MadSci Network: Botany

Re: Create an experiment showing how light & temperature affects transpiration

Date: Wed Jun 6 17:50:24 2001
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 991803087.Bt

From your description, it seems like you propose to have a cut shoot in a flask 
of water to measure transpiration. This is reasonable, however, it would not 
necessarily be the same as measuring transpiration of a potted plant which has 
a root system. Cut stems in water can get clogged by microbes or xylem blocked 
by air when they are cut. If you use cut shoots or cut flowers, you might also 
wish to investigate the effects of recutting stems under water and flower 
preservatives, such as Floralife, on transpiration of cut shoots or cut flowers.

Covering the shoot with a plastic bag may make it difficult to accurately 
measure transpiration because the high humidity in the plastic bag can reduce 
transpiration. Probably the best way to measure tranpiration is not to cover 
the plant with a plastic bag but to use a balance and measure weight loss. One 
milliliter of water weighs one gram. For a potted plant, enclose the pot in a 
plastic bag and tie it around the stem. This will prevent water loss from the 
soil surface. For a cut shoot in a flask, place plastic film over the flask 
opening to prevent evaporation from the water surface. 

Another common technique is the potometer (see websites under references). It 
uses a cut shoot but can be tricky to set up. The advantage of a potometer is 
that it can give rapid results.

Your treatments of light and dark are reasonable but having cold, warm and hot 
treatments in the dark probably will not give much difference because stomata 
close in the dark. If possible, it would be better to have temperature 
treatments in the light. One common treatment is to place plants in front of an 
electric fan to determine wind speed effects on transpiration. It is also easy 
to coat leaves with petroleum jelly to determine the effect of blocking stomata 
on transpiration. You can coat top leaf surface, bottom surface or both 
surfaces. Another type of experiment is to compare transpiration for different 
kinds of plants, such as cactus and tropical houseplants. With potted plants, 
you can also water the pots with salt or fertilizer solutions and see how that 
affects transpiration.

One of the nice things about short term transpiration experiments with potted 
plants is that you can often use the same plant over again. For example, one 
day the plant can be placed in light, next day in dark, next day in front of a 
fan, etc. 

Be sure to have replication, such as 3 to 5 plants per treatment. One thing 
that can add to the experiment is to measure the leaf area after the experiment 
is completed and calculate transpiration rates on a leaf area basis. Leaf area 
can be measured by tracing leaves on graph paper and counting the blocks. 
Another measurement often done is the stomatal density (stomata per square 
millimeter). The technique requires a microscope and nail polish.


Hershey, D.R. 1995. Plant Biology Science Projects. New York: Wiley.

Transpiration Lab


Experimenting with the Potometer

Transpiration Experiment

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