Plant HORMONES and
growth in plants
Chemical Coordination
- Plants are not passive objects. They respond to abiotic stimuli in terms of directional growth - this is a tropism.
- Limitations of plants include lack of mobility and nervous system. They use hormones secreted from cells to respond to their environment.
- Hormones are less immediate than the responses nervous systems animals provide, but their effects are often more long-term.
- Plant hormones can be used to protect themselves from pests, produce chemical defences against herbivores, communicate with other plants and more.
Hormone |
Role |
Importance |
Auxins |
|
|
Gibberellin |
|
|
Ethene |
|
|
Abscisic Acid |
|
|
Plant Hormones and Seed Germination
- A seed embryo is activated when it absorbs water, producing gibberellins.
- Gibberellins turn on the genes to code for amylases and proteases which break down food stores found in the seed.
- Cotyledons in dicotyledonous seeds, and endosperm in monocotyledonous seeds.
- The embryo plant uses these food stores to produce ATP for building materials to grow and break through the seed coat.
- Abscisic Acid (ABA) works against gibberellins to determine when the seed will germinate.
Experimental Evidence for the Role of Gibberellins in Germination
- Mutant breeds of seeds lacking gibberellins have not been able to germinate until gibberellins were applied externally.
- Seeds with gibberellin biosynthesis inhibitors do not germinate until the inhibitor is removed.
- Auxins are growth stimulants produced in plants. Small quantities can have significant effects.
- Auxins are made in meristems, in growing parts of the plant.
- Auxins can move around the plant in transport tissues.
- Auxins stimulate the growth of the main apical shoot. They affect the plasticity of cell walls, allowing them to stretch more easily for elongation.
- High concentration of auxins suppress growth of lateral shoots, resulting in apical dominance.
- Growth in the main shoot is stimulated by auxin IAA, but the lateral shoot growth is inhibited by it.
- Further down the stem where the concentration of IAA is less strong, lateral shoots grow stronger.
- If the apical shoot (apex) is removed, there is more lateral growth as the concentration of IAA is significantly less. If auxin is applied artificially then apical dominance returns.
- Low concentrations of auxins promote root growth.
- However, if the apical shoot is removed, then the concentration of auxins in the root is so low growth eventually stops.
- High auxin concentrations in the root also inhibit root growth.
Gibberellins in Growth and Apical Dominance
- Gibberellins are involved in the germination of seeds, elongation of plant stems, and the length of internodes (gaps between leaves on a stem).
- Plants with short stems produce very few gibberellins, whereas more are found in longer stems.
- Dwarf varieties of plants have been bred by interrupting the gibberellin synthesis pathway. Shorter stems reduce waste and protect the plant from bad weather.
Investigating the Effect of Hormones on Plant Growth
- Growing plants hydroponically in different serial dilutions of different hormones or applying different concentrations of hormones to the cut ends of roots and observing the effects can help us determine which parts of the plant grow best with which concentration of hormones.
- Studies usually involve large numbers of plants so a standard deviation can be done on the spread of data to find significance.
Synergism: Where two substances interact complementarily, prompting a greater response than they would alone.
Antagonism: Where two substances with opposite effects interact to balance each other, the balance of which determines the response.