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Hidden Light Signals: How the Red:Far‑Red Ratio Controls Plant Growth · Hydroponics Daily

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  Hello and welcome back to Hydroponics Daily, your go-to podcast for everything soilless cultivation.
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  I'm your host, Dr. Russell Sharp, and today we're talking about a very important
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  environmental variable that is often overlooked,
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  and that is the red to far red ratio in your grow room or your greenhouse or
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  anywhere where you're growing plants.
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  Often we think about light levels, but the light quality is very important,
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  so we're going to do a deep dive on the red to far red
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  ratio which scientists have discovered is very
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  critical in how plants grow and it's
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  why some grow lights are ideally suited for
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  growing basil whereas whereas those same lights
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  are terrible for growing things like legal cannabis so
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  we'll explain more in more detail or i'll explain more in more
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  detail as we go along often you'll see lighting manufacturers
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  oversimplifying things saying you know it's got high blue
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  light for compact growth or red light for photosynthesis but
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  it's actually the ratio of red light to far red
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  that's more critical it's often
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  abbreviated to capital r colon capital
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  f capital r so red to far red ratio it's critical
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  for steering your crop development and often you're hearing
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  a lot more about crop steering in many different crops so
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  plants perceive light not just as energy like the
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  chlorophyll absorbs it and it photosynthesizes but more
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  specifically they have photoreceptors that sense
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  the surroundings and the most important one is a group of pigments called phytochrome
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  and phytochrome is not involved in photosynthesis in harvesting it but it does
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  absorb red light and far red light it's an it's a pigment that absorbs both
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  and can switch its structure depending on which one it.
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  Absorbs and then that stimulates metabolic processes and signaling further around the plant.
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  So as you saw, the red to far red ratio can tell a plant if it's full sun,
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  if it's shaded by other plants, if it's near the sunset.
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  So high red to far red means open daylight,
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  no shade, and more far red singles competition
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  because the red will have been absorbed by
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  the chlorophyll and as it goes through the canopy and so
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  you the plants will receive more far red than
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  red and so a shaded plant will grow differently to an unshaded plant
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  it will have thicker leaves bigger
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  leaves it will grow often many
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  plants it will grow long and spindly spindly to
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  to get out of the shade or it can be a sign of dusk there's much more far red
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  at dusk and so plants perceive that it's now coming to dusk and it's going to
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  go night soon and so that can be a critical signal as well so the red to far
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  red ratio at the end of the day length can be important so they can change a
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  growth strategy of the plant.
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  In a greenhouse or a vertical farm we're not limited to just the sun spectrum
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  and the light coming through canopies and things like this we can adjust the red to far red ratio.
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  Used to be the case back when I was doing my PhD that really the
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  only way of doing that was through filter films so you could
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  have polytunnels with different filters on or glass with different filters
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  glass with different filters was very expensive and
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  and the poly tunnels were not much well
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  they were cheaper but they were they weren't as good they didn't they only
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  mildly affected the ratio of red to far red
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  ratio whereas now you can get lights that just have
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  the one wavelength that you're interested in emitting so i've got some lights
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  in my gray room that are just red at 660 nanometers and you look at their spectrum
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  the peak is just at that one wavelength or just a far red or just in the blue
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  so you can massively change the red to far red ratio and it's not just red light in general,
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  you have to be careful. There's one specific wavelength, 660 and 720 to 730 for far red.
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  So there's specific wavelengths. It's not just red in general.
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  That's important to note.
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  So if you're looking at a light spectrum, then look at 660 and 720.
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  Supplementary LEDs can add, you can manipulate them and you can change the LEDs.
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  So grow lights now, you can really have
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  a massive effect on the red to far red ratio and include
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  blues for compact growth and greens to
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  open stomata so there's a whole host of other things you can do with
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  the spectrum you can still use selective grazing or inter canopy lighting or
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  under canopy lighting as well if you're looking to increase the plant height
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  such as increasing by increasing far red or by reducing canopy height by having
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  red light so as a as opposed to using growth regulators and things like this,
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  so light recipes you'll often hear referring to light
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  refugee recipes depends on the crop you're growing so if
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  you're growing lettuce and leafy greens too much far red can cause
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  excessive stem elongation it's not what
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  you want if you want a marketable head of lettuce however
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  if you're growing basil especially if you're growing micro greens as i'm growing
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  at the moment you don't want them to be too short because the compost or the
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  grow media will get in the way of cutting them and they won't look as big so
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  you want more far red in one where you're growing basil.
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  Tomatoes and cucumbers, far red can improve light penetration,
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  canopy structure can accelerate flowering as well and then in ornamental crops
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  flowering in species like petunia and chrysanthemum can be strongly enhanced
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  by end of day far red treatments.
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  So a flowering crop you can get better flowering by just giving a spurt of far
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  red at the end of the treatment.
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  And then perhaps one of the most critical and powerful demonstrations of the
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  importance of this is seeds and germination.
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  Certain species use phytochrome status as a germination trigger.
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  So far red light inhibits germination and then red light promotes it.
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  So if you get a lettuce seed, normally things that promote.
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  Plants need to germinate it's oxygen moisture and warmth
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  you give them all three they'll germinate but in some plants
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  they need light as well they are photoblastic
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  i think that's the word i've googled that just off
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  the top of my head yeah and so if you give lettuce plants
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  far red and then put them in the dark they won't germinate
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  but if you give them red light and then put them in the dark they will
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  germinate and that's because the plant the phytochrome has been activated so
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  that's really powerful and that's the way that phytochrome was
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  discovered and then we found out about things like its
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  effect on stem elongation and flowering later on so
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  recent research has shown that far red is not just a signal but contributes
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  to photosynthesis as well so by extending the use of light spectrum through
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  what's called the emerson enhancement effect far red photons can increase photosynthetic
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  efficiency when paired with red light so we used to think of it as just being
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  this useless wavelength that wasn't actually useful because we can't see it really with our,
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  our eyes far red it's you can just about see
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  it it's it's not invisible to the naked eye but it's
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  pretty dull we don't perceive it very well i think
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  one of the areas that can really be exploited and probably could be exploited in
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  the next few years by growers is end of day far red treatments giving plants
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  a short pulse of far red at dusk used to be really difficult to do this but
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  now it's really easy with modern led lights you could even have a grow light
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  that was triggered to do this automatically so especially for flowering in long
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  day crops promote elongation in young transplants.
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  And reset phytogram status to mimic natural dusk so
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  you can much more natural grow growing conditions so obviously
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  it has to be applied carefully lettuce growers for instance you want to avoid that
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  but for basil crops could be a good one and i
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  think this is where i struggle when you see a
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  lot of led grow lights that are marketed for
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  legal cannabis growers or tomato
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  growers for indoor growers that contain far red light as an
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  as a as a benefit and i think why.
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  Would you want far red light on that light
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  because it's just going to increase stretch and the
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  grower is going to be reaching for the growth regulators and i think we should be
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  avoiding the use of growth regulators so maybe there's
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  i'm not a light manufacturer maybe they found in studies
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  that actually produces a better crop i don't know but for me it seems
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  a bit counterintuitive to include far red light in
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  in the in led grow lights for grow tents and
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  probably that's been borne out by the fact that one of
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  the main companies that that did include far red has
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  now gone out of business so let me
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  let me know your thoughts on the inclusion of far red
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  lights in in lights for indoor growers especially for
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  legal cannabis growers because that's seems a
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  bit bonkers to me so we said that phytocremes are the primary sensor
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  and then downstream effects involve changes in
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  auxin distribution one of the key flower hormones gibberellins
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  and transcription factors and things like this and these cascade
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  and change stem elongation and leaf expansion
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  flower initiation after that so that's been
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  hydroponics daily for today the red to far red ratio
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  i might do another couple of podcasts about spectral quality when the use of
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  green lights as safe lights for going into your grow tent when you're trying
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  to produce short day lengths and the effects of blue light and green light on
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  photosynthesis and plant growth that should be interesting as well okay thanks
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  very much for listening and i'll see you again tomorrow goodbye.