Root Pressure Basics

• Sap contains semi-impermeable minerals and sugar, plus water. If you cut a tomato plant at the base, root pressure or osmotic pressure, also called osmosis, will push the sap out, causing the cut stump to ooze. To understand how root pressure works, imagine sap in a thin tube that is the plant xylem. The water on the sides of the tube will be higher than the minerals and sugar in the middle, causing the surface to form a shallow, concave U-shape. As more water is added at the bottom of the root, the U-shape will flatten out. The water on the sides will then push higher, forming another U-shape. In this manner, the sap inches slowly higher.
  
  

Root Pressure Contribution

• Root pressure can only push water up a few yards at most. There is more root pressure in the spring when there is more water in plant sap and the rate of transpiration is low because leaves are still growing. In the summer, when leaves are mature and transpiration is high, the contribution of root pressure ceases. Root pressure forces out droplets of water to the tips of grass leaves. It is sufficient to force water to the tops of many herbaceous plants, those that do not have woody stems, but it moves water far too slowly to account for the total movement of water in most plants.
  
  

Root Pressure Absence

• Root pressure moves water only at night when transpiration through leaves is low. It does not occur when the air or soil is dry. Root pressure is absent from firs, pines, sequoias and other tall trees or long vines. To understand the limited contribution of root pressure to water movement, you need to understand how leaf transpiration more effectively pulls water upward.
  
  

Transpiration Pull

• Leaves "breathe" through pores called stoma. In the daytime, the leaves take in carbon dioxide and oxygen from the air. Through photosynthesis, they turn the carbon into the carbohydrates necessary for the plant to grow. In a process called transpiration, similar to evaporation, they exhale oxygen and water vapor. Hydrogen atoms in water bond, causing water molecules to cling together, or cohere. Water molecules cohered in a tube as thin as a plant xylem exert great pressure. Water exiting the leaves through transpiration creates negative pressure called tension, causing the water under pressure in the xylem to move higher. This is how transpiration effectively "pulls" the water upward from the roots to the leaves. Transpiration has pulled water to the top of a sequoia that was 370 feet tall and a Douglas fir that was 413 feet tall, and to the ends of 150-foot-long vines in Australia. An internal pressure in the xylem up to 270 lbs. per square inch is needed to pull water to those great heights and lengths.