Set up a prism in class to create a rainbow from sunlight. Ask students to explain the “rainbow.” Position the prism under incandescent and fluorescent light. Ask students to note differences in the “rainbows.” Direct the discussion toward the objectives of the lesson.

 

1 Examine the role of light quality on plant growth.

2 Describe the effects of light quantity on plant growth.

3 Discuss the effects of light duration on plant growth.

_ day-neutral plants _ electromagnetic spectrum _ etiolation _ fluorescent lights _ incandescent lights _ light quality _ light quantity _ long-day plant _ photoperiod _ photoperiodic _ photoreceptors _ phytochrome _ short-day plant

I. Light quality refers to the color or wavelengths of light.

A. The radiant energy given off by the sun is measured on the basis of wavelengths. The range of wavelengths is placed on the electromagnetic spectrum. Visible light, which drives photosynthesis, is a wavelength, in combination with blue wavelengths, influence flowering.

B. Fluorescent lights give off higher levels of blue wavelengths. They are cool and can be placed close to leaf surfaces. Fluorescent fixtures are useful for providing light to seedlings and tissue cultures. Incandescent lights give off red and orange wavelengths. They burn hot and should not be placed too close to plants. A combination of the two types of lights can provide plants with a range of wavelengths. Light intensity is important. The further the light source is away from the plants, the lower the light intensity. For indoor growing, it is very important to place lights close to the plants.

II. Light quantity refers to the intensity of light.

A. The intensity of sunlight varies with the season. Sunlight is typically most intense in the summer and least intense in the winter. The intensity of sunlight is also influenced by latitude and altitude. Sunlight is intense near the equator and at higher altitudes. Intensity diminishes as one moves further away from the equator and to lower altitudes.

B. The greater the light intensity, up to a saturation point, the greater the production of food through photosynthesis.

C. Most agricultural crops require a minimum of six hours of full sun a day. When plants that prefer full sun are grown in shaded locations or indoors, they appear to stretch. This stretching caused by an elongation of the internodes in response to low light intensity is termed etiolation.

III. Light duration is the length of time plants are exposed to sunlight or darkness. The period of exposure is termed photoperiod. Many plants respond to changes in day length. One of the main responses is flowering. Plants that respond to the periods of light are said to be photoperiodic. Plants can be placed in one of three groups based on responses to light duration—long-day, short-day, and day-neutral. More accurately, the plants respond to the length of uninterrupted darkness. For instance, a long-day plant is really responding to a short night.

A. Long-day plants initiate flowering when the nights, or periods of darkness, grow shorter. This happens naturally in the spring and early summer. Some examples of long-day plants include winter barley, red clover, oats, spinach, and winter wheat.

B. Short-day plants initiate flowering as the nights grow longer. These plants are seen flowering naturally in the late summer and fall. Examples of short-day plants include chrysanthemum, poinsettia, and soybean.

C. Day-neutral plants do not appear to respond to the change of day length. Corn, grapes, peas, and tomatoes are day-neutral plants.

D. In order for plants to respond to light, they must have photoreceptors, or light sensitive pigments, to absorb the light. The photoreceptor involved in photoperiodic responses and other responses is a blue-green pigment called phytochrome. Phytochrome is produced in all vascular plants. It has two forms that convert readily from one form to the other upon the absorption of certain light wavelengths. One form, phytochrome, or PR, strongly absorbs red light. In the process, it changes to the second form of phytochrome, PFR. This phytochrome strongly absorbs red light of much longer wavelengths than PR. When it absorbs the far-red wavelengths, it reverts back to PR. PFR is less stable than PR and reverts spontaneously to PR in the dark.

E. Sunlight has more red light than far-red light. During the day, more phytochrome, in the form of PFR, is present than PR, and during the dark, more PR is present. In short-day plants, PFR inhibits flowering. They flower under long periods of darkness when PFR is converted to PR. In long-day plants, PFR induces flowering.

F. Phytochrome can play a role in seed germination. Seeds with a light requirement for germination must be exposed to light in order to have PR change to PFR. Other biological functions influenced by phytochrome include shoot dormancy, leaf abscission, and pigment formation in flowers, fruits, and leaves.

 

 

1. What is the role of light quality on plant growth?

2. What are the effects of light quantity on plant growth?

3. What are the effects of light duration on plant growth?