The potometer
- a way of
measuring the uptake of water by a plant - it measures the rate of
transpiration
You are measuring the rate of uptake
of water by the plant, which is an estimate of the rate of water loss by
transpiration.
It is NOT completely accurate because some of the
water is used up in photosynthesis in making glucose.
You can lose water from leaks, which is why the
potometer system should be carefully sealed water tight.
A potometer consists of a vertical tube with a plant
shoot sealed in it plus a system to feed in water and some means of
measuring the water uptake of the plant.
The plant shoot should be cut
under water to prevent air entering the xylem and at an angle to
increase the surface area for water absorption.
Ideally, the whole apparatus is assembled under
water.
The tube is connected to a reservoir of water,
controlled by a tap, used to replace water lost by transpiration.
The horizontal capillary tubing, connected to the plant
tube and reservoir, has a scale set up beside it.
Inside the capillary tube, adjacent to the scale is an
air bubble which will move along to the left as water evaporates from
the leaves.
The whole apparatus must be airtight and watertight or the readings will be inaccurate and
initially have the tap open.
You need to let the plant
acclimatise ('settle down') to the laboratory conditions before
starting the experiment and then shut the tap.
Then remove the end of the
capillary tube from the beaker of water and let one air bubble into
the capillary tube which is then put back under the water in the beaker.
When the bubble reaches the scale, record the
initial reading and start the stop-watch..
When the bubble has moved a significant distance
along the scale, record the distance and the time.
relative rate of transpiration = distance
bubble moved / time taken
At the start and after a measurement of transpiration has been made, the
reservoir tap is opened to allow water to flow in and move the air
bubble to the right near the start of the scale.
Excess water runs out into the beaker which also acts as
a reservoir of water itself during an experimental run, rather than
sucking in air.
Making a measurement
You note the starting position of the
air bubble on the right.
You then measure the time it takes for the air bubble to move
from right to left and note the total distance moved.
To repeat the experiment you let
water in from the reservoir to bring the bubble back to near the start
of the scale.
Estimated transpiration rate =
distance air bubble travels / time taken
So your rate of transpiration units might be
mm or cm/min (an arbitrary scale based on the experimental setup).
BUT, note that the experiment assumes the water uptake by the plant through its roots is directly
related to the water loss by evaporation from the leaves.
The sorts of investigations you can do
with a potometer
Remember
Wherever possible keep everything constant
except the one factor you are investigating e.g. constant temperature,
constant air humidity in the laboratory.
You can now investigate various
environmental conditions by comparing the relative uptake of water.
1. Varying light intensity:
You can use a reasonably powerful light to increase
light intensity, placing it quite close to the plant, with the bulb at
the same height as the centre of the plant shoot.
You measure the distance of the centre of the lamp
bulb to the centre of the plant shoot (d).
Run the experiment and measure the rate of
transpiration.
You can then move the same lamp back,
measuring the new longer distance and re-measure the transpiration
rate. You should repeat all measurements for varying lamp distances.
The intensity of light on
the plant is proportional to1/d2 or you can
measure the intensity with a light meter.
You can then plot a graph of
transpiration rate against light intensity.
You must make sure the
temperature and humidity are constant and there is no air flow over the plant
(still air).
It would be easy to do
comparative experiments in a brightly lit room and a darkened room.
2. Varying air flow:
You could
use a hair dryer to blow cold air (room temperature) at different speeds
over the plant to see if increased air flow increases transpiration
rate. Difficult to get data to produce a meaningful graph, but no air
blowing, low blowing setting and a high blowing setting should give you
the trend.
You must make sure the
temperature, light intensity and humidity are constant and the light intensity stays the same.
3. Varying temperature:
This
is also tricky to get good quantitative data.
You can easily compare blowing
cold air and warm air over the plant, and that should give a
difference in transpiration rate.
BUT, not that easy to keep
conditions constant.
You can measure the temperature of
the air near the plant during the experiment.
It might be convenient to do
the experiment in a cold room and then in a warm room.
You must make sure the air flow
is constant (completely still air is the best condition), and the
humidity and light intensity stay the same.
