Plants normally absorb some ultraviolet, or UV, light along with visible light from the sun. Scientists perform experiments to determine the effects of ultraviolet light on plants, including growing plants under ultraviolet light alone, growing them with no ultraviolet light, and comparing these results to plants grown under normal light. According to Texas A & M University, glass greenhouse panes block UV light, while many types of plastic greenhouse panes do not block UV light.
Ultraviolet light activates plant defence mechanisms. According to Washington State University, plants produce 15 different defence proteins with exposure to ultraviolet light. The effect of UV exposure is similar to physically damaging the plant. The plants gradually increase the amount of defence proteins they produce as the amount of ultraviolet light increases.
Ultraviolet light is necessary for plants to produce some nutrients. Lima beans produce vitamins such as beta carotene and other carotenoids when grown with ultraviolet light. Plants produce these antioxidant vitamins to protect themselves from damage from the ultraviolet light. Plants which grow under ultraviolet light also produce less carbohydrates, according to Michigan State University. These compounds are necessary to protect the plant from attacks by mould and insects.
Ultraviolet light affects the growth rate of plants. Pea plants grow the greatest number of pea pods when gardeners block UV light, according to the University of Irvine. Pea plants in UV light alone produce an intermediate number of pea pods. Pea pods grown in regular sunlight produce less pea pods than in either of the other two situations, showing that peas use ultraviolet light to determine how many pods to grow.
Plant size changes along with plant exposure to different levels of ultraviolet light. Pea plants grown under normal sunlight produce the longest pea pods. Pea plants produce shorter pea pods without any ultraviolet light. Pea pods under ultraviolet light alone are the shortest. Ultraviolet light reduces the size of cucumbers, tomatoes, and impatiens, according to Texas A & M University. Size reductions can make the plants easier to transport to a market.
Ultraviolet light affects competition between plants. According to the University of Delaware, an invasive weed, Phragmites australis, produces a toxic acid to kill other plants. Increases in ultraviolet light degrade the toxin, changing it into another type of acid which causes additional damage to other plants. Some plants resist damage from ultraviolet light better than others, giving them a competitive advantage over the other plants.