Root pressure

Plants are complex organisms, and root pressure is one of the many fascinating processes of a plant. Root pressure is the concept that a plant’s roots can maintain a higher or lower pressure depending on their surroundings. It does this to encourage or discourage nutrient uptake. In other words, a plant’s root system can change 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 typically concerned with the former and how it affects the increase of water and nutrients in a plant.

Root Description:

The force that forces water absorbed from the soil to move through a plant’s roots and up its stem. This pressure can be demonstrated by severing a stem and allowing water to escape. To measure root pressure, a manometer can be attached to a plant stem. Root pressure is thought to be caused by both water osmosis from the soil into the root cells and active salt pumping into the xylem tissue, which maintains a concentration gradient along which the water will move. Also, see transpiration.

The transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves is referred to as root pressure. When the soil moisture level is high at night or when transpiration is low during the day, root pressure occurs in the xylem of some vascular plants. Because of transpirational pull, xylem sap is usually under tension rather than pressure when transpiration is high. Root pressure causes guttation or exudation of xylem sap drops from the tips or edges of leaves in some plants at night. The root pressure of a plant is studied by removing a shoot near the soil level. Due to root pressure, xylem sap will exude from the cut stem for hours or days. The root pressure can be measured by attaching a pressure gauge to the cut stem.

Theory of Root Pressure:

The active distribution of mineral nutrient ions into the root xylem causes root pressure. Ions accumulate in the root xylem without being carried up the stem by transpiration, lowering the water potential. Osmosis causes water to diffuse from the soil into the root xylem. The accumulation of water in the xylem pushes on the rigid cells, causing root pressure. Root pressure exerts a force that pushes water up the stem, but it is insufficient to account for water movement to leaves at the tops of the tallest trees. The maximum root pressure measured in some plants can only raise water to a height of 6.87 metres, while the tallest trees can reach heights of more than 100 metres.

When transpiration is low or zero, root pressure can transport water and dissolved mineral nutrients from roots to the tops of relatively short plants via the xylem. The maximum measured root pressure is approximately 0.6 megapascals, but some species never generate any root pressure. The transpirational pull is thought to be the primary contributor to the upward movement of water and mineral nutrients in vascular plants. Sunflower plants grown in 100% relative humidity, on the other hand, grew normally and accumulated the same amount of mineral nutrients as plants grown in normal humidity, which had a transpiration rate 10 to 15 times that of plants grown in 100% humidity. As a result, transpiration may not be as important in upward mineral nutrient transport in short plants as is commonly assumed.

Conclusion:

During the winter, xylem vessels may become empty. Root pressure may play a role in recharging the xylem vessels. In some species, however, vessels refill without the need for root pressure. Some deciduous trees have high root pressure before they leaf out. Without leaves, transpiration is minimal, and organic solutes are mobilised, lowering the xylem water potential. Early in the spring, sugar maple accumulates high concentrations of sugars in its xylem, which is the source of maple sugar. Some trees, such as maple and elm, “bleed” xylem sap profusely when their stems are pruned in late winter or early spring. This type of bleeding is similar to root pressure. Only sugars, and not ions, can reduce xylem water potential. Changes in stem pressure, rather than changes in root pressure, cause sap bleeding in maple trees.