Most of us have heard the expression, “One rotten apple spoils the whole barrel.” This activity, designed for classroom or home, will help students understand the science behind this old saying.

Background

Plants grow, develop and respond to changes within the environment and many of these growth responses are controlled by plant hormones, chemical substances produced by the plant that affect cellular processes. Ethylene, a plant hormone that exists in plant tissues as a gas, is released by the cells of many fruits to initiate the process of “ripening.”

An unripe tomato, for example, is hard and green; if you were to eat it, it would taste sour. When the fruit produces ethylene, a series of events occurs that leads to ripening. The cell walls begin to break down, making the fruit softer; the green plant pigment, chlorophyll, begins to break down while other pigments, including lycopene, which gives the tomato fruit its “ripe” coloring, are produced; and complex sugars break down into glucose and other simple sugars, which make the tomato fruit taste sweet.

These days, with produce shipped from all over the world to our local supermarkets, fruits are picked and packaged before they are ripe so that they will not be overripe on arrival. What happens between harvest and table? Many times, fruits are packed in special shipping trucks. En route to their final destination, the fruits are gassed with ethylene within the truck so that they are nearly ripe when they are shelved at the market.

Grocers will often place paper bags near peaches or apples that are not fully ripe and suggest that you place the fruit in a paper bag and close it tightly. Since fruit releases ethylene on its own, sealing it in a smaller space will hasten ripening.

Materials

3 very large zipper-locking plastic bags

1 bunch of 9 green, unripe bananas

3 overripened apples

The activity

Place three unripe bananas into each zipper-locking bag. Place two of the apples with the bananas in one bag and the third apple with the bananas in a second bag. Seal all three bags.

Set all three bags on a table or counter at room temperature where they will not receive direct sunlight. Leave them undisturbed for seven days.

Each student should record daily observations on the color of the bananas in all three groups. At the end of the week, ask the students to draw conclusions about the production of ethylene, based on their results.

The results

Ideally, the bananas with two apples will ripen more than those with one apple and the bananas without an apple will ripen the least. The overripe apples are producing ethylene that hastens ripening of the exposed bananas.

Remind the students that the changes associated with ripening are intended to capture the attention of animals, so they will eat the fruit. Ask them the question, “Why would a plant ‘want’ an animal to eat the fruit it has spent so much time and energy creating?”

Some fruits, such as grapes, cherries and pineapples, do not respond to ethylene; these fruits will not improve or continue to ripen after being picked from the plant. What does this mean for transporting these fruits to market?