Hormones are produced naturally by plants, while plant growth regulators are substances applied to plants to influence growth and development in some way. All plant hormones and plant growth regulators (PGR) are in fact ‘organic’ in that they contain carbon and nitrogen however these can be divided into either synthetic (man made) compounds (such as IBA used in rooting powders/gels, or cycocel) that mimic naturally occurring plant hormones, or they can be naturally occurring substances that have been extracted from plant tissue (e.g the cytokinin ‘zeatin’ is extracted from maize). Whether a plant growth regulator is naturally derived and extracted from plant tissue or synthetic can have a major effect in terms of the result on plant growth. Naturally derived PGRs, can be absorbed and deactivated inside plant tissues where as synthetic PGRs due to their chemical structure are resistant to break down inside the plant.
Plant growth regulator substances are usually applied in low concentrations to plants as only a few parts per million is sufficient to have the required affect. Most often these types of compounds are applied as a foliar spray, although some work best if applied as a seed soak, or media drench around the plants base. The method of application of various PGRs depends on what compound is being used for a particular purpose, but usually their effects are short lived, and repeated applications are required to achieve the desired result.
There are 4 groups of PGR compounds: these are Auxin, Gibberellins (GA), Cytokinin and Ethylene. While auxins, gibberellins and cytokinins are considered to be growth promoters. Ethylene is a gas which can cause leaf abscission and ripening of certain types of fruit, however when applied at the correct concentration as Ethrel (foliar spray) it has been used to induce femaleness in flowers of cucumber and other dioecious plants.
Auxins are most widely used to stimulate root formation on cuttings and are often used in the form of NAA or IBA, although they have other horticultural applications as well such as promoting fruit set in some plants such as tomatoes.
Gibberellic acid (GA) can have the effect of increasing plant height by elongation of the internodes on plants. GA also increasing the rate of germination and break bud dormancy. GA stimulates cell division and elongation, and will stimulate bolting or flowering in some plants by causing cells in the flower bud to divide and expand lengthwise more rapidly than normal. Synthetic compounds such as Cycocel have been produced which are ‘anti-gibberellins’ and force a plant to remain dwarfed by blocking the elongation effect of the plant’s natural Gibberellins.
Cytokinins (such as BA or BAP, or Kinetin, Zeatin): Cytokinins promote cell division, they also promote the growth of lateral buds and stimulate leaf expansion resulting from cell enlargement. Cytokinins also slow down ageing or senescence in leaves, allowing them to stay green and actively photosynthesising for longer.
When using PGRs the timing of application is important. Many PGRs only work when applied at a certain stage, and some are more effective if applied in the root zone where up take can be much greater than when spayed on the foliage.
Many of the other naturally derived PGRs are best applied to the plant part which they are to effect (i.e sprayed on the flower buds, foliage, or applied to where roots are to form).
Thiamine (Vitamin B1):
B1 is produced in the foliage of plants and transported down to the root system where it has an effect on root growth and development. In tissue culture and rooting preparations, B1 helps to stimulate the growth of roots on new plants but this is best used in combination with rooting hormones. B1 can assist at any time in a plant’s life with root regeneration where the root system has been damaged or stressed through high salinity, pathogens such as pythium, nutrient deficiencies and toxicities, high fruit loading etc but only if the foliage of the plant is unable to produce sufficient supplies for this purpose. Use of B1 is seen as a ‘back up’ or ‘insurance policy’ as it is difficult to determine if a plant which has come under stress is capable of producing sufficient B1 to send down to the root system to assist in cell development. Use of Vitamin B1 in plants is the same as in humans – it is most useful where a deficiency exists for some reason. B1 is best applied as a seed soak to speed up germination (root growth), or as a foliar spray.
B1 is an organic compound and as such is rapidly broken down by microbes in the nutrient solution (they love to eat carbon based compounds), adding high amounts of B1 may ensure sufficient thiamine stays in the nutrient for a few hours for some plant uptake, but generally microbes will break this down rapidly as well.