Muhammad Nadeem Ashraf Soil & Environmental SciencesFaculty of Agriculture, Rawalakot.
MECHANISM OF WATER UPTAKE BY PLANTSThe mechanism of absorption of water of two types active and passive often operating independently • Active Absorption: • Two types of theories: 1. Osmotic theory 2. Xon Osmotic theory
• Osmotic theory:• It explain that the root hairs act as ohmmeters and there exist a typical condition so that water can move accross osmotically from soil solution to root xylem.• The theory assume that translocation of water takes place by means of diffusion along a gradient of decreasing water potential or in creasing water solute.
• Non_Osmotic:• Non Osmotic theory assume that when the driving force are zero.• This theory explain the absorption of water by roots when the osmotic potential of soil water is greater than that of hair cell sap.
• Non Osmotic active water uptake can also be supposed by the following:1. There is a correlation b/w the rate of water absorption and respiration.2. Auxin_induced water uptake.3. The factors such as low temperature,oxygen,tension and respiratory inhibitors which affect respiratory rate and in turn the absorption rate.
Cohesion-Tension Theory• The major mechanism for long distance water transport is described by cohesion-tension theory;• Whereby the driving force of transport is transpiration,that is,the evaporation of water from the leaf surface.• Water molecules cohere(stick together), and are pulled up by the tension,or pulling force,excerted by evaporation at the leaf surface.
• Water will always move toward a site with lower water potential,which is a measure of the chemical free energy of water.• By definition,pure water has a water potential of 0 Maga pascal.• In contrast 20% relative humididy the water potential of the atmosphere is 500MPa.• this difference signifies that water will tend to evaporate into the atmosphere.• The water with in plants also has a negative potential, indicating water will tend to evaporate into the air from the leaf.
• The water in plants can exist at such low water potential due to the cohesive forces of water molecules.• The chemical structure of water molecules is such that they cohere very strongly.• By the cohesion tension theory, when sun light strikes a leaf, the resultant evaporation first cause a drop in leaf water potential.this causes water to move from stem to leaf,lowering the water potential in the stem,which in turn causes water to move from root to stem,and soil to root.• This severs to pull water up through the xylem tissue of the plant.