Subject Introduction

Calcium is an important nutrient needed for plant growth. In animals Ca²⁺ is found in bones and helps to maintain shape and structure. In plants too, Ca²⁺ is important for cell shape and plant form. High levels of free Ca²⁺ in the cell can be very toxic, and therefore cells have many ways to control and maintain a low level of free Ca²⁺. One way is to bind Ca²⁺ in plant cell walls and store Ca²⁺ inside the cell in internal compartments (called "organelles").

If the free Ca²⁺ increases in the cell, energy or ATP is needed to carry or transport the excess Ca²⁺ into the stores or out of the cell. The free Ca²⁺ in the cell is about 1000 times lower than in the cell wall or organelles.

A plant needs to be able to sense changes in its environment and adjust its growth accordingly or respond. As mentioned before, in many instances the change in environment or signal is detected in one part of the plant, but the growth response may occur in another part of the plant. The message therefore has to travel within cells and tissues rather like the way information from various parts of our bodies is carried to our brain.

This brings us to another very important role of Ca²⁺ in living cells. Because the plant controls where and how much Ca²⁺ is present, even very small changes in the amount of Ca²⁺ can be detected by the cell. Changes in the environment or signals (such as an increase or decrease in temperature) cause changes in the amount of free Ca²⁺ in the cell. Ca²⁺ acts as a second messenger so the cell can sense the signal and respond to it.


This particular experiment must be studied at school because it requires the use of chemical solutions.


Questions:

• Is Ca²⁺ required for the shoot gravitropic response?
• What effect do Ca²⁺ chelators ( chemicals that bind up all the Ca in a solution) have on the gravitropic response?
• Does pretreatment with Ca²⁺ or the chelator affect the gravitropic response?
  

• 3-4 week old Sunflower plants
  
• Foam or sponge plugs
  
• Styrofoam holder or a stand to hold the plugs so the plants are kept horizontal.
  
• Dark box, humid chamber or a cupboard- If you use a box be sure that the you place a pan of water inside or damp paper towels so that the plants do not dry out.
  
• Pure water
  
• Solutions
  
• Ca²⁺ solutions (50 mM - 0.735g CaCl.2H2o to 100ml H2O)
  
• Ca²⁺ chelator (10 mM EDTA = Disosium Ethylene Diamine Tetra Acetate)

Procedure (what to do)

1. Sunflower seeds may be planted in potting soil in small pots. Fill the pots with soil and wet thoroughly. Plant the seeds at a depth of about 1/2 inch and cover with soil. Water plants regularly. Keep plants in a greenhouse or on a sunny window sill.
   
   
2. Each treatment will need a control (this is a plant that is handled the same except it is not treated with Ca or EDTA. Pure water is a good control)
   
   
3. Choose plants of the same size and shape for experiments and controls.
   
   
4. Cut Sunflower stems at the soil level and immediately immerse the cut end in water and cut the stem again at a slight angle under water. (This will prevent the stem from drying out).
   
   
5. Soak the foam plugs in the solutions and set them in the holder.
   
   
6. Make a small opening in each plug to insert a plant.
   
   
7. Using a ruler and a marker pen, mark the stem of the plant in inches or cm so that you will be able to clearly see the region of the stem that will elongate or bend.
   
   
8. Set the holder with the plants in the dark, humid chamber so that the plant are vertical, to give the plants time to absorb the chemicals.
   
   
9. One hour later, turn the plants on their side, so that they are vertical (or gravistimulated)
   
   
10. Cover the chamber with a black cloth to prevent any light getting in to the plants.
    
    
11. Check the growth and bending every hour for about 2-3 hours.
    
    
    • Q: Of the three solutions used, is there a solution that inhibits growth or bending?
    • Q: Is there a solution that accelerates growth or bending?
    • Q: What do you think that this might suggest?

Introduction | Tropisms | Phototropism | Gravitropism | Calcium |