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