An Investigation Into Leaf Surface Temperature

An Investigation into leaf surface temperature Technical & Teaching Notes

Teaching Notes

This practical is designed to be used as the opening lesson of the transport in plants section of the A level specification. It aims to engage the students in an investigation-based introduction to this topic rather than one based on content. The simple investigation will ask, “What happens to the surface temperature of leaves if petroleum jelly is applied to their lower surfaces?”

Aims of the practical.

1. To provide an engaging introduction to the topic of transport in plants 2. To allow the students to plan an investigation to answer the question “What happens to the surface temperature of leaves if petroleum jelly is applied to their lower surfaces?” 3. To use hand-held infrared thermometers to produce quantitative data1 that will allow them to answer this question. 4. To develop their understanding of how science works.

As an introduction to plant physiology, and prior to any discussion of transpiration or stomatal structure, function and distribution, students will use hand-held infra-red thermometers to investigate the surface temperature of leaves. The students will measure the temperature of the upper side of each leaf before and after smearing the lower surface of the leaf with petroleum jelly. The investigation uses living plants with leaves still attached to the main stem and can either be carried out in the school grounds or can be a lab based investigation (see notes on apparatus).

Before carrying out the practical, the students must plan their investigation (with guidance) and must consider the variables in their investigation by answering the following questions;

 Should all the leaves be from the same species of plant? Yes, if possible  Should all the leaves be on the same plant? Yes, if possible  Should they all be the same size? Yes  Should all the leaves be at the same height? Yes  Should all the leaves be facing the same way? Yes  How many leaves should be tested? At least 30 so that a reliable mean is obtained and standard error and 95% confidence limits can be calculated  Should the temperature be measured from the same point on the leaf each time? Yes

Science & Plants for Schools: www.saps.org.uk Investigating leaf surface temperature – teaching notes

This document may be photocopied for educational use in any institution taking part in the SAPS programme. It may not be photocopied for any other purpose. Revised 2010.  How far away from the leaf should the thermometer be held? 1cm (see background information)  At what angle to the leaf? 90o  How long should the thermometer be pointed at the leaf for?  How long should we wait before we measure the temperature of the leaf after applying the Vaseline to the underside?  The investigation uses paired data i.e. the surface temperature of each leaf is measured first without Vaseline on the underside and then re-measured with Vaseline on the underside. How does this method improve the validity of the data?

Students can then carry out simple statistical analysis on their data (mean and Standard Deviation if AS, mean, Standard Deviation and Standard Error and 95% confidence limits if A2) to investigate the significance of the difference between the mean leaf surface temp without petroleum jelly and with petroleum jelly on the underside. They can then display their data in a bar chart.

The next question to pose is “What is the likely cause of this difference?” Hopefully, the students will make the connection between temperature change and evaporative cooling, and some will remember the role of stomata from GCSE.

Stomatal function and transpiration can then be introduced. Students can use the outcome of the statistical analysis to formulate a hypothesis to explain any observed trends in data (i.e. Evaporation of water from stomata [transpiration] cools the leaf).

Following this, the students outline how they could use the infrared thermometer to investigate differences in the stomatal density in the upper and lower epidermis of leaves. This exercise is a good opportunity for stretch and challenge as some students will need a lot of help to realise that this can be achieved by comparing the effect on leaf surface temperature of applying petroleum jelly to the lower epidermis (where there are lots of stomata) with the effect of applying petroleum jelly to the upper epidermis (where there are fewer, if any stomata).

The students can then be introduced to the procedure for investigating stomatal density2 on the upper and lower epidermis of the leaves, allowing them to collect data to further test their prediction that the leaf heating effect observed when the stomata are blocked will be greatest when the density of stomata in the epidermis is high.

This investigation and any extensions that the students devise provide opportunities for students to develop the skills outlined in AO2(application of knowledge) and AO3(how science works)3. Activity 8 should clarify the scientific method and hence how science works.

Scientific method solution for question 8.

This document may be photocopied for educational use in any institution taking part in the SAPS programme. It may not be photocopied for any other purpose. Revised 2010.

Step Description Observations There is a significant difference between the mean leaf surface temperature without Vaseline on the underside and with Vaseline on the underside

Hypothesis Evaporation of water from the stomata cools the leaf surface

Predictions The leaf heating effect observed when the stomata are blocked will be greatest when the density of stomata in the epidermis is high

Experiments Comparing the effect on leaf surface temperature of applying petroleum jelly to the lower epidermis (where there are lots of stomata) with the effect of applying petroleum jelly to the upper epidermis (where there are fewer stomata).

Investigating the stomatal density on the upper and lower epidermis of leaves

Analysis of Do results and conclusions support the original hypothesis? results and Yes - hypothesis is strengthened. drawing conclusions No - modify hypothesis in the light of the results of the experiment

Safety Notes

If carried out outdoors, students must be reminded of the potential risks of harm to themselves and others, the environment and its wildlife and the ethical implications of their investigation.

Check for hayfever/allergies before commencing the practical. Gloves may be worn by students who may suffer skin reactions when handling plants

Avoid plants with stings or thorns/prickles

The pocket infrared thermometer recommended in the apparatus list does not have a laser pointer. Some (more expensive) infrared thermometers do have a laser pointer to ‘spot’ the temperature target. These are not recommended for use in schools and should not be used.

Apparatus

 Pocket Infrared thermometer Science & Plants for Schools: www.saps.org.uk Investigating leaf surface temperature – teaching notes

This document may be photocopied for educational use in any institution taking part in the SAPS programme. It may not be photocopied for any other purpose. Revised 2010. Maplin Pocket Infrared thermometer, product code N19FR £17.99 (01/12/10)  Petroleum jelly (small tins of Vaseline are the most convenient)  Herbaceous plants with lots of leaves with a surface area of at least 1cm x 1cm e.g. Hairy Willowherb, Rosebay Willowherb or lots of herbaceous plants in the same area e.g. Dandelions, Cats-ear, Plantain (If carrying out the practical in the spring/summer, these may be available in the school grounds).

Shrubs such as Rhododendron can be used

Ready-grown pots of herbs such as Basil could be used in the laboratory if this topic is being introduced when there are few leafy plants about outside. These pots of herbs can be bought from supermarkets throughout the year if it is not possible to grow them in school.

Suppliers

Maplin Pocket Infrared thermometer, product code N19FR £17.99 (01/12/10)

Maplin Electronics, PO Box 534, Manvers, Rotherham, UK, S63 3DH

http://www.maplin.co.uk

Background information

The hand held thermometer listed in the apparatus section has an optics ratio of 1:1, so at a distance of 1cm from the object being measured (the leaf surface), the thermometer detects the infra-red radiation emitted from a circular spot with an area of 1cm2 on the object’s surface.

The funding for a set of 12 Pocket Infrared thermometers was obtained from the ENTHUSE award that I received from the National Science Learning Centre for participation in the Gatsby Plant Science Summer School 2010.

Sample data obtained in August 2010.

Plant species used: Salvia sp

This document may be photocopied for educational use in any institution taking part in the SAPS programme. It may not be photocopied for any other purpose. Revised 2010. Leaf Leaf surface Leaf surface Difference in number temperature temperature temperature/° before application of after application of C petroleum jelly to petroleum jelly lower leaf surface/°C to lower leaf surface/°C

1 20.9 24.8 3.9 2 20.2 19 -1.2 3 16.4 18.4 2 4 11.9 17.9 6 5 10.8 19.3 8.5 6 15.5 20.2 4.7 7 13 14.8 1.8 8 10.7 15.3 4.6 9 10.4 17.7 7.3 10 14.3 20.5 6.2 11 16.5 19.2 2.7 12 15.1 20.5 5.4 13 15.7 18.9 3.2 14 17.2 20.1 2.9 15 17.6 13.1 -4.5 16 12.4 15.9 3.5 17 9.4 11.4 2 18 13.2 13.9 0.7 19 9.1 13.7 4.6 20 9.2 19.9 10.7

This document may be photocopied for educational use in any institution taking part in the SAPS programme. It may not be photocopied for any other purpose. Revised 2010. 21 9.3 11.3 2 22 10.2 16 5.8 23 11.4 13.3 1.9 24 10.8 13 2.2 25 13.3 18.8 5.5 26 12.5 16.4 3.9 27 13.7 16.9 3.2 28 11 16 5 29 10.6 12.8 2.2 30 13.5 15.2 1.7 31 13.3 16.1 2.8 Total 409.1 520.3 111.2 Mean temperatur e 13.2 16.8 3.6 Standard deviation 3.1 3.2 2.8

Further Investigations

Which surface of the leaf has most stomata?

The students gather data comparing the leaf-cooling effect of applying petroleum jelly to the lower epidermis (where there are lots of stomata) with the effect of applying petroleum jelly to the upper epidermis(where there are fewer stomata). They use the temperature change data to suggest a hypothesis to answer this question. The students can then test their hypothesis by investigating stomatal density on the upper and lower epidermis of the leaves using leaf epidermal imprints. (Details of how to prepare stomatal imprints can be found on the SAPS website.)2

Comparison of stomatal density in different plant species

The students gather data on leaf surface temperature changes in different plant species using the infrared thermometer to help them to devise a hypothesis and then use leaf epidermal imprints of the different species to test their hypothesis. (NB- Differences in leaf temperature between different plant species may not result solely from differences in stomatal density. Leaf colour, thickness, area, shape and texture can all affect leaf temperature)

References

1An Investigation into the Ecological Significance of Leaf Temperature. Gatsby Plants Summer School 2010 Protocols for Practical Sessions Dr Jon Graves,University of Sheffield

This document may be photocopied for educational use in any institution taking part in the SAPS programme. It may not be photocopied for any other purpose. Revised 2010. 2How do I measure stomatal density? Available from SAPS website, www.saps.org.uk

3GCE AS and A level subject criteria for science Sept 2006 QCA/06/2864. Pages 5 to 7 http://www.ofqual.gov.uk/files/qca-06-2864_science.pdf

Acknowledgements

 Dr John Graves for ‘An Investigation into the Ecological Significance of Leaf Temperature’  Gatsby Plants and the Gatsby Charitable Foundation  National Science Learning Centre -Coordinators of teacher participation in Gatsby Plants Summer School and providers of an Enthuse award

This document may be photocopied for educational use in any institution taking part in the SAPS programme. It may not be photocopied for any other purpose. Revised 2010.