Investigating Rate of Transpiration

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AIMS
The aim of this experiment was to investigate the effect of temperature rise on the rate of transpiration of a plant (hibiscus) by measuring the plant’s water uptake in a period of time.

HYPOTHESIS
Transpiration is the process by which plants loses water to the atmosphere from their leaves. Water in the plant is lost to the atmosphere by evaporation. Evaporation of water can occur at any temperature. In the presence of sunlight, water in the plants evaporates into the intercellular air spaces and diffuses out through the stomata into the atmosphere. The diffusion of water from the plant into the air lowered the water potential in the outermost cell (mesophyll). The differences of water potential with the adjacent cell will cause water to move from the adjacent cell into the mesophyll cell which then established a water potential gradient in the plant. The water potential gradient will cause water to flow from the xylem along a chain of cells to the outermost mesophyll cell. Since, the experiment was dealing with rising the temperature of the plant’s surrounding air, water in the plant will have a greater tendency to evaporate and diffuses out into the atmosphere. Transpiration rate of the plant therefore should be increasing with temperature (to a certain extent).

INTRODUCTION
There are several factors which can affect the rate at which plants loses water from the leaves (transpiration). These factors can either be internal (e.g. no. of stomata, distribution of stomata, etc) or external factors (e.g. temperature, humidity, etc). For this particular experiment, the temperature surrounding the hibiscus plant was raise by placing a table lamp near it. A simple potometer (figure 1) where the distance travelled by an air bubble was used to measure the rate of water uptake from the plant.

Figure 1
The distance (in cm) travelled by the air bubble at a period of time (in h) was taken to be the transpiration rate for the plant. This rate can be calculated by the following formula; Transpiration rate= Final distance – initial distance Time taken

APPARATUS USED
1.Hibiscus shoot
2.Potometer
3.Stopwatch
4.Retort stand
5.Table lamp
6.Thermometer
PROCEDURE
A leafy hibiscus shoot was carefully cut from its stem. This leafy shoot was then immersed in water where the last one centimetre of the stalk was cut off under water. Next, the potometer was set up to receive the leafy shoot. Its capillary tube was filled with water via its reservoir and the water was left to drain into the sink for a while to ensure that there was no air bubbles along the tube.

As the potometer was then filled with water it was ready to receive the leafy shoot. The leafy shoot was carefully inserted into the rubber end of the potometer. A retort stand was used to ensure the leafy shoot stood upright throughout the experiment. When there were no air bubbles in the tube, the reservoir was shut down by clamping its rubber tube tightly. One air bubble was introduced at the starting end of the potometer to be use later as an indicator for water uptake measurement.

Three trial runs with the plant at room temperature which was 21.5oC were done to settle down the plant to a steady rate of transpiration before the real experiment began. These measurements were recorded in table 1.

The experiment began when a heat source (table lamp) was switch on to generate heat, raising the temperature of the plant’s immediate surroundings. The plant was allowed 5 minutes time lapse to enable it to adapt to the change in temperature. When the time was up, the stop watch was started. Initial distance of the air bubble, the temperature of the plant’s immediate surroundings was read and recorded. Distance and temperature were recorded at 1 minute interval for 5 minutes in table 2.

At the end of the experiment, all the leaves were carefully remove from the shoot. These leaves were then assembled...
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