Welcome to Control Systems in Plants!

Hello future biologists! This chapter explores how plants, despite being rooted in place, are masters of adaptation and control. They don't have nerves or muscles like us, but they use tiny amounts of powerful chemical messengers to coordinate their growth, respond to light, and even survive a drought. Understanding this chemical communication system is crucial for appreciating plant biology and answering those high-level exam questions!

Let’s dive into the fascinating world of plant hormones, officially known as plant growth substances.

3.4.5.1 Principles of Plant Control

Plants need control systems just like animals do, mainly to manage growth and respond to their environment (like where the sunlight is, or when water is scarce).

What are Plant Growth Substances?

These are the plant equivalent of hormones—chemical signal molecules that regulate biological processes.

  • Low Concentration: They are effective even when present in incredibly small amounts (very low concentrations).
  • Site of Action: They are produced in one part of the plant (often growing tips or roots) and either regulate target cells nearby, or are transported to other tissues to exert their effect.
  • Broad Functions: They control a vast range of activities, including:
    • Growth and differentiation (directional stimuli like tropisms).
    • Fruit development and ripening.
    • Leaf fall and dormancy.
    • Responding to stress (e.g., closing stomata during drought).

Analogy: Imagine a plant growth substance is a text message. It's short, delivered to specific recipients (target cells), and carries a vital instruction, like "Grow taller now!" or "Close the water gates!"

Quick Review: Key Characteristics

  • Signal molecules.
  • Active at very low concentrations.
  • Regulate processes like growth, stress response, and ripening.

3.4.5.2 Auxins and Tropisms: Directional Growth

One of the most important groups of plant growth substances is the auxins. The key example you must know is Indoleacetic acid (IAA).

What is a Tropism?

A tropism is a growth response to a directional stimulus. These responses ensure that the shoot and root maintain a favourable position for survival (shoots reach light, roots find water and anchorage).

  • Phototropism: Response to light (e.g., shoots growing toward light).
  • Gravitropism (or Geotropism): Response to gravity (e.g., roots growing downwards).

The Role of IAA in Cell Elongation

IAA influences growth by affecting cell elongation. However, shoots and roots have dramatically different sensitivities to IAA concentration. This is the crucial point!

IAA Concentration Effects: The Goldilocks Principle

Think of it this way: IAA is like a spicy seasoning. Shoots like a lot, roots can only handle a tiny sprinkle.

  • Shoots: Higher concentrations of IAA promote elongation.
  • Roots: Higher concentrations of IAA inhibit elongation. Roots require a much lower, optimal concentration for growth.
1. Phototropism (Response to Light)

When a shoot is illuminated from one side (unilaterally):

  1. The light stimulus causes IAA to move laterally (sideways) to the shaded side of the shoot.
  2. The shaded side now has a higher concentration of IAA.
  3. Since high IAA promotes cell elongation in the shoot, the cells on the shaded side elongate faster than those on the lit side.
  4. This differential elongation causes the shoot to bend towards the light (positive phototropism).
2. Gravitropism (Response to Gravity)

When a plant is placed horizontally, gravity affects IAA distribution, causing it to pool on the lower side.

In the Shoot (Negative Gravitropism):

  1. Gravity causes IAA to move to the lower side of the horizontal shoot.
  2. The lower side has a high IAA concentration, which promotes elongation.
  3. The lower cells elongate rapidly, causing the shoot to curve upwards (away from gravity).

In the Root (Positive Gravitropism):

  1. Gravity causes IAA to move to the lower side of the horizontal root.
  2. However, roots are highly sensitive. This high concentration inhibits cell elongation on the lower side.
  3. The cells on the upper side, having a lower concentration of IAA, elongate faster.
  4. This differential elongation causes the root to curve downwards (towards gravity).
Common Mistake Alert: Students often forget that high IAA inhibits root growth. Remember the key difference: shoots are stimulated by high IAA, roots are inhibited by high IAA.

3.4.5.3 Ethene and Abscisic Acid (ABA)

While auxins manage directional growth, two other key substances manage developmental timing and stress response.

1. Ethene: The Ripening Gas

Ethene (sometimes spelled ethylene) is unique because it is a gaseous plant growth substance.

Role: Ethene plays a crucial role in fruit ripening.

  • Ripening involves changes in texture (softening), colour, and sweetness.
  • Fruits that show a dramatic increase in respiration during ripening, like bananas and avocados, are called climacteric fruits, and they are highly responsive to ethene.
Real-World Application: Controlled Ripening

To ensure fruits like bananas arrive unspoiled at the supermarket, they are often picked green (unripe).

During shipping, ethene production is suppressed (often by refrigeration). Once they reach their destination, they are stored in rooms and artificially exposed to ethene gas. This controlled environment allows them to ripen quickly and uniformly, ready for sale.

Did you know? If you put an apple next to a bunch of bananas, the bananas will ripen faster! This is because apples naturally produce high amounts of ethene, which acts as a signal to speed up the ripening process in nearby climacteric fruits.

2. Abscisic Acid (ABA): The Stress Signal

Abscisic acid (ABA) is often called the stress hormone because it helps plants cope with challenging environmental conditions, particularly water stress or drought.

Primary Role: ABA controls the closure of stomata to reduce water loss via transpiration.

Step-by-Step Mechanism of ABA-Induced Stomatal Closure

When water availability is low, the roots detect the water deficiency and release ABA, which travels to the leaves and targets the guard cells surrounding the stomata.

  1. ABA Signal: Abscisic acid binds to receptors on the guard cell membrane.
  2. Ion Transport Out: ABA stimulates the active transport (or diffusion) of potassium ions (\(K^+\)) and chloride ions (\(Cl^-\)) out of the guard cells, into the surrounding epidermal cells.
  3. Water Potential Changes: The loss of solutes (ions) from the guard cells causes the water potential (\(\Psi\)) within the guard cells to increase (become less negative).
  4. Osmosis: Water moves out of the guard cells and into the adjacent epidermal cells (down the water potential gradient).
  5. Closure: The guard cells lose turgor pressure (become flaccid) and shrink, causing the stomatal pore to close.

This immediate closure prevents massive water loss, improving the plant's chances of survival during drought.

Key Takeaways: Plant Control Systems

  • Principles: Plants use low-concentration chemical signals (growth substances) for control.
  • IAA (Auxin): Governs tropisms (directional growth).
  • IAA Concentration: High IAA promotes shoot elongation (positive phototropism); High IAA inhibits root elongation (positive gravitropism).
  • Ethene: Gaseous hormone essential for fruit ripening (used commercially for climacteric fruits).
  • ABA: Stress hormone that causes stomatal closure in drought by stimulating the transport of \(\mathbf{K^+}\) and \(\mathbf{Cl^-}\) out of guard cells, leading to water loss by osmosis.