Root Pressure: Definition & Theory

Instructor: Brekke Peterson Munks
This lesson defines root pressure and its effect on plants. It also looks to the extent that root pressure has been scientifically proven, and how alternative theories come into play.

Root Pressure: Definition

Look outside your window. All of that plant life, even the tallest trees, have gravity-defying properties that allow nutrients to be 'sucked' up from the deepest depths of the soil and moved to the highest branches! This lesson takes a look at how plants have the amazing ability to transport water and nutrients upwards using a complex array of biological processes.

Plants are complicated organisms, and one of the many intriguing processes of a plant is root pressure. Root pressure is basically the idea that a plant's roots can either maintain a higher or lower pressure based on its surroundings. It does this in order to promote or discourage nutrient uptake. In other words, the root system of a plant can alter its pressure to either: a) help water and/or nutrients rise throughout the plant, or b) push water and/or nutrients out of the plant. Biologists are usually concerned with the former, and how it affects the rise of water and nutrients in a plant.

Background Concepts

In order to understand root pressure we have to take a look at key concepts in biology. We'll start with osmosis, which is the passive movement of water across a membrane due to a difference in concentration. Osmosis is key to root pressure because when the water concentration outside the root system of a plant is greater than inside the root system, it moves across the root membranes and into the plant's water and nutrient transportation system.

Here's an example that might help you better form an image. Imagine a wide open, five lane freeway. When there is traffic on the freeway, you'll often see the on-ramp traffic lights flashing. If no traffic lights are flashing, all of the cars approaching the freeway can enter quickly and easily. When the traffic on both the on-ramps, side streets, and freeway reach a similar traffic volume, the on-ramp traffic lights turn on. These lights then begin to meter the traffic that enter the freeway.

Now picture the on-ramp, along with the side streets leading to it, packed full of cars. In our example the wide open freeway represents the root water transportation system, while the on-ramp and side streets represent the soil moisture. The traffic lights represent the root membrane. When there is more soil moisture (the ramp and side streets packed full of cars) than in the root system (the freeway), the membrane (the traffic lights) begin to meter, or allow, water to move into the root cells. This is osmosis in action, and that's really where root pressure begins

Speaking of the plant's transportation system, we need to take a quick look at xylem and phloem. You can think of these two as the circulatory system of plants, and each has a specialized function. Xylem deals with transporting water (necessary for photosynthesis, among other things), and phloem deals with transporting nutrients like sugars and other organic compounds. When looking at root pressure, we're most concerned with xylem.

The last concept we should understand before seeing root pressure in action is transpirational pull. The general consensus among biologists is that transpirational pull is the process most responsible for shuttling water up a plant's xylem vessels. The idea is that transpiration, the evaporation of water from the surface cells of leaves, causes a pressure differential that favors the upward movement of water through the xylem of a plant. Or, in simpler terms, so much water is evaporated through the leaves of plants that it causes a 'sucking' action of water up through the plant. Transpirational pull is thought to cause the majority of the upward movement of water in plants, with hypothesizers claiming that root pressure lends a helping hand.

Root Pressure in Action

Explanation of water movement in a plant, beginning with water uptake by the root system.

As water enters the roots via osmosis, the xylem cells fill up and bloat, putting pressure on the more rigid outer cells of the root. This pressure, especially when levels are low outside the plant, causes the water to be forced up the plant, despite the force of gravity. But you may ask, if the pressure is low outside the plant, why doesn't the water just flow back down the roots with the force of gravity? Well, the process gets more complex as you look at the role of other nutrients and ions. For now, suffice it to say that the electrical charge of those outer root cells create a sort of 'one-way road' that doesn't allow the water back down and out of the roots. After all, allowing water to drain freely out of plants back into the ground would be terribly counterproductive to plant growth.

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