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United States Department of Agriculture

Agricultural Research Service

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Photobiology
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Communicating With Plants

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Photobiology in Plants

Plants depend on many environmental factors for survival. A plant's survival is, among other things, directly dependent upon light. In addition to using light for photosynthesis, plants determine information about their surroundings by the quantity and color of light that they receive.

How does a plant "know" what season it is?

Plants know what season it is according to the day length. They have a chemical called phytochrome that helps them measure the periods of light and darkness. Plants know when to flower and when to produce fruit according to the length of day. Some plants, such as clover plants, attempt to survive the winters and some do not. In order to survive the cold winter, clover plants store their energy in their roots instead of trying to grow taller and produce leaves. As the days get shorter in length, the clover plant will begin to store energy in the root to be used for early shoot growth the following spring. In this way, the amount of light they receive in a day tells them what they need to be doing to survive.

How does a plant "know" there are other plants around?

Plants compete with one another for light, so they need to know when other plants are around them. Although plants do not have eyes, they are able to sense that other plants are nearby. They do this through the light that they receive. When light hits the plant's leaves, some of the light is absorbed and some of it is reflected. The reflected light is different and is sensed by the other plants. As the light is sensed by the neighboring plants, they begin to try to outgrow each other.

How does a seed "know" when to germinate?

In order for a seed to sprout, there are certain environmental conditions that have to be right. Two factors that are very important for seed germination are proper temperature and water. For some very small seeds, light is a requirement for germination. By germinating only when there is light present, the seed is ensuring that it is near the surface of the soil. This is important for smaller seeds because they lack the energy that larger seeds have to push through a deep layer of soil.

These are a few examples of how light regulates the behavior and actions of plants in their different environments. In a sense, plants "know" what is going on around them. Humans can communicate with plants through light.

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Last Modified: 8/12/2016
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