Biology WATER MOVEMENT UP A PLANT

KEY TOPICS

star Root Pressure
star Experiment to study Root Pressure
star Transpiration Pull
star Experiment to study Transpiration

ROOT PRESSURE

● As color{violet}("various ions") from the soil are actively transported into the color{violet}("vascular tissues") of the roots, water follows (its potential gradient) and increases the color{brown}("pressure") inside the color{violet}("xylem.")

● This positive pressure is called color{brown}("root pressure,") and can be responsible for pushing up water to color{violet}("small heights")
in the stem.

● color{violet}("Effects of root pressure") is also observable at night and early morning when evaporation is low, and excess water collects in the form of droplets around special openings of veins near the color{violet}("tip of grass blades"), and color{violet}("leaves") of many color{violet}("herbaceous parts.")

● Such color{violet}("water loss in its liquid phase") is known as color{brown}("guttation.")

● color{violet}("Root pressure") can, at best, only provide a color{brown}("modest push") in the overall process of water transport.

● They obviously do not play a major role in color{violet}("water movement") up tall trees.

● The greatest contribution of color{violet}("root pressure") may be to color{brown}("re-establish the continuous chains") of water molecules in the color{violet}("xylem") which often break under the color{violet}("enormous tensions") created by transpiration.

● color{violet}("Root pressure") does not account for the majority of color{violet}("water transport;") most plants meet their need by color{brown}("transpiratory pull.")

EXPERIMENT TO STUDY ROOT PRESSURE

● Choose a color{brown}("small soft-stemmed plant") and on a day, when there is plenty of color{violet}("atmospheric moisture,") cut the stem horizontally near the base with a color{violet}("sharp blade,") early in the morning.

● You will soon see drops of solution ooze out of the cut stem; this comes out due to the color{brown}("positive root pressure.")

● If you fix a rubber tube to the cut stem as a sleeve you can actually collect and measure the rate of color{brown}("exudation,") and also determine the color{brown}("composition of the exudates.")

TRANSPIRATION PULL

● Despite the absence of a color{violet}("heart or a circulatory system") in plants, the flow of water upward through the xylem in plants can achieve fairly high rates, up to color{brown}("15 metres per hour.")

● A long standing question is, whether water is color{brown}("‘pushed’") or color{brown}("‘pulled’ ") through the color{violet}("plant.")

● Most researchers agree that water is mainly color{violet}("‘pulled’") through the color{violet}("plant,") and that the driving force for this process is color{brown}("transpiration") from the color{violet}("leaves.")

● This is referred to as the color{brown}("cohesion-tension-transpiration pull model") of water transport.

● Water is color{brown}("transient") in plants.

● color{brown}("Less than 1 percent") of the water reaching the leaves is used in color{violet}("photosynthesis") and color{violet}("plant growth.")

● Most of it is color{brown}("lost") through the color{brown}("stomata") in the leaves.

● This water loss is known as color{brown}("transpiration.")

EXPERIMENT TO STUDY TRANSPIRATION

● One has studied transpiration in an earlier class by enclosing a color{brown}("healthy plant in polythene bag") and observing the droplets of water formed inside the bag.

● One can also study water loss from a leaf using color{brown}("cobalt chloride paper") , which turns colour on absorbing water.