March 14, 2007

One Bad Apple Spoils the Whole Bunch: An Experiment on the Plant Hormone Ethylene

Objective

The goal of this experiment is to measure the ripening of unripe fruit induced by the plant hormone ethylene, by monitoring starch levels using an iodine solution.

Introduction

In general, plant tissues communicate using classes of compounds called hormones. These hormones 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.

Experimental Procedure and Additional Information

Used with permission. Copyright © 2002-2007 Kenneth Lafferty Hess Family Charitable Foundation. All rights reserved.

Webb, Sarah. 2005. The color of health. Science News for Kids (March 2). Available at http://www.sciencenewsforkids.org/articles/20050302/Feature1.asp .

ScienceFairZone
Browning in Apples
http://www.sciencenewsforkids.org/articles/20070221/ScienceFairZone.asp

LabZone
Vitamin C Versus Oxygen
http://www.sciencenewsforkids.org/articles/20060614/LabZone.asp

LabZone
Ripening Bananas
http://www.sciencenewsforkids.org/articles/20041208/LabZone.asp

For more science project ideas, go to http://www.sciencebuddies.org/mentoring/areas_of_science.shtml .

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