How Does the Spectrum of LED Grow Lights Affect the Plant's Growth?

Although results depend on other factors, you can follow a general rule of thumb when using the spectrum of LED grow lights to elicit different plant responses.

The following is an overview of how each band can be used for horticultural purposes so that spectral strategies can be tried in their growing environment as well as in selected crop varieties.

Ⅰ. UV wavelengths (100–400 nm) of LED grow lights

The UV band is not outside the PAR band, which may provide new applications for horticulture that are not yet well defined.

The most obvious is that UV radiation has effects on human health. We all know that prolonged exposure to UV rays will cause sunburn while short exposures usually lead to tanning. In this regard, plants and humans respond similarly to UV light.

Plants can be damaged by exposure to UV radiation like humans. Plants also naturally produce protective compounds to mitigate tissue damage from UV. Plants may become darker or more purple due to UV light. Research has shown that UVB light can increase the content of essential oil and phenolic compounds in some herbal species.

The potential of UV from LED grow lights includes increasing leaf color and thickness, as well as resistance to environmental stresses, pests and fungi. The amount of UV required to achieve these potential benefits has not been clearly defined. In addition, the risks associated with UV have not been well quantified.

Ⅱ. Blue light wavelengths (400–500 nm) of LED grow lights

Blue light has obvious effects on plant growth and flowering. In general, blue light can improve the overall quality of many green leaves and ornamental plants.

The amount of blue light required to maintain normal plant development is minimal. In terms of tunable spectrum lighting strategies, if equate the red light with the car engine, the blue light will be the steering wheel.

When combined with other spectral bands, the blue light of LED grow lights can promote plant compaction, root development and production of secondary metabolites. Blue light can be used as a growth regulator to reduce the need for chemical plant growth regulators (PGR). Also, blue light can improve the overall health of the plant by increasing chlorophyll accumulation and stomatal opening (facilitating air exchange).

In addition, blue light can promote other secondary metabolic compounds associated with improved flavor, aroma and taste. For example, blue light treatment has been shown to improve terpene retention in some cannabis varieties.

Blue light with high intensity (> 30 μmol·m-2·s-1) can inhibit or promote flowering for sun-sensitive crops. Blue light with low intensity (<30 μmol m-2 s-1) cannot modulate flowering so that it can be safely used at night to affect the other plant characteristics listed above.

Ⅲ. Green light wavelengths (500–600 nm) of LED grow lights

Since chlorophyll does not absorb green light as easily as other wavelengths, many people write off green wavelengths as they are less important for plant growth. Compared with blue and red lights, the low absorption rate of chlorophyll makes most plants green. Leaves typically reflect 10% to 50% of photons in the green band according to different plants.

Studies of green light in crop production have concluded that green light is important for photosynthesis, especially in the lower leaves of plants, which is the opposite of the hypothesis. About 80% of the green light is transmitted through the chloroplast while the leaves absorb about 90% of the light and transmit less than 1% of the red and blue light.

What does this mean? Chlorophyll reaches its saturation point when there is sufficient light and no longer absorbs red and blue light. However, green light can still excite electrons within the chlorophyll molecule deep inside leaves or in the chloroplast in the lower part of the plant canopy. Therefore, green light can improve the efficiency of photosynthesis under strong light conditions, which can potentially increase crop yields.

In addition, the ratio of green, blue and red wavelengths of LED grow lights signals to the plant the canopy position of leaves. It can lead to morphological changes to maximize light absorption. Green light also plays a role in regulating stomatal aperture (opening and closing plant pores that enable air exchange).