Light is essential for plant growth, and for many plants the quantity, duration, and quality of light a plant receives determines a complex chain of events that affects when flowering, seed production, and even dormancy occurs. Temperature plays a role in modifying many of the metabolic processes dependent on exposure to light.
We've all heard the term photosynthesis. Contrary to popular belief, photosynthesis does not make plants grow. Photosynthesis is the process by which plants change light into energy. Particles of light called photons excite the electrons located in chloroplasts causing a chemical reaction resulting in the formation of carbohydrates from carbon dioxide and water. Oxygen production is a by product of the process.
The amount of photosynthesis that takes place generally fluctuates during the day, and is also affected by temperature as well as the intensity of the light. Most activity occurs in the morning and late afternoon.
Plants convert carbohydrates produced by photosynthesis into energy by using the process of respiration. It is respiration that results in plant growth. It releases the energy stored in the carbohydrates to produce new plant tissue. Unlike photosynthesis which uses carbon dioxide, respiration uses oxygen and produces water and carbon dioxide as byproducts. Respiration is not dependent on light and takes place at night as well as during the day.
Plants do not use the full spectrum of sunlight. They absorb only three wave lengths -- red, yellow, and blue. Green light is reflected and is the reason plants look green to us. Blue light affects vegetative growth. The combination of red and blue light encourages flowering. This fact becomes useful when growing plants indoors under lights. Cool white fluorescent bulbs produce blue light and are great for growing non flowering plants or seedlings. Warm florescent bulbs or incandescent bulbs produce a redder light and are better for flowering plants. Bulbs marketed as full spectrum grow lights are generally costly and produces no better result than warm fluorescents.
For years scientists thought the number of hours of light a plant received affected flowering and growth of certain plants. Plants were referred to as long day, short day, or day neutral, depending on their needs. We now know that it is not the number of hours of light a plant receives, but rather the number of hours of uninterrupted darkness that controls flowering and growth. Short day plants, such as poinsettias and chrysanthemums, require at least 12 hours of uninterrupted darkness in order to flower, while day neutral plants are not affected much by day length. Long day plants that include many vegetables require less than 12 hours of darkness in order to flower and produce fruit.
Temperature and Light
Temperature is a major factor affecting how plants absorb light. During periods of high temperatures, the rate of respiration outstrips the rate of photosynthesis, causing plants shut down. Growth slows and in some cases produces unwanted results, such bitter taste in lettuce. Low day time temperatures slow photosynthesis, reduce energy use and increase sugar storage. This can result in lower yields and produce changes in preparation for winter dormancy. For optimum plant growth day time temperatures should be 10 to 15 degrees higher than night time temperatures.