1. Plant Growth Regulators for Greenhouse Use

2. Plant Growth Regulators
Compounds affect plant growth and development.
Natural (Endogenous)
Plant Hormones
Mediate plant response to environment
Synthetic (Man-made)
Control growth
Supplements/Mimics Endogenous Hormones
Restricts synthesis of Endogenous Hormones

3. Classes of Plant Hormones Include:
Auxins
Cytokinins
Gibberellins
Ethylene
Abscisic Acid

4. Auxins Responsible for cell elongation prior to differentiation
Produced in meristems; responsible for apical dominance
Phototrophism
Geotrophism
Indole-3-acetic acid (IAA)
(most common endogenous form)
Synthetic forms include:
Indole-3-butyric acid (IBA)
Napthalene acetic acid (NAA)

5. IBA and NAA used to promote rooting of stem and leaf cuttings
Exogenous auxins result in faster, more extensive and uniform rooting

6. Cytokinins Responsible for cell division
Associated with rapidly growing tissues, juvenility
Numerous endogenous forms based on Adenine (nucleotide)
Synthetic forms include:
Benzyladenine (BA)
Kinetin

7. Cytokinins interact with auxins in plant tissue culture
Cytokinin Auxin Shoots
Cytokinin Auxin Roots
Cytokinin = Auxin Callus

8. Gibberellins
Responsible for cell elongation and division (leaves, stems)
Produce more uniform growth than auxins
Endogenous forms are terpenoid compounds
Gibberellic Acid (GA3), first isolated from a fungus, is commercially available

9. Gibberellic acid may be used to:
Enhance flower size, number; decrease time to flower
Replace cold treatments (e.g. - azalea, camellia, hydragnea)
Elongate stems and peduncles

10. Ethylene Responsible for:
Flowering
Fruit formation and ripening
Leaf abscission
Senescence
Endogenous form is gas evolved by plant tissues
(partially dissolved in cytoplasm)
Ethephon is synthetic form.
(transformed to ethylene in plant tissues)

11. Increase lateral branching; retard height
Ethylene may be used to:
Increase lateral branching; retard height
Prevent stem topple
Promote flowering
Promote leaf abscission

12. Abscisic Acid Responsible for dormancy; maturity
Inhibitor – restrains growth and reproduction
Abscission of leaves, petals, fruit
Flowering in SD
Senescence
Only one endogenous form (ABA)

13. Abscisic Acid Promotes bud and seed dormancy
Causes leaf, petal and fruit abscission
In some SD plants, promotes flowering
Can block effects of IAA
Inhibitory effects on buds can be overcome by gibberellins

14. Greenhouse Use of PGRs Most applications made to regulate shoot growth
Plant Growth Retardants

15. Growth Retardants Reduce Plant Growth Rate by:
Inhibiting the biosynthesis of gibberellins
Gibberellin is the plant hormone primarily responsible for cell elongation
Releasing ethylene

16. Plant Growth Retardants
Primarily effects:
stem, petiole and flower stalk tissues
Less effect:
leaf expansion, resulting in darker green, thicker leaves

17. When growth retardants used:
leaf number remains the same but the internode length is reduced

18. Effects of Growth Retardants
Primary Effect of Growth Retardants:
Reduce elongation of stems, petioles and flower stalks
Secondary Effects of Growth Retardants:
Smaller, thicker leaves
Thicker stems (resistant to mechanical damage)
Darker leaf color
Increased stress tolerance (drought, pollution)
Higher aesthetic quality

19. PGRs for use in Greenhouses:
B-Nine
Cycocel
A-Rest
Bonzi
Sumagic
Florel

20. Mildly Active B-Nine (Daminozide) Cycocel (Chlormequat)
Low activity = relatively safe
Labeled rates range from ppm
Inactive in media; use only as foliar spray
Active on most crops
Enters plant through leaves
Cycocel (Chlormequat)
Use as spray or drench
Sprays may result in yellowing (≥ 1500 ppm)
Labeled rates range from ppm, with drenches having higher rates ( ppm)
Enters plant through leaves and stems

21. Tank Mixes
• Synergistic Response = mixing two chemicals results in enhanced activity
B-Nine / Cycocel Mix
More active than using either product alone
Lower rates of
Cycocel can be used
Optimal control,
reduced marginal
chlorosis

22. Intermediately Active
A-Rest (Ancymidol)
Use as spray or drench
Sprays absorbed through leaves
Labeled rates range from ppm, with lower rates given for drenches (2-4 ppm)
Broad label: active on most major greenhouse crops

23. Bonzi and Sumagic are newer growth retardants belonging to a class of chemicals called triazoles, which are highly active and have distinct differences from other plant growth retardants.
Triazoles much stronger than other retardants
Greater potential for control of excessive growth or stunting

24. Highly Active Bonzi (Paclobutrazol) Sumagic (Uniconazole)
Very persistent effect
Can be used as spray (1 to 200 ppm) or drench (1 to 30 ppm)
Active on almost all plants
Sumagic (Uniconazole)
Lowest rates
Most persistent
Use as spray (1 to 50 ppm) or drench (0.06 to 3 ppm), also bulb dip or soil spray
Active on a wide variety of plants

25. Effect of Increasing Volume of Spray Application on Height Control of Vinca
Untreated
2 qt/100 sq ft
3 qt/100 sq ft
4 qt/100 sq ft
Untreated, or treated with
1ppm Sumagic at the label recommended volume

26. Triazoles are absorbed primarily through the stems
Uniform coverage is important

27. Ethephon Florel/Pistill (Ethephon)
Ethephon is absorbed by the plant and converted to gaseous ethylene (senescence and ripening)
Produces different responses in various plants
Promotes defoliation (harvest, storage)
Promotes floral induction (bromeliads)
Facilitates hybrid seed production (cucurbits)
Reduces height, stem topple (bulbs)
“Chemically pinches” plants

28. Ethephon (Chemical Pincher)
Increases lateral branching, prevents flower initiation and development, inhibits internode elongation
Chrysanthemums, Azaleas, Geraniums
Removes unwanted flowers to encourage vegetative growth
Stock Plants, Young Bedding Plants, Plugs
Absorbed by foliage
Overdoses delay flowering

29. Timing of Application
PGRs do not reduce plant size! They reduce the plant’s growth rate. You must apply the PGRs prior to the “stretch”.
Apply prior to rapid shoot growth
On plugs after the roots are established and growth is resumed
On pinched plants, after shoots are visible, just starting to elongate

30. Interaction With Environment
Very water soluble PGRs (B-Nine and Cycocel)
move slowly through the cuticle
absorbed while the leaf is wet
spray under conditions of high humidity with little air movement
cloudy weather
early morning or evening.
Do not apply overhead irrigation for several hours after spraying.
Poorly water soluble PGRs (A-Rest, Bonzi, and Sumagic)
pass quickly through the cuticle and absorbed
leaf wetness is not as critical.

31. Spray Applications Label identifies target tissue
Sufficient foliage or stems must be present
Proper coverage of target tissue
B-Nine, target max foliage coverage
Triazoles, target max stem coverage

32. Drench Applications Generally less effect on flower or bract size
Tend to provide longer control
Easier to apply uniformly
More costly than sprays
Label recommendations are given in solution rates (ppm) and volume recommendations
Sometimes also as milligrams of active ingredient (mg.a.i.) per pot

33. Multiple Applications
vs. Single Application
More precise control of plant growth
More labor
If growing conditions are not good, a single application early at high rate may cause stunting
Excessive rates of triazoles - persistent growth reductions in the flat or even in the landscape
Very late applications may delay flower opening in bedding plants