What are the factors that affect leaf temp, is it mainly light intensity/distance or does environment play a role?
When I say to watch leaf temp, I guess I should have worded it better. What I meant was to watch the leaf temperature offset. (the difference between the air temp and the leaf temp).
Here's a brief (only a half cup of coffee required) explanation as to why it matters.
There are actually 2 VPD's at work. There is atmospheric VPD and plant VPD,but all you need to worry about is plant VPD, because if it's correct, or even close, atmospheric VPD is automatically correct. Here's what I mean.
Atmospheric VPD (AVPD)- The atmosphere's ability to wick water from everything. The atmosphere's drying ability.
Plant VPD (PVPD)- The plant's ability to transpire from itself to the atmosphere.
Example 1 - Air temp 76, leaf temp 74, RH 48 = PVPD 1.40 kPa.
That means the evaporation factor that occurs at the leaf is a suction of 1.40 kPa and it's caused by the difference in leaf temp to air temp.
The heat difference pulls water out the stomata that came in thru the roots.
So what happens to that water once it comes out of the stomata and condenses on the leaf?
It needs to evaporate or else it will flood the leaf and stomata will close to prevent the water from causing back pressure in the leaf from laying in a puddle over the stomatal opening. If it doesn't evaporate it will choke the PVPD process.
Enter AVPD, the atmospheres ability to wick water. It's a 2 part equation because there is no leaf to involve a leaf temp offset so it's formula is based off of 2 thing's, RH and air temp. To cancel out leaf VPD in a VPD calculator you simply set the leaf temp to equal the air temp, which tells the calculator to switch formulas, or use an atmospheric VPD calculator. This is designed so 1 meter, say a Pulse VPD Monitor, can be used to calculate either AVPD or PVPD. Otherwise you would need 2 meters. So in this example it would be air temp 76, leaf temp 76, RH 48. That equals an AVPD of 1.59. So the atmosphere is wicking water faster than the leaf is transpiring water. The air has a drying effect.
Example 2.
Inputs are air temp 76, leaf temp 75, RH 48.
PVPD = 1.49
AVPD = 1.59
The difference between the 2 is now smaller as the leaf is transpiring faster but the atmosphere's ability to wick water hasn't changed, so in reality the air can't dry the leaf as fast.
Example 3
air = 76, leaf = 76, RH =48
PVPD = 1.59
AVPD = 1.59
The air is wicking as fast as the leaf is transpiring. It never dries but also never floods the stomata, so in reality the plant now has wet stomatal openings, but they aren't yet under water.
Example 4.
Air = 76
leaf = 77
PVPD = 1.70
AVPD= 1.59
The leaf is now transpiring water faster than the atmosphere is wicking it. The stomatal opening is now under water and water has weight, so instead of water pulling out of the leaf it is stalked and starting to leak back into the leaf. The process has reversed and the plant is now drying the atmosphere, so the stomata slams shut and transpiration stops.
In summary. Air temp is air temp. The difference between leaf temp and air temp pulls the water out the stomata. It's called transpiration. If water transpires out the leaf faster than it can be mopped up you drown. If PVPD is correct, AVPD is automatically correct. You only need to maintain a 2 degree offset to have correct AVPD.
Now enter light intensity.
When light strikes a leaf the plant absorbs the radiation and uses it to photosynthesize.
If more light is striking the leaf than the plant can use to photosynthesize, then the excess radiation causes resistance and resistance causes heat. Too much light causes heat and the leaf temp rises.
A 2 degree offset is the sweet spot for PVPD. A plant will always do everything in it's power to maintain this 2 degree offset because that is where the plant's rootball's ability to intake water matches the ability to exhaust water, and have the atmosphere wick it away sufficiently.
It's running at the optimal rate for it's rootballs capability in the atmosphere provided.
The coolest part.
If you adjust light intensity to create a 2 degree offset, your PPFD is automatically correct for the size of your rootball. No need for PPFD or DLI now, she's dialed without a meter.
Use your meter to see what the PPFD is so you are educating yourself, but I grew plants my entire life without a PPFD meter.
I have no idea what her rootball can do in the current atmosphere, so arbitrarily assigning 1100 PPFD when a 2 degree offset occurs at 850 PPFD will 100% for sure cause detriment, so when you see someone call me an idiot for saying VPD is the best way to set your light intensity, and they profess to be a light guru and start quoting Bugbee and his belief that plants can easily use as high as 1500 PPFD, then you better have one helluva set of roots or else reconsider who the idiot is in the conversation, and follow a different light guru.
Yeah it happens and it causes huge nasty scenes, but now you know why one of the idiots in that convo was right, and one wasn't.
Pretty cool hey? Don't profess to know how much light she wants. VPD is smarter than us.
VPD is everything to a plant. It's literally the world she lives in above ground being put into sync with her world underground. It's your most valuable tool.
When someone scoffs at it you should probably not take their advice, they really have no idea.
As long as the roots match the shoots your good.
The delta on mine didn't change much even though rh went from 31 to 41. That's still too dry and I'll increase it over the next few days, but I thought more moisture would help the plant improve its performance, but sadly, no.
Maybe that happens once I get into proper range?
RH effects air temp. Wetter air is cooler air, so when you change RH it takes awhile for the temps to adjust.
Also this is why you want to read VPD late in her day. 10 hours after lights on. Thats when she's running full steam. That's the reading you do adjustments by.
Changing 1 of the 3 parts physically changes the other 2, so it takes awhile to get the hang of it.
Take readings all day long if you want, it will teach you how a plant changes her own leaf temps to control herself in her atmosphere, but only make adjustments at 10 hours or later, as that's when she is working her hardest and running her warmest as a result of her exercise all day long.
At night a plant will warm her own leaves to be 2 degrees warmer than air so she can reverse from transpiration to respiration.
Then when lights come on she doesn't immediately flip back, she graduates into it and fluctuates her leaf temps to control the graduation.
At 10 hours, graduation is complete for sure, so that's your reading without her influence. It's her maximum transpiration rate. You never want to over transpire her. Making changes earlier will magnify later.
Setting her VPD to 1.40 at lights on will definitely not be 1.40 anymore when she's really growing fast at 10 hours, and the result is she won't be growing at all at the 10 hour mark, she will be suffering.
She actually will still be growing, just not well, and brix are trashed for the day.
You only have 112-126 days, so every bad day knocks of almost 1% of her life.
You won't notice immediate results, but you certainly will notice it for better or for worse on harvest day.
Ok maybe it was a full cupper
).
. Merry Christmas Eve!
