Gibberellic Acid for Fruit Set and Seed Germination
John M. Riley
The following information is taken from a article by John M. Riley that
appeared in the 1987 CRFG Journal
(vol. 19, pp. 10-12).
See the back issue information
for price and ordering information.
The Germination Process
The first stage of germination consists of ingesting water and an awakening
or activation of the germ plasma. Protein components of the cells that were
formed as the seed developed, became inactive as it matured. After an uptake
of water, the system is reactivated and protein synthesis resumes. Enzymes
and hormones appear and begin to digest reserve substances in the storage
tissues and to translocate the digested substances in the storage tissues to
the growing points of the embryo. The sequence of the metabolic pattern than
occurs during germination involves the activation of specific enzymes at the
proper time and regulation of their activity.
Control is exercised by four classes of plant hormones: inhibitors such as
abscissic acid which block germination; auxins which control root formation
and growth; the gibberellins which regulate protein synthesis and stem
elongation; and cytokinins that control organ differentiation. Ethylene is
also believed to have a control function in some plants. Sometimes the last
three controls are used together to crash through dormancy in germinating
Gibberellic acid (actually a group of related substances called gibberellins)
was discovered as a metabolic byproduct of the fungus Gibberella fujikuroi,
which causes the stems of growing rice to elongate so rapidly the plant
collapsed. Synthetic forms of gibberellic acid are available commercially.
Gibberellic acid (GA) is a very potent hormone whose natural occurrence in
plants controls their development. Since GA regulates growth, applications of
very low concentrations can have a profound effect. Timing is critical: too
much GA may have an opposite effect from that desired; too little may require
the plant to be repeatedly treated to sustain desired levels of GA.
Effects of Gibberellic Acid
- Overcoming dormancy. Treatment with high concentrations of GA is effective
in overcoming dormancy and causing rapid germination of seed. Concentrations
of about 2 ppm can cause tubers to sprout earlier.
- Premature flowering. If a plant is sufficiently developed, premature
flowering may be induced by direct application of GA to young plants. This
action is not sustained and treatment may have to be repeated. Formation of
male flowers is generally promoted by concentrations of 10 to 200 ppm.,
female flowers by concentrations of 200 to 300 ppm. Concentrations of more
than 600 ppm markedly suppresses initiation of both male and female flowers.
- 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 some times 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. In a Department of Agriculture experiment, the GA was applied as a 1%
paste in a band around the terminal bud of trees. Treatment was repeated
three times during the summer. Walnut tee growth was 8.5 ft. for treated
trees, 1.5 ft. for untreated trees.
- 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.
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
||2400 (10 1/2)
||600 (2 1/2)
||5 ml (1 tsp.) lanolin
© Copyright 1987,1997, California Rare Fruit Growers, Inc.
Questions or comments? Contact us.