Root pressure has an effect on water movement in plants, particularly when transpiration is low, but it does not significantly drive or control the high rates of transpiration that occur during the day.
Understanding Root Pressure and Transpiration
Root pressure is a positive pressure that develops in the xylem of some plants roots. This pressure is primarily a result of the active absorption of mineral ions from the soil, which lowers the water potential in the root cells, causing water to move into the root xylem by osmosis.
Based on the provided information, we understand the timing of these two phenomena:
- Typically, root pressure builds throughout the night when absorption is highest and transpiration is lowest.
- During the day, transpiration is at its highest.
The Dominance of Transpiration Pull
Transpiration, the loss of water vapor from plants, mainly through stomata in the leaves, creates a powerful pulling force (tension) that extends down the xylem from the leaves to the roots. This transpiration pull is the primary mechanism responsible for moving water and dissolved minerals from the roots to the rest of the plant, especially in tall plants and during the day when stomata are open and water loss is high.
- High Transpiration: When transpiration rates are high during the day, the tension created by water evaporation from the leaves is much stronger than root pressure.
- Water Movement: This pull effectively overcomes any positive pressure from the roots and is the main force driving water upwards.
- Cell Response: The water lost via transpiration leads the guard cells and other epidermal cells to become flaccid. They, in turn, absorb water from the xylem. This process highlights how transpiration causes water movement into these cells from the xylem, further illustrating the pull effect originating from the leaves.
The Role of Root Pressure
While root pressure can push water upwards in the xylem over short distances, especially when transpiration pull is minimal (like at night), its contribution to overall water transport during active transpiration is considered minor compared to the transpiration pull.
Here's a simple comparison:
- Night (Low Transpiration): Root pressure builds, potentially causing phenomena like guttation (exudation of water droplets from leaf margins).
- Day (High Transpiration): Transpiration pull is the dominant force for water transport. Root pressure is often overcome by the negative tension in the xylem created by transpiration.
In essence, root pressure is most noticeable when transpiration is low, facilitating some water movement and potentially refilling collapsed xylem vessels, but it is not the force that significantly influences or enhances the high rates of water flow driven by transpiration during the day.
