Coordination in Plants: Tropisms and Phytohormones for CBSE Class 10

Unlike animals, plants have neither a nervous system nor muscle tissue. A sunflower cannot run, a tendril cannot leap, and a seedling has no brain. Yet, plants exhibit remarkable sensitivity to their surrounding environment: a potted plant on a windowsill bends gracefully toward sunlight, roots burrow unerringly downwards toward underground water, tendrils coil tightly around fences, and the leaflets of a Touch-me-not plant collapse within seconds when brushed by a fingertip.

In CBSE Class 10 Science, Chapter 6 (Control and Coordination) explores how plants accomplish these coordinated feats using electrical-chemical signals, turgor pressure adjustments, and specialized chemical growth regulators called phytohormones.

What You Will Learn

• Two primary categories of plant movements: Nastic movements vs. Tropic movements
• Mechanism of immediate response to touch in Mimosa pudica (turgor changes in pulvini)
• Directional growth movements (Tropisms): Phototropism, Geotropism, Hydrotropism, Chemotropism, and Thigmotropism
• How the plant hormone auxin drives phototropic curvature
• The five major plant hormones (Phytohormones): Auxins, Gibberellins, Cytokinins, Abscisic Acid, and Ethylene
• High-yield board exam experiments, diagrams, and common errors

1. Two Categories of Plant Movements

Plant movements are fundamentally classified based on whether they depend on directional growth:

                            Plant Movements
                                   |
       +---------------------------+---------------------------+
       |                                                       |
Nastic Movements (Non-Directional)                      Tropic Movements (Directional)
- Independent of growth                                 - Dependent on growth
- Very rapid (seconds/minutes)                          - Slow, cumulative (days/weeks)
- Response does not depend on direction of stimulus     - Direction of movement depends on direction of stimulus
- e.g., Drooping of Mimosa pudica leaves                - e.g., Shoots bending toward light

2. Nastic Movements: Immediate Response to Stimulus

The classic example of a growth-independent movement is observed in the sensitive plant Mimosa pudica (Chhui-Mui or Touch-me-not):

• When the tip of a leaf is touched, an electrical-chemical signal travels down the leaf.
• Within 1 to 2 seconds, all leaflets fold inward and the whole leaf droops downwards.

How Does Mimosa pudica Move Without Muscles?

Plants do not have specialized muscle proteins to create movement. Instead:

1. Leaves of Mimosa pudica have swollen leaf cushions at the base of petiole and leaflets called pulvini.
2. Cells inside the pulvinus maintain high internal water pressure (turgor pressure), keeping the leaf upright and erect.
3. When stimulated by touch, an electrical signal causes cells in the lower half of the pulvinus to rapidly lose potassium ions and water to the intercellular spaces.
4. With the sudden loss of water, the lower pulvinus cells become flaccid (limp). The sudden loss of turgor pressure causes the leaf to collapse and droop.
5. After 15–30 minutes, water is pumped back into the cells, restoring turgor and reopening the leaves.

3. Directional Growth Movements (Tropisms)

A tropic movement (or tropism) is a directional growth movement of a plant organ in response to an external environmental stimulus.

• Positive Tropism: Growth towards the stimulus.
• Negative Tropism: Growth away from the stimulus.

                             Types of Tropisms
                                     |
       +-----------------+-----------+-----------+-----------------+
       |                 |                       |                 |
 Phototropism       Geotropism              Hydrotropism      Chemotropism & Thigmotropism
 (Stimulus: Light)  (Stimulus: Gravity)     (Stimulus: Water) (Chemicals & Touch)

1. Phototropism (Response to Light)

• Shoots grow towards light $\implies$ Positively phototropic.
• Roots grow away from light $\implies$ Negatively phototropic.

How Does Auxin Cause Phototropic Bending? (CBSE High-Frequency Question)

1. The plant growth hormone auxin is synthesized at the growing tip of the shoot.
2. When light shines uniformly from above, auxin diffuses evenly down the stem, resulting in uniform upward growth.
3. When light comes from one side (directional light), auxin diffuses away from the light toward the shaded side of the shoot.
4. The higher concentration of auxin on the shaded side stimulates the cells on that side to grow and elongate longer than the cells on the illuminated side.
5. As a result, the shaded side grows faster than the bright side, forcing the stem to bend smoothly toward the light!

2. Geotropism (Response to Gravity)

• Roots always grow downward into the soil with gravity $\implies$ Positively geotropic.
• Shoots always grow upward into the air against gravity $\implies$ Negatively geotropic.

3. Hydrotropism (Response to Moisture)

• Plant roots grow towards regions of higher moisture content in soil, even if it requires bending away from gravity $\implies$ Roots are positively hydrotropic.

4. Chemotropism (Response to Chemical Stimuli)

• A classic example in plant reproduction: During pollination, the pollen tube grows down through the style towards the female ovule in the ovary in response to chemical attractants secreted by the ovule!

5. Thigmotropism (Response to Physical Contact/Touch)

• Climbing plants like peas, bitter gourd, and cucumbers use thread-like structures called tendrils.
• When a tendril touches a physical support (like a bamboo stick), the side of the tendril touching the support grows slowly, while the outer side away from the support grows rapidly. This uneven growth causes the tendril to circle and coil tightly around the support.

4. Plant Hormones (Phytohormones)

Phytohormones are chemical compounds naturally synthesized in minute quantities in one part of a plant and transported to other parts to regulate physiological growth and development.

Important: <u>While Auxins, Gibberellins, and Cytokinins are plant GROWTH PROMOTERS, Abscisic Acid (ABA) is a plant GROWTH INHIBITOR. ABA is often called the "stress hormone" because it signals stomata to close during water shortages.</u>

5. Summary and Examination Tips

Exam Tip: When explaining phototropism in board exams, clearly state that auxin accumulates on the shaded side and that auxin stimulates elongation of shoot cells.

Common Mistake: Confusing nastic movements with tropic movements. The drooping of Mimosa pudica leaves is NOT a tropism because it does not involve directional growth; it is an immediate nastic turgor response!