VPD and nutrients absorption

noreason · Nov 17, 2010

Hi ICmaggers :wave:

forgive me if I write something bad

I want to share with you my theory about VPD and nutrients absorption.maybe it's not only a my theory,maybe someone already know it or know something more or more accurate about this.You're welcome to reply,because I want to understand how this kind of stuff work.

VPD (link to wiki)

This is a value take in consideration RH, air temp. and leaf\canopy temp.

To be short,a low VPD value coincide with high umidity

on the other hand an high VPD value coincide with low humidity in the air

VPD is a shortcut,because it takes in consideration two values together: RH and temperature.

This is a little scheme I found in the co2 thread here on IC...it gives you values about VPD.As you can see the ideal VPD is 0.85 and you can reach this value considering the second scheme

Knowing this value we can know how much the stomata are open.

You know the plant (thanks to rubisco) hook the C atom in the co2 molecule just through the stomata.
A low VPD value let the stomata to open wide,so the plant has a great co2 availability.
In the other hand there is low h2o absorption from the roots,because the leaves already have a lot of water inside and need not much h2o due to little transpiration.

Having an high VPD value leads the stomata reduce their opening.This means the plants has little co2 availability but there is a lot of water absorption from the roots due to more transpiration through the leaves.More uptake of water from the roots means also more nutrients absorption,from the substrate to the top of the plants to be used after.

At now I'm thinking about a cycle alternating low VPD period with high VPD period.
This could be helpful because in the high VPD period the plant can absorb a lot of nutrients and then with a low VPD the plants can use those nutes faster due to the more co2 availability.

I wonder if in the night hours when the lights are off,plants usually absorb nutrients or not...but I think yes.Transpiration works also in the night cycle,isn'it?
So,having an high vpd (low RH) during the ''night'' can help the plants to absorb more nutrients to be used then,in the next light phase?

This is only a theory,I have no evidence of anything...but using the logic this stuff make sense to me,so feel free to give me\us your two cents or maybe more

!!! · Nov 18, 2010

I don't think the plant absorbs nutrients at night. The only thing I know happens during the night cycle is that energy from photosynthesis is stored in the roots, and I'm only basing this on something I read on a random 420 forum awhile back.

Rowdy420 · Nov 18, 2010

I think more folks need to take a look at VPD. It will exlpain alot about stomata activity during different times of relative humidity. In my opinion most people run RH to low and basically you are locking the stomata closed to fight against transparation.

Your theory is interesting with introducing both conditions, not sure if if the plant is using the nutes; rather storing and off gasing during night cycle. Would love to see more info on the subject fo shizzle.

spurr · Nov 18, 2010

noreason said:
Hi ICmaggers
forgive me if I write something bad

I have written very extensively about "Air to Leaf Vapor Pressure Deficit" (i.e. VPD) in the past. And my work was plagiarized by Urban Gardener magazine/blog (assholes)!

I can post up much info about this topic, and why we should use VPD and not RH if you'd like me to. I have a simple guide for people that will allow them to reach ideal VPD (i.e. 0.8-1.0 kPa) and close to ideal VPD (1-1.25 kPa) while keeping ADT (Average Daily Temp) high (ex. > 70'F) and keeping RH below ~65%, which is important for growers who use carbon scrubbers (they don't work once RH is ~65%+).

If a grower doesn't use a carbon scrubber than they should shoot for VPD of ~0.8-1 kPa for veg and ~1 kPa for flowering. But if they have a carbon scrubber than they should shoot for ~1.25 Kpa all the time.

To find VPD one needs to know the canopy temp (under shade), and RH, and ideally average leaf temp.

Using VPD is ideal because that allows us to provide ideal stomatal conductance, which in turn allows for ideal rate of transpiration ("E") and carbon fixation; the former in turn allows for ideal uptake (and translocation) of Ca and Bo because they need to be 'sucked' up into roots with water, unlike other elements.

Let me know if you'd like me to help you, I can also upload a thread I posted at a different forum (TCC) with tons of info and good studies, etc. I would prefer to give you my thread because I covered most topics and concerns about using ideal VPD, ex. why to use it, mold issues, etc. This thread would just bring up topics I already covered in my thread, so I would prefer to just upload my thread for people to read which will answer most all questions and concerns.

For veg and flowering I use a VPD of ~1.20-1.25 kPa due to my use of carbon scrubbers (which means I can't allow the VPD to go too low, ex. ~1.0 kPa).

I want to share with you my theory about VPD and nutrients absorption.maybe it's not only a my theory,maybe someone already know it or know something more or more accurate about this.You're welcome to reply,because I want to understand how this kind of stuff work.

VPD (link to wiki)

This is a value take in consideration RH, air temp. and leaf\canopy temp.

You are bit mixed up. VPD is RH, canopy temp and leaf temp; but because leaf temp is hard to accurately measure and also be representative of all leafs, just using canopy temp and RH is fine for most people.

To be short,a low VPD value coincide with high umidity

Kind of, low VPD coincides with higher RH and lower temp.

on the other hand an high VPD value coincide with low humidity in the air

Kind of, high VPD coincidences with lower RH and higher temp.

VPD is a shortcut,because it takes in consideration two values together: RH and temperature.

It's more than a shortcut, it's the only way to quantify what the plant's 'feel' and the effects it has upon stomatal conductance.

This is a little scheme I found in the co2 thread here on IC...it gives you values about VPD.As you can see the ideal VPD is 0.85 and you can reach this value considering the second scheme

Ideal VPD is 0.8-1 kPa, with up to 1.25 kPa being fine. Less than 0.2 kPa means much increased danger of molds, etc. Lower than 0.8 kPa reduces stomatal conductance (in resposne to too great of rate of transpiration or VPD directly) and higher than 1.25 kPa also means reduced stomatal conductance (in response to rate of transpiration or VPD directly).

There are two modes of action a plant uses with VPD, either directly (VPD via. leaf chemicals) or indirectly (via. rate of transpiration); and that depends upon wither or not the plant has been in drought conditions before...

A low VPD value let the stomata to open wide,so the plant has a great co2 availability.

Yes, but VPD below ~0.8 kPa is bad, the stomatal conductance is too high, thus the plant will close down stoma in response, to reduce water loss due to rate of transpiration.

Having an high VPD value leads the stomata reduce their opening.This means the plants has little co2 availability but there is a lot of water absorption from the roots due to more transpiration through the leaves.

Reduced stomatal conductance (due to too high VPD; and to too low VPD) means both reduced Co2 fixation and reduced rate of transpiration.

More uptake of water from the roots means also more nutrients absorption,from the substrate to the top of the plants to be used after.

Pretty much, but only Ca and Bo depend upon rate of transpiration to enter roots; other elemts can freely enter root tissue. The elements/amino acids traveling xylem also depend upon rate of transpiration.

At now I'm thinking about a cycle alternating low VPD period with high VPD period. This could be helpful because in the high VPD period the plant can absorb a lot of nutrients and then with a low VPD the plants can use those nutes faster due to the more co2 availability.

Not a good idea. High VPD means reduced rate of transpiration (and thus reduced uptake of Ca and Bo and translocation of elements via. xylem), and it also means reduced Co2 fixation.

It's best to keep VPD the same, ex. 1-1.25 kPa. Over 1.25 kPa is not good, just like under 0.8 kPa is not good.

I wonder if in the night hours when the lights are off,plants usually absorb nutrients or not...but I think yes.Transpiration works also in the night cycle,isn'it?

Yes and yes. But at night plants respire via. leafs and roots, unlike the day.

So,having an high vpd (low RH) during the ''night'' can help the plants to absorb more nutrients to be used then,in the next light phase?

No. They will absorb more Ca and Bo during light hours because E is increased during light hours. It's better to keep VPD at the higher end, ex. ~1.25-1.5 kPa during night due to incrased chance of fungi attack when it's dark.

This is only a theory,I have no evidence of anything...but using the logic this stuff make sense to me,so feel free to give me\us your two cents or maybe more

Done and done. Yrs, spurr. :tiphat:

noreason · Nov 18, 2010

Thanks all for reply,this is an interesting topic to me so any imput is welcome

Let me know if you'd like me to help you, I can also upload a thread I posted at a different forum (TCC) with tons of info and good studies, etc. I would prefer to give you my thread because I covered most topics and concerns about using ideal VPD, ex. why to use it, mold issues, etc. This thread would just bring up topics I already covered in my thread, so I would prefer to just upload my thread for people to read which will answer most all questions and concerns.

Feel free to post whatever you want.More info I can get the better

If a grower doesn't use a carbon scrubber than they should shoot for VPD of ~0.8-1 kPa for veg and ~1 kPa for flowering. But if they have a carbon scrubber than they should shoot for ~1.25 Kpa all the time.

This is another interesting topic btw.I have a friend using carbon filters...it could help him if you can explain why they don't work under 1.25Kpa

Using VPD is ideal because that allows us to provide ideal stomatal conductance, which in turn allows for ideal rate of transpiration ("E") and carbon fixation; the former in turn allows for ideal uptake (and translocation) of Ca and Bo because they need to be 'sucked' up into roots with water, unlike other elements.

So,my theory about nutrient uptake is wrong except for Ca and Bo?Other macro and micro elements don't need water to climb to the top of the plant...right?

Kind of, low VPD coincides with higher RH and lower temp.

Kind of, high VPD coincidences with lower RH and higher temp.

It's more than a shortcut, it's the only way to quantify what the plant's 'feel' and the effects it has upon stomatal conductance.

right

Ideal VPD is 0.8-1 kPa, with up to 1.25 kPa being fine. Less than 0.2 kPa means much increased danger of molds, etc. Lower than 0.8 kPa reduces stomatal conductance (in resposne to too great of rate of transpiration or VPD directly) and higher than 1.25 kPa also means reduced stomatal conductance (in response to rate of transpiration or VPD directly).

Values under 0.8 and over 1.25 are not good,got it.

There are two modes of action a plant uses with VPD, either directly (VPD via. leaf chemicals) or indirectly (via. rate of transpiration); and that depends upon wither or not the plant has been in drought conditions before...

This is not clear to me (forgive me but I can't understand everything,my native language is not English)...maybe can you spent two mins to explain better please?

Yes, but VPD below ~0.8 kPa is bad, the stomatal conductance is too high, thus the plant will close down stoma in response, to reduce water loss due to rate of transpiration.

Reduced stomatal conductance (due to too high VPD; and to too low VPD) means both reduced Co2 fixation and reduced rate of transpiration.

right

Pretty much, but only Ca and Bo depend upon rate of transpiration to enter roots; other elemts can freely enter root tissue. The elements/amino acids traveling xylem also depend upon rate of transpiration.

So as said before,only Ca,Bo and other aminoacids depends on traspiration?Elements like N P K are not linked to rate of transpiration?

Not a good idea. High VPD means reduced rate of transpiration (and thus reduced uptake of Ca and Bo and translocation of elements via. xylem), and it also means reduced Co2 fixation.

It's best to keep VPD the same, ex. 1-1.25 kPa. Over 1.25 kPa is not good, just like under 0.8 kPa is not good.

Yeah,you're right.But try to consider fluctuation from 0.8 to 1 - 1,25 Kpa,avoiding lower or higher values than this.

Yes and yes. But at night plants respire via. leafs and roots, unlike the day.

So the plant respire via roots in the day...the night also in the leaves,right?

No. They will absorb more Ca and Bo during light hours because E is increased during light hours. It's better to keep VPD at the higher end, ex. ~1.25-1.5 kPa during night due to incrased chance of fungi attack when it's dark.

Because E has effects only on Ca and Bo it's useless keep a low VPD value at night,right?

Thanks a lot for your reply buddy :wave:

spurr · Nov 19, 2010

noreason said:
Feel free to post whatever you want.More info I can get the better

Here is the VPD tread from TheCannaCabana, it's four full HTML pages. I compressed them into a zip file and encrypted it with the passphrase "ilovecanna" (without quotes). I uploaded them to a Russian file host, so it's 'safter' to download IP wise than a US file host:

http://www.file-upload.net/download-3019546/VPD.zip.html

I will post my easy to use guidelines later, I have to head off for dinner soon. The thread I uploaded should answer all your question, and much more. The studies I referenced you can read in full, I could upload them here, or you can use Google to search for them by title.

noreason said:
This is another interesting topic btw.I have a friend using carbon filters...it could help him if you can explain why they don't work under 1.25Kpa

You could use lower than 1.25 kPa. But doing so, while keeping temps around 75'F to 80'F at the same time (for high rate of photosynthesis), means the RH would be at least 65%. And that would stop carbon scrubbers from working.

noreason said:
So,my theory about nutrient uptake is wrong except for Ca and Bo?Other macro and micro elements don't need water to climb to the top of the plant...right?

So as said before,only Ca,Bo and other aminoacids depends on traspiration?Elements like N P K are not linked to rate of transpiration?

No, other elements also travel in xylem, thus transpiration effects the rate of translocation of all of them (in general). However, Ca and Bo rely upon rate of transpiration to be taken into root tissue, other elements can freely pass into root tissue. That is why low RH rooms often show Ca deficiency in plants, the rate of transpiration is too low to allow sufficient uptake of Ca into root tissue. Once all elements are in root tissue they (generally) use xylem to move to top of plant (translocate), etc.

noreason said:
spurr said:

There are two modes of action a plant uses with VPD, either directly (VPD via. leaf chemicals) or indirectly (via. rate of transpiration); and that depends upon wither or not the plant has been in drought conditions before...

Click to expand...

This is not clear to me (forgive me but I can't understand everything,my native language is not English)...maybe can you spent two mins to explain better please?

It's nothing you need to worry about, it's just very specific info about how and why a plant responds to VPD. Here is more info:

Leaf-to-Air Vapor Pressure Deficit (VPD) is basically a way to quantify the difference between water vapor in air outside the leaf and the inside the leaf. VPD controls stomatal conductance (Gs) directly or indirectly, and Gs affects many process in the plant. It has been found that Gs is controlled either directly by VPD, by the "feedforward" hypothesis. In which the leaf/plant senses VPD somehow and probably uses "abscisic acid" (ABA) to close stoma down (i.e, reduce Gs). Or Gs is controlled by the plant when it senses increasing rate of transpiration (E), which is controlled mostly by VPD (higher VPD means higher E). That is called the "feedback" hypothesis, where the plant changes its' Gs depending upon E. The plants use of either feedforward, or feedback hypothesis, seems to depend upon wither or not the plant has been in drought conditions (media water stress) in the past, ie., low media water status. If the plant has been under media water stressed it is thought the feedforward hypothesis controls Gs, otherwise the feedback hypothesis is thought to control Gs.

noreason said:
Yeah,you're right.But try to consider fluctuation from 0.8 to 1 - 1,25 Kpa,avoiding lower or higher values than this.

Yea, flux in that range is fine, in fact, it's hard not to have flux. But for flowering keeping it from 1-1.25 kPa is a good idea if mold or other fungal pathogens are a concern.

So the plant respire via roots in the day...the night also in the leaves,right?

No, plants do very little respiration in day, if they do so they are in trouble, called "photorespiration" and it means greatly reduced rate of photosynthesis and carbon fixation. Normally this is caused form too much light or heat.

Because E has effects only on Ca and Bo it's useless keep a low VPD value at night,right?

No, it's good to keep VPD lowish at night to keep E highish which allows greater translocation of Ca, Bo, N, etc., into tissue via. xylem. The stomatal conductance is reduced in darkness, but it's still wise to keep the VPD from 1.25-1.5 kPa...

Thanks a lot for your reply buddy

No problem, glad to help. This is a confusing topic for most people, esp. someone who's first language isn't English. But you are getting it faster than most people I have tried to teach!

Here is the Urban Gardener Blog post on VPD where the author fully plagiarized my work, I wrote it under the nic "gojo" (from the thread I uploaded for you). I bitched at them for the plagiarism, but they didn't care...assholes!

"Plantworks: Part 1 – Humidity and Vapor Pressure Deficit"
http://urbangardenmagazine.com/2010/07/plantworks-part-1-humidity-and-vapor-pressure-deficit

spurr · Nov 19, 2010

Edit:

I just re-uploaded the files.

noreason · Nov 19, 2010

Your posts are really interesting spurr.I'm understanding a lot of things,thanks.

You could use lower than 1.25 kPa. But doing so, while keeping temps around 75'F to 80'F at the same time (for high rate of photosynthesis), means the RH would be at least 65%. And that would stop carbon scrubbers from working.

just a little ot...why carbon scrubber would not work properly?I'm a noob on this,never used one.

No, other elements also travel in xylem, thus transpiration effects the rate of translocation of all of them (in general). However, Ca and Bo rely upon rate of transpiration to be taken into root tissue, other elements can freely pass into root tissue. That is why low RH rooms often show Ca deficiency in plants, the rate of transpiration is too low to allow sufficient uptake of Ca into root tissue. Once all elements are in root tissue they (generally) use xylem to move to top of plant (translocate), etc.

Are you saying the transpiration affect mainly Ca and Bo and in minor part other elements like NPK because these elements can pass trough the roots in any case?

Ca is an element plants use in a average amount...having low stomata conductance due both to too low VPD and too high VPD can bring problem in its upatake,am I right?

It's nothing you need to worry about, it's just very specific info about how and why a plant responds to VPD. Here is more info:

Leaf-to-Air Vapor Pressure Deficit (VPD) is basically a way to quantify the difference between water vapor in air outside the leaf and the inside the leaf. VPD controls stomatal conductance (Gs) directly or indirectly, and Gs affects many process in the plant. It has been found that Gs is controlled either directly by VPD, by the "feedforward" hypothesis. In which the leaf/plant senses VPD somehow and probably uses "abscisic acid" (ABA) to close stoma down (i.e, reduce Gs). Or Gs is controlled by the plant when it senses increasing rate of transpiration (E), which is controlled mostly by VPD (higher VPD means higher E). That is called the "feedback" hypothesis, where the plant changes its' Gs depending upon E. The plants use of either feedforward, or feedback hypothesis, seems to depend upon wither or not the plant has been in drought conditions (media water stress) in the past, ie., low media water status. If the plant has been under media water stressed it is thought the feedforward hypothesis controls Gs, otherwise the feedback hypothesis is thought to control Gs.

Thanks,I understood.Do you have any link to any sort of scentific tests?with value etc... (don't think I don't believe you)

No, plants do very little respiration in day, if they do so they are in trouble, called "photorespiration" and it means greatly reduced rate of photosynthesis and carbon fixation. Normally this is caused form too much light or heat.

So,plants use a little amount of oxygen during the day both trough the leaves and roots,right?
During night they suck a lot of oxigen and use it to produce ATP,is this correct?

No, it's good to keep VPD lowish at night to keep E highish which allows greater translocation of Ca, Bo, N, etc., into tissue via. xylem. The stomatal conductance is reduced in darkness, but it's still wise to keep the VPD from 1.25-1.5 kPa...

Do you mean ''to keep VPD high'' ?
Keep it high helps in mold prevention too.

Thanks again for your kindness,I'm sorry for that post on UGB

One more thing...reading your attachments I found this:

And for you, because you are so into nutrient and roots (pH, etc) I think VPD would matter because ideal VPD (or close to ideal) allows for ideal rate of transpiration (E) which allows for ideal nutrient uptake. Putting the nutrients in the water/media is the first step, then we should provide the plant with the ability to use those nutrients the best it can.

I'm definetely interested to know which elements are more or less involved in transpiration...if you have something more accurate to explain...it's welcome.

Have a nice day mate :wave:

Carl Carlson · Nov 23, 2010

spurr said:
\Here is the Urban Gardener Blog post on VPD where the author fully plagiarized my work, I wrote it under the nic "gojo" (from the thread I uploaded for you). I bitched at them for the plagiarism, but they didn't care...assholes!
"Plantworks: Part 1 – Humidity and Vapor Pressure Deficit"
http://urbangardenmagazine.com/2010/07/plantworks-part-1-humidity-and-vapor-pressure-deficit

on July 1st, 2010 I posted this:

https://www.icmag.com/ic/showpost.php?p=3659553&postcount=4

Previously on June 28, I posted the same stuff to another forum (t h c f a r m e r ......)

And on July 12, 2010, the ubgangarden blog post was published....

It could be a coincidence.

BTW, the text files you attached are not readable.

Scrogerman · Nov 23, 2010

Great thread & i know only too well the damage VPD can cause in leaf tissue if not in the correct balance, RH to AirTemps, can mimick & cause certain deficiencys too ive observed.
Not sure weather to put a de-huey in my cab now actually, canopy temp is 74.4f lights on & drops to mid 60's lights off(63-65f), nice winter atmosphere, lower RH etc. Now my undercanopy RH is 45% lights on & 50-54% light off, but never any higher, i think this is perfect but im wondering about putting more lighting in, which is gonna increase temps & reduce lights-on RH, but this will also increase transpiration, so i'll prolly need a de-huey im thinking?. (use Liquid 'Si' too)
anyway great thread man!

noreason · Nov 23, 2010

Carl Carlson said:
on July 1st, 2010 I posted this:

https://www.icmag.com/ic/showpost.php?p=3659553&postcount=4

Previously on June 28, I posted the same stuff to another forum (t h c f a r m e r ......)

And on July 12, 2010, the ubgangarden blog post was published....

It could be a coincidence.

BTW, the text files you attached are not readable.

Interesting post carl,thanks for sharing.That scheme is very useful.

Great thread & i know only too well the damage VPD can cause in leaf tissue if not in the correct balance, RH to AirTemps, can mimick & cause certain deficiencys too ive observed.

Hi buddy.My plants suffered in the past week a Ca def.All I had to do to fix it was add a little air movement trough the canopy,and decrease RH from 80-85% to ca. 70%.A couple of days and plants didn't show anymore signs on the leaves.I also noticed a great water uptake from the dwc bucket.This mean VPD has real effect and a trained eye can see it clear.

im wondering about putting more lighting in, which is gonna increase temps & reduce lights-on RH, but this will also increase transpiration, so i'll prolly need a de-huey im thinking?. (use Liquid 'Si' too)
anyway great thread man!

Well,if you put more light,you add more heat and decrease RH,so VPD will be high and you will need an huey,not a de-huey.
Waht do you mean with liquid silicon?how do you use it?

Keep it dank bro :wave:

Scrogerman · Nov 24, 2010

temps increase, plants are gonna wanna transpire more, release more water & work harder, no doubt increase in photosynth but my RH to temp ratio is gonna go out the window, causing VPD symptoms late inm bloom could cause major problems & vitually instant damage, ive suffrered VPD damage on mnore than one occasion. I note problems with k uptake in low RH atmosphere, pretty fast, also major Mg mimicking symptoms, but its VPD damage for sure. Major shit & not enough info on the web at all about it. environmental balance is as imporatant as any link in the chain. put it straight & the symptoms go away, its pretty obvious what it is!(knowledge is power etc), wish i knew more!

i cant stop thinking about 250-400w HPS LED combo trips atm, doing my head in & the advances with green light etc.

*mistress* · Nov 24, 2010

this may be helpful:

vpd, transpiration, water

Carl Carlson · Nov 24, 2010

It is not coincidence that Urban Gardener blog used the same images I used in my thread, nor that they used some of the exact same concepts I posted (and that are unique to my thread), nor some of the exact same text I used, nor two of the same studies/papers I used.

I posted my thread like a week before the Urban Gardner blog was posted...

They plagiarized me, plain and simple.

your friend, spurr

Actually, I was talking about my posts about VPD to icmag and another popular forum from July 1st, shortly before yours and the blog posting were published...

Scrogerman · Nov 25, 2010

Thanks Mistress! explains it all

Its this way around Noreason bro:
in general...
lower rh(high vpd)=increased transpiration, translocation, water uptake, greater calcium absorption/transport.

higher rh(low vpd)=slower transpiration, translocation, water uptake, slower evaporation, increased growth.

t,w. & b, h.:

edit
Now why didnt i read the whole link first like i should of done, its all there & more lol, duaaahhhaa, bed time me thinks lol! thanks M*

(I thought VPD is caused by the balance of RH to airtempreture, its not just high or low RH that causes it, ie- High RH90% & Low Airtemps like 50f for example will cause VPD. there is a balance of RH to tempreture & if the balance is wrong then various VPD damage will occur in leaf tissue. i 'usually' first see this as a yellow blotch(irregular elipticle), then more necrotic yellow blotches appear if enviromentals are not addressed & RH & Airtemp balanced again. Lights on & off RH & Temps.
Ive got a chart somewhere, see if i can dig it out, ill track back & edit this post to include a link when i find the chart which explains what im saying.

have i got this right, im quite high@

You'll see it when you first throw in your HPS on young startlets, sucking air out your cab, 80+f airtemp & 25% RH, this will cause VPD(& problems with K assimilation at low RH) & the same of the oppisite(minus the K problems) RH to Temp balance. of course this is where a humidifier comes in handy to balance enviromentals or use a CFL like me. Ive only known about VPD for a few years, i should learn this stuff inside out, its that important. seen the damage, mimicking Nute def's etc for years though, never realised what it was. it was VPD all along. enviromentals are important & there should always be a comfortable balance for the plants, the best way to avoid VPD & possible plant/tissue damage that comes with it. perfect/optimal atmosphere 24/7 ono)

noreason · Nov 25, 2010

*mistress* said:
this may be helpful:

vpd, transpiration, water, etc...teehee

Thanks for the link mistress

Do you have more info about this test?

t,w. & b, h.:
Quote:
The major “benefit” alleged to accrue from transpiration (the evaporative loss of water from plant surfaces) is that it is essential for the long-distance transport of mineral ions, but the possible interrelation between these two processes has rarely been tested. Transpiration was experimentally dissociated from mineral supply by growing ... in hydroculture and providing mineral nutrients only during the nights. These plants grew as well as a control group that received nutrients only during the day and transpired 12–15 times more water during the exposure period. It thus appears that convective water transport in the xylem, brought about by root pressure and the resultant guttation, “growth water,” and...phloem counterflow is in itself sufficient for long-distance mineral supply and that transpiration is not required for this function.

It's an interesting point here...plants can absorb minerals regardless from water uptake...but does water uptake influences mineral uptake or not at all?I mean,when water enter the roots does it carry on also minerals?The answer should be not,am I right?
If can't imagine feeding my plants only at night and still see them grow nicely...

Its this way around Noreason bro:
in general...
lower rh(high vpd)=increased transpiration, translocation, water uptake, greater calcium absorption/transport.

higher rh(low vpd)=slower transpiration, translocation, water uptake, slower evaporation, increased growth.

This is right...and having a too low or too high VPD leads however to a reduced stomata conductance,means the stoma tends to close.

(I thought VPD is caused by the balance of RH to airtempreture, its not just high or low RH that causes it, ie- High RH90% & Low Airtemps like 50f for example will cause VPD. there is a balance of RH to tempreture & if the balance is wrong then various VPD damage will occur in leaf tissue. i 'usually' first see this as a yellow blotch(irregular elipticle), then more necrotic yellow blotches appear if enviromentals are not addressed & RH & Airtemp balanced again. Lights on & off RH & Temps.

VPD has to do only with humidity in the air I think...but humidity in the air is linked to air temperature because you know,air at higher temperature can hold more water than air at lower temperature,so here comes the VPD instead of RH.RH is relative to temperature,and tell you only how many water (in %) is in the air at a certain temperature,but it doesn't tell you what plants ''feel''...thing that VPD does.Is this thought right?

Ive got a chart somewhere, see if i can dig it out, ill track back & edit this post to include a link when i find the chart which explains what im saying.

Every info on the topic is welcome.I just want to understand how all these stuffs work to give my plants a great setup.

You'll see it when you first throw in your HPS on young startlets, sucking air out your cab, 80+f airtemp & 25% RH, this will cause VPD(& problems with K assimilation at low RH) & the same of the oppisite(minus the K problems) RH to Temp balance. of course this is where a humidifier comes in handy to balance enviromentals or use a CFL like me. Ive only known about VPD for a few years, i should learn this stuff inside out, its that important. seen the damage, mimicking Nute def's etc for years though, never realised what it was. it was VPD all along. enviromentals are important & there should always be a comfortable balance for the plants, the best way to avoid VPD & possible plant/tissue damage that comes with it. perfect/optimal atmosphere 24/7 ono)

I agree with you that the best thing to do is to keep the right microclimate all the way through...and to do this you need an huey and a de-huey so you can keep RH forever in the right range.The best would be to have a controller,doing all the work itself.

I'm going to read some more about VPD,it's not so easy to me to read scentific text not in my language,so I'm slow but I try the same

noreason · Nov 25, 2010

Mistress if you have some time can you link me something that explain phloem counter-flow please?I know what phloem is but I don't know how it works and what is the counter-flow.

Scrogerman · Nov 25, 2010

Nice One Noreason,
hey man, its definately Airtemp Vs RH & a couple of other things that make up/cause & influence VDP< its why i said i should have read mistress's whole link, its when the balanace is from one extreame to another, for example, High RH Low temps or the other way Low RH & High airtemps, leaf temp & root temps also influence it, didnt know the last bit untill reading M's info either. its all in that link of M's, confirms what im saying.

youve sort of got it though. its when the two are not in balance, & there is a set RH balance at different airtempretures, if it way out leaves/tissue/plants cant function properly, the atmoshphere is not balanced & VPD is caused.

i need to find that chart because it lists the RH to Temp, safe ranges, to prevent VPD, like i said before its about the perfect enviroment 24/7.

like this: temps to rh safe range
80f or ??c & 60-70%RH
70f or 21c & 50-60%RH
60f or ??c & 40-50%RH, & so on...

if you have 80f airtemps & low 30% RH, your going to suffer VPD damage, but if the RH was brought back to safe range 60-70%, enviroment is balanced & we dont have to worry about VPD.

these numbers are not accurate, im just giving you the idea.
The way the numbers have been explained so far has just complicated it for people, what is Kpa when its at home, never heard of 0.8%-1.25%Kpa is there a meter that reads in this Kpa or whatever it is?, it would be better for people to understand & prevent like ive listed above imo, now if i can just find that chart you can see exactly what i mean! There is a set tempreture to RH balance to prevent VPD & my chart lists the numbers in laymans terms! Im quite sure *Mistress* will know the chart i speak of.

still i think this thread has been a major success in explaining what VPD is, & the damage it can cause, has anyone got photo's of VPD damage, like i explained in a previous post?, i had some really nice examples but my boy deleted them again, little bugger!

Cheers! best of Luck!

noreason · Nov 25, 2010

Nice One Noreason,
No man, its definately Airtemp Vs RH & a couple of other things that make up/cause & influence VDP< its why i said i should have read mistress's whole link, its when the balanace is from one extreame to another, for example, High RH Low temps or the other way Low RH & High airtemps, leaf temp & root temps also influence it, didnt know the last bit untill reading M's info either. its all in that link of M's, confirms what im saying.

SM I mean that the deficit between the leaf and the surround air is due to the quantity of water in the air,because leaf has 100% umidity inside.
In fact when RH is 100% (don't matter wich temperature) VPD is 0.At any temperature.
So I can assume that VPD is linked only to the quantity of water in the air,but water in the air in turn is linked to air temperature,because at the same level of water in the air but at different temperature we have different RH values.That's why RH is relatively to the temperature.

youve sort of got it though. its when the two are not in balance, & there is a set RH balance at different airtempretures, if it way out leaves/tissue/plants cant function properly, the atmoshphere is not balanced & VPD is caused.

VPD is not anything you can ''cause'' it's only a scale.It's a value like temperature or rh,co2 or whatever.

i need to find that chart because it lists the RH to Temp, safe ranges, to prevent VPD, like i said before its about the perfect enviroment 24/7.

like this: temps to rh safe range
80f or ??c & 60-70%RH
70f or 21c & 50-60%RH
60f or ??c & 40-50%RH,
these numbers are not accurate, im just giving you the idea.
The way the numbers have been explained so far has just complicated it for people, what is Kpa when its at home, never heard of 0.8%-1.25%Kpa is there a meter that reads in this Kpa or whatever it is?, it would be better for people to understand & prevent like ive listed above imo, now if i can just find that chart you can see exactly what i mean! There is a set tempreture to RH balance to prevent VPD & my chart lists the numbers in laymans terms! Im quite sure *Mistress* will know the chart i speak of.

I already saw and printed the VPD chart...found in one of the attachement posted by spurr (thanks mate

)Here is it:

ps:1 bar is 100.000 pascal or 2.088 pound\squarefeet or 750 Torr

I never heard of something to measure VPD in Kpa also if I'm pretty sure someone had made it already because it's pretty simple to made...however what I mean is to achieve the right VPD value using a chart like the one posted here.It's very simple and also a noob grower can use it easily.

still i think this thread has been a major success in explaining what VPD is, & the damage it can cause, has anyone got photo's of VPD damage, like i explained in a previous post?.
Cheers! best of Luck!

I'm growing 2 bluecheese in my dwc bucket,and a couple of weeks ago they had trouble with a Ca deficiency.All I needed to do is to add some ventilation trough the canopy and lower the RH from 80's till 70%.In a couple of days I didn't see anymore signs (brown path\spot) on the leaves.
I can assume this kind of stuff was due to the low transpiration levels of the plants.So for me this is a too low VPD damage,but it's not directely linked to VPD,but to Ca uptake,that is linked to the transpiration,that is linked to VPD.

A couple of pics of what I'm writing:

One more question is: does atmospheric pression affect VPD value???

Scrogerman · Nov 25, 2010

lol Right on, but the way i see it & i see direct damage, is the balance between airtemp to RH, extreames of one to the other cause damage. 100% RH to 50f airtemp & this is possible with de-huey/hueys etc will cause VPD damage to leaves?, necrotic blotches, seen a few different symptoms(but not at 100%, when is it ever 100%?), i suppose its extreames of RH to Airtemp that cause assimilation of element problems likle the Ca problems you had, ive seen problems with K at low RH & high airtemps too, this is VPD damage.
If mistress or spurr or even yourself can put me straight id really appreciate it.
cheers buddy! It doesnt matter at 100% RH then but does at 90%? i dont get this bit!????
(is this because the internal pressure of the leaf is 100% too?) if its 100% outside its gonna be 100% inside so that gives 0/zero VPD, is this right?

thanks for the chart man!

& the pics, funny how VPD breaks down the chlorophyl in the leaves, mine seemed to dissolve-(the green colour), leaves were showing like a Mg def but the leaf coulour turned 'Golden', very similar to your pics. nice example btw.

VPD is something You can cause, look at the damage to your leaves because of VPD, its messed up Ca(you think) assimilation & the plants have become deficient, or the damage is just mimicking Ca defs, not soo sure on that one either, wheres all the teachers gone when you need em!

VPD and nutrients absorption

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