Question

Give biological reason for the following:

Development of negative pressure in xylem sap.

Answer 1

Development of negative pressure

1. As the mesophyll cells of the leaves lose water through transpiration, the suction pressure increases in the xylem channels.
2. The water column in the xylem vessel comes under tension or negative pressure of around 10-20 atm.
3. The tension or negative pressure in the xylem vessel results in upward pull.
4. This negative pressure helps in the ascent of sap and is also the cause of transpiration pull.

Hence, loss of water through mesophyll cells due to transpiration creates the negative pressure for the ascent of sap in xylem channels.