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Thread: Do plants really grow towards a light source, surprisingly they don't?

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    Default Unanswered: Do plants really grow towards a light source, surprisingly they don't?

    Ahh, Like many I thought they did, but it appears they actually grow away from the dark, here's why.
    It's all down to cell elongation, Light striking one side of a plant actually inhibits cell elongation on that side of the plant, and the side away from the light source, isn't affected by this, therefore this causes the plant to start to bend away from the dark and 'Appear' to head towards the light.
    O.K. I know, it's a totally useless piece of information, as the end result is exactly the same, the plant gets the light it needs, but it just shows a commonly held belief is actually wrong.
    It shows its a 'Mechanical' kind of action, but at the end of the day, it Works!

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    That’s interesting but not sure how it works regarding internode spacing. Use a blue spectrum light such as MH during veg and the spacing between the nodes will be short, use a red spectrum light such as HPS for veg and the spacing would be much larger. So that would indicate the plant is reacting to the light rather than the dark.

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    Check out Technically Speaking: The Shade-Avoidance Response

    Quote Originally Posted by shadeavoidance
    Most plants can tolerate a wide range of light environments. They use different receptors, called photoreceptors, to perceive the different dimensions of light: intensity (or quantity), quality (or spectrum), and duration (or photoperiod). Photoreceptors then regulate the biosynthesis of different compounds that control plant attributes including leaf size, thickness, and coloration; branching; stem elongation; and flowering.


    Plants are innately competitive. When other plants are nearby, their photoreceptors detect changes in the light environment and modify their growth in an attempt to out-compete their neighbors. This phenomenon is called the shade-avoidance response. Understanding this response can help growers to promote or prevent this response, depending on the growth desired.

    Shade-avoidance signals. Two major changes occur when plants are shaded (or encroached) by others: light intensity decreases and the light spectrum changes. The decrease in light intensity depends on the density of the plant canopy overhead as well as the size of adjacent plants. Plant shading decreases the absolute amount of light, but the magnitude of that decrease varies by the color, or waveband, of light.

    When a light particle (a photon) strikes a leaf, it can be absorbed, transmitted or reflected. Most photosynthetic light (blue, green, and red light) is absorbed by leaves, and a relatively small percentage is reflected by or transmitted through them. In contrast, leaves absorb less far-red light and more of it is reflected or transmitted to other leaves. Therefore, plant shading decreases the ratio of red light to far-red light, which is the second signal of the shade-avoidance response.

    Shade-avoidance responses. When plants perceive shade signals, they alter their growth to increase light interception. New leaves increase in size and are thinner, and petioles and stems elongate. Growth of the primary stems increases and branching is suppressed. Therefore, most plants grown under shade (or with shade-avoidance signals) are less compact and have thinner stems and fewer branches.

    The magnitude of the shade-avoidance response increases as the shade signal increases. In other words, extension growth increases as the light intensity decreases and/or the red-to-far-red ratio decreases. Virtually all types of crops have a shade-avoidance response, but the response is stronger for plants that grow best outdoors under high light compared with shade-tolerant crops.

    Implications in controlled environments. The shade-avoidance response is triggered by low light intensity, close plant spacing, overhead hanging baskets, and lighting fixtures that emit at least a modest proportion of far-red light. Supplemental greenhouse lighting usually emits little (or sometimes no) far-red light. In contrast, the light emitted by some low-intensity “flowering lamps” intentionally includes the far-red spectrum. This is because the combination of red and far-red light is most effective at regulating the flowering of daylength-sensitive crops during light-limiting conditions. Therefore, growth under these types of LED fixtures, or incandescent bulbs that also emit a lot of far-red light, will display some of the shade-avoidance responses.

    Generally, shade-avoidance signals can decrease the crop quality of full-sun ornamentals. The effect is less on shade-tolerant plants (begonia, coleus, ferns, heuchera, impatiens, etc.) but can be noticeable when the signals are strong (e.g., under a dense canopy of ferns). The shade-avoidance promotion of extension growth can be countered by applications of plant growth retardants and/or other high-suppressing techniques, such as a cooler day than night, also known as –DIF. However, since light is the energy source for photosynthesis and thus growth, nothing can be done to overcome low-light conditions (other than supplemental lighting).

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    Hi Vlad, in answer to your question, it doesn't if the lights are directly overhead, it's when the light is to one side that this happens.
    I personally, start my seedlings off under a Cloche with a 24/0 light cycle, the LED light is a fair distance from the plants, as I don't want to 'Toast' them. I do find the plants are more 'Compact' and the internode spacing is acceptable. I flip the light to Bloom on an 18/6 light cycle after they declare. Obviously, that would be to 12/12 if they were Photoperiod plants, but these days, I only grow Autos, it works really well for me.
    I hope this answers your question.
    My biggest problem these days is what to do with all the HPS, MH, CFL and Ballast units I have accumulated and no longer use.

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    I prefer my description, which is, on the surface IMHO the same thing, but mine is a lot more simplistic description, I'm not saying it's 100% right though, also, does a Green Leaf absorb Green light to any extent, because some growers use a green light to check on flowering plants while in the dark, good post though.

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    Quote Originally Posted by Techead View Post
    Hi Vlad, in answer to your question, it doesn't if the lights are directly overhead, it's when the light is to one side that this happens.
    I personally, start my seedlings off under a Cloche with a 24/0 light cycle, the LED light is a fair distance from the plants, as I don't want to 'Toast' them. I do find the plants are more 'Compact' and the internode spacing is acceptable. I flip the light to Bloom on an 18/6 light cycle after they declare. Obviously, that would be to 12/12 if they were Photoperiod plants, but these days, I only grow Autos, it works really well for me.
    I hope this answers your question.
    My biggest problem these days is what to do with all the HPS, MH, CFL and Ballast units I have accumulated and no longer use.
    Hi Techead

    sorry but I don’t really understand what you mean. Are you saying that light spectrum (red or blue) doesn’t effect the spacing between the nodes if the light is directly overhead?

    I’m probably just being a bit thick, I’m recovering from a very heavy weekend.

    Vlad

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    Hi Vlad, no problems with clarification, Leds emit Photosynthetically Active Radiation, this is how they score over bulbs where a LOT of the power ends up as heat, I believe a lot of the early research into LED's was conducted by NASA. With a switchable LED light like mine there are two positions and each is independently switched, so you can have either one on or both together, as my light is really a bit big for my size of tent, I just use one or the other, the Veg light is richer in Blue light and the Bloom is richer in Red, but they do have other colours in the mix, some have Infra Red and Ultra Violet, some also have Far Red, albeit not a lot, most plants are very tolerant at the colour of the light, I have seen a decent grow under incandescent bulbs.
    I think in your case stretching i.e. the internodal distance may be caused by an overly long dark period, this or insufficient light causes the stretch, I get over this by leaving the lighs on, 24/0 and at about 30 inches to start, as the grow continues, I drop them down a bit, the amount would be dependant on the plant though and your setup, I sometimes Foliar spray the leaves when they are 18/6 and enter the Dark period, some Auto's can take 24/0 from start to finish though.
    If you have a Smart Phone, there are some Apps out there to measure the amount of Photosynthetically Active Radiation, I'm not saying these are accurate though but are a decent Free Guide to make comparisons. Rather curiously most LED light manufacturers recommend using BOTH Veg and Bloom switches on together, I have always wondered why put the switches there in the first place if they don't recommend having one-off, I do it because I grow indoors and heat tends to build up, hope this helps.


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    Thanks for the wrinkle, gents.

    B


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    Quote Originally Posted by Techead View Post
    Hi Vlad, no problems with clarification, Leds emit Photosynthetically Active Radiation, this is how they score over bulbs where a LOT of the power ends up as heat, I believe a lot of the early research into LED's was conducted by NASA. With a switchable LED light like mine there are two positions and each is independently switched, so you can have either one on or both together, as my light is really a bit big for my size of tent, I just use one or the other, the Veg light is richer in Blue light and the Bloom is richer in Red, but they do have other colours in the mix, some have Infra Red and Ultra Violet, some also have Far Red, albeit not a lot, most plants are very tolerant at the colour of the light, I have seen a decent grow under incandescent bulbs.
    I think in your case stretching i.e. the internodal distance may be caused by an overly long dark period, this or insufficient light causes the stretch, I get over this by leaving the lighs on, 24/0 and at about 30 inches to start, as the grow continues, I drop them down a bit, the amount would be dependant on the plant though and your setup, I sometimes Foliar spray the leaves when they are 18/6 and enter the Dark period, some Auto's can take 24/0 from start to finish though.
    If you have a Smart Phone, there are some Apps out there to measure the amount of Photosynthetically Active Radiation, I'm not saying these are accurate though but are a decent Free Guide to make comparisons. Rather curiously most LED light manufacturers recommend using BOTH Veg and Bloom switches on together, I have always wondered why put the switches there in the first place if they don't recommend having one-off, I do it because I grow indoors and heat tends to build up, hope this helps.


    One Shoe doesn't fit every foot......
    Wow, that’s all gone a bit over my head tbh. I like to use blue CFL for seedlings and cuttings, MH for veg and pure HPS for flower (not mixed spec) under a parabolic reflector.
    The blue spec in veg keeps my plants short and not stretchy. But as you say one shoe doesn’t fit every foot, It’s just how I like to grow and what suits my grow space.

    Thanks for the read.

    Vlad

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    Hi Vlad, not a problem as you seem to have a system that works well for you, I remember when all singing, all dancing LED's were just Red and Blue, then the must-have ones covered a few extra 'Bands', I deffo think the HPS at the end of the grow will surpass a LED, but for me, I couldn't be bothered to keep switching out Lights unless you have a Monster of a Grow Tent?
    Every kind of grow light has its followers, so I don't want to slate any as being better than another, generally, I do think a decent LED is a good compromise across them all, Heat and power consumption is a deal-breaker for me, cost is not the issue or the fact that one score's better than another at certain stages of plant growth, we only have the manufactures 'Blurb' on that, and as I am sure everybody has realized most of that is BS, a friend just bought a new LED that claimed a lot, rather oddly it was predominately white in radiation, I suppose if they made one that matched Sunlight and was Blue'ish when turned on, White about Midday and Red'ish at night, they would claim it to be unbeatable, it might very well be, perhaps even adding Moonlight, mind you, my LED's, (I have four BTW), just in Veg or Bloom, draw just 125 Watts of power, that's just two 60 Watt Incandescent light bulbs worth of power, I don't need anymore yet, but there's alway tomorrow and there are some new Ceramic based lights hitting the market, some Plasma ones appeared a few years back as well, I think I'll pass on them.

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