Photomorphogenesis involves plant responses initiated by light stimuli that are not from a specific direction or applied for any particular time period. An example of photomorphogenesis is etiolation, the elongation of plant stems caused by the absence of light.
In this project, you will have the opportunity to learn about the effect of light on the formation of plant tissues. The effects of the quantity and quality of light on etiolation will be determined. You will also look at the effects of light filtered by solar window film and of colored light on etiolation.
Purpose: To determine the effect of light on plant stem elongation.
- 27-ounce (210-ml) paper cups
- potting soil
- 2 saucers
- 8 pinto beans
- cardboard box about 18 inches (45 cm) tall
- masking tape
- Fill each paper cup with potting soil to within 1 inch (2.5 cm) from the top.
- Use the pencil to punch two holes into either side near the bottom of each cup.
- Set the cups in separate saucers.
- Plant four beans about 1 inch (2.5 cm) deep in each cup of soil.
- Moisten the soil in each cup with water and keep it moist during the entire experiment
- Place the cups together near a window.
- Put one cup inside the cardboard box. Seal all cracks in the box with the masking tape to prevent light from entering. Note: To add water to the plants inside the box, open the box inside a dark closet and seal the box before leaving the closet.
- At the end of two weeks, open the box and compare the length, diameter, and color of the stems of the plants grown with and without light (see Figure 14.1).
The plants grown in darkness have a spindly appearance. Their stems are longer, smaller in diameter, and paler in color compared to the short, sturdy, green stems of the plants grown in light. The development of the leaves of the plants grown in darkness is slower than that of the plants grown in light.
Photomorphogenesis is the term used to denote plant responses to light stimuli that are not specifically directional or periodic. The elongation of the stems of the plants grown in darkness is the result of a type of photomorphogenesis known as etiolation. The excessive lengthening of the cells in each stem is caused by the abnormally high levels of auxin and ethylene (growth-regulating hormones in plants).
The paleness of the stems of the dark-grown plants is a result of the lack of chloroplasts. Chloroplasts are cell structures that develop from small, colorless cell structures called proplastids. In the presence of light, the colorless proplastids develop into green chloroplasts. The green color is a result of the development, within the chloroplasts, of a green color pigment—chlorophyll. Chlorophyll is a light-sensitive molecule necessary to accomplish photosynthesis (light-stimulated, energy-producing reaction in plants). In the absence of light, the proplastids do not develop into chloroplasts, which results in the plants' pale color.
Light also affects the production of different kinds of growth hormones in the cells of the plants grown in sunlight. Low levels of these hormones cause the cells to be less elongated. Thus, plants grown with light have short, thick stems compared to the stems of plants grown without light.