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

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 sodiumhydrogen carbonate solution. The air is then sucked out of them so that they sink. They arethen 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 forphotosynthesis, e.g. shade plants, they will eventually rise as before. If the discs cannot usegreen light, e.g. sun plants, they will stay at the bottom of the container.

Reference

SAPS Student Sheet 3 - Investigating photosynthesis with leaf discs.

Classroom management

  • Since the apparatus used in this experiment is not expensive and probably readily available inmost schools/colleges, it may be appropriate for students to work individually or at the most inpairs.
  • Prior to students carrying out the experiment, arrange a maximum time to wait for the leafdiscs 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 isworth testing them quickly beforehand.
  • Timings will be shorter if good illumination is provided (e.g. light bank / warm, sunnywindowsill) and if the sodium hydrogen carbonate solution is warm (20 – 30ºC). (If thesodium hydrogen carbonate solution is cold to start with, heat from the lights will probablyincrease 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 allequipment and materials in e.g. a tray system for each student/group. Normal laboratoryapparatus should not be made available in kits but should generally be available in thelaboratory. 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 lightand 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.

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 checkpossible plants beforehand for this experiment.

Preparation of materials

  • 0.2 M sodium hydrogen carbonate (20 cm3 per group/student) made up with water that hasbeen 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 thesyringe.
  • 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 toseal the seam and to close the end at the tip of the syringe. (Over 20 such covers can bemade from one sheet).

Supply of materials

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

Science & Plants for Schools:

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.

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