While many are familiar with the role of light in photosynthesis, little is known about how day length, also known as photoperiod, can affect vegetative growth and reproduction of a plant. When the plant experiences uninterrupted darkness, typically at night, it uses this period for essential developmental processes such as dormancy, shoot growth as well as producing flowers.

Based on the absorption of either red (625-760 nm) or infrared (760- 850 nm) wavelengths of radiation, a photo-reversible pigment called phytochrome, is able to distinguish between different times of the day. As the phytochrome is very sensitive, just a brief flash of light is able to stimulate physiological conditions similar to that of during daytime and disrupt the development of the plant.

 

 

Especially for trees that are sensitive to day length, artificial lighting can cause them to perceive a longer daytime and hence, alter their flowering patterns. Additionally, it prevents trees from achieving a state of dormancy which is critical for withstanding harsh winter conditions.

Compared to older mature trees, younger trees have a higher tendency to grow longer naturally because of greater vigour. Hence, they are more likely to suffer from cold injuries as a result of growth prolonged by artificial illumination.

 

 

In Singapore, trees are planted along roads and highways to reduce carbon emissions from vehicles. However, these trees are planted at close proximity to streetlights. While Singapore does not experience winter, these trees are vulnerable to air pollution and water stress. Exposure to prolonged lighting and hence longer periods of photosynthesis, can result in stomata in leaves continuing to stay open for extended periods of time, absorbing more air pollutants and losing more water in the process.