Plant Growth Regulators for Greenhouse Use

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

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

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)

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

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

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

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

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

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)

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

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)

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

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

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

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

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

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

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

Mildly Active B-Nine (Daminozide) Cycocel (Chlormequat) Low activity = relatively safe Labeled rates range from 1000-7500 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 200-4000 ppm, with drenches having higher rates (2000-4000 ppm) Enters plant through leaves and stems

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

Intermediately Active A-Rest (Ancymidol) Use as spray or drench Sprays absorbed through leaves Labeled rates range from 0.5-132 ppm, with lower rates given for drenches (2-4 ppm) Broad label: active on most major greenhouse crops

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

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

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

Triazoles are absorbed primarily through the stems Uniform coverage is important

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

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

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

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.

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

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

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