Bruce Bugbee has never grown a high THC cannabis plant – Utah University only grows CBD strains. He also sells Apogee light meters, so never forget what his TRUE agenda is. To sell more light meters! He also contradicts himself a lot . . . but I digress.
There are some missing pieces of info in some of the above posts, namely that those high PPFD levels are being acheived with supplementary CO2. If you try growing at 1500-2000 PPFD in average atmospheric conditions (400PPM CO2) instead of 1000-1200PPM CO2 your plants are going to suffer.
A lot of people think that more CO2 facilitates more efficient photosynthsis – and it does – but the real benefit is that CO2 quenches chloroplasts (prevents them from overheating) so that they can absorb more photons. And that's why you can hit your plants with higher PPFD levels.
PPFD is not only CO2 dependent, it is strain dependent. Sativas can handle higher levels of light than indicas. They have skinnier leaves – less surface area – and a more open structure that allows light to penetrate instead of capturing most of the photons in the upper canopy, where it is concentrated. Large-scale defoliation makes things worse, because it increases light concentrations in any given area as there are fewer fan leaves to absorb the photons and protect the stems – which turn red from stress pigments (anthocyanins) produced in response to the added light stress.
Researchers only
think too much red light causes bleaching – they haven't proven it yet – but if it does, it could be as simple as the RGB ratio oversaturating the pgiments in chloroplasts with too much red light. In other words, chloroplasts are made of pigments that absorb different colors, but once those pigments start to saturate, excess photons are converted to heat instead of being photosynthesised and so leaf temperatures start to increase.
Most grow lights have 20% or less blue, but many have 50-60% or more red light, so it is feasible that this heavy red weighting is providing more red photons than can be photosynthesised compared to blue. If the light had a higher blue weighting, then we can assume that excess blue photons are going to bleach a plant faster than excess red photons, becaue blue photons carry about 50% more energy than red photons – which means each blue photon has 50% more latency.
The difference in "quality" between high red and high blue or UV lights on cannabis can be attributed to several schools of thought:
Cell contraction (UV, blue) negatively affects yield. If a plant produces the same amount of THC but less yield, the THC percentage gos up.
Cell expansion (far red, or rather increased far red:red ratio) positively affects yield, which has the potential to decrease THC levels if the plant does not produce more cannabinoids along with the extra yield.
But what if we combined the two? High levels of red – and especially far red – with moderate levels of blue and UVA? Do we see an increase in both yield and potency? Would a comparison between natural sunlight – which contains UV and high levels of far red – and indoor LED light – which usualy lacks these spectra – be a good comparison?
The fact is, outdoor cannabis – if grown in optimal conditions – will be more potent than indoor cannabis:
https://ecs-botanics.com/weed-grow-outdoor-vs-indoor/#:~:text=They've executed this experiment,, higher in THCv, etc.
Why? UV. And it doesn't have to be UVB, either – sunlight only has a very small percentage of UVB, as most of the UV is at the UVA end of the spectrum.
We've done our own tests that show the addition of UVA compared to no UVA increases THC levels without impacting yield. However, the addition of extra blue photons
does affect yield, as we have seen when comparing a ~3000K light to a ~3500K light.
But is it because of the cell contraction properties of blue light that yields suffer or is it something few people have thought to discuss? Namely, if you keep a plant compact, light can't penetrate to areas that are shaded compared to plants with more open structures that are going to grow bigger because they get more light to more areas of the plant?
Could it be as simple as that?
