What is the primary difference in auxin's effect on cell elongation in shoots versus roots?

In shoots, auxin promotes cell elongation, leading to increased growth. Conversely, in roots, auxin inhibits cell elongation, meaning higher concentrations can slow down or reduce root growth. This differential response is crucial for directional growth in response to stimuli.

How does phototropism differ from gravitropism in terms of the stimulus and plant response?

Phototropism is a plant's growth response to light, typically causing shoots to grow towards light (positive phototropism) and roots away from it (negative phototropism). Gravitropism, or geotropism, is the growth response to gravity, causing shoots to grow upwards (negative gravitropism) and roots downwards (positive gravitropism). Both involve hormone redistribution but are triggered by different environmental cues.

Compare the main roles of ethene and gibberellins in plant development.

Ethene is primarily known for its role as a gaseous hormone that controls fruit ripening and senescence, as well as cell division. Gibberellins, on the other hand, are crucial for initiating seed germination, breaking dormancy, and also play roles in inducing flowering and increasing fruit size. They regulate distinct phases of plant life.

What is a common misconception about auxin's role in plant growth, especially regarding roots?

A common misconception is that auxin always promotes growth in all plant parts. While it promotes cell elongation in shoots, it actually inhibits cell elongation in roots at higher concentrations. This differential effect is essential for roots to grow downwards in response to gravity.

What error might occur if an experiment investigating gravitropism does not control for light?

If light is not controlled, the plant's phototropic response (growth towards or away from light) could interfere with or mask its gravitropic response (growth towards or away from gravity). This would make it difficult to isolate and accurately measure the effect of gravity alone on plant growth, leading to inconclusive or incorrect results.

What is a potential negative consequence of using selective auxin-based weed killers?

A potential negative consequence is the reduction of biodiversity. While these weed killers target specific broad-leaved weeds, these 'weeds' often serve as food sources or habitats for other organisms in the ecosystem. Their destruction can disrupt local food webs and reduce the overall variety of life in an area.

Define 'auxin' and state its primary site of production.

Auxin is a class of plant hormones that primarily regulates cell elongation and plays a key role in tropisms. It is predominantly produced in the apical meristems, specifically the tips of growing shoots and roots, before diffusing to other parts of the plant.

What is phototropism?

Phototropism is the directional growth response of a plant or plant organ towards or away from a light source. Shoots typically exhibit positive phototropism (growing towards light), while roots often show negative phototropism (growing away from light).

What is the function of a clinostat in plant growth experiments?

A clinostat is a device used to negate the directional effect of gravity on plant growth. By slowly rotating a plant, it ensures that gravity is applied evenly from all sides over time, effectively canceling out any specific gravitropic response and allowing other stimuli to be studied in isolation.

How does auxin cause a plant shoot to bend towards light?

When light shines on a shoot from one side, auxin produced in the tip migrates to the shaded side. The higher concentration of auxin on the shaded side promotes faster cell elongation there compared to the illuminated side. This differential growth causes the shoot to bend towards the light source.

Plant Hormones | AQA GCSE Biology

1. Definition & Core Concepts

Plant hormones, also known as phytohormones, are chemical substances produced by plants that regulate growth and development. These hormones act as internal messengers, coordinating cellular activities across different parts of the plant in response to both internal cues and external environmental stimuli.
Auxins are a primary class of plant hormones, predominantly produced in the tips of growing shoots and roots. They play a crucial role in controlling cell elongation and are central to a plant's ability to respond to light and gravity, processes known as tropisms.
Tropisms are directional growth responses of plants to external stimuli. These responses ensure that plants grow in ways that maximize their chances of survival and reproduction, such as growing towards light for photosynthesis or roots growing into the soil for anchorage and water absorption.

2. Auxins: Regulation of Growth and Tropisms

Auxin production and diffusion: Auxins are synthesized in the apical meristems, specifically the tips of shoots and roots, and then diffuse to the cells located behind these tips. This diffusion gradient is critical for their regulatory effects on growth.
Differential effects of auxin: In shoots, auxins promote cell elongation, meaning a higher concentration of auxin leads to faster cell growth and elongation. Conversely, in roots, auxins inhibit cell elongation, so a higher concentration results in slower or reduced cell growth.
Phototropism: This is the growth response to light. In shoots, when light comes from one direction, auxin migrates to the shaded side of the shoot. The higher auxin concentration on the shaded side causes cells there to elongate faster than those on the illuminated side, leading the shoot to bend towards the light source.
Gravitropism (Geotropism): This is the growth response to gravity. In both shoots and roots placed horizontally, gravity causes auxins to accumulate on the lower side. In shoots, the higher auxin concentration on the lower side promotes faster cell elongation, causing the shoot to grow upwards (negative gravitropism). In roots, the higher auxin concentration on the lower side inhibits cell elongation, causing the root to grow downwards (positive gravitropism).

Diagram illustrating phototropism in a plant shoot. A shoot is shown bending towards a light source. The shaded side of the shoot has a higher concentration of auxin, leading to increased cell elongation and faster growth, which causes the shoot to bend towards the light. The sunny side has a lower auxin concentration and slower growth.

3. Other Key Plant Hormones

Gibberellins are another important class of plant hormones primarily known for their role in initiating seed germination. They break seed dormancy, allowing the embryo to grow when conditions are favorable, often after a period of cold or dry conditions.
Beyond germination, gibberellins also influence other developmental processes, including the induction of flowering and the growth and development of fruits. Higher levels of gibberellins can lead to larger fruit sizes.
Ethene (Ethylene) is unique among plant hormones as it is a gaseous hormone. It plays a significant role in controlling cell division and is most famously associated with the ripening of fruits. Ethene production often signals the end of a fruit's development and the beginning of its senescence.

4. Mechanisms of Tropism

The fundamental principle behind tropisms is differential growth, where one side of a plant organ grows faster or slower than the other. This unequal growth rate causes the organ to bend in a specific direction, either towards or away from the stimulus.
In phototropism, the redistribution of auxin to the shaded side of a shoot leads to enhanced cell elongation on that side. This differential growth pushes the shoot to curve towards the light, optimizing light capture for photosynthesis.
For gravitropism, gravity causes auxin to accumulate on the lower side of both shoots and roots. The contrasting effects of auxin (promotion in shoots, inhibition in roots) on cell elongation then dictate the direction of growth relative to gravity, ensuring shoots grow upwards and roots grow downwards.

5. Commercial Applications of Plant Hormones

Plant hormones are widely utilized in agriculture and horticulture to manipulate plant growth and development for human benefit, often leading to increased yields or improved plant propagation. These applications leverage the specific effects of each hormone.
Auxins are used as selective weed killers, targeting broad-leaved plants (weeds) while leaving narrow-leaved crops unharmed. They disrupt the normal growth patterns of weeds, causing them to die. Auxins are also the active ingredient in rooting powders, which stimulate rapid root development in plant cuttings, facilitating vegetative propagation.
Ethene is crucial in the food industry for controlling fruit ripening. Unripe fruits can be transported more easily without damage, and their ripening can be delayed by inhibiting ethene production or removing it from the storage environment. Conversely, ethene gas can be applied to ripen fruits quickly when they reach their destination, such as in supermarkets.
Gibberellins have several commercial uses, including ending seed dormancy to ensure uniform germination, promoting flowering in plants regardless of environmental conditions, and increasing fruit size in certain crops. These applications help farmers control crop cycles and enhance productivity.

6. Investigating Plant Responses

Studying plant tropisms often involves controlled experiments to isolate the effect of a single stimulus. For instance, to investigate gravitropism, light must be excluded as a variable to prevent phototropic responses from interfering.
A clinostat is a specialized device used in experiments to negate the effect of gravity on plant growth. By slowly rotating a plant, it ensures that gravity is applied evenly from all directions over time, effectively canceling out any directional gravitropic response.
Experimental setups typically involve comparing plant growth under normal conditions with growth under altered conditions (e.g., unilateral light, horizontal orientation) while controlling other environmental factors like temperature, water, and light (when studying gravity).

7. Environmental and Ethical Considerations

While commercial applications of plant hormones offer significant benefits, their widespread use can have environmental implications. For example, selective weed killers containing auxins, though designed to target specific weeds, can inadvertently affect non-target plant species in the ecosystem.
The destruction of 'weed' species, which may serve as food sources or habitats for other organisms, can lead to a reduction in biodiversity. This highlights the need for careful consideration and responsible application of these chemicals to minimize ecological impact.
The use of hormones to control ripening or enhance growth raises questions about natural processes and consumer perception. Balancing agricultural efficiency with ecological health and consumer preferences is an ongoing challenge in the application of plant hormones.

Plant Hormones

Summary

1. Definition & Core Concepts

2. Auxins: Regulation of Growth and Tropisms

3. Other Key Plant Hormones

4. Mechanisms of Tropism

5. Commercial Applications of Plant Hormones

6. Investigating Plant Responses

7. Environmental and Ethical Considerations

Plant Hormones

Summary

1. Definition & Core Concepts

2. Auxins: Regulation of Growth and Tropisms

3. Other Key Plant Hormones

4. Mechanisms of Tropism

5. Commercial Applications of Plant Hormones

6. Investigating Plant Responses

7. Environmental and Ethical Considerations