In a dicot pot herb, vaseline/vegetable oil was applied on the upper surface of one leaf (Experimental leaf 1) and on the lower surface of another leaf (Experimental leaf 2). Vaseline/Vegetable oil was not applied to the control leaf. The plant was deliberately not watered for several days. Which leaf will dry up last?
Experimental leaf 2
Understanding Leaf Drying and Transpiration
This question explores how blocking different surfaces of a leaf affects water loss and drying, especially when the plant is not watered.
Plants lose water primarily through a process called transpiration. Transpiration is the evaporation of water from plant surfaces, mainly through small pores called stomata. These stomata are typically more numerous on the lower surface of dicot leaves compared to the upper surface.
In this experiment, vaseline or vegetable oil is applied to block these stomata on different leaves:
- Control leaf: No treatment. Transpiration occurs normally from both upper and lower surfaces.
- Experimental leaf 1: Vaseline/vegetable oil applied to the upper surface. This blocks the stomata present on the upper surface. Since dicot leaves have fewer stomata on the upper side, the reduction in transpiration will be less significant compared to blocking the lower surface.
- Experimental leaf 2: Vaseline/vegetable oil applied to the lower surface. This blocks the stomata present on the lower surface. Since dicot leaves have many more stomata on the lower side, the reduction in transpiration will be much more significant.
The plant is not watered for several days, meaning the water supply from the roots is limited. The rate at which a leaf dries up depends on how quickly it loses water through transpiration. The less water a leaf loses, the longer it will stay hydrated and the last it will be to dry up.
Analyzing Water Loss Rates
Let's compare the rate of water loss from each leaf:
- Control leaf: Highest rate of transpiration because all stomata on both surfaces are open.
- Experimental leaf 1: Lower rate of transpiration than the control leaf because upper stomata are blocked, but significant water loss still occurs through the unblocked stomata on the lower surface.
- Experimental leaf 2: Lowest rate of transpiration because the majority of stomata (on the lower surface) are blocked. Some minimal transpiration might occur through the upper surface stomata (which are few) or through the cuticle, but this rate is much lower than the other two leaves.
Since Experimental leaf 2 loses water at the slowest rate due to the blocked stomata on its lower surface, it will retain water for the longest time and thus will be the last to dry up under conditions of water scarcity.
Summary of Leaf Drying Order
Based on the rate of water loss (transpiration):
| Leaf Type | Surface Coated | Stomata Blocked | Transpiration Rate | Drying Time (Relative) |
|---|---|---|---|---|
| Control leaf | None | None | Highest | Dries up first |
| Experimental leaf 1 | Upper | Few (Upper surface) | Moderate | Dries up after control leaf |
| Experimental leaf 2 | Lower | Many (Lower surface) | Lowest | Dries up last |
Therefore, the leaf that will dry up last is Experimental leaf 2 because coating the lower surface effectively minimizes water loss through transpiration.
Revision Table: Transpiration and Stomata
| Term | Description | Relevance to Experiment |
|---|---|---|
| Transpiration | Process of water loss from plants, mainly as vapor. | Directly affects how quickly leaves dry up. |
| Stomata | Small pores on leaf surface surrounded by guard cells, regulating gas exchange and transpiration. | Primary pathway for water loss; blocked by vaseline/oil. |
| Dicot Leaves | Leaves of dicotyledonous plants; typically have more stomata on the lower epidermis. | Explains why blocking the lower surface has a greater effect. |
| Cuticle | Waxy layer on leaf surface; provides some barrier to water loss (cuticular transpiration). | Minor pathway for water loss compared to stomata, less affected by coating. |
Additional Information: Factors Affecting Transpiration
Beyond stomatal control, several environmental factors influence the rate of transpiration:
- Light: Stomata usually open in light (for photosynthesis), increasing transpiration.
- Temperature: Higher temperatures increase evaporation rate, increasing transpiration.
- Humidity: High humidity in the air reduces the water potential gradient between the leaf and the air, decreasing transpiration.
- Wind: Wind removes water vapor from the leaf surface, maintaining a steep water potential gradient and increasing transpiration.
- Soil Water Availability: When soil water is scarce, the plant may close its stomata to conserve water, reducing transpiration.
In the described experiment, not watering the plant creates a condition of low soil water availability, which would naturally cause stomata to close. However, the physical blocking with vaseline/oil on specific surfaces is a more direct way to prevent water loss regardless of the plant's physiological response to drought.
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