Gibberellic acid (GA3) What is it? What does it do? and How to use it?

(1α,2β,4aα,4bβ,10β)-2,4a,7-Trihydroxy-1-methyl-8-methylenegibb-3-ene-1,10-dicarboxylic Acid 1,4a-Lactone Acetoxymethyl Ester  – or GA3 for short.

Gibberellic acid belongs to a group of organic chemicals known as PGRs (Plant Growth Regulators) and they are used in horticulture to modify the growth of plants. These chemicals are also known as ‘plant hormones’ because they are analogous to the hormones produced in animal bodies to regulate growth and body processes. There are several kinds of plant hormones: Auxins, Cytokinins, Abscisic Acid, and Gibberelins are the main ones, with various others having other functions in plant growth.

A healthy balance of these chemicals is essential for stable plant growth, and the external application of a PGR will ‘push’ the balance in a certain direction and encourage a particular type of plant growth. Auxins are used to promote rooting for instance, and cytokinins are used to encourage buds to break dormancy.

Gibberellins work by causing the cells to elongate causing the stems to stretch upwards, but the application of gibberellins can also have other interesting effects, as we shall examine, further below.

Is Gibberellic Acid (GA3) Safe?

Gibberellic acid has an excellent record of safety and it is unlikely to cause harm, especially when used in a responsible and safe manner. Gibberellic acid is routinely used by farmers to regulate the growth of seedless grapes and mandarins among other fruit crops, for instance. The degradation of gibberellic acid in the open environment is pretty quick, and it is normally used in very small concentrations.

Tests have shown that it is practically non-toxic to aquatic organisms, and that the lethal dose for mammals is very high. Tests have shown that GA3 is non-mutagenic, and being a substance already produced naturally by all plants and many fungi, this makes sense.

This means that Gibberellic acid is about as safe as any chemical – so long as it is handled and used correctly!

Seed Germination

Gibberellin synthesis is vastly upregulated in germinating seeds – in many cases it is possible to break the dormancy of hard-to-germinate seed by the external application of gibberellin, usually in the form of GA3 by soaking the seed immediately prior to sowing in a solution of 200 to 750ppm GA3. The precise concentration will vary from species to species, with some being badly effected by relatively low doses, and others requiring very high doses to show any effect.

Growth Stimulation

GA3 when used alone will cause unbalanced, tall and leggy growth, which can be counteracted with the simultaneous use of a cytokinin such as 6-BAP. Seaweed extracts used to promote plant growth contain a mixture of PGRs including gibberellins, and this balanced mixture is what contributes to plant growth stimulation.

Flower Stimulation

Some plants can be forced to flower with the application of GA3. Many coniferous plants can be induced to flower with just a tiny amount of GA3 applied to a shoot, and Cyclamen and Azalea are sprayed with GA3 to promote early flowering in time for garden centre sales requirements. This will not work for every species however, and it is often the case that some plants will be prevented from flowering by exposure to GA3 at the wrong time in its life cycle.

Production of Male or Female Flowers

Most common plants are hermaphrodite, producing flowers which have both male and female parts. Some plants however are dioecious, meaning that each plant is either male or female and will only produce flowers of that gender.

GA3 plays in interesting role in both these situations, and can be used to produce male-sterile flowers on some hermaphrodite species like sunflowers, or to produce the ‘wrong’ kind of flower on a particular dioecious plant.

Female hemp plants can be forced to produce male flowers (and pollen) with the application of GA3, allowing the production of all-female seed, whereas in Melons and other Cucurbits the application of GA3 encourages flowers of both genders to form on female plants which is beneficial to yields and breeding.

Plant Breeding

In addition to the sex-manipulating abilities detailed above, GA3 can also be used in various other aspects of plant breeding. Application at the time of flowering and pollination can encourage otherwise tricky crosses to actually produce viable seed, and can be used to stimulation germination of seed which might otherwise fail to germinate. GA3 is also one of the various PGRs used in culturing excised seed embryos and explants in sterile agar media.

How to Use Gibberellic Acid

Gibberellic acid usually comes in the form of GA3, which is a white crystalline powder. In its pure form, GA3 is not very soluble in water but needs to be dissolved in a solvent like isopropyl alcohol – this is possibly the most dangerous part of handling GA3 because the IPA (isopropyl alcohol) is very flammable.

Add 2-3ml of IPA to 50mg in a small sealable container and stir with a rod – pressing the crystals against the side to crush them will speed the process.

Gentle heat will also help, so seal the container and immerse in a hot water bath to heat the solution gently, while gently swirling the mixture around in the container.

Again, remember the IPA is flammable, and will produce fumes which are also flammable so keep away from flames, fire, burning cigarettes etc., when performing this process.

After 10 minutes the crystals should have dissolved, but if not, a small amount of IPA may need to be added and stirred until dissolved. This highly concentrated solution can now be added to water to create a stock solution which will keep for several weeks in a fridge (label it clearly), or diluted to make the required concentration immediately.

IPA and other alcohols are pretty harmless to plants and seeds in the low quantities of the finished solution, so this is usually not a concern.

Pro Tip: When using GA3 for germinating seeds, I have allowed the finished solution to breathe at about 30C for several hours before use, allowing the alcohol  to evaporate out – along with some water. The evaporated water needs to be added back again to the original volume to maintain the correct concentration.

Do your concentration calculations before you do your mixing – and check your workings.

It is easy to make a stupid mistake when converting between units. As a general rule, mg/Litre = ppm, so if you want to make a 500ppm solution, you will need to add the 50mg super-solution to about 100ml of distilled water.

The GA3 is inactive if it is in alkaline solution, however some gardeners will dissolve the GA3 in a small amount of strong alkali, and then titrate with HCl until mildly acidic – this may be worth a shot if the concentration needs to be high and the amount of IPA is a worry, however the effectiveness of the gibberellins treated in this way is likely to be lower.

As always, whenever handling chemicals follow the basic rules:

• maintain a clean working environment,
• minimise contact with skin,
• wash hands thoroughly, and
• dispose of chemicals in a safe manner.