Plant Hormones and Tropism
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Questions and Answers

How do auxins contribute to the growth of plant shoots?

  • By promoting cell elongation, resulting in increased growth. (correct)
  • By accelerating the process of photosynthesis within the cells.
  • By causing the shoots to grow downwards, anchoring the plant more firmly.
  • By inhibiting cell division, leading to slower growth.

What is the significance of shoots exhibiting a positive phototropic response?

  • It enables the shoots to absorb water and minerals from the soil.
  • It ensures the shoots grow upwards, away from gravity.
  • It allows the shoots to grow downwards into the soil.
  • It allows the leaves to absorb sunlight for photosynthesis. (correct)

Why do roots display a positive gravitropic response?

  • To grow upwards, away from the pull of gravity.
  • To maximize exposure to sunlight for photosynthesis.
  • To prevent the plant from growing too tall too quickly.
  • To anchor the plant and absorb water and minerals from the soil. (correct)

Where are auxins produced within a plant?

<p>In the tips of both the shoots and the roots. (C)</p> Signup and view all the answers

What would be the consequence of a plant hormone imbalance that resulted in excessive auxin production in the shoot tips?

<p>Rapid and excessive shoot elongation, potentially weakening the stem. (D)</p> Signup and view all the answers

How does gravity influence auxin distribution when a plant shoot is placed horizontally?

<p>Gravity causes auxin to accumulate on the lower side of the shoot. (C)</p> Signup and view all the answers

In plant shoots, what effect does a higher concentration of auxin have on cell elongation?

<p>Higher auxin concentration stimulates faster cell elongation. (A)</p> Signup and view all the answers

Why does a plant shoot bend towards the light when light is only coming from one direction?

<p>Auxin concentrates on the shaded side, promoting faster growth. (B)</p> Signup and view all the answers

In plant roots, how does auxin affect cell elongation and growth?

<p>Auxin inhibits cell elongation, leading to decreased growth. (B)</p> Signup and view all the answers

If a root is placed horizontally, how does the unequal distribution of auxin affect its growth?

<p>The lower side grows slower, causing the root to grow downwards. (D)</p> Signup and view all the answers

What is the primary role of gibberellins in plants?

<p>Initiating seed germination and inducing flowering. (C)</p> Signup and view all the answers

Where is auxin primarily produced in plant shoots, and how does it reach other parts of the shoot?

<p>Auxin is produced in the shoot tips and diffuses to the region behind the tip. (B)</p> Signup and view all the answers

Which plant hormone is responsible for the ripening of fruits?

<p>Ethene (B)</p> Signup and view all the answers

In the investigation of gravity's effect on seedlings, what is the purpose of using a clinostat?

<p>To ensure uniform exposure to gravity. (B)</p> Signup and view all the answers

What happens to auxin distribution in a shoot when light shines evenly around the tip of the shoot?

<p>Auxin is distributed evenly, causing the cells in the shoot to grow at the same rate. (C)</p> Signup and view all the answers

Which region of the shoot is most responsive to auxin in terms of growth by cell division and elongation?

<p>The region behind the tip of the shoot. (A)</p> Signup and view all the answers

What is the expected outcome regarding shoot growth in a petri dish placed on its side to investigate the effect of gravity?

<p>Shoots will grow upwards, against gravity. (D)</p> Signup and view all the answers

If a plant is showing signs of stunted growth and poor seed germination, which hormone might be deficient?

<p>Gibberellin (B)</p> Signup and view all the answers

A farmer wants to accelerate the ripening of harvested fruits to get them to market faster. Which plant hormone would be most effective for this purpose?

<p>Ethene (D)</p> Signup and view all the answers

What is the role of auxin in positive phototropism?

<p>Accumulating on the shaded side, causing cell elongation. (B)</p> Signup and view all the answers

In an experiment studying the effects of light on seedlings, if seedlings are placed in a dark environment with light coming from one direction, what growth pattern is expected?

<p>Seedlings will grow towards the light source. (D)</p> Signup and view all the answers

Why is it advantageous to transport unripe fruit over ripe fruit?

<p>Unripe fruit is less susceptible to damage and spoilage during transportation. (A)</p> Signup and view all the answers

What is the primary mechanism by which ethene controls fruit ripening?

<p>It stimulates the production of enzymes that soften fruit texture and alter its flavor profile. (D)</p> Signup and view all the answers

A fruit distributor wants to delay the ripening of a shipment of bananas. Which of the following strategies would be most effective?

<p>Lowering the temperature and adding chemicals that inhibit ethene production. (C)</p> Signup and view all the answers

How do gibberellins promote seed germination?

<p>By promoting the synthesis of proteins required for embryo growth. (C)</p> Signup and view all the answers

A plant breeder aims to produce larger grapes. How can gibberellins be utilized to achieve this goal?

<p>By applying gibberellins to stimulate fruit development and increase fruit size. (C)</p> Signup and view all the answers

Why is it important to conduct the gravitropism experiment within a light-proof box?

<p>To prevent phototropism from influencing the direction of growth. (A)</p> Signup and view all the answers

In a gravitropism experiment, what is the purpose of using a clinostat?

<p>To negate the effect of gravity on the seedlings. (A)</p> Signup and view all the answers

If a petri dish with bean seedlings is placed on its side for two days, what observation would support the principle of positive gravitropism?

<p>The plumules grow upwards, and radicles grow downwards. (B)</p> Signup and view all the answers

What would be expected if the clinostat was not functioning correctly and rotated very slowly?

<p>The radicles would grow downwards, but with a slight curve. (C)</p> Signup and view all the answers

In a well-designed experiment investigating gravitropism, what is the role of damp cotton wool?

<p>To maintain moisture levels essential for germination and growth. (A)</p> Signup and view all the answers

If, after two days in the lightproof box on a clinostat, some seedlings show a slight curve in their radicles, what is the most likely reason?

<p>Inconsistent seed viability across the sample. (C)</p> Signup and view all the answers

A student observes that the plumules in the petri dish oriented horizontally on its side still manage to grow upwards, even though gravity is acting perpendicular to their initial growth direction. What mechanism primarily facilitates this response?

<p>Differential cell elongation due to redistribution of auxin. (B)</p> Signup and view all the answers

During the gravitropism experiment, a student hypothesizes that increased humidity might enhance the gravitropic response. How can this hypothesis be tested using the described setup?

<p>By adding a humidifier to one light-proof box and comparing results with a control. (D)</p> Signup and view all the answers

How does the commercial application of plant hormones potentially impact biodiversity?

<p>By selectively inhibiting the growth of certain plant species considered weeds, which may serve as food sources or habitats for other organisms. (C)</p> Signup and view all the answers

What is the primary mechanism by which auxins act as selective weed killers?

<p>They disrupt the growth patterns of broad-leaved plants, causing them to die while sparing narrow-leaved plants such as grasses and cereals. (A)</p> Signup and view all the answers

A farmer applies an auxin-based weed killer to a field of wheat. What is a potential unintended consequence of this action?

<p>The weed killer spreads beyond the field, negatively affecting non-target plant species in nearby areas. (C)</p> Signup and view all the answers

A horticulturalist is propagating new plants from stem cuttings. How do auxins contribute to this process?

<p>By stimulating rapid root development at the cut ends of the stems. (C)</p> Signup and view all the answers

What is a key advantage of using tissue culture with auxins for plant propagation, compared to traditional cuttings?

<p>Tissue culture allows for the rapid production of a large number of clones from a small amount of starting material. (C)</p> Signup and view all the answers

A farmer observes that a certain weed is significantly reducing crop yield due to competition for resources. Which plant hormone-based strategy would be most effective for addressing this issue?

<p>Applying auxins as a selective weed killer to target the broad-leaved weeds without harming the narrow-leaved crops. (A)</p> Signup and view all the answers

A researcher is investigating the effects of different concentrations of auxins on root development in plant cuttings. Which experimental control is most important to include?

<p>A group of cuttings that do not receive any auxin treatment to establish a baseline for normal root development. (B)</p> Signup and view all the answers

Despite the benefits of using auxin-based rooting powders, what consideration is most important for a gardener to keep in mind?

<p>The potential for causing unintended harm to nearby plants if not applied carefully. (B)</p> Signup and view all the answers

Flashcards

Plant Hormones

Chemicals produced by plants that control growth and development.

Tropism

Growth response of a plant towards or away from a stimulus.

Phototropism

Growth response to light; shoots grow towards it (positive), roots away (negative).

Gravitropism

Growth response to gravity; roots grow down (positive), shoots up (negative).

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Auxin

A plant hormone that promotes cell elongation in shoots and influences tropisms.

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Auxin's Role in Phototropism

Plant hormone that promotes cell elongation, causing shoots to bend towards light.

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Gibberellins

A class of plant hormones involved in seed germination, flowering, and fruit growth.

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Ethene

A gaseous plant hormone that regulates cell division and ripening of fruits.

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Seed Germination

Seed's initial growth stage.

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Clinostat

Device that rotates plants to negate gravity's effect, used in experiments.

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Aim of Plant Growth Practical

Investigating the effect of light or gravity on seedling growth direction.

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Why shoots bend towards light?

Auxin accumulates on the shaded side of the shoot, promoting cell elongation on that side.

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Commercial Uses of Plant Hormones

Gibberellins are used to start to encourage seed growth and fruit production. Ethene is used to ripen fruit.

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Auxin effect on root cell elongation

In roots, auxin inhibits cell elongation, leading to less growth with more auxin.

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Factors affecting auxin distribution

Auxin distribution is affected by light and gravity in shoots, but mainly by gravity in roots.

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Phototropism via auxin

Plant shoots bend towards light due to unequal auxin distribution. Higher auxin concentration on the shaded side promotes faster cell elongation.

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Auxin's effect on sideways shoots

In shoots placed sideways, auxin accumulates on the lower side which promotes faster growth, causing the shoot to grow upwards.

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Auxin's effect on sideways roots

In roots placed sideways, auxin accumulates on the lower side which inhibits growth, causing the root to grow downwards.

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Auxin origin and action in shoots

Auxin is produced in the tips of growing shoots and diffuses to the region behind the tip, stimulating cell elongation.

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Growth region in shoots

Only the region behind the tip of a shoot contributes to growth through cell division and elongation.

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Unequal light effect on auxin

When light shines on a shoot from one side, auxin concentrates on the shaded side, causing it to elongate faster and bend towards the light.

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Damp cotton wool

Damp material used to provide moisture for germinating seedlings in a petri dish.

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Bean seedlings

Seedlings placed in petri dishes to observe root and shoot growth.

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Radicle

The part of a seed that develops into the root.

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Plumule

The part of a seed that develops into the shoot.

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Positive gravitropism

The response of a plant where the root grows downwards.

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Negative gravitropism

The response of a plant where the shoots grow upwards.

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Control variable

A factor kept constant during an experiment.

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Ethene's agricultural use

A gaseous plant hormone used to control fruit ripening during storage and transport.

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Inhibiting Ethene Production

Adding chemicals or reacting the ethene in the air around the fruit.

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Ending Seed Dormancy

Exposure to cold and dry conditions followed by water intake and warmer weather.

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Uses of Gibberellins

To end seed dormancy, promote flowering, and increase fruit size.

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Uses of Plant Hormones

Extracted or artificial substances used in horticulture and agriculture to control plant growth for increased yields.

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Hormones and Biodiversity

Using hormones as weed killers can negatively affect biodiversity by inhibiting the growth of natural plants.

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Auxins as Weed Killers

Plant hormones effective at killing broad-leaved plants, commonly used to remove weeds from narrow-leaved crops.

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Weed Competition

Farmers control weed growth to reduce competition with crops for resources, ensuring better yields.

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Uncontrolled Spread

Applying weed killers to crops cannot be controlled and may negatively affect other plant species.

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Auxins for Rooting

A technique where cuttings from a plant are dipped in auxins to encourage rapid root development, creating clones.

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Auxins in Tissue Culture

Using auxins to stimulate growth of cells in a petri dish to create clones, which are then planted in soil

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Study Notes

Tropisms Controlled by Hormones

  • Auxins are plant hormones that coordinate and control growth.
  • Plants respond to light (phototropism) and gravity (gravitropism/geotropism).
  • Shoots demonstrate a positive phototropic response and negative gravitropic response by growing upwards, away from gravity, to absorb sunlight.
  • Roots show a negative phototropic response and a positive gravitropic response by growing downwards into the soil to anchor the plant and absorb water and minerals.

Gravitropism and Phototropism Table

Stimulus Name of response Definition Positive response Negative response
Gravity Gravitropism (sometimes called geotropism) Growth towards or away from gravitational pull Growth towards gravity (e.g., roots) Growth away from gravity (e.g., shoots)
Light Phototropism Growth towards or away from source of light Growth towards light (e.g., shoots) Growth away from light (e.g., roots)
  • Auxins are created in the tips of roots and shoots, diffusing into cells behind the tips.
  • In shoots, auxin encourages cell elongation; more auxin results in more cell elongation and growth.
  • In roots, auxin hinders cell elongation; more auxin results in less cell elongation and growth.
  • Light and gravity impact auxin distribution in shoots, while gravity primarily affects auxin distribution in roots.
  • If a root or shoot is placed on its side, gravity causes auxin to accumulate on the lower side, resulting in a lower auxin concentration on the uppermost side.
  • Shoots grow upwards because the lower side grows faster, while roots grow downwards because the lower side grows slower.
  • Unequal auxin distribution causes unequal growth rates in both roots and shoots.

Auxin's Control of Shoot Growth

  • Auxin is mainly produced in the tips of growing shoots and diffuses to the region behind the tip.
  • Auxin stimulates cells behind the tip to elongate, with more auxin leading to faster elongation and growth.
  • Only the region behind the tip contributes to growth through cell division and elongation.
  • With even light distribution, auxin distributes evenly, and the cells in the shoot grow at the same rate.
  • Predominant light from one side causes auxin to concentrate on the shaded side, making those cells elongate and grow faster.
  • Unequal growth causes the shoot to bend towards the light.
  • Positive phototropism in plant shoots results from auxin accumulating on the shaded side.

Hormones in Plants

  • Auxins are just one type of hormone found in plants.
  • Seed germination is initiated with the help of Gibberellins.
  • Gibberellins also play a role in the growth of fruit and inducing flowering.
  • Ethene, a gas released by plants, regulates cell division and fruit ripening.
  • Gibberellins and ethene are used commercially

Required Practical: Plant Growth

  • The aim is to investigate the effects of light or gravity on the growth of newly germinated seedlings.
  • To investigate the effect of gravity on germinated seedlings:
    • Set up two petri dishes with 3 mustard seeds each and allow them to germinate.
    • Place one dish on a clinostat and the other on its side with support.
    • Record direction of growth for both shoots and roots for each seed.
  • Investigating gravitropic response (set-up):
    • Damp cotton wool is added to two petri dishes.
    • Place 3 bean seedlings in the cotton wool in each petri dish;
    • 'A' has the radicle facing downwards, 'B' is laid horizontally, and 'C' has the radicle facing upwards.
    • Cover each dish with a lid.
    • One petri dish is attached to a support on its side, and the other is attached to a clinostat.
    • Place both dishes in a light-proof box for two days, then observe seedling growth.
  • Gravitropic response (results):
    • Radicles in the first petri grow downwards (positive gravitropic response), and plumules grow upwards (negative gravitropic response).
    • Radicles and plumules in the second petri dish grow outwards in the direction they were placed, as the effect of gravity is cancelled by the clinostat.
    • The experiment should be done in a lightproof box to control the effect of light and gravity.

Uses of Plant Hormones

  • Plant hormones can be extracted or artificially made and used to control plant growth, like obtaining larger yields.
  • Auxins, ethene, and gibberellins are commercially beneficial for producing food and plants.
  • Using hormones as weed killers can negatively affect the growth of natural plants and biodiversity.
  • Many species of plants are classed as weeds, which affects food source and potential habitat.
  • Auxins can be used as selective weed killers, which negatively affects the growth of broad-leaved plants that are weeds.
  • Weedkillers disrupt the growth of weeds, causing them to die.
  • Once applied to a crop, weedkiller spread cannot be controlled.
  • Auxins encourage rapid root development in cuttings of desirable plants when dipped in 'rooting powders.'
  • Auxins promote growth in tissue culture to clone plants in a petri dish, which are then planted in soil.
  • Ethene is used to control ripening of fruit during storage and transport.
  • It helps with the transportation of unripe fruit as they are less easily damaged.
  • Inhibiting ethene production or removing it from the air can delay ripening.
  • Artificially produced ethene gas can speed up ripening in supermarkets.
  • Gibberellins can be used to end seed dormancy, encourage flowering regardless of weather, and increase fruit size.

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Description

Explore auxins, gibberellins, and tropism in plants. Understand how plant shoots respond to light (positive phototropism) and roots respond to gravity (positive gravitropism). Learn how gravity influences auxin distribution and its effects on cell elongation in both shoots and roots.

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