What is Photosynthetic Light?

Not all light is capable of helping plants grow, what is the difference?

In the most basic terms, photosynthetic light is what plants need to grow and develop. Photosynthetic light is the fuel for photosynthesis, but it is not as inert as you might imagine. Photosynthetic light comes in different colors and intensities, whether it is from the sun or from a grow light. These traits can cue biological changes in plants, so having the right light color and intensity can take your crops to the next level. Supplemental lighting for greenhouses needs to utilize photosynthetic light.

What Does Photosynthesis Do?

Photosynthesis is the primary energy source for plants
More than 90% of the dry matter of a plant is created from the photosynthetic conversion of atmospheric CO2
Elevations in CO2 can accelerate plant growth and improve photosynthetic productivity
Doubling CO2 concentration has been experimentally shown to increase total crop yield up to 30% in greenhouse and growth chamber conditions
Plants are highly capable of adapting to changing environmental conditions but certain levels are more optimal than others
Photosynthetic uptake of CO2 and production of plant biomass in the plant are temperature and light dependant, with ~25°C being ideal

Photosynthetic Light

Photosynthetic photon flux density (PPFD), measures the number of light energy particles (photons) in the photosynthetic active radiation (400-700 nm) visible light spectrum that falls on a one square meter area in one second.

Light particles (photons) in the 400-700 nm wavelength are able to be absorbed by plants and are also visible to the human eye. Chlorophyll is the primary pigment molecule in most plants and absorbs some red and blue light.

If light wavelengths are too short they can damage pigment molecules in a plant. If a light wavelength is too long it won’t carry enough energy particles to provide the energy needed for the photosynthetic process to activate.

Low PPFD (measured in μmol mol-1 or μmol m-2s-1) irradiance may limit photosynthesis at the leaf surface
High PPFD irradiance in excess of photosynthetic needs at the leaf surface may be a threat to plant metabolism

photosynthetic light absorbance

Absorbance Spectrum of Chlorophyll by Daniele Pugliesi

Photosynthetic Light vs Photoperiodism

In addition to providing energy for the plant, lighting can also influence growth stage maturation in photoperiodism plants like short-day plants. Short day plants will move from vegetative growth into floral growth when they receive 12 or fewer hours of light per day. Not all plants are sensitive to photoperiod, but remember when talking about lighting the goals of providing energy and controlling development are different.

Grow Light Bulb Color Temperature

The “whiteness” of a light is measured by its color temperature. Gases inside bulbs warm as they receive electricity. UV and visible light are self-absorbed by the gas vapor. The type of vapor used in a bulb will affect the amount of heat it emits. This is why HPS lamps have a warmer color spectrum than metal halide grow lamps.

Plants tend to respond better to metal halide lamps during vegetative growth because naturally those colors would be seen by the plant earlier in the natural outdoor growing season. Likewise, plants can benefit more from the warmer tones of HPS lamps during floral growth because those colors and intensities of light are naturally more prominent towards the end of the natural outdoor growing season. Plants have evolved with the sun, so they use those light cues to gain information about the environment and adapt as needed. Poor lighting can lead to maladaptations like hermaphroditism.

grow light color temperature

Color Temperatures of Common Light Bulbs by Mifsud26/Vladslinger

Grow Light Color and Intensity Features to Consider

While not exactly a grow light, fluorescent lamps come in a variety of spectrums. “Natural sunshine” fluorescent lights have a broad spectrum of peaks, where yellow and conventional fluorescent lights have just a few wavelength peaks around 540 nm and 630 nm.

Full Spectrum LED grow bulbs usually have two modes, the lower mode for vegetative growth will use less energy and less red light than the flowering mode. 32 watts per 1 square foot (1 plant) is recommended. Full spectrum LED provides 730 nm (IR), 660 nm, 630 nm, 610 nm, 580 nm, 460 nm, 430 nm, and 410 nm (UV) light wavelengths. LED bulbs produce about 40% less heat than incandescent bulbs so light can be applied closer to the canopy of the plants with less need for cooling and ventilation.

High-Pressure Sodium Lamps and Metal-Halide Lamps are two popular types of high-intensity discharge (HID) lamps used in growing.

High-pressure sodium (HPS) lamps and metal halide (MH) lamps are commonly used in cultivation. Plants respond better to a combination of HPS and MH lamps than to MH lamps alone but respond best to HPS lamps. Approximately two feet of space should be between HPS lamps and plants. With other light sources such as fluorescent lamps, only four inches of space is needed.

When the growing environment is cooled and ventilated, lamps can be moved closer to plants, but if light exposure is too intense and too close, it can bleach or burn the plant.

HPS lamp wavelength spikes at 632 nm, 605 nm, 589 nm, and 568 nm light wavelengths
MH lamp wavelength spikes at 674 nm, 630 nm, 583 nm, 564 nm, 540 nm, 497 nm, and 422 nm

HPS lamp spectrum

Light Spectrum for High-Pressure Sodium-Halide Lamps by LM Roberts

metal halide lamp color spectrum

Light Spectrum for Metal-Halide Lamps by Varistor60

FAQ on Photosynthesis with Hortitech's Biologist Sabine Downer

Hortitech's Biologist Sabine Downer is the person to ask if you are looking for deep details on plant biology. She explains some of the basic scientific concepts of photosynthesis for those who are looking for more information. If you don't want to dive too far into the science of photosynthetic light, you may want to just skip this section.

According to science, what are the best grow light options?

~1500 μmol m-2s-1 PPFD at 25-30°C with CO2 concentration elevated to 750 μmol mol-1 performed best in a 2008 study and 2010 study
Bright indoor light around 1500-2000 μmol m-2s-1 (high PPFD) is best for the plant
For plants in the vegetative growth phase, 400 to 600 μmol∙m-2∙s-1 PPFD can be sufficient but 600 PPFD bulbs/ballasts tend not to perform well so most growers opt for 1000+ PPFD
Flowering plants will benefit from more intense light that is at least 600 to 900 μmol∙m-2∙s-1 PPFD

What is red light?

Red light falls between 600 nm and 700 nm and is absorbed by the chlorophyll b pigment
The brightest red light occurs around 660 nm on the spectrum and will be the most efficient red wavelength for the plant to absorb
Red light is just as good as blue light at stimulating photosynthesis as blue light
Growing plants under only red light produces undesirable elongation
Some amount of blue light is always needed
The ratio of red light to far-red light also influences leaf growth and stem elongation (when far-red light increases relative to red light)
LED lights with little to no far-red light can stunt plant growth

Red light plays a major role in plant flowering! Low intensities of red light (1 μmol∙m-2∙s-1) that interrupt darkness inhibit flowering in short-day plants Lighting for the vegetative growth phase may emit red and far-red light, but those need to be avoided once flowering has begun.

What is blue light?

Blue light falls between 400 nm and 500 nm and is absorbed by the chlorophyll a pigment
Blue light is less productive than green and red light for photosynthesis
Minimal blue light is needed since it prevents irregular stem elongation and leaf shrinking
Plants grown under blue light tend to be shorter, thicker, and have darker green leaves
Plants that are deprived of blue and UV light can have purple leaves
Blue light is also important for regulating transpiration

Typical photoperiodic lighting intensity for blue light is 1–2 µmol∙m-2∙s-1, blue light intensities greater than 20 µmol∙m-2∙s-1 inhibit flowering in short-day plants.

What is photosynthesis in biology?

Photosynthesis is the process that plants use to convert light, CO2 gas, and water into sugar. Oxygen is produced as a waste product and is released out into the environment at night when plants open the pores on the underside of their leaves. Photosynthesis follows the following chemistry equation:

6CO2 + 6H20 + (energy) → C6H12O6 + 6O2
Carbon Dioxide + Water + Energy from light = Glucose and Oxygen

Why is photosynthesis important?

Without photosynthesis plants cannot grow and develop, they must have light from useful spectrums that can be absorbed by pigment molecules (chlorophyll).

Where does photosynthesis occur?

Photosynthesis mainly occurs in the leaves of plants, that is where chlorophyll is most abundant. The fan leaves essentially act like solar panels for the plant.