One Bad Apple: Synchrony in Ripening Fruit
abstract objectives background materials procedure home references credits

Curricular Materials to Accompany the McIntosh Apple Development Poster
Distributed by the Education Committee of the Botanical Society of America
Posted March 2001

Grade Levels: 6-8; 9-12

Objectives:
At the end of this exercise, students will be able to:

• Determine the ripeness of an apple or pear using an iodine staining procedure that tests for the presence of starch
• Generate a hypothesis about the role of hormones in synchronizing fruit ripening
• Design and conduct an experiment to test their hypothesis
• Present their results to the class and combine the results from all experiments to develop a general statement about the causes of synchrony in fruit ripening

Background:
   Does one bad apple spoil the whole bunch?  Does it require direct contact?  Contact through the air?  If so, what sort of mechanism accounts for the communication that occurs among the apples?
    In general, plant tissues communicate using classes of compounds called hormones.  These are defined as substances produced in one location that have an effect on target cells in a non-adjacent location.  In plants, germination, growth, development, reproduction, and environmental response are all coordinated through hormones.  Although most of the main plant hormones are transported in the vascular system of the plant, one class of hormones is transferred in gaseous phase.  This class includes the plant hormone ethylene. 
    Ethylene is manufactured and released by rapidly growing tissues (i.e., meristems) in roots, senescing flowers, and ripening fruit.  For example, the darkened spots on a ripe banana release great amounts of ethylene.  Ethylene has many effects on plants including being responsible for the stunting of plants in high winds or when repeatedly touched.   In addition, ethylene promotes fruit ripening.  Like many hormones, it does so at very low concentrations.  Apple growers take advantage of this by picking fruit when it is not ripe, holding it in enclosed conditions without ethylene, and exposing it to ethylene right before taking it to market.  This process is why we have newly ripened apples grown in temperate North America even in the spring and summer (apples ripen in the fall).
    During the process of ripening, apples convert stored starch into sugar.  In apples and many other commercial fruit, the sweet portion of the fruit evolved as a reward for animal seed dispersers.  When seeds are ripe and ready for dispersal, the fruit converts stored starch, which does not taste sweet, into sugar.  The hormone ethylene initiates the metabolic pathways that lead to this conversion.  The iodine staining procedure you use in this activity tests for the presence of starch.  Iodine turns black in the presence of starch.  So when the fruit is not ripe, starch is present, and the apple will stain black.  As ripening proceeds and starch is converted to sugar, the apple will no longer stain black.  This is best visualized in the Ripeness Chart.

Materials:

• McIntosh or red delicious apples [when possible, choose apples that are in the early stages of ripening, these may be obtained at a grocery store or an orchard; you may also use varieties of crab apples or other local ornamental apples that are in the early stages of ripening.  Store bought apples may already be too ripe to conduct a ripening experiment.  I that case, you may want to use pears that are usually sold with the expectation that they will ripen after purchase.]
• Potassium Iodide (KI) [Available from biological supply houses (e.g., Carolina Biological Supply Company].
• Iodine (I) crystals [Available from biological supply houses (e.g., Carolina Biological Supply Company]
• Shallow glass or plastic trays at least 5 cm (approx. 2 ") deep.
• Knife and cutting board for cutting apples/pears
• Students are asked to design their own experiments using common supplies such as:
  • other fruit like bananas, oranges, pineapples; perhaps at different stages of ripeness
  • plastic and paper bags
  • apple fragrance in shampoo or other perfumed product
  • different sizes or varieties of apples or pears

Procedure:

• Make the following iodine stain recipe:
• dissolve 10 g Potassium Iodide (KI) in 10 ml of water
• when properly dissolved, add 2.5 g of Iodine (I) Crystals; shake the crystals until thoroughly dissolved
• dilute this with water to make 1.1 liters
• this recipe should last for several days and the solution should be stored in brown glass bottles for longer shelf life.

• Familiarize students with the staining procedure by having them cut and stain several apples or pears. 

  CAUTION: Be careful with the iodine stain.  It can stain your skin and clothing.

  • pour the iodine test solution into the tray to a depth of 0.5 cm. 
  • Cut the apple or pear around the middle (in cross section).  Grasp the stem and place the cut section into the stain.  You may want to prevent the cut surface from adhering to the tray bottom by propping it using glass rods or plastic straws.
  • Let the apple or pear soak in the stain for one minute.  
  • Take the apple or pear out and rinse the face with water.   Do not let the rinse water fall into the stain. 
  • Determine a quantitative ripeness score by comparing their apples or pears with the Ripeness Chart.  It will be important for them to be able to assign a numeric score for their data analysis
• Bring in a rotten apple or pear and several ripe ones; ask students what would happen if these were all placed in a plastic bag together; encourage students to propose a mechanism that would cause the response they suggest
• Send students to their textbooks, the library, and web resources and have them develop a hypothesis about how apples or pears communicate and synchronize their ripening; it is important that students are asked to provide a justification for their hypothesis, in other words, they need to explain why their hypothesis is reasonable in light of what they have found out about the biological process of fruit ripening. 
• Have students design and conduct an experiment to test their hypotheses.  They may choose to test many different hypotheses; however, their experiments should all have a control and at least one experimental group and replication with at least five within each group.
• Students need to assign numeric scores to the ripeness of fruit in their experiment; they should summarize their data by calculating averages and comparing their results in a bar graph
• Have students present their results and compare them with their hypotheses in written form, in poster form, or in a class presentation
• After viewing the results from the entire class, have students write a 1-3 paragraph summary of the mechanisms of synchrony in fruit ripening

References:

Books:

Introductory plant biology books will discuss ethylene production and its role in fruit ripening.   

• Raven, P.H., R.F. Evert, S.E. Eichhorn. 1998. Biology of Plants. Worth Publishers Inc., New York.
• Stern, Kingsley R. 2000.  Introductory Plant Biology, 8th ed. McGraw Hill, Dubuque, IA.
• Uno, Gordon, R. Storey, and R. Moore 2001.  Principles of Botany, 1st ed. McGraw Hill, Dubuque, IA.

Websites:

A guide to determining ripeness in apples using the iodine staining procedure is found at the Midwest Tree Fruit Pest Management Handbook. See "Fruit Maturity Analysis".

Credits:

This activity is based on a procedure to determine ripeness in apples presented by the Midwest Tree Fruit Pest Management Handbook. See "Fruit Maturity Analysis".  The exercise was written by Steven Rice, Department of Biological Sciences, Union College, Schenectady, NY.