Plants and Solar UV Radiation

The spectrum of the Sun contains a strong UV component. In fact, astronauts are warned that unprotected skin that is exposed to sunlight through one of shuttle quartz windows (which are transparent to ultraviolet light) will cause severe sunburn in as little as seven seconds!

On Earth we are somewhat protected by the Earth's ozone layer which absorbs much of the Sun's ultraviolet light. Nevertheless, some ultraviolet light reaches the surface and many living organizims are harmed by this short wavelength solar radiation.

In this actvity we will explore some methods by which the ultraviolet light might be blocked and also observe the effects of direct sunlight on plant leaves which are not adequatly protected from solar UV.

Objectives

  1. To explore the effects of direct sunlight on UV protected and unprotected plant leaves.

  2. To examine and compare the differences in the leaf structure of and indoor and oudoor plant of the same species.

Transparency master 		For this experiment select a healthy green leaf plant that has been grown indoors and has not been exposed to direct sunlight for at least four weeks.

Indoor plants generally forego UV protection in their leaves so that they may absorb as much of the available (longer wavelength) photosynthetic light as possible.

Many indoor plants may be conditioned to live in direct sunlight.

If indoor plants are allowed time to "condition" themselves to solar UV by gradually exposing then to direct sunlight over a period of several weeks they will produce a thick waxy epidermal layer on their leaves. This layer will give the plant UV protection.

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Procedure

  1. Select a variety of materials that may be used to cover the individual leaves an indoor plant. Such material should include sunblock of various SPF values. Also include small swatches of thin fabrics and opaque materials such as aluminum foil. Try several thicknesses of plastic wrap.

    • Cover one leaf with one of the materials you have selected. Repeat, using other materials on other leaves.

    • Expose the plant to direct sunlight for one hour. Then bring the plant indoors and record the appearance of the leaves daily for a period of at least one week.

    • Supplemental: Prepare several plants identically and change only the exposure times in hourly increments. That is, expose one plant for one hour, another plant for two hours, a third plant for three hours and so on. Compare the results.

  2. Choose a healthy indoor plant. On the first day expose it to direct sunlight for only five (5) minutes, but keep one branch (with several leaves) covered, as a control sample, so that it is NOT exposed to direct sunlight (uncover it indoors).

    • On the second day increase the exposure time by 5 minutes (to 10 minutes total). Don't forget to re-cover the control leaves.

    • Continue for 10 days. Increasing the exposure time by five minutes each day. Examine the leaves carefully each day and document the results.

Discussion

  1. Many plants thrive in direct sunlight. They have adapted to the potentially lethal exposure to solar ultraviolet radiation.

    • What are the advantages of such adaptation?
    • What are the disadvantages of such adaptation?

  2. There are many sources of radiation which permeate our environment?
    • List other types of radiation.
    • Which sources and types of radiation are natural (indigenous to the Earth environment?)
    • Which sources and types of radiation are artifical (man-made)?

The Solar Spectrum

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