- All mechanisms of water absorption require a free energy gradient from soil water to the water transporting tissue of the root, the xylem. In order for water absorption to continue, that free energy gradient must extend through the leaf to the atmosphere. This gradient is referred to as the soil-plant-atmosphere system.
- Osmosis (also Active water absorption) is the flow of one constituent of a solution through a membrane while the other constituents are blocked and unable to pass through the membrane.
- Osmotic water absorption occurs in warm, moist, well-aerated, fertile soils, the factors that enable the root to absorb minerals.
- Experimentation is necessary to determine which membranes permit selective flow, or osmosis because not all membranes act in this way. Many membranes allow all or none of the constituents of a solution to pass through; only a few allow a selective flow.
- In the classic demonstration of osmosis, a vertical tube containing a solution of sugar, with its lower end closed off by a semipermeable membrane, is placed in a container of water. As the water passes through the membrane into the tube, the level of sugar solution in the tube rises visibly.
- A semipermeable membrane that may be used for such a demonstration is the membrane found just inside the shell of an egg, that is, the film that keeps the white of the egg from direct contact with the shell.
- In this demonstration, the water moves in both directions through the membrane; the flow is greater from the vessel of pure water, however, because the concentration of water is greater there, that is, fewer dissolved substances exist in this solution than in the sugar solution.
- The level of liquid in the tube of sugar solution will eventually rise until the flow of water from the tube of sugar solution, under the influence of hydrostatic pressure, just equals the flow of water into the tube.
- Thereafter, no further rise in level will occur. The hydrostatic pressure establishing this equality of flow is called osmotic pressure.
- A variety of physical and chemical principles is involved in the phenomenon of osmosis in animals and plants.
- Consider the following figure:
- The experiment shown above demonstrates the process of osmosis. Water flows through a semipermeable membrane into a sugar solution, diluting the solution. The sugar molecules cannot pass through the membrane, so the water outside remains pure.
THE ROLE OF OSMOTIC WATER ABSORPTION
- Although osmotic water absorption may induce guttation, root pressure, exudation, and maintain the water balance of slowly transpiring plants, it has many limitations that prevent it from being the sole or even major mechanism of water absorption.
- Root pressure has not been observed in many tree and other plants. Root pressure is necessary if water is to be moved from the root to the shoot by osmotic water absorption. Even in species that demonstrate root pressure, the pressure observed under the best conditions would only support of the hydrostatic head of about ten meters, much less than ordinary tree height.
- Under rapid transpiration the mineral concentration of the stele will be diluted, destroying their gradient from soil to stele. The low ψ gradient associated with osmotic water absorption does not allow a sufficient pressure difference for rapid absorption.
- Osmotic water absorption may be able to maintain water balance in short, slowly transpiring plants, but another mechanism is responsible for absorption in trees and most plants.
EFFECTS OF OSMOTIC WATER ABSORPTION
Osmotic water absorption can maintain water balance in a slowly transpiring plant that is not very tall. A root xylem with 0.1 MPa has sufficient osmotic potential under ideal circumstances to raise a water column to a height of 10 meters. This is sufficient for many shrubs and herbs, but not for trees. In addition, related phenomena to this have not been observed in conifers, the tallest trees.
Osmotic water absorption may cause xylem sap to leak from special pores, hydrothodes, in leaves. Hydrothodes are common at the tips of grass leaves. This is called guttation and it occurs under conditions of very low transpiration when the root acts as an osmometer. Salts accumulate in the stele, soil water flows into the stele in response to the ψ gradient created by the minerals in the stele, and continues to do so even as pressure develops inside the root. This pressure forces xylem sap through the hydrothodes. If the water of guttation dries, the remaining xylem salts and may cause tip burn of the leaf.
If the top of a plant using osmotic water absorption is detached, and a pressure gage such as a mercury manometer attached to the root stump, pressure may be measured. This is called root pressure, and it is a plant expression of osmotic pressure. Root pressure may reach as much 0.15 MPa under unusual circumstances. Root pressure has not been detected in many tree species. However, root pressure may be demonstrated by tying a portion of a child’s balloon on the stump of grape in the spring, and watching the balloon swell with exuded sap until it bursts.
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