⚡ Quick Revision: Transpiration Fundamentals
🔹 What is Transpiration?
- ✔ Definition: Loss of water in the form of water vapor from the aerial parts of the plant.
- ✔ Demonstration: Usually shown by covering a potted plant with a polythene bag (water droplets condense inside).
- ✔ Quantity: Plants transpire about 98-99% of the water they absorb; only 1-2% is used for photosynthesis.
🔹 The Three Types of Transpiration
| Type | Location | Contribution (%) |
|---|---|---|
| Stomatal | Through Stomata (Leaves) | 80 - 90% (Maximum) |
| Cuticular | Through Waxy Cuticle | 3 - 10% |
| Lenticular | Through Lenticels (Woody stem) | < 1% (Minimum) |
Potometer: A device (e.g., Ganong's Potometer) used to measure the rate of water intake by a plant, which is almost equal to the rate of transpiration.
Transpiration with Evaporation. Remember: Transpiration is a physiological/regulated process by living cells, while Evaporation is a simple physical process from any free surface.
⚡ Quick Revision: Stomatal Structure & Mechanism
🔹 Anatomy of a Stoma
- ✔ Guard Cells: Two bean-shaped (in dicots) or dumb-bell shaped (in monocots) cells containing chloroplasts.
- ✔ Differential Walls: Inner walls (facing the pore) are thick and inelastic; outer walls are thin and elastic.
- ✔ Stomatal Pore: The opening between the guard cells through which gas exchange and transpiration occur.
🔹 Mechanism: Potassium Ion Theory
- ✔ Opening (Day): $K^+$ ions enter guard cells → Water potential decreases → Endosmosis occurs → Cells become turgid → Pore opens.
- ✔ Closing (Night): $K^+$ ions leave guard cells → Water leaves the cells → Cells become flaccid → Pore closes.
Stomatal Frequency: The number of stomata per unit area of the leaf surface, usually higher on the lower surface of dicot leaves.
Guard Cells with Subsidiary Cells. Remember: Guard cells contain chloroplasts and regulate the pore; Subsidiary cells are surrounding epidermal cells that support the guard cells.
⚡ Quick Revision: Factors Affecting Transpiration
🔹 External (Environmental) Factors
- ✔ Intensity of Light: Increases transpiration as stomata open wide in bright light.
- ✔ Temperature: Higher temperature increases the rate of evaporation from leaf surfaces.
- ✔ Velocity of Wind: Blows away water vapor, maintaining a steep concentration gradient.
- ✖ Humidity: High humidity decreases transpiration as the air is already saturated.
🔹 Internal (Plant) Factors
- ✔ Sunken Stomata: Stomata hidden in pits (e.g., Nerium) to reduce water loss.
- ✔ Thick Cuticle: Reduces non-stomatal water loss in desert plants (Xerophytes).
- ✔ Modified Leaves: Leaves reduced to spines or scales (e.g., Opuntia).
Anti-transpirants: Chemicals (like Phenylmercuric acetate or Abscisic acid) that can be used to artificially reduce transpiration.
The effect of $CO_2$. Remember: If $CO_2$ concentration in the air increases beyond 0.03%, stomata tend to close, decreasing transpiration.
⚡ Quick Revision: Significance & Comparison
🔹 Importance of Transpiration
- ✔ Cooling Effect: Evaporation of water lowers the plant's temperature, protecting enzymes from heat.
- ✔ Suction Force: Creates the Transpiration Pull responsible for the ascent of sap in tall trees.
- ✔ Mineral Distribution: Helps in the upward translocation of minerals absorbed by roots.
🔹 Transpiration vs. Guttation (Exam Favorite)
| Feature | Transpiration | Guttation |
|---|---|---|
| Form | Water Vapor | Liquid Droplets |
| Purity | Pure water | Water with salts/sugars |
| Opening | Stomata/Cuticle/Lenticels | Hydathodes |
| Time | Mostly during the day | Night or early morning |
Necessary Evil: Curtis described transpiration this way because while it causes huge water loss (Evil), it is essential for cooling and nutrient transport (Necessary).
Do not say plants transpire to get rid of excess water. Transpiration is an inevitable consequence of stomata being open for $CO_2$ intake during photosynthesis.