Plant Reproduction
What Is the Influence of Light on Plant Growth and Development?
Light Quality, Intensity, and Duration
Sunlight is a necessary trigger for photosynthesis, which produces sugars for the plant. However, certain plants have specific light requirements. Also, variations in properties of light, such as quality, intensity, and duration, may affect plant growth.
The quality of light refers to its color or wavelength. Sunlight consists of visible light, infrared light (IR), and ultraviolet light (UV). Light within the range of 400 to 700 nanometers encompasses all visible light and a small amount of IR and UV wavelengths. Light in this range is important for plant growth and is considered photosynthetically active radiation. The relative amounts of light at different wavelengths within this range are important in determining the efficiency of photosynthesis and plant growth.
In this investigation, you will control variables to see the effect on plant growth. How does a plant respond when it receives too little light?
The amount of light received by a plant impacts its growth. While more intense light increases the rate of photosynthesis, light that is too intense can actually damage plant structures, drying them out or burning them.
The duration of light exposure, also called the photoperiod, is important to the growth of plants. The photoperiod refers to the amount of time that a plant is exposed to sunlight relative to the amount of time it is exposed to darkness. Of course, many factors can influence the photoperiod. The length of the day and the amount of shading and location of the plant can increase or decrease the photoperiod. Different plants require unique photoperiods for optimal growth.
Plant Responses to Light
Although they do not have specialized sensory organs like animals, plants are still capable of sensing and responding to environmental factors. The directional growth of plants in response to environmental cues is called tropism. Growth in response to light is called phototropism. One way to show a plant’s response to light is by placing newly planted seeds near a sunny window. The plants will grow toward the sunlight. How does this process work? Different parts of the plant may react to various wavelengths of light. Light falling on the plant reduces the amount of the auxin in its cells. This means that the cells that get the least light contain more auxin. Auxin makes these cells grow longer. The cells on the side with the least light grow more than the ones on the illuminated side of the plant. This causes the plant to grow toward the light. Individual parts of the stem (such as branches and leaves) also orient themselves towards the light in the same manner. Roots also exhibit phototropism, but in this case, it is negative. They grow away from the light. The mechanism is similar, but in roots, auxin works in the reverse manner—it reduces cell elongation. This makes the roots grow away from the light.
Plants respond in other ways to light. For example, some flowers, such as the snow buttercup, turn according to the daily movement of the sun. The flowers turn to face the sun, an important warming mechanism in cold climatic conditions. This is called heliotropism. Many flowers close at night to protect themselves from the cold. This is not the same as tropism because it is not a directional response. This phenomenon is called photonasty.
