RESPONDING TO THE ENVIRONMENT: PLANTS Flashcards
(36 cards)
What are plant hormones?
Chemicals that regulate plant growth, development, and differentiation of cells and tissues and occur in extremely low concentrations
what is the major means of response to stimuli by plants?
Plant hormones
How are plant hormones produced
Each cell in the plant is capable of producing hormones. They usually interact with each other to bring about the necessary responses
What do plants have to respond to ?
They respond to stimuli in their environment as a safety measure to ensure their survival. They respond to stimuli more slowly than animals.
Examples of stimuli from the environment
Light
Gravity
Chemicals
etc.
What are auxins
One of the most important group of plant hormones
Most common naturally occurring auxin
Indoleacetic acid (IAA)
Where are auxins made and how are they distributed in the plant?
Auxins are made by cells in the tip of a shoot, the apical bud (meristem). Very small amounts are also produced in the root tip. The auxins then diffuse downwards towards the roots. This creates a gradient down the main axis of the plant, the concentration being highest at the tip and lowest at the root.
Functions of Auxins
- Promote growth
- Cause apical dominance
- Play a part in tropic movements
How do auxins promote growth?
Auxins stimulate both cell division and cell elongation which will bring about growth (these cells can then differentiate and develop into various tissues).
How does the auxin concentration affect growth in different parts of the plant?
The further the stem tissue is from the shoot tip, the more sensitive they become to auxin concentrations (need less auxins to bring about elongation.) If the concentration of auxins is too high cell elongation will stop. Root elongation requires much less auxin than stems; higher concentrations will inhibit cell elongation in the roots.
How do auxins cause apical dominance and why is it important?
The concentration of auxins near the apical bud inhibits the growth of the higher lateral buds. If they developed, these buds would compete with the apical tip for light and nutrients. This is known as apical dominance
Explain why some plants have a pyramidal shape.
Lower down the plant, the auxin concentration becomes less so these lower lateral buds can produce shoots which grow into lateral branches. This gives plants the pyramidal shape.
How does pruning make plants bushier
If the apical bud is removed, lateral buds will grow as they are no longer inhibited by auxins and a much bushier plant will develope. This is the theory behind pruning by horticulturists.
How do auxins play a part in tropic movements?
External stimuli bring about tropic movements. This is brought abput by auxins causing cell elongation on only one side of the stem or root, which results in uneven plant growth
Why do tropic movements happen?
Plants cannot move if they happen to grow where conditions are not suitable. However plants can alter their growth so they can grow into more favourable conditions.
Tropisms
Growth movements of a part of a plant in response to an environmental stimulus. (Plants may respond positively by growing towards the stimulus or negatively by growing away from it.)
Phototropism
The growth response to light stimulus.
How do shoots respond to light?
Auxins are light sensitive hormones that will move away from the light source. If light comes from one direction, auxin from the growing tip moves away from the light source. It will concentrate in the cells on the shady or darker. The increased concentration makes these cells elongate more than those on the brights side, so the shoot bends towards the light
Are shoots positively or negatively phototropic
positively phototropic
Are roots positively or negatively phototropic
negatively phototropic
Geotropism
Growth movement of part of a plant in response to the gravitational pull of the earth
How do shoots and roots respond to gravity?
If a young seedling is placed horizontally in a dark chamber and left to continue its development, after a few days the shoot will bend upwards and the root downwards.
Why do stems curve upward in response to gravity?
Under the influence of gravity, the auxin tends to accumulate on the lower side of the shoot and root. In the shoot, this high concentration results in more growth taking place on the lower side than the upper, resulting in an upward curvature. Main stems are therefore negatively geotropic.
