Difference Between Root Pressure and Transpiration Pull

The key difference between root pressure and transpiration pull is that root pressure is the osmotic pressure developing in the root cells due to movement of water from soil solution to root cells while transpiration pull is the negative pressure developing at the top of the plant due to the evaporation of water from the surfaces of mesophyll cells.

Xylem and phloem are the two main complex tissues that are in the vascular bundle of plants. Xylem transports water and minerals from the root to aerial parts of the plant. The ascent of sap is the movement of water and dissolved minerals through xylem tissue in vascular plants. Plant roots absorb water and dissolved minerals from the soil and hand them over into the xylem tissue in the roots. Then the xylem tracheids and vessels transport water and minerals from roots to aerial parts of the plant. The ascent of sap takes place due to passive forces created by several processes such as transpiration, root pressure, and capillary forces, etc.

CONTENTS

1. Overview and Key Difference
2. What is Root Pressure
3. What is Transpiration Pull
4. Similarities Between Root Pressure and Transpiration Pull
5. Side by Side Comparison – Root Pressure vs Transpiration Pull in Tabular Form
6. Summary

 What is Root Pressure?

Root pressure is the osmotic pressure or force built up in the root cells that pushes water and minerals (sap) upwards through the xylem. Due to root pressure, the water rises through the plant stem to the leaves. There is a difference between the water potential of the soli solution and water potential inside the root cell. Root hair cell has a low water potential than the soil solution. Hence, water molecules travel from the soil solution to the cells by osmosis. When water molecules accumulate inside the root cells, a hydrostatic pressure develops in the root system, pushing the water upwards through the xylem. Therefore, root pressure is an important force in the ascent of sap.

Root pressure can be generally seen during the time when the transpiration pull does not cause tension in the xylem sap. When the stem is cut off just aboveground, xylem sap will come out from the cut stem due to the root pressure. Moreover, root pressure can be measured by the manometer.

Some plant species do not generate root pressure. In short plants, root pressure is largely involved in transporting water and minerals through the xylem to the top of the plant. In tall plants, root pressure is not enough, but it contributes partially to the ascent of sap. When transpiration occurs rapidly, root pressure tends to become very low.

What is Transpiration Pull?

Transpiration pull is the negative pressure building on the top of the plant due to the evaporation of water from mesophyll cells of leaves through the stomata to the atmosphere. When transpiration occurs in leaves, it creates a suction pressure in leaves. Hence, it pulls the water column from the lower parts to the upper parts of the plant.

The transpiration pull of one atmospheric pressure can pull the water up to 15-20 feet in height according to estimations. It is the main contributor to the movement of water and mineral nutrients upward in vascular plants. Furthermore, transpiration pull requires the vessels to have a small diameter in order to lift water upwards without a break in the water column.

What are the Similarities Between Root Pressure and Transpiration Pull?

• Both root pressure and transpiration pull are forces that cause water and minerals to rise through the plant stem to the leaves.

What is the Difference Between Root Pressure and Transpiration Pull?

Root pressure is the osmotic pressure developing in the root cells due to the movement of water from the soil to root cells via osmosis. On the other hand, transpiration pull is the force developing in the top of the plants due to the evaporation of water through the stomata of the mesophyll cells to the atmosphere. So, this is the key difference between root pressure and transpiration pull.

Moreover, root pressure is partially responsible for the rise of water in plants while transpiration pull is the main contributor to the movement of water and mineral nutrients upward in vascular plants. Therefore, this is also a difference between root pressure and transpiration pull. In addition, root pressure is high in the morning before stomata are open while transpiration pull is high in the noon when photosynthesis takes place efficiently.

Summary – Root Pressure vs Transpiration Pull

Root pressure and transpiration pull are two driving forces that are responsible for the water flow from roots to leaves. Root pressure is the force developing in the root hair cells due to the uptake of water from the soil solution. In small plants, root pressure contributes more to the water flow from roots to leaves. In contrast, transpiration pull is the negative force developing on the top of the plant due to the evaporation of water from leaves to air. It is the main contributor to the water flow from roots to leave in taller plants. This is the summary of the difference between root pressure and transpiration pull.

Reference:

1. “Cohesion Hypothesis.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 4 Feb. 2011, Available here.
2. “Xylem.” Wikipedia, Wikimedia Foundation, 20 Dec. 2019, Available here.

Image Courtesy:

1. “Image from page 190 of “Science of plant life, a high school botany treating of the plant and its relation to the environment” (1921)” By Internet Archive Book Images (No known copyright restrictions) via Flickr
2. “Transpiration Overview” By Laurel Jules – Own work (CC BY-SA 3.0) via Commons Wikimedia