Bugbee on light spectrum. Excerpts from 2018.

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Grow lighting Masterclass with Dr Bruce Bugbee - Grow Light Spectrum Discussion
The following are excerpts from an interview of Dr. Bruce Bugbee.

This is pretty old now but still interesting. The economic rational for
LEDs has strengthened, making them more appealing for getting some
'green' light spectrum.



“The question is did you find an optimal light spectrum for
plants?”


“There’s not a single optimum, but taking that specific study one
of the things we found was that green photons are fine — people
thought, oh my, the green photons aren’t used as efficiently as
other photons. No, they are when you have a plant canopy and
overlapping leaves that green photons penetrate better, and that
penetration really helps the plant because the lower leaves still get
light. If you have red and blue photons only, the lower leaves don’t
get any light and they start to die. So green photons were helpful—
that was one of the findings.

“Blue photons are very powerful because they reduce cell division
and cell expansion so you get more small compact plants. And
generally small compact plants are very good but you got to plant
more plants to capture all the photons so a plant that’s less compact
spreads its leaves out and captures the photons more efficiently.”

“So are blue photons, are they good or are they bad?”

“It depends on how you capture the photons. Sometimes we want
a lot of leaf expansion and even commercially you could argue
that a salad with big thin leaves would it’s better than a salad with
smaller thicker leaves. We can use blue photons to manipulate the
thickness. For example, particularly in a crop like lettuce. Some
other crops are much less sensitive to those blue photons but
lettuce really is. As far as amount of blue photons to interact with
the total light just 10% might be optimum in low light. But in very
high bright light, there’s more total blue photons so the optimum
might be more like 5% blue.”

“Let’s have a range of percentage blue in the spectrum. What
would you suggest is in the correct range for most of us?”


“I would say five to ten percent is okay and now we’re finding it
interacts with some other wavelengths. If you add far red into this
spectrum, maybe the optimum might be 15% blue, but my high
pressure sodium has 6% blue photons. We didn’t realize this, but
my that was close to optimum! Some said high pressure sodium is
the gold standard because its light looks gold and that is 6% blue
and was close to an optimum for plant growth.”

“To your most recent papers where you just saw light recipes and
the effects of different light spectrum and on growth characteristics.
Why red is not often discussed?”


“Reds really a filler color. Red’s the most efficient color of LEDs,
so you want lots of red because it’s very efficient, but pure red is—
for most species—pure red is not good. So you add a little blue.
And maybe in a cooking analogy, where red is like the potatoes
and then you add this spice to the background of red. It’s not exactly
neutral because it does have some effects. But as a general rule
of thumb, it’s a filler color.”

“I believe that blue, green,and red will perform better than
blue and red only, in most plants from the photosynthetic efficiency
point of view.”


“So some people have said give me an optimum spectrum
and I draw something and they say, ‘Well that looks just like
sunlight!” And I say, yeah, the color the plants have evolved to
use, is to use sunlight. There’s two questions here: one, what
colors result in best plant growth, and two, which ones are the
most economical? Because green light might be fine but there
are no efficient green LEDs. We need green light to be able
to see the plants more naturally, so we add the green light in.
Even though it’s less efficient, we add it in so we can see the
plants and diagnose them more easily.

“We’ve done a lot of studies with blue and red and oh I hate it!
I mean you can’t see any abnormalities in the plants.

“I use some old Chinese proverb that said the best fertilizer is
the footsteps of the farmer. Well, if you’ve got purple LEDs
and the plants look black, those footsteps of the farmer aren’t
working for you. If he could be able to see; so we add green
light so we can see the plants and study and make sure they’re
growing right.”

“From a photon color perspective, if you could make believe
that all spectrum were all the same cost, would it be preferable
to have an element of green with the blue and red?”


“Absolutely yeah. Well there’s the physiological reason that
they penetrate better into canopies so there’s a direct physiological
benefit.”

“Is it because it reflects off leaves and bounces in further or is
it because it goes through?”


“It does both. When a proton hits a leaf it rattles around,
you know, and sometimes it bounces down and sometimes it
bounces up. So it is true that some of those photons bounce
up and they’re lost, but the reflection of green protons are
still 90% absorbed, maybe 80% absorbed. It’s not like they’re
wildly reflected and that it’s true a few do bounce up and those
are lost, so the green is less captured. But that reduction of
radiation capture is not as big as most people think. Fundamentally,
in a canopy, the green photons are still 90 percent
captured.
...

“Now having said that, there are no green LEDs. But white
LEDs have lots of green. That’s the dominant coloring of
white LEDs.”

“So that HPS, which has been proven over the years to be 40
percent green?”


“That’s right.”

“Do you have any things to be discovered about UV A or B?”

“Now on the other end of the spectrum, oh my, I think there is a lot to learn
about the beneficial effects of UV photons for plants! I don’t call it UV light
because we can’t see it. Light is something we can see. So it’s better to say
UV photons. Sometimes we say UV radiation, but it’s technically the number
of photons. That causes the response.

“We now know UV light is good. Some UV is good for people. We’ve got
people putting lots of sunscreen on and, guess what, the incidence of vitamin D
deficiency is going up in the population! There’s a lot of beneficial effects of
UV and the reason we haven’t studied them is there are no efficient UV LEDs.
Those are very, very low efficiency so it hasn’t had commercial implications,
because they’re hard to produce. But that doesn’t matter in the laboratories
or science; we can still add them. And the wavelengths matter a lot: 390
nanometers is really different than 360 or even 320.

“That’s why we call them UV-A, -B and -C, to separate those effects, but they’re
so potent that small amounts of UV can have big effects. It increases cuticle thickness,
for example, and that helps plants be just more rugged and more water efficient.
UVs, they’re really powerful in inhibiting some diseases because they killed micro organisms on leaf surfaces.

“There’s a recent paper ... it was terrific because they showed that pulsed UV
was far more effective than steady-state UV. Steady-state caused the plants
to synthesize UV blocking pigments in the cuticle, so they don’t penetrate the leaf.

“But if they turned them on — and I think their frequency was on one day and
not the next day — they were able to get the UV protons into the leaf, and one
of the things they showed was increase in what we call secondary metabolism,
which is flavor compounds. These phenolic compounds such as anthocyanins
and things like that make up interesting colors and flavors in plants.

“So I think we’re gonna see the addition of UV can be used to really beneficially
improve plant quality and plant durability.”
...

“Do you think we can say that some photons are worth more, with more
value to you for photosynthesis, than another?”


“... It may be from a user point of view that it just makes it easier for them
to achieve their goal keep the plant shorter. That’s a really good reason to
use metal halide, is for their 30% blue photons. That’s a lot of blue protons in
terms of the optimum PPFD level for indoor growers [ed. but the goal might
be shorter plants, thus blue protons are worth more]”

“Would you have a suggestion in terms of just the light intensity?”

“... There’s an economic optimum and there’s a physiological optimum. Outside
plants get up to 2,000 micro moles of protons per meter squared per second.
Well, that’s not economic to provide that much light inside! Especially when a
crop like lettuce grows okay at 200 PPFD.

“But cannabis! We give cannabis a thousand PPFD all the time to enhance
quality of the plant. So that gives you a sense of the range: two hundred to a thousand, depending on the species.

“As a simple rule of thumb we would like to begin with up to four to five
hundred micro moles per meter squared per second for growth of robust
plants. But if that’s not always the economic optimum, if the economic
optimum might be a little less, because the efficiency has to increase the light the efficiency is less.

“It’s a curved line in terms of daily light integral. You would like to be getting
up above 10 moles per meter squared per day to get decent growth. Yes then
20 is even better. 30 and the curve is starting to curve over quite a bit.
At 30 moles per meter squared per day, except for very high light crops like cannabis....”
 
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