Tag: water absorption and ascent of sap in plants

Godlewski proposed relay pump theory to explain ascent of sap. According to him there was a rhythmic change in the osmotic pressure of the living cells of xylem parenchyma and medullary rays, which brought about a pumping action of water in an upward direction.

The tracheids and vessels acted as water reservoirs. Janse supported Godlewski and showed that if the lower portion of branch was killed, the leaves above were affected.

Capillarity Theory: It is given by Boehm (1809). According to this theory, water is translocated because water molecules adhere to the surfaces of small, or capillary, tubes. This adhesion causes water to somewhat creep upward along the sides of xylem elements. In glass tubes this upward movement is visible as the curved or crescent-shaped (concave) meniscus. However, it was shown that capillarity (or capillary rise) alone in tubes of similar diameter as that of a xylem element raises water less than 1 meter.

Hopkins (1999) explained that the relationship between the rise of water in a capillary tube and the size of the tube is inversely proportional. This means that the thinner is the tube, the higher will be the rise of water.

The passage of absorbed water from root to the aerial parts of the plant is known as the ascent of sap. It occurs through xylem (lumen of tracheary elements or vessels and tracheids) with a rate of 10 to 75 centimetres per minute. This can be demonstrated by ringing experiments (conducted by Malpighi, 1672; Stephen Hales, 1727; and Hartig in 1837) or dipping the cut end of a fresh shoot (e.g., Balsam) in 2 percent eosin solution for about 30 minutes. Then, the veins of the leaves turn red. Sections through stem and leaves indicate that the xylem vessels/tracheids turn red.

Downward translocation of the organic solutes takes place through phloem. This can be proved by the ringing experiment. Ringing experiment was carried by Hartig, Malpighi and Stephen Hales.
They took a branch with leaves immersed in beaker which is filled with water. Now remove all the tissues which are present above the xylem [cortex, cambium and phloem]. Now placed this experiment in a suitable environment for 2 - 3 days. Then they observed that leaves did not wilt. This experiment makes it clear that water moves up only through vessels and tracheids of the xylem. 
Ringing [girdling] experiment has been done only in dicotyledons because their vascular bundles lies in a ring. While vascular bundles are scattered in monocotyledons, so that this experiment can not be possible.

Atmospheric pressure theory was advanced to explain ascent of sap in plants. The protagonists of this theory have assumed that plants are closed systems. When water escapes by transpiration from the surface of the leaves, it is believed that vacuum will be created within the plant body. As the root system is submerged in soil water, with the atmospheric action on the soil water, in order to fill up the vacuum created in the xylem vessels, water just enters passively; thus, the water is translocated upwards. But it was later on concluded that plants are not closed systems but they exhibit openness, for, the gases can diffuse into and out of the plant system with ease and facility. Added to this, atmospheric pressure can support the water to be lifted only to a height of 34 feet; but there are plants which are taller than this and still there is transport of water. Hence it can be concluded that atmospheric pressure could not be the force for ascent of sap.

Imbibitions theory was proposed (1868) and supported by Sachs (1878). According to this theory the ascent of sap occurs due to imbibitonal activity of the cell walls of the xylem elements.

This theory is discarded as it has been experimentally found that water rises up through the lumen of the xylem and not through the walls.

Transpiration is the process of water movement from soil solution into the plants and then to the atmosphere. The direction and intensity of water movement are determined by the difference in the water potential gradient between the two regions. The water potential in the leaf is lower which allows the movement of water in the upward direction. When the water potential is low, the solute potential is high.