Maximize Cannabis Growth @ 700, 1,500 or 2,000 µmol/m2/s? And, What About CO2?

How 'high' must Photosynthetic Photon Flux Density (PPFD, µmol/m²/s) be to induce peak photosynthesis in Cannabis? In 2008, Chandra measured a maximum rate of photosynthesis at 1,500 µmol/m²/s. In 2015, Chandra conducted a similar assessment and measured a maximum rate of photosynthesis at 2,000 µmol/m²/s. So, problem solved... right? Not necessarily.

Chandra, 2008.

Chandra's 2008 Cannabis analysis is a two part study. First, he measured the photosynthetic response to a range of light intensities at various temperatures. The light and temperature combination that produced peak photosynthesis was then applied to a series designed to gauge the photosynthetic response to different concentrations of CO2.

Part 1:

• Cannabis strain: "high yielding Mexican" sativa (in Chandra's other studies, "high yielding" means a high level of THC).
• PPFD range: 000, 500, 1,000, 1,500 and 2,000 µmol/m²/s.
• Temperature range: 20, 25, 30, 35 and 40 C.
• CO2: 350 µmol/mol (defined as ambient).
• Humidity: 55 % (+/-5 %).
• At 30 C, the rate of photosynthesis started to level off above 1,000 µmol/m²/s, then peaked at 1,500 µmol/m²/s.
• At 30 C, water use efficiency started to level off above 500 µmol/m²/s, then peaked at 1,500 µmol/m²/s.

Part 2:

• Cannabis strain: "high yielding Mexican" sativa (in Chandra's other studies, "high yielding" means a high level of THC).
• PPFD: 1,500 µmol/m²/s.
• Temperature: 30 C.
• CO2: range 250, 350, 450, 550, 650 and 750 µmol/mol.
• Humidity: 55 % (+/-5 %).
• Compared to the predefined ambient CO2 of 350 µmol/mol, a concentration of 250 µmol/mol inhibited net photosynthesis and depressed water use efficiency.
• All CO2 concentrations above the predefined ambient of 350 µmol/mol, increased net photosynthesis and improved water use efficiency.
• At 750 µmol/mol, net photosynthesis increased by 49.4 % and water use efficiency improved by 111.4 % over measurements observed at 350 µmol/mol.
• The photosynthetic rate and water use efficiency produced at a CO2 concentration of 750 µmol/mol, don't represent peak responses. They appear to represent plateaus.

Chandra, 2015.

In 2015, Chandra repeated aspects of his 2008 study. This time, he measured the photosynthetic rate of four different Cannabis strains in response to a range of light intensities:

• Cannabis strains: "high THC yielding drug type" sativas (HPM, K2, MX and W1).
• PPFD range: 000, 400, 800, 1,200, 1,600 and 2,000 µmol/m²/s.
• Temperature: 25 C (+/-3 C).
• CO2 350 µmol/mol.
• Humidity: 55 % (+/-5 %).
• All four strains demonstrated their highest rate of photosynthesis at 2,000 µmol/m²/s. These results are not peak responses. They appear to be plateaus.
• Water use efficiency among all four strains started to level off above 800 µmol/m²/s and peaked at 1,600 µmol/m²/s.
• Above 1,600 µmol/m²/s, water use efficiency of K2 decreased, while W1, HPM and MX plateaued.

What's the answer? 1,500 or 2,000 µmol/m²/s?

Although a variety of Cannabis strains appear to thrive at very high levels of Photosynthetically Active Radiation (PAR), Chandra's analyses are constrained by his materials and methods. In both 2008 and 2015, Chandra used an LI-6400, 6400-01, 6400-02 Portable Photosynthesis System. The LI-6400-02 only produces light at 670 nm (+/-10 nm), so Chandra's results were exclusively induced by a narrow sliver of red light. Although biochemical processes peak at or near 670 nm, a broad spectrum like that of the Zenith Bud Cultivator is known to potentiate growth. Repeating Chandra's research with a broad spectrum may validate his results, but until then, Chandra has not yet fully rationalized the real life utility of broad spectrum light intensity as it applies to growing Cannabis.

What about 700 µmol/m²/s?

Putting aside the limitations of Chandra's results, reaping an average intensity of 2,000 µmol/m²/s with natural light isn't possible. And, although maintaining 2,000 µmol/m²/s over a typical 18 hour light cycle is possible with artificial light, it may not be cost-effective. So, what's a realistic light intensity that's less than 2,000 µmol/m²/s? It seems that a practical maximum has already been determined. When Chandra studies Cannabis, he maintains a crop of test plants and relies on 700 µmol/m²/s in the following setup:

• Area: 335 to 350 square feet.
• Lights: 14 1,000 watt HID's with cooling fans attached to each.
• Light height: 3 to 4 feet from the canopy.
• PPFD: 700 µmol/m²/s (+/-24 µmol/m²/s) at the canopy.
• Light cycles: 18/6 and 12/12.
• Temperature: 25 C (+/- 3 C).
• Humidity: 55 % (+/-5 %).
• Containers: 30 cm diameter x 28 cm high.
• Growth medium: 1:1:1 of topsoil, sand and manure.
• Ambient CO2 is assumed, because it's not mentioned.

Although the above doesn't probe the limits of Cannabis growth, I assume Chandra utilizes this setup, because it's an effective use of resources that produces a reasonable crop. So, if you replicate this environment and target 700 µmol/m²/s as a light intensity average, your grow has as much potential as Chandra's.

But Questions Remain.

With a slight modification, the question still remains, "What level of broad spectrum PPFD will induce peak photosynthesis in Cannabis?" To answer this, a broad spectrum light needs to be used to confirm/contribute to Chandra's research.

A question of greater importance, "How do broad spectrum light intensities and CO2 concentrations affect the photosynthetic rate of Cannabis?" Exploring this question may be more significant, because understanding responses to a range of PPFD and an associated range of CO2 will allow professionals to consider costs and benefits of investing in lights, gas or both. For example, this research could provide information to resolve and exploit a question like, "Will elevated concentrations of CO2 combined with a 'sub-maximal' broad spectrum light intensity of 700 µmol/m²/s potentiate photosynthesis?"

Improving Cannabis growth may be more cost-effective if coaxed by pumping gas, not by burning more lights. Many plants grown under moderate PPFD, develop greater biomass in an environment supplemented with CO2. Moreover, it's well known that elevated CO2 directly affects flowering plants by increasing the number and size of flowers. Light intensity is certainly critical, but elevated CO2 appears to further leverage cultivation. So, the overarching question is, "What combination of broad spectrum PPFD and CO2 induces maximal photosynthesis in Cannabis?"

Digging into the Details.

The above isn't an exhaustive exploration. If you know of additional scholarly Cannabis-specific research, please post the associated link in the comments. In the meantime, I urge you to personally examine Chandra's research and post comments, concerns, challenges, etc.

• Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions (Chandra, 2008).
• Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2 (Chandra, 2011).
• Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L. (Chandra, 2011).
• Light dependence of photosynthesis and water vapor exchange characteristics in different high THC yielding varieties of Cannabis sativa L. (Chandra, 2015).

Happy growing from Zenith LED Grow Lights.

Words and images by @SuperFunker.

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