In an effort to encourage the work of growers everywhere, Fohse would like to share previously classified company information. The secret aim of our company is to make lights that produce photosynthesis within a plant in the most efficient way possible. The way we do that is by creating LED fixtures that provide tons of photons. We produce a lot of light, plain and simple. And, by increasing the amount of light, CO2, and other inputs in a grow room, you, too, can increase the rate of photosynthesis in your crop.

That’s great news! More light means bigger and better-looking plants grown at a faster rate! In fact, we are still learning the extent you can engender a plant to flourish by increasing the amount of light. That’s why we made the most powerful light on the market.

Now, you may not need that kind of power in your setup, and with our company’s secret in hand, you have all you need to look for the light that’s right for you. But there’s one more thing you should keep in mind. Some lights emit heat in the form of infrared light, and this invisible menace can increase leaf surface temperature, killing your green buds before they can even bloom. It all has something to do with an enzyme known as RuBisCo.

PHOTOSYNTHESIS CHECK-UP

You’re probably familiar with photosynthesis, the way plants convert light into energy. Photosynthesis takes place inside an organelle, called the chloroplast. Chloroplasts are found in any green plant tissue, but it is most abundant in leaves.

Inside the chloroplast, floating around in fluid known as stroma, are disc-like structures called thylakoids. Within the membranes of the thylakoid is chlorophyll.

When a substance absorbs visible light, it’s called a pigment. Chlorophyll is a class of pigments, of which the two most abundant forms are Chlorophyll A, which drives photosynthesis, and Chlorophyll B, an accessory pigment that helps the plant absorb more diverse photons.

The biochemistry of photosynthesis is broken down into two cycles, happening simultaneously.

LIGHT-DEPENDENT REACTIONS

The Light-Dependent Reactions take place in the thylakoid membrane. Photons emitted from a grow light interact with chlorophyll and water, and are converted into chemical energy in the forms of ATP and NADPH. It is in this reaction that the oxygen we inhale is created as a byproduct, and released through little pores in the plant known as stomata.

Photons + H2O -> Thylakoid membrane ->  ATP + NADPH (+O2)

LIGHT-INDEPENDENT REACTIONS

Light-dependent reactions, otherwise known as The Calvin Cycle, take place in the stroma. The enzyme RuBisCo (ribulose bisphosphate carboxylase/oxygenase) creates organic carbon from carbon dioxide in the air, which filters in through the plant’s stomata. RuBisCo takes the carbon dioxide and attaches it to ribulose bisphosphate, a short sugar chain with five carbon atoms. RuBisCo then halves the new chain into two identical phosphoglycerate pieces, each with three carbon atoms, otherwise known as 3-phosphoglycerate. Any plant that creates 3-phosphoglycerate during this process is known as a C3 plant, and that includes cannabis. 

RuBisCo is actually slow compared to most organic enzymes. Typically, enzymes can process about a thousand molecules per second, but RuBisCo only fixes three carbon dioxide molecules per second. Plant cells compensate for its slow rate by having it in abundance. Half of the chloroplast is actually RuBisCo, making it the most plentiful single enzyme on the Earth.

Most of the phosphoglycerate made by RuBisCo is recycled to build the ribulose bisphosphate which is used again in the carbon-fixing cycle. However, one of every six molecules is skimmed off and used to make sucrose to feed the rest of the plant, or stored away in the form of starch for later use.

ATP + NADPH + CO2 -> RuBisCo -> 3-Phosphoglycerate (+Sugar/Starch)

RUBISCO AND TEMPERATURE

When grow-room engineers discuss temperature, they usually reference the ambient room temperature. In most growing environments, however, the leaf surface temperature, or LST, will be higher than the ambient air temperature surrounding the plant. This is especially true for plants grown under HPS lights, which emit infrared radiation that is absorbed as heat by the plant.

Cannabis and other C3 plants have a unique disadvantage when it comes to photosynthetic efficiency and temperature. Under stressful conditions, such as an excessive LST, RuBisCo will accidentally perform its reaction with Oxygen instead of CO2, creating 2-Phosphoglycolate, which is ultimately useless for the plant. This will kick off a process known as photorespiration. To try and salvage the 2-Phosphoglycolate, it will have to react with 2-Phosphoglycolate Phosphatase to give glycolate. Glycolate will then have to react with glycolate oxidase to give glyoxylate, which will then have to react with glutamate glyoxylate transaminase to get glycine. Glycine reacts with glycine decarboxylase complex to give serine, which will then react with serine-glyoxylate-transaminase to give pyruvate, which then reacts with pyruvate reductase to give glyceride. Finally, glyceride must then react with glycerate kinase to give a single molecule of 3-phosphoglycerate, which will finally allow the cycle to begin again. Just to deal with 1 molecule of 2-phosphoglycolate, the cell must waste 2 NADH and 2 ATP. This whole process of photorespiration is long and drawn out, slowing down an already slow enzyme while wasting compounds that the plant could otherwise use, and all because the temperature of the leaves were off.

There are, however, certain conditions under which RuBisCo performs at a higher rate, increasing the speed of photosynthesis, and therefore, the rate at which the plant is creating sugar and starch to grow.

With the right light and ambient CO2 concentrations, the rate at which CO2 is converted by RuBisCo increases as the temperature increases from 5°C to 27°C (about 41ºF-80ºF). While 5ºC technically works, it’s on the low end of a big bell curve. Right around 25-27ºC (77ºF-80ºF) is a goldilocks zone at the crest of the curve where plants are at their highest photosynthetic efficiency. This means RuBisCo is working fast converting CO2, ATP, and NADPH into those byproducts of sucrose and starch, growing the plant at the fastest rate they can sustain. As temperature rises above 27º, the plant begins to enter photorespiration. RuBisCo starts to work too fast, gets confused, and grabs oxygen instead of CO2. As leaf temperatures approach 40°C, the plant burns more carbon than it gains, making the net photosynthesis negative.

HPS’ DIRTY SECRET

HPS bulbs emit a large infrared spike between 800 nm and 900 nm. This infrared spike significantly increases leaf temperatures at the top of the canopy through radiative heat transfer, where most of the infrared light is absorbed. With HPS lamps, the high temperatures are often apparent at the top of the canopy, and all the chilled air pumped into the room to compensate results in low leaf surface temperatures lower in the canopy, creating top-heavy crops.

Since the rate of carbon fixation by RuBisCO is affected by leaf temperature and CO2 concentration, growers with HPS systems have to compensate for excessive heat. They can do this by increasing CO2 exponentially. By increasing CO2 levels to somewhere around the amount that would make humans dizzy, it would increase the ratio of reactants (ATP + NADPH + CO2) to products (3-Phosphoglycerate), allowing plants to continue to fix CO2 into sucrose at leaf temperatures up to about 36°C (almost 97ºF!).

Or, they could decrease ambient room temperature through air conditioning and ventilation. Either way, it will be a hit to the budget and a loss in efficiency.

LEDs have no Infrared spike, and as a result, do not increase leaf surface temperature significantly above the ambient room temperature. In fact, when HPS growers switch to LEDs with efficient heat disposal systems, they can increase the ambient room temperature to achieve optimal leaf surface temperatures for plant growth. LED grown crops also report a more uniform leaf surface temperature throughout the plant, resulting in a more even crop.

Keeping RuBisCo working efficiently is the key to a fast and flourishing plant. It’s a finicky, slow enzyme that can drive the plant into the ground if not properly cared for. But, if kept at the right temperature, with enough light and CO2, there may be no limit to your crops potential photosynthetic efficiency.