Photosynthesis Under Solid State Light. Setting the Standards .

Light and Photosynthesis. Quantitative analysis

Through photosynthesis, plants gets the required energy to dissociate water molecules (H2O) into its components (H and O) and build, together with the C from the air's CO2 (carbon dioxide), the organic matter used to build the plants. Some other elements are required in small amounts, and they are uptaked by roots (they are called plant's nutrients). So a plant need mainly air (CO2), water and light to thrive.

The process of photosynthesis requires light, water and CO2 and produces O2 (oxygen). So its possible to measure accurately photosynthesis by measuring CO2 uptaken or O2 produced. Photosynthesis is limited for the required factor (light,water,CO2) which is depleted earlier. On indoor growing, it shouldnt be a lack of enough water, and the grower should provide enough CO2 by air renovation or directly CO2 supplementing if want to optimize the grow, so the main limiting factor is the light available. Its a must on a well designed grow room.

Photosynthesis is tightly linked with total amount of photons absorbed. This concept is the base of all, and it should be clear for any grower. So im going to analyze it deeper:

-Amount of photons. Not of watts, or lm. Plants use photons, so the number of photons is the essential figure to consider. The more the photons which reach the plant, the better (up to a limit).

Its important to note that same energy (for example 1 watt) of blue (450nm) have 33% less photons than of red ones (670nm) (450/670=0.67 : as noted before, energy carried by a photon is inversely proportional to its wl) if we take the amount of red photons as base. If we take the amount of blue photons as reference, then 1 watt of red ones carries 49%, near half, more photons (670/450=149). So very often, producing as more red photons possible is the most effective way of using artificial light for growing plants (if the efficiency of producing 1 watt of each are similar).

This effect is what does that plants have adapted their systems to use 670nm photons the more efficiently, as sunlight reaching Earth's surface has higher number of then than of any other wl, while the higher energy (watts) received is of green light.

-Absorbed. Plants absorb differentially photons of different wl, and it depends too slightly of the light density they are receiving. One of the ways of improving plants lighting is by optimizing the light level and the spectrum in order to get the max absorption (and less reflection) of photons possible.

This way is how cannabis reflect different wl:


Note that although the higher reflection is of green (around 550nm), and thats why see the plant green, just a small fraction of green is reflected back, as 15%, and not all as you can read many times on the net.That plants reflect back all the green light is a false statement.

This false statement is found very often linked to other false one, that plants dont use green photons for photosynthesis. Plants reflect back more green photons than of other wl, as they use them with lower efficacy, but as max its used at half the efficacy of red ones. Lower efficacy of green, yes, but its not wasted at all. That green light is wasted is another false statement. Ill analyze this topic deeper later, as its qualitative and not quantitative analysis.

Quantitative analysis II. Photosynthesis and irradiance

Irradiance refers to the light falling on a given surface. It measures the light density at a given point. The unit used is uE/m2 (per second). Its often found as PPFD (Photosynthetic Photon Flux Density).

Its the radiometric equivalence to the more known iluminance (photometric, for humans, as lm) which is measured on lm/m2=lux. Light concepts preceded by a "i" are referred to the lighted object or surface, and not to the light's source.

The term light's "intensity" is too used for irradiance, but i prefer to avoid using it, due there is other light "intensity" concept which is referred to the light source, which is measured in Cd (candles) or W/sr (optic watt per steroradian). It could lead to confusion, thus ill only use the irradiance term, which not only is clearly referred to the lighted surface, but its too a radiometric concept: radiometric units refers to physical entities (watts, photons) while photometric units refers to how human sense the light (lm, cd), thus they are very misleading when used for plants lighting.

There is a relatively simple way of measuring irradiance with a standard luxometer (light meter) knowing the spectrum of the light: its explained on the "Bulb Comparison" thread which is absolutely complementary to this thread.

Irradiance depends strongly on the point where its measured. It drops sharply with the increased distance and its very affected by how the reflector distribute the light. A concept very similar is when we calculate the average light available, by dividing lm by sq meter (or sq ft). If instead of using the photometric lm, we use number of photons and divide it by sq meters where it's distributed, we get an average uE/m2 figure. Irradiance is too given on uE/m2, but it relates to a point with a given position and distance to the lamp while the calculated uE/m2 is just an average of the light thrown to a given space, without taking into account position or distance.

Sorry for the long introduction to the term irradiance, but its a very bad understood concept which is very important to know how light affect plants, thus i want it very clear. Please ask later any doubt about it: optimizing the lighting of a grow is a task mainly of improving irradiance distribution along the grow room so the better you understand it, the better you will can improve your lighting.

(Im trying to condense on a few post what is often studied along a year. If you dont have previous knowledge about this topic, is very probable you dont understand all on a first read, neither on a second one. But trying to understand it worth, and its not as complex as it seems, just take your time and ask for explanations

Photosynthesis behaves on a typical way depending of the irradiance level al leaves, as this graph shows:

(From The photosynthesis 'light response curve')
(Read too for more in deep graphs and explanations on Eutrophication - light and growth)

There is a first part of the curve which is near linear, meaning that equal increase in irradiance level lead to a equal increase of photosynthesis. The slope of this line determine the maximum photosynthetic rate of the plant. This is called the "light limited" part of the P-E curve (photosynthesis(P) vs irradiance(E)), because what limit the photosynthesis is the amount of light. Along levels of irradiance of this part of the curve (the lowest), more light produces more photosynthesis.

But there is a point where the curve goes flattening, called max Photosynthesis rate point, often noted as A. This is the part of the curve called "CO2 limited", because is internal CO2 concentration in leaves what limit P. Plants is using more CO2 than its able to absorb from the air, thus part of the light cant be used for photosynthesis. The higher the irradiation from this point, the more light is wasted. For cannabis, this point is about 300 uE/m2 at ambient CO2 concentrations: at higher CO2 levels, this point happen at higher irradiance, as well as the flattening of the curve is less pronounced, because plant is able to keep internal CO2 higher.

Finally, there is a point when further increase of irradiance dont get any increase on P, wich is called the "saturation" point. If we still increase irradiation, plant finally protect itself from damage due to excess light and P decreases. There are different ways used by plants to do it, but at really high irradiance plants deactivate photosynthetic systems and chlorophyll is retired from the leaves, producing the effect known as "light bleaching" (because leaves becomes white), which is irreversible (permanent damage).

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There are many practical consequences of this pattern of the P-E relationship:

-Light use is higher as closer to the max P rate point (A) the irradiance we use, thus higher productivity of light (g/uE).

-The most efficient use of light is when we can distribute the light on a way all leaves works on similar irradiance, close to A. This mean zenital lighting (from top) isnt desiderable ideally, as it produces high irradiance at the top of the plant (thus, wasting light) and low irradiance at the bottom (wasting the ability of those leaves to produce more photosynthesis if they have more light available). Although is desiderable a slightly higher irradiance at top than the bottom, because old leaves are less efficient doing P than new ones, in general we must try as even lighting along all the grow volume (in 3 dimensions) as we can. This is one of the main advantages of LEDs over other ways of lighting: small sources of light may be distributed along the grow, without the requirement of use light from top, as with HIDs, due to heat. NASA has achieved up to 35% higher yields using same light just by moving part of the LEDs arrays from top to side lighting. With our plant, doing it not only increases light productivity, but allows to harvest fat buds from the bottom part of the plant instead of small ones.

-CO2 enrichment is little useful at low irradiance levels, but very useful as higher the irradiance used.

This article models C3 plants P-E behavior on each part of the curve based on the limitating factors very deep. I only recommend reading it with strong botanic background, its intended as a model for predicting P-E after genetic engineering.

PS: P-E curve of C4 plants is different due the different way of keeping internal CO2 concentration, but i wont enter on that as Cannabis is a C3 plant

Qualitative analysis. Photosynthesis and spectrum

We understand light quality for the distribution of wl that emits the light. The graph which show it is called SPD (Spectral Power Distribution), many times abbreviated as spectrum.

Reading other threads about lighting, it seems that spectrum is the only parameter which determines the efficacy of light for growing. But its not true, what determines mainly the efficacy is the energy efficiency of the bulb, which determine the amount of PAR watts delivered. But spectrum affect the efficacy by two ways: changing the amount of photons that carries each PAR watt and because there are wl which are more efficients promoting photosynthesis than others.

Opposite to general belief, differences in efficiency promoting P are relatively small, and almost always is below a 20% for wide spectrum lights (white of different tones). Using LEDs is possible to get somewhat higher enhancement, but its up to 50% in best cases (meaning an optimized spectrum may reach 50% more photosynthesis for the same amount of photons than for example, an HPS). But its very important to note than this enhancement may be applied to the number of photons, which, again, is the base of all. If you use half photons with an average 50% more efficacy promoting P you get a final efficacy of just 75%.

If you use half photons, dont mind how good is the spectrum, you never get as much photosynthesis (thus, plant growth). This is very often forgotten by LED grow lights designers, which largely overstates the importance of spectrum alone. This mistake is due many people use "action spectrum" curves, which are curves build from laboratory absorbency of photosynthetic pigments, and not from measuring P promoted by each wl with live plants.

Main general studies about this topic were performed by McCree and Inada in the 70's of last century. Their results have been repeated many times for specific plant species, so their results are widely accepted as valid. Both performed the studies by irradiating the plants with narrow bandwidths of light (2nm) and measuring the photosynthesis (by O2/CO2 changes).

The most used today is the McCree curve, wich shows P for each mol of photons absorbed. You may find it many times plotted as background of horticultural lamp's SPDs:

The curve of Inada, however, shows the P promoted by watt of incident energy, and not of absorbed photons. Thus, it gives together the effect of photon's absorbency and its efficacy promoting P:

1 is the average of 27 herb plants and 2 the average of 7 trees.

Studying both arises some conclusions:

-Maximum photosynthetic efficacy is achieved by red photons. And differences between different red wl are small. The max is at 670nm, but all the range between 610 and 670nm gets very good efficacies. There is a sharp drop in efficacy at 685nm, so is important to try not use LEDs emitting part of the light there.

-Blue and green photons efficacy is similar.

-There is a drop at the end of the blue range, where is the minimum efficacy, and not in the green. 470nm blue leds are the worst, but Royal Blue leds emitting at 450nm or sightly less are very good.

-Use of photons almost all the PAR range is similar. The curves are pretty flat, with the minimum being near 65% of the max. Enhancement due to optimized spectrum maybe nice, but it may be up to an 25% against white sources in best cases (25% of the max: if we take the lower efficacy spectrum as baseline, as that of HPSs, it may be up to 50%. But in the practice is going to be very difficult get enhancements over 20%). Claims of LED sellers of 8x (800%) or higher enhancements are completely off base, as many people has checked, unfortunately.

These curves have a severe limitation: dont take into account possible synergies between differents wl. a famous synergy is known as the "Emerson effect": if a plant is irradiated with 660 and 700nm light at same time, P produced is higher than if its irradiated with them separately. Its possible there are more synergies like that we still dont know. However, there have been studies that has calculated the P of white light by adding the P promoted by each wl showing than measured results are between a 7% error margin of calculated values using the McCree curve. Its a decent error margin which confirms the validity of the method.

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But things are more complex, due photosynthetic systems of plants are very adaptable, and they change to use the light quality they are receiving the best. Plants grown under HPS may have up to double chlorophyll b than those grown using sunlight, in order to use better the yellow light of them. So we always may expect that after plants acclimatization to a given light quality (after about a week under it), P promoted is going to be higher than calculated. This effect still reduce further the improvement margin by spectrum optimization.

Another effect to keep in mind is that most plants have shown better results when exposed to wide spectrum than to a nearly monochromatic one. There are exceptions, as wheat, so we must check it with cannabis, but my personal results point toward cannabis liking wl along all the PAR range, still being a little demanding plant specie in terms of light quality. The main task of LED grow lights researchers is to find the best wl distributions for cannabis, which is still unknown.

In order to show the importance of this task, i strongly recommend to read OPTIMIZATION OF LAMP SPECTRUM FOR VEGETABLE GROWTH. It shows how tomato, which is known to be a very little demanding specie about spectrum (as cannabis), still produce more when is grown under decent amounts of blue and green light (for the same energy used), but it produces more when using higher red proportion than with cucumbers, which need way more green to perform fine.

Other article of that page shows how plants adapted to lighting of different colors perform different than those grown under sunlight. It shows too how different quality of lights get saturated at different irradiance:

That point out that higher content of blue light may worth when using high irradiance. It as well shows how performance under different light qualities is affected by leaves age:

All the articles on the same page worth the reading: International Lighting in Controlled Environments Workshop. Some of the comments about leds are clearly obsolete (its from 1994), but most of the lighting concepts are completely valid and very accurate.

Qualitative aspects of lighting (non photosynthesis related)

Until now, ive talked mainly about photosynthesis. But light affect plants on other ways that arnt related to photosynthesis. Im going to resume the most known:

-Blue light requirement

Most plant's species requires some amount of blue light in order to grow healthy. Cannabis not seem to be one of them. Ive read of MJ grown successfully under LPS, which are yellow monochromatic lights, although with poor productivity, but health. Ive never seen it, so im not sure of this, and i never find any article about it. But it seems cannabis isnt specially demanding in terms of light quality. Not requiring blue strictly not mean MJ wont benefit of using it, of course, as it probably does.

But, independent of it required or not, blue light play a role on some aspects which advice to use it:

-Phototropism. (Check the full page, excellent botany resource online). The movement of plant towards the light. 450nm light (blue) has by far the stronger effect over phototropism. It may be a concern if strong blue light is delivered laterally to plant producing unexpected reactions. Anyway, phototropism effect is stronger on the taller tip of the plant, where auxins concentration are higher.

-Internodal distance control. Blue light has an strong effect reducing internodal length. This effect is exponential at low doses, and gets stabilized at irradiance about 30 uE/m2 of blue light. At 40 uE/m2 near the minimum internodal distances are achieved. As total irradiance levels of 400-500 uE/m2 seems to be the most adequate for MJ growing, the blue fraction to keep internode distances short is relatively small, about 10% of total or still less. But providing some blue light is a must to avoid stretching.

-Stomata aperture. Blue strongly promotes stomata opening, while red light has the opposite effect. Stomatas aperture increases transpiration, thus water consumption, but help the plant keeping internal CO2 concentration high enough. And this strongly affect photosynthetic efficacy (we saw how low CO2 internal concentration is the main limitation of P at moderate-high irradiance). So when growing with LEDs, or any reddish spectrum, there is only two ways of avoiding low internal CO2 concentrations: compensate it with enough blue light or grow in CO2 enriched environment.

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Probably this is the main reason to use moderate amounts of blue when growing with LEDs, as previous cited factors advising to use blue only requires it at minimal amounts. On a CO2 enriched environment, probably percentages of blue around 10% are enough, but if not, percentages at least double that are required to keep internal CO2 high enough. This issue places a dilemma of what is better: if use more blue than required, with lower photosynthetic efficacy, or enrich with CO2, which is costly too but allows to use redder spectrum. This dilemma shows too that there is no a only way of building LED grow lights, but there is different optimal spectrum distributions depending of the setup.

This issue must be kept in mind when trying to find optimal spectrum distribution for MJ growing, so it must be always referred to a given CO2 level.

- Near Red/Far Red ratio

This ratio affect many biological parameters, through Phytochrome sensing. It affect strongly phenotype, by some ways:

-Internodal distance. The higher the ratio R/Fr, the shorter internodes. This is complementary with the effect of blue light, although R/Fr effect is weaker. Strong far red spectrum promotes stretching (incandescents, for example).

-Affects branching. Again, as blue light. More blue or higher R/Fr promotes branching.

-Determine leaves morphology, together with irradiance level. High irradiance and high R/Fr promotes small but thick leaves, with high chlorophyll concentrations, while low irradiance and low R/Fr promotes large but thin leaves with low chlorophyll density. Its called respectively sun and shade adaptation. Shade adaptation works better at low light levels and sun one works better at high irradiance.

On early veg, makes sense to use a lower R/Fr ratio to promote a fast covering of ground to reach an higher light capture, which allows to faster growth. While on flower clearly is better a sun adaptation which allows to use higher irradiance and results on more compact plants. But an interesting question for what i dont have any answer is if this adaptation affect to resin production and tricomes density.

R/Fr ratio strongly affect the Phytochromes (Phy) photo stationary equilibrium, which is what in the last instance determine these effects and which affect too to photoperiod sensing of plants. Short day plants as cannabis has proven to require up to 2 and a half hour less of dark period to continue flowering when exposed to strong Fr environment at the last hour. Manipulation of Phy sensing is still very unknown and seem a promissory field of experimentation on cannabis.

I thought to continue with the technical aspect of building LED arrays, but now im thinking its probably better on a own thread. What do you think? (this mean the thread is now open )

Hope this thread help you understanding light and plants better.

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Inda, most photoperiodical cannabis strains needs between 9 and 10h of darkness to flower. Using flashes of far red at the end of the light period has proven to reduce dark period requirement on other plant species by 20-120min.

So for a typical photoperiodical cannabis strain to flower under a 16/8 photoperiod you need an average reduction of 90min, 60min as best. It lies between the range achieved on other species, but its somewhat demanding to get it at first. I think you should try to obtain first a less ambitious reduction, say 30min, by trying to flower under 15/9 (on a strain first proven to veg under it without far red supplementation). That way you could concentrate first on tuning the intensity and duration of the far red blast. I would suggest to start with 30min far red blast at the end of light period by using 720-730nm LEDs (Edison Opto has them), trying different intensities.

Once achieved the goal, you could go shortening the dark period duration in order to find what reduction in dark period is achievable for cannabis.
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LEDs are improving continuously very fast. They are following the Moore law's of computers (called Heitz for LEDs), that states an improvement of cost/performance of 100% each 18 months. LEDs are accomplishing it since at least 2003. At the end, both are semiconductor based.

The problem is LEDs is yet a relatively immature technology. Computers in the early 80's were on a similar way, improving very fast but starting from a low point of development. Once they reached the point price/performance was good enough to most people start thinking to buy one, on the mid-end of 80's, many new companies joined the market, there was a huge investment on production capacity so prices went down fast, with a race along the 90's for the lower price. LEDs, on the other hand, not only are still on that point where price/performance is not good enough for most people thinking on using them (actually many people dont know them at all), but there are other lighting alternatives. So the point to massive investment for mass production is not here yet, and that is what drives prices down fast. Surely its going to be realized at some point between 2012 and most likely, 2013. Once those investment are done, there is some delay to the factories are already built and offering products, but it is to expect than in 2014 we start to notice the price drop that should continue strongly at least until 2020 or so. LED Industry projections are that in 2015-2016 LED lighting will be more competitive than any other alternative.

This past year there was very good improvements on cool white LEDs. The performance level achieved by top white LEDs (mainly Crees, but followed closely with just a six months delay or so by Phillips and Osram) is now so good that from now on, just small improvements are expected, way smaller in percentage than in the past. Now it is a matter smaller manufacturers with older technology (mainly asian) reach that level of performance to see a price drop race. Red LEDs gave a big jump past year too, and that is very relevant for us.

In short, and applied to growing, price/performance level has improved enough to make LED lighting an option to consider on many situations. Specially along next year, many new modern LED lamps using last technology are going to be launched so growers finally are going to have true good LED lamps working fine and priced decently. However, for the moment probably they won't be cost effective on large grows compared to HIDs.

But for vegging lamps, probably along this year you will have choices way better than any other lighting alternative. And for supplementary lighting on large grows and for the lighting of small-medium grow areas, probably this new lamps mostly still to appear (but expected soon) are going to be the best choices. As last for 2013 for sure that commercial LED lamps are going to be cost effective, maybe except for large grows that maybe will need to wait a little longer.

For 2015, Im sure LEDs lighting are going to be way more cost effective than HIDs and improving each year.
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All the starting posts are based on solid scientific studies. I tried to give a general view of what is well know. In general, concepts explained are basic ideas of Botany.

In order to go deeper in any topic, Google Scholar is your friend For example, in this case search by "blue light internode distance" and add "capsicum pepper" if you want to narrow results.

Anatomical Features of Pepper Plants (Capsicum annuum L.) Grown under Red Light-emitting Diodes Supplemented with Blue or Far-red Light
Light as a Growth Regulator: Controlling Plant Biology with Narrow-bandwidth Solid-state Lighting Systems
Plant Productivity in Response to LED Lighting
Journal of Plant Biology, Volume 49, Number 2 - SpringerLink
http://www.springerlink.com/content/...3/fulltext.pdf
http://hortsci.ashspublications.org/...5/427.full.pdf
knna, Dec 15, 2011 #29

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SupraSPL said:

To add to that, is there a difference between photosynthesis for growing leaf material versus growing flower material?

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Positivity said:

Does appropriate photosynthesis spectrum include benefits of uv and ir levels?

I would assume ir since it's so important for normal flowering

This will be an interesting topic

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Positivity said:

Is that spectrum appropriate for our plant species ?
From which latitude is that spectrum taken ?
Why?
For which type of plants that Standard is intended ?
For which types of cultivation ?
Where and when ?
Spectrum looks good except for flowering. So something like that you could possibly lead to fastest growing plants possible. Veg. Although the level of red may give more stretch in veg than desired..maybe not. The amount of blue would leave airier flowers I'd assume. So a modification of the perfect spectrum would be needed to suit morphological preferences of a particular plant

Din5031-.10....isn't it just a global action spectrum for a averaged vegetation type?

Bout' all I can muster..will read a bit more

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