Stimulation of Plant Growth

Author: Marc Rosner

When young rice plants are infected with the fungus Gibberella fujikuroi, they grow extraordinarily tall. Some 70 years ago, a Japanese plant physiologist discovered that an extract of the fungus produced the same effect. Gibberellic acid is now commercially available from garden supply stores. It has remarkable growing effects on plants—a speck smaller than a grain of sugar can spur the growth of an entire window box. This chemical is useful to florists, farmers, researchers, and home gardeners.

A fungus is a group of spore-producing organisms that feed on dead or decaying matter. Spores are seedlike reproductive units.

Gibberellic acid, a hormone (a chemical substance that stimulates growth or other chemical activity), is sold by nursery supply houses for plant breeding and hybridizing. If you have trouble locating it, you can substitute another chemical sold for the purpose of stimulating growth. Try to find a stimulant with only one active ingredient, rather than a fertilizing blend, in order to study just one variable.


  • 4 packages of pea seeds (same variety)
  • water
  • paper towels or sawdust
  • bowl
  • notebook and pencil
  • 32 paper drinking cups
  • metric ruler
  • sifted topsoil or potting soil
  • pen and masking tape (for labeling cups and bottles)
  • 7 dropper bottles with a capacity of approximately 100 mL (available at drugstores, medical supply stores, or housewares stores. Ideally, they should be marked on the side with cubic centimeters or milliliters.)
  • metric balance
  • gibberellic acid or other growth-stimulating plant or chemical
  • thermometer
  • fluorescent lamp or plant lamps
  • meterstick or tape measure
  • graph paper
  • electronic balance or postal scale (optional)


  1. Germinate 45 to 50 pea seeds by putting them inside moist paper towels or sawdust. Keep them warm and dark, with a bowl inverted over them to keep in the moisture. Only the seeds that germinate will be used in this experiment.
  2. Do not select ungerminated seeds. You are studying the effect of the stimulant on growing plants, not on germinating seeds—although you could design an experiment to do that, too.
  3. While the seeds are germinating, start your notebook. You will record every detail of your experiment, including (a) the date you bought the gibberellic acid or other growth stimulant (in case the compound should deteriorate with time), (b) the date on which the peas were placed in the towels for germination, (c) the temperature, (d) the date of planting, (e) when the acid was first administered, (f) the date of the second treatment, and so on.
  4. When the peas have germinated, fill 32 paper drinking cups to within 1 cm of the rim with sifted topsoil or potting soil moistened just enough to form a crumbly lump when a pinch of it is squeezed.
  5. Place one sprouted pea in each cup so that the tip of the shoot is pointing up and flush with the surface of the soil.
  6. Arrange the cups in eight groups of four and label according to the concentration of gibberellic acid the group is to receive: 5%, 0.5%, 0.05%, 0.005%, 0.0005%, 0.00005%, and 0.000005%. One group of four cups is reserved as a control and receives only tap water. Label these cups "control."
  7. Stimulation of Plant Growth

  8. In one of the dropper bottles, dissolve 2.5 g of the gibberellic acid in 50 cc or 50 mL of tap water. Label this bottle "full strength (5%)." This is approximately a 5 percent solution by weight. The gibberellic acid will not be sold in pure form, but will be mixed with some inert substance.
  9. In a second dropper bottle, prepare the second dilution by putting 5 cc of the 5 percent solution in the bottle and adding enough tap water to make 50 cc. Label this bottle "0.5%." Each cubic centimeter of this solution contains one-tenth as much gibberellic acid as the 5 percent solution.
  10. Inert chemicals in products have no effect. They are there as filler or are left over from the production process.
    The old name for commercial gibberellic acid is Brellin. If you use Brellin no. 10, each milliliter contains 5 mg of gibberellic acid; hence each cubic centimeter or milliliter of the 5 percent solution will contain a 0.25 mg of acid.
  11. Place 5 cc of the 0.5 percent solution in a third bottle and add water to make 50 cc. Label this bottle "0.05%."
  12. Continue this process until seven dilutions have been prepared. Label each bottle to indicate the dilution it holds: 0.05%,0.005%, and so on (see step 5). Fresh dilutions must be prepared for each treatment, because the acid gradually loses its activity in solution.
  13. Stimulation of Plant Growth

  14. To eliminate the influence of variations in environment during the experiment, place the growing plants in a dark room in which the temperature does not vary appreciably from 20° C. The plants should be exposed daily for 11 hours to a fluorescent lamp or plant lamp of at least 40 watts placed lengthwise above the cups at a height of 60 cm. Each experimental plant should receive a treatment with 10 cc of the appropriate fresh acid dilution, as indicated by its label, every 48 hours for at least 14 days. No water or other solution should be administered—except to the four control plants, which receive 10 cc of tap water. The acid should always be administered in a uniform manner. Pour the dilutions on the soil near the base of the plant.
  15. Record the height of each plant daily, beginning with 0 cm on the day the experiment starts, when the shoots are flush with the top of the soil. Make a table in your notebook with columns for recording the date and the height of each plant.
  16. Plot graphs of the plant growth, either for each individual plant or for averages of each group. Construct similar tables for the number of leaves or other measurable quantities you identify.
  17. Stimulation of Plant Growth

    At the end of the experiment, you can measure the final weights of the plants with an electronic balance.
  18. As an extension, you can explore the effect of gibberellic acid on the growth of other plant types, such as algae, molds, mushrooms, mosses, or ferns.


Down to Earth: Garden Secrets! Garden Stories! Garden Projects You Can Do! by Michael J. Rosen (Orlando, Fla.: Harcourt Brace, 1998).

Green Thumbs: A Kid's Activity Guide to Indoor and Outdoor Gardening by Laurie Carlson (Chicago: Chicago Review Press, 1995).

Let's Grow It: Funstation by Brenda Walpole (San Diego: Advantage/Silver Dolphin, 1998).

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