Design and Carry out an investigation to see what effect light intensity has on how quickly water is lost from a shoot, loss of water from the upper surfaces of a plant is known as transpiration.

Planning

The investigation brief was to Design and Carry out an investigation to see what effect light intensity has on how quickly water is lost from a shoot, loss of water from the upper surfaces of a plant is known as transpiration.

Hypothesis

The Stomata are responsive to light, they open in light and close in darkness to allow Carbon Dioxide into the leaf for photosynthesis. As most of the water is lost via stomata transpiration rate will be increased at greater light intensity, up to a point where the stomata are fully open and transpiration is at its maximum.

The water loss can be measured by enclosing the pot of the plant in a plastic bag and putting the plant on top of a top pan balance (see Fig ______), the mass will gradually fall as the water evaporates off the surface of the plant, this in not very accurate at the plant is growing, so the loss in the mass is offset to an extent by an increase in the mass of the plant tissue due to growth. It is easier to measure water uptake by a plant.

Water uptake can be measured by using a bubble potometer, but the water uptake in a plant is not only for transpiration it is also for consumption, i.e. for cell expansion and photosynthesis.

The question to ask is will the light that I use effect the rate of photosynthesis and is it possible to limit the cell expansion, or keep both variables constant, so that the difference in values is due to a change which equals transpiration and not a change in consumption. The other question to ask is will the rate of photosynthesis, which with an increased light intensity cell expansion, significantly affect the results.

On further investigation it transpires that 1% of water uptake is used for photosynthesis and the same amount for building protoplasm. Ninety-eight percent is lost via ‘transit’. Thus the possible resultant change in photosynthesis with an increased light intensity will not be significant so, the rate of water loss can be calculated by the distance a bubble moves in a potometer (see Fig ______)

The variable, which needs to be adjusted, is the light intensity, with the limited facilities available to me, the best way to do this is to use a lamp and move it closer and further away form the plant, but this would also adjust the temperature that the plant is in, i.e. increased as the source (lamp) moves closer.

The intensity of the light will be measured using a luxmeter, this will be kept at a stationary position and angle on the plant, so that any discrepancies will be nullified i.e. if the angle of the luxmeter is not optimal to obtain the current reading, then it will not be optimal for all the readings, thus the inaccuracy will be constant and therefore irrelevant.

The rate at which water is lost is not only affected by the intensity of the light, it is affected by other factors, which affect transit and are claimed to be external and internal factors. These factors need to be minimal so that the only variable, which is adjusted, is light intensity.

External Factors

Humidity – This is concentration of water vapour in the air, as this affects the water potential gradient both outside the leaf and within the leaf. If the air is dry the potential gradient is great thus temperature increases.
Temp – The higher the temperature, the greater the kinetic energy of the molecules thus increasing transpiration. The surrounding air can also hold more water, so thus it increased the potential gradient.
Air Movement – If a layer of moist air around the leaf is constantly blown away, then this maintained the potential gradient thus increases the rate of transpiration.
Light intensity – See Hypothesis

Internal Factors

There are a number of anatomical and morphological differences between plants/leaves, which influence transpiration rate i.e.

Leaf Area – A proportion of the water loss via cuticle thus increase in area leads to increase in transpiration irrelevant to concentration of stomata (which will also increase as leaf area increases.
Cuticle – This is a waxy covering outer leaf surface, which eliminates water loss. The thinner this is, the increased rate of transpiration.
Stomata – The greater the number of Stomata per area, the greater the rate of transpiration.

The internal factors ...