Understanding the spectrum of light that plants can see and use is imperative as up to 90% of plant genes are regulated by light. Each spectrum of light uniquely influences how a plant grows and how quickly it develops, all of which ultimately impact plant morphology and rates of photosynthesis. Promising new developments are aiding in our understanding of how light can influence the terpene and cannabinoid content, maximizing the true expression and medicinal value of the plant.
The McCree curve has been our guide to understanding the range of light (400nm to 700nm) that directly drives photosynthesis. Further studies show that spectral and temporal light recipes help direct plant growth and development as well as regulating the different biochemicals that affect plant physiology. Certain spectrums impact stretching and internodal spacing (red light) and bulking (blue light), while others have little to no direct impact on photosynthesis (green/yellow/orange)..
Photomorphogenesis is the development of form and structure (not occurring from photosynthesis) within plants.. It is regulated by different photoreceptors that manage structural developments such as height, leaf size, and flowering. These changes ultimately effect photosynthesis through the architectural changes that it causes to take place within the plant. As an example, while green light has little photosynthetic value, recent studies are proving out the discrete effects it has on plant biology.
Sunlight isn’t purple so why do some LED lights only have blue and red? There are two good reasons for this: (1) greenhouses many times only need light for photoperiod purposes and blue/red LED were the first good quality colors accessible; (2) red LED has the highest ppf/w values and it looks good on spec sheets and fills in DLI requirements (on paper) the cheapest way possible. Unfortunately you can’t use blue/red LED effectively for indoor cultivation as it is missing a huge portion of the sun’s spectrum.
Adhering to the McCree curve has led many to ignore indirect benefits of certain wavelengths of light, such as those longer than 700 nm. In 1957, Robert Emerson discovered that when plants were simultaneously exposed to both deep red (660nm) and far red (730nm) wavelengths, the rate of photosynthesis was far higher than the sum of the deep red and far red light separately. When used in synergy with deep red, far red light reduces the dissipation of absorbed light as heat, increasing photosynthesis rates.