Water Transportation

Principal:

Water transport in the plants occurs in the form of a continues column of liquid inward from the root surface to the xylem and upward in the stem from root to shoot and out into the leaves. The liquid water turns to vapor and diffuses out into the atmosphere by the process called transpiration in leaves. A continues supply of water is needed to replace water lose from the leaves by transpiration. This replacement water moves upward in plant tissue (xylem), adapted for rapid, long distance transport.

The rate of transpiration is affected by several factors namely light, temperature, wind and humidity. They all affect the transpiration rate in different ways. Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata. Light also speeds up transpiration by warming up the leaf. Plants transpire more rapidly at higher temperature because water evaporates more rapidly as the temperature rises. As for humidity, we know that the rate of diffusion of any substance increases as the difference in concentration of substance in two regions increases. When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly. As for the wind, when there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air.

The transpiration rate can be measured using a potometer. We will use a cut end of the shoot stem which is inserted into a flexible tube containing water and connected to a pressure sensor. Uptake of water by the plant from the tube causes a decrease in pressure in the water column and this decrease is detected and quantified by the pressure sensor and is displayed on the screen connected to the sensor.

Result:

Table showing the transpiration rate in two plants in different ambient conditions

In different lab conditions

Rate (kPa min-1)

Rate (Pa min-1 mm-2)

Rate (kPa min-1)

Rate (Pa min-1 mm-2)

Macaranga Tanarius

Small leaves

Ambient

-2.788

-0.0412

-1.827

-0.0482

Ambient + Fan

-3.148

-0.0465

-3.173

-0.0837

Ambient + Lamp

-2.354

-0.0348

-3.062

-0.0808

Lamp + Fan

-1.92

-0.0286

-3.134

-0.0827

...