Light saturation in a greenhouse

Watching a video of Bill Mollison lecturing on trees, I was interested by his description of light saturation- after a certain amount of exposure to sunlight in a day, plants stop photosynthesizing.

This is very intuitive to me, living in North Texas- I can't imagine any living thing thriving on a full day's exposure to the punishing midsummer sun. It feels like something physically pushing on your skin, the light is so strong.

I wonder how this plays into selection of glass for a greenhouse? I know normally greenhouses are for more temperate climates, but according to Bill the amount of light plants receive at the 60-70 degree marks (Alaska, Northern Russia) is great for plant growth! Is there something I'm missing that leads to folks being so picky about light penetration? It seems like the focus is less on the spectrum, more on just total penetration...

I love polycarbonate greenhouse glazing. Because it filters out UV light. Therefore I can move seedlings directly into the greenhouse and out to the field a week later without worrying about sunburn.

My small greenhouse is covered with "suntuff" panels.  They are very clear and work great.  They have been up for several years now. I would like to add "something" that would provide a more diffuse light and maybe a bit more insulative values.    I can also attest to the fact that the sun, and the number of hours of sun, we receive up here during our growing season is really conducive to great growth.  One of my hobbies in the summer is growing giant veggies for entry in our state fair that is held toward the end of August each year.  I've grown 20+ pound turnips and kohlrabi, 50+ pound cabbage, 50+pound rutabaga and vegetable marrows that went over 60 pounds.  For some things you need the genetic potential there in the seed, but some things just grow big.  Turnips, for example, are just run of the mill off the seed rack purple top turnips.  They get more room to grow and a bit more fertilization and care than the garden turnips do.  I can take seed from the same package of cabbage that someone in the lower 48 plants and grow the same cabbage and mine will be close to twice as big, if not larger just because of our climate and 20+ hours of daylight in the summer.

Light requirements also vary a lot by plant species.

Certainly, there is a reason you can buy greenhouse shadecloth that blocks 30,40, 50% or more of the sunlight for those plants that need less in some climates and circumstances.


One of the things that make the alaskan sunlight so very optimal for some plants and veggies is the extremely long day length in the summer.

Hi Cody,

I am not sure I totally understand the question, but here's some background that may be helpful. There are a few different aspects to light:

-intensity (how strong the light is)
-duration (how long it's shining)
-spectrum, often called light 'quality' (what wavelengths are coming through)

Indeed the combination of the first two plays a much greater role in yields and productivity in a greenhouse. (Also, plants use a different type of spectrum called PAR light -- if you want to go deeper you can look that up.)

It's also true that plants can only use so much light. There are curves on this from various labs.

More likely the summer sun at your location is going to have other adverse effects -- plants getting way too hot --  before they reach their photosynthetic potential. Off the top of my head, I remember the curves for most plants are pretty high -- i.e. they have to be exposed to light for much longer than the typical day length for this to take effect, so it's a rare problem to encounter (again, my recollection from a few examples of trials; it varies by plant). It's an inverse curve though: the benefit of each additional hour of sun diminishes as you add more and more.

Some thoughts.

Lindsey

This is an interesting topic to me also. As far as I'm aware any glass will block the UV B rays which is the wavelength that reacts with the cholesterol on our skin to create that very important hormone Vitamin D. Regular glass will not block UV A light though which actually decreases Vitamin D production. Using polycarbonate will block all UV light and I wonder if that is healthier for the plants than having the UV wavelengths split? Seems there are always correlations between our bodies and the rest of nature.....?

This is a very big topic.
To continue from Lindsey:

-Instantaneous light level
 Full sunlight on a cloudless day at noon is about 2000 umol photons m-2 s-1.  That level is higher than the level to saturate photosynthesis for most leaves.  Most leaves, even leaves on plants in high sunlight, are light saturated by around 500 umol photons m-2 s-1.
  A greenhouse would cut this by about 50%, so maximum level in a greenhouse is likely around 1000 umol photons m-2 s-1, maybe a bit more with careful cleaning, materials and design.  1000 umol photons m-2 s-1 is still more than enough to saturate leaf photosynthesis for most species.

 But leaves do not lie flat on the ground, they have an orientation and are usually in a canopy of other leaves that lowers the instantaneous light.
 (Also, humans have a logarithmic response to light; typical room light is ~40 umol photons m-2 s-1, 1/50 the level of full sunlight.  So it is tricky for us to gauge light levels.)
  Exposing a leaf to light above its saturation level often leads to photoinhibition; a decline in photosynthesis or growth rate caused by excess light, which has toxic effects.  In most cases it is not the absolute level of light, it is whether light is above or below saturation.  The saturation level varies with species, developmental history, temperature, nutrient status and prior light acclimation; most leaves have some capacity to acclimate over time to changing light.
  And, saturation of leaf level photosynthesis is not the same as light saturation of growth, nor of production of the desired crop.  Growth usually  light saturates at levels below leave light saturation.

-Photoperiod
  The length of the light exposure in the day.  Plants have many photoreceptors aside from the photosynthetic system, and some are sensitive to very low levels of specific light.  Some plants are able to exploit long photoperiods (tomatoes in Alaska etc.); others are not, and some respond to changing photoperiod to complete their developmental cycle.
  Depending upon species light acclimation can flatten the response of growth to photoperiod somewhat; long low light photoperiods can sometimes drive growth at a rate similar to higher light shorter photoperiods.

-Spectrum
 The colour of the light has regulatory effects upon the photoreceptors, with different plants responding differently.  Some wavelengths (UVB) are strongly inhibitory.

-Photon dose
  Total photons m-2  in a given period.  This can be a factor; it is the product of instantaneous light and photoperiod (actually the integral of instantaneous light over photoperiod).  Think of a laser flash that delivered a whole days light in a ms; the plant would be incinerated.  Then think of a very dim light for a very long time; the plant is unable to grow at all because the light level is insufficient to drive net photosynthesis, so no matter how much total light is delivered slowly there is no growth.  The useful photon dose also interacts with nutrient status, temperature, presence of mycorhizae, species, developmental history etc.

-UVB & UVA effects are complicated, dose and species specific.

sunnily yours, Doug Campbell (Canada Research Chair in Phytoplankton Ecophysiology, Mount Allison University)

Don't remember hearing about light saturation before but in the old days we were told not to use window glass for greenhouses because the iron in the glass (as demonstrated by the green color seen by looking at the pane on edge) blocked some frequencies needed by the plants.  Glass blocks the UV that darkens my "Transistion" lenses but do not know whether that is UVA or UVB.  Not sure how much effect that has as greenhouses were built with regular glass for decades before the iron free glass could be formulated.
In these new days, with my fancy glasses, I have observed that they do not darken outside until the sun is about 15degrees.  That might indicate that a lot of the light in Alaska does not have strong UV at the lower angles.