Growth Stimulants, Retardants, and Rooting Hormones

Growth regulators

 Hormones  Organic chemicals that act and interact to affect growth rate  Auxins – accelerate growth by stimulating cell enlargement

Gibberellins

 Stimulate growth in stem and leaf by cell elongation  Stimulates premature flowering, growth of young fruits and breaking of dormancy

Cytokinins

 Stimulate cell division  Work along with auxins  Will not work without auxins present

Inhibitors

 Abscisic Acid and Ethylene Gas  inhibits seed germination  Inhibits stem elongation  Hasten ripening of fruit – ethylene gas

 Auxins and Gibberellins promote cell enlargement  Cytokinins stimulate cell division

Growth Hormones

 Organic chemicals produced by actively growing plant tissue  Shoot tips and young leaves  Move throughout the plant and can be found in most tissues

 These chemicals react with one another in a very complex system in the plant  In some cases a concentration of one hormone stimulates growth and a different concentration restricts growth

Apical Dominance

 Dominance of the terminal bud  Apical dominance exists there  Terminal bud secretes chemicals that inhibit or prevent the growth of axillary buds on the same shoot

Apical Dominance

 Axillary buds are found in the axil – angle between a leaf and the stem.

 Causes the plant to grow tall and not branch  Once the plant reaches flowering age and the terminal bud becomes a flower, the chemicals are no longer secreted

Apical Dominance

 The plant then starts to send out side branches  This allows the plant to grow above competing plants  Once height and access to sunlight are secured, the plant spreads out over its competitors

Apical Dominance

 Pinching off terminal buds removes the hormone and causes branching to happen faster  Common practice in greenhouses and nurseries to produce bushier plants with many flower buds

Stimulants

 Allow plants to grow taller  Most common is Gibberellic Acid (GA)  Causes stems of plants to stretch out  Nodes are farther apart

Growth Stimulants

 Natural growth stimulants have been found in alfalfa  It causes stimulation of growth  It is the alcohol called Triacontanol  Stimulation is brought about by mulching plants with alfalfa hay

 Or watering plants with a “tea” made from soaking alfalfa feed pellets in water  Amount needed is very small  Also sold commercially

Chemical Retardants

 Chemicals are used to retard growth of plants causing them to be shorter and more compact  Plants are more attractive and the plants themselves are stronger  Used commercially  B-Nine is commonly used on Azaleas

 Newest uses of Plant Growth Retardants (PGR) is the application to lawns  After first mowing in the spring “Limit” is applied and absorbed by the roots  Restricts growth for 6-8 weeks

 “Embark” is absorbed by the leaves and also restricts growth  PGR’s absorbed by the leaves move to the growing point where it interrupts cell division, stem elongation and seed head formation  Roots continue to grow  Applied after first or second mowing

Sumagic

 New growth retardant  Reduces height of plants by inhibiting production of GA  Chemical is taken up by leaves and moves through the plant to the terminal bud

 Used to control growth of shrubbery and hedges  PGR Atrimmic applied after pruning can last an entire season  Reduces or eliminates the need to prune

Atrimmic

 A systemic  Penetrates the plant, enters the plant sap, and moves through the plant  Blocks plant hormones that stimulate growth  Applied as a foliar spray

Rooting Hormones

 Important when propagating plants by cutting  Helps cuttings to develop more roots faster than without a rooting hormone  Indoleacetic Acid (IAA) naturally occurring, causes roots to form on plant stems

IBA – Indolebutyric Acid

 Most widely used rooting hormone  Most effective rooting hormone

Rooting Hormones

 Either mixed with talc and used as a powder or dissolved in water and used as a wet dip  Liquid can be slightly more effective  All rooting hormones should contain a fungicide

Fungicide

 Helps prevent cuttings from rotting

Dwarfing Rootstock

 Used in fruit trees  Shorter trees enable you to pick fruit without using a ladder  First research done to prevent apple trees from growing very tall  Trees growing from certain types of roots didn’t grow as tall

 These trees also bore fruit at an earlier age  Complete series of rootstock known as malling rootstock developed in England  Controls size and rate of growth or apple trees

 Stock has also been developed for peach and pear trees as well  Dwarf trees can be purchased commercially

Chemical Blossom Set

 Used on tomato blossoms early in the season  Causes earlier development of fruit  Causes seedless tomatoes to set on first blossoms and results in tomatoes ready to eat as much as 10 days earlier than normal

Plant Biostimulants

 Natural products  Organic  Work to stimulate soil microbial activity and improve soil cation exchange capacity, stimulate plant growth and promote disease resistance

Humic Acid

 Product of rotting organic matte is an example of a soil microbial stimulant

Root growth biostimulants

 Improve water and nutrient uptake  Increase number of fibrous roots  High levels found in kelp plants

Biostimulants

 Greatly reduce the need for fertilizers, especially N  Saves money, reduces pollution

Methanol

 A form of alcohol  Speeds plant growth  Research in California showed an increase in plant yield of 36-100%  Works by blocking photrespiration  Plants use water for growth rather than transpiring it into the air

Methanol

 Works best on plants frown in full summer sun  Water use reduced by as much as 50% in some plants

Allelopathy

 Production of a chemical compound in one plant that slows or stops the growth of another plant  Natural herbicide  Black Walnut, Millet