I'm Bruce Bugbee, president of Apogee Instruments and professor of Crop Physiology at Utah State
University. In this short video, we're going to talk just about UV photons and what they do to
plants. Apogee makes a couple different sensors for UV. It makes a SU-200, which is just for UV.
Some of the extended range now start to measure UV. And most importantly for UV
is a spectroradiometer. And they're more expensive, but if you're really doing UV research
you really need to buy the more expensive instrument. Now let's take a look at this
because it's very confusing to people. Here's our what we call the electromagnetic spectrum,
and the two key points are 400 and 700 nanometers. And this is visible light, and it's been well
discussed in previous videos so we'll just draw a box for this. Now as everybody knows,
these colors have different effects on plants. They're not all equal, but for photosynthesis,
they're close to equal. Suddenly right at 400 nm we call this UV light. But the wavelengths
of UV are very different, and so we give letters to them. And let's put UV in blue. So if we go to
320 nm. Now some people say this is 315 nanometers – the transition. Right here this is called UV-A,
and it's just “A” the letter A because it's close to the visible. This radiation is… LEDs
can produce this UV radiation pretty effectively, but it is much less powerful to a plant
than the next group of wavelengths. And now I'll show these in blue. There's a band right here.
This is “B” UV-B radiation. And then finally we get a band down here,
which is UV-C. So this is pretty much review for people that study UV: “A” is fairly
benign, “B” is a lot more powerful, and “C” is really powerful/very damaging.
Let's take a look at two things: 1) how much of these wavelengths are in
sunlight, and 2) how do those photons cause sunburn? Because we know that sunburn in people
is caused by these. So let's take a look at sunlight first. I'll put that in yellow.
Sunlight starts… this wavelength right here by the way is 280 nm. Below 280 nm all of the
radiation from the sun is filtered by our atmosphere. The sunlight starts here. And
if we're going to make a curve for sunlight, it's vanishingly small… and get this in yellow here…
it's vanishingly small. And then right about 380 nm starts to go up. And then
there's a big jump right at 400 nm. And it curves… starts to look like that – sunlight.
So this little tiny curve/graph like this looks like there's hardly anything there.
But these photons are hundreds and thousands of times more powerful than these photons. So
the trace of radiation here is a very big deal for light. Then it starts to jump up.
How about sunburn? We have weighting factors for sunburn... and let's put those in white…
The waiting factors for sunburn start very high here, and they look like that. This is
called the erythemal response curve. And a huge weighting factors. One photon in UV-B is equal
to 100 of UV-A. It's tremendously powerful for causing sunburn. The glass in greenhouses
filters out pretty much all of the UV-B. So if you worked all day inside a glass greenhouse,
the glass is kind of like SPF 60 in terms of the sunscreen effect. You could get a
sunburn inside a greenhouse, but you'd have to be working a long long time to get it. So these
photons are not at all the same big weighting factors. And you can see why you need to know
are you on this curve? Are you right here on the curve? Are you right there? Are you right there?
Enormously different. How does this look for plants? This is the erythemal response curve.
When we graph this for plants, the curves look really similar to sunburn curves. There's a series
of plant growth curves depending on what we're interested in. Some are DNA damage. But they all
come down something like this. And the exact shape is important, but they all exponentially
decrease. UV is so damaging down here, we want to use it very sparingly.
But up here, it can have some beneficial effects. It can help make plants tougher. Plants make
their own sunscreen. So you give them some UV, they synthesize pigments in the leaves and they
protect themselves against UV photons. So that's a beneficial effect. But these photons in here
that are close to the visible light can be beneficial to plants. And we can use them
in various ways: help keep plants short, help keep them make the cuticles thicker,
make the layers on the plants thicker. The same thing happens here, but when we get into these
we need to have extremely small doses. We have done research on UV-C in our laboratory,
and oh my you only have to turn that on for seconds per day and you hope you don't burn
the plants. The “B” we've also studied, and we've burned our share of plants with UV-B
by turning them on one hour, two hours, three hours a day. And then you start to see
burned parts of leaves because the plants couldn't adapt to that much. “A” is far
less powerful, and we can give them a lot more UV-A. So all three of these are potentially
beneficial depending on the dose. The rule of thumb is some UV is good for people,
it's just the right amount. And some UV is good for plants, it's just the right amount.
We got so concerned worldwide about skin cancer and sunburn, everyone’s using sunscreen. Guess
what? The incidence of Vitamin D went up in the population because people were blocking
all the UV. And we need UV light/UV photons to synthesize Vitamin D. So it's a powerful
example of some UV is beneficial for people. It's just not too much. Same thing for plants.
Let's go to one other screen… can I get to another screen… There. Let's take another screen
just to get a clean screen… Here's 400 nm. Here's 700 nm. Same graph. Apogee makes the…
sorry this is not right, I'm getting the wrong model number on here… The SU-200.
This is just a UV sensor, and its response curve looks something like this. It gets mostly
UV-A. It doesn't get any visible – it cuts off in here. This is a useful sensor,
but you’ve got to couple it with what types of UV that it measures. To be able to separately
get UV-B, UV-C, UV-A, and all the weighting factors then you need a spectroradiometer,
which is a powerful tool on this. It’s just more expensive. So this sensor is good,
but it doesn't tell you the exact wavelengths. And of course the other ePAR sensors now
get some of this in here because this is beneficial radiation right below
400 nm that causes photosynthesis. So I hope this helps as a summary of UV light in plants.
We really need to know the wavelengths to be able to tell whether we can have big doses of a
fairly safe wavelength or tiny doses of a really powerful wavelength. But we have electric lights
to give all of these different wavelengths. And we are actively studying these now in my
laboratory at the university these effects of UV. I can tell you our early research is… it's tricky
to get the doses just right because these are damaging wavelengths. But the goal
is to use them to improve product quality. One of the things plants do when they get UV photons
is make what we call secondary metabolites. Secondary metabolites includes cannabinoids,
it includes terpenes, it includes vitamins – a lot of beneficial things for people.
So if we can get the doses of UV just right, we have a potential to
improve product quality. Thanks for listening. We'll talk to you again in another video.