NAA in the
central core of plants.
In recent studies, Cycocel and Alar at 5000ppm and Ethrel at 100-200ppm
have been shown to induce flowering in mango during an off year.
In Jasminum grandiflorum, the flowering period is extended by the
application of Cycocel at 500ppm.
Flowering can also be induced in certain vegetables such as radish, beet
root and carrot with the application of GA.

15. Fruit-set
Various growth regulators like IAA, IBA, IPA, NAA, 2, 4-D, 2, 4, 5-T and GA have
been found to improve fruit set in many crops. Among these chemicals 2, 4-D and
NAA (Planofix) have been found in general to be most effective in increasing the
fruit set.
Optimum concentrations for this purpose are 10-20 ppm of auxins and 10-100ppm
of GA in different crops.
Spraying the flower cluster thoroughly 4-6 days after full bloom with 100 ppm GA
increased the fruit set in grape.
It has been found that in chillies spraying of Planofix @ 1ml in 4.5 litres of water at
60th and 90th day after planting is beneficial for good fruit setting.

16. Fruit drop
Losses resulting from pre-harvest drop of fruits have long been a serious problem.
When the growth regulators have been put in to use in apples and pears,
preharvest fruit drop can be checked by the application of 2,4-D and 2,4,5-T
effectively.
Preharvest fruit drop in citrus is controlled with 2,4-D at a concentration of 20ppm
2,4-D, 10-15ppm of NAA and 2,4,5-T at 15 to 30ppm at pea stage and marble stage
and 2,4D at 20ppm and 2,4,5-T at 10-15ppm in mandarins.
At 10ppm and NAA at 20ppm have effectively prevented fruit drop in mango.
Application of planofix containing NAA at pea seed and marble size of the fruits
completely controlled early fruit drop in Guava.

17. Parthenocarpy
Partenocarpic fruit set could be induced in a no. of vegetables like cucurbits,
bhendi, brinjal, chillies and tomato and fruits like guava, straw berry, citrus,
watermelon etc.
IAA, IBA, NAA, 2,4-D, IPA and GA are effective in different plants.
Application of GA at 100 ppm induced complete seedlessness in grape varieties Viz.,
Anab-e-shahi, Pachadraksha etc.
Problem of development of seeds in Poovan variety of Banana in Trichy area of
Tamil Nadu is controlled by application of 2,4-D at 25ppm in the bunches when the
last hand is opened.

18. Fruit ripening
The plant growth regulators can be employed to hasten or delay fruit ripening.
Plant growth regulators like 2, 4, 5-T at concentrations of 25 to 100ppm has been
found to hasten the ripening in some varieties of plums and peaches.
In banana ethrel treatment at 2500ppm induces ripening in 24 hours.
Application of 2, 4-D at 16ppm delays ripening in Washington navel oranges.
In Calymirna fig maturity and ripening of the fruit is greatly hastened by spraying
2, 4, 5-T, while in apples in addition to this B-Nine also hastens ripening by about
1-4 weeks.
Ethephon has been shown to hasten ripening in grapes.

19. Fruit ripening
In tomatoes all fruits on a plant won‘t mature and ripen at a time. This
is a serious disadvantage for mechanical harvesting.
Ethephon applied 1-2 weeks before harvest promotes degreening and
ripening of tomatoes.
Application of smoke is commercially employed to hasten and ripen
bananas, the active ingredient responsible being ethylene.
Ethephon is also employed for degreeing and colour development of
harvested fruits.

20. Fruit size and quality
Increase in berry size in Anab-e-shahi,Kismis and Bhokri varieties was reported
when GA was applied at 40ppm at bud and flower stages.
Higher concentrations resulted in the increase in the length of berries.

21. Sex expression
Plant regulators can be employed to modify the sex expression in crops.
In cucurbitaceous vegetables the production of male flowers will be always more in
number than the female flowers and this sex ratio can be narrowed down by the
application of ethrel at 100 to 250ppm, if sprayed four times at weekly intervals
commencing from 10 to 15 days after sowing.
This growth regulator not only increases the number of female flowers to male
flowers, but also produces female flowers at earlier nodes.

22. Sex expression
Application of GA, the sex ratio is shifted towards maleness in several cucurbits.
Certain plant regulators are employed to induce male sterility in crop plants, so that
such male sterile plants can be used as a female plant in the hybridization work.
This process dispenses the expensive work.
Complete male sterility in bhendi can be obtained by spraying with 0.4% of MH.
A single spray one week before floral bud initiation offers male sterility for 10 days
and a subsequent spray at floral initiation extends the effect to 22 days.

23. APPLICATION
Among these PGRs auxin compounds especially IBA is of great use in rooting
of cuttings. They can apply in dust, liquid (or) paste formulation. The commercial
dust formulations are Seradix A seradix B and Rootamone.
Dust formulation
The active ingredient of IBA has been dispersed in talcum to get required strength
of the dust formulation. While treating the cuttings, a fresh cut is made at the base
and immediately smear by dipping in the dust. The moisture exuding out of fresh
cut will help proper adherence of dust over the cut surfaces.
The cuttings are gently tapped with fingers to remove the excess dust and planted
for rooting. Cuttings can also be treated in bundles.

24. APPLICATION
Dilute solution method (soak method)
The basal part of cutting is soaked in dilute solution of the auxin compounds for 24 hr in 20
ppm prior to planting for easy rooters and 200 ppm for shy and never rooters.
Concentrated solution method (Quick dip method)
The concentration of compounds vary from 1000 to 10,000 ppm where in the base of the
cuttings are dipped for few seconds before planting. The remaining solution may be reused by
storing in dark far about a week. High concentration for longer period leads to yellowing and
dropping of leaves, blackening and death of the cuttings.
Paste application
The PGR like IAA (or) IBA in lanolin paste is applied at the basal end of the bark removed
portion.