30 Hormonal Influences on Growth

In animals, endocrine glands produce hormones (chemical messengers) which are secreted into the bloodstream.

e.g.1 The pituitary gland is situated at the base of the brain and produces growth hormone (GH) and thyroid stimulating hormone (TSH). Both hormones affect growth and development.

Growth hormone stimulates bone and muscle growth by increasing the rate of mitosis and the rate of amino acid uptake (and protein synthesis) in bones and muscles. Another name for growth hormone is somatotrophin

Over or under-production of somatotrophin at various stages in life can lead to growth abnormalities

e.g. Robert Wadlow

e.g. 2 The thyroid gland produces the hormone thyroxine in response to thyroid stimulating hormone.

Thyroxine controls metabolic rate and, therefore, growth.

1. Auxins

The most common auxin is indole acetic acid (IAA)

Produced by root and shoot tips (and leaf meristems)

Original work on IAA done on seedling coleoptiles:

IAA stimulates mitosis in apical meristems Promotes cell elongation in apical and

lateral meristems Is necessary for cell differentiation in the

area in shoot and toot tips behind the growing tips

Top tip & handy hint:C-MED

Cell level – Mitosis, Elongation, Differentiation

IAA also affects plant organs such as buds, leaves and fruits

The apical bud produces high concentrations of IAA which inhibits the growth of the lateral buds.

This is called apical dominance

Leaf fall is caused by a decrease in IAA concentration in the leaf fall. The decrease in IAA causes the formation of an abscission layer in which the cells become weakened and eventually the leaf or fruit falls off.

Following fertilisation, the seed produces IAA, causing the swelling of the ovary to produce the fruit coat e.g. Grapes etc

Top tip & handy hint:OLAF

Organ level – Leaf abscission, Apical dominance, Fruit formation

This is the term used to describe a plant’s movement towards the direction light is coming from and is caused by IAA

Phototropism is caused by a higher concentration of IAA being produced in the side of a shoot not exposed to light

Plants love being next to windows!

Parthenocarpy: Fruit development without fertilisation can be induced by treating unfertilised flowers with auxins, producing seedless fruits

Delaying abscission: Spraying fruit crops with synthetic auxins near the end of the growing season delays the formation of the abscission layer, stopping fruit from dropping too early before it is ripe

Rooting powders: when applied to cut stems, the auxins in rooting powders promote the formation of adventititous roots

Herbicides: Auxins can be used as selective weedkillers, particularly against broadleaved weeds on lawns. Since the weeds absorb more auxins than the narrow-leaved grasses, their growth rate is highly accelerated, causing them to die of starvation whilst the grasses are relatively unharmed.

Plant Growth Substances2. Gibberellins

Originally discovered in Japan in the 1920’s where plants infected with the fungus Gibberella grew abnormally tall due to the fungus producing a growth substance.

Gibberellins, like auxins, stimulate cell division and elongation (but they do not contribute to bending of shoots or phototropism). The most common gibberellin is gibberellic acid (GA)

Effect of GA on dwarf pea plants

Dwarf pea plants grown in the presence of GA were compared with dwarf pea plants and normal tall pea plants, both grown in water.

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The dwarf plants grown with GA grew as tall as the normal tall plants. Growth was due to an increase in the length between (not number of) internodes. Dwarf pea plants have an inherited deficiency in the gene that produces gibberellic acid.

Effect of GA on dormancy of buds: The dormancy during winter of the buds of deciduous trees is broken by the production of GA in the spring. Dormancy can be artificially broken by application of GA in commercial plantations.

Effect of GA on germinating barley grains

GA induces the production of α-amylase in barley grains which breaks down the stored starch into maltose sugar, giving energy for the embryo to germinate. The GA stimulates the cells of the aleurone layer to produce α-amylase:

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