Auxins are a class of plant hormones that are involved in all phases of plant
growth and development, but are most commonly associated with the stimulation of roots from plant cuttings.
The majority of naturally occurring auxin in plants is indole-3-acetic acid (IAA) produced in the new growing points, hence the word “auxin” is derived from the Greek word “auxein,” meaning “to increase” or “to grow.” Other than the initiation of new roots, auxins are also responsible for cell expansion and division and have shown to be involved in shoot elongation, adventitious root development, fruit and flower development, and tropisms (plant movements).
The amino acid tryptophan is the precursor for indole-3-acetic acid and other indole derivatives known as auxins. The most common forms of synthetic auxins are indole-3-butyric acid (IBA) and naphthalene acetic acid (NAA). While there are naturally derived auxins, such as IAA, they are generally chemically unstable once extracted from the plant and thus are not typically used in rooting compounds. The effective exogenous application rate of IBA or NAA has been shown to be between 500-2000 ppm for herbaceous cuttings up to 10,000 ppm for difficult to root hardwood cuttings. When calculating the percentage of IBA or NAA simply multiply the percent of auxin on bottle by 10,000 to convert from percentage to ppm. So for a product that shows 0.3% IBA, the product would contain 3000 ppm IBA in solution.
When a cutting is taken from a plant, the auxin (mostly IAA) that was stored in the new growing point of the cutting (apical meristem) is transported towards the freshly cut end to begin the process of cell differentiation to initiate root cell development. While the plant contains small amounts of auxins stored in the newly expanding tissues of the plant, adding an auxin compound directly to the cut portion of the plant where new roots will increase rooting potential. This is why growers who propagate plants from cuttings will utilize rooting gels, powders, and dips that contain auxin to increase root initiation and vigor of root formation.
– Brandon Jewell