PLANT CONTROL SYSTEMS

Tropisms

• Stimuli: environmental factors that induce a response from an organism

  • Examples: animals

    • Bee flies toward flowers

    • Deer runs from predator

  • Examples: plants

    • Stomata opens and closes due to water

    • Growth, reproduction

Plant Responses to Stimuli

• Plants can respond to light, gravity, and touch

• Tropism: a growth movement whose direction is determined by the direction from which the stimulus strikes the plant

Positive and Negative Tropisms

• Positive tropism: the plant, or a part of it, grows in the direction from which the stimulus originates

• Negative tropism: growth away from the stimulus

Phototropism

• Plants respond to light

• Stems are positively phototropic

• Roots are negatively phototropic

• Adaptive value: roots growing down and/or away from light are more likely to find the soil, water, and minerals they need, while stems growing up and toward the light can expose their leaves for photosynthesis

Gravitropism

• Plants respond to gravity

• Stems are negatively gravitropic

• Roots are positively gravitropic

• Adaptive value: roots growing down and/or away from light are more likely to find the soil, water, and minerals they need, while stems growing up and toward the light can expose their leaves for photosynthesis

Other Tropisms

• Hydrotropism

Phototropism and Auxins

• Phototropism: the growth of plants towards a light source

• Maximizes the amount of light absorbed by the plant's leaves

• Increased absorption of light yields more photosynthesis and further plant growth

• Plant cells respond to light by growing at different rates

• Auxins: plant growth chemicals produced in the plant tips that stimulate growth

• Auxin moves from the plant tip to the shaded side of the stem when light shines on a plant from one side

• Auxin causes cells on the shaded side of the stem to grow longer, causing the stem to curve toward the light

Gravitropism and Auxins

• Gravitropism: plant growth response to the force of gravity

• Involves auxins

• When a plant is placed on its side, more auxin collects in the cells on the stem's lower side

• Cells on the lower side grow longer than those on the upper side, causing the stem to curve upward

• In roots, decreases in auxin concentration inhibit root growth

Examples of Tropisms

• Positive gravitropism

• Negative gravitropism

• Positive phototropism

• Negative phototropism

• Positive hydrotropism

• Negative hydrotropism