Plant Hormones:
Plant hormones are chemical messengers that affect a plants ability to respond to its environment. Hormones
are organic compounds that are effective at very low concentration; they are usually synthesized in one part of
the plant (the meristems) and are transported to another location. Because hormones stimulate or inhibit plant
growth, many botanists also refer to them as plant growth regulators. Botanists recognize five major groups of
hormones: auxins, gibberllins, ethylene, cytokinins, and abscisic acid.
Auxins: A well-known natural auxin is indoleaceric acid, or IAA, which is produced in the apical meristem of
the shoot. Before a cell can elongate, the cell wall must become less rigid so that it can expand. IAA triggers an
increase in the plasticity, or stretch ability, of cell walls, allowing elongation to occur.
Responsible for tropism
Promotes cell division and cell elongation.
Promotes root development.
Responsible for apical dominance.
Gibberellins: Gibberellin is found in small quantities by plants themselves. It has many effects on a plant, but
primarily stimulates elongation growth. Spraying a plant with gibberellins will usually cause the plant to grow
to a larger than expected height.
Bring about elongation of the internodes of stems.
Stimulates root growth.
Promotes development of flowers/seed germination.
Promotes sprouting of buds, which have been dormant.
Increases fruit size
Abscisic Acid: Abscisic acid, or ABA, generally inhibits other hormones, such as the auxin IAA. ABA helps
bring out dormancy in a plants bud and maintains dormancy in seeds. Dormancy in seeds is advantageous
because the seeds can wait for favourable conditions and the seeds have a chance to get dispersed.
ABA causes the closure of plants stomata in response to a drought. Water stressed leaves produce large
amounts of ABA, which triggers potassium ions to be transported out of the guard cells. This causes stomata to
close, and water is held in the leaf.
Causes dormancy of seeds by slowing down germination and the dormancy of apical buds.
Induces flowering in some plants.
Helps the closing of the stoma in times of water stress.
Promotes aging in leave.
Tropisms: A tropism is a growth movement in response to an external stimulus.
Phototropism is a response to light, geotropism is a response to gravity, thigmotropism is a response to
touch, and hydrotropism is a response to water.
If the part of the plant grows toward the stimulus it is said to be positively e.g. phototropic. If the part of the
plant grows away from the stimulus it is said to be negatively e.g. phototropic.
Experiment to demonstrate phototropism in stems:
Method: Two gas jars were prepared as follows: Mustard seeds with straight plumules were placed on the
gauze mat suspended above water.
Gas jar A (experiment apparatus) was covered with a box, which had a slit, cut on one side and the rotated on a
horizontal clinostat.
Gas jar B (control) was placed on a horizontal clinostat, which rotated constantly, ensuring light from all sides.
The plants were examined after 2 days.
Result: The stems of the experimental apparatus had grown toward the light. The stems of the control had
grown straight.
Conclusion: Stems are positively phototropic.
Plant hormones are chemical messengers that affect a plants ability to respond to its environment. Hormones
are organic compounds that are effective at very low concentration; they are usually synthesized in one part of
the plant (the meristems) and are transported to another location. Because hormones stimulate or inhibit plant
growth, many botanists also refer to them as plant growth regulators. Botanists recognize five major groups of
hormones: auxins, gibberllins, ethylene, cytokinins, and abscisic acid.
Auxins: A well-known natural auxin is indoleaceric acid, or IAA, which is produced in the apical meristem of
the shoot. Before a cell can elongate, the cell wall must become less rigid so that it can expand. IAA triggers an
increase in the plasticity, or stretch ability, of cell walls, allowing elongation to occur.
Responsible for tropism
Promotes cell division and cell elongation.
Promotes root development.
Responsible for apical dominance.
Gibberellins: Gibberellin is found in small quantities by plants themselves. It has many effects on a plant, but
primarily stimulates elongation growth. Spraying a plant with gibberellins will usually cause the plant to grow
to a larger than expected height.
Bring about elongation of the internodes of stems.
Stimulates root growth.
Promotes development of flowers/seed germination.
Promotes sprouting of buds, which have been dormant.
Increases fruit size
Abscisic Acid: Abscisic acid, or ABA, generally inhibits other hormones, such as the auxin IAA. ABA helps
bring out dormancy in a plants bud and maintains dormancy in seeds. Dormancy in seeds is advantageous
because the seeds can wait for favourable conditions and the seeds have a chance to get dispersed.
ABA causes the closure of plants stomata in response to a drought. Water stressed leaves produce large
amounts of ABA, which triggers potassium ions to be transported out of the guard cells. This causes stomata to
close, and water is held in the leaf.
Causes dormancy of seeds by slowing down germination and the dormancy of apical buds.
Induces flowering in some plants.
Helps the closing of the stoma in times of water stress.
Promotes aging in leave.
Tropisms: A tropism is a growth movement in response to an external stimulus.
Phototropism is a response to light, geotropism is a response to gravity, thigmotropism is a response to
touch, and hydrotropism is a response to water.
If the part of the plant grows toward the stimulus it is said to be positively e.g. phototropic. If the part of the
plant grows away from the stimulus it is said to be negatively e.g. phototropic.
Experiment to demonstrate phototropism in stems:
Method: Two gas jars were prepared as follows: Mustard seeds with straight plumules were placed on the
gauze mat suspended above water.
Gas jar A (experiment apparatus) was covered with a box, which had a slit, cut on one side and the rotated on a
horizontal clinostat.
Gas jar B (control) was placed on a horizontal clinostat, which rotated constantly, ensuring light from all sides.
The plants were examined after 2 days.
Result: The stems of the experimental apparatus had grown toward the light. The stems of the control had
grown straight.
Conclusion: Stems are positively phototropic.
