University of Illinois Extension


Barbara Larson,
Unit Educator, Horticulture
Boone & Winnebago Counties

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Plants and Light

In the darkness of December, light is important in fact and symbolism. Plants have three basic responses or reactions to light. They are photosynthesis, phototropism and photoperiodism.

Photosynthesis is, of course, the process on which all life on earth depends. Radiant energy from the sun is converted into chemical energy. The energy is stored in chemical bonds in sugars like glucose and fructose.

Phototropism is the plant's movement in response to light. All of us have seen the houseplant that leans toward the window. That is phototropism. Growth hormones are produced which cause the stem cells on the side away from the light to multiply causing the stem to tilt. The leaves are then closer to the light source and aligned to intercept the most light.

The most interesting response is photoperiodism. This is the plant's reaction to dark and is controlled by the phytochrome pigment in the leaves. The pigment shifts between two forms based on whether it receives more red or far red light. The reaction controls several different plant reactions including seed germination, stem elongation, dormancy, and blooming in day length sensitive plants.

Some of our common ornamental plants are day length sensitive plants requiring specific light conditions to initiate bloom. Poinsettia and chrysanthemums are short-day plants. To set bloom, the night (dark period) must be longer than 12 consecutive hours. Some plants are so sensitive that if the dark is interrupted by even a blink of light the plants will not bloom. Conversely, long day plants require over 12 hours of light to bloom. A common long-day plant is the Easter lily. Many plants do not have this requirement and are called day-neutral. Asters have a combination requirement of long-day followed by short-day.

Some seeds are also light sensitive. Germination is controlled by the reaction in the phytochrome pigment. Many lettuce varieties must have light to germinate. Lettuce is packaged and distributed in foil packets to prevent sprouting before planting. Most weed seeds are in this category. Have you noticed how every time you till the soil more weeds shoot up? Weed seeds lie dormant in the soil for years waiting for you to stir up the soil so they get enough light to germinate.

Phytochrome also controls lengthening or elongation of stems. Leggy plants in low light are one example. The light reaction in phytochrome also guides the germinating seedling stem through the soil toward light.

The last photoperiod response is stimulation of dormancy. Several things trigger dormancy, but a major one is the shortening day length. This is critical when we move plants out of the area where they evolved. For example, a sugar maple grown in the north but from southern seed will not become dormant early enough to escape winter cold injury. Therefore it is important to buy perennial plants from seed sources at similar latitudes to our own.

The ways plants respond to light is a fascinating topic. On these dark days remember your plants are reacting to the light or dark in multiple ways.


December 2000 - January 2001: Winter Gardening Tips | Plants and Light | Botrytis (Gray Mold): A Disease for Many Plants | Choosng a Christmas Tree Variety | Key Questions for Garden Catalogs

Past Issues

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