Until now, ive talked mainly about photosynthesis. But light affect plants on other ways that arnt related to photosyntesis. Im going to resume the most known:
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Blue light requeriment
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 sucessfully under LPS, wich are yelow 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 strickly 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 aspect wich advice to use it:
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Phototropism. (Check the full page, excelent 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 unespected reactions. Anyway, phototropism effect is stronger on the taller tip of the plant, where auxins concentration are higher.
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Internodal distance control. Blue light has an strong effect reducing internodal lenght. This effect is exponential at low doses, and gets estabilized at arradiances about 30 uE/m2 of blue light. At 40 uE/m2 near the minimun internodal distances are achieved. As total irradiances levels of 400-500 uE/m2 seems to be the most adecuate for MJ growing, the blue fraction to keep internodes distances short is relatively small, about 10% of total or still less. But providing some blue light is a must to avoid stretching.
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Stomata aperture. Blue strongly promotes stomata opening, while red light has the opposite effect. Stomatas aperture increases transpiration, thus water consuption, 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 irradiances). 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.
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 dilema of what is better: if use more blue than required, with lower photosynthetic efficacy, or enrich with CO2, wich is costly too but allows to use redder spectrums. This dilema 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 refered to a given CO2 level.
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Near Red/Far Red ratio
This ratio affect many biological parameters, through
Phytochrome sensing. It affect strongly phonotype, 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 spectrums 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 irradiances and high R/Fr promotes small but thick leaves, with high chlorofill concentrations, while low irradiances and low R/Fr promotes large but thin leaves with low chlorofill density. Its called respectivelly sun and shade adaptation. Shade adaptation works better at low light levels and sun one works better at high irradiances.
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, wich allows to faster growth. While on flower clearly is better a sun adaptation wich allows to use higher irradiances 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) photostationary equilibrium, wich is what in the last instance determine these effects and wich 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 unkonwn and seem a promisory 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
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Hope this thread help you understanding light and plants better.
knna