Uses of Gibberellic Acid
Robert L. Morris
March 10, 2009
Gibberellic acid (also called Gibberellin A3, GA, and (GA3) is a hormone found in
plants. Gibberellic acid is a very potent hormone whose natural occurrence in
plants controls their development. Gibberellic acid promotes growth and
elongation of cells. It affects decomposition of plants and helps plants grow
if used in small amounts, but eventually plants develop tolerance for it.
Gibberellic acid stimulates the cells of germinating seeds. Since GA regulates
growth, applications of very low concentrations can have a profound effect
while too much will have the opposite effect. It is usually used in
concentrations between 0.01-10mg/L.
Gibberellins have a number of effects on plant development including
rapid stem and root growth and increase seed germination rate.
Gibberellins are used
in agriculture for various purposes. GA-3 is sprayed on seedless grapes to
increase grape size and yield, and it is used on navel oranges, lemons, blueberries,
sweet and tart cherries, artichokes and other crops to decrease or increase
fruit set, delay rind aging, etc. These effects are highly dependent on
concentration and stage of plant growth. GA is used to trigger flowering of
sweet potatoes in breeding programs, to help tomatoes set fruit at high
temperatures in the tropics, overcomes the need for chilling or long days to
trigger flowering, and so is used in the tropics for breeding.
Developing seeds are active sites of containing GA and studies have found increases in GA levels in seeds during germination. The germination of old seeds has been improved with use of GA. Applied GA-3 may trigger dormant seed germination, in many cases overcoming the need for special or prolonged dormancy-breaking conditions such as cold treatment, light, after-ripening, etc.
- Overcoming dormancy. Treatment with high concentrations of GA is effective in overcoming dormancy and causing rapid germination of seed.
- Premature flowering. If a plant is sufficiently developed, premature flowering may be induced by direct application of GA to young plants.
- Increased fruit set. When there is difficulty with fruit set because of incomplete pollination, GA may be effectively used to increase fruit set. The resulting fruit maybe partially or entirely seedless. GA has increased the total yield in greenhouse tomato crops both as a result of increased fruit set and more rapid growth of the fruit.
- Hybridizing. Pollination within self-incompatible clones and between closely related species may sometimes be forced by the application of GA and cytokinin to the blooms at the time of hand pollination.
- Increased growth. GA applied near the terminal bud of trees may increase the rate of growth by stimulating more or less constant growth during the season.
- Frost protection. Spraying fruit trees at full-blossom or when the blossoms begin to wither can offset the detrimental effects of frost.
- Root formation. GA inhibits the formation of roots in cuttings.
Concentration
parts/million |
GA
mg |
Water
ml (cup) |
Purpose
|
50
|
125
|
2400 (10 1/2)
|
Early flowering
|
200
|
125
|
600 (2 1/2)
|
Early flowering
|
800
|
125
|
160 (2/3)
|
Blossom set
|
2000
|
125
|
60 (1/4)
|
Seed germination
|
1% paste
|
125
|
5 ml (1 tsp.) lanolin
|
Growth promoter
|
Gibberellic acid is sometimes used in laboratory and greenhouse settings
to stimulate germination in seeds that would otherwise remain dormant. It is
also widely used in the grape-growing industry as a hormone to induce the
production of larger bundles and bigger grapes, especially Thompson seedless grapes.
In the Okanagan and Creston Valleys of Canada it is used in the cherry industry
as a growth regulator.
Effects of Gibberellic Acid
Although
GA is not listed as a "poison", the following precautions should be
observed: Flush with water any GA that may get into the eye. Avoid skin contact
if possible. If skin contact is suspected, wash with soap and water. Do not
re-enter an area after spraying until the GA spray is fully dry. Avoid
ingestion of GA. The
powder may be dissolved as specified below to give the desired concentration.
Thinning and Increasing the Size of Table Grapes
In varieties like
Thompson Seedless, Flame Seedless, Perlette and a few other varieties (seedless
grapes primarily) GA3 (gibberellic acid) is used for either thinning out the
number of flowers which set fruit, a bloom thinning spray, or for increasing
the size of the berries (referred to as a sizing spray which is applied about
one week following bloom).
In some varieties both a bloom spray and one or two
sizing sprays are generally applied. Not all varieties respond to bloom sprays
for thinning. These varieties may use a gibberellin spray only for increasing
the size of the berry. Grapes which produce seeds are generally larger than
seedless grapes because seeds produce natural gibberellins which increase the
size of the berries.
One must be
extremely careful when using GA3 on grapes. Each variety has a different
tolerance level and one should not ‘experiment” or use more of the material
than is recommended for any given variety.
The gibberellin
produced for commercial use in grapes is manufactured by Abbott Laboratories,
Merck, and Agtrol. It is produced in both a powder formulation and a liquid
formulation. For most home owners, the 4% liquid formulation (containing 1.0
gram/fluid ounce of formulated product) is the easiest to use. Before applying
gibberellin to your vines, refer to the gibberellin product label and the
variety write-up for levels which each variety will tolerate. Do not exceed the
recommended dosages. Higher than recommended levels can severely injure the
plant. Do not apply gibberellic acid to grape varieties not listed on the
product label.
The spraying
guide put out by the companies for the use of 4% liquid on grapes refers to
actual grams of gibberellic acid applied in the finished spray per acre.
Conversion for only a few grapevines is often difficult; so, refer to Table I
below in which the values have been converted to parts per million (ppm) in a
one gallon water solution. One gallon of solution is enough to treat two mature
grapevines.
The rate of
gibberellin used increases as the season progresses. Bloom sprays are generally
much lower than sizing sprays which are applied about one week later.
References
J. L. Hudson Seeds
website. http://www.jlhudsonseeds.net/GibberellicAcid.htm
Norton, Maxwell.
2007. Presentation at the University of Nevada Cooperative Extension Orchard
Workshop.
Riley, John M.
Gibberellic Acid for Fruit Set and Seed Germination. 1987 CRFG Journal (vol.
19, pp. 10-12). And can be found at http://www.crfg.org/tidbits/gibberellic.html
Wikipedia.
Gibberellic Acid. Found at http://en.wikipedia.org/wiki/Gibberellic_acid