Is there any way of measuring transpiration rate without using a potometer? I am investigating the relationship between leaf area and transpiration rate. Can you suggest how I can change the area of a leaf?

A potometer is literally a "drinking meter", so anything which measures the uptake of water by a shoot or leaf is a potometer.

The simplest potometer is a small jar/beaker/tube of water with a leaf stalk dipping in it. Usually the surface of the water is covered with oil (eg cooking oil) to prevent evaporation from the water surface. The whole apparatus can be weighed and the loss of mass represents the loss of water from the leaves. The rate of loss in (say) mg per 15 mins is the transpiration rate.

You can change the area of leaves by carrying out the experiment with large and small leaves. You will need to look for leaves of about the same age etc so that the only real difference is their area. You should choose several (at least 3) of each size leaf. You can measure leaf area by tracing round the leaf on graph paper and counting the squares (count any square which is less than a whole one as half a square).

Naturally, you would predict that the large leaf will transpire more quickly than the small leaf. However, it will be interesting to see whether each leaf transpires at the same rate PER SQUARE CM of leaf surface (divide the transpiration rate by the leaf area). Perhaps the small leaves tend to be younger and more fleshy. Perhaps the older leaves are more woody and have a thicker cuticle. Perhaps there are the same number of stomata on a small leaf as on a large leaf and the space between the stomata increases as the leaf grows. Perhaps larger leaves have more stomata and more stomata are formed in the spaces as a leaf gets bigger. Each of these hypotheses could lead you to a different prediction of the "rate of transpiration per cm²" in large compared with small leaves.

We would be interested to know how your experiments go.

John Hewitson