Plant Responses

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  • Created by: abyrne
  • Created on: 07-02-17 10:11

Chemical coordination

Scientists are still unsure about the details of many plant responses. There are a no. of reasons for this:

  • Plant hormones work at very low concentrations, so isolating them and measuring changes in concentrations is not easy
  • The multiple interactions between the different chemical control systems also make it very difficult for researchers to isolate the role of a single chemical in a specific response.
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Plant hormones and seed germination

When the seed absorbs water, the embryo is activated and is triggered to begin to prodcue Gibberellins. They in turn stimulate the production of enzymes that break down the food stores found in the stores i.e starch stores-----> Glucose. The food store is in the cotyledons in dicot seeds and the endosperm in monocot seeds. The embyro plant uses these food stores to produce ATP for building materials so it can grow and break out through the seed coat. Evidence suggests that gibberellins switch on genes which code for amylases and proteases- the digestive enzymes required for germination. There is also evidence suggesting that another plant hormone, ABA, acts as an antagonist (interferes with the action of gibberellin), and that it is the realtive levels of both hormones which determine when a seed will germinate.

Experimental Evidence:


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Plant Hormones, growth, and apical dominance - Aux

Auxins:

Auxins such as indoleacetic acid (IAA) are grotwth stimulants produced in plants. They are mad in cells at the tip of the roots and shoots, and in the meristems. Auxins can move down the stem and up the root both in the transport tissue and from cell to cell. The effect of auxin depends on the concentration and its interactions with other hormones.

Have a no. of major effects on plant growth:

  • Stimulate the growth of the main apical shoot. Evidence suggests that auxins affect the plasticity of the cell wall- the presence of auxins means the cell wall stretches more easily. Auxin molecules bind to specific receptor sites in the plant cell membrane, causing a fall in the PH of about 5. This is the optimum temp for the enzymes needed to keep the walls very flexible and plastic.  As the cells mature, auxin is destroyed. As the hormone levels fall, the PH rises so the enzymes maintaining plasticty become inactive. As a result, the wall becomes rigid and more fixed in shae and size and the cells can no longer expand and grow.
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Plant hormones, growth, and apical dominance - Aux

  • High concentrations of Auxin supress growth of lateral shoots. This results in apical dominance. The lateral shoots are inhibited by the hormone that moves back down the stem, so they do not grow very well. Further down the stem, the auxin concentration is lower and so the lateral shoots grow more strongly. There is alot of experimental evidence for the role of auxins in apical dominance. E.g, if the apical shoot/bud is removed/cut, the auxin-producing cells are removed and so there is no auxin. The dormant lateral buds of the apical shoot grow faster. If auxin is applied artificially to the cut apical shoot to the cut apical shoot/bud, apical dominance is reasserted and lateral shoot growth is suppressed.
  • Low concentrations of auxin promote root growth. Up to a given concentration, the more auxin that reaches the roots, the more they grow. Auxin is produced by the root tips and auxin also reaches the roots in low concentrations from  the growing shoots. If apical shoot is removed, amount of auxin reaching the roots is greatly reduced and root growth slows and stops. Replacing the auxin artificially at the cut apical shoot restores the growth of the roots. High auxin concentrations inhibit root growth.
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Plant hormones, growth, and apical dominance - Gib

Gibberellins:

Involved in the germination of seeds. Also important in the elongation of plant stems during growth. Gibberellins affect the length of the internodes- the regions between the leaves on a stem. Gibberellins were discovered because they are produced by a fungus from the genus Giberella that affects rice- the infected seedling grew extremely tall and thin. The scinetists investigated the rice and siolated - gibberellins- which produce the same spindly growth in the plants. It was then discovered that plants themselves produce the same compounds.

Plants that have short stems produce few or no gibberellins.

Scientist have bred many dwarf varieties of plants where the gibberellin synthesis pathway is interrupted. Without gibberellins the plant stems are much shorter. This reduces waste and also makes the plants less vulnerable to damage by weather and harvesting.

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Plant hormones, growth, and apical dominance - hor

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Plant hormones and growth in plants- synergism and

Synergism and antagonism:

Most plant hormones do not work on their own but by interacting with other substances. 

  • If different hormones work together, complementing each other and achieving/ giving a greater response then they would on their own- interaction known as synergism.
  • If one substance promotes growth for instance and one inhibiting it, the balance between them will determine the response of the plant. This is known as antagonism.
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Plant responses to abiotic stress- Abscission or l

Abscission: 

The falling light levels result in faling concentrations of auxin. The leaves respond to the falling auxin concentrations by producing the gaseous plant hormone ethene. At the base of the leaf stalk is a region called the abscission zone, made up of two layers of cells sensitive to ethene. Ethene seems to initiate gene switching in these cells resulting in the production of new enzymes. These digest and weaken the cell walls in the outer layer of the abscission zone, knwon ad the separation layer. 

The vascualr bundles which carry materials into and out of the leaf are sealed off. At the same time fatty material is deposited in the cells on the stem side of the separation layer. This layer forms a protective scar when the leaf falls, preventing the entry of pathogens. Cells deep in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain on the already weakened outer layer. Further abiotic factors such as low tempsor strong winds finish the process- strain is too mcuh and the leaf separates from the plant. A neat waterproof scar is left behind.

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Plant responses to abiotic stress- Preventing Free

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