Root pressure is a positive pressure that forms in the xylem sap of some plants’ roots. It’s an example of active water absorption in action. During certain seasons, root pressure is detected, which favours optimal metabolic activity and reduces transpiration. It is at its peak in tropical regions during the rainy season, and in temperate settings during the spring. 1-2 bars or atmospheres of root pressure are typically encountered in plants. Higher pressures (e.g., 5-10 atm) are infrequently seen. Under situations of malnutrition, low temperature, drought, and poor oxygen supply, root pressure is decreased or nonexistent.
The mechanism of root pressure development may be seen from three perspectives:
(a) Osmotic:
Salts and sugars build in the xylem’s tracheary components. Water is withdrawn from the surrounding cells as well as from the typical water absorption pathway when the solute concentration is high. As a result, the sap of the xylem creates positive pressure.
(b) Electro-osmotic:
The xylem channels and surrounding cells have a bioelectric potential that favours the flow of water into them.
(c) Nonosmotic:
Differentiating xylem components secrete hormones that act as metabolic sinks, causing water to flow towards them. Water can be actively pumped into the xylem by the living cells that surround it.
Objections to the Root Pressure Theory include the following:
(i) Root pressure isn’t observed in every plant. In gymnosperms, which include some of the world’s tallest trees, little root pressure has been seen.
(ii) Root pressure is only seen during the most favourable growth times, such as spring or the rainy season. The xylem sap is significantly hypertonic to the soil solution at this time, and the transpiration rate is low. The root pressure is often absent in the summer when the water requirements are high.
(iii) Root pressure is often low, preventing sap from reaching the tops of plants.
(iv) Even in the absence of roots, water continues to move upwards.
(v) There is no root pressure in the quickly transpiring plants. In most plants, however, there is negative pressure.
(vi) When compared to the pace of movement through the xylem, the quantity of exudation caused by root pressure is fairly minimal.
(vii) When compared to intact plants, absorption in de-topped plants is relatively low.
(viii) In unfavourable environmental circumstances, root pressure diminishes but sap ascent remains unabated.
(ix) At night, when evapotranspiration is low, root pressure is commonly observed. It might aid in the re-establishment of continuous water chains in the xylem, which is frequently broken due to the huge stress caused by transpiration.
Root pressure :
Water received from the earth is forced through the roots and up the stem of a plant under pressure. Cutting a stem, which will discharge water, will exhibit this pressure. To measure root pressure, a manometer can be affixed to a plant stem. Both osmosis of water from the soil into the root cells and active pumping of salts into the xylem tissue, which maintains a concentration gradient along which the water will travel, is thought to be responsible for root pressure.
When the soil moisture level is high at night or when transpiration is low during the day, root pressure develops in the xylem of several vascular plants. Because of the transpirational pull, xylem sap is frequently under tension rather than pressure when transpiration is high. Root pressure produces guttation or exudation of xylem sap from the tips or margins of leaves in certain plants at night.
The active dispersion of mineral nutrient ions into the root xylem causes root pressure. If there isn’t enough transpiration to move the ions up the stem, they build up in the root xylem, lowering the water potential. Due to osmosis, water diffuses from the soil into the root xylem. The buildup of water in the xylem pushes on the stiff cells, causing root pressure.
Root Pressure’s Importance:
Although root pressure is largely responsible for the ascent of water in vascular plants, it is inadequate for the flow of sap against gravity, particularly in the tallest trees. Furthermore, the fact that root pressures are lowest when water loss from leaves (transpiration) is maximum, indicating that root pressure is not driving sap movement, provides evidence that root pressure is not driving sap movement.
Instead, the lifting force created by evaporation and transpiration of water from the leaves, as well as the cohesive and adhesive forces of molecules in the vessels, and potentially other variables, play a far larger part in plant sap rise.
The active dispersion of mineral nutrient ions into the root xylem causes root pressure. If there isn’t enough transpiration to move the ions up the stem, they build up in the root xylem, lowering the water potential. Due to osmosis, water diffuses from the soil into the root xylem. The buildup of water in the xylem pushes on the stiff cells, causing root pressure. Although root pressure pulls water up the stem, it is insufficient to account for water circulation to leaves at the summit of the tallest trees. The largest trees are almost 100 metres tall, while the highest root pressure observed in certain species can only elevate water to 6.87 metres.
Conclusion :
When transpiration is minimal or non-existent, root pressure can carry water and dissolved mineral nutrients from roots to the crowns of relatively short plants via the xylem. During the winter, xylem vessels may empty.