Light spectrum and its effect on plant growth: Outdoor vs. greenhouse vs. indoor
Growing plants can take many forms, from traditional outdoor cultivation, through greenhouses to indoor cultivation under artificial lighting. Each of these methods offers unique advantages, but also has some disadvantages. In today’s article we will focus on the differences in the light spectrum when cultivating outdoors, in a greenhouse and indoors under LED grow lights.
Plants perceive light using photosensors called photoreceptors. Most of them are tasked with capturing photons of light and converting them into energy during the process of photosynthesis. Plants are also equipped with specialized photoreceptors that function differently from the others, do not always participate in photosynthesis, and some even detect light outside the visible spectrum. These photoreceptors are important because they influence circadian processes, developmental signals, gene regulation and much more.
Different colors of light
To understand the light spectrum it is important to realize that light is electromagnetic radiation that can be characterized both as a particle (photon) and as a wave. Types of electromagnetic radiation are divided according to wavelength and the corresponding frequency. The term “spectrum” originally referred to the color spectrum visible to the human eye (the colors of the rainbow), but over time other types of radiation that humans cannot see were discovered.
Visible light: The visible part of the light spectrum with wavelengths of 400–800 nanometers. The individual colors in the light spectrum are called spectral colors (red, orange, yellow, green, cyan, blue, violet).
Photosynthetically active radiation: PAR (photosynthetically active radiation) overlaps with visible light and denotes the range of wavelengths (400 to 700 nanometers) that plants use for photosynthesis. Most cultivation LED fixtures include only photosynthetically active wavelengths.
UV: Ultraviolet radiation (10–400 nanometers) is dangerous to humans and plants, damages DNA and can cause carcinogenic growth. Most UV radiation is absorbed by the Earth’s atmosphere, but a small amount reaches the surface.
Infrared radiation: Infrared radiation has a wavelength between 760 nanometers - 1 nanometer and is further divided into near-IR, mid-IR and far-IR.
X-rays: X‑ray radiation with wavelengths of 10 – 0.1 nanometers is used in practice due to its ability to penetrate many materials (radiography, CT). It has no relevance for plant cultivation.
Gamma radiation: Radioactive radiation produced in nuclear processes. It has no relevance for plant cultivation.
Growing outdoors: All colors of light
It will not surprise anyone to say that natural sunlight is the most complex and covers the widest possible spectrum. Plants grown outdoors are exposed not only to the visible part of the light spectrum including photosynthetically active radiation, but also to infrared, UV and other types of radiation. While the effects of extremely short or extremely long wavelengths are not well documented in relation to plants, some invisible wavelengths, such as UV and far-red radiation, can be crucial for plants even though they do not affect photosynthesis.
Greenhouses: Absence of UV radiation
