Ch#14 |Transportation | Lec #07 | ASCENT OF SAP (B) Root Potential (FSc Bio part 1)@riffatjahan

Root Pressure:
Second force involved in the movement of water and dissolved minerals up in the xylem tissue is the root pressure.
Root pressure is created by the active secretion of salts and other solutes from the other cells into the xylem sap.
This lowers the water potential of xylem sap.
Water moves through apoplast/symplast or vacuolar pathway.
Water enters the xylem cells by osmosis. The hydrostatic pressure in the xylem cells increases.
As a result the water is pushed upward and the level of the in the xylem cells increases.

Limitations of Root Pressure:
(i) As a result of root pressure the sap in the xylem does not rise to enough height in most plants.
(ii) The root pressure is also least effective during the day, when the transpiration pull is the active
force involved in pulling the sap in the xylem cells upwards.

(ii)It has been estimated that a positive hydrostatic pressure of around 100 to 200 KPa (exceptionally 800 KPa) is generated by root pressure.
(iii)This pressure is not enough to push water upwards to required height in most plants.
(iv) However it has a role in plants which transpire slowly, and are smaller in size.

Guttation or Exudation:
Closely associated with the root pressure is a phenomenon called guttation or exudation.
Guttation is loss of liquid water through water secreting glands or hydathodes.
The guttation is due to the root pressure (positive pressure), developed in xylem tissue of roots.
Guttation or exudation is more notable when transpiration almost stops and the relative humidity is high (as at night).
The dewdrops that can be seen on the tips of grass leaves or strawberry leaves are actually guttation droplets exuded from hydathodes.

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