BEHAVIOURAL ADAPTATIONS

Behaviour in Plants (Or is it physiological)

  What stimuli impact on plants and how do plants respond as a result?

– The effect of environmental stimuli on plants and their effect on plant growth

– Rhythmic activities of plants in response to external stimuli

Plant Behaviour

TROPISMS •  External factors, such as light, gravity and touch, exert an

influence on plant growth and development. •  The growth of a plant in response to a stimulus such as

light or water is called a tropism •  When a plant grows towards a stimulus, it is termed a

positive tropism •  When a plant grows away from a stimulus, it is termed a

negative tropism

TROPISMS Light as a stimulus

When a plant moves in response to light, it shows phototropism

Movement towards the light is a – Positive phototropism

TROPISMS •  Investigations by Charles Darwin which had covered and

uncovered growth tips of plants showed the tip had an influence that passed from the tip to the area where bending occurred

•  Auxin – a plant hormone is produced in the tip of a coleoptile and causes growth of cells in the coleoptile.

•  The tip is the site of reception of the light stimulus. •  The growing region below the tip is the effector

TROPISMS •  Auxin moves away from light

•  Auxin influences the growth rate of plant cells. More auxin, increased growth rate.

•  When a seedling is evenly illuminated, auxin is evenly distributed throughout the growth tip.

•  When light is concentrated on one side of the plant, auxin moves away from the light, to the darker side of the tip

•  Increased concentration of auxin in one side of the plant results in accelerated growth (elongated growth) of the cells on that side.

•  The uneven growth of cells results in bending of the plant.

TROPISMS Gravity as a stimulus

Auxin stimulates the growth of cells in shoots but inhibits the rate of growth in the cells of roots.

Lie a seed on its side and the auxin settles in the lower part of the shoot and the roots due to gravity

Which way will the roots go? Which way will the shoots go?

TROPISMS Gravity as a stimulus

In a horizontal seedling, auxin accumulates (settles) along the lower horizontal part of the shoot (due to gravity) causing the cells in that part to grow faster Hence, the shoot turns up away from gravity In the roots, it is the upper part that grows faster so that the root turns downwards towards gravity. Auxin slows the cell growth in the roots Geotropism is the plant responding to gravity

TROPISMS Gravity as a stimulus

Shoots show negative geotropism

Roots show positive geotropism

The response of plants to distribution of auxins allows plants to orient themselves to a more favourable position within their environment

How do tropisms assist the plant in photosynthesis?

CLIMBING PLANTS

Thigmotropism is the change in growth because of contact with another object.

Explain what would be happening in regard to the distribution of auxin in a climbing plant when it comes in contact with another object. How does auxin contribute to thigmotropism?

RHYTHMIC ACTIVITIES

•  Plants, like animals exhibit a range of rhythmic behaviours.

RHYTHMIC ACTIVITIES Time as a stimulus

Biological clock – the internal clock or timing system for sleep patterns. The sleep pattern occurs every 24 hours

An activity that follows a 24 hour cycle is called a Circadian Rhythm (or circadian cycle)

Plant behaviours that follow a circadian rhythm – opening and closing of flowers, nectar and perfume production

This movement is independent of the direction of the stimulus – Nastic Movement.

RHYTHMIC ACTIVITIES

Solar Tracking or Heliotropism Leaves and flowers that are able to move during the day so they are oriented either perpendicular or parallel to the sun’s direct rays. Eg. Sunflowers How can heliotropism as a behaviour assist in survival?

RHYTHMIC ACTIVITIES

Photoperiodism Timing of Flowering The relative length of day and night is the photoperiod The response of plants to particular periods of light and dark is called photoperiodism. The length of day and night to which a plant is exposed is very important.

RHYTHMIC ACTIVITIES

Photoperiodism Photoperiodism differs between plants Short day plants – eg Chrysanthemums flower

only when the day length is shorter than some critical period.

Long day plants – eg Carnations flower only when day lengths are longer than certain critical minimum.

Day neutral plants – eg Dandelion flower regardless of the length of the day.

RHYTHMIC ACTIVITIES

Photoperiodism How would classify plants that flower in the spring?

Although the terms short-day and long-day plants suggests the exposure to light is critical. The length of darkness is also very critical.

RHYTHMIC ACTIVITIES

Photoperiodism Example – Cocklebur (short-day plant) would only

flower when the dark period exceeded nine hours; the day length does not matter. When the cocklebur is exposed to even a brief flash of light during the dark period, the plant no longer flowers. Uninterrupted dark is important for flowering Short-day plants are really long-night plants. Long-day plants are really short-night plants.

RHYTHMIC ACTIVITIES

Photoperiodism Phytochromes, light sensitive pigments in leaves react to a photoperiod that is appropriate for the particular plant. As a result, the leaves produce a hormone This hormone then travels to buds which then flower Photoperiod can also be important in breaking the dormancy of seeds and other parts of plants.