Significance of Phototropism and Geotropism

Stems

• Positive phototropism and negative geotropism of stems.
• The stems will grow towards the light and up away from gravity.
• This places the leaves in better light with increase in photosynthesis.
• More food means better growth and reproduction.

Roots

• Negative phototropism and positive geotropism of roots.
• The roots grow away from light and down in the gravity of direction.
• The roots are more likely to find ‘soil’ in this direction.
• Soil is important for plants for anchorage, water and mineral nutrients.

Plant Growth Regulators

Tropisms are controlled and moderated by special chemicals called growth regulators.

A plant growth regulator is an organic substance that is made in tiny amounts by the plant and has very definite specific effects on tissue metabolism and growth.

The target tissue of the growth regulator may be the local tissue or tissue in a different part of the plant.

The growth regulator affects the cell cycle, cell enlargement and cell differentiation.

Natural plant growth regulators that move to their target are called plant hormones.

The transport of plant hormones to distant targets may be by diffusion, in xylem or in phloem.

Five Major Groups of Plant Growth Regulators

• Auxins: growth promoters – stimulates stem cell elongation, flower and fruit formation.
• Gibberellins: growth promoters – stimulates stem cell elongation and seed germination.
• Cytokinins: growth promoters – stimulates cell division and differentiation.
• Abscisic Acid: a growth inhibitor – causes seed and bud dormancy, represses cell elongation.
• Ethylene: often a growth inhibitor – fall of leaves and fruit.

Many growth responses are not cause just by one growth regulator but by a combination of different regulators and the concentration of each in the ‘mix’.

Auxin and gibberellin are together involved in stem cell elongation – each affecting a different part of the process.

Auxin and cytokinin are together involved in the terminal bud suppressing the development of lateral buds – this is termed ‘apical dominance’.

Auxin

Production Sites

• meristems – apical and lateral,
• young leaves,
• developing fruit and seeds.

Functions

• increase the plasticity of plant cell walls for enlargement and shaping.
• influences the expression of specific genes involved in growth.
• role in stimulating cell division.

Effects

• change in growth direction of stem and root,
• apical dominance – prevent lateral bud growth,
• fruit development,
• formation of adventitious roots.

Mechanism of a Plant Response

E.g. positive phototropism of a stem to unilateral light.

• Auxin is produced in the apical meristem of the stem.
• In unilateral light much auxin moves to the shaded side of the stem apex.
• Auxin moves away from the stem apex towards the elongation zone.
• There will be an unequal distribution of auxin in the elongation zone.
• The shaded side will have a higher auxin concentration.
• High auxin concentration in stems stimulates cell elongation.
• The shaded side cells are stimulated to greater elongation than the cells on the other side.
• This unequal growth causes the stem to bend towards the light.

Synthetic Plant Growth Regulators
These are inorganic substances made by physical chemistry that affect plant growth.
Some mimic the natural growth regulators in structure and action.
Many are completely different to natural regulators both in structure and action.

Commercial Use of Plant Regulators
Any two of:

• Rooting Power: increase the success of stem cutting by promoting extensive early rooting.
• Cytokinin: use in tissue culture to stimulate cell differentiation.
• Ethelene: quick ripening of mature ‘green bananas’ for the market.
• Auxin: as a selective weed killer to reduce competition and so promote crop growth.
• Gibberellins: production of seedless fruits e.g. oranges.

(Practical use of abscisic acid has not yet been extensively developed.)

Plant Protection Adaptations

• Cuticle: protection against leaf infection by bacteria, fungi and viruses.
• Cork: protection against insect pest damage.
• Cuticle and Stomata Closure: protection against excessive water loss.
• Stinging Dermal Hairs: protection against ‘large’ herbivores.
• Spines and Thorns: protection against ‘large’ herbivores.
• Toxic Substances: protection against insect pests and ‘large’ herbivores.
• Foul Tasting Chemical: discourage ‘large’ herbivores.
• Warning Chemicals: to alert neighbouring to start making protective chemicals.
• Heat Shock Proteins: prevent specific proteins from denaturing so they remain functional.

Mandatory Activity

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