How does The Light Spectrum Effect on Your Plants Growth?

As you can see from figure 1, this is a very small range. In fact, it is less than 1 percent of the total spectrum. Photosynthetically active radiation (PAR), or photosynthetic photon flux density (PPFD), is the range of light that can be used by plants to photosynthesise. However, because the PPFD is a summation of all photons in the 400-700nm range, two very different spectral distributions can have the same PPFD. This means that there is no one-to-one relationship between PPFD and spectral distribution. It also means that when we compare light sources, we need to consider spectrum distribution data as well as PPFD.

How a plant senses light/spectrum

As well as providing the energy for photosynthesis, light also acts as an information source for plants. Different light spectra give the plant an indication of its environment and therefore how it should survive, and hopefully thrive and reproduce. In this sense, the composition of the light is as important as the total quantity of light used for photosynthesis. The light spectrum in the range of 300 to 800 nm causes a developmental response in the plant. Additionally, UV and infrared (IR) light are known to play a role in plant morphogenesis.

A plant gains information from the light that reaches it by means of special pigments, called photoreceptors. These photoreceptors are sensitive to different wavelengths of the light spectrum.

Blue light (400 – 500 nm)

Under the condition of Blue light, photosynthesis on the plants is not as strong as red light ,  but it could help chlorophy A/B to absorb more light among the rang of  400~500nm, promote its root well-developed and produce more synthetic proteins and amino acids in the early stage of  plant’s growing.  Therefore, appropriate blue lights can make the plants grow more evenly and healthily.

Interaction between red (600 – 700 nm) and far-red (700 – 800 nm) light

Because red and far-red light have a higher wavelength, they are less energetic than blue light. Combined with the profound influence of the red-induced phytochromes on plant morphogenesis, relatively more red and far-red light is needed for plants to develop.

The Pr phytochrome has a light absorption peak at a wavelength of 670 nm. When the Pr absorbs red light, it is converted to the Pfr form. The Pfr form acts the other way around – when it absorbs far red light at a peak of 730 nm, it converts into a Pr form. However, because Pfr molecules can also absorb red light, some of the Pfr molecules are converted back to Pr. Because of this phenomenon, there is not a linear relationship between PSS and the ratio of red to far red. For example, when the ratio of red to far red light exceeds two, there is barely any response in the PSS and thus plant development is not affected. It is therefore better to speak about PSS than the red to far red ratio of the light.

Influence of the light spectrum on flowering

Flowering is also influenced by the Pr and Pfr forms. The length of time for which Pfr is the predominant phytochrome is what causes the plant to flower. Basically, the levels of Pfr tell the plant how long the night is (photoperiodism). As the sun sets, the amount of far red light exceeds the amount of red light. During the darkness of the night, the Pfr forms are slowly converted back to Pr. A long night means that there is more time for this conversion to happen. Consequently at the end of the night period, the concentration of Pfr is low and this will trigger short-day plants to flower (see figure 4).