The Response of Leaf Discs from Sun and Shade Plants to Green Light

The response of leaf discs from sun and shade plants to green light Teacher/Lecturer guide

Type and purpose of activity

This experiment can be used to:  provide evidence for assessment of Outcome 3  develop knowledge and understanding of compensation points in sun and shade plants  develop problem solving skills and in particular Outcome 2 PC: (d) Experimental procedures are planned, designed and evaluated appropriately.

Background Information

 Leaf discs in 10 cm3 syringes from a sun plant and a shade plant are put in a sodium hydrogen carbonate solution. The air is then sucked out of them so that they sink. They are then allowed to photosynthesise in white light, the O2 produced making them re-float.  The experiment is then repeated in green light. If the leaf discs can use the green light for photosynthesis, e.g. shade plants, they will eventually rise as before. If the discs cannot use green light, e.g. sun plants, they will stay at the bottom of the container.

Reference

SAPS Student Sheet 3 - Investigating photosynthesis with leaf discs. http://www.saps.org.uk/secondary/teaching-resources/284-investigating-photosynthesis- with-leaf-discs

Classroom management

 Since the apparatus used in this experiment is not expensive and probably readily available in most schools/colleges, it may be appropriate for students to work individually or at the most in pairs.  Prior to students carrying out the experiment, arrange a maximum time to wait for the leaf discs to rise - say 15 or 20 minutes. Assume they will never float after that time.  For reasons that are not immediately obvious, only certain plants work in this practical so it is worth testing them quickly beforehand.  Timings will be shorter if good illumination is provided (e.g. light bank / warm, sunny windowsill) and if the sodium hydrogen carbonate solution is warm (20 – 30ºC). (If the sodium hydrogen carbonate solution is cold to start with, heat

Science & Plants for Schools: www.saps.org.uk The response of leaf discs from sun and shade plants to green light: p. 1

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 2012. from the lights will probably increase its temperature as the experiment proceeds, influencing the results.  Even when working individually, the experiment should take less than one hour. Working in pairs will reduce this time considerably.

Supply of materials

In order to satisfy the core skill in problem solving, students will be required to “identify and obtain resources” required for themselves. It is therefore not appropriate to provide all equipment and materials in e.g. a tray system for each student/group. Normal laboratory apparatus should not be made available in kits but should generally be available in the laboratory. Trays could be provided containing one type of specialist equipment or materials.

Extension work

 Does the response of the same species vary if one plant has previously been in bright light and one in the shade?  Vary the bathing solution e.g. concentration of sodium hydrogen carbonate or pH.  Response to different colours of light using filters.

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 2012. Technical Guide

Materials required

Materials required by each student/group:  2 x 10 cm3 syringes  about 20 cm3 0.2 M sodium hydrogen carbonate solution  stop clock  2 green syringe covers

Materials to be shared  Strong source of light  Sheets of glass or perspex can also be provided to prevent heat from the light reaching the syringe. cork borer no. 3  Shade plant e.g. aspidistra  Sun plant e.g. punnet of cress, ’fast’ plant cotyledons  Other plants are possible but they must be in good health and growing rapidly. It is best to check possible plants beforehand for this experiment.

Preparation of materials

 0.2 M sodium hydrogen carbonate (20 cm3 per group/student) made up with water that has been allowed to come up to room temperature (20 – 30ºC). 4–5 drops of detergent should be  added to a litre of solution. This helps prevent the discs from sticking to the sides of the syringe.  Green syringe covers - Green colour filter sheet from Philip Harris, Catalogue no. Q59854/0.  Cut a piece of filter sheet 10.5 x 5.5 cm. Fit this around a 10 cm3 syringe. Use sellotape to seal the seam and to close the end at the tip of the syringe. (Over 20 such covers can be made from one sheet).

Supply of materials

It is not appropriate to provide all equipment and materials in e.g. a tray system for each student/group. Equipment and materials should be supplied in a way that students have to identify and obtain resources. Normal laboratory apparatus should not be made available in kits but should generally be available in the laboratory. Trays could be provided containing one type of specialist equipment or materials.

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 2012.